Papers

We present an exact gravitational instanton solution on a five-dimensional, conformally invariant, Kerr-like warped Riemannian brane-world manifold. The geometry can be described as the Kähler manifold $\mathbb{C}^1\times\mathbb{C}^1\times \mathbb{R}$. By applying a double cover of $S^3$ through stereographic projection onto $\mathbb{C}P^1\times \mathbb{C}P^1$ of the effective four-dimensional manifold, together with the Klein surface construction, we exploit the underlying $\mathbb{Z}_2$ symmetry. The instanton is then obtained by fibering over the antipodal $S^2$. The metric is determined by a first-order differential equation containing an integer parameter, while the equation governing the angular momentum component decouples from the remaining field equations. Finally, we show that our solution admits an analytic complex transformation to a locally conformally related Kähler manifold possessing a Kähler potential, thereby making the self-dual structure manifest.

Recently, an order completeness property has attracted attention in some low regularity spacetime geometry literature, where it was named `chronocompleteness'. In this work we prove that, in the framework of standard Lorentzian geometry, this property is equivalent to global hyperbolicity. The equivalence of global hyperbolicity with other more traditional forms of order completeness is also established.

We study whether varying the topological boundary of the doubled Virasoro TQFT can produce distinct torus pairings for an ensemble interpretation of 3D gravity. In the ordinary nondegenerate sector, every bounded pairing that commutes with the modular transformations is scalar, and vacuum normalization selects the diagonal pairing. The proof identifies the modular transformations with the even Weil representation and applies the Cowling–Steger lattice-restriction theorem. Under the ordinary block-diagonal vacuum–continuum ansatz, positivity and the two vacuum marginals also make a Borel-measure pairing bounded, without assuming absolute continuity; modular invariance then forces the same diagonal pairing. These two classes therefore contain no distinct torus pairings to average. A nontrivial Virasoro boundary ensemble must involve genus-one data outside these classes, additional sectors or vacuum–continuum couplings, or information invisible in the torus pairing. Elementary proofs in explicit regular sectors explain the rigidity mechanisms, and numerical calculations provide independent checks.

We investigate the conservative dynamics of relativistic charged particles in a Schwarzschild spacetime threaded by an externally supported dipolar magnetic field. The gravitational sector is treated exactly in isotropic coordinates, while the electromagnetic field is described by the leading asymptotic term of the relativistic exterior dipole solution. This formulation provides an analytically tractable relativistic extension of the classical Størmer problem and allows the circular-orbit and stability structure to be studied explicitly. For the outward-Lorentz branch, characterized by a positive signed magnetic coupling for positive azimuthal motion, we find a non-trivial competition between radial and vertical stability. Weak magnetic coupling shifts the radially marginal orbit inward, whereas at stronger coupling vertical instability becomes the limiting mechanism and the inner boundary of stable circular motion moves outward. The associated vertical epicyclic mode progressively softens and vanishes at the vertical stability boundary. The marginally bound sequence exhibits a distinct strong-coupling behaviour, illustrating that energetic and stability boundaries need not evolve together. The radial and vertical epicyclic frequencies also generate characteristic commensurabilities that may provide natural sites for nonlinear orbital coupling. Because the magnetic field is treated as a test field, these charged-particle effects leave the vacuum Schwarzschild null-geodesic structure unchanged. Magnetic signatures in compact-object observations must therefore arise through the dynamics and radiation of charged matter, plasma propagation effects, or physics beyond the test-field approximation. The framework developed here provides an analytical basis for studying nonlinear dynamics, radiation reaction, and more realistic compact-object magnetospheres.

We extend the classical relativistic Størmer problem by incorporating radiation reaction through the Landau–Lifshitz formulation, thereby providing a self-consistent description of dissipative charged-particle motion in a static dipole magnetic field. We derive the complete dimensionless equations and distinguish them from a reduced drag-only model that preserves the exact energy-loss law while omitting directional effects associated with magnetic field gradients. For planar motion, the reduced system yields exact instantaneous evolution laws for the particle energy and canonical angular momentum, together with averaged transport equations for regular bound librations. When the motion is constrained to the instantaneous circular branch, the secular evolution can be integrated in closed form and approaches a simple large-radius power law. This analytical solution provides a useful benchmark, although the circular branch is radially unstable and therefore does not describe generic nearby trajectories. Numerical integrations further illustrate the nonuniform dissipative deformation of planar rosette-like orbits. In three dimensions, the complete Landau–Lifshitz force produces local exponential damping of small vertical perturbations, with the leading contribution arising from the field-gradient term absent from the reduced model. These exact, averaged, conditional, and local results establish a controlled analytical framework for studying radiation-driven phase-space transport in strongly inhomogeneous magnetic fields and provide a foundation for future global simulations, kinetic descriptions, and calculations of the associated electromagnetic emission.

