Papers

We review the promise of upcoming gravitational-wave (GW) observations in addressing some of the important open questions in astrophysics and cosmology. A decade after the first direct detection of GWs, the field is entering a transformative phase driven by progressive detector upgrades and the anticipated advent of space-based and next-generation ground-based observatories. These instruments will not only push the distance horizon of GW observations to cosmological redshifts, but will dramatically extend the accessible frequency band, opening entirely new source classes that lie beyond the reach of current detectors. The source landscape is rich enough that surprises are likely. This review surveys our current understanding across this landscape and offers some perspective on the discoveries that upcoming GW observations are poised to deliver.

We study the stability of a thin shell of ordinary matter that separates a flat interior from a Schwarzschild exterior, which is the simplest model of a subluminal warp drive. The matter content of the shell is fixed by the junction conditions and satisfies all the standard energy conditions except very close to the horizon. For a shell made of causal matter we find an exact radius below which it cannot be stable under radial perturbations. We then place a black hole inside the shell and show that this radius grows with the black hole mass, so that the flat interior is the most favourable case. Since such shells are known to be unstable to non-radial perturbations, our result marks the boundary of the region in which any stabilising mechanism would have to operate.

We study static, spherically symmetric black holes surrounded by dark matter halos in the presence of a positive cosmological constant. Assuming vanishing radial pressure, we derive the constraints imposed by the null, weak, strong, and dominant energy conditions on the halo matter. The halo is confined to a finite radial interval separated from both horizons, and the total mass is bounded above by the Nariai value. We construct a Hernquist-type profile and study black hole images, showing that a positive cosmological constant can substantially change the images. We also compute scalar quasinormal modes of the Hernquist-type spacetime. When the dark matter mass is sufficiently small, the relative difference of a selected mode is approximately linear in the compactness of the matter halo, i.e., the ratio of mass parameter $M_{\text{DM}}$ and scale parameter $a$, with a coefficient that depends on the cosmological constant.

We study disformal transformations of $\mathcal{N}$ scalar fields coupled to a Dirac fermion. The resulting axial coupling generically introduces higher derivatives in the scalar field equations of motion; for field-dependent free functions, these are absent if and only if the disformal transformation acts along a single field-space direction. We show that these higher derivatives do not signal an Ostrogradsky ghost. After the transformation, the fermion registers the derivatives of the matter-frame tetrad only through the axial current, so that every coupling to the derivatives of the scalar fields multiplies a fermion bilinear, which is Grassmann even and nilpotent. Such terms do not contribute to the body of the constraint matrix, and at each level of the Grassmann expansion the field equations are linear, with the principal part of the theory without fermions. Through a Hamiltonian analysis, we show that the number of degrees of freedom is that of the bosonic theory plus that of the fermion, both in the Einstein frame, for free functions depending on the scalar fields and their kinetic matrix, and for minimally coupled fermions in multi-field DHOST theories. The same mechanism applies to disformal transformations with higher derivatives of a single scalar field, for which fermions impose no condition beyond those required by bosonic matter.

In this work we construct three-dimensional Chern–Simons gravity theories for the six resonant $JPZ$ algebras generated by the Lorentz generators $J_a$, translations $P_a$, and an additional Lorentz-like generator $Z_a$. The resonance condition together with the Jacobi identities leaves precisely six configurations arising from the semigroup expansions. Our main result is that three of them, $B_4$, $\widetilde C_4$, and $C_4$, are isomorphic to $iso(2,1)\oplus so(2,1)$, although they yield different gravitational realizations when we keep the original $JPZ$ field identification fixed. For each case $JPZ$, we determine the invariant tensors, gauge curvatures, Chern–Simons action, field equations, and the corresponding maximal-rank coupling sectors. The field equations can be written in the compact form $\Omega_\sigma\mathbf F=0$, which makes explicit how the additional field $h^a$ contributes to the curvature, torsion, and the remaining gauge-curvature components. We use the direct-sum isomorphisms to classify the local metric sectors and to relate them to known global and asymptotic solutions, while keeping the gravitational interpretation tied to the original $(\omega,e,h)$ fields.

