Papers

The gauge structure of the Weyl Gravitoelectromagnetic (GEM) formalism has been investigated, showing that although the theory admits additional propagating modes, only an effective spin-2 sector contributes to physical observables. Building on this result, the formalism is applied to Bhabha scattering mediated by the tensor field $A_{\mu\nu}$, and the Newtonian gravitational potential is derived in the zero-temperature limit. Finite-temperature effects are incorporated through the Thermo Field Dynamics (TFD) formalism. While the Newtonian interaction is recovered at low temperatures, it becomes progressively suppressed in the high-temperature regime, revealing a thermal screening mechanism. The physical origin of this behavior and its possible dependence on the underlying scattering process are briefly discussed.

Matter density ($\rho$) and radial pressure ($p$) are often used as the matter Lagrangian density ($\mathcal{L}_{m}$) because both are thermodynamically consistent and produce the same Einstein field equation (EFE) in general relativity (GR). New gravity models with explicit links between matter and geometry instead involve second-order derivatives of $\mathcal{L}_{m}$ relative to the metric tensor. So, picking either $p$ or $-\rho$ for $\mathcal{L}_{m}$ gives different effective EFEs. This confusion appears because one usually treats the four-velocity ($u_\mu$) and the metric tensor ($g_{\mu \nu}$) as independent. Here, we revisit the basics and offer a consistent framework by relaxing that assumption, thereby making the modified gravity theory independent of the choice of $\mathcal{L}_{m}$. Finally, we test this approach on neutron and quark stars (ultraviolet region) and on cosmological situations with radiation-dominated ($p=\rho/3$) equations of state (infrared region), showing how it clarifies the ambiguity in picking $\mathcal{L}_{m}$ for gravity models.

In the standard model of cosmology, it is assumed that dark matter is pressureless with equation of state $w=0$ and dark energy has $w=-1$. We test these assumptions jointly using DESI DR2 distance measurements, including the recent Lyman-$\alpha$ full-shape Alcock-Paczynski (AP) information, DES supernovae, and two complementary CMB treatment. When constant $w_{dm}$ and $w_{de}$ are varied together, we find $w_{dm}=0.000968^{+0.000501}_{-0.000496}$ and $w_{de}=-0.9380^{+0.0259}_{-0.0262}$ (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are $w_{dm}=0.000870^{+0.000408}_{-0.000410}$ and $w_{de}=-0.9353^{+0.0258}_{-0.0254}$. Both standard $\Lambda$CDM values are disfavored at approximately $2\sigma$ in the joint extension. Neither parameter departs significantly from its standard value when only that parameter is varied. This behavior arises because late-time distances favor $w_{de}>-1$, while maintaining the early-Universe physical matter density requires a compensating positive $w_{dm}$, which changes the mapping to the matter density today. Allowing dynamical dark energy further highlights the complexity of the situation: phantom crossing for dark energy makes $w_{dm}=0$ consistent with the data, whereas a positive $w_{dm}$ preference persists when crossing is forbidden. Interestingly, the Pad'e-$w$ parameterization that provides a flexible description of a class of quintessence models (with no phantom crossing), along with $w_{dm}$ free, is even mildly favored over the phantom-crossing $w_0w_a$ model according to both the best-fit $\chi^2$ and the DIC under both CMB treatments. One can conclude that the apparent preference for phantom crossing may instead reflect deviations in the dark-matter sector rather than dark-energy dynamics alone. [abridged]

We propose a quantum description of the ultracold Reissner-Nordström de Sitter black hole via a specific flat space limit of the double-scaled SYK model. Fluctuations in this regime reduce to flat JT gravity, which is ill-behaved as a quantum system. We show that including the first subleading correction to the dilaton potential breaks the leading order Hagedorn degeneracy but does not resolve the structural problem. Finally, we leverage the gauged description of the sine dilaton gravity model to propose a novel quantization of the near-horizon dynamics of ultracold black holes. This leads to a new proposal for the partition function where states in the asymptotics of the spectrum are suppressed dynamically, and features of a discretized spacetime arise. We obtain a non-perturbative low temperature behavior of the partition function, which is quite different than the cold and Nariai physics. Our result can be interpreted as a dynamical way to enforce cosmic censorship on the quantum system.

