Papers

We construct a holographic dark-energy model using Kaniadakis entropy with the particle horizon as the infrared cutoff, consistently modifying both the holographic density and the Friedmann background. In standard Einstein gravity, noninteracting particle-horizon holographic dark energy (HDE) does not produce the sufficiently negative pressure required for late-time accelerated expansion. We show that the Kaniadakis deformation changes this behavior while the particle horizon is retained. For the representative parameter choice $K=1.90\times10^{-36}$, with $\Omega_{\rm DE0}=0.7$ and $c^2=0.64$, the deceleration parameter changes sign at $z\simeq0.585$ in the noninteracting case. Including the interaction strengths $b^2=0.03$ and $0.06$ changes the transition only slightly, giving $z\simeq0.586$ and $0.589$, respectively. Thus, for the parameter set considered here, the interaction does not generate the acceleration; rather, the transition is already present in the noninteracting Kaniadakis model and the interaction produces only a small shift in its timing. We derive the autonomous evolution equations, the effective dark-energy equation of state, and the deceleration parameter. The adiabatic squared sound speed is also examined as a diagnostic of the effective-fluid stability, while statefinder variables are used to characterize deviations from $\Lambda$CDM. Finally, we verify that the $K\to0$ limit continuously recovers standard Einstein-gravity particle-horizon HDE, for which the noninteracting model remains decelerating.

In the nanohertz band, spatial correlations between pulsar timing residuals provide a key observable for characterizing stochastic gravitational-wave backgrounds and probing their polarization content. In torsionless Palatini spacetime, nonmetricity can generate two additional shear modes, referred to as the shear-$x$ and shear-$y$ modes. In this work, we investigate the pulsar timing response produced by the shear-induced motions of the emitting pulsar and the receiving Earth. Assuming that the electromagnetic field is minimally coupled to the physical metric and that the Earth and pulsar possess non-negligible effective hypermomentum responses, we derive the single-pulsar redshift, the frequency-domain two-point correlation function, and the corresponding spatial correlation. For a stationary and isotropic stochastic shear background, the normalized overlap reduction function (ORF) for distinct pulsars reduces in the short-wavelength limit to the pure dipolar form $\Gamma_{ab}^{\mathrm{sh}}(\zeta)=\cos\zeta$, where $\zeta$ is the angular separation between the two pulsars. The same dipolar correlation can also be produced by isotropic Solar System ephemeris errors, leading to a spatial degeneracy between the two signals. This degeneracy highlights the importance of information beyond the angular correlation for identifying Palatini shear signatures in PTA data.

Phenomenological deviation parameters used in gravitational-wave tests of general relativity need not represent universal fundamental properties of the sources. Instead, a deviation parameter can be a derived quantity that depends on source properties. Apparent violations of general relativity arising from waveform systematics or data-quality issues can also depend on source properties. In addition to diagnosing individual events, it is helpful to identify systematic trends at the population level. We introduce a nonparametric framework that uses mutual information to test whether the deviations depend on source properties. We validate the framework using simulations with massive graviton waveforms. Assuming we do not have prior knowledge of the underlying theory, massive graviton effects can be captured by a parameterized post-Einstein coefficient that is expected to depend on the luminosity distance and source mass. The results show that the mass and distance dependencies become clear for graviton masses of $\geq 1\times10^{-22}\,\mathrm{eV}$ at detector sensitivities representative of the current fourth observing run, while the controlled general relativity simulations yield null results. We then analyze public real-data products for tests of general relativity from GWTC-4.0 and 4-OGC, including phenomenological parameterized tests, modified dispersion tests, and parity violation tests. The phenomenological deviation parameters have tentative correlations with effective spin, and correlations of modified-dispersion or parity-violation parameters with source mass are visible. These results demonstrate that catalog-level dependence can identify patterns and thus be used to diagnose the origin of deviations in testing general relativity. The framework therefore complements single-event analyses and parameterized hierarchical population tests.

