ALT Nonlinear tides couple stellar oscillations and reshape the gravitational wave (GW) signal from inspiralling binaries. We present hydrodynamic simulations of nonlinear tides in inspiralling neutron-star binaries and, within a mode-coupling model, derive a semi-analytic quadrupolar fundamental mode-frequency shift through second order in the tidal strength. The two approaches agree in the perturbative regime, but diverge at stronger tidal fields, where perturbation theory breaks down. In the late inspiral, nonlinear tides reduce the fundamental-mode frequency by ∼10% for equal-mass binaries and add a GW phase shift of O(1) radian beyond the linear tidal response. These effects make nonlinear tides a promising target for next-generation GW detectors.

ALT Radio pulsars emit highly polarized radiation whose polarization properties are commonly described by the Rotating Vector Model (RVM). In this work, we investigate the effects of gravitational light bending and parallel transport on the polarization profile of a pulsar in a hypothetical binary system with a black hole (BH) companion. We combine the RVM with numerical integration of null geodesics to calculate the polarization direction along the photon trajectory and model the orbital-phase-dependent variation of the observed polarization position angle (PPA). We consider both non-spinning and spinning black hole companions and include the evolution of the orbital geometry due to relativistic spin-orbit coupling. For a non-spinning black hole, the change in the observed polarization direction arises from the geometric shift associated with gravitational light bending. For a spinning black hole, frame dragging introduces an additional geometric shift, while parallel transport produces a

ALT In this paper, we introduce a metric describing rotating acoustic black holes in spherical coordinates. This formulation allows us to obtain exact analytical solutions of the massless Klein–Gordon equation governing scalar perturbations in the corresponding background spacetime. By imposing the appropriate boundary conditions, we derive an exact analytical expression for the quasibound state frequencies of the system. Quasibound states play a central role in the dynamics of black hole analog systems, as they characterize long-lived resonant modes trapped in the vicinity of the effective horizon. Their investigation provides valuable insights into wave propagation, mode stability, and energy confinement in analog spacetimes. Moreover, because acoustic black holes offer experimentally accessible realizations of horizon physics, the study of quasibound states may establish a direct connection between theoretical predictions and laboratory observations, thereby contributing to a deeper und

ALT We construct reflection-asymmetric thin-shell wormholes by joining a Schwarzschild exterior to its minimal geometric deformation, which shares the same mass and redshift function, so that the asymmetry is carried entirely by the gravitational-decoupling sector. The deformed side is a black hole sourced by a scalar field coupled to nonlinear electrodynamics, in its branch with a real scalar. We show that the total radial null energy of shell and bulk obeys an exact identity and becomes positive for a sufficiently strong deformation. The momentum flux across the shell equals the radial null-energy density of the bulk, and retaining it yields the linear stability condition in closed form. We prove that stability with a conventional shell response requires the bulk to violate the radial null energy condition at the throat; the deformation instead enlarges the stable domain and moves the transition radius inside the photon sphere. For a stationary extension built on Kerr, the junction is so

ALT We study the cosmological dynamics of a general class of viscous dark matter models using a dynamical system analysis. In the Eckart formalism, these viscous cosmologies are characterized by a dimensionless bulk viscous pressure ΩΠ≡ ΩΠ(Ωₘ), where Ωₘ is the matter density parameter. Our analysis demonstrates that a viable early radiation era requires the complete absence of viscosity, a proper matter-dominated epoch demands a small viscous pressure, and the only physically admissible late-time attractor is the cosmological constant-dominated fixed point; consequently, the background expansion must closely track standard ΛCDM evolution. We validate these general results using the power-law parameterization ΩΠ=-ξ̂₀ Ωₘˢ, where ξ̂₀ and s are real parameters; for s ≥ 1 and ξ̂₀ ≪ 1, the model reproduces the ΛCDM background, while for s < 1 the system encounters a past-time integration boundary where the dark matter density transitions into negative values and its viscous pressure evaluates to

ALT We study periodic orbits of charged particles in the equatorial plane of a five-dimensional (D=5) rotating Myers–Perry black hole (BH) immersed in a uniform magnetic field, and the gravitational waves emitted by these orbits in an extreme mass-ratio inspiral. All results are compared with the Kerr BH in the uniform magnetic field. We find that a charged particle is much more weakly bound to the D=5 dimensional BH, i.e. its energy at the innermost stable circular orbit is close to unity, 0.95–0.99, compared with 0.78–0.94 for the Kerr BH, and the depth of the potential well is several times smaller. As a result, the periodic orbits of the D=5 dimensional BH lie much closer to the separatrix. The magnetic field can make the azimuthal frequency of the orbit smaller than the radial one and produces loops in the orbits far from the BH. Using the numerical kludge method, we compute the gravitational waveforms for a compact object of 10M⊙ orbiting a BH of 10⁶M⊙. At the same magnetic field, th

