Papers

This work investigates whether the trace anomaly can diagnose the stellar response to a topological scalar field in tensor multi-scalar gravity. Eleven cold tabulated equations of state (EoSs) were examined, with six selected after conservative GR stability and speed of sound checks over the tabulated density range. Their fiducial topological configurations were compared with the corresponding GR models under complementary matching prescriptions. After controlling for stellar mass and EoS dependence, the GR trace source strength $S_T$ remained strongly correlated with the topological mass response, with a partial Spearman coefficient $\rho = 0.975$ within the sampled fiducial sequences. Near $1.4\,M_\odot$, the topological configurations were systematically more compact, with reductions in the Jordan frame radius of $6.7$–$8.7\%$ at fixed baryonic mass and $0.87$–$1.29\,\mathrm{km}$ at fixed gravitational mass. These shifts are comparable to current uncertainties from the Neutron Star Interior Composition Explorer (NICER) and their observational impact depends on the source. Despite the global deformation, the interior effective source remained dominated by matter, with a median topological contribution of about $0.8\%$. The GR matter trace therefore emerges as a useful diagnostic of the fiducial topological stellar response.

We study semiclassical tunneling in scalar field theories with slowly evolving homogeneous backgrounds. We formulate the problem by combining tadpole subtraction with an adiabatic expansion: the former maps the rolling system to an instantaneous multi-field tunneling problem, while the latter systematically tracks the slow evolution of the background. This formulation is particularly useful for tunneling during slow-roll inflation, where the tunneling direction may be transverse to the rolling inflaton direction. For a time-reflection-symmetric bounce on a fixed on-shell geometry, we show that the real tunneling exponent receives no direct correction linear in the rolling velocity. The tunneling rate nevertheless drifts along the rolling trajectory because the frozen tadpole-subtracted potential evolves. We also find that the exponent approaches a finite but nonanalytic limit as the tangential curvature vanishes, suggesting that a weakly tachyonic tangential direction can yield an exponent of the same order.

Warm inflation in $F(\Phi)R$ scalar-tensor theories of gravity is investigated in the `defining' frame, where the theory and its parameter values are specified. Translating the resulting dynamics to the Einstein frame, we find that the dissipation ratio is suppressed by the modified-gravity effects. Thus, although the effective warm-inflation dynamics can be consistently analysed in either frame, the dissipative regimes need not coincide between them. In particular, we find that quantum perturbations can dominate over thermal fluctuations in the scalar power spectrum even in a high-temperature, strong-dissipation regime in the defining frame. Finally, we compute the scalar spectral index and tensor-to-scalar ratio for a non-minimal coupling function $F(\Phi) = 1+\xi (\Phi/m_{\rm P})^2$ with a quartic potential and both constant and quadratic field-dependent dissipation coefficients, and identify benchmark points compatible with current CMB constraints.

Kilonovae provide a key electromagnetic window into binary neutron star mergers, revealing the properties of the merging system while probing r-process nucleosynthesis of the Universe. However, only a few kilonova candidates have been detected to date, with AT2017gfo being the only one associated with a gravitational wave event, GW170817. In this work, we present Genova, a probabilistic framework for predicting kilonova spectra and light curves directly from gravitational wave posterior samples of binary neutron star mergers. This method uses a conditional normalising flow to learn the distribution of rest-frame spectra conditioned on the source-frame component masses, tidal deformabilities, viewing angle and time since merger. Other kilonova model parameters, such as ejecta opacities, are marginalised over during training, so that their effects are propagated into the predicted spectra as predictive uncertainty. In the self-consistency test, the flow model reproduces the median light curves with residuals typically below $\sim 0.1$ mag, and the ratio of the predicted central 68% interval widths remains predominantly between $0.8$ and $1.4$ over $\sim 0.4$–$8.0$ days. Comparisons with a physically distinct kilonova model show that the probabilistic prediction can remain informative beyond the model used for training. We apply Genova to GW170817/AT2017gfo using multi-band observations, including newly re-reduced $Y$-, $J$-, $K_s$-band photometry from the Visible and Infrared Survey Telescope for Astronomy, which we present in this work. The resulting predictive intervals broadly encompass the observations while capturing both gravitational wave posterior uncertainty and the variation induced by marginalised kilonova model parameters.