Our central premise is that quantum mechanics may be the statistical limit of a more fundamental discrete theory: any such theory equips a physical system with a finite information capacity, and its departure from quantum statistics is controlled by how much of that capacity the system uses. We show that commercial cloud photonic quantum processors have reached the precision required to bound this capacity from below, using the binary-sequence model of Powers et al. as the concrete test theory: outcome probabilities arise from counting discrete sequences of length $n$, quantum mechanics is recovered as $n \to \infty$, and $n$ measures the information capacity of the register behind a prepared state. Photon Fock states $|1\rangle$, $|1,1\rangle$, heralded $|2\rangle$, and cascaded beam-splitter pairs are measured on programmable interferometers with dominant systematics determined in situ. The model's composition-consistent parametrization, singled out by requiring that rotations compose, recovers quantum mechanics with deviations $1.24/n$; a random-effects likelihood analysis calibrated by parametric bootstrap excludes all $n \le 100$: the information capacity of the register carrying the two-photon state, if finite, exceeds $10^2$. Cascaded beam splitters test the composition law directly: the data are split-invariant, excluding naive count composition at $8\sigma$ and confirming the interference-sign rule. Model-independently, curve-averaged deviations from the quantum partition law larger than $2.3\times10^{-2}$ are excluded at 95% CL, and the originally published linear parametrization is excluded outright. Because the compilation offset is frozen per circuit it is calibratable, opening the $10^{-3}$ floor ($n \sim 10^3$) to current hardware: cloud photonic processors are quantitative instruments for quantum foundations, and information capacity an experimentally boundable quantity.

Constraint-satisfying initial data for binary black holes require the repeated solution of a nonlinear elliptic boundary-value problem throughout the physical parameter space. We present a para\-metric, differentiable initial-data model that combines sparse-grid spectral interpolation with Newton refinement over a family of binary configurations. Interpolation is used only to provide warm starts for the underlying elliptic solver, rather than to replace it. A small number of Newton iterations refines each interpolated solution to a discrete, row-equilibrated constraint residual $\|R\|_\infty\le10^{-10}$, independent of the interpolation error, so that every returned initial-data set satisfies the discretized constraint to that tolerance. Our JAX implementation provides analytic derivatives with respect to the physical parameters, enabling gradient-based parameter targeting, optimization, and sensitivity analysis. We demonstrate the method for quasi-circular binary-black-hole initial data using conformally flat Bowen–York punctures by constructing four-dimensional aligned-spin and eight-dimensional general-spin models. The resulting initial data are validated against TwoPunctures, and the method is applied to certified targeting of the physical Arnowitt–Deser–Misner mass and angular momentum and to effective-potential eccentricity reduction using a dedicated separation–tangential-momentum model.

We constrain a damped harmonic oscillator (DHO) dark-energy equation of state using the full cosmic microwave background (CMB) likelihoods in combination with DESI BAO and three distinct Type Ia supernova compilations: Pantheon+, DES-Dovekie, and Union3. The equation of state obeys a second-order damped oscillator equation in number of $e$-folds, so that its frequency $f$, damping rate $b$, and equilibrium value $w_{\rm m}$ fully specify the late-time dynamics. The model exhibits oscillatory behavior only at low redshifts, around the equilibrium value $w=-1$, with distinct characteristics for the different supernova compilations: an underdamped solution for DES-Dovekie and Union3, and an overdamped solution for Pantheon+. At higher redshifts, the model closely mimics $\Lambda$CDM and deviates significantly only at $z<0.6$, with the magnitude of the deviation depending on the supernova compilation. We further identify a region of the $(f,b)$ parameter space, corresponding to rapid variation of the equation of state at low redshift, in which the perturbation equations become numerically stiff and cannot be integrated with a canonical dark-energy sound speed i.e., $c_s^2 =1$. We show that reducing the rest-frame sound speed removes this obstruction while leaving the observables unchanged at the $10^{-3}$ level, and therefore treat it as a numerical prescription rather than a physical modification of the model. The model yields $H_0 = 67.53^{+1.22}_{-1.18}$ km/s/Mpc for Pantheon+, $H_0 = 69.08^{+1.23}_{-1.16}$ km/s/Mpc for DES-Dovekie, and $H_0 = 70.67^{+1.89}_{-1.87}$ km/s/Mpc for Union3. The present-day equation-of-state parameter is constrained to $w_0 = -0.521^{+0.891}_{-0.414}$, $-3.01^{+1.14}_{-1.21}$, and $-3.16^{+1.11}_{-1.16}$ for Pantheon+, DES-Dovekie, and Union3, respectively.