We develop a quantitative Fredenhagen–Haag like approach for describing Hawking radiation using massless scalar fields on controlled spherically symmetric Vaidya space-times. The local thermal character of the system is supplied by the universal scaling limit of Hadamard two-point functions at an outer trapping horizon \cite{KurpiczPinamontiVerch2021}. On regular detector–horizon windows, we construct globally hyperbolic developments and compare the nonautonomous Vaidya evolution with a frozen Schwarzschild propagator on Sobolev energy spaces. We derive an explicit Duhamel estimate that is uniform in angular momentum, calculate the exact linear null-peeling coefficient, and bound the quadratic remainder of the ray map. Combining these estimates with positivity gives a two-sided detector-response inequality relative to the local thermal reference. Its error terms quantify operator variation, stationary scattering tails, finite Hadamard scaling, horizon localisation, and the outgoing channel. The inequality is valid at finite parameters because each of these contributions is retained. Under the decay hypotheses, the detector response converges to the corresponding Fredenhagen–Haag form for asymptotically stationary accretion and for evaporation–accretion turnaround profiles. We also construct a Hadamard state by Cauchy transport from an eventually stationary Unruh covariance and show that a finite evaporating slab does not determine a late-time response without a prescribed future extension. For asymptotic evaporation with $m(u)>0$ at every finite time and $m(u)\to0$, a mass-rescaled conformal formulation yields a scale-covariant finite-window estimate for scale-following detectors.

Optical coatings with minimal thermal noise are essential for high-precision, laser-based metrology experiments. Substrate-transferred crystalline GaAs/Al$_{0.92}$Ga$_{0.08}$As coatings have the lowest thermal noise for high-reflectivity mirror coatings with $\lambda =900$ nm – 12 $\mu$m. Time-frequency stabilization experiments have demonstrated that GaAs/AlGaAs mirror coatings have a lower noise floor than amorphous mirror coatings. These mirrors are now being developed at sizes $\ge 300$ mm for gravitational-wave detectors. In order to understand the coating's fundamental loss mechanisms, which cause thermal noise, we have performed the first measurement of the mechanical loss at cryogenic temperatures using a cryogenic, multimodal, gentle nodal suspension system, which operates from 12 K to room temperature. This initial measurement used a silicon substrate whose asymmetry produced excess friction with the nodal support. Accounting for this friction and Akhiezer loss in the substrate and coating, the mechanical loss agrees well with the thermal noise observed in experiments using cryogenic fixed-cavities with GaAs/AlGaAs mirrors.

Testing Lorentz symmetry in strong gravitational fields provides a unique probe of extensions to standard model. The orbiting motions of the S-stars around the supermassive black hole Sgr~A* provide a natural laboratory for such tests. In this paper, we analyze the S2 orbital data focusing on a static and spherically symmetric black hole within Kalb–Ramond gravity, where the deviations from general relativity are encoded in a single Lorentz-violating parameter $\ell$ introduced by the Kalb–Ramond tensor field. Using a full 14-dimensional Markov Chain Monte Carlo analysis under uniform and Gaussian priors, we obtain $\ell = {1.60 \times 10^{-5}}^{+1.38 \times 10^{-4}}_{-1.76 \times 10^{-4}} $ and $\ell = {-1.02 \times 10^{-5}}^{+1.26 \times 10^{-4}}_{-1.19 \times 10^{-4}} $ at $1\sigma$ confidence level, respectively. These constraints are about three orders of magnitude tighter than those from Event Horizon Telescope imaging of Sgr~A*. We also perform MCMC simulation by fitting data of S38 and S55 stars, as well as their joint analysis. Our results show that the best fit values of $\ell$ in all simulations are always of $10^{-5}$ order, but S2 star provides the most stringent constraints on the parameters because S2 star has higher precision observational data comparing to the fewer public data for other two stars.