The worldwide network of gravitational-wave detectors have detected more than 350 binary coalescence events till date. Future third-generation detectors, like Einstein telescope, are expected to detect orders-of-magnitude more signals from sources with more complicated characteristics, including eccentric orbits and high-mass ratio binaries. It is well-established that the computational cost of parameter estimation for signals from these kinds of sources will be extremely high. In particular, the process could be sped-up if generating theoretical waveform predictions, used for likelihood calculation becomes faster. Recently, various machine-learning techniques has been proposed to this end. In this work, we propose a two-stage deterministic conditional-autoencoder model for generating four-parameter SEOBNRv4 waveforms. The first-stage of the model generates amplitude and phase series of the waveform, while the second-stage calibrates the residual error in the predictions. Our model achieves a median mismatch of around $10^{-2}$ with the target polarization waveforms, while the calibrated amplitude/phase series achieve $10^{-6}$ level cosine distance error. We then propose a waveform conditioning step to enable use of these surrogate waveforms for downstream parameter estimation tasks. Finally, we perform extensive parameter estimation tests, with ML and EOB waveform injections and try to recover posterior estimates for the source parameters. We find that when ML waveforms are used to recover EOB target parameter estimates, the inferred posterior have some systematic bias. This inherent bias can be estimated and corrected for, and then importance reweighting of posterior samples can enable use of low-accuracy surrogate waveforms at low SNRs.

We use lattice methods to study inflation in the scalar Gauss-Bonnet (SGB) gravity theory. We focus on the ultra-slow-roll scenario with the peak frequency falls into the PTA band. In the parameter range we consider, we find that the lattice results exceed the perturbative predictions, which differs from the case in Einstein gravity. We further find that lattice corrections become significant when the peak of the primordial curvature spectrum reaches $\sim 10^{-2}$. We calculate the energy density spectra of second-order scalar induced gravitational waves (SIGWs) using the primordial power spectra obtained from both the lattice method and the traditional perturbative method, and analyze the SGB model in light of current pulsar timing array (PTA) observations. Our results indicate that lattice corrections enhance the ability of the SGB model to dominate the PTA observations.

We develop a semi-analytic and semi-numerical formulation of the harmonic-gauge second post-Newtonian (2PN) equations of motion for a general point-mass N-body system within Hadamard regularization. The equations of motion are separated into a closed analytic contribution and a non-closed integral contribution. We analyze the singular structure of the latter and further regularize it into a numerically evaluable representation. We apply the formulation to the Sun-Jupiter-Saturn and Sun-Mercury-Venus systems, evaluating the instantaneous non-closed 2PN acceleration along Newtonian trajectories and its leading finite-time relative-distance response through the corresponding perturbation equations. In both benchmarks, the non-closed acceleration remains a small fraction of the complete 2PN acceleration, while the induced relative-distance perturbation remains oscillatory and can reach larger oscillation amplitudes at later times.

We construct an analytical relativistic two fluid star characterized by a mixed core, where ordinary matter and dark matter coexist as two independently conserved incompressible perfect fluids, and an envelope composed exclusively of ordinary matter. The fluids exchange neither matter nor energy and interact only through the common spacetime geometry, with the ordinary component extending across the core envelope interface while the dark component is confined to the core. Despite the mixed core–single-fluid envelope structure and the internal interface, the system remains analytically tractable, allowing us to obtain explicit expressions for the pressures and metric functions and to follow directly the effects of the dark-matter fraction and relative core size. We determine the physically admissible parameter space and derive a Buchdahl like critical compactness associated with the divergence of the central pressure, whose value depends on the relative dark matter density and the size of the mixed core. The Schwarzschild constant density star and its standard critical value, $2M/R=8/9$, are recovered in the corresponding one fluid limit. The mass–radius analysis further shows that configurations with the same global compactness can correspond to distinct internal matter distributions. Beyond providing an analytically controlled description of a core-confined second component, the construction offers a useful benchmark for identifying qualitative trends that may subsequently be examined in more realistic dark matter admixed neutron star models, whose detailed treatment lies beyond the scope of the present work.