Recent hints of dynamical dark energy, including a possible crossing of the phantom divide, have motivated the search for viable theoretical explanations. Scalar fields non-minimally coupled to gravity with a coupling of the form $F(\varphi)=1+\alpha\varphi^2$ have emerged as promising candidates, providing an improved fit to the data compared to $\Lambda$CDM and even the $w_0w_a$CDM parametrization. However, in the absence of a screening mechanism, the regions of parameter space examined exhaustively so far (with $\alpha<0$) violate stringent local constraints on the strength and time variation of gravity and enhance the matter growth rate at low redshifts, thereby compromising the viability of these solutions. In this work, we investigate a largely unexplored region of parameter space (with $\alpha>0$) that realizes the crossing while remaining consistent with local constraints. This setup, which is subject to some fine-tuning, respects the BBN constraints and leads only to deviations of the effective gravitational coupling from Newton's constant below $0.3%$ in the past. We review the background and linear perturbation dynamics of the model, deriving analytical results that provide physical insight into its behavior, and update the constraints on the model using the latest cosmological data, including the DES-Dovekie supernova sample. We perform for the first time a full-fledged Monte Carlo analysis with $\alpha>0$, studying the impact of the variation of $G$ on the supernovae absolute luminosity, and compare the results with those obtained for $\alpha<0$, using in both cases a linear potential. In the context of the model with non-minimal coupling and $\alpha<0$, the standard model is excluded at the $2.50\sigma$ CL, whereas for $\alpha>0$, the exclusion significance decreases to $\sim 2\sigma$ CL, indicating a moderate preference for these models over $\Lambda$CDM. [abridged]

Minimal Geometric Deformation (MGD) applied to a static Schwarzschild black hole seed generates a single decoupler function $h(r)$, obtained by solving the $\theta$-sector field equations together with an equation of state. Once $h(r)$ is fixed, the resulting one-parameter family is controlled by the coupling strength $k$ through $F(r;k)=1+k\,h(r)$. We show that, whenever the deformation develops a simple outermost root that crosses the seed horizon, the same fixed decoupler leads to two mutually exclusive branches associated with different global completions: on one side of the critical coupling the deformed metric preserves the seed horizon as a black hole, whereas on the other side the root $r_*>2M$ lies in the exterior and cannot be interpreted as an interior modification of the black hole geometry. We prove that this root forces a loss of Lorentzian signature on the interval $(2M,r_*)$, so that no smooth extension of the exterior metric through the seed horizon $r=2M$ exists once $r_*$ lies outside it. Within the static, spherically symmetric class considered here, the corresponding smooth Lorentzian completion is a two-ended wormhole obtained by excising $(2M,r_*)$ and doubling the region $r\geq r_*$ across the minimal sphere $\mathcal T=\{r=r_*\}$. No topology change of any single spacetime is claimed or required: $k>k_c$ and $k<k_c$ simply correspond to two different, non-diffeomorphic manifolds, and Lemma~1 below shows that the metric itself dictates which of the two is the admissible completion for a given $k$. We compute the second homology group of both completions explicitly, $H_2(\Sigma_{\rm BH},\mathcal H)=0$ for the black hole exterior relative to its horizon and $H_2(\Sigma_{\rm WH})\cong\mathbb Z$ for the completed wormhole manifold, giving a discrete invariant that distinguishes the two branches.

We investigate the impact of self-interactions on the Rényi and entanglement entropies of a scalar field on $(2+1)$-dimensional spacetimes, whose spatial sections are modeled by fuzzy spaces, specifically the fuzzy sphere and the fuzzy disc. We compute the first-order perturbative correction induced by a $\lambda\phi^4$ interaction using the Green's function approach. In contrast to the free theory, where the entanglement entropy is dominated by degrees of freedom near the entangling boundary and obeys an area law, we find that the interaction correction has an extensive bulk contribution, receiving significant contributions from degrees of freedom throughout the fuzzy space. For the fuzzy sphere, the correction exhibits strong infrared sensitivity associated with the zero mode. We isolate and resolve this zero-mode IR divergence by projecting out the zero mode, thereby obtaining a physically meaningful quantity. In the commutative continuum limit, the interaction correction has the same degree of UV divergence as the free entropy but does not obey a pure area law. Furthermore, we analyze the Moyal plane limit, where the interaction correction exhibits a distinct IR divergence. We discuss the physical origin of these extensive bulk features and examine their possible connection to the celebrated UV/IR mixing phenomenon in noncommutative quantum field theories.