Gravitational-wave parameter estimation requires millions of waveform evaluations per event, a cost that constrains real-time inference and population studies. We present a fast, fully differentiable neural-network surrogate for the precessing numerical-relativity model \NRSur{}, spanning its full intrinsic parameter space $\lambda=(q,\,\boldsymbol{\chi}_1,\,\boldsymbol{\chi}_2)$ together with the reference orbital frequency $\omega_0$. Rather than a single polarization at a fixed orientation, the surrogate predicts the inertial-frame spherical-harmonic modes $h_{\ell m}$ ($\ell\leq 4$), so that both polarizations $h_+,h_\times$ at an arbitrary orientation $(\iota,\varphi,\psi)$ are reconstructed from one network evaluation through an analytic, differentiable mode-to-strain projection. Trained on $6\times10^5$ waveforms, it attains a fixed-orientation match of mean $0.975$ (median $0.988$) and an orientation-averaged match of mean $0.940$ (median $0.975$) for $q\in[1,4]$, $|\boldsymbol{\chi}_{1,2}|\leq 0.8$, while keeping the overall strain amplitude physical (median ratio $0.98$). It generates a waveform in $\SI{12}{ms}$ (single) and ${\sim}3.5\times10^4$ per second in batches on a single GPU. Because the mode-to-strain projection is analytic, the surrogate is differentiable in both intrinsic and extrinsic parameters, yielding a full 13-dimensional Fisher matrix (validated against finite differences) and gradient-based (HMC/NUTS) parameter estimation.

A Newtonian halo density does not uniquely determine a relativistic spacetime, and the resulting completion ambiguity can dominate predicted horizon-scale signals. We make this dependence explicit for the feedback-cored Einasto profile calibrated on FIRE-2 simulations. In Schwarzschild gauge, the Einstein equations give an asymptotically flat, density-preserving geometry supported by an effective anisotropic fluid; no microscopic description of collisionless dark matter is assumed. By contrast, a commonly used rotation-curve completion replaces the finite seed core by an inverse-square source cusp. For the density-preserving solution we derive the enclosed mass in closed form, obtain a sharp one-horizon criterion and the extremal boundaries of a three-horizon phase, and establish the fixed-halo first law. More generally, the leading fractional shift of a spherical black-hole shadow is the environmental mass enclosed within the vacuum photon sphere divided by the central black-hole mass. Applying this result to published Event Horizon Telescope shadow-deviation summaries gives illustrative one-sided 95% credible limits of $4.8\times10^{23}\,\Msun\,{\rm pc}^{-3}$ for $\sgr$ and $4.1\times10^{17}\,\Msun\,{\rm pc}^{-3}$ for $\mseven$, far above realistic smooth-halo densities. A Milky-Way calibration predicts a fractional shadow shift of $1.7\times10^{-25}$, whereas the rotation-curve completion gives a shift 19 orders of magnitude larger for the same galactic inputs. A covariant polarized thin-disk calculation shows the same weak-core suppression. Thus current horizon-scale images do not constrain a smooth kiloparsec-scale core-Einasto halo; an observable environmental signal would instead require a compact inner component or a physically different relativistic source.

We investigate horizon-brightened acceleration radiation (HBAR) and a strong-deflection expansion for the deflection angle of light rays scattered in the vicinity of a degenerate photon sphere, within the context of a quantum-corrected black hole spacetime. We characterize the horizon structure and thermodynamics, and we extract the divergent part of the deflection-angle integral from the near-marginal-orbit contribution using a nonsingular prescription at marginality, obtaining a unique leading power-law term. In terms of the closest-approach radius, the strong-deflection leading coefficient factorizes into a universal branch constant and a local factor involving the third derivative of the effective potential at the degenerate photon sphere. On the quantum side, we develop the near-horizon reduction relevant to HBAR, demonstrating that the dominant sector governing the detector response exhibits conformal behavior and yields a thermal excitation spectrum characterized by the horizon temperature. We adopt a Lindblad master-equation framework for the radiation field, establish the existence of a thermal steady state, and obtain an HBAR entropy-energy relation that satisfies a Clausius-type first-law structure. Also, we derive a Wien-type displacement law for the HBAR spectrum, connecting the peak wavelength to horizon thermodynamics and thereby providing an additional observable probe of quantum gravity via near-horizon radiation.