Buchdahl transformations provide a geometric route from vacuum spacetimes to scalar-field solutions. We formulate this mechanism as a target-covariant solution generator for Einstein gravity coupled to nonlinear sigma models in $D>3$ dimensions. On any connected region in which a Ricci-flat seed admits a non-null hypersurface-orthogonal cyclic coordinate with fixed-sign nonzero norm, the logarithm of that norm defines a Buchdahl potential $\Sigma$. A one-parameter redistribution of the cyclic norm then generates the exact Ricci source $R_{AB}=C_D(\beta)\partial_A\Sigma\partial_B\Sigma$, with $C_D(\beta)=\frac{D-2}{D-3}(1-\beta^2)$, while $\Sigma$ remains harmonic in the deformed metric. Composing $\Sigma$ with an affinely parametrized geodesic of a non-degenerate target metric whose squared speed is $C_D(\beta)$ therefore yields an exact Einstein–sigma-model solution. For positive-definite targets, the corresponding pullback condition also implies that the nontrivial scalar map has one-dimensional image; for indefinite targets, the geodesic ansatz remains a sufficient solution-generating sector but need not exhaust all possible realizations of the same rank-one spacetime source. Through the Einstein/Jordan-frame correspondence, the construction gives vacuum tensor–multiscalar solutions as well. We illustrate the unified generator with flat, spherical, hyperbolic and axion–dilaton targets and with Tangherlini, Weyl, Kasner, Rosen-wave, Gowdy, Kaluza–Klein bubble and C-metric seeds. The result separates cleanly the geometric Buchdahl source from its target-space realization and extends this rank-one cyclic sector across static, cosmological, wave and accelerating geometries.

Spin inversions appear in several settings. Analytical work predicts a single spin flip during unequal-mass supermassive binary inspiral, while numerical relativity and post-Newtonian calculations show repeated flip-flop motion in comparable-mass binaries. Secular spin evolution also predicts additional cases driven by spin-induced mass quadrupoles. Whether these effects can be distinguished in gravitational-wave data is still unclear. We combine the secular spin angle equations with a quasi-circular second post-Newtonian frequency evolution and build a restricted waveform weighted by the sky-averaged LISA sensitivity. We study five near-equal-mass injections with detector-frame total mass $M=2\times10^5M_\odot$, including Kerr flip-flops and one quadrupole-induced case. Each injection is compared with physically evolving waveforms constrained to have no orbital-plane crossings. We search all four no-inversion sectors, vary the masses, spin magnitudes, and initial spin angles, and maximize over time, phase, and overall amplitude. Large spin motion does not by itself lead to a clearly different waveform in this restricted model. A weak spin that sweeps through $133.26^\circ$ leaves a residual SNR of $0.806$ at reference signal SNR $\rho_\star=100$. A case where both spins cross the orbital plane many times gives the largest residual, $1.641$. The quadrupole case has a secondary-spin range of $128.11^\circ$ with five crossings, yet the best no-inversion candidate found leaves a residual SNR of only $0.454$. All largest matches found exceed $0.999865$. Within this restricted model, the spin inversion cases are therefore strongly degenerate with no-inversion binaries at $\rho_\star=100$. More complete waveforms, including observer-frame precession modulations, higher harmonics, separate polarizations, and the full LISA response, are needed to test whether this degeneracy can be broken.

Freezing gravity (FG), a phantom-crossing dark energy model, was recently proposed in the absence of matter. In this work, we study FG minimally coupled to a perfect fluid through a detailed cosmological perturbation analysis and derive the conditions for the absence of ghost and gradient instabilities. The ghost stability conditions are obtained analytically without taking the high-$k$ limit, yielding the ghost-free range of scales and a finite cutoff when it exists. In the branch where a finite cutoff is already present in vacuum, we find that matter coupling lowers it relative to its vacuum value. Moreover, in the high-$k$ regime, the gradient stability condition guarantees a positive effective gravitational coupling for cold dark matter. We further show that two characteristic properties of FG persist in the presence of matter. First, the background and perturbation sectors remain separated, in the sense that they are controlled by independent sets of parameters, allowing FG to realize arbitrary background evolutions, including phantom crossing, while maintaining stable perturbation dynamics. Second, the scalar degree of freedom of FG becomes non-dynamical in the large-scale limit at linear order, while propagating with a finite speed of sound on small scales. Whether this behavior persists to arbitrarily high orders in perturbation theory, thereby avoiding the potential strong-coupling issue, requires further investigation through a nonlinear perturbation analysis. To make contact with large-scale structure and gravitational lensing observables, we also derive the effective gravitational coupling and gravitational slip within the quasi-static regime and express our results in terms of the EFT parameters.

