Papers

We investigate the geodesic dynamics, eikonal quasinormal modes (QNMs), and extended phase space thermodynamics of a Schwarzschild-de Sitter-like black hole coupled with a global monopole in Bumblebee gravity. Our analysis reveals that spontaneous Lorentz symmetry breaking and topological defects modify the effective radial dynamics, leading to lower effective potential barriers and a reduced magnitude of the effective force. For null geodesics, higher monopole parameters expand the photon sphere and critical impact parameter, implying an enlarged black hole shadow, while mitigating dynamical instability. For massive particles, the stable circular orbit window narrows significantly as the Outermost Stable Circular Orbit (OSCO) shrinks, alongside a marked enhancement in perihelion precession. In addition, we examine the correspondence between the eikonal QNMs and the properties of null geodesics. The excellent agreement between the eikonal approximation and the WKB method for scalar and electromagnetic perturbations supports the validity of the correspondence for the spacetime under consideration. In the extended thermodynamic phase space, we evaluate the black hole as a holographic heat engine. We demonstrate that classical efficiency is mathematically blind to Lorentz violation; however, introducing a quantum-corrected modified entropy tightly couples the macroscopic work output to the symmetry-breaking framework. Ultimately, the necessity to respect the Carnot bound imposes a strict macroscopic constraint on the Lorentz-violating parameter, safeguarding the Second Law of Thermodynamics.

We study quasibound states of a massive neutral Dirac field in the Newman-Janis-generated rotating charged geometry used in Lorentz-violating Kalb-Ramond (KR) gravity. Rather than assuming that the Kerr separation survives the deformation, we first recast the metric in four-dimensional off-shell Carter form. The corresponding principal closed conformal Killing-Yano tensor then provides the hidden symmetry needed to separate the torsion-free, minimally coupled Dirac equation. We derive the horizon and large-radius boundary conditions, including the non-Minkowskian asymptotic normalization, and determine the complex spectrum from coupled angular and matrix radial continued fractions. The code reproduces the Kerr spectrum and is checked by truncation studies and independent two-sided radial integrations. In scans at fixed metric parameters, increasing the KR parameter moves most real frequencies toward the mass threshold and reduces most decay rates. These trends are not invariant, however, because the same variation changes both the asymptotic potential scale and the distance from extremality. A scan in which these two quantities are held fixed changes the absolute ground-state trend but preserves a reversal in the real-frequency ordering of the maximal-$m$, $j=3/2$, $\ell=1$ pair. The location of the crossing shifts with the normalization convention. Thus the level reordering is more robust than the individual monotonic trends in binding energy or lifetime within this background family.

Parity violation in the gravitational and electromagnetic sectors has been extensively investigated, yet the two are conventionally treated as independent phenomena. This separation, however, may be a four-dimensional prejudice. In higher-dimensional spacetime, gravity and electromagnetism may share a common geometric origin—and so, perhaps, does their parity violation. In this paper, we pursue this idea by constructing parity-violating Kaluza-Klein models in both Riemannian and teleparallel geometries. In Riemannian geometry, the simplest five-dimensional parity-violating term reduces to several complicated four-dimensional terms, including the familiar gravitational Chern-Simons term, which suffers from a ghost instability. In teleparallel geometry, however, the result is strikingly simple. The simplest five-dimensional parity-violating term reduces to only two ghost-free terms—the familiar Nieh-Yan term and the standard electromagnetic Chern-Simons term. Remarkably, the model predicts that the helicity-dependent dispersion shift for electromagnetic waves is exactly six times that for gravitational waves, $\Delta\omega^{2}_{ EM}=6\,\Delta\omega^{2}_{GW}$, a background-independent relation. This relation offers a falsifiable test of unification through joint cosmic microwave background and gravitational wave birefringence observations.