ALT Under explicit and physically motivated assumptions, we identify a local-to-global thermodynamic mechanism that organizes several previously disconnected inequalities on black holes. We first focus on asymptotically flat, static, and spherically-symmetric black holes, and provide a unified thermodynamic perspective on various Penrose-like inequalities. We also discuss the generalizations in asymptotically Schwarzschild anti-de Sitter black holes/branes. In holographic settings, these bounds imply monotonic behavior of normalized thermodynamic quantities in the dual field theory. Notably, one central integrated inequality can be established directly under the NEC in general static planar symmetric asymptotically Schwarzschild anti-de Sitter black branes. Our purpose is not to claim a fully general proof of the proposed local bounds, but to expose a unified structural perspective connecting local thermodynamic constraints to global geometric inequalities. Establishing the local bounds be

ALT We identify the phase of a Bargmann invariant of two matter-wave interferometers as a quantum gravity signature that cannot be reproduced by any branchwise-definite classical gravitational field. For matter coupled linearly to linearized gravity, the interferometric coherence factorizes exactly into a conservative Newtonian phase, the characteristic functional of the graviton state, and a commutator phase. For Wigner-positive graviton states, the first two are classically reproducible; therefore, decoherence alone cannot certify that gravity is quantized. The Bargmann phase isolates the commutator, which is independent of the graviton state and of all Newtonian interactions and is bounded by which-path decoherence. For gravity, this radiation-reaction effect is sixty orders of magnitude below experimental reach, but trapped-ion analogs can test it at order unity.

ALT In this work, we construct an asymptotically conical regular black hole in four-dimensional nonpolynomial quasitopological gravity. Starting from a linear choice of the reduced function, which yields an asymptotically conical Schwarzschild-like solution, we introduce a nonlinear deformation that preserves the leading asymptotic cone while producing an exact black hole solution with a de Sitter core and finite curvature invariants. We further establish the corresponding black hole thermodynamics for both solutions within the Hamiltonian and Wald frameworks. The entropy contains rational and logarithmic terms in the regular model, and both solutions satisfy the first law of black hole thermodynamics.

ALT We study tensor perturbations and quasinormal modes (QNMs) of thick branes in f(R,T)=R+αT gravity. Starting from the background thick brane solutions, we derive the Schrödinger-like equation for gravitational perturbations and construct the corresponding effective potential. The effective potential exhibits a double-barrier structure whose properties depend on the model parameters. We analyze transmission spectra and find good agreement between resonance peaks and the real parts of the quasinormal frequencies. The parameters b and α mainly control the width and height of the potential barriers, respectively. QNMs are extracted using both time-domain evolution and direct integration methods, showing consistent results for low-lying modes. Higher overtones become increasingly difficult to extract due to stronger damping and numerical contamination. Our results indicate that f(R,T) corrections significantly affect both the effective potential and the quasinormal spectrum, providing a poss

ALT We prove the C⁰-inextendibility of the full Kerr realizations considered in this paper and all their connected covering spacetimes. The result holds for positive mass and nonzero rotation in the subextremal, extremal, and overextremal regimes. The obstruction at the singular ring is expressed by unbounded proper time in a fixed-endpoint causal homotopy class. It persists under every positive continuous conformal rescaling and under continuous widening of the Kerr causal cones, without any global causality assumption. We give sharp bounds in a fixed coordinate norm for stationary and axisymmetric perturbations and describe complete timelike geodesics that encounter arbitrarily large curvature near the ring.

ALT We construct a new exact solution to the five-dimensional Einstein-phantom-scalar system that describes a rotating wormhole connecting two asymptotically flat regions. The solution represents a rotating generalization of the Ellis-Bronnikov wormhole with equal angular momenta. Under the cohomogeneity-one ansatz, we demonstrate that the radial equations reduce to an integrable A₁⊕ A₂ Toda system, from which the general local solution can be obtained in closed form. This integrable structure enables us to implement the regularity and asymptotic-flatness conditions analytically, thereby singling out a three-parameter family of solutions characterized by the mass, the equal angular momenta, and the scalar charge. The resulting spacetimes are geodesically complete, horizonless and stably causal, and contain no curvature singularities. The Toda structure further allows us to prove the uniqueness of the wormhole throat and to derive a sharp upper bound on the angular momentum for a fixed thro

ALT We construct smooth Riemannian metrics and scalar potentials that are invariant under a circle action and satisfy the static Einstein equations with positive cosmological constant. The underlying compact three-manifold is a ball with two disjoint open balls removed. The potential (or lapse function) is positive in the interior and vanishes at the boundary, consisting of three non-degenerate horizons. The areas of the two inner boundary components are controlled by two arbitrary parameters, namely the masses of the black holes, which are taken to be sufficiently small and comparable. Each inner horizon radius scales linearly with the individual mass parameters, while the separation of their reference centers scales as the cube root of the total mass. These two scales are assumed to be sufficiently small at different rates, whereas the cosmological length remains fixed. The main analytical challenge is to control the linearized equation and the nonlinear remainder simultaneously near the

