We give a new rigorous construction of outer trapped surface formation in smooth vacuum spacetimes without symmetry assumptions. The outer trapped surface is produced by the collision of two transversely localized, nearly plane-fronted gravitational waves described by short pulse data. The outer trapped surface is obtained by joining two smooth pieces along a curve with their future-directed outer null normals matching continuously. The incompleteness of the maximal future development is then established by adapting Penrose's original argument. This construction is inspired by Penrose's argument in constructing a marginally outer trapped surface in the collision of two ultra-relativistic particles modeled by the Aichelburg--Sexl solution. less

Quantum-gravitational spacetime foam varying in space and in time imprints, respectively, a transverse phase decoherence and an achromatic, stochastic arrival-time jitter on light from distant sources. We show that the jitter modifies the first-order temporal coherence $g^{(1)}(τ)$ and hence the observed spectrum, providing a spectroscopic probe of the temporal variation, complementary to existing imaging-channel constraints on the spatial variation. In the dynamical-foam limit, where the foam decorrelates within the light's coherence time, the spectrum splits into a coherent line core suppressed by $e^{-σ_φ^2}$, with $σ_φ^2$ the variance of the foam-induced phase jitter, and an incoherent pedestal, yielding a spectral shape-distortion bound on the foam exponent $α$. Three probes follow: the outright survival of broadband flux, the fractional width of a spectral line, and a chromatic deficit across a broadband continuum. Applied to the MeV emission lines of GRB\,221009A we obtain $α\gtrsim 0.63$, a weak, lever-arm-dominated floor. For the broadband CMB the foam induces an $e^{-σ_φ^2(ν)}$ deficit ($σ_φ^2 \propto ν^2$); a fit to the COBE/FIRAS monopole spectrum reaches $α\gtrsim 0.52$ ($95\%$ CL), and next-generation spectrometers reach only $α\gtrsim 0.54$--$0.55$ (forecast), limited by the foam--temperature degeneracy. The pedestal-independent line-width and CMB bounds are floors below the holographic value $α= 2/3$, consistent with and excluding no part of the imaging window reported for GRB\,221009A; only the conditional TeV survival bound ($α\gtrsim0.74$), which holds only for near-Planck-refresh foam ($τ_{\rm foam}\lesssim4\times10^{-28}$\,s), would exclude it --- and, with it, the window. less

As the rate of gravitational wave detections continues to grow, the probability of observing a gravitationally lensed gravitational wave also increases. When the Schwarzschild radius of the lens is comparable in size to the gravitational wave wavelength, wave-optics effects induce a frequency- dependent modulation on the signal. CPU-based parameter estimation analyses for wave-optics lensing are computationally intensive, limiting the scale of studies to determine the significance of any potential lensing candidate. In this work, we implement a point-mass lens model in Jim, a high-performance GPU-based parameter estimation pipeline. This enables, for the first time, GPU-accelerated Bayesian inference in full-wave optics, including the computation of Bayes factors, without the need for training data. We call this framework Jim-lensing. Using this framework, we are able to speed up a full parameter estimation run under the point-mass lens hypothesis from more than 10 hours to the order of 10 minutes compared to the current state-of-the-art CPU-based pipeline. We show that our implementation is stable and obtains comparable results to this state-of- the-art pipeline. The substantial speed gain over existing CPU pipelines paves the way for analyzing gravitational wave data under a broader range of lensing models with greater capacity to produce robust frequentist significance estimates. less

We show how static, spherically symmetric regular black holes and black-hole mimickers with radius-dependent mass function $m(r)$ can be emulated in a quasi-two-dimensional Bose--Einstein condensate, with the aim of probing inner-horizon and inner-light-ring instabilities in the laboratory. Matching the acoustic and gravitational line elements on an equatorial disc fixes the ratio of radial flow velocity to sound speed and reproduces both horizon locations and surface gravities exactly. The analogue geometry nevertheless contains a residual radially varying sound-speed factor in $g_{tt}$, which shifts the light-ring positions and reduces the range of the regularization parameter $\ell/M$ for which a light-ring pair exists. We derive the interaction profile that removes this shift. Combining the metric correspondence with the continuity and Bernoulli equations, we then obtain closed-form expressions for the external potential and interaction strength realizing the full target spacetime, with $M$ and $\ell$ directly tunable through parameters of the potential. Finally, we translate the construction into experimental requirements (quasi-two-dimensional confinement, healing-length resolution, three-body lifetime, and stationary-flow lifetime) and present three fiducial designs achievable with present-day $^{39}\mathrm{K}$ platforms. less

We develop a first-principles description of the linearized gravitational field of localized sources in spatially flat Friedmann-Lemaître-Robertson-Walker universes with arbitrary smooth expansion histories. The construction determines the complete retarded metric and the induced perturbation of the surrounding cosmic fluid from prescribed source stresses and a cosmologically stationary past field. It includes all multipole orders and accounts for curvature tails, through which the field depends on the source's past history inside the light cone. Generalizing the Blanchet-Damour multipolar formalism, we identify gauge-invariant mass and current moments together with a canonical cosmic fluid moment that determines an independent scalar response. This response vanishes in Minkowski and pure de Sitter spacetime, although curvature tails persist in de Sitter. The gauge-invariant multipoles are obtained directly in position space. This gives an exterior description without the nonlocal spatial projection through the source interior required by the usual scalar-vector-tensor decomposition. We also relate the canonical multipole metric to the standard variables of cosmological perturbation theory. The retarded construction requires neither a slow-motion nor a geometric-optics approximation. We define effective multipolar sources by truncating the moments of slowly varying sub-Hubble extended sources at a prescribed Cartesian multipole order, recovering the earlier quadrupolar and octupolar models in de Sitter spacetime. We recover the known Minkowski and de Sitter multipolar fields and obtain an analytical representation for gravitational propagation in the $Λ$CDM model including radiation. less