Cavity optomechanical models typically treat a sensor either as a perturbative polarizable object in a pre-existing optical field, or solve the field at a fixed equilibrium and linearize in a fixed optical-mode basis. Neither self-consistently determines the spatially resolved cavity field, the optical force, the mechanical equilibrium, and the back-action when a finite-thickness dielectric plate intercepts the entire optical mode and restructures the intracavity field. In this companion paper to [1], we develop that treatment by extending the vacuum two-photon formalism to finite-thickness planar dielectrics, incorporating their Fresnel reflection, internal propagation, optical forces, and mechanical back-action directly into the field equations. We derive the carrier and sideband transfer functions needed to describe the coupled optical-mechanical system, including optomechanical cooling and the response to external perturbations. The result is a systematic starting point for calculating detector response and noise budgets for cavity experiments with intracavity dielectrics, including gravitational wave detectors deploying levitated sensors.

The geometrical configuration of the Einstein Telescope (ET) gravitational-wave observatory is one the most important design choices to be made for this future scientific facility. Two baseline configurations are currently being considered, a single triangle or 2 distant L-shaped detectors tilted by about $45^{\circ}$. In this document we present a study of the scientific performance of the ET baseline configurations in both stand-alone mode and in the presence of either an L-shaped Cosmic Explorer detector with 40-km long arms located in the United States or the existing LIGO-Virgo-KAGRA network. We also include in the study an alternative ET configuration, which is currently not being considered and consists of a triangle plus a distant L-shaped detector. We find that in stand-alone mode this alternative configuration performs significantly better than the other two in both the compact binary coalescence and stochastic gravitational wave background scientific cases. The differences in performance between the ET configurations are reduced in the presence of the considered, additional non-ET detectors. We also show that the impact of possible correlated noise in co-located detectors on the stochastic gravitational-wave background performance can be addressed with the addition of extra detectors. Finally, we study the impact on the ET scientific performance of the relative orientation of the future detectors in the network. We find that, while the configurations that include a triangular ET detector are basically insensitive to this orientation, a balanced compromise for the ET configuration consisting of two L-shaped detectors could be represented by a tilt angle between the two detectors of about $30^{\circ}$.

We investigate the early-universe cosmological implications of Unimodular Gravity coupled to Power-Law Nonlinear Electrodynamics in the presence of quantum-corrected horizon thermodynamics derived from the principle of thermodynamic duality. By incorporating both logarithmic and inverse-area corrections to the Bekenstein-Hawking entropy, we derive modified Friedmann equations exhibiting novel $H^4$ and $H^6$ terms that represent genuine quantum gravity effects. We demonstrate that these higher-order corrections become dynamically significant in the high-energy regime of cosmic inflation, naturally modifying the slow-roll dynamics without requiring a fundamental inflaton potential. A comprehensive phase-space analysis reveals that the quantum corrections drive early-time de Sitter-like attractors, providing a robust mechanism for sustained inflationary expansion. Confronting the model with the Swampland program, we show that the correction factor $\mathcal{F}(H)$ modifies the effective slow-roll parameters, enabling the model to satisfy the Swampland de Sitter and Distance conjectures during the inflationary epoch. We calculate the inflationary observables, including the spectral index $n_s$ and the tensor-to-scalar ratio $r$, and demonstrate that the model's predictions are in excellent agreement with the Planck 2018 observational constraints. Our results suggest that thermodynamic duality-induced horizon corrections offer a viable and testable pathway for realizing successful inflation within a well-motivated quantum-gravitational framework.

We derive a local upper bound on the coordinate-time Lyapunov exponent of unstable circular null orbits in static, spherically symmetric, asymptotically flat Einstein–Gauss–Bonnet gravity in $n\geq5$ dimensions with nonnegative Gauss–Bonnet coupling. On the regular Einstein branch, the tangential null energy condition suffices; no additional density-pressure inequality is required for the principal result. The bound retains the local matter-density contribution dictated by the field equations. Using the spherical mass relation, we express the departure of its dimensionless coefficient from the Einstein value $n-3$ in terms of the difference between the local density and the enclosed generalized Misner–Sharp mass per areal volume. This gives a sufficient density-contrast criterion for the Einstein optical bound to remain valid at nonzero coupling. Optical Gaussian curvature and tangential null convergence provide geometrical interpretations that distinguish radial tidal instability from the focusing of a null congruence. Vacuum solutions saturate the bound, while a Maxwell field with nonzero charge produces a strictly positive gap. The smooth Einstein limit and explicit vacuum and charged solutions provide consistency checks. A separate matter condition yields a density-free corollary. We also discuss consequences for strong-deflection lensing and test-field eikonal damping within their respective regimes of validity.