We study the directed completion of Lorentzian pre-length spaces, and we show that, under some natural assumptions which cover the case of smooth globally hyperbolic spacetimes, it coincides with the future causal completion of Geroch–Kronheimer–Penrose. Moreover, we provide applications of our findings by characterizing the directed completion of the Kruskal–Szekeres spacetime in terms of its radial null geodesics.

We consider a real massless scalar field in punctured Minkowski spacetime, endowed at the removed origin with the stable one-parameter family of Robin boundary conditions $G(0)-\beta G'(0)=0$, $\beta\geq0$. We obtain the boundary-induced part of the static ground-state Wightman function in closed form and study the response of a uniformly accelerated Unruh-DeWitt detector. The puncture breaks the boost symmetry underlying the stationary Unruh response, so the boundary-induced pullback is nonstationary in proper time. The corresponding subtracted detector contribution can either enhance or suppress the ordinary Minkowski response and depends on the Robin parameter, the detector gap, and the portion of the trajectory sampled. For the unshifted, radially aligned trajectory studied in detail, we prove that the boundary-induced pullback is absolutely integrable in the two proper-time variables. Consequently, as the smooth interaction is extended over the full detector history, this contribution approaches a finite limit and remains $O(1)$, producing no additional term linear in the interaction duration. The formal Neumann limit is singular: the boundary-induced two-point function grows logarithmically because of an infrared singularity in the $s$-wave sector, and the numerical response exhibits growth consistent with this asymptotic behavior.

The recently-discovered long-period radio transients represent a puzzling new class of astrophysical sources, some of which are thought to be compact binary systems emitting a pulse once per orbit. If this interpretation is correct, the orbital period–and therefore the gravitational-wave frequency–is directly encoded in the radio signal, which typically lies in the millihertz band. In this work, we explore whether these systems can be detected by the Laser Interferometer Space Antenna (LISA). Assuming that this phase-locking between radio pulses and orbital motion applies broadly across the population, we construct synthetic source catalogues informed by both observations and theoretical models of related systems, such as cataclysmic variables. We estimate that between $\sim 0.05\%$ and $\sim 6\%$ of long-period transients will be detectable within four years of observation with LISA, depending on astrophysical assumptions. For detectable systems, we find injected frequencies are recoverable to one part in $\sim 10^{5}$, amplitudes to within a factor $\sim 2$, and the sky positions to within $\sim$30 square degrees. Our results demonstrate that gravitational-wave observations can provide direct evidence for the binary nature of these sources and, importantly, can guide future radio surveys by predicting pulse periods, sky positions, and orbital properties. The mismatch between extracted frequency derivatives and that imposed by gravitational-wave decay is also computed to show that the likelihood of electromotive losses driving orbital evolution can be assessed for each detectable candidate.

We study how holographic phase transitions imprint their scaling laws on the local Kasner geometry inside Einstein-scalar black holes. At fixed double-trace coupling, we derive the near-critical scaling of the deviation of the first-epoch Kasner exponent from the Schwarzschild value: $p_t^{(1)}+\frac{1}{3} \propto O^2 \propto (T_c-T)^{1/r}$, where $O$ denotes the boundary condensate and $r$ labels ordinary criticality ($r=1$), tricriticality ($r=2$), and higher multicriticality ($r\geq3$). A super-exponential scalar potential generates a sequence of Kasner epochs separated by scalar-field bounces. We further find that any later epoch that can be tracked continuously across the transition inherits the same temperature exponent, while its coefficient depends on the epoch. Numerical solutions confirm these predictions for the ordinary and tricritical cases. Finally, we show analytically that the leading irrelevant exponent of the infrared fixed point governs the low-temperature Kasner scaling, and verify the resulting scaling numerically for the first Kasner exponent.