We study static and slowly rotating traversable wormholes embedded in two contrasting dark matter environments, the cuspy Navarro-Frenk-White (NFW) halo and the cored solitonic wave dark matter ($\psi$DM) profile, using observational parameters set by the rotation curve of the dwarf galaxy NGC\,2366. For each profile, we solve the Einstein field equations in the Morris-Thorne framework to obtain the shape and redshift functions, then extend to slow rotation via the Teo metric with a Lense–Thirring frame-dragging term. Across the four resulting spacetimes and six throat radii, we trace null geodesics, compute specific intensity profiles and polar shadow maps, and construct accretion disk images including the full relativistic Doppler effect. We noticed these two profiles differ sharply in photon dynamics. The NFW potential is too shallow to support a detached photon sphere, so its critical impact parameter stays close to the throat radius. The soliton core, by contrast, is focused enough to host a genuine unstable photon orbit that raises the critical impact parameter to about $1.18$–$1.26$ times the throat radius. This difference in photon-sphere structure propagates through nearly every observable, including photon-ring sharpness, shadow size, accretion-disk appearance, and, in the rotating case, the degree of shadow asymmetry from frame dragging. The soliton shadow size also tracks the mass of the underlying ultralight boson through $\rho_c\propto m_b^{-2}$, with a transition from total capture to mainly deflecting lensing near $m_b\sim10^{-18}$\,eV, a feature absent for NFW that offers a direct probe of dark matter microphysics. These results show that wormhole photon dynamics are shaped by the dark matter profile, pointing to a possible observational route to probe the nature of dark matter.

For the Fierz-Pauli action the Bessel-Hagen construction does not produce a preferred local gauge-invariant energy-momentum tensor; it identifies only a gauge-invariant equivalence class of Noether currents, because the action is built from first derivatives of $h_{\mu\nu}$ whereas the first local gauge-invariant spin-2 field strength, the curvature, contains two. It is natural to ask whether a theory built directly from the linearised curvature recovers the electromagnetic-like feature of a strictly gauge-invariant local representative. We examine linearised Weyl-squared (conformal) gravity, whose action is built from the linearised Weyl tensor $C^{(1)}_{\mu\nu\rho\sigma}$, the irreducible spin-2 part of the curvature and hence the curvature counterpart of the spin-1 field strength of electromagnetism. The Bessel-Hagen construction extends naturally from the Poincaré to the full conformal group, realised actively on the fixed Minkowski background, and the resulting Noether current is gauge invariant as a class for every conformal generator. Nevertheless there exists no nonzero strict local, polynomial, symmetric, dimension-four rank-two tensor quadratic in $h_{\mu\nu}$ that is invariant under both gauge symmetries and conserved on the Bach shell: the two gauge symmetries force any candidate to be built from $C^{(1)}$ and its derivatives, while the dimension-four and quadratic-order hypotheses leave only expressions quadratic in $C^{(1)}$ with no additional derivatives, and the four-dimensional Weyl identities collapse these to a pure trace, which cannot be conserved. Linearised Weyl-squared gravity therefore behaves like Fierz-Pauli, not electromagnetism. This suggests that a first-order gauge-covariant field strength, rather than a curvature-built action, is the natural route to an electromagnetic-like local representative.

Static and slowly evolving regular black holes with a non-extremal inner horizon are generally expected to suffer from mass inflation. Here we consider the scenario of asymptotic gravitational collapse into extremal regular black holes. To that end, we first construct geometric models of static spherically symmetric single-horizon extremal and double-horizon inner-extremal regular black holes for generic black hole mass as the only dimensionful scale. These spacetimes may be interpreted as non-fine-tuned vacuum solutions of modified gravitational theories defined implicitly by the requirement that their spherical reduction yields an integrable two-dimensional Horndeski theory. As such, these so-called general quasi-topological gravities admit exact Vaidya solutions in which the mass becomes a time-dependent function. We use this effectively two-dimensional second-order dynamical framework to model the asymptotic formation of extremal regular black holes without a violation of energy conditions. The latter is illustrated explicitly by the equivalence between the strong energy condition for generalised Vaidya solutions of general relativity, and the kinematic timelike convergence condition reformulated dynamically as an onshell condition on the theory-dependent functions characterising the spherical reduction and correspondingly Vaidya solutions of a general quasi-topological gravity. The defocusing of geodesics necessary for regular black holes is thus triggered by the modified gravitational dynamics rather than by the addition of exotic matter degrees of freedom.

