Papers

We formulate a generally covariant extension of London electrodynamics in four-dimensional curved spacetime and investigate the propagation of electromagnetic perturbations in a superconducting medium surrounding a black hole. The ordinary London relation is promoted to a gauge-invariant four-dimensional constitutive equation involving the electromagnetic four-potential and the phase of the superconducting condensate. When combined with the covariant Maxwell equations, the resulting system is equivalent, in the London (unitary) gauge, to a Proca theory with an effective photon mass set by the inverse London penetration depth. We derive the corresponding Schrödinger-like wave equations for electromagnetic perturbations in the axial and polar sectors of the Schwarzschild spacetime in the presence of the superconducting plasma. The axial sector is governed by a single Regge–Wheeler-type equation whose effective potential contains the London mass term, whereas the polar sector contains two coupled physical degrees of freedom. Quasinormal-mode boundary conditions are formulated for the Schwarzschild geometry. The London penetration depth shifts the oscillation frequencies, modifies the damping rates, and can generate long-lived quasibound configurations when the effective field mass is sufficiently large. This formalism provides a framework for studying electromagnetic relaxation in curved superconducting media, compact objects containing superconducting phases, and effective massive-vector fields around black holes.

In this work, we analyze a two-parameter $(\mu, b)$ asymmetric family of traversable wormhole solutions generated by a massless conformally coupled scalar field. We show that, for a suitable range of parameters, the spacetime is everywhere regular and has two asymptotically flat regions. We then define the wormhole throat and verify the flare-out condition, which leads to violations of the classical energy conditions in GR, thereby requiring exotic matter to hold the throat open. We further analyze the motion of photons governed by null geodesics through an effective potential. The resulting trajectories are computed numerically and visualized using embedding diagrams. Following this route, we investigate scalar perturbations governed by the massless Klein-Gordon equation and find the QNM spectrum semianalytically and numerically. Interestingly enough, we show that such perturbations are governed by only one parameter $\mu$. Therefore, in $D = 4$, we find an isospectrality between the wormhole and the BBMB/Extremal Reissner-Nordström black hole. In the spirit of the seminal work of Morris and Thorne, we derive the conditions for traversability and compute the proper time for a traveler to traverse the wormhole. Finally, we discuss possible extensions and future directions for our work.

The renormalization of asymptotically AdS spacetimes in New Massive Gravity is studied using boundary terms constructed with the extrinsic curvature. For generic values of the theory couplings, a single extrinsic boundary term produces a consistent holographic description, as the variation of the total action is finite and expressible in terms of the holographic source. This prescription applies to constant-curvature geometries and non-Einstein solutions with Brown-Henneaux asymptotic conditions. A suitable asymptotic expansion shows the agreement of the resulting holographic stress tensor with the one obtained from the auxiliary-field formulation. At the degenerate point, where the two maximally symmetric vacua coalesce, the Fefferman-Graham expansion admits an additional mode associated with relaxed AdS boundary conditions. A quadratic extrinsic counterterm removes the new divergences and leads naturally to a holographic picture with two independent sources. The corresponding responses obey a modified holographic Ward identity and determine finite conserved charges. Finally, the proper use of the Noether-Wald formalism reproduces the correct charges of the BTZ black hole, non-Einstein AdS waves, and the rotating hairy black hole.

We study linearized perturbations of the subextremal Kerr-Newman black hole solution of the Einstein-Maxwell equations. We show how to overcome what Chandrasekhar called the "apparent indissolubility of the coupling between the spin-1 and spin-2 fields" by using a matrix integrating factor for suitably transformed first-order systems that describe the radiative degrees of freedom. For the radial ODEs obtained after angular separation, we prove mode stability on the real axis, i.e., the absence of quasinormal modes with real frequencies, using a Whiting-type transform.

