Preprints in GR/QC

Apratim Ganguly, Radouane Gannouji, Akshay Kumar

Published: 2026-06-18

Categories: gr-qc

We show that higher-curvature Lovelock terms do not restore local cosmic censorship in spherical dust collapse, but instead promote the local visibility of central shell-focusing singularities. On the collapse branch with positive highest-order Lovelock coefficient \(c_N\), the highest nonvanishing Lovelock order \(N\) controls both the near-singularity collapse and the formation of trapped surfaces. In noncritical dimensions, \(D-1-2N>0\), the apparent-horizon curve approaches the singularity curve with trapping exponent \(β_N=(D-1)/(D-1-2N)\). Comparing this scale with the first nonvanishing correction \(r^\ell\) to the singularity curve gives the local-visibility condition \(\ell<β_N\), provided the singularity curve opens outward. Thus increasing \(N\) enlarges the class of inhomogeneous initial data producing outgoing radial null rays from the central singularity. In the critical odd-dimensional branch, \(D=2N+1\), no apparent horizon forms sufficiently close to the center, so any outward opening of the singularity curve gives local visibility. The locally visible singularities are Królak-strong along the emerging null rays, with Tipler strength reached at threshold. For bound and unbound collapse, the noncritical exponents are unchanged: the energy function modifies the opening of the singularity curve, while in the critical branch it enters the leading terminal collapse velocity.

Diogo S. Gorgulho, Jacob A. Litterer, João G. Rosa

Published: 2026-06-18

Categories: hep-ph

We study the effects of string axion emission on dark matter production by light primordial black holes (PBHs), through both evaporation and superradiance. We show, in particular, that the Hawking emission of $\mathcal{O}(100-10^5)$ light axion species predicted in realistic string theory constructions can significantly enhance the efficiency of superradiance, given the associated increase in the PBH spin. The string axiverse thus significantly expands the parametric regions (dark matter mass and PBH mass and spin) for which a sizeable fraction of dark matter may presently be in the form of ``micro-boson stars'': the self-gravitating remnants of superradiant dark matter clouds. Conversely, for too large a number of axion species PBHs evaporate too quickly for superradiant clouds to attain their maximum mass. Finally, assuming that all dark matter is produced by PBHs (through both superradiance and Hawking emission), we show that the axions emitted during PBH evaporation give an immeasurably small contribution to the relativistic degrees of freedom at recombination.

Karen Yagdjian

Published: 2026-06-18

Categories: gr-qc

The explicit formulas for the spherical solutions of the Dirac equation in the expanding universe are given. The initial value of the solution can be, in particular, a wave function of the hydrogen-like atom or a spherical wave in the Minkowski space, that then propagates in the Friedmann-Lemaître-Robertson-Walker space-time, which is expanding with the de~Sitter scale

Marco van den Beld-Serrano

Published: 2026-06-18

Categories: math.DG

Given a fixed past-directed timelike vector field, does there exist a time function whose gradient is optimally aligned with it? We address this question by introducing a functional that, on the one hand, captures the misalignment between the timelike vector field and the gradients of suitable Sobolev functions, and, on the other hand, penalizes null gradients. Our analysis focuses on compact subsets of smooth stably causal spacetimes. More precisely, we prove that, under suitable assumptions on the Sobolev index and the strength of the null gradient penalization, there exists a unique smooth temporal function which minimizes the considered functional. We refer to this minimizer as the \emph{alignment time function}. Furthermore, several useful properties of the alignment time function are established: there exists a canonical procedure to improve its steepness, it is stable under $C^{p}$ convergence of the underlying metrics and vector fields and it inherits the symmetries shared by the metric and the given vector field.

Maarten Grothus, V. Vilasini

Published: 2026-06-18

Categories: gr-qc

In PRL 129, 110401 it was shown that it is theoretically possible to have operationally detectable causal loops without violating the principle of no superluminal signalling (NSS) in (1+1)-Minkowski spacetime. Whether or not such causal loops are also possible in $d > 1$ spatial dimensions, has remained a key open question. We resolve this question by showing that in a wide class of "conical" spacetimes, including Minkowski with d > 1, NSS does rule out all operationally detectable causal loops, in classical, quantum and post-quantum theories. This establishes that the relationship between the relativistic principles of NSS and no causal loops depends inherently on the geometry of spacetime.

Hyukjung Kim, İlayda Kuzu, Kerem Özsoy, Zeynep Kahraman, Wan-Il Park, Heeseung Zoe

Published: 2026-06-18

Categories: hep-ph

We investigate the first-order phase transition that terminates thermal inflation and evaluate the associated stochastic gravitational-wave signals. The transition is first characterized through semi-analytic calculations of the bounce action, which are compared with numerical results obtained using CosmoTransitions. We then study its real-time evolution in a three-dimensional Langevin lattice simulation that incorporates Hubble expansion and the corresponding temperature evolution throughout the transition. The lattice dynamics are consistent with the bounce-action estimates: the transition proceeds through localized bubble nucleation and subsequent bubble growth, rather than through a phase-mixing instability. Using the resulting transition parameters, we estimate the gravitational-wave spectra generated by bubble collisions and acoustic motions in the plasma. The predicted stochastic background lies within the projected sensitivity ranges of future gravitational-wave observatories, including BBO and DECIGO.

Samir D. Mathur

Published: 2026-06-18

Categories: hep-th

The black hole information paradox challenges us to do something that is seemingly impossible: find a violation of the semiclassical approximation in a region where all curvatures are low. The vecro hypothesis proposes a structure of the gravitational vacuum that can accomplish this task. In this article we explain the hypothesis, and give a lattice model to describe the essence of its idea. The Hamiltonian of the model is completely local, but the vacuum exhibits correlations among planck scale fluctuations which fall off relatively slowly with distance. These extended-scale correlations are able to `feel around' the region where a closed trapped surface is about to form, and to react by nucleating fuzzball structure that destroys semiclassical spacetime.

Feng-Yin Cheng, Pei-Ming Ho, Wei-Hsiang Shao

Published: 2026-06-18

Categories: hep-th

We demonstrate that, for a large black hole of radius $a$, Hawking radiation terminates around the scrambling time $u_{\text{scr}} \equiv 2a \log(a/\ell)$ due to the nonlocal, exponential suppression of trans-Planckian interactions inherent in string field theory (SFT). Modifying a massless scalar field's interaction with a dynamical black hole background via the smearing operator $e^{\ell^2\Box}$ (where $\ell$ denotes the string length scale), we calculate the time-dependent number expectation value $\langle \hat{N}(u) \rangle$ of outgoing Hawking particles at retarded time $u$. While the standard Planck spectrum at the Hawking temperature is reproduced at early times ($u \ll u_{\text{scr}}$), the particle number approaches zero shortly after the scrambling time. This early shutoff reflects the property that the collapsing shell becomes effectively invisible to trans-Planckian modes, offering an alternative resolution to the black hole information paradox via a macroscopic remnant.

Ariel Guzmán, Mohsen Fathi, J. R. Villanueva

Published: 2026-06-18

Categories: gr-qc

We study high-frequency electromagnetic propagation in the rotating García--Díaz solution of Einstein gravity coupled to NLED. In this system, light is not governed only by the null cone of the spacetime metric, because the NLED field also behaves as an optical medium whose constitutive response determines the physical optical cones. Starting from the mixed electromagnetic potentials, we project the field $F$ and the excitation $P$ on a principal tetrad and obtain the aligned scalars $E$, $B$, $D$ and $H$. These scalars allow us to reconstruct the regular local constitutive branch connected with Maxwell theory through the map $(D,B)\mapsto(E,H)$. We then insert the resulting response matrix into the Fresnel characteristic problem. At the perturbative order considered here, the Fresnel quartic factorizes into two quadratic branches, each defining an effective optical metric. Both optical metrics admit Carter-type separation of the Hamilton--Jacobi equation and possess their own radial and angular potentials, critical constants and unstable critical families. By projecting these families onto the celestial sphere of a finite-distance observer, we obtain two critical contours, $Γ_+$ and $Γ_-$, which coincide in the Maxwell limit and split when the nonlinear constitutive response is active. We quantify this birefringent splitting through the maximum angular separation, the relative diameter shift and the normalized birefringent width. Numerical scans over the nonlinear coupling, spin and observer inclination show that the splitting is generated by the constitutive response, redistributed by rotation and stable under local projection changes within the perturbative domain. This provides a direct geometrical link between the local NLED response and a polarization-dependent critical structure on the observer screen.

Avi Vajpeyi, Giorgio Mentasti, Quentin Baghi, Ollie Burke, Lorenzo Speri

Published: 2026-06-18

Categories: gr-qc

The Wilson-Daubechies-Meyer (WDM) time-frequency transform has been widely used in gravitational-wave astronomy, yet a self-contained, mathematically explicit reference for practitioners remains lacking. This is especially true for those wishing to adopt the transform in modern Python and JAX inference workflows. We present wdm_transform, an open-source Python package implementing the WDM wavelet-packet time-frequency transform, and document its mathematical foundations, statistical properties, and practical implementation for gravitational-wave data analysis. The package supplies NumPy and JAX backends, both transforms (forward and inverse) validated to floating-point precision, with the JAX backend enabling GPU-accelerated transforms of million-point data streams in tens of milliseconds. As a worked example, we verify that the WDM-domain likelihood reproduces frequency-domain posteriors for a resolved LISA galactic binary under a shared stationary noise model, confirming numerical equivalence of the two representations in that controlled setting. This work paves the way for systematic optimisation of WDM tilings, a particularly promising direction for the non-stationary noise, stochastic backgrounds, and data gaps anticipated in future detectors, and for direct comparisons with alternative time-frequency representations needed to meet the challenges of future gravitational-wave data analysis.

Bayram Tekin

Published: 2026-06-18

Categories: gr-qc

Born-Infeld New Massive Gravity (BINMG) completes New Massive Gravity to all orders in curvature through the determinant of the metric shifted by the Einstein tensor. We recast it with an independent auxiliary metric $q_{μν}$, whose algebraic equation of motion $q_{μν}=g_{μν}+\fracσ{m^2}G_{μν}(g)$ recovers the determinant action exactly on the regular branch and resums the infinite curvature series into a single relation. In the densitized variable $P^{μν}=\sqrt{-q}\,q^{μν}$ the three-dimensional action is polynomial, with all derivative dependence carried by the coupling $P^{μν}G_{μν}(g)$. The formulation makes known properties follow with substantially less algebra: the unique vacuum follows in one line, and the quadratic action yields a single Pauli-Fierz massive spin-2 field with the Fierz-Pauli tuning generated rather than imposed. On locally AdS backgrounds the conserved charges, BTZ mass and angular momentum, central charge, and entropy reduce to the Einstein results times a common factor. The formulation also isolates the nonlinear degree-of-freedom problem in the right variables, leaving the full Dirac count to separate work.

Ratikanta Swain, Sayantan Ghosh, Bharat Kumar

Published: 2026-06-18

Categories: nucl-th

In energy-momentum squared gravity (EMSG), the spacetime inside a neutron star is sourced by effective thermodynamic variables that need not coincide with the physical fluid pressure and energy density. It is therefore an open question whether the trace anomaly of dense matter -- the QCD measure of how strongly conformal symmetry is broken -- still organizes interior profiles and curvature in the same way it does in general relativity (GR). We adopt a clear matter-geometry separation: the trace anomaly is computed from the fluid sector alone, while spacetime curvature scalars are built from the variables that actually source the modified Tolman-Oppenheimer-Volkoff equations. For five relativistic mean-field equations of state, the radial trace-anomaly profiles increase monotonically from core to surface in all accepted EMSG models, as in GR, but split systematically with the EMSG coupling strength; the splitting grows with stellar compactness. Despite this deformation, curvature invariants still fall onto organized bands when plotted against the trace anomaly, extending the GR thermodynamic-geometric correspondence. The Ricci contraction shows the tightest organization, whereas the Ricci scalar remains the most equation-of-state sensitive. EMSG effects are modest for observationally accessible stars but largest in stiff, ultracompact configurations, indicating that the trace anomaly remains a useful thermodynamic label for interior geometry even when gravity couples nonlinearly to matter.

Hassan Firouzjahi

Published: 2026-06-18

Categories: gr-qc

We study regular black hole solutions sourced by an anisotropic energy momentum tensor. It is well known that the geometry of the interior of a spherically symmetric regular black hole approaches the dS metric. Having decomposed the energy momentum tensor into its isotropic and anisotropic components, we assume a hydrodynamic equation of state, $P= P(ρ)$, for the pressure, and look for spherically symmetric, regular black hole solutions. We consider different forms of $ P(ρ)$ which yield the previously known regular black hole solutions, as well as various new metrics. We show that the profile of $ P(ρ)$ has a root and a maximum as it approaches $0^+$ at large distances. Consequently, the square of the sound speed of perturbations, $c_s^2$, changes sign at the point where $P$ reaches its maximum, indicating a potential hydrodynamic instability. In addition, imposing the subluminal bound on $c_s$ puts strong constraints on the model parameters, excluding models in which the energy density has an exponential fall off. We establish a universal hierarchy among the relative positions at which the strong energy condition is violated, at which $P$ has its root, and at which $P$ attains its maximum.

Jack C. M. Hughes, Joudy F. Jamal Beek, Fedor V. Kusmartsev

Published: 2026-06-18

Categories: gr-qc

In Plebanski's formulation the coupling of matter is less direct than in the metric formulation since the energy-momentum tensor $T_{μν}$ is symmetric, while the Plebanski variables are naturally valued in the self-dual/anti-self-dual Hodge decomposition of 2-forms. An explicit construction of the Plebanski matter source $T^i$ is obtained by lifting the trace-free energy-momentum tensor $\hat T_{μν}$ into the $(1,1)$ component of the algebraic curvature space using the Kulkarni-Nomizu product, and then extracting its chiral components. This construction reproduces the definition for $T^i$ in terms of the self-dual basis $Σ^i$ and $\hat T_{μν}$ introduced by Krasnov. We also verify that the matter-coupled chiral field equations imply the usual conservation law $\nabla_μT^{μν}=0$ through the chiral Bianchi identity $d^A F^i=0$. As an application, the construction is applied to a spherically symmetric electromagnetic stress-energy tensor, where the anti-self-dual part of the matter-coupled Plebanski field equations yields the Reissner-Nordström-de Sitter solution. The result gives a systematic prescription for translating metric matter sources into the anti-self-dual source terms required by the Plebanski field equations.