The nonlinear gravitational-wave memory, or Christodoulou memory effect, is a permanent displacement produced by the self-interaction of gravitational waves predicted by General Relativity. In this work, we present GWMemoryLab, a modular numerical framework for investigating leading-order Christodoulou memory during the inspiral of non-spinning binary black hole systems within the post-Newtonian approximation. The framework implements modules for binary dynamics, post-Newtonian inspiral evolution, oscillatory waveform generation, gravitational-wave energy flux, and nonlinear memory accumulation. Analytical comparisons and convergence tests demonstrate numerical stability. We investigate the dependence of accumulated memory on binary mass ratio, total mass, and detector low-frequency cutoff. The simulations reveal an optimal total mass that maximizes the observable inspiral memory for a given detector bandwidth. Within the explored parameter space, the optimal mass follows an approximately inverse dependence on the detector low-frequency cutoff. This behaviour arises from the competition between increasing gravitational-wave luminosity and decreasing inspiral duration as the binary approaches the innermost stable circular orbit. These results provide insight into the detectability of nonlinear gravitational-wave memory and demonstrate the utility of GWMemoryLab for systematic parameter studies.

The new era of multi-messenger astronomy requires the accurate and self-consistent derivation of the nuclear equation of state at high temperature. In the present work, we focused on the calculation of hot hybrid equations of state, studying the different evolution stages of a proto-neutron star with a quark matter core (proto-hybrid star). For the hadronic matter we used two distinct Skyrme effective interactions, while for quark matter the well-known vector MIT bag model was employed. To model the era of trapped neutrinos in the system we considered the global conservation of lepton fraction which resulted in an equation of state with an extended mixed phase. For periods following the neutrino diffusion phase of a proto-neutron star, the equations of state were modelled using both the Maxwell and the Gibbs construction depending on the assumption for either local or global electric-charge conservation. With the use of the derived hybrid models, we solved the Tolman-Oppenheimer-Volkov equations to describe the corresponding hybrid star configurations. Finally, we investigated how the structure of proto-hybrid stars evolves, using constant rest mass sequences. We found that regardless of whether electric-charge is globally or locally conserved, the earlier stages of a hybrid star's life may play a crucial role on the determination of its maximum possible gravitational mass in later stages.

We extend the modified double-copy construction to quasitopological gravity (QTG) coupled to matter. Spherically symmetric QTG solutions in $D$ dimensions are generated from an auxiliary gauge field in a flat $(D+1)$-dimensional spacetime. The gravitational field equations reduce to two relations determined by the QTG generating function and the matter stress–energy tensor. For a broad class of spherically symmetric matter sources, solutions of the auxiliary gauge-field equations can be used to construct exact solutions of the QTG field equations. The framework applies to Maxwell theory, nonlinear electrodynamics, and Yang–Mills theory, encompasses both static and Vaidya-type solutions, and reproduces the Einstein-gravity limit.

We develop a variational calculus for curves on Fréchet manifolds and apply it to the Lagrangian formulation of the Einstein evolution equations on the manifold $\Mm$ of Riemannian metrics of a compact manifold. For mechanical Lagrangians associated with weak pseudo-Riemannian metrics, we establish the Euler–Lagrange equations, energy conservation, and an infinitesimal Noether theorem that does not require the symmetry vector field to generate a flow. Applied to the DeWitt Lagrangian, this framework identifies the momentum constraint with the vanishing of a Noether momentum map and gives a direct proof of its propagation. We also derive the pointwise transport identity \[ \partial_t\bigl(\Ham_\lambda\,\dv_g\bigr) = 2\alpha\,\delta_g(\delta_g^-k)\,\dv_g\,, \] which implies propagation of the Hamiltonian constraint. Finally, we prove a Maupertuis– Jacobi theorem for weak pseudo-Riemannian metrics on Fréchet manifolds and show that, away from the zero set of the total scalar-curvature potential, constrained Einstein evolutions are reparametrized unit-speed geodesics of the conformal DeWitt metric $-4\alpha S_\lambda G^-$.

We present a model in which asymptotically de Sitter universes of various types, and de Sitter (dS) radii $R_n$, live in the interiors of black holes in a maximally entropic flat $p = \rho$ Friedmann-Robertson-Walker universe. These dS universes clearly have many quantum states. We argue that they decay and equilibrate with the maximal entropy universe on time scales of order $\alpha_n R_n {\rm ln} (R_n /\delta_n)$, where $\alpha_n$ are model-dependent dimensionless constants and $\delta_n$ are the initial distances between the shell, satisfying the Israel junction conditions, and the dS horizon. These are time-scales as viewed by a detector following a trajectory far from the would-be cosmological horizon. These times are all exponentially shorter than dS recurrence times, which have no meaning in this model. It is impossible for a detector inside one of the dS universes to determine whether it is actually part of such a structure. This model could be used as the basis for claiming that certain constants of nature or cosmological initial conditions were chosen to have mathematically unnatural values because other values could not lead to any form of intelligent life.