This work explores asymptotically de Sitter black-bounce spacetimes, obtained as a regular extension of the Schwarzschild-de Sitter geometry via the Simpson-Visser prescription. The resulting family of solutions provides a smooth interpolation between regular black holes, extremal configurations, and traversable wormholes within a single geometrical framework. In particular, we verify the regularity of the spacetime by showing that (some of) curvature invariants, including the Kretschmann and Ricci scalars, remain finite at the origin for any non-vanishing value of the regularization parameter $a$, ensuring geodesic completeness. An analysis of the effective stress-energy tensor reveals a violation of the null energy condition, which can be consistently interpreted in terms of a composite source involving a phantom scalar field coupled to nonlinear electrodynamics. Subsequently, we investigate the observational signatures of these geometries by studying: i) null geodesic propagation and ii) constructing the associated optical appearance. In particular, we determine the shadow radius and characterize the structure of photon rings through numerical ray-tracing procedure. Finally, we compute the spectrum of axial gravitational perturbations using a pseudospectral approach, obtaining the quasinormal mode frequencies and their overtones. Our results show a non-trivial interplay between the cosmological constant and the regularization scale $a$, which imprints itself on both the light-ring structure and the quasinormal spectrum. These features suggest potential observational discriminants that could distinguish these regular spacetimes from their singular counterparts in future gravitational-wave and high-resolution electromagnetic observations.

We study singular generalized Dirac-oscillator couplings in the near-horizon region of a nonextremal BTZ black hole. An effective covariant construction adapted to the stationary horizon congruence relates the two singular profiles to the proper acceleration and its radial gradient, whose leading near-horizon behaviors are $1/\rho$ and $1/\rho^2$. This motivates the family $\mathcal{L}_p(\rho)=\lambda_0+\beta_p/\rho^p$, with $p=1,2$. The $p=1$ system has a regular singular point and is Fuchsian, whereas $p=2$ is the first irregular case; more generally, a leading $\rho^{-p}$ interaction with $p>1$ has Poincaré rank $p-1$. For $p=1$, a constant spinor transformation reduces the matrix-Coulomb system exactly to the Whittaker equation, while elimination in the original basis gives an equivalent confluent-Heun representation with an apparent finite singularity. For $p=2$, the local branches contain the essential factors $e^{\pm\beta_2/\rho}\rho^{-1/2}$, and their coefficient expansions exhibit generic factorial late-order growth. The finite-radius response $\mathcal{Z}_p(\Omega;\rho_0)=\psi_{2,p}(\rho_0)/\psi_{1,p}(\rho_0)$ obeys an exact Riccati equation whose large-damping expansion first becomes sensitive to the radial gradient of the interaction at order $\gamma^{-3}$. Even when the two couplings are matched at $\rho_0$, this response retains information about their different pole orders. The response functions provide local spinorial data for subsequent matching to the complete BTZ exterior.

We investigate a charged anisotropic radiating stellar configuration undergoing gravitational collapse in the presence of shear and heat flux within the Einstein Maxwell framework. The interior spacetime is described by a time dependent spherically symmetric geometry and is matched to an exterior charged Vaidya spacetime. The electromagnetic field is incorporated explicitly through Maxwell equations, allowing the electric charge to contribute to the matter variables, mass function, and boundary evolution. The charged junction condition is reduced to a Riccati type differential equation with suitable transformations and exact solution is obtained. The physical properties of the resulting shearing solution are examined through the energy density, radial and tangential pressures, pressure anisotropy, heat flux, electric charge, energy conditions, sound speeds, Herrera cracking criterion, and complexity factor. The energy density and radial pressure remain positive and decrease towards the stellar surface, whereas the tangential pressure remains negative, confirming the anisotropic character of the configuration. Heat transport and electromagnetic effects are strongest in the inner stellar region. The energy and causality conditions are satisfied. The cracking function indicates potential stability against cracking. The complexity factor remains positive, with electric charge providing an additional contribution alongside pressure anisotropy, density inhomogeneity, and dissipative heat flux. These results provide a comprehensive picture of the physical behavior and internal structure of the charged shearing radiative collapse model.