Universal relations among macroscopic properties of neutron stars provide a powerful framework to probe their internal structures while minimizing uncertainties associated with the equation of state (EoS). Although such relations have been extensively studied for uniformly rotating stars, their extension to differentially rotating and strongly magnetized configurations is relatively less explored. We systematically investigate equilibrium configurations for a wide range of EoSs, rotation profiles, and magnetic field strengths with XNS numerical simulation code. For sequences with fixed angular momentum, we analyze the dependence of normalized moment of inertia on compactness. We establish a generalized quasi-universal relation between the moment of inertia and compactness that remains remarkably insensitive to the underlying EoS across uniformly rotating, differentially rotating, and strongly magnetized toroidal configurations. For sequences with fixed angular momentum, the normalized moment of inertia exhibits a quasi-universal dependence on compactness, with deviations primarily due to magnetic field strength and degree of differential rotation. We derive analytic expressions for these dependencies, enabling a unified phenomenological model applicable over a wide range of stellar configurations. As astrophysical applications, we quantify the systematic bias in magnetar luminosity estimates and the rotational kinetic energy of post-merger remnant of GW170817. These results extend quasi-universal relations beyond the standard assumptions of uniform rotation and weak magnetization, providing a robust framework for interpreting observations of highly magnetized and differentially rotating neutron stars, and enabling more reliable constraints on their astrophysical properties.

Cloud-of-strings (CS) geometries provide an effective description of one-dimensional string distributions. However, their central singularity cannot be removed through the standard regular black holes (RBH) mechanism based on an effective mass function, since the string sector contributes independently to the ultraviolet structure of the spacetime. Motivated by this observation, we investigate whether string-supported black holes can be consistently regularized while preserving the CS asymptotics and admitting a physically meaningful string-fluid interpretation. Using the gravitational decoupling method, we construct a RBH supported by an effective anisotropic string fluid. We show that the string sector deforms the de Sitter core, modifies the local topology of the spacelike slices, and introduces a longer-range correction dominating the usual Hayward/LQG term. The geometry admits non-extremal and extremal RBH, as well as a regular horizonless compact object. Moreover, the string parameter qualitatively modifies the thermodynamic evolution by shifting the Davies phase transition and the size of the black-hole remnant. Finally, the scalar quasinormal-mode spectrum exhibits systematic changes in both the oscillation frequencies and damping rates. These results show that regularizing string supported black holes is a physically distinct problem, with the matter sector governing the ultraviolet structure, thermodynamics, and dynamical response of the spacetime.

Polymer quantization program applied in Loop Quantum Gravity/Cosmology leads to nonseparable Hilbert spaces. Commonly, one sidesteps this problem by singling out and working with a separable superselection sector. This is however often no longer accessible in more involved models beyond isotropic ones. In an alternative approach one builds a separable Hilbert space as an integral over all available sectors. Here we test the dynamics following from the latter on the example of a flat isotropic Universe admitting negative cosmological constant and a massless scalar field. There, numerical evolution of (initially) semiclassical states shows that there is no relevant difference in their long term semiclassicality properties in comparison to those in a single sector approach. Further, the older (problem-specific) numerical methods are compared against an application of more common and more efficient standard tools (eigen library).

This article derives static and spherically symmetric wormhole and black bounce spacetimes as vacuum solutions of metric gravitational theories in four and higher dimensions. To this end, I extend previous results on quasi-topological gravities to encompass cases in which the metric depends on two arbitrary functions of one variable — instead of the usual single function. I then explain how, given (almost) any spherically symmetric metric, one can reverse engineer a well-defined higher-dimensional theory having such a metric as solution. I apply these results to three paradigmatic examples, including the Simpson–Visser black bounce, to showcase the versatility of this method, as well as its limitations. Moreover, I consider the coupling to a Vaidya-like matter source, so as to generate time-varying solutions and investigate what kind of dynamical evolution these theories can account for.

