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

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

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

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

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

We investigate a class of traversable wormhole geometries supported by an effective Casimir source corrected by gravitational memory. The construction is motivated by the fact that a time-dependent gravitational background can leave a permanent positive shift in the vacuum polarization of a quantum field confined to a Casimir cavity. By promoting the plate separation to an effective radial scale in the Morris-Thorne spacetime, we obtain a density profile composed of the usual negative Casimir contribution, proportional to $r^{-4}$, and a positive memory-induced correction, proportional to $r^{-7}$. The corresponding shape function is derived directly from the Einstein equations and satisfies the throat condition by construction. We determine the redshift function from a constant barotropic equation of state together with the requirement of regularity at the throat, which fixes the barotropic parameter in terms of the Casimir and memory coefficients. The flare-out condition defines the admissible range of the memory parameter and separates a Casimir-dominated sector from a phantom-like sector, with the transition point associated with a singular limit of the constant-barotropic description. We analyze the curvature scalar, the embedding structure, the energy conditions, and the Tolman-Oppenheimer-Volkoff equilibrium of the anisotropic matter source. The radial null energy condition is necessarily violated at the throat, while the tangential sector depends on the redshift gradient. We also examine the shadow radius as a phenomenological diagnostic and show that admissible solutions can overlap the Event Horizon Telescope range for M87*. The results indicate that gravitational memory can deform Casimir-supported wormholes by softening the ordinary Casimir contribution, modifying the near-throat geometry, and reshaping the internal stress balance required to sustain traversability. less

We investigate the observational appearance of a rotating black hole (BH) in Rastall gravity by analyzing its shadow and accretion signatures under different illumination environments. The spacetime geometry is characterized by the Rastall parameter $μ$, the structure parameter $γ$, and the rotation parameter $a$. To visualize the BH environment, we employ a ray-tracing algorithm that follows photon trajectories from the observer's screen to the emission region. We analyze how the shadow radius, distortion, and photon ring morphology respond to changes in the spacetime parameters. For a fixed value of $a$, the shadow observables exhibit a pronounced dependence on the Rastall gravity parameters. In particular, increasing the structure parameter $γ$ leads to a gradual enlargement of the shadow radius, indicating an expansion of the photon capture region surrounding the BH. At the same time, the distortion parameter decreases, implying that the shadow boundary becomes progressively more circular and less deformed. These results suggest that larger values of $γ$ tend to suppress the asymmetry induced by rotation and enhance the apparent size of the shadow. Similar modifications are observed for different values of the Rastall parameter $μ$, demonstrating that the combined effects of $μ$ and $γ$ leave distinct signatures on the shadow morphology. Consequently, shadow observations may provide an effective tool for constraining the parameter space of rotating BHs in Rastall gravity. less

We study the ringdown of the regular Jusufi-Singleton black hole, whose nonsingular core is controlled by a zero-point length arising from a non-local, T-duality-inspired gravitational model. Scalar, electromagnetic and Dirac perturbations are considered. The zero-point-length parameter raises the effective scattering barrier and produces a systematic increase of the oscillation frequencies, while also making the damping faster over most of the parameter range. High-order WKB results are checked against time-domain integration and show very good agreement for the dominant modes. We also compute excitation factors, which characterize the source-independent strength of the quasinormal-mode poles and show a smooth dependence on the new length scale. less

We investigate the construction of regular compact objects in the recently proposed lower-dimensional Einstein--Gauss--Bonnet (EGB) gravity obtained through regularized dimensional reduction. Unlike the standard BTZ black hole, the corresponding vacuum EGB solution develops a genuine curvature singularity at the origin, providing an interesting setting in which higher-curvature corrections deteriorate the ultraviolet behavior of spacetime. To address this issue, we reconstruct matter sectors capable of restoring regularity while preserving the BTZ-like asymptotic structure. First, we derive regular black-hole solutions supported by nonlinear electrodynamics and determine the corresponding electromagnetic Lagrangians directly from the field equations. We then extend the analysis to Simpson--Visser black-bounce geometries, obtaining smooth throat configurations with finite curvature invariants throughout the spacetime. As an alternative regularization mechanism, we formulate a unimodular extension of lower-dimensional EGB gravity and show that standard Maxwell fields can support regular geometries through a dynamical exchange between the vacuum and matter sectors mediated by a spacetime-dependent cosmological function. We further investigate the thermodynamic properties of the regular black-hole and black-bounce solutions, showing that the matter sector modifies the evaporation process, allows for remnant formation, and produces nontrivial phase transitions. In the black-bounce case, the thermodynamic quantities smoothly recover the EGB-BTZ behavior in the appropriate limit. These results demonstrate that lower-dimensional EGB gravity provides a useful laboratory for exploring the interplay between higher-curvature corrections, regular compact objects, nonlinear electrodynamics, and unimodular gravity. less

We develop an affine-null characteristic formulation of the Einstein-Maxwell system coupled to a charged complex scalar field in $n$-dimensional spherical symmetry. By introducing suitable auxiliary variables, the main field equations are cast into a hierarchical system of radial hypersurface equations, supplemented by a transport equation for the scalar field. We discuss the associated characteristic initial-boundary value problem for asymptotic, vertex and null-boundary configurations, and derive the corresponding asymptotic quantities and balance laws. As a consistency check, we recover the scalar-free Reissner-Nordström-Tangherlini family, including both the non-extremal and extremal branches, directly from the hierarchy. The resulting framework provides a systematic setting for the study of charged scalar dynamics and exact black-hole solutions in higher-dimensional affine-null coordinates. less