We establish a minimal nonrelativistic realization of scalarization in a self-gravitating bosonic condensate coupled to a scalar response field. A local effective-mass shift and nonlinear saturation generate a multibranch structure with two distinct transition routes. In the linearly stable regime, a finite perturbation drives a first-order transition with type-I logarithmic scaling near threshold. Beyond the linear onset, small perturbations grow tachyonically, and the response time controls both the growth and the subsequent breathing dynamics. These results identify a common mechanism for scalarization across relativistic compact objects and nonrelativistic condensates, and point toward laboratory analogues in coherent media.

The millihertz gravitational-wave sky will be dominated by $\sim10^{8}$ double white dwarfs while $\sim10^{4}$ be resolvable in the Milky Way, whose three-dimensional spatial distribution traces the Galaxy's structural parameters. We present Galena, a hierarchical Bayesian pipeline that recovers these parameters from a double white dwarf catalogue through an inhomogeneous Poisson-process likelihood. Each source's $3\times3$ Galactic position covariance is computed from the waveform Fisher information matrix and propagated analytically through a $3\times5$ Jacobian; the resulting measurement-error convolution is factored into a pre-computed sparse weight matrix, rendering the nested-sampling inference tractable. Applied to GBSIEVER-reported LISA Data Challenge sources ($N=2151$ after quality cuts), Galena recovers the disk scale length $R_d=2175^{+51}_{-52}$~pc and scale height $z_d=282^{+7}_{-7}$~pc, consistent with a canonical thin disk, together with the bulge fraction $A=0.187^{+0.011}_{-0.012}$ and bulge scale radius $R_b=773^{+26}_{-25}$~pc; the bulge fraction, now recovered much closer to the literature value of $0.25$ than in our earlier exact-position fit, is measured at $\sim6\%$ statistical precision. An independent particle-swarm optimisation of the same likelihood reproduces these values to within a fraction of a percent, supporting the attribution of the improvement to the Fisher-matrix error propagation rather than to the sampler. The search-stage astrophysical prior improves per-source distance estimates but leaves the hierarchical inference essentially unchanged.

We consider 4-dimensional asymptotically flat vacuum spacetimes near future null infinity endowed with the most general allowable Carroll geometry. We show that the near-boundary radial expansion (to a certain order) can be organised in terms of connections that can be obtained by gauging the conformal Carroll algebra. The only non-vanishing curvatures in this gauging procedure are those that are associated with the special conformal generators and we will refer to these as the $K$-curvatures. The vanishing of these $K$-curvatures defines an asymptotic vacuum spacetime and we use this to construct a boundary (i.e. Carroll) covariant expression for the vacuum (soft) shear in terms of two boundary Carroll scalar fields. The $K$-curvatures transform in a hierarchical fashion into one another under Carroll boosts. This leads to a classification of 4 types of spacetimes: vacuum, strongly and weakly non-radiative, and radiative spacetimes. It is shown that the $K$-curvatures correspond to 5 of the 10 Weyl tensor components at leading order in their $1/r$ expansion. We furthermore observe that one of the $K$-curvatures is equal to the recently found Carroll boost anomaly. Finally, we show that the Bondi loss equations for the boundary energy-momentum-news complex can be cast into a form involving another energy-momentum tensor with vanishing energy flux that is traceless and whose non-conservation is entirely captured by the $K$-curvatures. The BMS currents can be obtained by contracting this latter energy-momentum tensor with a Carroll conformal Killing vector.

Can propagating gravitational waves serve as operational probes of the quantum nature of gravity? We address this question by developing a unified theoretical framework that combines spacetime geometry, quantum information, and gravitational-wave physics. Starting from the geodesic deviation equation in linearized General Relativity, we derive the effective detector Hamiltonian directly from spacetime geometry and construct the complete quantum dynamics for detector subsystems interacting with both classical and quantized propagating gravitational-wave fields. This unified formulation enables a direct comparison between classical and quantum descriptions of gravitational radiation within the same physical framework. We demonstrate that classical gravitational-wave backgrounds can induce mixedness in the detector state but cannot generate genuine quantum correlations between the detector subsystems. In contrast, quantized gravitational waves coherently mediate gravity-induced entanglement, quantum coherence, quantum memory, and nonclassical correlations, providing clear operational signatures of the quantum nature of propagating gravitational radiation. We further discuss how mesoscopic quantum mechanical oscillators offer a promising route towards experimentally probing these effects. Our results establish a geometric and quantum-information-based framework for exploring quantum gravity through propagating gravitational waves.