One of the fundamental questions we ask in cosmology is what the spatial curvature of the universe is. Experimental observations show that the universe is very close to being spatially flat. However, new results, mainly from measurements by the Planck satellite, indicate that the curvature parameter might be positive. We will, however, examine completely new theoretical results that suggest that we actually live in a closed Universe.

Late-time wave tails decay at different rates along future null infinity and along timelike worldlines at finite radius. A compactified numerical evolution must represent both the slower decay at null infinity and the faster interior decay, producing an increasingly sharp transition between the two regimes. We address this difficulty for semilinear wave equations in Minkowski spacetime using homothetic hyperboloidal coordinates adapted to the scaling structure of the tail. In these coordinates, the tail approaches a smooth radial profile with the same decay rate at every compactified radius. The formulation therefore avoids the steepening of the radial profile seen in stationary hyperboloidal evolutions, and it reaches late times in a number of steps that grows only logarithmically with retarded time. We demonstrate this approach using pseudospectral simulations in 3+1 dimensions and reproduce the generic decay rates conjectured by Rinne. We also provide numerical evidence consistent with a nongeneric codimension-one cancellation of the leading tail coefficient at null infinity, resulting in a faster decay rate.

We investigate the application of Physics-Informed Neural Networks (PINNs) to the numerical solution of Einstein's vacuum field equations for static spacetimes. We first reproduce the Schwarzschild solution and then extend the method to the axisymmetric $q-$metric, a nontrivial exact solution characterized by a mass quadrupole moment. We analyze the influence of boundary conditions, domain decomposition, and equation redundancy on the convergence and stability of the training process. The proposed framework accurately reconstructs the metric functions in the computational domain while maintaining small residual errors. Our results demonstrate that PINNs provide a robust and flexible approach to solving Einstein's equations and offer a promising foundation for investigating gravitational configurations for which exact analytical solutions are unknown.

We derive an exact radial monotonicity law for static, spherically symmetric spacetimes in metric \(f(R)\) gravity. For \(ds^2=A(r)dt^2-dr^2/B(r)-r^2d\Omega^2\), the matter contribution and the scalaron Hessian combine into an effective radial-convergence numerator that fixes the derivative of \(\Q=B/A\). On every connected static interval with \(A>0\), \(B>0\), and \(f_R\equiv df/dR>0\), its sign therefore determines the monotonicity of \(\Q\). The integrated identity retains the finite, generally nonzero value of \(B/A\) at a regular nondegenerate Killing horizon; consequently, a fixed-sign convergence condition orders the horizon endpoint ratios rather than excluding two horizons. A zero-integral obstruction arises for equal endpoint values, including boundaries where \(B\to0\) while \(A\) remains finite and nonzero. Saturation is equivalent to \(B/A=\mathrm{const}\), and in vacuum requires a scalaron profile linear in the areal radius. We illustrate the strict non-saturated branch, within the General-Relativity sector, using the exact constant-density stellar interior, and apply the equality and consistency diagnostics to the constant-\(X\) and power-law solutions of Multamäki and Vilja. In particular, in the Schwarzschild–de Sitter two-horizon parameter range, one constant-\(X\) solution crosses \(f_R=0\) inside the complete static patch, while a direct substitution into the original radial equation exposes an unresolved exponent mismatch in the displayed power-law family. The results provide a model-independent static-sector diagnostic and a precise starting point for a future horizon-regular treatment of black-hole interiors.

We investigate whether squeezing can provide an observable signature of quantum gravitational waves. Because a realistic detector couples only to a particular wave-packet mode, squeezing in global source modes need not remain observable. We show that inflationary two-mode squeezing reduces to an unsqueezed thermal state in the accessible one-mode sector, phase incoherence washes out squeezing in stochastic backgrounds, and the limited coverage of the solid angle of detectors strongly suppresses squeezing from isolated sources. We then show that source squeezing is not essential, i.e., a quantized gravitational wave can generate a positive squeezing witness if the detector state is initially prepared in a squeezed state, whereas a classical external gravitational field cannot, producing only a displacement. However, the resulting signal is bounded by the extremely small graviton–detector coupling. Thus, detector squeezing can remove the need for squeezed incident waves, but not the suppression caused by weak gravitational interaction.