The static Love numbers of black holes vanish in classical, four-dimensional, asymptotically flat General Relativity. We study how this changes for the response of near-extremal Reissner–Nordström black holes to a minimally coupled massless scalar, at energies where the Schwarzian mode of the near-horizon throat fluctuates strongly. The black hole then responds as a quantum system with a spectrum of states. Its static response is a polarizability, given by virtual transitions between them. For $\ell=0$ the static Love number vanishes, by the shift symmetry of the scalar, but for multipoles $\ell\geq1$ the static Love numbers are nonzero: the finite rotational energy of the spin-$\ell$ intermediate states spoils the cancellation responsible for the classical result. Since the throat no longer has a classical geometry from which to read off a tidal response, we define it through the retarded Green's function, and compute its full frequency dependence at leading order in $r_+\omega$. Its conservative part requires renormalization: the near-horizon theory fixes the coefficient of its logarithm, and with it a dependence on the energy that no local term can remove, while its finite local terms are matching data of the four-dimensional theory. The dissipative part is finite and scheme independent. For $\ell\geq1$, it vanishes at low frequencies when the energy is below the rotational gap, while at the gap threshold it becomes non-analytic in frequency and produces a late-time $t^{-3/2}$ tail.

We adapt to the context of quantum theory the treatment, familiar in relativity theory, of a reference frame as a tetrad. Since a tetrad is a set of four orthonormal vectors at a spacetime point, it encodes a timelike direction, three orthogonal spatial directions, and units; and so it gives a configuration in spacetime (a spatial orientation and state of motion) of an idealized infinitesimal apparatus (“rulers and a clock"). So a natural “first guess" is that the state of a quantum reference frame should be an assignment of complex amplitudes to a set of tetrads. We develop this idea, in terms of bundles over spacetime of bases of the tangent spaces. This paper is a companion to two other papers. It emphasises the main conceptual aspects of our proposal, while the other two papers give physical details. It also explains the main contrast between those two papers. Namely: (i) the first considers a single spacetime geometry, so that a quantum reference frame, on our proposal, gives a superposition of perspectives on that geometry; while (ii) the second considers the spacetime manifold with no geometry given a priori, which will imply that a quantum reference frame, on our proposal, can describe a superposition of perspectives on a superposition of geometries—not “just" a superposition of perspectives on a single geometry. The main mathematical idea for representing this contrast will be choosing between (i) the orthonormal frame bundle, for a given spacetime geometry; (ii) the frame bundle in the usual sense of containing all bases of all spacetime points' tangent spaces.

Our understanding of gas torques on massive black hole binaries (MBHBs) relies on hydrodynamical (HD) simulations of circumbinary disks (CBDs) in which an $\alpha$-viscosity models angular momentum transport. However, CBDs around MBHBs near merger are hot, magnetized plasmas where transport is driven by magnetohydrodynamical (MHD) turbulence sustained by the magnetorotational instability (MRI). We test how well an $\alpha$-viscosity captures MRI turbulence with 3D global Athena++ simulations of a locally isothermal CBD around an equal-mass, circular MBHB, run in HD and MHD from the same initial torus on identical grids, so that only the transport mechanism differs. Excising the region within twice the binary separation, $r<2~a_{\rm bin}$, allows $1000$ orbits of evolution, several times longer than previous MHD simulations. Both disks deliver the same accretion rate, and the gas beyond $2~a_{\rm bin}$ exerts a positive torque in both, yet the MHD torque is $3.5$ times larger and $3$ times more variable. Near the midplane, transport is similar in the disk body but three times weaker in MHD at the cavity edge, where $2$-$3$ times more mass piles up in a more eccentric cavity, raising the torque. The accretion rate is modulated mainly at twice the binary frequency in HD, but by a persistent lump at the cavity edge and its beat with the binary in MHD. The outer-disk torque therefore implies a contribution to the gas-induced phase shift in the gravitational-wave signal of LISA binaries $3.5$ times larger than in $\alpha$-disk models, and a different variability signature for LSST and Roman.