Manchak and Weatherall (2018) formulate the black hole information paradox as a failure of predictability in black hole evaporation spacetimes, diagnosed by non-global hyperbolicity. I offer a strategy for resolving this paradox. I argue that failures of predictability in black hole evaporation are not well diagnosed by non-global hyperbolicity. I then consider two weakenings of global hyperbolicity: prediction and retrodiction friendliness, the failure of which could ground a new paradox. However, deidealized black hole evaporation models can be prediction and retrodiction friendly. Therefore, the information paradox cannot be based upon failures of global hyperbolicity, nor either retrodiction or prediction unfriendliness.

Neutron star-black hole (NSBH) mergers are promising multimessenger sources, but only a subset is expected to produce detectable electromagnetic (EM) counterparts, depending on the binary mass ratio, black hole (BH) spin, and neutron star (NS) equation of state (EoS). We investigate the prospects for detecting kilonova counterparts to NSBH mergers in the LIGO-India era using the Vera C. Rubin Observatory (Rubin). We simulate GW230529-motivated NSBH populations with two effective tidal deformability ranges, estimate gravitational-wave detections for the LHV (LIGO-Livingston, LIGO-Hanford, Virgo) and LHVA (LHV with LIGO-India at Aundha) detector networks, and model the detectability of the associated kilonova emission with Rubin. We find that adding LIGO-India approximately doubles the number of EM follow-ups, primarily by increasing the duty cycle and reducing the median sky-localization area, and notably increases the number of such detections at large distances. For fixed-exposure strategies, the EM detection rate saturates at exposure times of a few hundred seconds, reflecting the trade-off between depth and sky coverage. We therefore introduce an event-specific exposure-time optimization strategy based on the expected counterpart brightness and sky-localization area. With this strategy, the expected NSBH GW+EM detection rate per year is 4.19(-3.58,+9.66) for the optimistic and 0.79(-0.68,+1.83) for the conservative population with the LHVA network, compared to 1.46(-1.25,+3.37) and 0.27(-0.23,+0.62), respectively, for LHV. The required Rubin follow-up time remains within the expected Target-of-Opportunity allocation. These results show that in the next decade, a few such NSBH follow-ups may take place, with a fraction of them in the redshift range of about 0.1-0.2, which will not only be useful for measuring the Hubble constant but can also help us in probing the Hubble parameter.

In space-based gravitational-wave detectors, laser phase locking transfers the phase of weak received light to a local laser and maintains the interspacecraft heterodyne frequencies within the phasemeter bandwidth. Fluctuations of the onboard ultrastable oscillators introduce clock noise into these heterodyne phase measurements, requiring additional calibration in time-delay interferometry (TDI). Existing formulations based on six independently formed one-way measurements do not explicitly describe the common-reference clock structure of a master–slave phase-locking configuration. In this work, we formulate clock-noise propagation and calibration directly for this configuration. Starting from carrier and sideband readouts, we derive the clock-noise couplings and construct an ordered sideband-calibration rule. The rule retains noncommuting delays and applies to two-branch TDI combinations satisfying propagation and clock-coefficient closure. For mutually independent onboard clocks with identical fractional-frequency noise spectra, we compare matched realizations with and without phase locking. In the adopted frozen equal-arm model, 45 second-generation combinations with up to 16 links considered here have smaller clock-noise residuals in the phase-locking realization throughout 0.1–10~mHz, before final clock calibration. Numerical simulations of two representative 16-link combinations reproduce the analytical spectra and verify the predicted reduction and calibration. These results show that the phase-locking realizations reduce the clock-noise contribution before final calibration, while the cancellation of master-laser phase noise still relies on TDI.

In this work, we extend the thermodynamic analyses of the neutral and specially charged Kerr-Bertotti-Robinson black holes to the general Kerr-Newman-Bertotti-Robinson family, in which the electric and external-field parameters are independent. Using covariant surface charges and canonical integrability methods, we determine the total angular momentum and the canonical mass. The angular momentum follows analytically from the combined gravitational and electromagnetic surface charges. However, the infinitesimal charge associated with coordinate time translations is not integrable in solution space, so the mass must be associated with a more general symmetry generator. Imposing the canonical integrability conditions on this generator, together with the Kerr-Newman mass as the zero-field boundary condition, selects the Christodoulou-Ruffini mass and determines the associated thermodynamic potentials. The first law and Smarr formula take the same form as the ones in usual Kerr-Newman case, and the two previously studied Kerr-Bertotti-Robinson cases are recovered as special limits.