We propose an experiment to test Bell nonlocality, a genuine nonclassicality, for the gravitational field. Two masses with embedded entangled spins (e.g., two diamonds with their NV-centre spins entangled) are placed well outside each other's light cones, ensuring a locality-loophole-free scenario. The spins are then coupled to the motion of their respective masses to generate spatial superpositions. Finally, local measurements are performed only on the gravitational fields of the two masses. If gravity is quantum, then the two entangled masses would entangle their gravitational fields, leading to correlations certifying Bell nonlocality of gravity. Trapped and ground-state cooled nano-objects with micron-sized spatial superposition are sufficient for this test. This goes beyond the recent proposals to test nonclassicality of gravity by providing, for the first time in the literature, a minimal tool to (i) create Einstein-Podolsky-Rosen (EPR) state of gravitational curvatures, (ii) witness entangled gravitational curvatures, (iii) rule out any local-realist description of gravity, and (iv) achieve a loophole-free test of gravity's nonclassicality in a fully device-independent way.

We present a new avenue to the usual Schwinger effect via the open string pair production for a system of two Dp branes with $1 \le p \le 6$ in Type II superstrings. The two Dp are placed parallel at a separation with one of them carrying the most general constant electromagnetic background allowed for the pair production. By taking the so-called field theory limit, we obtain systematically, from the open string pair production rate, the respective explicit Lorentz invariant QED pair production rate in diverse dimensions for a pair of charged/anti-charged massive particles, which can be scalars, spinors or vectors.

In its standard black-hole realization, the Penrose process uses an event horizon to remove a negative-energy fragment. We show that, at the kinematic level, horizon absorption can instead be replaced by confinement within a compact ergoregion, without imposing an absorbing or reflecting inner boundary. For a broad class of regular, stationary, axisymmetric, asymptotically flat horizonless spacetimes, we prove that every smooth connected component of the spatial ergosurface is a compact torus, independently of the field equations and matter content. Future-directed geodesics with negative Killing energy encounter a forbidden neighborhood of each such boundary component and are therefore confined. We derive an exterior no-barrier condition and identify an open set of on-shell, future-directed, four-momentum-conserving splittings producing both a confined negative-energy fragment and an amplified partner. Under equatorial reflection symmetry, once the latter enters the exterior channel on an outward branch, it necessarily reaches infinity. A rotating boson star explicitly realizes the complete process without a horizon.

In the present paper, we derive a new charged black hole solution embedded in a quintessence field within a framework combining the Rastall and massive gravity in an asymptotically flat spacetime. We interpret the integration constant associated with the structural properties of the quintessence field as a thermodynamical variable. In this context, several limiting cases have been explored with the aim to shed light on the role of the model parameters. Then, we analyse the thermodynamic properties and stability of the charged black holes in flat spacetime, where the quintessence field is characterized by the state parameter \( \omega_q = -2/3 \). Detailed studies are perfomed to see how the various parameters affect phase transitions between small and large black holes. In particular, we establish a connection between the impact parameter and unstable circular photon orbits with the thermodynamic phase transitions. Our analysis suggest that the photon sphere observables could remarkably provide an indirect observational signature of black hole thermodynamic critical transitions. Generally, thanks to the effective contribution of the quintessence parameter, we found that the overall thermodynamic behavior basically resembles that of the charged RN–AdS black hole with thermal features analogous to of a Van der Waals fluid ones. Finally, we have checked these findings using thermodynamic geometry with the Quevedo metric. Our results showed that the critical behaviour derived from the standard thermodynamics is fully consistent with geothermodynamics description, where the phase structure and divergence points exactly coincide with those identified from heat capacity and Gibbs free energy analyses.