We revisit the scenario of critical Higgs inflation, in which inflation is driven by the Standard Model Higgs boson with parameters chosen close to a critical point, giving rise to an inflection point in its potential. After reviewing this model, we perform a systematic study of its viability centered around three criteria: (1) consistency with electroweak-scale measurements, (2) ability to reproduce the cosmic microwave background (CMB) observables, and (3) sensitivity of the model predictions to unknown UV physics, based on a breakdown of perturbative unitarity. We first show that compliance with electroweak-scale measurements is controlled by the parameters in the Higgs potential at inflationary scales, also updating the literature values. We then demonstrate how for any choice of potential parameters, agreement with the CMB observables can be achieved through the choice of the nonminimal coupling, in both metric and Palatini formulations of gravity. However, we also find that there exist no regions of parameter space, in either the metric or Palatini formulations, in which compliance with all three physical criteria is possible. While this does not exclude the possibility of the Higgs field driving inflation, it means that predictions for any observable are in principle sensitive to the details of the model's UV completion.

Tidal Love numbers characterize the conservative response of compact objects to external perturbations and provide a powerful probe of their nature. While the static Love numbers of Schwarzschild, Kerr, and Reissner–Nordström black holes remarkably vanish, the tidal response of a generic Kerr–Newman black hole has remained unexplored due to the intrinsic coupling between gravitational and electromagnetic perturbations. Exploiting a recent breakthrough in the separability of these perturbations, we analytically determine the static tidal response of the most general stationary black hole in four-dimensional Einstein–Maxwell theory. For arbitrary multipole order, charge, and spin, we show that the entire matrix of conservative Love numbers vanishes, including the mixed gravito-electromagnetic couplings. We also compute the nonvanishing static dissipative response induced by frame dragging.

Bartnik's conjecture states that a timelike geodesically complete cosmological spacetime satisfying the timelike convergence condition splits as a static product. We show that any counterexample exhibits a peculiar global causal structure: two disjoint open regions, the future and the past cosmological developments, separated by a non-empty closed causally convex set: the cosmological wall. The developments are bounded by $C^0$ null hypersurfaces, the cosmological horizons, and a component of each horizon contains a lightlike line. These objects are natural and hence carry physical significance, as they do not depend on the choice of Cauchy hypersurface. They can be defined through the Cauchy Busemann functions $b^\pm$ of a compact Cauchy hypersurface $\Sigma$, the developments being their finiteness domains. Using a product trick we show that $b^+$ is the graph function of a Cauchy horizon in $M\times \mathbb{R}$, and so inherits the regularity and dynamical properties of horizons: $b^+$ is continuous, locally Lipschitz and locally semi-concave on its finiteness domain, and its calibrated curves are timelike rays. We prove that if the two cosmological developments intersect, which happens whenever $b^+$ is $C^1$ on its finiteness domain, then the spacetime contains a timelike line and hence splits, a result which recovers several positive results of the literature. We also obtain a localized rigidity version of Hawking's singularity theorem: if some compact Cauchy hypersurface has non-positive mean curvature on the future development, the spacetime splits. Hence, in a counterexample, every compact Cauchy hypersurface expands somewhere in the future development and contracts somewhere in the past one. If the wall has empty interior, it coincides with both horizons and is ruled by lightlike lines, and under null completeness its components are smooth totally geodesic null hypersurfaces.