A complete classification of exact solutions of Maxwell vacuum equations for invariant electromagnetic fields in null homogeneous Petrov spaces with unsolvable groups of motions $G_3$, acting simply transitively on null hypersurfaces of null homogeneous Petrov spaces, is conducted. The classification of exact solutions of Maxwell vacuum equations is based on using the representation of components of the contravariant metric tensor and components of the invariant electromagnetic field in terms of vector fields of a canonical frame. The classification results are presented in a form convenient for further use.

Gravitational-wave (GW) observations offer a powerful probe of new fundamental fields. One well-motivated source is black hole (BH)–boson cloud systems, in which an ultralight scalar field forms a cloud around a rotating BH via superradiance and emits long-lived nearly monochromatic GWs. In this work, we compute the stochastic GW background (SGWB) from such systems, extending previous work by including scalar self-interactions, and discuss its detectability with next-generation ground-based GW detectors. We find that self-interactions can suppress the SGWB and thereby relax existing LIGO-Virgo-KAGRA constraints inferred from null searches. Namely, the LIGO detectors at design sensitivity are insensitive to a SGWB produced by scalar fields with a decay constant $f_\mathrm{s} \lesssim 3\times 10^{17}$ GeV, independently of the scalar field mass. Looking ahead, we show that a moderately self-interacting cloud can still produce a detectable SGWB with next-generation detectors. Under conservative assumptions, for a decay constant $f_\mathrm{s} = 10^{17}$ GeV, the Einstein Telescope (ET) will be sensitive to scalar masses in the range $\sim[10^{-13.0},10^{-11.8}]$ eV, while Cosmic Explorer (CE) will be sensitive to scalar masses in the range $\sim[10^{-13.2},10^{-11.7}]$ eV. We also find that the minimum decay constants that still yield a detectable SGWB for ET and CE are $f_\mathrm{s}\sim 6\times10^{16}\,$GeV and $\sim 3\times10^{16}\,$GeV, respectively.

We study linear perturbations of the Maldacena-Milekhin-Popov (MMP) traversable wormhole. On a magnetically charged background, polar metric perturbations couple to axial perturbations of the gauge field. We treat this sector and reduce it to two coupled master equations of Zerilli-Moncrief type. The wormhole has two regions. In the mouths the geometry is a nearly extremal magnetic Reissner-Nordstrom black hole with no quantum source. In the throat the Casimir energy of the lowest Landau level of charged fermions holds the wormhole open. We show that the throat equations are consistent only if the fermions respond to the perturbation, computed exactly from the two-dimensional conformal anomaly with the full stress tensor including the trace part that MMP discard. This response must be accompanied by an induced Hall current of the fermions. With both effects included, the linearised Bianchi identities hold at first order in the fermionic backreaction $\alpha$. Deriving the Hall current from the fermion effective action instead requires an Euler term with coefficient $c/48\pi$, which becomes a coupling between magnetic flux and curvature similar to a Wen-Zee term. At $\alpha=0$ the throat is exact AdS$_2\times S^2$ and the modes decouple into two Poschl-Teller problems with masses $l(l-1)$ and $(l+1)(l+2)$, with levels spaced by $1/\ell$ in frequency. We give the $O(\alpha)$ corrections in closed form. They mix the two modes and depend on frequency, and match the mouths in the overlap region. The symmetric part of the potential is positive throughout the wormhole, excluding purely exponential growth. The corrections lower the normal frequencies and split the levels shared by the two modes, with real shifts at first order in $\alpha$. This sector has no growing mode for $l\ge2$ at this order.

Bell's theorem establishes that a local and spatially separable physical theory with no superdeterminism or retrocausality must be a theory of many local worlds. The local many worlds formalism of quantum mechanics is a transparent separable physical theory in spacetime with a Lorentz covariant causal structure. Unlike the Everett many worlds interpretation, there is no nonseparable universal wavefunction in configuration space, but like Everett, there is only local unitary evolution which conserves probability (fluid) current, and no wavefunction collapse. Each event in spacetime has its own local wavefunction which is built up using only the local wavefunctions and local unitary evolution operators within its past light cone, and which is entirely disconnected from spacelike separated local wavefunctions. We call this the local Schrödinger picture. Local many worlds can also be described in the Heisenberg picture by ascribing Deutsch-Hayden (DH) descriptors to each individual system, each of which evolves under local unitary operations. Both the local wavefunctions and DH descriptors contain records of past interactions, and while the local wavefunctions provide a clear and intuitive physical narrative, the descriptors contain all of the same information in a much more compact form. Unfortunately, the compact form is also somewhat cryptic, which has made the local Heisenberg picture relatively inaccessible. The purpose of this article is to show that the local wavefunctions for any $N$-qubit system can always be constructed directly from the DH descriptors, allowing us to combine the benefits of both pictures.