Dejiang Li, Tonghua Liu, Kai Liao, Beining Xia, Cuihong Wen, Jieci Wang

Published: 2026-06-18

Categories: astro-ph.CO

LISA-band gravitational waves from massive binary black holes can be diffractively lensed by low-mass dark matter halos and subhalos, so their frequency-dependent amplification can probe the inner density profile. We isolate the generic finite-core part of this signal by comparing fixed-mass Navarro-Frenk-White (NFW) and cored-NFW lenses and propagating both profiles to the complex wave-optics amplification factor. A finite core smooths the time-delay response and reshapes the diffraction peak; an NFW template with a lower concentration can mimic part of the effect, but structured complex residuals remain after time and phase alignment. The residual peaks for intermediate cores, $r_c/r_s\simeq0.25$--$0.3$. An SIDM-inspired isothermal-core profile gives the same qualitative response, showing that the signal is not an artifact of one cored parameterization. For a fiducial LISA source, an appreciable mismatch requires favorable near alignment and $M_{\rm vir}\gtrsim 10^7M_\odot$. The result is a finite-core baseline for isolated line-of-sight halos and for subhalos perturbing strongly lensed macro-images.

Jan Kelleter, Patrick Schillings, Jonathan Kuckert, David Bertram, Markus Bachlechner, Achim Stahl, Johannes Erdmann

Published: 2026-06-18

Categories: astro-ph.IM

The gravitational effects of seismic waves, so-called Newtonian noise, will likely limit the low-frequency sensitivity of future ground-based gravitational wave detectors, such as the Einstein Telescope. It has been proposed to mitigate this noise source by predicting it from measurements of the surrounding seismic displacement field using an array of seismometers. In this paper, we investigate the Newtonian noise prediction abilities of neural networks based on synthetic data from such seismometer arrays and compare the results with the Wiener filter as benchmark. We developed a simulation that generates density fluctuations of random plane waves and Gaussian wave packets, and that calculates the resulting Newtonian noise and displacement field. We investigate the performance on approximately stationary wave fields and single dominating long- and short-term events. For the first case, we observe comparable performance of neural networks and the Wiener filter with the networks performing slightly better. For the second case, however, we find that convolutional neural networks and graph neural networks can outperform the Wiener filter by factors of 15-80, depending on the frequency and the array configuration, and that they can reduce the corresponding Newtonian noise amplitude spectral density by factors of 10-30.

Akbar Davlataliev, Abdurakhmon Nosirov, Odil Yunusov, Bobomurat Ahmedov, Jackson Levi Said

Published: 2026-06-18

Categories: gr-qc

Cubic Galileon cosmological models provide a well-motivated framework for investigating late-time cosmic acceleration beyond the standard $Λ$CDM paradigm. In this work, we study observational constraints on cubic Galileon models within the teleparallel gravity framework, where deviations from the standard teleparallel equivalent of general relativity are encoded through the model parameter $b_1$. We consider two scalar-field potentials, namely quadratic and exponential potentials, and analyze four representative scenarios: quadratic and exponential potentials with $b_1$ treated as a free parameter, together with the corresponding cases in which $b_1=2$ is fixed. Using the $\text{Pantheon}^+$ Type Ia supernova sample, cosmic chronometer measurements, SH0ES information, and baryon acoustic oscillation data, we constrain the cosmological and model parameters and compare the observational viability of the different scenarios. We find that the considered teleparallel cubic Galileon models can accommodate the late-time expansion history, although the statistical preference depends on the choice of potential and on whether $b_1$ is fixed or varied. In particular, the fixed-$b_1$ model with a quadratic potential provides the most competitive fit among the Galileon scenarios when BAO data are included, showing a lower $χ^2_{\min}$ than $Λ$CDM and comparable support according to the AIC criterion. However, the BIC criterion continues to favor the minimal $Λ$CDM model because of the larger parameter space of the extended models. These results suggest that teleparallel cubic Galileon cosmologies remain phenomenologically viable, while a stronger claim regarding the Hubble tension requires further consistency tests.

Eduardo Bittencourt, Mariam Campbell, Peter K. S. Dunsby, Sergio E. Jorás

Published: 2026-06-18

Categories: gr-qc

We study static, spherically symmetric stellar configurations in an extended class of Energy--Momentum Squared Gravity using the covariant \(1+1+2\) semi-tetrad formalism. For perfect physical fluids, we show that the nonlinear matter corrections can be reinterpreted as an effective perfect fluid, so that the stellar equilibrium equations retain the standard Tolman--Oppenheimer--Volkoff form when written in terms of effective variables. The resulting covariant structure equations are formulated in both metric and dimensionless variables and, whenever an effective closure relation exists, reduce to an autonomous planar dynamical system. This provides a global qualitative description of the stellar phase space in terms of finite and asymptotic critical points. Specializing to linear physical equations of state, we recover the general relativistic benchmark and identify sectors that are exactly, asymptotically, or piecewise equivalent to general relativity, as well as sectors -- particularly dust configurations -- for which the planar reduction breaks down and the full three-dimensional covariant flow must be considered. We further recover the standard metric Tolman--Oppenheimer--Volkoff equation in terms of effective variables and show that, although the exterior spacetime remains Schwarzschild, the natural matching condition at the stellar surface is \(p_{\rm eff}(R)=0\), which need not coincide with \(p(R)=0\) for self-bound matter.

Roberto Giambò, Camilla Lucamarini

Published: 2026-06-18

Categories: gr-qc

We extend the analysis of photon surfaces in spherical dust collapse to the general, non-marginally bound case, i.e.\ allowing the \textit{energy function} $k(x)$ of the Lemaître--Tolman--Bondi (LTB) model to be non-zero. Starting from the dynamical-systems formulation developed in our previous work for the marginally bound case (arXiv:2509.01368), we derive the photon surface equations for the LTB metric with $k(x)\neq 0$, and we recast the geometric condition for a timelike hypersurface to be a photon surface as a non-autonomous first-order dynamical system. Even though the LTB evolution equation does not integrate in closed form when $k(x)\neq 0$, the implicit solutions available in the literature, together with comparison theorems for ordinary differential equations, are sufficient to show that the only physically acceptable extension of the exterior photon sphere $r=3M$ into the collapsing dust cloud is a null hypersurface, generated by outgoing radial null geodesics. Combining this fact with the known results on the causal structure of the general LTB model, we then establish that the photon surface reaches the central singularity if and only if the singularity is naked, thereby extending to the general case the picture already known in the marginally bound regime. The structural dichotomy between the naked and covered end states is also discussed in connection with possible observational signatures in the early-time evolution of black-hole shadows.

Takamasa Kanai

Published: 2026-06-18

Categories: gr-qc

We investigate the impact of higher-curvature corrections on photon propagation within an effective field theory framework and explore their observational consequences in strong gravitational fields. In particular, we consider polarization-dependent modifications to photon trajectories induced by higher-order curvature terms and analyze their effects in static and spherically symmetric spacetimes, focusing on Schwarzschild and Reissner-Nordström backgrounds. Using the geometrical optics approximation, we derive the effective metric governing photon propagation and study the resulting shifts in the photon sphere. Based on this modified propagation, we compute the quasinormal modes in the eikonal limit and examine their dependence on the polarization modes. We further analyze gravitational lensing observables, focusing on the deflection angle, incorporating the polarization-dependent corrections. Our results clarify how contributions from beyond-general-relativity effects manifest in both quasinormal mode spectra and strong gravitational lensing observables. These findings further suggest the possibility of placing meaningful constraints on effective field theories.

Himeka Matsuo, Asuka Ito, Kazunori Kohri, Teruaki Suyama, Ryutaro Tomomatsu

Published: 2026-06-18

Categories: gr-qc

It has been observed that the Universe is permeated by the cosmic photon background, ranging from radio waves to gamma rays. We investigate the conversion of the photon background into gravitons in the presence of background magnetic fields in the Milky Way Galaxy and in blazar jets. We find that the resulting graviton background is dominated by the contribution generated in blazar jets. Importantly, this graviton background constitutes a graviton floor for high-frequency gravitational wave detectors searching for new physics, analogous to the neutrino floor.

Salih Kibaroğlu, Mustafa Senay

Published: 2026-06-17

Categories: gr-qc

We study a $q$-deformed extension of Brans-Dicke gravity in a spatially flat Friedmann-Lemaître-Robertson-Walker space-time. The deformation enters through a coupling function that modifies the effective gravitational strength and leads to generalized Friedmann equations. In the matter-free sector, we obtain exact analytic solutions for the scale factor and the Brans-Dicke scalar field, and recast the scalar contribution as an effective fluid. We show that the corresponding equation-of-state parameter and the deceleration parameter are constants and depend only on the Brans-Dicke coupling $ω$ and the deformation function, allowing the scalar sector to mimic radiation-, matter-, or dark-energy-like behavior for a restricted region of parameter space.

Jinn-Ouk Gong

Published: 2026-06-17

Categories: astro-ph.CO

We formulate reheating as a constrained variational problem in the space of equation-of-state histories, rather than attempting to describe it through microscopic models. We introduce a regularized functional framework that identifies reheating histories which extremize a given cosmological observable under minimal physical assumptions. As illustrative applications, we consider prompt gravitational waves, induced gravitational waves, and primordial black holes. We find that different observables select qualitatively different regions of reheating-history space. These examples demonstrate that cosmological observables define distinct extremal directions in reheating-history space and can therefore be used to systematically explore the space of post-inflationary expansion histories.

Zheng Zhang

Published: 2026-06-17

Categories: physics.comp-ph

When stochastic field dynamics are cast into a path-integral formulation, perturbation theory becomes systematic but the resulting expansion quickly grows combinatorially large. The setting targeted here includes multi-component, multi-dimensional fields with matrix propagators, tensor-valued couplings, and non-Gaussian driving noise specified by arbitrary $n$-point cumulants. Wick pairings grow factorially, and component indices must be routed through the tensor-valued vertices. The useful output is not a raw contraction list, but a diagram table: one entry per topology, with multiplicities, coupling sums, signs, and causal constraints resolved. We present sft-wick, an open-source Python package that constructs these diagram tables and computes their integrals numerically. Given an action and an observable, it enumerates topologically distinct Feynman diagrams, derives their algebraic coefficients, and evaluates the resulting diagram integrals from user-supplied response and cumulant functions. The core algorithm enumerates spatial topologies before routing component indices, avoiding contraction-by-contraction Wick expansion. Response-field constraints, including vanishing response-response contractions, the ito prescription, and the absence of causal response loops, are enforced during enumeration. Predictions are validated against direct Langevin simulation, agreeing to within the simulation's statistical noise.

Zheng Zhang, Philip Bull, Chris Clarkson, Andrina Nicola

Published: 2026-06-17

Categories: astro-ph.CO

Standard weak-lensing calculations treat lensing as a linear remapping of the matter field along the line of sight. We instead formulate lensing as a stochastic field theory for the Sachs optical scalars, driven by random Ricci-focusing and Weyl-shearing fields. The resulting path integral generates a diagrammatic expansion for arbitrary $n$-point correlation functions of lensing observables, organised into linear response, nonlinear propagation, and driving-field cumulants. The conventional calculation emerges as the lowest-order, linear-propagation limit. Beyond it, nonlinear Sachs evolution couples to driving-field non-Gaussianity, mixing the matter cumulant hierarchy into the lensing hierarchy. A selection rule governs the couplings: an $n$-point observable receives a direct contribution from the $n$-point driving-field cumulant, and its leading hierarchy-mixing correction from the $(n+1)$-point cumulant via one nonlinear Sachs interaction, with higher cumulants entering only at higher order. The two-point function, for instance, is corrected by squeezed three-point cumulants of Ricci focusing and Weyl shearing, letting small-scale modes source larger scales and feeding the lensing $E$- and $B$-modes equally. Rather than a restrictive approximation scheme, the formalism is a paradigm shift: a unified framework naturally accommodating path corrections, higher-order matter statistics, stochasticity, and small-scale effects.

Thomas Apostolidis, Valerio De Luca, Leonardo Gualtieri, Takuya Katagiri, Paolo Pani, Luca Santoni

Published: 2026-06-17

Categories: gr-qc

We investigate the fully relativistic dynamical tidal response of neutron stars up to second order in the frequency. Combining the worldline effective field theory for extended gravitating bodies with perturbation theory of relativistic stellar models, we derive the tidal deformation induced by an external time-dependent field, including a universal logarithmic running term. In the effective theory, we work in dimensional regularization and, through a consistent matching procedure, obtain for the first time the complete leading-order dynamical tidal corrections to both the conservative dynamics and the gravitational-wave signal of compact binaries, including the scheme-dependent finite terms in addition to the running. We show that, in the relativistic regime, dynamical effects cannot be fully captured by mode excitations alone. The magnitude of the additional contribution depends on the stellar compactness, the equation of state, and the running term. Dynamical Love numbers are significantly enhanced with respect to their static counterparts for relatively small compactness. As a result, although they formally enter the gravitational-wave phase at 8th post-Newtonian order, dynamical tidal effects yield a non-negligible contribution during the late inspiral. Using a Fisher-matrix analysis, we show that third-generation detectors such as the Einstein Telescope could measure dynamical Love numbers for a range of neutron-star masses and equations of state. Conversely, neglecting these effects can lead to significant biases in the inference of static Love numbers, and hence on the nuclear equation of state. Our results highlight the importance of dynamical tidal effects for high-precision gravitational-wave modeling with future detectors.