The motion of a spinning test particle in a static, spherically symmetric spacetime with a cosmological constant is studied within the Mathisson-Papapetrou-Dixon formalism, working to linear order in the particle's spin. By exploiting the symmetries of the background, the radial and latitudinal dynamics are reduced to first-order equations governed by a spin-deformed quintic polynomial. A complete classification of the real root structure of this quintic is carried out, from which the allowed orbital types for spinning particles in the Schwarzschild-anti-de Sitter and Schwarzschild-de Sitter geometries follow directly. The parameter dependence of the root structure and of the stable circular orbit interval is analyzed. In particular, for values of the cosmological constant beyond the Stuchlík limit, where spinless particles admit no bound orbits, spinning particles can still remain bound. The radial motion is expressed piecewise in terms of Lauricella hypergeometric functions, with the simple positive real roots of the quintic acting as branch cuts. The latitudinal motion consists of small oscillations about an equatorial plane, and its phase, governed by the same quintic, admits an analogous analytic representation.

We present an explicit three-parameter family of toric Einstein-Maxwell gravitational instantons. These are complete Riemannian manifolds with vanishing scalar curvature which satisfy the Riemannian Einstein-Maxwell equations. We argue that the metric can be extended to a space with two asymptotic ends diffeomorphic to $\mathbb{H}^2 \times S^2$ with a non-collapsing two-cycle (bolt) in the bulk. The global metric is produced by suitably extending an analytic continuation of a local family of Lorentzian black hole geometries constructed by Ovcharenko-Podolsky.

We investigate whether quantum fluctuations of a circular probe string develop a quantum-chaotic response while traversing a wormhole throat. The classical circular-string embedding is periodic and radially stable, but its two physical transverse polarizations experience time-dependent tidal potentials. Expanding the world-sheet action to quadratic order, we canonically quantize these modes and construct out-of-time-ordered correlator(OTOC) amplitudes from their unequal-time commutators. For the Ellis–Bronnikov wormhole, both polarizations exhibit finite intervals of approximately exponential OTOC growth associated with the first throat passage. The corresponding dimensionless rate measured with respect to physical time is positive over the parameter range studied and generally decreases as the probe energy is increased relative to the throat scale. In the global-monopole extension, increasing the solid-angle deficit narrows the band of locally amplifiable modes and suppresses the extracted rates; a sufficiently strong defect can nearly quench the radial signal, while the angular channel retains a polarization-dependent non-monotonic structure when the energy-to-throat-scale ratio is small. These quantities characterize finite-time dynamical sensitivity in the Gaussian fluctuation sector and should not be identified with asymptotic many-body chaos or a thermodynamic phase transition. Although the numerical analysis uses two representative wormhole geometries, the construction depends only on covariant world-sheet fluctuations and real-time commutators. It therefore provides a transferable, non-holographic framework for applying quantum-chaos diagnostics directly to quantum probes in curved spacetimes.

We investigate the non-equilibrium thermodynamics of a Swiss-cheese (SC) braneworld universe. By employing Hayward's unified first law and the Clausius relation at the apparent horizon, we first show that the standard equilibrium treatment of the effective SC braneworld fluid, together with the usual Hawking temperature, reproduces the Bekenstein-Hawking area law without any braneworld correction to the entropy. This demonstrates that the brane quadratic energy density corrections cannot, by themselves, generate a modified entropy area relation within the equilibrium framework. We therefore formulate a non-equilibrium thermodynamic description in which deviations from the Bekenstein-Hawking entropy are accompanied by an internal entropy production term. For a general horizon entropy functional, we derive the corresponding entropy production rate and then specialize to a logarithmic and inverse area quantum-corrected entropy. The resulting irreversible contribution is expressed in terms of the Hubble parameter and its derivative. We apply the formalism to a nonsingular matter-bounce solution in the SC braneworld. During the expanding phase, the total entropy production remains positive satisfying the GSLT. Moreover, the late-time behavior of the second derivative of the total entropy becomes negative, indicating a decelerating entropy growth and an asymptotic approach toward thermodynamic equilibrium. Far from the bounce, the equation of state parameter $w \approx -1$, indicating a dark energy-dominated regime. Significant departures from $w = -1$ are confined to the bounce vicinity, where high energy braneworld corrections drive the nonsingular transition. Thus, the model naturally transitions from a high energy bounce into an asymptotic dark energy phase during late-time expansion.