We study transition amplitudes of the quantum universe in $D$-dimensional spacetime within the de Sitter minisuperspace approximation in Einstein gravity. Using the Lorentzian path integral combined with Picard-Lefschetz theory, we identify the saddle points that contribute to the integral and derive a well-defined transition amplitude under Dirichlet boundary conditions. We further show numerically that a resurgence analysis based on Borel-Padé resummation is effective in resolving the residual Stokes ambiguities that arise in certain cases. Analyzing three-, four-, and five-dimensional cases, as well as the large-$D$ limit, we find that the tunneling proposal is commonly realized for the creation of the universe from nothing. This result implies a universal feature of quantum cosmology based on Einstein gravity across dimensions.

We study scalar and electromagnetic quasinormal modes of the Hayward black hole and the axial spectrum of a specified Regge–Wheeler-type effective model, from the Schwarzschild limit to exact extremality. Multiprecision Chebyshev collocation reproduces most available low-lying oscillatory benchmarks and supplies further overtone candidates. The spin-2 equation is model dependent. It differs from the effective-source potential used in recent Hayward studies, and we quantify representative frequency differences. The discretisation also produces purely imaginary eigenvalue sequences, whose physical classification requires special care because highly reproducible imaginary-axis ladders need not be poles of the continued Green function. Accordingly, the imaginary-axis entries reported here are retained as unclassified spectral candidates, not established QNMs. We quantify their finite-table gap dispersion and sensitivity to the number of included gaps. Their spacings near one quarter in $M\omega$, and the extremal product near $1/3$ with the throat radius, remain properties of the tabulated candidates, and they do not establish universal QNM spacing or throat-controlled damping.

Entanglement harvesting transfers quantum field correlations to localized probes, yet whether these correlations can alter stable strategic behavior has remained unclear. We formulate a quantum-input game in which two Unruh-DeWitt detectors harvest vacuum entanglement from a free massless scalar field in $(3+1)$-dimensional Minkowski spacetime. A referee converts this state into payoffs using independent question qubits and phase-calibrated local Bell measurements. To leading perturbative order, the mean reward is proportional to the signed entanglement margin $\sqrt{\mathcal{H}^2+\mathcal{Q}^2}-\mathcal{P}$, where $\mathcal{H}$ is the Hadamard correlation contribution, $\mathcal{Q}$ is the Pauli-Jordan causal contribution, and $\mathcal{P}$ is the local excitation noise. Away from payoff degeneracies, allowing each player to exit or to choose an early or late interaction yields $1+2n_+$ trembling-hand perfect equilibria, where $n_+$ counts the positive-reward timing equilibria. For one fixed protocol, five analytically certified settings realize the sequence $7\to3\to1\to7\to1$. They represent the spacelike region, the first commutator lobe, central carrier cancellation, the second lobe, and the post-wavefront Huygens regime. Thus, within this fixed quantum-input verification protocol, distinct field-correlation regimes generate different stable strategic structures.

We construct a class of global naked singularity solutions for the $(3+1)$-dimensional spherically symmetric Einstein–Maxwell-charged scalar field equations. The solutions arise from $C^{1,\alpha}$ asymptotically flat one-ended characteristic initial data with $0<\alpha\ll1$ and develop a curvature singularity that is not enclosed by an event horizon. The resulting spacetimes are genuinely charged, with a nonzero Maxwell field that satisfies a quantitative positive lower bound up to the singularity and admits an explicit asymptotic profile. As part of the construction, we develop a general framework for constructing naked singularity exteriors, namely, the causal future region of the past light cone emanating from the singularity. The framework accommodates a broad range of singular characteristic data and, in particular, allows for singular behavior of the Maxwell field near the singularity.