We study how three physically motivated dark matter equations of state alter the spacetime geometry of static, spherically symmetric black holes in General Relativity. The models considered are anisotropic perfect fluid dark matter (PFDM), isotropic constant-$\omega$ dark matter, and Bose-Einstein condensate (BEC) dark matter with a polytropic equation of state (EoS). For each model, we derive the Einstein field equations and solve for the metric function analytically in the PFDM and constant-$\omega$ cases, and numerically via the Tolman-Oppenheimer-Volkoff equations for BEC dark matter. We then compute the key strong gravity observables: event horizon radius, photon sphere radius, black hole shadow radius, innermost stable circular orbit (ISCO), and circular orbital velocity profiles. The shadow radii obtained for each model are compared against the Event Horizon Telescope constraints to place bounds on the dark matter parameters. Physical viability is assessed through the null, weak, dominant, and strong energy conditions, along with causality requirements on the sound speed. Our results show that PFDM produces near Schwarzschild geometry with mild anisotropic corrections, the constant-$\omega$ model is strongly restricted by causality to $0 \leq \omega \leq 1$, and BEC dark matter generates smooth, bounded deviations governed by the bosonic self-interaction parameter K. Taken together, these findings show that strong gravity observables can serve as practical tools for distinguishing between competing dark matter models through their geometric imprints on black hole space times.

We study a static, circularly symmetric deformation of the non-rotating BTZ black hole obtained by inserting a smooth transition function into the inverse radial metric component, $g^{rr}=S_\delta(r)F(r)$ with $S_\delta=\tanh[(r-r_h)/\delta]$, leaving $g_{tt}=-F$ untouched. This was motivated by the proposal that such a construction realizes a Lorentzian-to-Riemannian signature change at the horizon; we show that it does not. In coordinates $r-r_h=q^2$ with an advanced time, the metric extends real-analytically across $r=r_h$, and the extension is Lorentzian: $q=0$ is a regular null hypersurface, a degenerate Killing horizon with vanishing surface gravity, beyond which lies a second, isometric copy of the exterior. The areal radius has a minimum there, so the geometry is a black bounce; the would-be Riemannian branch is a separate geometry the Lorentzian sector never reaches. We give the effective source in closed form, an invariant account of the energy conditions, and identify the near-throat geometry as AdS$_2\times S^1$. The scalar effective potential is proven strictly positive for every mode, and the throat circle is a minimal surface whose length gives an entropy $\pi r_h/2G$, reproduced independently by the Wald–Noether charge and by a Cardy estimate from the computed Brown–York mass – concordant results for which no first law is available since $\kappa=0$. The throat carries an Aretakis-type instability, with a conserved leading transverse derivative and a linearly growing subleading one. We also record a negative result: smoothing $g_{tt}$ instead, as in the Lorentzian-Euclidean Schwarzschild proposal, is singular at the horizon for any finite smoothing width. We state explicitly what the construction does not establish.

Recently, Comisso and Asenjo proposed a new mechanism for energy extraction based on magnetic reconnection of plasma within the ergosphere. In this paper, we analyze the power and efficiency of energy extraction through magnetic reconnection in a rotating four-dimensional Einstein-Gauss-Bonnet gravity black hole. Firstly, we analyze the background properties of this spacetime and its physical quantities, including the event horizon, the boundary of the ergosphere, and the size of the ergosphere. We analyze the magnetic reconnection in circular orbits and investigate the energy extraction region, as well as the power and efficiency of energy extraction. Our results indicate that energy extraction remains feasible even for a low spin parameter of $0.4$, which is well below the previously reported threshold. We also find that the energy extraction power exceeds that of the Blandford-Znajek mechanism. The Gauss-Bonnet coupling parameters $\alpha$ lower the spin threshold for energy extraction. Similarly, we analyze the energy extraction region in the plunging regime, together with the corresponding power and efficiency. We find that energy extraction is possible even for a low spin parameter as $0.22$. We also observe that the Gauss-Bonnet coupling parameter $\alpha$ further lowers the spin threshold. This behavior is consistent with that observed in the circular orbit case. Finally, we compare the energy extraction power in the plunging and circular orbit regimes and find that the plunging regime yields a higher energy extraction power than the circular orbit case.