We study quasibound states of a massive charged Dirac field on the Einstein–Euler–Heisenberg (EEH) black hole, the extension of Reissner–Nordström (RN) by the leading weak-field vacuum-polarization term of QED. We derive the background to first order in the Euler–Heisenberg coupling $a$. We show that it admits a nondegenerate horizon at $Q=M$ and an extremal charge $Q_{\rm ext}=M[1+a/(40M^2)+\mathcal{O}(a^2)]>M$, with horizon data that depend on $\sqrt a$ near extremality. From the separated radial equations, we derive an identity that excludes real-frequency quasibound states on any background of this class, so damped branches cannot become unstable. Since the EEH corrections first enter at $\mathcal{O}(r^{-5})$ in the electrostatic potential, the trapping condition $M\mu^2-qQ\omega_R>0$ and the hydrogenic spectrum coincide with RN, and no fine-structure-order splitting arises. Two-sided shooting with a Muller root finder shows that the real frequencies shift only by near-zone amounts that are linear in $a$ and strongly suppressed with angular momentum. The damping rates respond in a branch-dependent way. At $Q/M=0.99$ the $\ell=0$ ground state becomes slightly longer-lived, while the $(n_r,j,\ell,\lambda)=(0,\tfrac32,1,-2)$ state decays up to $80\%$ faster for $a/M^2\le1$, and near the trapping threshold the damping rate increases by up to $60\%$. Switching on the metric and potential corrections separately, together with a WKB analysis of the inner barrier, identifies the mechanisms: Coulomb screening always increases leakage, while the metric correction controls the sign and size of the $\ell$-dependent effect. Characteristic time-domain evolutions with Richardson-extrapolated matrix-pencil extraction reproduce the frequency-domain shifts.

Black-hole interiors are hidden from exterior observers, yet their geometry can enter the high-overtone spectrum of linear perturbations. We study how this spectrum encodes metric and supporting-matter information in analytic Einstein–Maxwell–dilaton black holes. Coupling a neutral spectator to the dilaton at fixed UV dimension provides a matter-sensitive channel alongside the momentum response. For real-endpoint spacelike connections, the momentum response and mass-label dependence separate the metric exponents from the signed dilaton slope. Near the causal threshold, a complex endpoint controls the spectral family: the mass profiles approach a common value, producing a shared fractional correction, while regular propagation retains label dependence. This mechanism is present at criticality without an inner horizon and continues locally into timelike interiors. Exterior numerical spectra support the reconstruction and the leading complex-endpoint responses. Local scaling and probe coupling set the candidate responses, while wave connection determines their imprint on the exterior spectrum.

We present $\texttt{EccHybIMR22}$, a fully analytical $\textbf{Ecc}$entric-$\textbf{Hyb}$rid-$\textbf{I}$nspiral-$\textbf{M}$erger-$\textbf{R}$ingdown model for the quadrupole ($\ell$=$2$, $|m|$=$2$ or simply the $\textbf{22}$) mode. The model is obtained by smoothly attaching an eccentric-inspiral to a quasi-circular-merger-ringdown prescription and is trained on eccentric numerical relativity (NR) simulations with mass ratios $q\leq 10$. In particular, we focus on obtaining a model that is sensitive to large mass ratios ($q>3$). Leveraging the recent expansion of SXS catalog of NR simulations, we construct a set of 91 hybrids (obtained by combining waveforms from post-Newtonian (PN) theory with NR simulations) with a target low total mass limit ($M_{\rm low}$) of $20M_{\odot}$ at an averaged 22 mode frequency of 20Hz, inaccessible with existing NR simulations alone. Employing a little less than a third of these hybrids as targets, we construct $\texttt{EccHybIMR22}$ and recover $65\%$ of the hybrids from the remaining set with mismatches smaller than $\sim3\%$ for a total mass of $40M_\odot$ (chosen arbitrarily) and orbital eccentricity $e_{\rm 20Hz}\lesssim0.33$ ($e_{\rm 20Hz}\lesssim 0.12$) for mass ratios $q\leq3$ ($q>3$). Furthermore, when tested against 175 SXS simulations, the model is able to recover nearly $80\%$ of the simulations with similar mismatches for systems with $0\lesssim e^{\rm NR}_{\rm 20Hz}\lesssim 0.3$.