We construct and compare two charged rotating extensions of the Kerr–Levi-Civita geometry. In Einstein–Maxwell theory, a fixed magnetic Kerr–Newman Ernst representative is inverted, and covariance of the coupled Ernst equations, integrability of the dragging and electric-potential quadratures, and the field equations are verified. The resulting local line element agrees with a strong-field representative obtained independently, but no global equivalence of the azimuthal quotients is assumed. In low-energy heterotic string theory, the Hassan–Sen map is instead applied after the vacuum Kerr–Levi-Civita inversion, generating Maxwell, dilaton, and Kalb–Ramond fields. Both branches possess regular local Killing horizons. For the Einstein–Maxwell branch, an exact denominator factorization proves that the subextreme exterior contains neither Ernst zeros nor azimuthal closed timelike curves. The former Kerr ring has finite curvature, although it lies inside an interior region of timelike azimuthal orbits. Its Kretschmann scalar has the form $8\mathcal P_{\rm N}/\mathcal H^6$ and approaches a parameter-independent Levi-Civita law at fixed off-axis latitude. The heterotic branch is qualitatively different: the real component connected to the horizon terminates at a finite-radius $\Lambda=0$ surface. Exact string-frame slice factorizations and independent Einstein-frame calculations show that this surface is a curvature singularity rather than a conformal-frame artifact. Both families are generically Petrov type I. The results distinguish exact local solution generation from the unresolved construction of complete global spacetimes and the identification of possible distributional sources.

We investigate strong gravitational lensing by an Ellis-Bronnikov (EB) wormhole in the presence of effective field theory (EFT) corrections to photon propagation. We derive the modified photon propagation law induced by non-minimal couplings between the electromagnetic field and spacetime curvature, which leads to polarization-dependent photon trajectories in the EB wormhole spacetime. Using the strong deflection limit, we derive the corrections to the photon sphere and the logarithmically divergent part of the deflection angle. Although the EFT corrections are parametrically small, their contributions can become appreciable near the photon sphere. In particular, the $R_{\mu\nu}F^{\mu\rho}F^{\nu}{}_{\rho}$ interaction, which is nonvanishing for the EB wormhole but absent in the Schwarzschild spacetime, contributes to the strong-deflection coefficients. This may provide a distinctive lensing signature for observationally distinguishing wormholes from black holes.

The construction of accretion environments is fundamental to black hole imaging. From a purely geometric perspective, we construct a novel analytic accretion environment comprising a geometrically thick disk, ring-like bumps with a Gaussian profile, and localized compact emission regions modeled by Gaussian distributions. This environment offers high flexibility, enabling independent adjustments of disk thickness, vertical structure, and the positions and morphologies of localized spots, thereby allowing it to qualitatively mimic high-energy astrophysical phenomena. Applying this model to the Kerr spacetime, we investigate the resulting images via radiative transfer and ray-tracing simulations. The results validate the effectiveness of our accretion model and reveal novel observational signatures of Kerr black holes under multi-component illumination, including multiple bright spots and arc-like structures. This work provides a convenient and fully analytic framework for modeling accretion in curved spacetimes, and offers a new perspective on inferring accretion mechanisms and transient high-energy processes from image features.

Stochastic gravitational-wave backgrounds (SGWBs) provide a unique opportunity to probe both unresolved astrophysical populations and fundamental physics in the early Universe. Future space-based gravitational-wave (GW) detectors, such as LISA and TAIJI, will enable cross-correlation observations that are particularly sensitive to SGWBs and their polarization content. In this work, we investigate the capabilities of two proposed LISA-TAIJI networks, LISA-TAIJIp and LISA-TAIJIm, for detecting and discriminating tensor, vector, and scalar polarization components of isotropic SGWBs. Using the cross-correlation between the two networks, we evaluate their sensitivities and signal-to-noise ratios for SGWBs containing different combinations of polarization sectors. We find that the LISA-TAIJIm configuration, which has a larger relative inclination between the two constellations, exhibits substantially weaker polarization degeneracies and significantly improved performance in separating polarization components, particularly for tensor-vector-scalar backgrounds. Fisher-matrix forecasts for power-law SGWBs show that the two configurations yield broadly comparable constraints on spectral parameters, reflecting the different roles of component separation and parameter estimation. Our results demonstrate that large-angle LISA-TAIJI networks provide a more favorable geometry for model-independent SGWB polarization measurements.