Andrzej Udalski, Przemek Mróz

Published: 2026-06-17

Categories: astro-ph.GA

Recent preprints by Key et al. reported the discovery of a short-lived brightening of a star (nicknamed "Phoebe") located in the Large Magellanic Cloud that was interpreted as a short-timescale gravitational microlensing event produced by a lunar-mass primordial black hole (PBH) in the Milky Way dark matter halo. Here, we present an independent re-analysis of the publicly available DECam observations of this object, incorporating additional data from 2020 and 2021. The object underwent at least three distinct, low-amplitude brightenings (one of which was misinterpreted as a short-timescale microlensing event) in addition to long-term variations of its mean magnitude. These characteristics indicate that Phoebe is an ordinary variable star rather than a microlensing event. This finding resolves the apparent tension with the results from earlier microlensing experiments that rule out the hypothesis that a substantial fraction of dark matter is composed of lunar- and planetary-mass PBHs.

Steven B. Giddings, Madhur Mehta

Published: 2026-06-17

Categories: gr-qc

We investigate top-down derivations of effective descriptions for radiation from gravitational systems such as binaries. With a specified cutoff prescription, one can derive worldline effective field theories, but the cutoff dependence also complicates their description. We investigate a related effective approach, based on parameterizing dynamics in terms of an action on the boundaries of cavities encompassing individual bodies. We give examples of such cavity descriptions for black holes and for simple models for modifications of their behavior. We also show how cavity effective descriptions connect to observable quantities -- detailed wave profiles, and importantly, accumulated phase shift of emitted signals. A primary motivation is to have a systematic approach to inferring effects of modification of classical black hole behavior, such as those motivated by the need for black hole evolution to be consistent with quantum mechanics, or by other models for new BH behavior, on gravitational wave signals; the latter phase shifts have in particular been argued to provide sensitivity to small new effects from the inspiral phase. To illustrate basic principles and methods, this paper largely focuses on examples with scalar radiation, but we outline extension of the analysis to gravitational wave contexts.

Reynan A. Dulinayan, Kevin T. Grosvenor

Published: 2026-06-17

Categories: gr-qc

We present for the first time the exact solution of the massive Klein-Gordon equation in the Schwarzschild star (perfect-fluid, uniform-density, spherically-symmetric star), including the non-minimal curvature-scalar coupling. The solution is expressed in terms of the general Heun function. A geometry-induced algebraic coordinate transformation reveals a hidden Fuchsian structure that underlies the exact solvability. Known leading- and next-to-leading-order results are recovered in the low-compactness limit. In the Buchdahl limit, we derive a regularity condition for static modes and describe analytically the divergence in amplitude and oscillation wave vector of dynamic modes as they approach the pressure singularity at the center of the star.

A. I. Lonappan, K. Ramesh, T. Namikawa, F. J. Qu, B. Keating

Published: 2026-06-17

Categories: astro-ph.CO

A network of cosmic strings is one of the few well-motivated cosmological sources of vector and tensor metric perturbations on the largest observable scales. Such perturbations imprint a characteristic curl component in the deflection angle of cosmic microwave background (CMB) photons that, unlike the scalar lensing potential, vanishes for adiabatic density fluctuations at linear order. We exploit the curl-mode lensing reconstruction released as part of the Atacama Cosmology Telescope (ACT) Data Release~6 (DR6), based on five seasons of temperature and polarization data covering $9400~\mathrm{deg}^2$ of sky, to set new constraints on the dimensionless string tension $Gμ$ and the inter-commutation (reconnection) probability $P$. Modelling the string-induced curl power spectrum within the velocity-dependent one-scale framework, we obtain a $2σ$ upper bound on the combination $GμP^{-1}\le 3.5\times 10^{-5}$ in the small-$P$ regime, and $Gμ\le 5.0\times 10^{-5}$ at $2σ$ assuming the canonical Nambu-Goto value $P=1$. Combining the ACT DR6 curl bandpowers with the Planck 2013 curl-mode reconstruction, which extends down to $L_{\rm min}=2$, tightens these bounds to $GμP^{-1}\le 3.2\times 10^{-5}$ and $Gμ\le 4.3\times 10^{-5}$ ($2σ$). These represent the tightest constraints on cosmic strings derived from the curl-mode CMB lensing power spectrum to date. Using the ACT data alone, compared to the ACT 2008-season analysis, the ACT DR6 constraint on $GμP^{-1}$ is nearly an order of magnitude tighter.

Gabriel Assant, Daniel F. Litim, Manuel Reichert

Published: 2026-06-17

Categories: hep-th

We compute spectral functions of graviton modes in Lorentzian quantum gravity, interpolating between classical general relativity and an asymptotically safe ultraviolet fixed point. Using functional renormalisation adapted for theories in Lorentzian signature, and enhanced by new symmetry conditions to account for underlying Ward identities, we derive and solve flow equations directly for the Källén-Lehmann representation of propagators. Consistent results are found for several sets of renormalisation conditions yielding normalisable spectral functions for the graviton and the scalar graviton mode, in agreement with effective theory in the infrared. We further calculate the full quantum effective action to quadratic order in curvature, extract graviton-induced form factors, and discuss implications for unitarity of quantum gravity.

Hong-Xuan Jiang

Published: 2026-06-17

Categories: astro-ph.HE

This dissertation studies the electromagnetic signatures of accreting supermassive black holes using general relativistic magnetohydrodynamic simulations and covariant radiative-transfer calculations. It develops a unified numerical framework for modeling black-hole accretion, jet launching, flaring activity, and multi-band variability in Kerr, non-Kerr, and binary black-hole spacetimes. For isolated Kerr black holes, I investigate how magnetic-field geometry affects accretion dynamics and transient emission. Multi-loop magnetic configurations naturally produce reconnection events and flux-rope structures that can power near-infrared flares from Sagittarius A*, while the evolving optical depth of expanding plasma explains delayed millimeter emission. I also show that in tilted magnetically arrested disks, magnetic torques can drive retrograde disk and jet precession. The dissertation then applies the same framework to more complex spacetimes. Simulations of accretion onto regular loop-quantum black holes show that quantum-gravity corrections can modify photon-ring size, polarization structure, and jet power, leading to observational constraints from Event Horizon Telescope data. Finally, simulations of supermassive binary black holes in time-dependent spacetimes reveal how gravitational self-lensing, shock activity, and spin-orbit coupling shape multi-wavelength light curves and jet precession. Together, these results connect relativistic plasma dynamics with current and future observations of black-hole systems.

Debanjan Debnath, Rikpratik Sengupta, Kaushik Bhattacharya

Published: 2026-06-17

Categories: gr-qc

We present a class of horizonless, regular ultra-compact objects arising in a theory of gravity which allows curvature-fluid coupling. The non-minimal interaction between fluid variables and the Ricci scalar generates a vacuum-like equation of state in the interior, while the exterior remains exactly Schwarzschild. The two spacetimes are glued through a shell at the junction. The interior metric is non-singular, the shell acquires a stiff-matter equation of state, and near-horizon compactness can potentially mimic black-hole phenomenology without event horizons. Unlike the Mazur-Mottola gravastar and its variants, the present model naturally selects a typical ultra-compact mass-radius window, with masses in the range $1.4$-$2.1 M_\odot$ and radii in the range 5-7 km. This framework predicts a unique geometric-thermodynamic shell temperature in the ultra-compact limit distinctly different from the Hawking expression and the other unique observational feature of the model is the prediction of mass independent luminosity.

Mikhail P. Solon

Published: 2026-06-17

Categories: hep-th

Black hole static Love numbers vanish, but their dynamical counterparts do not. We present the scheme-independent dynamical response $\bar{F}_{\ell,s}$ of a Schwarzschild black hole in closed form, to all orders, and for every spin $s$ and multipole $\ell$. The result is $\bar{F}_{\ell,s}/4πR_S^{2\ell+1}=Φ_{\ell,s}(\bar{y})-\tfrac12η\,Φ_{\ell,s}'(\bar{y})$ with $\bar{y}=-\tfrac12η^2τ$ and $η=iωR_S$. Here $Φ_{\ell,s}$ is simply the leading-log solution to the renormalization group equation, but lifting the running logarithm to $τ=\log(R_S/R)-2\sum_{k\ge2}ζ_k\,η^{k-1}$ resums it to all orders. This tower of Riemann zeta values is the Newtonian phase in disguise: it originates from the same far-zone $Γ(1-η)$ that governs long-range scattering, and is universal across multipole and spin. Our result exhibits a factorization pinned to three ingredients: the hard matching coefficient at the horizon, the anomalous dimension in the near zone, and the dressed log in the far zone. Using shell effective field theory, we independently verify our formula for scalar, electromagnetic, and gravitational perturbations, reaching $\mathcal O(G^{15})$.

Yehui Hou, Jiewei Huang, Bin Chen

Published: 2026-06-17

Categories: gr-qc

We show that the near-horizon expansion of the linear polarization vector for synchrotron emission in a Kerr background admits a distinct analytic form. For emission from a stationary, axisymmetric, degenerate electromagnetic field, the leading-order polarization pattern depends only on the Kerr spin and the source polar angle, while the next-to-leading-order correction further encodes the geometric and rotational structure of the electromagnetic field. Our result extends the equatorial analysis of [Hou et al. (2024)] and the off-equatorial leading-order result of [Chael et al. (2026)]. Near-horizon polarization thus offers a potential probe of the fundamental properties of rotating black holes and of gravito-electromagnetic interactions.

Luca Negri, Thomas C. K. Ng, Thibeau Wouters, Tim J. Kuhlbusch, Harsh Narola, Robin Chan, Kailib Ryan Doney, Francesco Cireddu, Isaac C. F. Wong, Fabian Gittins, Peter T. H. Pang, Anuradha Samajdar, Achim Stahl, Justin Janquart, Chris Van Den Broeck, Tjonnie G. F. Li

Published: 2026-06-17

Categories: gr-qc

The geometry of the Einstein Telescope, the proposed next-generation European gravitational-wave observatory, is yet to be finalized. Two competing designs are under consideration: a nested triangular configuration (ET-Δ) and two separated L-shaped detectors (ET-2L). Extensive prior comparisons of ET designs established the scientific landscape using the Fisher-information-matrix formalism and identified that duty-cycle-induced single-detector operation is precisely the regime where this approximation becomes less reliable, underscoring the need for a refined, principled treatment of the duty cycle. In this manuscript, we build on that foundation by revisiting the comparison with full Bayesian parameter estimation of gravitational-wave signals from binary black-hole mergers, projected onto a simulated Einstein Telescope that incorporates a refined duty cycle modelled via continuous-time Markov chains and testing different detector maintenance strategies. We find that the redundancy inherent in the ET-Δ design enables it to maintain at least two operational detectors for the majority of the observing time, whereas the ET-2L configuration is often limited to a single detector. Crucially, we show that, during partial network operation, ET-Δ often outperforms ET-2L, and that the increased multi-detector uptime translates into tighter constraints on the luminosity distance and source-frame component masses. Notably, this remains true even when gravitational-wave events have a lower signal-to-noise ratio in ET-Δ than in ET-2L.

Mario Juvenal S. Onglao, Yuki Inoue, Daiki Tanabe

Published: 2026-06-17

Categories: physics.ins-det

Ground-based gravitational-wave detectors operating in the sub-Hertz regime are expected to be strongly limited by environmental gravity-gradient fluctuations, commonly referred to as Newtonian Noise (NN). At the same time, this frequency band provides unique opportunities to probe terrestrial gravitational perturbations associated with seismic and atmospheric processes. In this work, we investigate the feasibility of using the proposed Cryogenic sub-Hz cROss torsion-bar detector with quantum NOn-demolition speed meter (CHRONOS) as a platform for studying gravity-gradient noise and detecting prompt gravitational signals from earthquakes. We model gravity-gradient contributions from Rayleigh-wave-induced seismic fields, atmospheric infrasound fluctuations, and transient mass redistribution during earthquakes, and project these onto the CHRONOS torsion-bar response. CHRONOS achieves a peak strain sensitivity of order ~1e-18 Hz^(-1/2) near ~2 Hz. Rayleigh-wave NN is found to be the dominant environmental contribution below approximately 0.5 Hz, while atmospheric NN remains several orders of magnitude smaller throughout the frequency range considered. We further assess the detectability of prompt gravitational signals from earthquakes. For a representative Mw = 5.2 event, sources within approximately 90 km may produce detectable signals. At 40 km distance, we obtain a signal-to-noise ratio (SNR) of approximately 3.62 integrated over the sub-Hz band, with a corresponding strain amplitude reaching the CHRONOS sensitivity curve around 0.2 to 0.6 Hz. The gravitational signal is expected to precede seismic P-wave arrival by several seconds, depending on the assumed propagation velocity. These results demonstrate the potential of CHRONOS to probe both gravity-gradient noise and transient geophysical gravity signals in the sub-Hertz regime.

Chen-Hao Wu, Xiao Liang, Ya-Peng Hu

Published: 2026-06-17

Categories: gr-qc

Kiselev solution is a metric that describes black holes immersed in a quintessence-like dark energy background. By introducing a dynamic state parameter $w_q$, the Kiselev solution is supposed to help comprehend the effect of quintessential matter on black holes. In this work, we study the evaporation behaviors of Kiselev black holes. By varying the state parameter $w_q$, we find that the decreasing state parameter lowers the non-final stage temperature and markedly prolongs the evaporation lifetime. We also find that the ultra-slow evaporation mechanism of Kiselev black holes differs vastly from the perfect fluid dark matter (PFDM) black holes and Horndeski black holes, which share the analogous ultra-long lifetime. These results illuminate the effects of dynamic dark energy background on black hole evaporation, provide a potential laboratory to constrain the value of $w_q$, and may complement cosmological and astrophysical observations, e.g., the DESI's preference for thawing dark energy and the observation of exploding black holes based on ultra-slow evaporation.