5We study homogeneous cosmological perturbations in the quadratic model $F(R,\mathcal G)=R+\alpha R^2+\beta\mathcal G^2$. Linearization of the modified Friedmann equation together with dust conservation yields a second-order equation for the homogeneous Hubble perturbation, or equivalently a third-order equation for the matter contrast. We then restrict the asymptotic analysis to backgrounds that satisfy the zeroth-order equations of the same quadratic theory. Besides the low-curvature dust branch, which approaches $H=2/(3t)$, the model admits a nontrivial vacuum de Sitter solution with $\beta H_{\rm dS}^6=1/96$. Its homogeneous curvature modes are stable only when $\alpha<-1/(6H_{\rm dS}^2)$. At high curvature the background equation also admits a vacuum big-rip asymptotic branch beginning with $H=7/(t_s-t)$; the first corrections are fixed by the quadratic couplings. The corresponding $\mathcal G^2$-dominated perturbation has one divergent homogeneous mode. The simultaneous Einstein limit is taken only as a consistency check and recovers the standard dust behavior.

Linear perturbations of a Schwarzschild-MOG black hole separate into scalar and gravito-vector sectors in the reduced strong-field theory at leading order in $\mu\bar G M\ll1$. The field redefinition $\widehat h_{\mu\nu}=h_{\mu\nu}-\psi\bar g_{\mu\nu}$ gives a decoupled Klein–Gordon equation for the scalar and the Reissner–Nordström equations for the gravito-vector sector. The scalar is stable under the stated spectral boundary conditions for $m_G^2\ge0$. A scalar-only excitation produces a conformal metric perturbation with null Hamiltonian contribution $H_1^{(s)}=-\psi H_0$. For fixed observer worldlines and conformally corresponding orthonormal frames, every linear scalar-to-critical-curve transfer coefficient vanishes, including monopole breathing, provided the conformal factor is smooth, positive, and preserves the capture boundary. The exceptional charged axial dipole instead induces a constrained metric perturbation. A first-order radial integral reconstructs this perturbation in a gauge regular at the future horizon and on the sphere, separating the radiative amplitude from stationary angular momentum without division by frequency. For a unit-incident canonical vector amplitude with zero angular momentum, matched asymptotic solutions give an explicit charge-dependent local metric response that vanishes as $\omega^2$ at fixed radius and fixed subextremal background. A separate infinitesimal Kerr–Newman benchmark yields the stationary impact-parameter midpoint shift and recovers $\delta C_b=-2a_{\rm rot}$ in the neutral limit. These results exclude linear scalar breathing of the specified angular critical curve and distinguish the low-frequency vector-induced metric response from a stationary rotation effect.

First law consistency alone does not determine the black hole state space of NUT-charged spacetimes, because several inequivalent macroscopic formulations can yield consistent thermodynamics. We develop a two-horizon sector diagnostic for the electrically charged Kerr-Newman-NUT family by organizing the inner- and outer-horizon entropies and inverse temperatures into sum ($\Sigma$) and difference ($\Delta$) sectors. The charged rotating geometry gives a more stringent test of the uncharged Taub-NUT sector result, since the mass $M$, ordinary electric charge $Q$, ordinary angular momentum $J$, NUT charge $N$, and the NUT secondary hair $J_N=MN$ all enter the same two-horizon thermodynamics. For the physically anchored minimal homogeneous state spaces defined and tested here, we find a clear sector separation: the $\Sigma$ sector closes with the charge-like variables $Q$ and $N$, whereas the $\Delta$ sector additionally requires the rotation-like variables $J$ and $J_N$. Natural reductions that eliminate either $J_N$ or $N$ fail to reproduce the first law coefficient fixed by the sector temperature, and their associated Bekenstein-Smarr relations do not close with the assigned scaling weights. Within this class, electric charge and ordinary rotation therefore do not absorb the NUT secondary response, and the charged rotating family supports the NUT bi-hair organization: $N$ has a charge-like sector role, while $J_N=MN$ is a rotation-like thermodynamic secondary hair. The latter is defined off shell in the homogeneous equation of state, not as a new metric parameter or asymptotic conserved charge.