We present new hairy stationary and axisymmetric black hole solutions. These solutions belong to a subclass of Horndeski scalar-tensor theories, in which there is no non-minimal coupling to curvature. The full rotating solutions are obtained numerically using the Kadath library based on spectral methods. The metric does not rely on an explicit ansatz but rather on the use of a set of numerical coordinates. Black holes obtained differ from Kerr in general, and depend on a new independent parameter, a primary hair, coming from a linear time dependence in the scalar field. These solutions admit a Killing horizon like the Kerr black hole, but are non-circular, and break the zeroth law of black hole thermodynamics. less

An exact rotating geometry need not define a black hole in equilibrium when its surface gravity varies across the horizon. We construct an asymptotically flat, non-Kerr subfamily of the aligned null solutions of generalized Proca theory whose outer horizon is exactly Killing and isothermal to second order in rotation. The horizon conditions constrain the vector hair, and integrability of the normalized horizon charge fixes the remaining second-order amplitude. Computing the complete gravitational and nonminimal vector surface charges, we obtain the asymptotically normalized mass and total angular momentum, and derive a logarithmic Hamiltonian entropy. The first law holds in both independent directions of the resulting two-parameter solution space. We express the mass as a function of entropy and angular momentum, recover the geometric thermodynamic potentials, and establish a scale-corrected Smarr relation that retains the dimensionful coupling. The exact metric defines the geometry, while equilibrium thermodynamics is consistently restricted to second order in spin. less

We demonstrate that the swirling universe can be recovered through a coordinated horizon-degeneration and scaling procedure applied to the Kerr C-metric. The construction exploits the horizon structure of accelerating, rotating black holes, which allows their inner and outer horizons to coalesce while simultaneously approaching the acceleration horizon. By magnifying the near-acceleration-horizon region faster than the black hole horizons approach it, the pair of accelerating, rotating black holes is pushed to an infinite proper distance, while a finite gravitomagnetic twist survives. The resulting geometry reproduces precisely the swirling universe, providing a physical interpretation of its intrinsic rotation as the gravitational remnant of rotating black holes sent to infinity. less

Hydrodynamic modeling of viscous neutron stars requires a choice of theory of relativistic hydrodynamics. That distinct formulations contain the same physical content in the long-wavelength limit is referred to as ``hydrodynamic universality.'' It has been recently shown that hydrodynamic universality can break down in radial neutron star oscillations: causality and finite-size effects restrict access to the long-wavelength universal hydrodynamic regime in sufficiently viscous stars. We show that this breakdown occurs for a broad range of causal hydrodynamic frames in Bemfica--Disconzi--Noronha--Kovtun (BDNK) theory. Increasing viscosity collectively pushes this class of causal frames away from the BDNK regime of validity, eventually forcing all to lie outside it at bulk viscosities in the range $ζ\sim10^{30}$--$10^{31}\mathrm{g}/\mathrm{cm}/\mathrm{s}$. This breakdown of universality is accompanied by material changes in the oscillation mode structure: non-hydrodynamic modes become damped on timescales comparable to those of hydrodynamic modes, the first overtone can become longer lived than the fundamental mode, and unphysical instabilities with sub-millisecond timescales can emerge. Even in regimes where universality has broken down, we find that the inviscid threshold of gravitational collapse remains unchanged in BDNK theory. These results provide evidence that first-order theories of relativistic hydrodynamics may be insufficient for modeling sufficiently viscous neutron stars. less

A confident detection of orbital eccentricity in gravitational-wave signals from black hole-neutron star systems would be a strong indicator of their dynamical formation origin. While eccentric waveform models have recently been developed, their validity for analyzing black hole--neutron star signals has yet to be tested against numerical-relativity simulations. Building upon our previous study of quasicircular black hole--neutron star mergers, we perform simulations of a subset of these systems with orbital eccentricities around five times larger than those inferred for GW200105_162426. Additionally, we perform simulations with magnetized matter to assess whether trapped $g$-mode oscillations persist in strongly magnetized disks. Regarding the emitted gravitational-wave signal, we find that the effective one-body waveform model SEOBNRv5EHM shows reasonable agreement unless tidal contributions become important, and the inspiral-only post-Newtonian models pyEFPE/pyEFPEHM show larger disagreement in the late inspiral. For the equal-mass system, which has the largest effective tidal deformability, eccentric tidal corrections have a small but noticeable impact on the phase, $\sim0.1$ rad. We find an almost negligible impact of eccentricity on the properties of the remnant black hole, and differences of up to a factor of two in the remnant disk mass. Trapped $g$ modes in the accretion disks have the same frequency regardless of the eccentricity, confirming that they depend mainly on the properties of the remnant black hole. We find that when accretion disks are strongly magnetized, they become less dense and have less pronounced structure, and that the trapped $g$ modes previously identified in the unmagnetized cases are efficiently damped. less