Recent results from the Dark Energy Spectroscopic Instrument (DESI) have suggested that dark energy, long considered to be a cosmological constant, may actually be \lq{}dynamical\rq{}. To clarify what follows from these results, we distinguish three claims: (D1) the data disfavour the \LCDM expansion history; (D2) within the phenomenological Chevallier–Polarski–Linder (CPL) parametrisation, the data prefer $(w_0,w_a)\neq(-1,0)$; and (D3) there exist one or more genuine dynamical dark-energy degrees of freedom in the matter-sector, within GR and coupled to the standard sectors only gravitationally, whose dynamics account for the departure from the \LCDM expansion history. We demonstrate that DESI establishes D2, which supports D1, but does not establish D3. The \lq{}designer\rq{} construction in $f(R)$ gravity provides a gravitational realisation of the same background history, so that no observable determined solely by that history can discriminate, even in principle, between its \lq{}matter\rq{} and \lq{}gravitational\rq{} readings. Furthermore, non-minimally coupled models admit regular effective phantom-crossing realisations of the relevant phenomenology, while the selected crossing places additional pressure on a single-component, minimally coupled realisation of the matter-sector reading. We characterise the resulting underdetermination between matter and gravitational interpretations—exact at the level of background history, but breakable beyond it—and draw the corresponding norm for survey reporting.

We present an analysis of the perturbative modes of dissipation in the minimal warm inflation model where an axion-inflaton is coupled to a gluon thermal bath through the axion-gauge coupling. The corresponding friction coefficient, which generically is governed by IR physics, for the processes we consider is dominated by Landau damping of soft space-like gluons and the plasmon decay of hard on shell gluons. Since both of these processes are IR dominated, the friction coefficient becomes sensitive to the non-perturbative magnetic mass scale $m_{g}^{-1} \sim (\alpha T)^{-1}$ which in turn leads to the friction coefficient's overall linear proportionality to $\alpha$, which is much less suppressed in $\alpha$ powers compared to the well understood term from Chern-Simons (CS) diffusion $(\propto \alpha^5)$. On the other hand, this contribution to the friction coefficient is suppressed due to slow roll. We show that this can be viewed as a suppression of local spontaneous CPT violation for derivative couplings in general and that time correlations of nonlocal variables which do not fall off at large time separations are what allow an unsuppressed friction coefficient.

We investigate the thermodynamics of the apparent horizon in an exponential $f(Q)$ gravity model characterized by the two parameters $b$ and $n$, within a spatially flat Friedmann–Lemaître–Robertson–Walker background and the coincident gauge. Focusing on the $n=1$ solution, we use the approximate cosmological solution for the Hubble parameter up to second order in the exponential parameter $b$ to study the redshift evolution of the apparent-horizon radius and the Kodama–Hayward temperature. We formulate Hayward's unified first law in terms of the Misner–Sharp–Hernandez mass, the work density, and the energy-supply vector, and show that the horizon dynamics admits an equilibrium thermodynamic description. The associated entropy differential is proportional to $f_Q+2Qf_{QQ}$, yielding exponentially suppressed corrections to the Bekenstein–Hawking area law and recovering $S=A/4$ in the limit $b\to0$. We then examine the generalized second law (GSL) by including the entropy of matter inside the apparent horizon. When the matter and horizon temperatures are identified, we adopt the GSL viability criterion $f_Q+2Qf_{QQ}\geq0$ previously derived in the literature. The observationally motivated best-fit values of $b$ satisfy this condition over the redshift interval studied and produce departures from $\Lambda$CDM mainly at late times. In contrast, sufficiently large positive values, approximately $b>0.26$, can violate the GSL in the future region $z<0$. These results show that apparent-horizon thermodynamics provides a complementary constraint on the parameter space of exponential $f(Q)$ cosmology.

Relativistic quantum-information studies in noninertial and black-hole settings often determine resource behavior through explicit calculations for particular input states and state functionals, leaving unclear whether the recurring monotonic trends originate from those choices or from a common underlying structure. In this work, we formulate the effective single-mode fermionic horizon transformation as a pair of complementary exterior and interior quantum channels, corresponding respectively to the physically accessible and inaccessible sectors, and establish exact post-processing orders in opposite directions. As the relativistic channel parameter $q$ increases, the exterior channel becomes progressively degraded, whereas the interior channel is ordered in the reverse direction. These relations extend to arbitrary multipartite settings. Consequently, every state functional that is non-increasing under the corresponding intermediate maps is non-increasing in homogeneous exterior sectors and non-decreasing in homogeneous interior sectors. The framework therefore applies to broad classes of quantum resources and correlations, including entanglement and occupation-basis coherence monotones, optimized Bell-functional quantities, and contractive-divergence correlation measures. We further numerically evaluate collective coherence based on the quantum Jensen-Shannon divergence (QJSD) in the Garfinkle-Horowitz-Strominger (GHS) dilaton-black-hole spacetime, illustrating the predicted homogeneous monotonicity. The recurring trends are therefore traced to a common channel-ordering structure, while the physical setting determines the parameterization of $q$ and the resource-theoretic monotonicity determines which output-state quantities inherit the order.