De Sitter unitarity and reflection positivity are logically independent. We study their relation for a free scalar field on a fixed de Sitter background, restricting attention to the one-particle sector. Motivated by Witten's de Sitter pairing and the radial adjoint of Bousso, Maloney and Strominger, we compose the invariant Hilbert product with equatorial reflection. In the standard principal-series realization, this form vanishes identically and its Osterwalder-Schrader quotient contains no states. For the complementary series, the reflected product becomes a nonzero reflected kernel on the unit ball. A result of Neeb and Ólafsson shows that this kernel is positive semidefinite precisely when the lower weight satisfies the scalar conformal unitarity bound. The resulting Hilbert space with the reflected inner product furnishes a positive-energy representation of AdS isometries according to Neeb and Ólafsson. We identify that the range coincides with the Klebanov-Witten alternative-quantization window from physics point of view. This correspondence suggests a novel relation from the representation viewpoint between AdS and dS without the conventional analytic continuation of radius.

The post-Minkowskian expansion of general relativity expands the metric in Newton's constant G around flat space-time, but much effort currently focuses on ways to extend its range of applicability by summing the perturbative series to all orders. A regime of particular interest is a specific high-energy limit where a formula derived recently provides the momentum kick of binary scattering problem to all orders, and even suggests an approach towards strong coupling. We show that this is an asymptotic expansion. Although the series diverges we give evidence that it is Borel summable, and the full function in integral form is finite. As a by-product we determine the transcendentality properties of the coefficients in the expanded expression.

Dimensionless observables of neutron stars, such as the moment of inertia, the tidal deformability, the spin-induced quadrupole moment, and the compactness, satisfy the I–Love–Q and Love–$C$ universal relations to percent-level accuracy over a wide range of equations of state. We investigate analytically the origin of this insensitivity in the stellar structure equations. We derive asymptotic expansions of these relations directly from the differential equations and boundary conditions for slowly rotating, tidally deformed stars described by a general piecewise-polytropic equation of state. Although the differential equations allow several forms of the asymptotic expansion, we find that only one class is consistent with the observed universality, and we use this class to analyze the universal relations. Because the observables are determined at the stellar surface, information about the deep-interior equation of state can enter them only through the parameters that survive in the asymptotic expansion at the surface. We find that the universal relations depend only on three parameters: two integration constants and one polytrope index of the outermost segment. By determining how these parameters deform the relations, we show that, throughout the region of parameter space occupied by realistic equations of state, the resulting deviations remain at the percent level, consistent with the observed accuracy of the universal relations. Within the same formalism, we classify violations of universality according to the additional degrees of freedom or input data responsible for them. Since the construction relies only on the underlying differential equations and boundary conditions, the same procedure can be applied to other systems once the corresponding equations and boundary conditions are specified.

In previous work arXiv:2604.00160, we found soliton solutions in a deformation of the sine-Gordon theory in AdS spacetime that, in the infinite-radius limit, reduce to single-soliton solutions in flat space. In this paper, we find another single soliton solution that has no analog in flat space.

Quintessential $\alpha$-attractor models of single-field inflation and evolving dark energy were constructed about a decade ago. Recently, it was pointed out that some of them might be disfavored due to dark-radiation constraints on gravitational waves and the higher values of $n_s$ favored by ACT. Here we present a class of updated quintessential $\alpha$-attractor models in which a single field describes both inflation and evolving dark energy, yields higher values of $n_s$, and admits reheating scenarios consistent with the dark-radiation bound on $\Delta N_{\rm eff}$. Depending on the value of the cosmological constant $\Lambda$, these models interpolate between $\Lambda$CDM with $\Lambda > 0$ (future dS universe), dynamical dark energy with $\Lambda = 0$ (future Minkowski universe), and dynamical dark energy with $\Lambda < 0$ (future cosmological collapse). We also study quintessential $\alpha$-attractor models based on an axion-inflaton complex scalar field with hyperbolic geometry, which describe inflation and dark energy of a “phantom illusion” type compatible with DESI DR2.