Guo-Hong Du, Tian-Nuo Li, Lu Yin, Sheng-Han Zhou, Hao Wang, Jing-Fei Zhang, Xin Zhang

Published: 2026-06-17

Categories: astro-ph.CO

In the JWST and DESI era, the JWST high-redshift galaxy observations and DESI baryon acoustic oscillation (BAO) measurements severely challenge the standard $Λ$CDM model, while the $H_0$ tension becomes increasingly prominent. In this work, we investigate the capability of the early dark energy (EDE) model to alleviate the $H_0$ tension utilizing cosmic microwave background data from Planck, ACT, and SPT, BAO data from DESI, and ultraviolet luminosity function observations from the JWST. Within the canonical axion EDE framework, the CMB+DESI+JWST data significantly increase the $H_0$ value to $71.58\pm1.05\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, alleviating the $H_0$ tension to the $1.0σ$ level. Simultaneously, this model improves the fit to the JWST data and exhibits statistical performance significantly better than the $Λ$CDM model, with $Δχ^2_{\mathrm{tot}} = -18.26$ and $Δ\mathrm{DIC} = -11.89$. Our results highlight the complementary advantages of JWST high-redshift galaxy data alongside early- and late-time observations in testing EDE and alleviating the $H_0$ tension.

Davide Astesiano, Matteo Luca Ruggiero, Federico Re

Published: 2026-06-17

Categories: gr-qc

We propose an alternative interpretation of dark matter effects within the framework of General Relativity. In particular, we suggest that, in astrophysical and cosmological contexts, different initial assumptions about a system inevitably lead to different interpretations of the same phenomena. As a concrete example, we examine self-gravitating systems composed of an axially symmetric rotating dust fluid and show that effects typically attributed to the presence of additional matter, can instead be reproduced through an appropriate choice of initial and boundary conditions for the equations governing the system.

Boyu Li, Kohta Hatakeyama, Matsuo Sato, Yuji Sugimoto, Gota Tanaka

Published: 2026-06-17

Categories: hep-th

Color graphs and their subgraphs, referred to as bubble graphs, correspond bijectively to the simplicial complexes of pseudomanifolds and their subsimplices, respectively. In this paper, we introduce matrix representations for colored graphs and their associated bubble graphs. By using this correspondence, we define simplicial-complex matrices and subsimplex matrices that encode the simplicial complexes of pseudomanifolds and their subsimplices. Moreover, we formulate mutation and crossover operations on colored graphs. Through the established correspondence among simplicial complexes, colored graphs, and simplicial-complex matrices, we extend these operations to simplicial complexes and simplicial-complex matrices. We further implement an algorithm generating simplicial-complex matrices and a genetic algorithm performing mutation and crossover of them to produce pseudomanifolds exhibiting diverse topologies. In addition, we implement procedures for decomposing the generated simplicial-complex matrices into simplex matrices, reconstructing the simplicial complexes of the associated pseudomanifolds from this information, and computing geometric quantities such as the volume, circumcenter, and dual-simplex volume of each simplex.

Pramit Rej

Published: 2026-06-17

Categories: gr-qc

A possible approach for preventing compact astrophysical objects from gravitational collapse into singularities is the idea of dark energy. Since it is the cause of our universe's accelerated expansion, it has the greatest impact on the cosmos. As a result, it appears that dark energy can interact with any compact astrophysical stellar object [Phys. Rev. D 103, 084042 (2021)]. In this study, our primary objective is to develop a simpler model of a charged strange star coupled with anisotropic dark energy admitting the Adler-Finch-Skea solution [J. Math. Phys. 15, 727 (1974); Class. Quantum Grav. 6, 467 (1989)] within Einstein gravity. To develop this model, the Karmarkar condition was employed to determine the radial metric component, while Adler's methodology was used to choose the time-metric component. For this purpose, we explored a particular strange star, Her X-1, with observed values of mass $(0.85 \pm 0.15)M_{\odot}$ and radius $= 8.1_{-0.41}^{+0.41}$ km. In this context, we proceeded to model dark energy using the equation of state (EoS), such that the density of dark energy is proportional to the density of isotropic perfect fluid matter. The unknown constants in the metric were determined by smooth matching using the Darmois-Israel criterion. We conduct an in-depth examination of the stability and force equilibrium of our suggested star framework, as well as several physical characteristics of the model such as the metric function, pressure, density, mass-radius relation, and dark energy parameters. Thus, the physical consistency and stability of the present model are investigated. Therefore, following a comprehensive theoretical investigation, we discovered that our proposed model is singularity free and meets all the stability requirements to be a stable and physically realistic stellar model.

Tsvetan Vetsov

Published: 2026-06-16

Categories: hep-th

We investigate the thermodynamic stability and the stochastic thermal fluctuations of the warped black hole solution in three-dimensional General Massive Gravity. We demonstrate that the black hole is thermodynamically unstable and identify the nontrivial Davies phase-transition curves from the behavior of its admissible heat capacities. Going beyond the classical stability analysis, we study thermal fluctuations within a modified finite-time nonequilibrium extension of Ruppeiner's Hessian-based fluctuation theory. For a class of isentropic and isoenergetic processes, we derive exact on-shell angular momentum trajectories in the thermodynamic state space and compute the corresponding thermodynamic lengths. These quantities characterize relaxation processes between macrostates and provide an estimate of the associated relaxation times. Furthermore, we show that the thermodynamic geodesic equations do not admit constant-angular-momentum solutions, suggesting a continuous change of the black hole's angular momentum. Our results consistently reproduce the warped AdS$_3$ black hole limit of Topological Massive Gravity.

Waleed Sherif

Published: 2026-06-16

Categories: gr-qc

The Ashtekar-Lewandowski (AL) volume operator of loop quantum gravity is central to the Hamiltonian constraint, but its vertex action is usually obtained from dense spectral decompositions of finite recoupling matrices, obstructing numerical analysis on large kinematical Hilbert spaces or high-valence vertices. We formulate a matrix free action of the $SU(2)$ AL vertex volume operator in standard recoupling basis, making use of the Brunnemann-Thiemann expression for the oriented AL volume density $Q_{v}$ whose matrix elements can be generated locally from recoupling theory without forming the full matrix. Based on the Balakrishnan-Stieltjes representation of $(Q_{v}^{2})^{1/4}$ we approximate the volume by shifted-resolvent quadrature (SRQ). The resulting action uses only repeated applications of $Q_{v}$ and shifted positive linear solves, making it compatible with multi-shift Krylov methods. We prove exact preservation of the volume kernel, provide operator-norm and residual error estimates, discuss sector-wise scaling bounds, and validate the method on an embedded $K_{5}$ graph at small spin cutoffs against exact dense local-block operators. Numerical simulations show rapid convergence of vertex expectation values, controlled dependence on bound parameters, and exact preservation of zero-volume modes. We further demonstrate matrix free Monte Carlo estimates at doubled-spin cutoff $2j=250000$ beyond dense materialisation, and show that SRQ can be combined with stochastic Lanczos quadrature to estimate fixed-sector volume spectral measures without dense volume matrices.

Luca Buoninfante

Published: 2026-06-16

Categories: hep-th

We elucidate the physical nature of ghosts above the multi-particle threshold by contrasting them with unstable particles in quantum field theory. We first consider the asymptotic formulation, where ordinary positive-norm one-particle states can be unstable and decay, whereas ghosts survive asymptotically without decaying, yet admit no particle interpretation due to interference with the multi-particle component which masks the negative-norm one-particle state. This distinction originates from two different analytic structures of the dressed propagator, whose complex conjugate poles lie in the first or second Riemann sheet in the ghost or ordinary case, respectively. Ghost resonances are, in principle, phenomenologically distinguishable from ordinary ones, being narrower and exhibiting weaker interference between positive- and negative-energy peaks. We then formulate the quantum field theory in a finite interval of time and, working within a suitable approximation for the dressed propagator, find that finite-time effects amplify differences in the resonant behavior and give rise to new features, such as higher peaks in ghost resonances. Distinct temporal regimes are also identified: for times shorter than the inverse width, an approximate free-particle description is valid, whereas at later times interactions and interference effects dominate, leading to decay or multi-particle masking. Complex poles in the dressed propagator emerge only at late times and become complex-conjugate pairs asymptotically, determining the asymptotic dynamics. This study supports the absence of freely propagating ghost particles in the asymptotic limit.

Bing-Yang Han, H. Lu

Published: 2026-06-16

Categories: hep-th

This paper generalizes the method used in the previous article 2512.09930 to black $p$-brane thermodynamics in arbitrary dimensions containing black holes and strings as special cases: thermodynamic quantities can be derived without constructing the corresponding black $p$-brane solutions. We further extend the discussion to black holes or $p$-branes involving a general scalar coset.

Benjamin Knorr

Published: 2026-06-16

Categories: hep-th

We derive an analytical approximation for the graviton propagator from Asymptotic Safety. We find neither extra poles nor indications of unitarity or causality violations in the spin-two sector. Our results strengthen the case that Asymptotic Safety does not introduce new degrees of freedom, and thus propagates the same field content as General Relativity. We also identify the underlying mechanism: the residues of spurious poles in finite-order derivative expansions approach zero as the order is increased.

Albert Radulea, Marcelo E. Rubio, Konstantinos Kritos, Andrea Maselli

Published: 2026-06-16

Categories: gr-qc

Extreme mass-ratio inspirals embedded in accretion disks provide a natural arena for studying the interplay between relativistic orbital dynamics and environmental effects. In this work, we develop a framework to investigate the secular evolution of compact objects repeatedly crossing an accretion disk around a supermassive black hole. The orbital motion is modeled through Kerr geodesics, while disk interactions are encoded through effective prescriptions for mass accretion and dynamical friction. We find that disk-induced dissipation generically drives a two-stage evolution characterized by rapid alignment of the orbital plane with the disk, followed by slower eccentricity damping. By systematically comparing the dynamics with a purely Keplerian treatment, we show that cumulative relativistic effects produce deviations even at large orbital separations, where the Keplerian approximation would naively be expected to remain accurate. These discrepancies grow through repeated disk crossings and become increasingly pronounced in more relativistic orbital configurations. We further investigate the impact of the accretion-disk model by comparing the Sirko-Goodman and Novikov-Thorne prescriptions. Relativistic disk structures predict systematically lower densities and larger scale heights, leading to weaker orbital dissipation and slower secular evolution. By contrast, the spin of the central black hole has only a minor effect on the overall circularization efficiency. Our results demonstrate the importance of consistently modeling both relativistic orbital dynamics and disk structure when studying compact objects embedded in AGN disks, and provide a framework for exploring their long-term evolution, as well as a possible connection to quasi-periodic eruptions.

Boris Goncharov, Gabriela Sato-Polito, Xiaoming Bi, Matias Zaldarriaga

Published: 2026-06-16

Categories: astro-ph.HE

We introduce, from first principles, a joint model of the gravitational wave background (GWB) and brightest supermassive black hole binary (SMBHB) sources that may be individually resolvable in Pulsar Timing Array (PTA) searches for gravitational waves. We propose the characteristic number of SMBHB sources, $N_{\rm c}$, as a detection statistic for the astrophysical origin of the GWB. We then demonstrate how the brightest SMBHBs assist in resolving $N_{\rm c}$. Applying our method to the simulated NANOGrav 15-year data, which replicates all aspects of real data's known noise, observations, and the inferred GWB power spectrum, we demonstrate direct astrophysical limits on the strain amplitude of individually resolvable SMBHBs. We find that 21 of 114 SMBHB candidates from active galactic nuclei observations are in tension with the NANOGrav's observations. In contrast, only one candidate is in tension with the NANOGrav data based on the upper limits reported in the original analysis. Constraining the Poisson-specific characteristic number of SMBHBs, $N_{\rm c}$, at ${\rm yr}^{-1}$, we outline implications for the population properties of SMBHBs. Based on our new model applied to the simulated NANOGrav data, we calculate the probability of detecting GWs from isolated SMBHB in the 15-year data to be 2\% at the ${\rm SNR}=5$ level. Our projection towards the expected NANOGrav 20-year data suggests an increase to 5\%. With this, we estimate the probability of finding an outlier with an SNR of 2 in the NANOGrav 20-year data to be $40\%$.

S. D. Campos

Published: 2026-06-16

Categories: gr-qc

We present a paraxial wavepacket model for collimated gravitational-wave bursts and derive the coherent response of detector networks to these structured signals. For current LIGO-Virgo baselines, analytic mismatch estimates and overlaps show that PWM waveforms are effectively indistinguishable from standard sine-Gaussian bursts, validating the plane-wave approximation. We then identify a regime relevant to third-generation networks in which finite transverse structure produces non-negligible geometric phase shifts. A toy event-level Monte Carlo compares a standard burst-search ranking with a paraxial wavepacket model-constrained statistic that penalizes geometric inconsistencies across detectors; in this controlled setup, the PWM prior yields a factor of $\sim3$-$4$ gain in detection efficiency at a fixed false-alarm rate, while maintaining performance on plane-wave-like signals.

Kostas Tzanavaris, Latham Boyle, Neil Turok

Published: 2026-06-16

Categories: gr-qc

We study the action principle for space-times whose boundary is singular. We suggest that it is natural to treat the singularity as a {\it free} boundary, where the variation is unconstrained. Demanding that the action is stationary under such free variations then implies certain (on-shell) boundary conditions at the singularity. We derive these boundary conditions for the case of Einstein gravity coupled to matter and show that, when applied to an initial spacelike singularity, they exclude Kasner-like or BKL space-times, but admit conformally regular space-times (including FLRW models) sourced by fluids satisfying $0 \leq P < ρ$. For standard hot big bang FLRW cosmologies, the admissible linear (scalar, vector, tensor) perturbations satisfy reflecting boundary conditions at the bang, in agreement with large-scale cosmological observations.

Abylaikhan Tlemissov, Arman Tursunov, Maciek Wielgus

Published: 2026-06-16

Categories: astro-ph.HE

We investigate the polarimetric signatures of orbiting hot-spots around a Schwarzschild black hole in the presence of an external magnetic field, accounting for the electromagnetic interaction between the charged emitter and the field. Using a general-relativistic model that incorporates synchrotron emission and ray-tracing of light propagation, we analyze how the electromagnetic interaction parameter modifies the observed polarization patterns, with particular emphasis on the behavior of the electric vector position angle (EVPA) and the time-evolving polarization loops in the $Q$-$U$ plane. Applying the model to millimeter wavelength ALMA observations of Sagittarius~A*, we explore the parameter space that best reproduces the asymmetry, time ratio, and area ratio of the observed polarization loops. We find that the inclusion of a small positive interaction parameter increases the symmetry of the loops and the time ratio, while a negative magnetic parameter introduces strong asymmetry and fails to reproduce the data. Our results indicate that electromagnetic interaction can lead to ambiguity in the estimation of the system parameters such as orbital inclination or hot-spot velocity.