Theoretical interpretations of horizon-scale observations often rely on the idealized assumption of an isolated vacuum Kerr geometry. However, astrophysical black holes are expected to be embedded within dense dark matter distributions that can modify the local spacetime geometry. In this work, we propose a theoretical framework to model a rotating compact object surrounded by a bounded dark matter envelope governed by a Modified Chaplygin Gas (MCG) equation of state. To ensure strict adherence to the Einstein Field Equations, we construct a piece-wise, three-region spacetime using a fully coupled Tolman-Oppenheimer-Volkoff (TOV) integration, allowing the fluid's pressure to taper naturally to zero and dynamically define the outer boundary. Rotation is introduced via a pressure-corrected Kerr-form ansatz where the temporal component is obtained directly from the integrated hydrostatic potential. Using this geometrically rigorous configuration, which explicitly evaluates the exact 4D equatorial metric determinant rather than relying on vacuum approximations, we solve the circular equatorial geodesics and determine the innermost stable circular orbit (ISCO). Evaluating the thin accretion disk thermodynamics via the Novikov-Thorne formalism reveals that the deep gravitational potential well of the MCG envelope acts as a strong driver for viscous dissipation, systematically shifting the peak thermal flux, effective temperature, and multi-colour blackbody spectral luminosity to higher energy bands. Furthermore, we identify a clear structural degeneracy: a static or slowly rotating black hole embedded in a dense MCG structure can elevate radiative efficiencies up to $\eta \approx 6.5\%$. This framework is presented as a structured proposal to quantify environmental systematic uncertainties in standard black hole spin-estimation techniques.

Hawking radiation sets stringent constraints on Primordial Black Holes (PBHs) as a dark matter candidate in the $M\sim 10^{16}$ g regime based on the evaporation products produced by the black hole, motivating the need to rigorously model the photon, electron, and positron emission spectra. This manuscript is the second in a series of two papers (see Vasquez et al. [Phys. Rev. D, 112:063002 (2025)] for the first paper in the series) with the goal of proving the stochastic emission of electrons and positrons, known as the "stochastic charge effect" and first predicted by Page [Phys. Rev. D, 16:2402 (1977)] using semi-classical arguments, arises from quantum electrodynamics (QED) on a Schwarzschild spacetime. We derive the corrections to the $e^\pm$ from the relevant term in the Hamiltonian (the long-range monopole: $H_{\rm int,0L}$) to first order in the fine structure constant ($\alpha$), and show that the semi-classical terms are also present in the QED calculation. We also highlight which terms in our analysis only appear within our quantum mechanical approach and cannot be explained otherwise. We find that due to some cancellation of terms, over the range of $2.2\times 10^{16}$ - $1.7\times 10^{17}$ g, the net correction to the $e^\pm$ emission rate is less than 2%, versus the suppression of up to 5% previously found by semi-classical arguments.

In this study, we investigate higher-dimensional static and stationary Bianchi-type solutions in an $R^2$ gravity theory using the formalism of exterior differential forms. We construct a broad class of exact higher-dimensional Bianchi-type solutions in an $R^{2}$-corrected gravity theory. It is shown that the theory admits hyperscaling violating black hole solutions associated with homogeneous but anisotropic horizon geometries, encompassing both static and stationary configurations. We further analyze some thermodynamic properties of these backgrounds and demonstrate that higher-curvature corrections give rise to unconventional features. In particular, we show that within the critical sector, the solutions possess a finite Hawking temperature despite an identically vanishing Wald entropy, signaling an unusual thermodynamic regime. These configurations arise at the critical point of the theory, where the effective gravitational coupling vanishes and the space of admissible constant-curvature geometries is enlarged.

We extend the Lee-Yang phase transition framework to charged anti-de Sitter (AdS) black holes in four-dimensional trace anomaly gravity. By treating the horizon radius as a complex variable, we derive a fully resolved complex phase diagram that uncovers novel supercritical phenomena within this modified gravity setting. Relative to the standard Reissner-Nordström-AdS black hole, the trace anomaly shifts the location of the critical point: for a representative set of anomaly parameters, both the critical pressure and critical temperature are suppressed. The Widom line is rigorously identified as the projection of the complex Lee-Yang zeros onto the real physical phase plane, a trajectory that demarcates the small-black-hole-like and large-black-hole-like phases throughout the supercritical regime. We also independently recover this same Widom line via the thermodynamic response function method, demonstrating that the two definitions are in excellent quantitative agreement in the near-critical region and share identical universal scaling behavior. Furthermore, our analysis reveals a smooth, continuous crossover across the Widom line as probed by thermodynamic response functions–a behavior fundamentally distinct from the discontinuous first-order phase transitions that occur below the critical point. These results offer new, physically concrete insights into the thermodynamics of quantum-corrected black holes.