We identify the complete set of fundamental building blocks for a 4D pre-geometric theory of gravity. Based on a gauge theory of $SO(1,4)$ or $SO(2,3)$ coupled to a Higgs-like field under the rigid constraint of general covariance in the unbroken phase, these building blocks - $\mathcal{L}_\text{MM}$, $\mathcal{L}_\text{W}$, $\mathcal{L}_{J}$, $\mathcal{L}_\theta$ and $\mathcal{L}_\vartheta$ - constitute the minimal generating set of independent field monomials from which any pre-geometric action, including arbitrary functionals thereof, can be constructed. While the most general pre-geometric theory can extend beyond linear combinations, these five irreducible invariants serve as the 'atomic' constituents of all possible pre-geometric dynamics. Upon spontaneous symmetry breaking, they collectively generate an emergent gravitational theory consisting of the Einstein-Hilbert action, the cosmological constant term and all 4D topological invariants: the Gauss-Bonnet, Pontryagin, Holst and Nieh-Yan terms. The unification of gravity's dynamical and topological sectors from a common pre-geometric source represents the central result of this work. We also uncover a see-saw mechanism linking the Planck mass and the cosmological constant, as well as several novel relations for the coupling constants of the topological sector, inclusive of the Barbero-Immirzi parameter. This framework establishes the pre-geometric foundations from which all aspects of gravitation can dynamically emerge, providing a unified starting point for quantum gravity, dark energy phenomenology and the study of topological phases in gravitational theories.

We show that a ringdown waveform can admit a constant-coefficient quasinormal-mode (QNM) representation while its individual QNM-pole contributions remain driven. To establish this result, we construct a first-principles rational approximation to the Kerr Green's function from its QNM poles, physical residues, and horizon-frequency zeros. For particles plunging from the innermost stable circular orbit into Kerr black holes with spins 0.5-0.9, each QNM-pole contribution initially follows the source's instantaneous complex frequency and decouples only after the source decays faster than the corresponding mode. At late times, the source approaches a sum of damped oscillations at the third and higher horizon frequencies. The corresponding zeros in the Green's function cancel these source-frequency components in the coherent waveform, while the continuing drive generates oscillations at QNM frequencies. Thus, an apparently free QNM superposition can emerge before its individual contributions dynamically decouple from the source.

We investigate axial gravitational perturbations and shadow formation in black-bounce geometries supported by anisotropic fluids within general relativity. Two distinct classes of solutions are analyzed: a symmetric Simpson–Visser-like black bounce and an asymmetric deformation of the Reissner–Nordström metric, which can feature either a bounded interior region or an unbounded wormhole-like extension. Our findings indicate that configurations with horizons, whether symmetric or asymmetric, exhibit single-barrier effective potentials and quasinormal frequencies nearly indistinguishable from those of standard black holes. In contrast, horizonless symmetric configurations may develop multiple potential barriers, giving rise to gravitational-wave echoes whose amplitude and separation depend sensitively on the bounce parameter $a$ and the density parameter $\rho_0$. Horizonless asymmetric solutions, on the other hand, exhibit regularized effective potentials that do not produce echoes and remain phenomenologically close to the Reissner–Nordström case. From the optical perspective, both classes of solutions with horizons exhibit a single relevant photon sphere and shadow profiles nearly indistinguishable from those of their black-hole counterparts, while horizonless symmetric configurations can sustain multiple photon rings. Horizonless asymmetric configurations may instead support either a single photon ring or none. Finally, we recover the standard eikonal correspondence between quasinormal modes and the instability of circular null geodesics for configurations with horizons. For horizonless geometries, however, the emergence of multiple potential barriers prevents a straightforward extension of this relation, although the strong similarity between the wave and optical sectors suggests that a meaningful phenomenological correspondence may still persist.

Inspired by a recent ground-breaking space-based measurement of Newton's gravitational constant, we outline a proposal for a general layout of an optimized space-based approach that combines the advantages of a largely disturbance-free environment with lock-in techniques, null coordinates and the method of covariances. The method of covariances, and cross-covariances, is used for an optimized statistical analysis of data originating from two distinct experimental channels. One of two distinct measurement channels relies on an approximately constant acceleration in the gravitational field of static sources masses, while the second channel involves a lock-in measurement based on test and source masses with oscillating coordinates. Conceivable mission parameters are analyzed in terms of the optimum scale for the source masses and launch capabilities of currently available space launch systems. A candidate location for a largely disturbance-free position in geostationary orbit is identified.