Wenkang Xin

Published: 2026-06-16

Categories: astro-ph.HE

A tidal disruption event (TDE) occurs when a star is scattered onto a near-radial orbit and is torn apart by a black hole (BH)'s tidal field. The angular momentum threshold for disruption is set by general relativistic tidal dynamics, while the supply of stars to the disruption zone is governed by Newtonian stellar dynamics. A spinning BH breaks the spherical symmetry of the disruption boundary, so a star's survival depends on both the magnitude and the orientation of its angular momentum. Existing treatments either assume a non-spinning BH or rely on numerical simulations of spinning BHs. We develop the first semi analytical framework that incorporates spin-dependent loss cone boundaries into TDE rate theory. Using a novel tidal tensor formalism, we compute inclination-dependent thresholds for tidal disruption and direct capture by the event horizon. We then revisit the classical one dimensional loss cone problem with nested disruption and capture boundaries, deriving a closed form capture fraction valid across all loss cone regimes. Finally, we formulate a two dimensional Fokker--Planck equation describing simultaneous diffusion in angular momentum magnitude and orientation. Through a perturbative treatment, we demonstrate that while the Kerr disruption boundary induces a first-order bias favouring the disruption of retrograde stars, the global TDE rate is remarkably insensitive to black hole spin. This approach offers a tractable route to including spin and orbital inclination in population-level TDE studies.

Hongsheng Hou, Sai Wang, Zhi-Chao Zhao, Xin Zhang

Published: 2026-06-16

Categories: astro-ph.CO

We present a robust assessment of cosmological constraints on the sum of neutrino masses ($\sum m_ν$) when relaxing the standard assumption of purely adiabatic primordial initial conditions. Allowing for a neutrino density isocurvature (NDI) component alongside the adiabatic mode, we analyse the latest CMB-SPA combination (Planck 2018, ACT DR6, and SPT-3G), DESI DR2 baryon acoustic oscillation data, and the DES Year 5 supernova sample. Within the $Λ$CDM model, the 95\% upper limit weakens only marginally from $\sum m_ν< 0.052$ eV (purely adiabatic) to $< 0.057$ eV (including NDI), with the NDI amplitude consistent with zero. In the CPL dynamical dark energy model, the adiabatic limit is $< 0.111$ eV, shifting to $< 0.115$ eV with NDI, yet the isocurvature mode remains undetected. While these limits are robust against the inclusion of isocurvature perturbations, they are highly sensitive to both the assumed dark energy equation of state and the prior lower bound on $\sum m_ν$. Notably, the adiabatic $Λ$CDM limit of $0.052$ eV lies below the minimum sum required by the normal neutrino mass hierarchy ($0.05878$ eV), indicating that this bound is an artifact of the statistical prior extending to zero. Imposing a physically motivated hierarchy-informed prior raises the limit to $< 0.092$ eV. Our results demonstrate that current data show no evidence for NDI modes and that the inferred neutrino mass upper limit is robust against this extension, but a definitive, model-independent bound requires addressing prior dependencies and dark energy uncertainties. This work provides the first joint constraint on $\sum m_ν$ and NDI using the full CMB-SPA+DESI DR2+DES dataset.

A. J. Foraci, C. Litos, R. P. Woodard

Published: 2026-06-16

Categories: gr-qc

We review the mechanism by which loops of matter fields contribute to the graviton self-energy during de Sitter inflation. The self-energy is used to quantum-correct the linearized Einstein equations. A Green's function method is employed to obtain exact 1-loop corrections to the plane wave mode functions of gravitational radiation, subject to the usual ambiguity in the initial state. Conformally coupled matter, which does not experience inflationary particle production, makes only a logarithmic enhancement of the rate at which the imaginary part of the mode function goes to zero after horizon crossing. These corrections can be understood, and even summed up, using a variant of the renormalization group. However, massless, minimally coupled scalars, which experience massive inflationary particle production, induce a much stronger enhancement of the rate at which the real part of the mode function approaches a constant. One interpretation of this effect is as a shift of the inflationary Hubble parameter.

Wen-Biao Han

Published: 2026-06-16

Categories: gr-qc

Near-extremal Kerr black holes support zero-damped modes (ZDMs), whose small time-domain damping rates make them long-lived probes of the near-horizon region. We show that bound extreme-mass-ratio inspirals (EMRIs) can resonantly drive this response in vacuum general relativity. Using frequency-domain Teukolsky amplitudes for eccentric-inclined Kerr geodesics, we identify a source-supported orbital harmonic whose real frequency falls within one pole half-width of the fundamental gravitational ZDM. In the complex response, the pole contribution is enhanced by this small half-width, while complex-response tomography recovers the independently computed Kerr pole from real-frequency orbital data. After subtracting the smooth non-pole component, the residual exhibits the phase winding of a coherent simple pole, with a pole contribution comparable to the smooth non-pole part of the EMRI-sourced Teukolsky amplitude. The driven branch also lies in the superradiant regime and carries negative horizon flux. These results establish a pole-resolved, resonantly driven ZDM response by EMRIs and make the recovered pole half-width a route to measuring the horizon surface gravity.

Kalin V. Staykov, Violetta Sagun, Lorenzo Cipriani, Daniela D. Doneva, Stoytcho S. Yazadjiev

Published: 2026-06-16

Categories: gr-qc

We study the effect of differential rotation on hybrid stars with the first-order deconfinement phase transition from hadronic to color superconducting quark matter. The differential rotation is introduced within a realistic, four-parameter phenomenological rotation law, in which the maximum angular velocity of the rotating configuration is shifted away from the center. We focus on two classes of differentially rotating solutions, namely quasi-toroidal (type C) and quasi-spherical (type A), and study the changes in the star global properties and angular velocity profiles due to the presence of a phase transition. Thus, we demonstrate the existence of quasi-toroidal hybrid star configurations in which deconfined quark matter forms a ring around the center of mass, while hadronic matter remains at the center and outer layers. Furthermore, we show that when increasing the angular momentum $J$ the turning points of the $J=const$ sequences shift towards lower energy densities, shrinking considerably the region where differentially rotating neutron stars with phase transitions exists. Interestingly, for both type A and type C solutions, the angular velocity profile is continuous throughout the star despite the discontinuity in the energy density. Moreover, we show that at the crossing points where the mass-radius curves for different equations of state intersect, the rotational profiles of the solutions are very close despite large differences in the energy density profiles. This reveals a possible degeneracy between the post-merger remnant properties for models with and without phase transitions, emphasizing the need for complementary multi-messenger observables to distinguish between them.

Rustam Ibadov, Najibullokhon Shukurullokhon

Published: 2026-06-16

Categories: gr-qc

We investigate the geometric, dynamical, and thermodynamic properties of a novel class of regular black holes in scalar-tensor gravity non-minimally coupled to nonlinear electrodynamics (NLED). By incorporating a purely magnetic NLED source, we circumvent the classical "no-go" theorems that prohibit regular configurations for purely electric fields under the weak energy condition. The geometric analysis demonstrates the complete resolution of the central Penrose singularity, replacing it with a regular, de Sitter-like vacuum core characterized by a globally bounded energy density ($ρ_c < \infty$). To assess the physical viability of these configurations, we analyze their dynamical stability against odd-parity (axial) linear gravitational perturbations. The derived Regge-Wheeler-like effective potential is strictly positive and convex outside the event horizon. Numerical time-domain integration, independently corroborated by the semi-analytical WKB approximation, confirms the total absence of tachyonic instabilities, revealing a stable quasi-normal ringing phase followed by an exponential decay. Furthermore, our thermodynamic analysis of the mass-radius relation reveals a strict mass gap corresponding to an extremal configuration ($M \ge M_{min}$). This indicates that the semi-classical Hawking evaporation must terminate at this extremal limit, leaving behind a massive thermodynamic remnant, thereby providing a theoretical framework toward resolving the black hole information loss paradox.

Aimeric Colléaux, Ivan Kolář, Tomáš Málek

Published: 2026-06-16

Categories: gr-qc

We classify 4-dimensional gravitational theories with integrability properties analogous to quasi-topological gravity, but for metrics with the symmetries of spherical, hyperbolic, and planar Schwarzschild and Taub-NUT solutions, their double-Wick-rotated counterparts - the B-metrics, the near-horizon extreme Kerr, and the swirling universe - and the Eguchi-Hanson instanton. These are the symmetries that allow consistent reductions (principle of symmetric criticality) with 4 Killing vectors and 3-dimensional orbits. Considering theories depending only on the Riemann tensor, we show that, for these metrics, only those with third-order equations (second-order after trivial integration) can be analytic in the Riemann tensor. We show that there is a unique theory with first-order field equations (algebraic after trivial integration, with the same integrability as general relativity) at each order in curvature and construct regular static black holes from infinite towers of these high-energy corrections to general relativity. For these theories, we obtain closed-form solutions for all the symmetries listed above, which we analyze to ensure they have a clear physical interpretation.

Miguel Zumalacárregui, Xikai Shan

Published: 2026-06-16

Categories: astro-ph.HE

As natural telescopes, Gravitational lenses enable the observation of sources that would otherwise be too distant and faint. Stellar-mass objects, or microlenses, act as impurities in the lens, producing subtle distortions of the source. These effects are necessary to correctly interpret observations, and may in some cases be themselves evidence of gravitational magnification. Gravitational waves (GWs) observed by ground detectors and magnified by galaxies and clusters will undergo microlensing by fields of stars and remnants: describing these systems requires not only considering a large number of small-scale lenses (microlenses), but also including wave-optics effects, leading to frequency dependent modulations of the signal. Here we present novel models for Reduced-Order Stochastic Diffraction (ROSD), which overcome these challenges in the search for GW lensing signatures: an effective description is synthesized from numerical simulations of wave-optics lensing by stellar fields via a singular value decomposition. The procedure yields an optimized orthonormal basis to describe microlensing distortions and a probability density function for the coefficients, which can be used as priors or to verify the consistency with stellar-field lensing. We present SVD-stellar-I5-aLIGO as an example of this model category, discuss the role of truncation order and demonstrate how it can be applied to GW data via injection and recovery in Bayesian parameter estimation. ROSD can be tailored to account for detector sensitivity and the type of source under analysis, and extended to different microlens populations and external potentials. ROSD models open a new window to probe small-scale objects (stars, remnants and potentially dark matter) and facilitate the discovery of the most distant compact binary mergers.

Badis Ydri

Published: 2026-06-16

Categories: hep-th

We study the large--\(d\) dynamics of the mass--deformed bosonic \(\mathrm{BFSS}_{d+1}\) matrix quantum mechanics using a Hubbard--Stratonovich localization of the Yang--Mills interaction. After integrating out the matrix coordinates, the theory reduces to a holonomy--dependent effective action for an auxiliary adjoint kernel. We introduce a commuting--symmetric saddle and its maximally symmetric specialization, in which the interaction is encoded in a single dynamically generated mass shift \(k_0\). The resulting large--\(d\) description is a gauged matrix harmonic oscillator with self--consistent frequency \(s^2=m+k_0\), fixed by a gap equation. We analyze the low--temperature \(X\)-space physics, the holonomy effective action, the Yang--Mills observable, and the associated phase structure. We then identify a correlated double--scaling limit in which \(d\to\infty\), \(m\to\infty\), and \(κ=m^{3/2}/d\) is held fixed. In this limit the Yang--Mills interaction and the explicit mass deformation remain parametrically balanced: the theory interpolates between the commutator--dominated BFSS regime and the mass--dominated Gaussian regime. The double--scaled theory exhibits two complementary large--\(d\) regimes. At low temperature, the enhanced gap pushes the deconfinement scale upward and opens a parametrically large uniform--holonomy region, where the bulk dynamics behaves as weakly coupled \(\mathrm{BFSS}_2\)--type gauged harmonic--oscillator sectors. At the same time, the high--temperature branch reveals an overlap window in which the Gaussian description remains self--consistent while the commutator contribution per matrix pair is parametrically suppressed. The resulting dynamics is therefore \(\mathrm{BFSS}_2\)--like in its enlarged uniform--holonomy sector and IKKT--like in its almost--commuting matrix behavior.

Juan Calderón Bustillo, Adrián del Rio, Nicolás Sanchis-Gual, Koustav Chandra

Published: 2026-06-16

Categories: gr-qc

Precessing binary black-holes generically produce an imbalance of right- and left- handed gravitational waves, reflecting the breaking of mirror symmetry by the merger dynamics. We study this phenomenon using the observer-independent quantity $V_{\rm GW}$, a gravitational analogue of the optical Stokes parameter that quantifies the intrinsic handedness of the emitted radiation. Using 91 LIGO-Virgo-KAGRA black-hole mergers from the O1-O4a observing runs, we find that $92\%$ of the analyzed events favour non-vanishing $V_{\rm GW}$, indicating a predominance of precessing dynamics across the events. Through a recently established relation between $V_{\rm GW}$ and the remnant black hole recoil, we further constrain the retention of merger remnants in dense stellar environments, finding that at most $8\%$ could remain gravitationally bound to globular or nuclear star clusters and subsequently participate in hierarchical merger channels. We finally investigate the cosmological distribution of black-hole merger handedness. The observed $V_{\rm GW}$ distribution is consistent with symmetry under $V_{\rm GW}\rightarrow -V_{\rm GW}$, and yields an average value $\langle V_{\rm GW}\rangle=-1.9^{+6.1}_{-6.6}\times10^{-3}$ ($90\%$ credibility), consistent with the absence of a preferred handedness and with expectations from large-scale statistical isotropy. In particular, the inclusion of O4a events reduces uncertainties in $\langle V_{\rm GW} \rangle$ by $\sim 40\%$ with respect to O1-O3 events. These results establish black-hole merger handedness as a unified probe of orbital precession, remnant recoil, hierarchical formation, and cosmological mirror symmetry.