We study the entanglement entropy of a conformally coupled scalar field at its ground state in $(3+1)$-dimensional AdS space in global coordinates. We consider spherical entangling surfaces centered at the origin of AdS and allow for general boundary conditions at the conformal boundary. Through numerical and analytical means, we show that the UV-divergent part of the entropy has a universal form, while the UV-finite part is sensitive to the boundary conditions. We determine the dependence of the latter part on mixed boundary conditions that interpolate between Dirichlet and Neumann.

Being agnostic about the field content of a gravitational system, we consider a general disformal transformation of the metric, $g_{\mu\nu}=C h_{\mu\nu}+D t_{\mu\nu}$, on a four-dimensional Lorentzian manifold. Using the Cayley-Hamilton theorem, we derive an explicit formula for the inverse disformed metric. Implementing the Hawking-Ellis classification, we categorize disformal transformations into four types: Type I, II, III, and IV, based on possible Jordan block structures. We further identify the Segre subclasses from the eigenvalues. We derive explicit relations between the Cayley-Hamilton degree of $t_{\mu\nu}$, its Hawking-Ellis type, and its Segre subclass, restricting the possible types from the degree alone. In some cases, the type can be determined without even performing a full Jordan decomposition. For singular transformations, when new dynamical degrees of freedom emerge, we obtain the form of their corresponding mimetic energy-momentum tensor $T^\star_{\mu\nu}$. Within the polynomial sector, the Hawking-Ellis types of $t_{\mu\nu}$ and $T^\star_{\mu\nu}$ always coincide for Type I. For Types II and III it can differ, while Type IV is preserved generically but can reduce to Type I when the complex pair is mapped to a repeated real eigenvalue. This makes it possible to infer physical properties of $T^\star_{\mu\nu}$ directly from the Hawking-Ellis type of $t_{\mu\nu}$. We apply our setup to scalar and gauge fields. We find a new pure-disformal scalar-gauge family yielding irrotational perfect fluids, including barotropic fluids with an arbitrary constant equation-of-state parameter. Two scalar fields realize all four types and the reductions from Types II and IV to Type I. This framework applies to the kinematical and dynamical study of invertible and non-invertible disformal transformations with different field content.

We investigate an analytically tractable sector of the Extended Cuscuton model, a non-dynamical dark-energy realization within the framework of viable Horndeski gravity. We focus on four benchmark submodels and constrain them with current background probes, namely cosmic chronometers, Type-Ia supernovae, and BAO, while imposing theoretical viability, Lunar Laser Ranging, and Big Bang Nucleosynthesis bounds. We then forecast third-generation bright-standard-siren constraints with Einstein Telescope and Cosmic Explorer networks, considering prompt-emission, afterglow, and kilonova counterparts. Current data already restrict the viable parameter space to small departures from $\Lambda$CDM and do not remove the calibration-driven offset between the CC+SN and CC+BAO determinations of $H_0$. In principle, future bright sirens substantially sharpen the constraints, especially for kilonova catalogues and extended detector networks. Across the forecast configurations, the relative uncertainty on $H_0$ remains below $13.18\%$ and can reach $0.21\%$ in the most constraining cases, while $\Omega_\Lambda$ is recovered at the percent level in the best cases. These results show that third-generation standard sirens can provide a precise complementary test of non-dynamical dark energy beyond $\Lambda$CDM.

We point out that the asymptotically Bertotti-Robinson multi-black hole solutions of Einstein-Maxwell theory can be constructed using only three-dimensional sigma-model techniques, without resorting to the monodromy-matrix approach.