We investigate the Fulling-Davies-Unruh effect in a local Lorentz-violating effective field theory, focusing on the quantum-field response of accelerated detectors. We canonically quantize a real scalar field exactly in the coefficients for Lorentz violation. The dispersion relation, Hamiltonian, and massless and massive Wightman functions are obtained. We determine the conditions under which the scalar dynamics preserves boost symmetry and distinguish these from the weaker conditions required for stationarity along a single accelerated worldline. The Wightman functions are used to construct the finite-time Unruh-DeWitt response for a uniformly accelerated detector. For generic coefficients the response is nonstationary and depends on both the duration and temporal location of the measurement. The spectrum, which is the resulting excitation rate as a function of the energy level difference in the detector, is obtained numerically. It shows discernable differences with the usual massive thermal spectrum for dimensionless coefficients on the order of $10^{-7}$. Experimental implications of the result are discussed. As an alternative viewpoint, we place Lorentz violation in the point-particle sector, derive the exact trajectory generated by a constant electric field, and study the proper-time Fourier response of a conventional scalar wave along the modified trajectory.

First-order phase transitions in neutron-star interiors can leave signatures in the polar quasi-normal-mode spectrum. We compute the complex quadrupolar $f$- and $p_1$-mode spectra in full general relativity for cold, nonrotating hybrid stars with a sharp hadron–quark interface. We model the equation of state by matching a hadronic SLy4 phase to a constant-sound-speed quark-phase. We compare slow phase conversion, in which fluid elements retain their phase and the interface moves with the fluid, with rapid phase conversion, in which matter changes phase as it crosses the boundary and the interface follows phase equilibrium rather than the fluid, together with an algebraic interpolation between these limits. On identical equilibrium backgrounds, the two physical limits produce a sizable separation in the real part of the $p_1$-mode frequency. This $p_1$ frequency separation is largest when the local tangential-displacement fraction of the eigenfunction changes rapidly across the interface. Across the surveyed EOS parameter space, rapid conversion generally lowers the $p_1$-mode frequency relative to slow conversion, with compactness-averaged separations reaching $\sim3.5\,{\rm kHz}$ on disconnected twin-star branches. The largest separations occur at relatively low transition pressures and large density discontinuities. By contrast, the $f$-mode is only weakly affected by the interface condition for individual stellar models. Across the sampled constant-sound-speed parameter space, departures exceeding the hadronic SLy4 reference residual occur mainly at low transition pressure, large density discontinuity, or both, with different boundaries for the real- and imaginary-part fitting variables. Thus, the $p_1$-mode probes the local interface response, whereas the tracked $f$-mode reflects the global departure of the hybrid sequence from the hadronic calibration.

Studying Gravitational Waves (GWs) provide a unique probe of the interplay between quantum matter and the dynamics of spacetime. We develop a QED framework for spontaneous emission in the presence of a weak classical GW by quantizing the electromagnetic field in a linearized gravitational background. We show that a passing GW dynamically phase-modulates vacuum electromagnetic modes, dressing photon states into a Floquet ladder of sidebands separated by the GW frequency. While the total spontaneous decay rate remains unchanged to first order in the GW strain, the emission pattern develops a characteristic spin-2 quadrupolar angular redistribution modulated with the GW phase. For finite interaction times, interference between the carrier and Floquet sidebands generates phase-dependent spectral asymmetries in the transient emission spectrum. To explore experimental observability, we formulate a cavity-QED master equation description and propose a differential cross-cavity detection protocol employing Rydberg ensembles or highly charged ions. These results establish cavity QED as a promising platform for exploring quantum electrodynamics in dynamical spacetime and for high-frequency gravitational-wave detection.