Although the Alcubierre Warp Drive is theoretically capable of providing faster-than-light travel, it may be difficult to use for this purpose. But is it useful for slower-than-light travel? We begin by observing the that the warp bubble will act to protect the ship from dust particles and other space debris (a potentially serious hazard even at 10% the speed of light). We then explore several modifications of the Alcubierre Warp Drive, e.g. a "deflector shield", with the perspective of keeping a ship safe from collisions with particles, projectiles, rogue planets, and other dangers of space travel.

We construct a novel class of black bounce solutions within General Relativity, sourced by a canonical scalar field non-minimally coupled to linear electrodynamics, establishing a self-consistent framework in which regular black holes and traversable wormholes are supported by ordinary bulk matter, with the necessary exoticity confined to an infinitesimally thin defect at the bounce.

We challenge the common belief that if there is no absolute time parameter in physics, a quantum system described without reference to an external clock can be assumed to be in a stationary state of its Hamiltonian. We present a time-reparameterisation invariant quantum evolution law, which for a given initial condition predicts the same trajectory in state space as the Schrödinger equation, except that for nontrivial trajectories it does not predict the speed at which the trajectory is traversed. The solutions of this evolution law are all time-reparameterised solutions of the Schrödinger equation. We show how the predictions of the Schrödinger equation are recovered relative to an internal clock in this framework. In contrast to the Page-Wootters formalism or Dirac's quantisation of the Hamiltonian constraint, here the global sate is not stationary. We discuss the assumptions leading to the common conclusion that the state can be taken stationary and suggest that they need to be revisited.

We present a numerical method that constructs geometrically defined coordinates on black-hole horizons, from which the multipole moments are computed without assuming axisymmetry. This method, which we denote the conformal-mapping method (CMM), provides a numerical realization of the conformal construction proposed by Ashtekar et al. in 2022, combining discrete Ricci flow, spectral embedding onto the unit sphere, and Möbius gauge fixing by the vanishing-area-dipole condition. We first test the CMM against analytic Kerr benchmarks, and then apply it to an equal-mass, non-spinning binary black-hole merger. We also compare it with an approximate-symmetry-based method. The CMM allows the multipole moments to be expressed in a fixed reference frame, whereas the symmetry-adapted frame can reorient abruptly when the preferred approximate axis changes. In a frame aligned with the orbital angular momentum, the amplitude of the quadrupole mode grows during inspiral and decays after merger, displaying a qualitative ringdown behavior. These results show that the CMM is a useful tool for studying horizon geometry in dynamical situations where no stable symmetry axis is available.

We consider the properties of the gravitational radiation emitted by massive stars in binary systems with time varying mass, orbiting around each other in Keplerian orbits under the influence of the gravitational force. When this effect is combined to the quadrupole expression of the gravitational radiation, some supplementary terms in the standard formulae describing radiative effects do appear. By using the generalized quadrupole formalism, with time dependent masses, we obtain the expressions describing the time variation of the angular momentum, semimajor axis, eccentricity and periastron shift of the binary system. As a simple application of the developed formalism we consider in detail two cases of the mass variation, by assuming a linear and an exponential time dependence, respectively. The gravitational radiation characteristics corresponding to these models are fully investigated numerically with the help of a dedicated software package \href{https://github.com/croi900/GRAV-T}{GRAV-T}, specifically developed for the study of the gravitational radiation of the mass varying systems. The impact of the gravitational wave emission on the merger of two magnetars losing mass due to neutrino heated winds is also investigated in detail. The obtained results could be important for the understanding of general relativistic effects in the case of the variation of the gravitational mass with time, which sensitively influences the properties of the gravitational radiation emission.