We use physics-informed neural networks (PINNs) to solve the gravitational quasinormal-mode (QNM) eigenvalue problem for Kerr spacetime directly in the two-dimensional hyperboloidal formulation of the Teukolsky equation. This formulation does not require separation of variables and thus retains the coupled radial–angular structure. Such a scheme provides a prototype for calculating the QNMs of beyond-Kerr black holes for which the perturbation equations are non-separable. Sequences with increasing angular momentum are constructed, reaching close to the extremal limit. We focus on the fundamental modes $(\ell,m,n)=(2,0,0)$, $(2,1,0)$, $(2,2,0)$, $(3,3,0)$ and $(4,4,0)$, together with the first overtone $(2,2,1)$. Independent benchmark evaluation shows that every reported real and imaginary frequency component remains below $0.5\%$ error, with a median deviation of $0.1\%$. This accuracy is maintained in the near-extremal regime, where the damping rate becomes small and the modes are longest-lived. The results establish a non-spectral numerical route to multidimensional black-hole perturbation eigenproblems which does not match the substantially higher precision of dedicated Kerr solvers but offers greater flexibility and requires less analytical pre-processing. Non-separable rotating backgrounds and coupled systems, such as gravitational–electromagnetic Kerr–Newman perturbations, are natural extensions of the same construction.

Several recent observational analyses have suggested the possible existence of anisotropy in our Universe. It is often thought that this would contradict the inflationary picture. We have argued that this is not the case: if we relax the artificial restriction that spacetime torsion must be zero, then such anisotropies arise naturally, indeed almost inevitably, in a theory of torsional Inflation. This is seen most clearly if one uses a “$1\,+\,3\,$” analysis of a torsional spacetime, leading naturally to concepts of intrinsic torsion and extrinsic torsion. An anisotropic quantum fluctuation of the inflaton leads to an extrinsic torsion field which is anisotropic and which grows throughout the inflationary era, even though the intrinsic geometry of the spatial sections is isotropised in the conventional manner. This leads to an anisotropic Hubble field at the end of Inflation. The anisotropy takes a very specific and characteristic form, which in principle could be observable on relatively short cosmic scales. On the other hand, an unusual feature of torsional geometry leads to a short interval, early in the inflationary era, during which there is a torsion-induced energy flux, which leaves an anisotropic signal in superhorizon modes and thus ultimately at very large scales in the present Universe. Thus in principle torsion can explain anisotropies both on relatively short cosmic scales and also at the largest scales.

Gravitational microlensing can provide a possible observational method for distinguishing between the signatures of massive and massless phantom wormholes. In this work, we consider Galactic microlensing by the bounded Lobo-Parsaei-Riazi phantom wormhole (LPR), assuming source stars located in the Galactic Bulge and in the Large Magellanic Cloud (LMC). We derive the weak-field deflection angle up to fourth post-Newtonian order and compute the Einstein radius, Einstein-radius crossing time, and idealized point-source Paczyński-type light curves. We also estimate the optical depth and event rate in a simplified model in which the wormhole lenses are assumed to be gravitationally bound to the Galaxy. The LPR parameter $\gamma$, which is related to the radial equation-of-state parameter by $\omega = 1/ \gamma$, affects the ADM mass, the Einstein radius, and the microlensing timescale. In the massive phantom branch $-1 < \gamma < 0$, the leading deflection term is proportional to $1/r$, and the resulting point-source light curves are Paczyński-like. The massless comparison case $\gamma = 1$ is qualitatively different because the leading $1/r$ term vanishes and gutters may appear. The idealized observational predictions are compared with those for a Schwarzschild black hole.

We study tensor perturbations propagating through a viscous relativistic medium in a spatially flat FLRW universe, with particular emphasis on the correction produced by spatial free streaming beyond a local causal relaxation model. We start from the relaxation-time Boltzmann equation used by Baym, Patil, and Pethick to describe the response of matter to a gravitational wave. In the zero-streaming limit, the tensor stress obeys a Maxwell-Cattaneo, or linear Muller-Israel-Stewart, relation. We then retain the spatial streaming term and evaluate the resulting angular response analytically for an ultrarelativistic isotropic medium. The resulting kinetic response is coupled to the tensor Einstein equation and solved numerically in a flat matter-plus-Lambda background, with comparison to the local MIS propagation model and an independent kinetic WKB calculation. For the illustrative normalization considered, the full numerical calculation produces a nonmonotonic correction to the tensor power transfer function, with a maximum of about 0.8 percent near x equal to 0.37 and a minimum of about minus 5.6 percent near x equal to 1.97. These features are stable under angular-resolution and ODE-tolerance tests and are reproduced by the kinetic WKB calculation. We also find a corresponding finite-momentum phase correction. The effect is a property of the specified single-relaxation-time kinetic model and should not be interpreted as a universal transport law.