A black hole horizon carries exactly one nat of information per Planck cell, without naming what the black hole is made of. Two inputs suffice: entropy is irreducible uncertainty (the configurations no observation can resolve at the Planck floor, an objective bound, not ignorance), and the horizon is a complete scrambler, so by the Ehrenfest time its Wigner function has folded below the floor and no cell can be certified empty. A single cell then holds unresolved filament crossings; a cell crossed $n$ times admits $n!$ orderings that scrambling erases, so cardinality $n$ carries weight $1/n!$ and its count is $\Omega_1=\sum_n 1/n!=e$, one nat. The $N=A/\ell_P^2$ independent cells give $\Omega=e^N$, hence $S=N$. Multiplied by the kinematic factor $\tfrac{1}{4}$ (the causally accessible fraction of the null phase space, established separately), this returns $S_{BH}=A/4\ell_P^2$. The derivation invokes no field equations, no microscopic theory, and no posited Hilbert space.

We construct exact, static, circularly symmetric charged regular AdS$_3$ black hole solutions in a three-dimensional scalar-vector-tensor theory obtained from regularized Gauss-Bonnet couplings. While the Maxwell-charged counterpart of this theory suffers from a logarithmic curvature singularity at the origin, we demonstrate that coupling the action to non-linear Born-Infeld electrodynamics cures this pathology. We obtain the metric function and show explicitly the regularity of the core. Depending on the choice of parameters, the solution exhibits various horizon structures, including single-horizon, double-horizon, extremal black holes, and horizonless regular geometries. Finally, we discuss the significance of this regular spacetime as a testbed for microscopic entropy calculations in singularity-free setups.

We prove that 3d Lovelock gravity, the Horndeski theory obtained from the regularized Lovelock invariants in three dimensions, admits a holographic c-theorem at all orders and for arbitrary values of the couplings. In the minisuperspace of domain wall solutions, the equation of the Horndeski scalar is a total derivative at every order, which locks the derivative of the scalar to that of the warp factor along the flows we consider and allows the null energy condition to be integrated into a monotonic c-function. The linear scalar profile of the resulting vacuum breaks the special conformal transformations, so the dual theory is scale-invariant but not conformal, and the central charge is defined as the coefficient of the logarithmic divergence of the on-shell action on the sphere. We compute it exactly for the Gauss-Bonnet case and asymptotically for the full tower, and find that it is linear in the couplings and matches the UV value of the c-function. Since all results depend on the couplings only through two generating functions, we give the resummed Lagrangians, c-functions and central charges in closed form for the choices of couplings that lead to regular black holes, and classify the resummed theories by the positivity of the effective Newton constant on the vacuum and the uniqueness of the vacuum with a non-trivial scalar.

Starting from the constant-roll condition, which involves the constant-roll parameter $\beta$, which is a fundamental constant in the constant-roll inflationary models, we derive the scalar power spectrum and obtain an analytic expression for the scalar spectral index $n_s$ in terms of $\beta$ and the number of remaining e-folds $\tilde N$. The resulting relation is non-monotonic in $\beta$, allowing two distinct constant-roll branches, one with $\beta<0$ and one with $\beta>0$, to reproduce nearly the same observed scalar tilt. We constrain $\beta$ using Planck 2018 CMB data, and further using the combined Planck 2018 + ACT DR6 data. For Planck 2018, the posterior distribution of $\beta$ exhibits a bimodal structure associated with the two analytical branches, while the combined Planck+ACT analysis tightens the allowed range of $\beta$ and shows continued consistency with the $\beta=0$ slow-roll-like spectral point. The data constrain $\beta$ to be small, but do not require it to vanish exactly. We also estimate the inflationary mass scale $M$ by using the observed scalar-amplitude normalization at the pivot scale. Both the negative and positive $\beta$ branches give a high-scale inflationary normalization of order $M\sim10^{-5}M_{\rm Pl}$. We show that the $\beta \to 0$ limit is only spectrally slow-roll like, but does not reproduce standard slow-roll dynamics. Our results show that constant-roll inflation is observationally competitive with the usual slow-roll inflationary paradigm. The constant roll inflation model is supported by the latest CMB observations equally well or marginally better when compared with the slow roll inflation model.