ALT We investigate how a local geometric critical point of the odd-parity Proca potential relates to the global quasibound spectrum in Schwarzschild spacetime. After normalization by μ², the axial potential Vₐₓ=(1-2M/r)[ℓ(ℓ+1)/r²+μ²] has the same radial form as the timelike-geodesic effective potential. Its outer minimum and inner maximum merge at r=6M, the Schwarzschild innermost stable circular orbit, when (Mμc)²=ℓ(ℓ+1)/12. With δ=1-(Mμ)²/(Mμc)², we derive exact fold scalings: stationary-point separation proptoδ¹/², barrier–well depth proptoδ³/², and curvature magnitudes proptoδ¹/². Continued-fraction calculations for the first three axial quasibound branches with ℓ=1,2,3, cross-checked for selected modes by independent Riccati shooting, show regular frequency continuation across the threshold and approximately linear near-critical behavior. At fixed radius the radial operator is affine in δ, supporting a conditional simple-root analytic interpretation. The main result is the explicit di

ALT We discuss Einstein equations with a non-conserved energy-momentum Tμν. The energy-momentum tensor T̃μν on the right hand side of Einstein equations is expressed in the form 8πGTμν-gμνΛ+Σμν where G is the Newton constant, Λ is the cosmological term and Tμν is the energy-momentum of an ideal fluid. The remaining part Σμν describes the viscosity and a heat flow. We discuss space-time-dependent Newton “constant" G and the cosmological term Λ. The covariant conservation law for T̃μν imposes a relation between the energy-momentum (non)conservation and the variation of G and Λ. The covariant derivative ∇μTμν can be related to the first law of thermodynamics and expressed by the temperature T and the entropy S. The energy-momentum non-conservation in our models is a consequence of a diffusion of its constituting particles. We describe a gas of diffusing massive particles in a homogeneous isotropic space-time as a system with a time-dependent linear barotropic equation of state Pₜ=wₜρₜ with 0<

ALT In this work, we investigate the properties of a charged anti-de Sitter (AdS) black hole in the presence of a NGC 4649-type dark matter halo and a surrounding quintessence field. Starting from the modified Einstein field equations (EFEs), we construct the corresponding black hole solutions by incorporating the effects of electric charge, cosmological constant, dark matter distribution, and quintessence characterized by a variable equation of state. We analyze the resulting spacetime geometry through the metric function and horizon structure, showing that the dark matter halo strengthens the gravitational attraction, whereas the quintessence field provides a repulsive contribution that substantially alters the asymptotic behavior of the spacetime. Furthermore, the thermodynamic properties are investigated by applying the Duan-φ-mapping topological theory together with the generalized free energy method. This framework allows us to examine the topological structure, critical behavior, an

ALT We calculate finite-distance images of stationary, pressure-supported tori around genuine, electrically neutral Kerr–Bertotti–Robinson black holes using general relativistic ray tracing with gray emission and absorption. The emitting volume is bounded by a constant-specific-angular-momentum equilibrium free surface, and the observer is placed at a radius of 50M. Along the investigated external-field sequence, integrated brightness and apparent size both increase strongly as the field modifies the equilibrium source and photon propagation. Higher torus filling has a different effect: in a representative sequence, brightness grows by a factor of 5.9 while the intensity-weighted image size increases by only 10%. Increasing viewing inclination enhances lateral brightness contrast. Stronger self-absorption can coexist with higher absolute brightness because emission and absorption scale together in the adopted gray prescription. At common pressure center and filling, increasing the dimensio

ALT The nonlinear iterative repetitive Penrose process established recently by Ruffini et al. is generalized to the magnetic Penrose process. The spacetime background is the Kerr metric, the external magnetic field is described by the Wald solution, the electromagnetic field is treated as a test field, and the charge is treated as a test charge. We analyze the optimal conditions for maximum energy extraction on the equatorial plane and find that under these conditions the core mechanism of the iterative process is charging feedback. After charging, the black hole charge is approximately half of the Wald charge. When the neutral parent particle decays, charge conservation forces the infalling fragment to carry charge while the other fragment carries the opposite charge, so the black hole charge accumulates monotonically. The monopole voltage of the Wald solution decreases linearly with the charge Q and crosses zero to reverse sign at a certain point, so the magnetic gain is gradually neutra

ALT This Comment clarifies the physical interpretation of the black hole spacetime metric employed by Shaymatov et al. [Phys. Dark Univ. 34 (2021) 100891]. We point out that the metric, originally derived in Li and Yang (2012), describes a specific dual-component system consisting of cold dark matter and a phantom field background. Treating this geometry as a single-component “Perfect Fluid Dark Matter” (PFDM) with a total equation of state wₜₒₜₐₗ = -1 is physically incompatible with the gravitational attraction required for dark matter halos. Furthermore, we clarify the distinct parameter regimes of the original model to prevent potential misapplications in extreme astrophysical scenarios.

ALT We study the asymptotic behavior of small-data global solutions to the Einstein–Klein–Gordon system with a positive-mass real scalar field in wave gauge. We show that the linearized trace component of the metric perturbation admits a t̂⁻¹≃ t⁻¹ optimal decreasing rate. Moreover, a quantitative one-point observability estimate is established: the initial Klein-Gordon energy is controlled by this leading order coefficient.