Motivated by recent findings that the nHz signal of Pulsar Timing Arrays (PTAs) may be dominated by supermassive black hole (SMBH) binaries ($M\gtrsim {10}^9{M}_{\odot }$), and high redshift quasar observations revealing unexpectedly massive SMBHs, we calculate the implications of early seeded SMBHs for the PTA signal. As an application, we explore two prominent scenarios of high-$z$ SMBH seeding mechanisms: direct collapse black holes (DCBHs) and collapse of Dark Stars. We show that Dark Star seeded SMBHs with comoving seed number density of $\mathcal{O}({10}^{-3}){\mathrm{Mpc}}^{-3}$ can be the dominant contributor to the PTA signal while the DCBH channel may contribute subdominantly. We also suggest ways to place an upper bound on the seed number density.

One of the most intriguing proposals for wave-function collapse is the Diósi-Penrose model, in which collapse is driven by stochastic fluctuations of the Newtonian potential. We argue that a closely related effective structure can emerge in string theory if, as recently suggested, the present cosmic acceleration is sourced by instant folded strings and their decay products. A key difference, however, is that in this stringy setting the noise is naturally colored in time rather than white. As a result, the scenario is significantly less constrained by existing experiments than the standard Diósi-Penrose model.

Quantum timeless approaches solve the problem of time by recovering the usual unitary evolution of quantum theory relative to a clock in a stationary quantum universe. For some Hamiltonians of the universe, such as those including an interaction term with the clock, the dynamics is substantially altered and can be nonunitary. This work derives necessary and sufficient conditions for the relative dynamics to be unitary and finds the general form of the unitary evolution operator. A physical interpretation of these conditions is given in terms of the clock’s rate. Unitary dynamics is associated with rates that are constant in time and independent of the clock’s internal structure.

With the Event Horizon Telescope and future Very Long Baseline Interferometry arrays poised to image supermassive black holes, there is a strong motivation to understand the dynamic aspects of the accretion flow near the black hole. Interestingly, in such highly relativistic regime, the finite light-travel time should be taken into account to correctly simulate the images, known as the slow-light effect. This paper investigates the impact of the slow-light effect on the observational signatures of hot spots corotating with the Keplerian disks. It is found that the magnification can be modulated by the hot spot’s orbital velocity. Specifically, the corotating hot spots at the maxima of magnification are located at the image positions shifted relative to the positions behind the black hole. This subsequently causes the peak of the magnification profile to shift toward alignment with the peak of the redshift factor profile, ultimately leading to flux enhancement relative to the results of previous studies. This enhancement becomes particularly pronounced for corotating hot spots in the strong-field regime at large inclination angle, indicating that the slow-light effect is essential for accurately modeling high-energy emission in the vicinity of the black hole.

We identify the complete set of fundamental building blocks for a 4D pregeometric theory of gravity. Based on a gauge theory of $SO(1,4)$ or $SO(2,3)$ coupled to a Higgs-like field under the rigid constraint of general covariance in the unbroken phase, these building blocks (${\mathcal{L}}_{\mathrm{MM}}$, ${\mathcal{L}}_{\mathrm{W}}$, ${\mathcal{L}}_J$, ${\mathcal{L}}_{\theta }$, and ${\mathcal{L}}_{\vartheta }$) constitute the minimal generating set of independent field monomials from which any pregeometric action, including arbitrary functionals thereof, can be constructed. While the most general pregeometric theory can extend beyond linear combinations, these five irreducible invariants serve as the “atomic” constituents of all possible pregeometric dynamics. Upon spontaneous symmetry breaking, they collectively generate an emergent gravitational theory consisting of the Einstein-Hilbert action, the cosmological constant term, and all 4D topological invariants: the Gauss-Bonnet, Pontryagin, Holst, and Nieh-Yan terms. The unification of gravity’s dynamical and topological sectors from a common pregeometric source represents the central result of this work. We also uncover a seesaw mechanism linking the Planck mass and the cosmological constant, as well as several novel relations for the coupling constants of the topological sector, inclusive of the Barbero-Immirzi parameter. This framework establishes the pregeometric foundations from which all aspects of gravitation can dynamically emerge, providing a unified starting point for quantum gravity, dark energy phenomenology, and the study of topological phases in gravitational theories.