Youri Carloni

Published: 2026-06-16

Categories: astro-ph.CO

We formulate a novel extension of the $Λ$CDM model, named $Λ_{ω_s}$CDM, in which we consider an additional term at early times in order to alleviate the Hubble tension. This additional component, referred to as \emph{matter with pressure}, indicates a barotropic fluid that is subdominant to dust and radiation as the Universe expands, thereby recovering the $Λ$CDM paradigm at late times. We constrain the $Λ_{ω_s}$CDM cosmology by performing a Markov Chain Monte Carlo analysis with Planck 2018 CMB, DESI DR2, and Pantheon+\texttt{SH0ES} data. The results suggest that the barotropic factor and the normalized density of the new fluid are given, respectively, by $ω_s=0.294_{-0.004(0.023)}^{+0.014(0.015)}$ and $10^{5}Ω_s=1.62_{-0.56(0.91)}^{+0.36(1.02)}$. With these two additional parameters, the Hubble constant is increased to $H_0 = 71.51^{+0.72(1.43)}_{-0.74(1.46)}$ km/s/Mpc, alleviating \emph{de facto} the Hubble tension.

G. L. Comer, N. Andersson, T. Celora, I. Hawke

Published: 2026-06-16

Categories: gr-qc

We develop an action principle for a relativistic two-fluid system with dissipation. The specific constituents of the model - which serves as a proof of principle - are particles and entropy. The linchpin of the action is the assertion that a given flux is dissipative if its covariant divergence is non-zero. For our model, the particle flux is taken to be conservative while the entropy flux is dissipative. This allows for a "top-down" approach where the general question is geometric. Previous work has shown that new terms (the proper time derivative of matter space "metrics") must be included in the Lagrangian in order to produce equations of motion with terms representing bulk and shear viscosity. In addition to including these terms we show that further terms - interpreted as velocities - can be included. The new action-based model recovers known relativistic formulations of the Cattaneo equation, which results in causal heat propagation. We further advance our understanding by exploring the single-fluid limit by locking the entropy four-velocity to that of the matter component. This reduces the system to a single field equation along with a constraint equation. We show that this constraint leads to a dynamical extension of the standard Tolman red-shift condition. Finally, we provide three example actions (of increasing complexity) which demonstrate that the model is able to reproduce (in the single-fluid limit) the anticipated terms from the relativistic Navier-Stokes equations. In the general case, the action based approach allows for a much richer structure, which may be relevant for realistic models of non-linear dissipative relativistic fluid systems.

Ya-Nan Du, Ji-Yu Song, Jing-Fei Zhang, Xin Zhang

Published: 2026-06-16

Categories: astro-ph.CO

Gravitational-wave (GW) events and galaxies both trace the cosmic matter distribution, but the mergers of astrophysical black holes and primordial black holes (PBHs) are expected to populate different environments and therefore to cluster with different biases. The GW clustering bias is thus a statistical observable that can separate the two populations. We assess how well this can be done by cross-correlating the photometric galaxy survey of the Chinese Space-station Survey Telescope (CSST) with mock GW catalogs from two future detector networks: the third-generation ET2CE network (the Einstein Telescope and two Cosmic Explorer detectors) and the multi-band BDET2CE network, which adds the space-based baseline Decihertz Interferometer Gravitational-Wave Observatory. We find that CSST combined with 10 years of ET2CE observations can reveal a PBH contribution once its fraction in the total merger rate exceeds about $40\%$, while the much sharper sky localization of BDET2CE lowers this threshold to about $20\%$. The improvement comes from recovering the small-scale clustering information that localization errors would otherwise erase. These results show that combining future GW detector networks with CSST galaxy clustering offers a promising and largely independent route to identifying PBHs statistically.

Konstantinos F. Dialektopoulos

Published: 2026-06-16

Categories: astro-ph.CO

We present a proof of concept for the joint reconstruction of the Hubble parameter $H(z)$ that assumes no dark energy equation of state and the growth rate of large scale structure $fσ_8(z)$ using a physics informed neural network. Rather than fitting these two observables separately and checking their consistency post hoc, we couple them through the linear growth equation of general relativity directly during training, using the equation residual evaluated at collocation points via automatic differentiation as an additional loss term. The network employs a shared backbone feeding two independent output heads, one per observable. We train an ensemble of 100 independently seeded networks on a compilation of 50 $H(z)$ measurements from Cosmic Chronometers and Baryon Acoustic Oscillations and 63 $fσ_8(z)$ measurements from Redshift Space Distortions, and study four values of the physics coupling weight $λ\in \{0,\,0.01,\,0.1,\,1.0\}$. We then anchor the $H_0$ normalization using two independent local distance scale determinations: the SH0ES result $H_0 = 73.04 \pm 1.04$\,km\,s$^{-1}$\,Mpc$^{-1}$ and the Local Distance Network consensus $H_0 = 73.50 \pm 0.81$\,km\,s$^{-1}$\,Mpc$^{-1}$. With either prior the Hubble constant is recovered exactly at the prior value, and the two reconstructions are indistinguishable in $fσ_8(z)$. The reconstructed $fσ_8(z)$ sits systematically below the $Λ$CDM prediction at all redshifts, consistent with the $σ_8$ tension, while the $\mathrm{Om}(z)$ null test shows a marked departure from the flat $Λ$CDM expectation at low redshift. The results establish that coupling the two observables through the growth equation during training is both feasible and beneficial, and that the reconstruction is robust to the choice between the two local $H_0$ determinations.

Salvatore Capozziello, Giuseppe Meluccio

Published: 2026-06-16

Categories: gr-qc

The so-called Geometric Trinity of Gravity is based on three distinct geometric features of spacetime, i.e.\ curvature, torsion and non-metricity, which give rise to equivalent dynamics for General Relativity (GR), Teleparallel Equivalent of General Relativity (TEGR) and Symmetric Teleparallel Equivalent of General Relativity (STEGR). Pre-geometric gravity, on the other hand, offers a unifying framework from which all metric-affine theories can emerge. Starting from a gauge formulation \textit{à la} Yang--Mills with a Higgs-like field, a mechanism of spontaneous symmetry breaking can give rise to an effective metric as well as to the classical dynamics of the gravitational field. In particular, the emergence of gravity in the spontaneously broken phase is shown to be consistent with all the different formulations of the Geometric Trinity of Gravity, in terms both of actions and of gauge choices for the affine connection. This general result is achieved by deriving and analysing suitable expressions in the unbroken phase for pre-geometric actions and for pre-geometric gauge-fixing conditions respectively.

Saddam Hussain, Simran Arora, Qiang Wu, Tao Zhu

Published: 2026-06-16

Categories: astro-ph.CO

We investigate a dark-energy equation of state governed by a damped harmonic oscillator equation, admitting underdamped, critically damped, and overdamped solutions. Confronting the model against Planck CMB, DESI BAO, BBN, Cosmic Chronometers, and three Type~Ia supernova compilations, we find that the underdamped solution yields $H_0 = 70.9 \pm 1.1$ km/s/Mpc, with DESY5 and $H_0 = 72.0^{+1.4}_{-2.1}$ km/s/Mpc with Union3, reducing the tension with SH0ES to $\sim\!1.4σ$, while Pantheon+ strongly favors a near-critically damped solution with positive $w_0$ and $H_0 = 66.23 \pm 0.85$ km/s/Mpc, revealing a significant systematic tension among supernova datasets. Bayesian evidence relative to $Λ$CDM is inconclusive for DES and Union3 data, demonstrating that $H_0$ tension alleviation is achievable at no statistical cost relative to the standard model.

Suriyah R. Kannagi, Aadarsh Singh, Sudhir K Vempati

Published: 2026-06-16

Categories: hep-ph

We present an endpoint-safe Fourier method for multi-field vacuum tunnelling. The field-space tunnelling path is written as a straight-line interpolation between the false and true vacua, plus sine-mode deformations that vanish at the endpoints. This gives a finite-dimensional path optimisation problem, which we implement using automatic differentiation in the JAX numerical framework. The method is studied both as a standalone variational ansatz for curved tunnelling paths and as a preconditioner for existing bounce solvers. On the OptiBounce benchmark potential for $N_φ=3,\ldots,20$ and on a nested random-coefficient potential family up to $N_φ=50$, the Fourier result agrees with FindBounce, OptiBounce, and CosmoTransitions at the sub-percent level in the regular benchmark cases, while requiring only a modest number of modes. We also compare several endpoint-safe basis families and find that Fourier sine modes provide a robust default for smooth tunnelling paths. When used as an initialiser, the Fourier path supplies useful geometric information to existing solvers before the final bounce calculation is carried out. In the CosmoTransitions tests, this reduces the number of steps in subsequent path deformation, while in the FindBounce point-injection tests, it gives large runtime improvements in the high-dimensional cases up to 90 $\%$. These results suggest that endpoint-safe Fourier paths provide a useful bridge between simple analytic path ansätze and fully numerical multi-field bounce algorithms.

Jiawei Hu, Hongwei Yu

Published: 2026-06-16

Categories: gr-qc

Stimulated transitions are among the most fundamental processes in light-matter interaction, underlying resonant absorption and emission in atomic systems. Here we show that uniform acceleration can convert this familiar response into a frequency-selective absence of response. Specifically, when an incident photon has a nonzero momentum component transverse to the acceleration, the stimulated transition probability vanishes at a discrete set of frequencies fixed by the acceleration, the atomic transition frequency, and the photon propagation angle. At these spectral blind spots, both ordinary stimulated absorption and acceleration-induced excitation are simultaneously suppressed, rendering the atom effectively unresponsive to the incident radiation. The effect arises from the nontrivial response of accelerated atoms to quantum vacuum fluctuations and provides a distinctive signature of the Unruh effect through the absence, rather than the enhancement, of stimulated transitions. We further provide an order-of-magnitude estimate showing that an electron-based implementation with spin splitting in combined electric and magnetic fields could access the required parameter regime. These results reveal an unexplored form of acceleration-modified light-matter interaction and identify spectral blind spots as a new manifestation of the Unruh effect.

Jin Kozaki, Yasha Neiman

Published: 2026-06-16

Categories: hep-th

We consider cubic interactions of massless fields of arbitrary spin in 4 spacetime dimensions, within the lightcone formalism. We extend two key results from flat to (Anti-)de Sitter spacetime. First, we present a simple explicit expression for all the cubic vertices. Second, we prove that the various self-dual/chiral theories, such as Self-Dual Yang-Mills, Self-Dual General Relativity and Chiral Higher-Spin Gravity, are fully consistent with no need for vertices beyond cubic. The key observation behind our results is that all cubic vertices in the lightcone formalism can be expressed as a direct generalization of the "abelian" vertices formed by multiplying linearized curvatures. The simple relationship between flat and (Anti-)de Sitter then follows essentially from the conformal invariance of such linearized curvatures. The price for such simplicity is that our vertex expressions are non-local. It is however easy to bring them into a local form, which we also present.

Kuantay Boshkayev, Ainur Urazalina, Aidana Kurmanbek, Manas Khassanov, Daniya Utepova

Published: 2026-06-15

Categories: gr-qc

We study the shadow and strong-field optical properties of a static, spherically symmetric dyon-like dilatonic black hole. The photon sphere radius, critical impact parameter, and shadow radius are obtained and analyzed in terms of the charge $Q$ and the dilatonic coupling parameter $a$. We show that increasing these parameters decreases the photon sphere and shadow radii, leading to a smaller apparent shadow. The predicted angular diameter is compared with the observational data for M87$^{*}$ and SgrA$^{*}$, and the model parameters are constrained. We also estimate the high-frequency energy emission rate in the geometric-optics approximation and derive the leading Bozza coefficient $\bar{a}$, which characterizes the logarithmic behavior of the deflection angle in the strong-field regime. Astrophysical implication of the obtained outcomes are discussed.

Chen Zhang

Published: 2026-06-15

Categories: astro-ph.HE

Recent anomalous compact-star observations challenge the conventional neutron-star interpretation in complementary ways: HESS J1731--347 and XTE J1814--338 favor unusually small radii at low mass, while the secondary component of GW190814 appears too massive for an ordinary neutron star under GW170817-based maximum-mass inferences. We examine whether neutron stars with a strong first-order hadron--quark phase transition can address these tensions via the extended stable hybrid branches that arise when the phase conversion is slow compared to radial oscillations, while remaining consistent with GW170817 and NICER constraints. Using a piecewise-polytropic (PP) benchmark, supplemented by an independent speed-of-sound (CS) parametrization comparison, we find two viable patterns in a general parameter scan over both the hadronic and hybrid branches. Scenario 1 realizes an all-at-once solution: the same EOS has a pure hadronic branch compatible with both the GW190814-scale mass and HESS J1731--347, while its slow stable hybrid branch reaches XTE J1814--338. Scenario 2 retains the GW190814-scale hadronic branch but reaches XTE J1814--338 and HESS J1731--347 on slow stable branches in different transition-strength regimes.

R. Rai, R. Verma, Somya

Published: 2026-06-15

Categories: gr-qc

Traditional matched filtering has been the standard for Gravitational waves (GW) detection ever since LIGO was established, even though it requires pre-computed waveform templates and provides no accounts of information about which signal drove the decision of classification. Deep-learning alternatives showed competitive sensitivity, but system biasesincluding class overlap, imbalanced class weighting, limited sample variation, and traintest mismatchcontinue to cause problems with generalisation in real detector noise. We introduce CASPER-Classification with Attribution via ShaPlEy in Residual neural networks, an end-to-end pipeline combining residual convolutional neural network (CNN) classifier with a FastSHAP explainer. 260 distinct events from the Gravitational Wave open Science Centre were fetched across SNR range of 7-42 from both H1 and L1 detectors with no synthetic augmentation. The classifier achieves AUC (Area Under Curve) of 91% across the model with a low false alarm rate. Focal Loss and Platt Calibration were used to improve decision boundary and generalisation. FastSHAP attribution maps recover the complete chirp morphology and provides detailed maps for a visual interpretation of the decision. The complete pipeline contains fewer parameters than standard deep learning models and requires no hardware except a standard CPU making our model an effective lightweight pipeline for Gravitational Wave Detection under real life conditions.