We formulate a local geometric criterion for weak cosmic censorship in black hole overcharging and overspinning thought experiments. Under the null convergence and generic conditions, matter injection turns a horizon cross section into a closed trapped surface. Any final spacetime unable to accommodate this surface is ruled out. This trapped surface criterion excludes superextremal Reissner-Nordström, Reissner-Nordström-de Sitter, and Kerr-Newman final states, as well as Weyl-class naked singularities. Our criterion does not rely on asymptotic charges or on an extremal condition characterizing naked singularities. less

This paper develops an epistemological constraint on quantum gravity grounded in the empirical meaning of general relativity. The central claim is that a complete recovery of general relativity requires an effective metric, a continuum limit, or Einstein-like dynamics together with the physical conditions under which relational geometrical quantities can be objectively determined. These conditions concern the dynamical stability of measuring devices and reference systems, causal accessibility among physical systems, record formation, and invariance under admissible descriptions. In classical general relativity, they are usually implicit in the use of clocks, rods, light signals, freely falling bodies, detectors, and gauge-invariant observables. In quantum gravity, however, they become non-trivial because spacetime geometry may be emergent, effective, thermodynamic, relational, or frame-dependent. This claim is developed through four cases: Rindler horizons and the Unruh effect, black-hole thermodynamics and Jacobson's equation-of-state derivation, gravitational-wave detection, and Weyl and conformal gravity. The latter is discussed as a critical limiting case in which conformal invariance raises a sharp question about whether scale-dependent measurements of space and time can be physically fixed. Implications for quantum gravity are also discussed using emergent gravity and quantum reference frames as examples. The perspective developed in the study suggests a general epistemological constraint on quantum gravity: any viable approach must recover the physical possibility of objective geometrical measurement together with geometry itself. less

We study the ability of the upcoming Laser Interferometer Space Antenna (LISA) to constrain gas torques acting on extreme-mass-ratio inspirals (EMRIs) when these are embedded in accretion disks, using recently developed relativistic models for the binary-disk interaction. Using a fully Bayesian setup, we find that, contrary to previous forecasts based on Newtonian results, these observations can provide simultaneous estimates of the disk surface density and the accretion rate (or, equivalently, its total luminosity) without the need for an electromagnetic counterpart. Our analysis also indicates that simpler measurement constraints based on the linear-signal (Fisher matrix) approximation are not valid for these systems. For typical EMRI observations, the torque amplitude can be constrained to within ~10%, strengthening the prospect of probing accretion physics at (sub)microparsec scales, deep in the strong-field gravity regime and complementing electromagnetic observations. This also strengthens LISA's ability to help answering questions such as how massive black holes grow and coevolve with their host galaxies and, by helping to identify the EMRI's host galaxy through cross-correlation with AGN catalogues, to improve the use of these sources as (dark) sirens for cosmology. less

Gravitational wave (GW) observations provide an unprecedented laboratory for testing general relativity (GR) in the strong-field, highly dynamic, and relativistic regimes. Within the parameterized post-Newtonian (PN) formalisms, waveform generation tests have conventionally been limited to constraining inspiral coefficients up to the 3.5PN order. Leveraging the recent theoretical breakthrough that extended the analytical compact binary phasing to the 4.5PN order, we present the first observational constraints on these higher-order effects. Our analysis utilizes two exceptional events detected by the LIGO-Virgo-KAGRA (LVK) network: GW250114\_082203, which boasts the highest signal-to-noise ratio (SNR) recorded to date, and GW230627\_015337, which features a uniquely prolonged inspiral phase and the highest inspiral phase SNR to date. By performing Bayesian inference on the dimensionless deviation parameters ($δφ_i$) associated with the 4PN and 4.5PN coefficients, we find that our results are fully consistent with the predictions of GR. While the current 90\% credible intervals for the four deviation parameters are of order $\mathcal{O}(1) \text{-} \mathcal{O}(10)$, the general relativistic null values ($δ\hatφ_a= 0$) are entirely encapsulated within the bounds. This investigation establishes the first empirical baseline for 4PN and 4.5PN inspiral tests of GR, paving the way for high-precision null tests of GR with current and next-generation GW detectors. less

The detection of high-frequency gravitational waves (HFGWs) above 10 kHz provides a crucial probe of exotic astrophysical phenomena and new physics. We report the first search for HFGWs via their conversion to electromagnetic radiation through the inverse Gertsenshtein effect in Earth's magnetic field, utilizing radio telescopes including the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA). Since no statistically significant signal is observed, we obtain new upper limits on the characteristic strain across the 1 GHz -- 1 THz band, with the most stringent constraint reaching $h_c \lesssim 10^{-18}$, improving upon existing bounds by up to three orders of magnitude. These results significantly advance the exploration of uncharted parameter space for exotic gravitational-wave sources, paving the way for future discoveries with next-generation facilities such as the Square Kilometre Array (SKA). less