In this paper, we investigate the thermodynamic properties and information recovery of Schwarzschild and Reissner–Nordstr"om black holes surrounded by perfect fluid dark matter. We show that, while the Bekenstein–Hawking entropy remains unchanged, dark matter significantly modifies the Hawking temperature by introducing a positive contribution that enhances thermal effects, particularly for small black holes. We find that the phase structure is preserved: Schwarzschild black holes remain unstable, whereas Reissner–Nordstr"om black holes exhibit the standard small/large black hole transition, in which small black holes are stable and large black holes are unstable. Furthermore, we demonstrate that dark matter accelerates Hawking evaporation, thereby reducing black hole lifetimes. We further investigate the black hole information loss paradox using the island formula. In the absence of islands, the entanglement entropy of Hawking radiation grows linearly with time and diverges at late times, thereby violating unitarity. By including island contributions, the entanglement entropy of Hawking radiation saturates at twice the Bekenstein–Hawking entropy, reproducing the Page curve and restoring information recovery for both Schwarzschild and Reissner–Nordstr"om black holes surrounded by perfect fluid dark matter. We derive analytical expressions for the Page time and demonstrate that it is directly determined by the thermodynamic parameters of the black hole. Furthermore, we establish a correspondence between thermodynamics and information recovery by showing that the Page time is governed by the Hawking temperature and the event horizon. Finally, we find that the presence of dark matter reduces the Page time, thereby accelerating information recovery.

We model the compact object Cen X-3, which is considered to be a good strange quark star candidate of known mass and radius, incorporating a negative anisotropic factor, and we compute the frequencies of the ten lowest radial oscillation modes. We introduce the anisotropy in three different manners and investigate its impact on the spectra.

Gravitational waves originating from unresolved sources, or stochastic gravitational wave backgrounds (SGWBs), carry precious information about the physics underlying their diverse sources, and represent an important target for future experimental searches. Here, we model the SGWB due to core collapse supernovae. Using an extensive collection of state-of-the-art, three-dimensional, multi-second supernova simulations, we characterize the two main components of this background: one at $f \sim 10^{-2} - 1$ Hz, due to anisotropic neutrino emission (the gravitational wave memory); the other at $f \gtrsim 10^{2}$ Hz, from the near-core matter dynamics. We find that the memory component offers the best prospects of detection, as its characteristic peak at $f\sim 0.1$ Hz is within the reach of future space-born detectors. At the peak, the energy density might be comparable to that of backgrounds from slow roll inflation and from possible cosmological relics, thus potentially impacting searches of these important signals.

Universal hallmarks of quantum chaos—such as the ramp and plateau in the spectral form factor—and the ramp's gravitational duals involving wormholes are widely interpreted as consequences of spectral or ensemble averaging. In this paper, following an earlier proposal of~\cite{Liu25c}, we develop an alternative approach: these phenomena arise as macroscopic smooth structures hidden within erratic microscopic data, which can be isolated through a smooth filter projection. Using the semiclassical Gutzwiller trace formula as a paradigmatic example, we illustrate how many features characteristic of random matrix models—including the ramp, the plateau, the spectral curve, and single-eigenvalue instantons—can be derived in the semiclassical limit without invoking ensemble or explicit spectral averages. We postulate the existence of a minimal Gutzwiller-like structure in the large-$N$ limit of holographic systems and explore its consequences. Beyond deriving the ramp and the plateau, this Gutzwiller-like structure predicts universal rapid macroscopic oscillations in the density of states and the possible existence of hyper-instantons, both of which involve double exponentials in $1/N^2$. On the gravity side, we demonstrate how spacetime wormholes enable the construction of emergent hyper-non-perturbative objects—such as baby-universe and wormhole condensates—which yield double exponential effects in $G_N$. This mirrors the postulated boundary Gutzwiller-like structure and provides a dual gravitational derivation of the universal rapid macroscopic oscillations in the density of states and the spectral plateau.

We compute the energy and angular momentum radiated by compact binaries through gravitational waves, scalar and vector fields, using the field-theoretic approach. We observe that each emitted graviton carries away an angular momentum of $\dot{J}/\dot {N}\simeq 2\hbar$, not only for elliptical orbits (as has already been pointed out by Page [arXiv:2409.00305]) but also for hyperbolic orbits. We extend the analysis to scalar and vector radiation and find that the angular momentum carried by each emitted quantum of radiation is approximately $\hbar$, i.e., $\dot{J}/\dot{N}\simeq \hbar$ for both types of radiation. We demonstrate that the radiated energy and angular momentum of binary systems can be determined from the evolution of the orbital angular frequency and the eccentricity of the quasi-Kepler orbit. However, a gauge-invariant decomposition of the angular momentum carried by the emitted particles into the spin and orbital components is not possible.

Spontaneous scalarization is a well-known phenomenon in scalar-tensor theories, featuring large deviations from general relativity. However, some of its signatures are hard to detect, because it is generally studied as a continuous phase transition. We show that scalarization occurs in a discontinuous (first-order) manner in many astrophysically relevant scenarios. This leads to loud signals that provide new tests of gravity. We demonstrate this in the case of gravitational wave breathing modes from a neutron star that scalarizes due to accretion, and discuss other possibilities.