We study linear gravitational perturbations and the corresponding quasinormal-mode spectra of electrically and magnetically charged black holes in the presence of higher-order corrections to the Einstein-Hilbert action. In particular, we consider a four-derivative interaction that couples the Riemann tensor to the electromagnetic field strength in the unique (Horndeski) combination that preserves second-order equations of motion. We provide a comprehensive analysis of the stability conditions. We numerically confirm the absence of exponentially growing modes throughout the physically admissible region of parameter space where the theory remains free of pathologies. We then investigate in detail the properties of the quasinormal-mode spectra and identify several distinctive features induced by the higher-order interaction.

We develop an area-constrained Willmore theory for closed spacelike surfaces in four-dimensional Lorentzian manifolds. Since the Euler–Lagrange equation is normal-bundle-valued, we introduce directional criticality along a prescribed normal direction and identify a class of connection-compatible directions determined by the normal connection. Our main result is the sharp inequality $2\lambda|\Sigma|+\int_\Sigma |\vec H|^2,d\mu\leq 8\pi\chi(\Sigma)$ for criticality in a connection-compatible spacelike direction under an Einstein-tensor sign condition. Under the dominant energy condition and $\lambda\geq0$, this yields $\int_\Sigma |\vec H|^2,d\mu\leq16\pi$. Equality is rigid: $\Sigma$ is round and satisfies Minkowski causal-diamond rigidity. We also prove an analogous sharp inequality for null critical directions. These results give positivity, rigidity, stationarity, and infinitesimal monotonicity statements for the Hawking quasi-local energy. A hypersurface formulation recovers the previously studied Hawking-surface problem as criticality in the spatial normal direction. Explicit examples in spherical symmetry, Kerr, and FLRW spacetimes distinguish directional from full criticality and show the necessity of the sign assumptions in the rigidity theory.

We investigate the high-energy collision of two spinless bodies within the Self-Force Effective Field Theory (EFT), taking one of them to be massless and highly energetic, such that it generates a gravitational shockwave (GSW) background. The GSW is perturbed by the second body, either massless or massive, satisfying respectively $E_2/E_1\ll1$ or $m_2/E_1\ll1$. Although the Self-Force expansion for shockwaves is frame-dependent, we freely choose a frame in which this EFT manifests its power, obtaining results valid to all orders in Newton's constant that make the underlying resummation patterns evident. Specifically, we push the expansion to 1SF order and prove the all-order vanishing of the so-called recoil operators in light-cone gauge. We obtain the exact graviton propagator on the shockwave background, matching previous results, and use it to compute the on-shell waveform at 1SF order in momentum space, specialising to the massless-massless collision. The waveform is computed to all orders in the Post-Minkowskian, or weak-field, expansion. In the trans-Planckian regime, its resummed form is proved to yield a UV-finite angular spectrum of radiated energy.

Matter fields localized around black holes can produce small secondary bumps in the effective potentials governing black-hole perturbations, potentially leading to quasinormal-mode spectral instabilities and late-time echoes. Considering physically motivated models of anisotropic fluids surrounding a spherically symmetric black hole, we show that the appearance of such a secondary bump is accompanied by either singular matter profiles or divergent characteristic speeds in the fluid. These features indicate that the corresponding matter configurations are unphysical. We further demonstrate that a secondary bump in the effective potential does not necessarily imply the existence of additional light rings in the spacetime.

We study the gravitational collapse of a shell, modeled as a coherent state of quantum matter. Without adding any physics beyond semiclassical gravity, we show that the universal gravitational coupling between matter and geometry induces an interaction between the shell and the negative-frequency modes present in the $|in \rangle$ vacuum state. We find that this interaction occurs unavoidably immediately after a trapping horizon forms, making it a universal feature of semiclassical gravitational collapse. The resulting transfer of energy from the collapsing matter to outgoing Hawking radiation acts as a gravitational quantum friction, opposing the formation of trapped regions while simultaneously imprinting the information carried by the collapsing matter onto the emitted quanta. We discuss the implications of this mechanism for the black hole information-loss problem and speculate about the possible endpoints of collapse, potentially preventing horizon and singularity formation.