Sohrab Rahvar

Published: 2026-06-15

Categories: gr-qc

We investigate particle production in the dynamical curved spacetime of a binary black hole system. Particle production is a well-known feature of quantum field theory in curved spacetime, underlying the Hawking and Unruh effects. Here we extend it to the time-varying gravitational perturbation sourced by a binary black hole. Treating a massless scalar field coupled to the binary metric, we compute the particle flux and radiated energy to leading order in the metric perturbation $h_{μν}$, using both the Bogoliubov transformation method and the S-matrix formalism. The perturbation is modeled with the standard quadrupole formalism, retaining the time-domain quadrupolar ($\ell=2$) contribution that dominates gravitational-wave emission. Our calculation is valid in the weak-field, large-separation inspiral regime and is not expected to capture the strong-field, nonlinear merger phase. In this regime we find a characteristic non-thermal, power-law emission with $dE/dt \propto M^{10/3}ω^{16/3}$, in contrast to a thermal Hawking spectrum. Extending the analysis through merger that uses the numerically-relativistic metric is left to future work.

Silvester G. A. Borsboom, Manus R. Visser

Published: 2026-06-15

Categories: hep-th

We systematically develop a framework for equilibrium thermodynamics in terms of thermodynamic representations, which are choices of thermodynamic potential and independent variables, one from each conjugate pair. In each representation, equations of state arise from first derivatives of the potential and response functions from its second derivatives. We apply this framework to ideal gases and to Schwarzschild black holes in a spherical cavity in various representations, interpreting the cavity area and its conjugate surface pressure as the thermodynamic volume and pressure of a holographically dual system. For this quasi-local Schwarzschild black hole, stability depends on the thermodynamic representation, or equivalently on which variables are held fixed under equilibrium perturbations. The large black hole branch is thermally stable at fixed volume but mechanically unstable under isothermal compression, while the system is mechanically stable under adiabatic compression everywhere. At fixed pressure, the black hole is thermally unstable throughout the physical state space. We also find that the thermal expansion coefficient is negative everywhere and show that isenthalpic expansion always cools the black hole. More broadly, the framework provides a systematic route to deriving equations of state and response functions for a wide class of black hole systems using quasi-local gravitational thermodynamics.

Steffen Gielen

Published: 2026-06-15

Categories: gr-qc

We construct Hilbert spaces for the minisuperspace quantum cosmology of a closed Universe in the limit of extreme slow-roll inflation, in which the scalar field is approximated as constant. In this setting, the potential energy in the scalar field is an integration constant depending on initial conditions, equivalent to the cosmological constant as it appears in unimodular gravity. If one fixes the value of this integration constant, the Wheeler-DeWitt equation admits two independent solutions, and a natural inner product picks out one of them (essentially Vilenkin's tunnelling wavefunction) with positive norm. The physical Hilbert space is then one-dimensional, in agreement with some recent discussions of closed universes in quantum gravity. However, if the potential energy is left arbitrary, the theory allows for an infinite-dimensional Hilbert space corresponding to energy eigenstates of an effective Hamiltonian. Requiring that this Hamiltonian be represented as a self-adjoint operator leads to a one-parameter family of boundary conditions at the singularity, generalising the DeWitt criterion of a vanishing wavefunction. The boundary condition always leads to a mixture of Hartle-Hawking (no-boundary) and tunnelling wavefunctions, but a particular choice "almost" singles out the Hartle-Hawking wavefunction, with exponentially suppressed corrections.

Orhan Donmez, M. Yousaf, G. Mustafa

Published: 2026-06-15

Categories: astro-ph.HE

Scalar tensor theories of gravity provide a broad as well as physically rich extension of general theory of relativity by allowing the gravitational interaction to be mediated not only by the spacetime metric but also by scalar degrees of freedom. In this manuscript, we present a new exact solution in the Freund-Nambu scalar-tensor (FNST) gravity scenario, representing a nontrivial scalar-tensor generalization of the Janis-Newman-Winicour naked-singularity geometry, characterized by an additional coupling parameter q in the scalar sector. We also numerically solve the general relativistic hydrodynamic equations in order to investigate the shock-cone mechanism formed by Bondi-Hoyle-Lyttleton accretion around this compact spacetime on the equatorial plane. We show that stronger scalar-tensor deviations modify the shock-cone morphology, significantly increase the amount of matter accumulated near the central compact object, and enhance the oscillatory behavior of the shock cone. The Lorentzian-like peaks obtained from the numerically computed power spectral density are interpreted as hydrodynamically generated QPO-like modes. These modes are driven by shock cone oscillations and by the compression and rarefaction of the plasma trapped inside the cone. Finally, for a compact object with mass parameter M = 10M_sun, the numerically extracted frequencies are found mainly in the range from a few Hz up to approximately 100 Hz. These frequencies overlap with the QPO ranges reported in stellar-mass black-hole-candidate systems. In particular, the frequencies obtained for the FNST2-FNST4 models fall within the range of timing features reported for the source GRS 1915+105. These results suggest that the exterior hydrodynamical variability of FNST compact spacetimes may provide phenomenological diagnostics of scalar-field-induced deviations from the Schwarzschild reference case.

Ashoke Sen

Published: 2026-06-15

Categories: hep-th

Many string compactifications down to four non-compact space-time dimensions with N=8, N=6 and N=4 supersymmetry have BPS black holes carrying pure D-brane charges and preserving four supersymmetries. The string coupling does not flow in these backgrounds and can be set to any arbitrary value. Therefore the supersymmetric index of these black holes must be independent of the string coupling. On the other hand, explicit computation of one loop correction to the index from gravitational path integral is sensitive to the choice of ultraviolet cut-off. We show that if the cut-off scale is chosen to be the string scale in accordance with the rules of string theory, then the dependence of the index on the string coupling disappears in accordance with the expectation from supersymmetry. Similar results are obtained for type II string theories compactified on Calabi-Yau manifolds with zero Euler number. For non-zero Euler number we encounter a puzzle that we discuss but do not fully resolve.

Francesco Iacovelli, Digvijay Wadekar, Javier Roulet, Emanuele Berti, Alessandra Corsi

Published: 2026-06-15

Categories: gr-qc

Rapid parameter estimation for neutron star-black hole (NSBH) mergers is essential for deciding whether, where, and how electromagnetic facilities should follow up gravitational-wave alerts. Current low-latency analyses typically use only the dominant quadrupole harmonic, leaving strong degeneracies among luminosity distance, inclination, and intrinsic binary parameters. We show that mode-by-mode filtering of the $(2,2)$, $(3,3)$, and $(4,4)$ signal-to-noise-ratio (SNR) time series enables low-latency marginalization over higher-order-mode information at a computational cost comparable to quadrupole-only analyses. Applied to simulated NSBH detections in a LIGO-Virgo network at design sensitivity, our method improves constraints on luminosity distance, viewing angle, localization volume, and source-frame secondary mass, thereby sharpening crucial estimates of electromagnetic detectability and host-galaxy association. We also validate the approach on public data for previously detected NSBH events, finding the largest improvement for the asymmetric, higher-SNR event GW190814.

Chih-Hung Wu

Published: 2026-06-15

Categories: gr-qc

Classical critical collapse provides a dynamical route from smooth initial data to a naked singularity, representing a sharper violation of predictability than ordinary black hole singularities. We argue that this distinction is erased by quantum backreaction. Building on the semiclassical interior analysis, where quantum self-energy of the collapsing matter generates a universal growing mode and a finite mass gap, we study the exterior naked singularity region that determines global visibility in the Einstein-scalar system. We analyze controlled exterior models in both $2+1$ and $3+1$ dimensions. In the former, smooth matching and physical boundary conditions analytically select a vacuum polarization state, whose backreaction cloaks the classically naked region by a quantum trapped branch. In the latter, numerical horizon tracing shows that near a quantum-shifted threshold the exterior develops finite-mass marginally trapped surfaces rather than a zero-mass naked endpoint. These results suggest a global quantum picture in which the Choptuik naked singularity shares the fate of an ordinary black hole singularity: quantum effects push the putative Cauchy horizon behind a quantum-generated horizon, thereby reducing the loss of predictability to the standard black hole evaporation problem.

Georgios K. Karananas, Mikhail Shaposhnikov

Published: 2026-06-15

Categories: hep-ph

We present a dynamical mechanism for the erasure of inflationary isocurvature perturbations of the non-compact QCD axion. The key ingredient is an early-time runaway exponential potential, which drives the axion onto the well-known scaling cosmological attractor after inflation. Once on the attractor, the axion tracks the dominant component of the Universe, radiation, and isocurvature modes are erased even if the field is effectively massless during inflation. When the QCD potential turns on, the axion carries nonzero velocity, and kinetic misalignment can become operative. The exponential potential induces residual CP violation, potentially accessible to future electric dipole moment searches. This mechanism requires that the axion be effectively non-compact over the field range relevant for its post-inflationary evolution.

Alexandre Landry

Published: 2026-06-15

Categories: gr-qc

A covariant reconstruction framework for electromagnetic Kantowski--Sachs (KS) geometries in teleparallel $F(T)$ gravity is developed using the coframe/spin-connection (CSC) formalism and the Coley--Landry invariant approach. In a restricted Maxwell-compatible branch, the electromagnetic conservation laws strongly constrain the anisotropic KS scale factors and lead to the scaling $ρ_{\mathrm{em}}\propto A_3^{-4}$. The corresponding symmetric and antisymmetric field equations are derived and used to reconstruct the functional form of $F(T)$ directly from the KS dynamics. Power-law and exponential ansätze generate distinct invariant reconstruction branches associated with electric, magnetic, and transverse electromagnetic sectors. The exponential branch naturally admits reduced teleparallel de Sitter limits and shifted models of the form $F(T)=f(T_0-T)$. The reconstructed branches describe anisotropic cosmological sectors together with local BH-interior-like sectors that may reproduce reduced BH-interior-like or RN--dS-type behaviors at the level of the KS dynamics. These branches are organized through the invariant coframe/spin-connection classification and screened using the necessary leading-order viability conditions $F_T>0$ and $F_{TT}>0$. The local and branch-dependent nature of the construction is emphasized throughout.

Carlo Branchina, Vincenzo Branchina, Filippo Contino, Riccardo Gandolfo, Arcangelo Pernace

Published: 2026-06-15

Categories: hep-th

We have recently shown that a natural way to implement the Wilsonian paradigm in gauge theories is through the introduction of a ``spectral cutoff", a cut on the eigenvalues of the covariant Laplacian, pointing out that this provides the route toward the renormalization group (RG) construction. Here we apply this idea to quantum gravity, resorting to two realizations of the spectral running cutoff: ``hard" and ``smooth". We derive the RG equations for the Newton and cosmological constant and find the RG pattern of the asymptotic safety scenario, with a non-Gaussian UV-attractive fixed point.

Edilberto O. Silva

Published: 2026-06-15

Categories: gr-qc

Static Love numbers of four-dimensional Schwarzschild black holes vanish in general relativity. We study how the fixed-quadrupole static tidal solution is modified by the parity-even cubic Weyl operator in the gravitational effective field theory. Working perturbatively in $ε_{\rm e}=λ_{\rm e}(Λr_s)^{-4}$, we construct the reduced quadratic radial action for static even-parity $\ell=2$ perturbations, order-reduce the higher-derivative equations, and solve the resulting boundary-value problem directly in metric variables. The order-$ε_{\rm e}$ equations reduce to a first-order two-dimensional inhomogeneous system for $X_0$ and $X_K$, with $X_2$ fixed by an algebraic constraint. Horizon regularity leaves one constant, but matching to infinity shows that this freedom only renormalizes the applied tidal branch. After removing this tidal renormalization, the decaying branch is unambiguous. Calibrating the spatial sector at fixed $\ell=2$ against the associated-Legendre branches $P_2^{2}$ and $Q_2^{2}$, we obtain a fixed-quadrupole response amplitude $Δ(B/A)=-2400ε_{\rm e}$. Equivalently, the scalar fixed-$\ell$ quotient gives $Δk_{2,\rm sc}^{\rm fix}=-20ε_{\rm e}$. The second number is a scalar fixed-$\ell$ conversion of the metric branch ratio, not, by itself, the analytically continued, gauge-invariant electric Love number. A comparison with canonical Teukolsky-based Love numbers requires an additional continuation in $\ell$ and a precise map of normalizations. The result should therefore be viewed as a reproducible metric-sector benchmark for the cubic Weyl EFT, complementary to gauge-invariant master-equation approaches.

Dimitrios Tsimpis, Govind Venugopal

Published: 2026-06-15

Categories: astro-ph.CO

We derive analytic templates for the scalar and tensor primordial power spectra describing cosmologies that transition from kinetic dominance to slow-roll inflation in the presence of spatial curvature. Our results extend recent works in the literature, allowing us, in particular, to recover the scalar and tensor tilts analytically. We revisit the case of curvature-assisted single-exponential models in light of this framework. In the case of an open universe, the phase space of such models naturally includes cosmologies that start out in a kinetic-dominance regime followed by a parametrically controlled quasi-de Sitter phase. However, they do not fit in the framework of the templates, as their second Hubble slow-roll parameter remains of order one in the quasi-de Sitter regime.

David Fajman, Elliot Marshall

Published: 2026-06-15

Categories: gr-qc

We numerically study non-linear perturbations of the Einstein-de Sitter spacetime as a solution to the Gowdy-symmetric Einstein-Euler system for a polytropic equation of state. The results suggest that the Einstein-de Sitter spacetime is stable for sufficiently small but otherwise generic perturbations. This is in stark contrast to the well known instability of this spacetime when the matter model is dust. Moreover, this indicates a previously unknown stable regime of the Einstein-Euler equations with direct implications for cosmology.