We develop a stochastic extension of the Wheeler–DeWitt equation in FLRW minisuperspace and show that quantum backreaction can dynamically regulate the big bang singularity without imposing external boundary conditions. Using Laplace–Beltrami quantisation and an open-system treatment of coarse-grained graviton modes, we obtain a stochastic Hamiltonian evolution equation in which the diffusion coefficient takes the form $\sigma(a)\propto a^2$. This multiplicative noise vanishes at the origin and renders $a=0$ an entrance boundary in Feller's classification, leading to super-exponential suppression of the Laplace–Beltrami weighted stationary density and zero probability flux into the singular point. At large scale factor, the global behaviour depends on the cosmological sector: de Sitter and positive potential-dominated regimes exhibit power-law stationary tails, whereas confining potentials or negative effective cosmological constant lead to an entrance boundary at infinity and a globally normalizable steady state. Taken together, these results indicate that stochastic backreaction arising from semiclassical coarse-graining provides a consistent and dynamical mechanism for singularity avoidance in minisuperspace quantum cosmology.

In this paper we employ the open quantum system framework with the influence functional formalism to non-perturbatively analyze the response of an Unruh-DeWitt detector modeled as a harmonic oscillator which interacts with a massless scalar field in impulsive plane wave spacetimes. Subtracting the Minkowski results, we obtain the expectation values for $\langle Q^2 \rangle$, $\langle P^2 \rangle$, and $\langle \{Q,P\} \rangle$, along with transition probabilities $P_{0\rightarrow 1}$ from ground state to the first excited state of the detector due to the influence of the wave. Explicit calculations are performed for both a pure gravitational wave (vanishing Ricci tensor) and a null electromagnetic wave (vanishing Weyl tensor). In both scenarios, the wave suppresses excitation transitions. Since our approach is non-perturbative, we are able to consider cases with both weak and strong coupling constants.

We investigate static, spherically symmetric regular black holes with anti-de Sitter (AdS) asymptotics in arbitrary spacetime dimensions. They are solutions of Einstein gravity, sourced by an anisotropic fluid whose radial pressure corresponds to vacuum energy, while the tangential pressure satisfies a generalized polytropic equation of state. By solving the Einstein field equations, we derive a generic class of asymptotically AdS black hole solutions and determine the conditions required for spacetime regularity. We then investigate the dynamical formation of these regular black holes within the thin-shell formalism, assuming a linear barotropic equation of state for the shell matter. Next, we study the thermodynamics of the regular AdS black holes in arbitrary dimensions by verifying the first law of black hole thermodynamics and the corresponding Smarr relation. We analyze the thermodynamic stability and phase structure of solutions in four, five, and six spacetime dimensions, demonstrating the existence of dimension-dependent phase transitions.

Inspired by Mashhoon's bound state method, we propose a new bound state method for computing quasinormal modes (QNMs). By a two-step coordinate transformation where a real parameter $\alpha$ is introduced, a QNM problem is mapped to a bound state problem, whose eigenvalues $E_n(\alpha)$ are inversely mapped to the QNM frequencies $\omega_n$ via analytic continuation. With this method, we numerically calculate various QNM frequencies for a Schwarzschild black hole directly from the bound state spectrum of the inverted Regge-Wheeler potential for the first time. It is found that the method yields QNM frequencies of high accuracy for low-lying modes with overtone $n\leq\ell$ ($\ell$ is the multipole number), while the accuracy degrades or the calculation fails for higher overtones. To identify the origin of this limitation, we analyze the singularity structure of the eigenvalues $E_n(\alpha)$ using Padé approximants in the complex $\alpha$-plane. For higher overtones, the singularities of $E_n(\alpha)$ lie within the analytic continuation circle, providing a direct explanation for the limitation of the method. To mitigate this limitation, we suggest a homotopy deformation to the potential, which improves the method and enable us to compute a few more high overtone modes reliably.