We study the conformal coupling in the torsion-free Palatini formulation with an ordinary scalar kinetic term. Tuning the curvature coupling cannot restore Weyl invariance in a Palatini theory with an ordinary scalar kinetic term because, for an independent and inert connection, the scalar-curvature term is separately Weyl-invariant, and no finite curvature coupling cancels the Weyl variation of the kinetic term. This can be demonstrated by evaluating the energy-momentum trace on the scalar equation of motion and noting that it is explicitly independent of the Palatini coupling. Eliminating the connection, we construct a map between the curvature coupling and the effective coupling for the equivalent metric theory, and demonstrate that the scalar continues to propagate even if the reduced kinetic coefficient vanishes. We also show that it becomes a Weyl compensator with a scale-invariant potential only in the rescaled infinite-coupling limit, which in four dimensions is directly related to Palatini ${\cal R}^2$ gravity. We calculate the running of the couplings at one loop, and we find that the conformal shift in the metric-curvature coupling's beta function generates a metric-curvature coupling on a fixed metric-affine background, whereas the independent-curvature coupling runs homogeneously. Furthermore, we follow an alternate prescription in which the connection is allowed to respond to the scalar fluctuations, which results in a different scalar Hessian and therefore different beta functions. Nonetheless, we find that both prescriptions admit the formal infrared limit $(\xi_{eff},\lambda_{eff})\to(1/6,0)$ through different limits of the original couplings in the case of positive perturbative quartics. Finally, we note that trajectories starting on the positive gravitational branch encounter a vanishing gravitational coefficient at a finite scale before this endpoint.

We find recurring signatures of intermittency in molecular-gas velocities from cloud to galactic scales. In 31 selected galaxies from PHANGS-ALMA, velocity-centroid exponents through sixth order show nonlinear order dependence and nearly linear relations between consecutive exponents across galaxies. Positive estimated offsets from single-exponent scaling occur in all 52 galaxies with a valid power law fit and 99 of 100 clouds. A generalized She-Leveque hierarchy accommodates these signatures, supporting a hierarchical description of turbulent scaling in molecular gas across these environments.

General Parallel Relativity is a dynamically equivalent formulation of Einstein gravity in which the parallel frame supplies explicit boundary reference data. In this formulation, finite-region energy-momentum and horizon entropy arise as distinct Hamiltonian surface charges. Diffeomorphisms along canonical frame directions define a quasi-local energy-momentum including a matter surface term, while normal-plane boosts at a smooth horizon give entropy. These covariant formulas are applied to the Reissner-Nordström black hole and to a dilatonic charged black hole. In both examples, the matter contribution and its Legendre transform complete the Misner-Sharp mass into the Hamiltonian energy appropriate to the chosen electromagnetic boundary datum. The resulting horizon charges satisfy the thermodynamic relation obtained independently from the Euclidean boundary action.

Since Brill's work of 1959, the conformal factor of finite-amplitude Brill waves, the axisymmetric and time-symmetric vacuum initial data on $\mathbb{R}^3$ that describe pure gravitational radiation, has been obtained only numerically, up to the most recent studies. Here an explicit one-parameter family of such data is constructed: the first explicit real-analytic, non-trivial, asymptotically flat vacuum initial data with trivial topology and a single end. The free metric function is a closed-form combination of elementary functions and a dilogarithm. The data have the toroidal conformal symmetry of Beig and Husa, and a change of variable of Natanzon type reduces every mode equation to the Gauss hypergeometric equation, so that the conformal factor is a series with explicit Gamma-function coefficients. The quantities that have so far required numerical work are thereby obtained exactly. A ground-state identity gives a sharp existence criterion: complete, asymptotically flat data exist precisely for $k<k_c=0.8048016772\ldots$, and the explicit series is identified with the unique variational solution. The ADM mass is an absolutely convergent digamma series, with $m/a=\pi k^2/10+O(k^3)$ for small amplitudes and an explicit residue at its critical divergence. Sufficiently strong waves contain trapped surfaces: it is proved that for $k$ close to $k_c$ the rescaled conformal factor converges to an explicit function and explicit tori and spheres become outer trapped, so that the data contain an apparent horizon. A numerical search, restricted to equatorially symmetric star-shaped surfaces, finds marginally trapped (minimal) spheres, which appear in a fold near $k=0.7670$. No thin-torus or weak-field approximation is used.

In this article, we present an investigation of observational signatures of a black hole (BH) in effective quantum gravity (EQG), particularly gravitational weak lensing and shadow analyses with the assumption that the BH is surrounded by uniform and non-uniform plasma. First, we review the spacetime by comparing it with well-known BH spacetimes such as Schwarzschild and Reissner–Nordström BHs. Subsequently, we explore the effect of the uniform and non-uniform plasma on the BH shadow. The results indicate that shadow radii decrease under the influence of the BH charge and both uniform and non-uniform plasma parameters. In addition, we obtain theoretical constraints on the spacetime parameters using observational data from the Event Horizon Telescope (EHT) collaboration. Finally, we study gravitational weak lensing around the BH in EQG surrounded by uniform and non-uniform plasma. It was found that increasing the uniform plasma frequency leads to a rise in the deflection angle and vice versa for the non-uniform plasma.