The next generation of gravitational wave detectors, such as the Einstein Telescope (ET), will observe orders of magnitude more binary black hole mergers than current facilities. Most of these events will lack an electromagnetic counterpart, also known as dark siren events, yet will still enable percent-level cosmological constraints. However, the likelihood traditionally used in Hierarchical Bayesian Inference (HBI) for population-level analyses becomes computationally prohibitive as the size of dark siren catalogues and population parameters grow. In this work we compare HBI against simulation-based inference (SBI) as a scalable alternative for cosmological population inference in the ET era. Studying a proof-of-concept example of a mock ET inference, we build a catalogue of $O(10^4)$ binary black hole events, then perform inference on the Hubble constant $H_0$ and matter density $\Omega_m$ in a flat $\Lambda$CDM cosmology, using both a hierarchical analytical likelihood and Marginal Neural Ratio Estimation (MNRE). We find excellent agreement between the two approaches, with SBI reproducing the HBI posteriors to high accuracy, while requiring orders of magnitude less computation once the simulation and training cost is amortized. We further demonstrate that SBI extends straightforwardly to a joint cosmology-plus-astrophysics analysis, simultaneously constraining $(H_0,\Omega_m)$ together with the parameters of the star formation rate density, at negligible additional cost compared to the significant increase in complexity such an extension would require within the HBI framework. Our results indicate that SBI is a promising and scalable tool for population inference with third-generation GW detectors.

$\Lambda$CDM provides a remarkably successful description of the Universe, yet it suffers from persistent discrepancies, with one of the most prominent being the $\sigma_8$ tension between early-Universe Cosmic Microwave Background (CMB) measurements and late-time structure growth probes. To address this mismatch, we investigate generalized interacting dark sector models governed by a quintessence field coupled to dark matter via conformal and disformal transformations. By generically engineering a quintessence potential that ensures the combined evolution of dark matter and dark energy exactly mimics $\Lambda$CDM, our models satisfy all background constraints while signaling modified gravity effects within the linear perturbation sector. We perform a statistical analysis using late-time structure growth data anchored to Planck 2018 constraints. Furthermore, we address the initial conditions problem inherent to purely disformal models, demonstrating the necessity of a non-zero initial kinetic energy to initiate dynamical evolution. Our results indicate that purely conformal couplings efficiently transfer energy from dark matter to the quintessence field, suppressing late-time structure formation and successfully accommodating CMB data within late-time modified structure growth. This purely conformal limit yields a strong statistical preference over the $\Lambda$CDM baseline according to both the Akaike and Bayesian Information Criteria. Conversely, while non-zero disformal couplings introduce a kinetic friction that dampens the dark sector energy exchange, their expanded parameter space incurs heavy statistical penalties without providing a decisive improvement over the purely conformal scenario.

A topological geon is an asymptotically (anti-)de Sitter or flat spacetime with topology $\mathbb{R}\times M$, where $M$ is the punctured projective space, $M=\mathbb{R}P^3\backslash\{p\}$, and the removed point $p$ corresponds to the spacelike infinity. The manifold $M$ is conventionally obtained from a spherically symmetric spatial slice of a wormhole spacetime as the quotient manifold by the isometric action of the group $\mathbb{Z}_2$. We study the general properties of static, spherically symmetric, charged topological geons supported by a selfgravitating, minimally coupled scalar field with negative kinetic energy and an arbitrary selfinteraction potential. In the most general case, it turns out that the gravitational mass of such a geon is completely determined by its electric charge and size (for a given scalar field), that is, the size of the corresponding wormhole throat. We discuss the properties of the charged Ellis-Bronnikov-Sorkin geon and argue that it can be considered as a possible classical model of elementary particles beyond the Standard Model, in particular, as dark matter particles.

We calculate the vacuum polarization energy (VPE) of a scalar field in the background of a nonsingular cosmic string in $2+1$ spacetime dimensions. Our calculation expresses the VPE as a renormalized sum and integral over scattering data, which can be expressed in terms of Legendre and Bessel functions for the "ballpoint pen" model, analogous to a square well in curvature, and which can be obtained numerically for a generic string profile. We show how relationships between the local density of states, expressed in terms of the Green's function, and the global density of states, expressed in terms of the Jost function, extend to this curved spacetime background and allow for precise implementation of perturbative renormalization conditions.