We determine the optimal local extension regularity of the future black-hole event horizon in the exact spatially flat McVittie solutions sourced by a positive cosmological constant and a barotropic fluid with constant equation-of-state parameter $w>-1$. Let $H_\infty$ be the asymptotic Hubble constant, $\kappa$ the surface gravity of the limiting black-hole root, and $p=3(1+w)H_\infty/\kappa$. Ingoing radial null geodesics reach the horizon in finite affine length. A parallelly propagated angular curvature component is asymptotic to $C s^{p-2}$, with $C\ne0$ and $s$ the remaining affine distance, which excludes every anchored $C^2$ extension for $0<p<2$. For $p\ge2$ we construct a parameter-uniform Gaussian-null compactification and an explicit two-sided Lorentzian collar. If $p=N+\vartheta$ is nonintegral, with $N\ge2$ and $0<\vartheta<1$, the optimal regularity is the standard big Hölder class $C^{N,\vartheta}$: extensions of this class exist, whereas no $C^{N,\vartheta'}$ extension exists for $\vartheta'>\vartheta$. Every integer $p\ge2$ instead belongs to an analytic island and admits a real-analytic local extension. At the critical value $p=2$ the boundary Einstein endomorphism has a nonzero rank-one nilpotent part. The ratio of cosmological decay to horizon redshift therefore determines a sharp, arithmetic hierarchy of geometric regularity.

We introduce the reader to the continuous posets approach to spacetime geometry, a topic pioneered by Martin and Panangaden (MP), supplying the relevant domain-theoretic proofs to ease the transition for readers from Lorentzian geometry and general relativity. We then present new results for the spacetime interpretation. Identifying the way-below relation with the chronological relation $I$, we determine the unique choice of $\le$ and of causality condition yielding each poset property: continuous posets require $\le\,=D_p$ and “past-distinction, (SC) and future-reflectivity”, bicontinuous posets are causally continuous spacetimes with $\le\,=D$, and globally hyperbolic posets are precisely globally hyperbolic spacetimes with $\le\,=J$, a necessity result beyond MP's sufficiency. Causal simplicity is recovered through a chain realizability condition, stable causality through the Hausdorffness of a topology $\tau$, so the causal ladder is fully determined at the abstract level, solving a problem posed by MP. Being phrased entirely in terms of a poset, the theory is of low regularity. We introduce topological Kronheimer–Penrose causal spaces, general enough to encompass the Lorentzian length spaces in the literature, and give weak conditions making them (bi)continuous posets. Domain-theoretic constructions then apply directly; e.g., Lawson's round-ideal completion, the most natural for preserving continuity, is proved to equal the future GKP completion, as is the directed completion of Gigli et al., for which we remove the (SC) assumption under strong causality. Reversing the logical order of the theory, we characterize the relations that induce a continuous poset having them as way-below relation. Finally, physical considerations on the key role of past-reflectivity in black hole evaporation lead us to suggest that spacetime is, at the fundamental level, a co-continuous poset.

While the recent Baryon Acoustic Oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) collaboration are largely consistent with a flat $\Lambda$CDM cosmology, the preferred parameters are in mild tension with those determined from the cosmic microwave background (CMB). A late-time dynamical dark energy (DDE) solution has been proposed by the DESI collaboration to address this tension. In this work, we investigate whether the statistical preference for DDE is a genuine late-time phenomenon or an artifact of unresolved early-universe physics. To do so, we simultaneously allow for both early- and late-time modifications to the expansion history by combining the Early Dark Energy (EDE) framework with the Chevallier-Polarski-Linder (CPL) parametrization. Excluding the DESI BAO measurements, our joint analysis of the CMB+Pantheon+ datasets demonstrates that within an EDE-extended framework, the CPL parameters remain statistically consistent with the standard $\Lambda$CDM model. This supports the hypothesis that a DDE signal at low redshifts can be effectively accounted for by an EDE component within the $\Lambda$CDM background. However, upon the inclusion of the DESI BAO measurements in the joint analysis, a statistically significant deviation from a cosmological constant emerges. Within this combined framework, the best-fit CPL parameters robustly indicate a departure from the standard $\Lambda$CDM model, favoring a phantom-to-quintessence transition in the DE equation of state. This demonstrates that the DESI preference for the late-time DDE is a robust signature that cannot be absorbed by modifying the physics of the early Universe.