We investigate Hawking radiation of a quantum-corrected Schwarzschild black hole with vacuum polarization and conformal anomaly effects. The corrected geometry has two horizons, a non-monotonic Hawking temperature that vanishes at a critical mass, and a logarithmic entropy correction. We compute greybody factors, spectra, and emission rates for massless bosonic fields of various spins and angular momenta. Quantum corrections suppress low-energy greybody factors and enhance high-energy ones. During late evaporation, where quantum influences become significant, the decreasing temperature strongly suppresses emission. In this case, the higher-spin modes become negligible, and the scalar $s=l=0$ mode dominates. The black hole lifetime is greatly extended as it asymptotically approaches a stable extremal remnant.

The compact four-dimensional manifold $\mathbb{CP}_2\# 3\overline{\mathbb{CP}_2}$ is known to admit a toric Kähler-Einstein metric $g_{\text{KE}}$, but the metric is not known in closed form, which makes it difficult to draw conclusions about its geometry. In this article, we use a combination of analytic and computer-assisted techniques to produce an approximate Einstein metric $g$ described explicitly herein, and also prove that the true Einstein metric $g_{\text{KE}}$ is close to $g$, where both the closeness and the topology are described explicitly. As an application, we prove bounds on the first invariant eigenvalue of the Laplace-Beltrami operator, and prove that this Kähler-Einstein metric does not have positive holomorphic sectional curvature everywhere.

The problem of thermal equilibrium in general relativity is discussed and the unique role played by the fluid associated with the cosmological constant is emphasized. In addition to being the only fluid that is always in thermal equilibrium, it is shown here that its energy can also be seen as a universal quantity given by $E_\Lambda=\rho_\Lambda V$, where $V$ is a spatial volume. Furthermore, if the cosmological constant turns out to be a universal constant, then the conclusion that any spatial volume $V$ has an intrinsic energy follows naturally from the approach considered here. A possible application to quantum theories of gravity that postulate the existence of a minimal length scale is discussed. In studying the energies, the so-called teleparallel equivalent of general relativity is used, and the difficulty with the zero-point energy is discussed in light of the gravitational energy problem. The quasi-local energies of the Reissner-Nordström-de Sitter black hole and of a regular black hole are calculated. The results obtained here with the teleparallel theory are consistent with our expectations.

In this paper, we study the evolution of curvature and entropy perturbations in three-field inflation with non-canonical kinetic terms. Using the kinematic basis in field space, we characterize the background trajectory through its turning rate and non-planar torsion. We also emphasize the inherited structure of perturbations from the kinematic system, which detaches the distinct roles of turning rate and torsion in the interactions among perturbations. We present and study numerically a three-phase model with an intermediate decelerated expansion stage in which two separated transient epochs amplify two distinct families of modes, generating a double-peaked curvature power spectrum. This double-peaked power spectrum leads to the formation of primordial black holes with two characteristic masses, $M_1\sim2\times10^{-12}M_\odot$ and $M_2\sim52M_\odot$ explaining, simultaneously, both dark matter and the LIGO-Virgo-KAGRA binary black hole mass range.

A spectator spin-2 field can mix linearly with the metric tensor perturbations. We study the coupled tensor system in an inflationary background, including a parity violating effect parametrised by $\theta$. Without expanding in the linear mixing between the two fields, we solve the system exactly for arbitrary mass while treating the mixing strength nonperturbatively. The mode functions of both helicities are constructed by acting with an operator-valued Gauss hypergeometric function ${}_2F_1$ on Whittaker modes, and the late-time power spectrum of each graviton helicity is expressed in closed form in terms of the generalised hypergeometric function ${}_3F_2$. We find that one helicity is exponentially enhanced, showing that strong mixing with a spectator spin-2 field can generate a large and highly chiral primordial gravitational wave spectrum. The degree of circular polarisation is bounded by $\tanh(\pi\theta)$, independently of the spin-2 mass and the mixing strength. The same solution also gives the late-time power spectrum of the spectator field and its cross spectrum with the graviton. In the weak mixing limit, our exact result reproduces the perturbative Schwinger-Keldysh result. These solutions provide a nonperturbative framework for studying primordial gravitational waves sourced by additional tensor modes.