We derive the leading logarithmic soft-photon theorem in $d>4$ spacetime dimensions and its classical radiative counterpart. A direct one-loop analysis in massive scalar quantum electrodynamics (QED) yields a factorizing soft term of order $\omega^{d-4}\ln\omega$, although the charged-particle S-matrix is infrared finite. The logarithm is generated by the scale-invariant loop-momentum region $\omega\ll|\ell|\ll\Lambda$, where $\Lambda$ denotes a characteristic hard-particle energy scale. At leading radiative order, the corresponding logarithmic contribution to the classical electromagnetic waveform arises from the long-range acceleration of the asymptotic charged particles. In even $d\geq6$, the straight-line waveform at this radiative order is distributionally supported in retarded time within an interval whose width is set by the characteristic size of the hard-scattering region, whereas the logarithmic acceleration term produces universal early- and late-time radiative tails proportional to $|u|^{-(3d-10)/2}$. In odd $d\geq5$, straight-line motion already gives a late-time tail at the same radiative order proportional to $u^{-(d-4)/2}$. The acceleration correction adds universal late- and early-time terms proportional, respectively, to $\ln u/u^{(3d-10)/2}$ and $|u|^{-(3d-10)/2}$. A comparison of Feynman and retarded boundary conditions separates the classically radiative contribution from the intrinsically quantum part of the logarithmic soft factor.

Gravitational-wave (GW) cosmology is currently bottlenecked by the scarcity of electromagnetic counterparts for bright sirens and the systematic uncertainties of galaxy catalogs for dark sirens. We propose a purely gravitational resolution using the Integrated Cosmological Memory (ICM)-the cumulative GW strain encoded in the spacetime geometry of an expanding Universe. While the GW transient emitted by the source provides the luminosity distance, the ICM accumulates a mathematically distinct integral of the cosmic expansion history. We demonstrate that extracting both observables from a single binary merger completely breaks the distance-redshift degeneracy within the gravitational sector. This establishes a novel, catalog-free dark siren framework for third-generation GW detector networks. Crucially, the resulting constraints on late-time dark energy are only weakly sensitive to the local expansion rate, providing a robust cosmological probe that can potentially mitigate the impact of the H0 tension.

Departures from standard slow-roll (SR) inflation have attracted increasing interest in recent years. In particular, scenarios that strongly enhance the power spectrum of the curvature perturbation are often proposed as a mechanism for producing primordial black holes, which could account for part or the totality of dark matter. An ongoing debate in these models is whether non-linear interactions of cosmological perturbations on small scales, namely loop corrections, can become sufficiently large to backreact on CMB scales. In this work, we adopt a non-linear framework, dropping spatial-gradient terms, to study loop corrections at super-Hubble scales. Our focus is to clarify the role of spatial-diffeomorphism invariance, especially dilatation invariance, which is the symmetry under overall rescaling of spatial coordinates, in demonstrating the suppression of loop corrections to the CMB power spectrum. This analysis is valid for any inflaton potential, both with smooth and sharp transitions, and at any loop order, as long as the CMB scales cross the horizon during an SR phase. We compare this result with the different explanations proposed in the literature using various gauge choices, and show our analysis is consistent both in the comoving and in the spatially-flat gauges. In particular, we show that in the spatially-flat gauge, the cubic interaction contains a term that diverges in the sharp transition limit. However, this contribution is exactly cancelled by the quartic interaction.

Two-dimensional incompressible fluids conserve energy and enstrophy in the inviscid limit. Fjørtoft's argument uses this pair to constrain nonlinear energy transfer. We examine the corresponding question in General Relativity using the magnetic Weyl functional $\mathcal Z=\int B_{ab}B^{ab}\sqrt{\gamma}\,d^3x$. Its transverse radiativ e part carries an additional frequency-squared weight relative to gravitational-wave energy: $\delta\mathcal Z_k\simeq\omega_k^2W_k$ in the radiation zone, with $W=8\pi G E_{\rm GW}$. We derive its balance law from the Bianchi identities, obtaining the algebraic cancellations, bulk sources and boundary fluxes. Conservation of positive e nergy and curvature norm through nonlinear transfer gives a Fjørtoft constraint when their weights are ordered. Resonant weak-wave interactions generally change the c urvature norm; $2\leftrightarrow2$ dynamics instead conserves energy and wave action. We examine the competition between transfer and linear losses in Kerr and AdS backg rounds, where long-lived or confined modes provide time for nonlinear evolution. In the hydrodynamic regime of AdS$_4$, a fluid-weighted magnetic Weyl functional inherit s fluid enstrophy conservation in the inviscid limit. We compute its proportionality coefficient to boundary enstrophy in closed form at leading gradient order, giving a bulk diagnostic for holographic turbulence.