We study the propagation of a neutral vector test field non-minimally coupled to the Einstein tensor on a Reissner–Nordström black hole. The interaction vanishes on a vacuum background, so electric charge is required to activate the curvature-induced response. The parity potentials have one symmetry: reversing the sign of the coupling exchanges the electric and magnetic susceptibilities. Because of this symmetry, the two polarizations respond identically at first order in the coupling. The dominant propagation-level effect is therefore a common shift of the effective photon sphere and of the shadow radius, not a birefringent splitting. The variation of shadow size can mimic the charge variation of a black hole. When the coupling is weak, polarization splitting appears only in the second-order term, distinguishing this interaction from corrections involving only charge. We follow this ordering through full-frequency absorption and scattering, the semiclassical glory, and ray-traced double shadows. The absorption parity asymmetry and the shadow rim splitting probe the difference between the effective potentials. The polarization-summed backward scattering isolates the helicity-reversing response, which is zero without the coupling. At the positive-coupling boundary the splitting grows but stays finite. Beyond it the even-parity potential has a pole outside the horizon. In the high-multipole limit, the two boundaries of the stability window both correspond to the thresholds of bound-state instability, but their physical properties differ: the negatively coupled boundary forms a regular potential well, while the positively coupled boundary marks the breaking of regularity. A Frobenius analysis shows a forced logarithmic branch at that pole. Without an additional physical connection condition, the continuation across the pole is not unique.

We present CosmoAgent, an agentic AI framework using large language models (LLMs) to search for dark-energy and modified-gravity models. We define a covariant grammar that allows LLM agents to explore the space of modified-gravity Lagrangians. An in-house computational pipeline derives the equations of motion, solves the background dynamics, and evaluates the likelihood using late-time expansion data, including the DES 5-year type Ia supernova sample and the DESI Data Release 2 BAO measurements. Theories are ranked by the Akaike Information Criterion (AIC) to balance simplicity and likelihood. We benchmarked 8 frontier LLMs from ChatGPT, Claude, Gemini, Kimi, DeepSeek, and Qwen, and all LLMs are able to find theories with a lower nominal AIC than flat $\Lambda$CDM, with a best improvement of $\Delta$AIC = -3.328. Seven recover quadratic quintessence. Exploratory searches also return induced-gravity and scalar-Gauss-Bonnet structures. The modest AIC improvements do not establish an observational preference for these models over flat $\Lambda$CDM. The code is publicly available at wsunamoutlook-prog/AI-Modified-Gravity.

Recent measurements from DESI baryon acoustic oscillations (BAO) have reported a moderate preference for dynamical dark energy, challenging the standard $\Lambda$CDM paradigm. Nonetheless, such a trend could reflect a tension between the shape and the offset of the Hubble diagrams predicted by current BAO and Type Ia supernovae (SN) data, which may underpin a possible breakdown of the cosmic distance duality relation (CDDR) linking luminosity distances $D_{\rm L}$ and angular-diameter distances $D_{\rm A}$. We explore this possibility by parametrizing deviations from this relation as $\eta(z) \equiv D_{\rm L} / [{(1+z)}^2 D_{\rm A}] = \alpha{(1+z)}^\varepsilon$, and investigate how the preference for a Chevallier-Polarski-Linder (CPL) dynamical dark energy is modified once $\varepsilon \!\neq\! 0$ is allowed. Using DESI DR2 and uncalibrated SN data (Pantheon$+$, DES-Dovekie, Union3, and Union3.1), we find that allowing $\varepsilon$ to vary freely in both $\Lambda$CDM and CPL reduces the CPL preference with respect to $\Lambda$CDM by a larger amount than the addition of spatial curvature $\Omega_k$; only Union3.1 exhibits comparable reductions. We also find that the preference reduction is robust to alternative $\eta(z)$ parametrizations, and that the $\eta(z)$ constraints remain stable under the removal of BAO tracers or low-redshift cutoffs for Pantheon$+$, whereas the DES-Dovekie preference largely disappears once $z < 0.1$ SN are removed. In a separate calibrated analysis, illustrative Planck sound-horizon and SH0ES absolute-magnitude priors give $\alpha \simeq 0.942 \pm 0.014$, quantifying the normalization mismatch under those assumptions. We also map the CDDR violation onto photon-loss and varying-$G$ scenarios, concluding that the cosmological constraints are much weaker than existing constraints on these physical mechanisms.