Grigorios Giotopoulos, Hisham Sati

Published: 2026-06-15

Categories: hep-th

We produce the dimensional reduction to 10D IIA supergravity (SuGra), via cyclification, of the remarkable result that full 11D SuGra is put on shell just by imposing the duality-symmetric Bianchi identities on C-field super-flux densities over supertorsion-free superspace. Generally, we highlight that when duality-symmetric superspace Bianchi identities are characterized by Whitehead bracket $L_\infty$-algebras $\mathfrak{l}\mathcal{A}$ of a classifying space $\mathcal{A}$, their dimensional reduction is characterized by the cyclic loop space $\mathrm{Cyc}(\mathcal{A})$. We promote this to a general mechanism of dimensional reduction on super-spacetime, compatible with the global (infrared) completion of supergravity theories by flux quantization in non-abelian cohomology with coefficients in $\mathcal{A}$ and $\mathrm{Cyc}(\mathcal{A})$, respectively. In the case of 11D SuGra, the characteristic $L_\infty$-algebra is $\mathfrak{l}S^4$ and hence we obtain that full on-shell 10D IIA SuGra is equivalent to $\mathfrak{l}\mathrm{Cyc}(S^4)$-Bianchi imposed identities on NS/RR super-flux densities over supertorsion-free 10D super-spacetime. This implies that any space which is $\mathbb{R}$-rationally equivalent to $\mathrm{Cyc}(S^4)$ classifies an admissible flux quantization law, which provides a global completion of 10D IIA SuGra that admits oxidation to 11D.

Wen-Tao Fu, Ming-Fei Ji, Yu-Sen Zhou, Li-Ming Cao

Published: 2026-06-15

Categories: gr-qc

We investigate the entropy of a dynamical black hole arising from second-order perturbations of a general stationary background with a bifurcate Killing horizon. Using Gaussian null coordinates, we study the geometry of the apparent horizon perturbatively up to second order. Within the covariant phase space formalism, to explore the contribution of matter fields, we introduce a new modified canonical energy, and establish a balance law relating the second-order variation of the entropy to the energy flux entering the black hole. We show that the entropy is given precisely by the area of the apparent horizon at second order when the null energy condition holds for the infalling matter, and that the variation of the entropy also obeys the second law. We also discuss the possibility that the area law continues to hold when the null energy condition is violated.

Marc Schneider

Published: 2026-06-15

Categories: hep-th

We derive the Lehmann-Symanzik-Zimmermann reduction formalism for a massive spin-2 particle on Minkowski spacetime and extend the formalism to cosmological spacetimes. The reduction formalism allows for a versatile proof that the Goldstone boson equivalence theorem holds in Friedmann-Lemaître-Robertson-Walker space-times. For the de Sitter and the radiation filled universe, we investigate the Goldstone dynamics and perform an analysis of the range of validity provided by the effective kinetic operator.

Rui Yang, Jun Tao, Peng Wang, Mian Zhu

Published: 2026-06-15

Categories: astro-ph.CO

The Lyth bound asserts that the field excursion of inflaton must be sub-Planckian, thereby imposing an upper bound on the amplitude of the tensor power spectrum in inflationary scenario. This bound is conventionally derived assuming a scale-invariant curvature power spectrum, i.e., $n_s = 1$. However, astrophysical observations confirm a red-tilted spectrum with $n_s < 1$. In light of recent results from the Atacama Cosmology Telescope (ACT) DR6, we revisit these constraints using the newly implied scalar spectral index of $n_s \simeq 0.9743$. Incorporating the ACT data yields a different upper bound on the tensor-to-scalar ratio $r$, which can potentially exclude inflationary scenarios previously robust under the original Lyth bound with $n_s = 1$. Our result highlights the urgent need to combine theoretical Lyth bound considerations with the most up-to-date astrophysical data.

Bob Osano

Published: 2026-06-15

Categories: gr-qc

We derive and compare three mechanisms of cosmological magnetogenesis: the Thomson-scattering velocity-difference mechanism of Takahashi et al.\ (2005), a new density-gradient source identified here for the first time, and the Harrison bulk-flow mechanism of Cembranos et al.\ (2020). Starting from the coupled Maxwell-Boltzmann equations, the complete kinetic theory chain is derived in a single document -- from the BBGKY hierarchy and Thomson collision term, through the generalised Ohm's law, to the second-order magnetic induction equation. The Ohm's law correction terms are each bounded by $m_e/m_p\approx5.4\times10^{-4}$, confirming the standard single-fluid approximation to better than $0.1\%$. At second order in cosmological perturbations, products of first-order scalar source vorticity, we identify a coupling between the photon density contrast $δ_γ\equiv δρ^{(1)}_γ/ \barρ_γ$ and the electron-photon velocity difference $(u_e-u_γ)^{(1)}$ that was implicitly present in previous treatments but never isolated. Numerical evaluation with CAMB~v1.6.6 at $z=1100$ shows that this term contributes at ${\approx}0.97\times B_{\rm Tak}$, giving a scattering-mechanism total ${\approx}1.4\times$ the Takahashi result. The Harrison mechanism at the Planck bulk-flow limit ($β<8.5\times10^{-4}$) yields $B\approx5.7\times10^{-24}$~G at 1~Mpc today and dominates for $β\gtrsim2\times10^{-3}$, mildly above the Planck limit. All seed fields exceed the galactic dynamo threshold by many orders of magnitude.

Elsa M. Teixeira

Published: 2026-06-15

Categories: astro-ph.CO

We present a new class of interacting dark sector models in which the intrinsic entropy of a dark matter fluid couples to a scalar field describing dark energy. These interactions are constructed within a covariant Lagrangian framework, including algebraic and derivative entropy couplings, effectively leading to pure momentum exchange in the dark sector. A key feature is that the background cosmology remains unchanged and therefore indistinguishable from $Λ$CDM or uncoupled quintessence. However, at the level of the linear perturbations, the dark matter Euler equation exhibits scale-dependent contributions, while the continuity equation is unmodified. We show that these classes of models are compatible with current CMB constraints and can potentially produce observable signatures in large-scale structure.

Yiwen Deng-Huang, Daniele Bertacca, Sabino Matarrese

Published: 2026-06-15

Categories: astro-ph.CO

We extend the Schrödinger approach to large-scale structure formation beyond the Newtonian regime by working at first post-Friedmann (1PF) order. The standard Schrödinger--Poisson system gives a useful reformulation of the dynamics of a self-gravitating pressureless fluid, but it corresponds to the leading post-Friedmann, or Newtonian, limit. It therefore misses the relativistic corrections that enter at next-to-leading order and become relevant on horizon scales and for high-precision cosmological surveys. Starting from the 1PF continuity and Euler equations in a flat $Λ$CDM background, we identify the conserved density variable associated with covariant mass conservation. In terms of this variable, the continuity equation takes a Newtonian-like conservative form. However, even for vanishing covariant vorticity, the spatial velocity field in the cosmological frame contains a transverse 1PF component. Thus the full 1PF mass flux cannot be represented solely by the gradient of a scalar phase. We show that the Schrödinger-like formulation at 1PF order requires an effective vector potential fixed by this transverse velocity component. This vector potential contains the post-Friedmann metric vector perturbation, related to relativistic frame-dragging effects, together with nonlinear scalar terms required by the zero-vorticity condition. Equivalently, when the equation is written in scalar form, these corrections appear as an imaginary contribution to the effective potential. At leading order our system reduces to the usual Schrödinger--Poisson formulation, while at 1PF order it provides a relativistic extension of the Schrödinger description of cold matter dynamics.

Ayan Kumar Naskar, Avijit Sen Majumder, Sourav Bhattacharya

Published: 2026-06-15

Categories: hep-th

We investigate the bending of massless fields by a massive object in the presence of a curvature-scalar $\sqrt{-g}ξR φ^2$ non-minimal coupling up to one loop, using the perturbative quantum gravity computations. It is well known that without such coupling a self interacting scalar field theory cannot be renormalised in the presence of gravity. The massive object is modelled by a massive scalar $φ$, and it is assumed to be non-relativistic, e.g., a star. We compute the 2-2 scattering of massless scalar and photons off this object via graviton exchanges. Assuming both $ξ$ and the bending angle to be small, we use the eikonal approximation to compute the angle up to ${\cal O}(ξG^2)$. At tree level $({\cal O}(ξG))$ we find no bending, and hence the ${\cal O}(ξG^2)$ result happens to be leading in this case. The non-minimal vertices are qualitatively different from that of the standard minimal ones, e.g. $ \sqrt{G} h_{μν} T^{μν}$, as the former contains explicit momenta of the gravitons instead of the scalar, complementing the second. The bending angle is found to behave like $\sim b^{-7}$, where $b$ is the impact parameter. We have emphasised the qualitative differences of our results from that of the well studied minimal case.

Júlio C. Fabris, Alexander Yu. Kamenshchik

Published: 2026-06-15

Categories: gr-qc

The generalized unimodular theory is revisited and its consequence for cosmology is discussed. The usual matter components of the universe are obtained in a pure geometric way. This result gives a new perspective to the studies of the dark sector of the universe. A background and perturbative analysis are carried out, recovering the corresponding results obtained through the general relativity theory but with a different interpretation.

Han Lai, Atsuhisa Ota

Published: 2026-06-15

Categories: gr-qc

We study the real-time linear response of gravitational waves in a time-dependent resonant scalar field in a Minkowski background. In the Schwinger-Keldysh formalism, we develop an adiabatic regularization scheme for unequal-time correlation functions and use it to extract the ultraviolet structure of the one-loop response. The leading divergence reproduces the familiar $\Box^2 h_{ij}$ structure, whereas the time-dependent background induces additional local divergences proportional to $\Box h_{ij}$, $\partial_0 h_{ij}$, and $h_{ij}$. These are renormalized by local counterterms associated with the Weyl-squared term, a time-dependent Ricci-scalar term, and a time-dependent cosmological constant. We also compare the renormalization of the linear response with that of the tadpole stress tensor and find a mismatch beyond leading adiabatic order in the present toy model. By considering a covariant completion of the resonance, we further argue that this mismatch is tied to the off-shell nature of the fixed background, and is expected to disappear once the background is treated on shell.

Yi-Han Huang, Sen Guo, Yu Liang, Lin Wen, Kai Lin

Published: 2026-06-15

Categories: gr-qc

Extreme mass-ratio inspirals (EMRIs) constitute one of the most promising probes of strong field gravity for future space borne gravitational-wave observatories. As a representative higher-curvature extension of General Relativity (GR), four-dimensional Einstein-Gauss-Bonnet (4D EGB) gravity is distinguished by its strictly linear geometric coupling. By this mathematical property, the pathological Fisher-matrix singularities that typically plague conventional modified black hole models are effectively evaded, thereby providing an ideal framework to test topological deviations from classical spacetimes. Through the classification of equatorial periodic orbits via an integer taxonomy $(z,w,v)$, it is demonstrated that even modest Gauss-Bonnet couplings ($α\sim 0.1M^2$) imprint measurable geometric signatures onto the zoom-whirl architecture. Although the global conservative energy budget is shifted by a mere $\sim 0.2\%$, the short-range repulsive EGB core severely alters the strong field whirl dynamics, whereby a resolvable macroscopic dephasing of several radians per orbit is accumulated. Through semi-relativistic waveform modeling, it is revealed that this temporal compression manifests as a rigid, high-frequency stretching of the gravitational-wave harmonic comb -- a clean, amplitude-independent spectral signature ideally suited for detection by LISA, Taiji, and TianQin. A rigorous Fisher information analysis confirms that for a typical four-year observation at a signal-to-noise ratio of $ρ=20$, the marginalized error on the EGB coupling can be tightly bounded to $σ_α\sim \mathcal{O}(10^{-6}) M^2$, with virtually negligible parameter degeneracy with the orbital eccentricity.

Peter Mészáros, Daniel Račko

Published: 2026-06-15

Categories: gr-qc

We derive the stress-energy tensor for a pair of fluids with a novel form of interaction that depends on the relative velocity of volume elements of the two fluids. The interaction is described through quantities measuring the local particle density of one fluid through the metric tensor induced on hypersurfaces perpendicular to the 4-velocity field of the other fluid -- the particle density of one fluid measured in reference frames of volume elements of the other fluid, as opposed to the standardly defined particle density of a fluid measured in reference frames of its own volume elements. This introduces an explicit dependence of the stress-energy tensor on the scalar product of 4-velocities of the two fluids, which can be expressed through the relative physical speed of their volume elements. We also investigate the effect of the studied interaction on the evolution of Bianchi type-I spacetimes, under the assumption of small anisotropy. This represents the simplest nontrivial application of the studied form of interaction. The evolution of the anisotropy does not change qualitatively, which implies compatibility with models of our Universe.

Sai Swagat Mishra

Published: 2026-06-15

Categories: astro-ph.CO

Forthcoming cosmological surveys require inference pipelines that are both statistically reliable and computationally scalable. In this work, we perform a systematic comparison of three complementary inference strategies for late-time $Λ$CDM cosmology: exact Markov Chain Monte Carlo (MCMC), Gaussian Process (GP)-assisted MCMC, and neural Simulation-Based Inference (SBI). Using a common analysis framework based on Cosmic Chronometers, DESI DR2 baryon acoustic oscillation measurements, and the Pantheon+ Type Ia supernova compilation, we consider two dataset combinations of increasing complexity, namely CC+DESI and CC+DESI+PP, under identical cosmological assumptions and priors. For CC+DESI, both GP emulation and SBI reproduce the exact posterior constraints on $(H_0,Ω_{m,0})$ to better than $0.3σ$. For the more constraining CC+DESI+PP combination, modest method-dependent shifts emerge, reaching at most $\sim1.5σ$ in a single parameter. Despite these differences, all methods recover a nearly identical expansion history, with percent-level agreement across the full redshift range. From a computational perspective, GP emulation accelerates model evaluations but remains limited by MCMC sampling, whereas SBI achieves order-of-magnitude reductions in total runtime through amortized posterior learning. We further investigate the convergence of SBI as a function of simulation budget and identify the number of simulations required to obtain stable posterior constraints. Overall, our results demonstrate that accelerated inference techniques can deliver reliable cosmological constraints for realistic late-time datasets at a fraction of the computational cost of conventional likelihood-based analyses.