We develop a general-relativistic framework for two-component quantum dark fermion stars: equilibrium configurations of two degenerate fermion species governed by gravity, a Yukawa-mediated dark fifth force and a globally relevant Bohm quantum-pressure correction. The treatment retains the full covariant form of the nonlinear Klein–Gordon equation in the Schwarzschild interior, with closure relations valid at arbitrary compactness, from the ultralight baseline up to densities of order $0.16\,\mathrm{fm}^{-3}$ at which relativistic scalar densities and self-consistent effective fermion masses become unavoidable. Two Lagrangian parameters, the dark-fermion mass and the dimensionless ratio between the Yukawa channel and gravity, together fix the equilibrium structure; a joint measurement of mass and radius therefore constrains the dark-sector microphysics directly from gravitational-wave observables. For particle masses in the band $10^{-11}$–$10^{-10}$ eV, the configurations exhibit radii of $3$-$24\,\mathrm{km}$ and compactness in the range $0.14$-$0.34$, behaving as dual mimickers: at moderate compactness they overlap with neutron stars in mass, radius, and inspiral frequency; at the most compact end ($\kappa\sim 0.34$, $R_T/R_S\simeq 1.5$) they cross the photon sphere and could masquerade as low-mass black holes in the mass-gap region. Tidal-deformability measurements discriminate against both populations: the dimensionless tidal deformability is measurably smaller than the neutron-star value yet remains non-zero, unlike that of a genuine black hole. These objects populate the sensitivity bands of LISA, the Einstein Telescope, and Cosmic Explorer, producing astrometric microlensing signatures individually resolvable by Gaia. The framework thus provides a reproducible and falsifiable template for constraining dark-sector properties through multi-messenger observations.

The gravitational frequency shift of light is well understood in the theory of classical electromagnetism. Nevertheless, its description in quantum theory is not yet fully developed. Recent work pointed out inconsistencies in previously developed models aimed at describing the gravitational redshift as an effective multi-mode mixer (MMM) acting on modes of light, but so far a complete solution of these issues was not obtained. Here, we identify the root cause of the MMM model's inconsistency and provide two complementary approaches to correct it: a "natural" one from a field-theoretic perspective, and another adapted to the language of quantum mechanics of finite-dimensional systems. We show that the second approach allows for modeling of the redshift in a multi-mode transmission setup as a quantum channel that can be characterized using standard quantum information-theoretic techniques when restricting the input states to Gaussian states of light.

Recent numerical studies of semiclassical gravity suggest that, in spherically symmetric black holes with both outer and inner horizons, the semiclassical instability of the inner horizon can drive the complete evaporation of the trapped region on timescales shorter than the standard Hawking evaporation time. Independent simulations further indicate that the disappearance of the trapped region is followed by the formation of an anti-trapped region, i.e.~a dynamical white hole. In this work, we develop an analytic treatment of quantum effects in trapped and anti-trapped regions, showing how these numerical results can be understood within simplified two-dimensional models. We consider collapse models describing the formation of charged and regular black holes and compute the renormalized stress-energy tensor of the $|\textit{in}\rangle$ vacuum state. We show that, within this framework, the emergence of an anti-trapped region is a generic consequence of the amplification of negative energy fluxes propagating along the outgoing direction inside the initial trapped region. This provides an analytic explanation for the black-hole-to-white-hole transition observed in numerical simulations. Our analysis further suggests that the fluxes generated by the subsequent anti-trapped region, now propagating along the ingoing direction, may trigger the formation of a new trapped region. This raises the possibility of a cascade of black-to-white-hole transitions, potentially ending in a horizon-free, bouncing spacetime without invoking additional quantum-gravitational dynamics. Although establishing the complete evolution requires a self-consistent treatment of semiclassical backreaction, our framework identifies which features of the mechanism are universal and which depend on the geometry, laying the groundwork for a systematic investigation of semiclassical black-hole-to-white-hole transitions.