We study classical Bopp–Podolsky (BP) electrodynamics on four-dimensional de Sitter spacetime, focusing on its transverse retarded Green function and gauge-invariant electromagnetic response. Because de Sitter spacetime is maximally symmetric, its vector Green functions admit a covariant expansion in a finite basis of invariant bitensors, with scalar coefficients depending only on the invariant separation. For conserved sources, the fourth-order field operator factorizes into Maxwell and massive-vector operators, yielding the BP retarded kernel as the difference of their transverse Green functions. Using the invariant bitensor decomposition, we reduce the propagator equations to a hypergeometric master equation and determine the distributional structure of its solution. The common leading short-distance singularity of the Maxwell and massive-vector sectors cancels, leaving a logarithmic master singularity and a softened vector-potential wavefront. After one curl, the gauge-invariant field-strength kernel contains a null-cone delta distribution and a regular timelike tail. The smooth Maxwell master tail is annihilated by the curl, so the interior electromagnetic tail is generated entirely by the massive sector. We also derive an exact horizon-regular static potential, demonstrate the cancellation of the Coulomb divergence, recover the flat-space BP potential, and formulate the generalized Gauss law. On global de Sitter spacetime, the compactness of the spatial sections requires the total charge to vanish; we implement this constraint using an antipodal charge configuration with smooth factorized initial data. The analysis is classical and does not assume a quantum Lee–Wick contour or a preferred de Sitter vacuum.

Gravitational waves from core-collapse supernovae may provide a unique probe of the equation of state (EOS) of dense nuclear matter. In previous work, we assessed the potential of machine-learning methods for EOS classification for rapidly rotating supernovae using individual gravitational-wave detectors. Here, we extend the framework to detector networks and investigate the impact of detector configuration, source distance, arrival time, and sky location on the classification. Using convolutional neural networks, we perform 10-class EOS classification directly from noisy detector data for both second- and third-generation observatories. We find that detector geometry strongly influences classification performance. For Galactic events, current detector networks can achieve promising classification performance, but the reach depends sensitively on the detector network antenna response. Adding detectors improves time-averaged performance and produces more uniform classification across the sky. Third-generation observatories extend the classification reach to 100 kpc for favorable source orientations but do not remove the dependence on detector geometry.

In this work, using nonlinear electrodynamics (NLED) as a source in the field equations of extended scalar-tensor-Gauss-Bonnet (E-STGB) gravity, a novel type of static and spherically symmetric spacetime is found. We show that this novel type of spacetime can be used to explain the rotation curves and gravitational lensing of spiral galaxies. The NLED model used to derive our solution, in the weak and strong field limits, reduces to Maxwell electrodynamics. Furthermore, for a wide range of parameter values where our solution describes the gravitational field of a spiral galaxy, the NLED model satisfies the null and weak energy conditions. Hence, our results suggest that the extended scalar-tensor-Gauss-Bonnet gravity minimally coupled to nonlinear electrodynamics (E-STGB-NLED) provides a viable alternative to explain flat rotation curves of spiral galaxies without invoking dark matter.

We report the first accurate computation of the leading-order shifts in the quasinormal-mode frequencies of rapidly rotating black holes surrounded by massive scalar fields with nonminimal curvature coupling. For fields with reduced Compton wavelengths no shorter than the black-hole mass scale, the numerical errors in the frequency shifts are typically $10^{-8} - 10^{-5}$ for dimensionless spins between 0 and 0.9, and at most $\mathcal{O}(10^{-3})$ next to the scalar-mass threshold. Our results open up the possibility of searching for ultralight scalar particles, prime dark-matter candidates, using existing gravitational-wave signals.