The Kerr-Bertotti-Robinson black hole is an exact Einstein-Maxwell solution describing a rotating black hole in an asymptotically uniform magnetic field. It differs from the Kerr black hole in the Wald solution: the magnetic field lines are no longer cylindrical, with the constant-\(A_\phi\) lines saturating at large distances, and the interface \(\vec{E}\cdot\vec{B}=0\) is no longer a cone but contracts with radius and eventually disappears. Determining the acceleration of charged particles pointwise, we find that protons are driven outward and electrons inward in the polar region, and vice versa in the equatorial region; the polar region shrinks as the magnetic field grows. Captured protons carry negative energy and angular momentum, whereas captured electrons carry positive energy and angular momentum. Fixing the capture domain by the gauge-invariant flux criterion and integrating over the \((r,\theta)\) plane, we find that in weak magnetic fields the black hole gains mass, while in strong magnetic fields energy extraction occurs. Energy extraction can be achieved under the usual spherically symmetric density, without introducing the anisotropic density distribution required in the Wald solution for a Kerr black hole.

We interpret the $248~\mathrm{keV_{nr}}$ nuclear-recoil candidate reported by LZ through elastic dark matter scattering mediated by a light Majoron. The Majoron provides a natural origin for the required pseudoscalar interaction: it is the pseudo Nambu-Goldstone boson associated with spontaneous lepton number breaking, directly connecting the recoil signal to neutrino mass generation. After a model independent analysis of this idea, we present a concrete realization in a scotogenic ultraviolet completion in which the lightest singlet Majorana fermion is dark matter and acquires both its mass and Majoron coupling from the symmetry breaking dynamics. A vector like up type quark and a type-I Dirac seesaw in quark sector communicate the Majoron interaction to the up quark. At low energies, the resulting portal generates the nonrelativistic operator $\mathcal O_6$, whose momentum dependence reproduces the high-recoil event without overpopulating the lower energy bins. Its spin structure removes the dominant heavy element solar capture channels, while capture on hydrogen is suppressed by poor kinematic matching and small momentum transfer. This construction simultaneously reproduces the LZ recoil spectrum and the observed thermal relic abundance while strongly suppressing solar capture.

The gravitational-wave (GW) memory effect is a prediction of general relativity, characterized by a permanent change in spacetime geometry. Although it has not yet been observed, next-generation detectors such as the Einstein Telescope (ET) and the Laser Interferometer Space Antenna (LISA) are expected to provide the sensitivity required for its first detection. GW memory comprises two distinct contributions: the nonlinear memory, generated by the self-interaction of GWs, and the linear memory, originating from the anisotropic emission of matter and radiation associated with astrophysical transients. In this work, we quantify both nonlinear and linear memory contributions produced during binary neutron star mergers (BNSs), including those associated with GWs, gamma-ray bursts (GRBs), afterglows, neutrinos, dynamical and disk-wind ejecta, and kilonova emission. We evaluate the detectability of the resulting memory signals with ET and LISA through two applications. First, we apply our analysis to the multi-messenger event GW170817 - GRB 170817A - AT2017gfo. We find that the combined memory signal from this event would have been detectable by ET with a signal-to-noise ratio of 10.7 dominated by the nonlinear memory. We also assess the detectability of the GRB memory across a synthetic population of GRBs, and find that only extreme-energy and jointly favorable configurations could produce a detectable signal in ET. Together, these results demonstrate that while next-generation detectors will probe nonlinear memory, observing linear memory from BNS counterparts will remain elusive except in the most favorable scenarios.