Some authors have argued that spacetime in general relativity should be given a radically different interpretation from the ones given to spacetime in other theories in virtue of it being a gauge theory. In this article I review this sort of argument and argue against this view. That is, I argue that spacetime in general relativity can be understood analogously to spacetime in other models and that the argument from the diffeomorphism invariance of the theory misapplies the concepts of gauge theory to a context in which they are of limited application.

Ray et al. [1] have claimed that the acceleration of the Hubble expansion rate inferred from Type Ia supernovae in the Pantheon+ catalogue "is equally strong in the CMB dipole ($q_m = + 1.45$) and anti-dipole hemispheres ($+ 1.55$), directly contradicting the SRS26 anisotropy argument". Here SRS26 refers to our analysis [2] which showed that "locally $q_0$ has a strong dipole anisotropy aligned approximately with the bulk flow, and only a small monopole component remains at distances exceeding a few hundred Mpc". We find that these authors confused Equatorial with Galactic coordinates, thereby getting the CMB dipole direction wrong. Hence their conclusion is baseless.

Regular black hole metrics are usually studied kinematically, but a finite-curvature static core does not guarantee that the underlying theory can consistently evolve generic matter through a regular center. We derive a necessary local consistency condition within the most general class of action-based, identically conserved, second-order gravitational field equations in spherical symmetry. Regularity requires that the two functions defining the theory have opposite parities under reversal of the signed radial coordinate, together with additional center-regularity and nondegeneracy conditions. In the integrable sector, this criterion is equivalent to requiring the generalized Misner–Sharp–Hernandez mass to be odd across the center, to vanish cubically there, and to contain no point-mass contribution. For theories reconstructed from static one-parameter vacuum families, the condition becomes covariance under simultaneous reversal of radius and mass. The theories associated with the Hayward and Dymnikova geometries satisfy this selection rule. In contrast, the Bardeen theory does not, demonstrating that curvature regularity of a static solution is insufficient for dynamical consistency with generic matter. We also characterize an infinite class of admissible theories containing Hayward-like black holes with de Sitter cores. The selection rule provides a necessary condition for theories intended to describe regular collapse, but does not by itself establish well-posedness or guarantee a nonsingular endpoint.

We present a class of cosmological scenarios in which a preferred spatial direction in the matter sector coexists with an exactly homogeneous and isotropic Friedmann–Lemaître–Robertson–Walker (FLRW) geometry. The Cosmological Principle is realized on shell, with the vector-field equations enforcing a vanishing momentum density and suitable interactions eliminating the anisotropic stress. Consequently, the construction yields an entire FLRW branch without tuning the matter-field initial conditions. The preferred direction reemerges in perturbations, producing direction-dependent propagation and mixing among scalar, vector, and tensor modes already at linear order. In particular, the mixing opens a linear channel through which perturbations in the scalar sector can in principle source gravitational waves. Our construction thus reveals a new route by which preferred-direction physics can leave observable cosmological signatures while remaining hidden in the background geometry.

We study subleading asymptotic charges of a U(1) gauge field coupled to a charged massless scalar field in Minkowski spacetime. We consider a phase space for which the scalar-field initial data on null infinity have compact support, while the gauge-field initial data admit an asymptotic expansion with inverse-power tails in retarded (advanced) time, which we refer to as initial-data tails. We identify three finite charges at the null infinities which are conserved under scattering. One is a renormalized version of the standard subleading soft charge. The second is a logarithmic charge which is the difference of the coefficients of the leading order tails at the future and past ends of null infinity. The third is a new charge that probes the average of the tail coefficients. We conjecture that the third charge is associated with a new class of large gauge transformations which diverge linearly in retarded time.

We show that the recently discovered “mass symmetries" of the radial Teukolsky equation for Kerr-deSitter black holes can be understood from a corresponding symmetry of the Painlevé VI equation via the classical theory of “isomonodromic deformations". It is known that a subset of the mass symmetries transforms the Teukolsky equation to a wave equation on a “dual" Lorentzian spacetime. We find that this dual spacetime again represents a black hole, and use this insight to re-obtain a previously known partial mode stability result in a geometric manner. Further special geometric features of the dual spacetime are pointed out.