With evidence for a nanohertz gravitational-wave background now established by pulsar timing arrays, the search focuses on identifying individual supermassive black hole binaries. We show that these binaries produce a distinct spatial correlation pattern across the array, acting as a deterministic analog to the stochastic Hellings and Downs curve. We derive a closed analytic expression for this single-source overlap reduction function, ${\mathrm{\Upsilon }}_{ab}$, factorizing the signal into a source-dependent amplitude and a purely geometric fingerprint. Using simulated datasets, we demonstrate that this fingerprint breaks the degeneracy between an individual binary and a stochastic background. Including these cross-correlations yields Bayes factors of 1611 favoring the continuous-wave model over a Hellings and Downs correlated background model and 159 favoring the continuous-wave model over an uncorrelated red-noise model. Furthermore, these new cross-correlations improve sky localization by a factor of $11\mathsf{x}$ over an uncorrelated search. Finally, while coherent matched filtering offers higher theoretical sensitivity, we argue that a cross-correlation-based search for individual binaries provides a robust alternative that hedges against the possibility of overfitting to noise fluctuations by focusing on the evidence for the correlations. Indeed, the geometric fingerprints we present here show that spatial correlations can also be used to identify the first nanohertz gravitational-wave sources.

Inspired by Mashhoon’s framework connecting black-hole quasinormal modes to bound-state resonances in inverted potentials, Völkel’s recent numerical analysis of asymptotically flat Schwarzschild black holes revealed a counterintuitive phenomenon: highly excited bound states rapidly delocalize, become extremely weakly bound, and exhibit wave functions highly sensitive to far-field perturbations. To analytically explain this phenomenon and extend the investigation to Schwarzschild–de Sitter (SdS) black holes, we derive the characteristic equation for excited bound-state resonances in SdS spacetime and obtain compact closed-form analytical expressions for their resonance energies. In the $\mathrm{\Lambda }\to 0$ limit, our SdS-derived spectrum aligns perfectly with recent results for Schwarzschild black holes. We analytically demonstrate that the rapid and infinite delocalization of highly excited resonances is a universal feature of asymptotically flat Schwarzschild systems. More significantly, we prove that SdS black holes support only a finite number of bound-state resonance levels—in sharp contrast to the infinite spectrum of the asymptotically flat case. This finiteness implies an upper bound on the oscillatory domain of the bound-state resonances in SdS geometries, thereby preventing infinite delocalization and offering a fundamental distinction in the resonance structure of black holes in different asymptotic backgrounds. Surprisingly, we also find that delocalized half-bound states exist in SdS black holes when the $\mathrm{\Lambda }$ takes specific discrete values. This is a unique feature of SdS black holes and is absent in asymptotically flat Schwarzschild black holes. We also reveal the deep connection between the half-bound states and the number of bound-state resonance energy levels.

In the strong deflection regime, light rays passing close to an astrophysical black hole may remain trapped near unstable photon orbits for a long time before escaping to infinity. The traditional strong-deflection limit, which accurately describes the logarithmic divergence of the deflection angle for spherically symmetric and slowly rotating black holes, breaks down when the relevant prograde critical photon orbit coincides with the degenerate horizon of an extremal rotating black hole. We present a new strong deflection limit expansion for this horizon critical orbit, covering a general class of extremal rotating black holes. We show that the deflection angle exhibits a stronger power-law divergence in addition to the logarithmic divergence. For an adequate description of higher-order images, additional terms in the expansion must be retained. We first study prograde gravitational lensing in the equatorial plane and then extend the analysis to quasiequatorial motion, which allows us to calculate the magnification of the higher-order images and the position of the caustic points. We finally apply the general framework to explicit examples, including the Kerr, Kerr-Newman, and Kerr-Sen metrics.

Neutron star asteroseismology offers a unique opportunity to probe nuclear physics through stellar oscillations. Although the pressure inside of a neutron star is typically assumed to be isotropic, pressure anisotropy can arise from various physical mechanisms, including elasticity, viscosity, and magnetic fields. Previous studies of nonradial stellar quasinormal mode oscillations with anisotropic pressure have focused primarily on fluid modes. In this paper, we compute, for the first time, spacetime oscillation modes (so-called $w$-modes) of anisotropic neutron stars. Using a perturbative framework for stellar oscillations with pressure anisotropy, developed previously by some of the authors, together with a phenomenological anisotropy model, we find that both the real and imaginary parts of the $w$-mode frequencies decrease as the tangential pressure becomes dominant over the radial pressure. Although we do not find any unstable $w$-modes within the physically viable parameter space, unstable $w$-modes appear in an unphysical branch of solutions when the tangential pressure strongly dominates the radial one. We also find that the relation between the real part of the $w$-mode frequency and the stellar compactness is quasiuniversal with respect to variations in the equation of state and may become insensitive to the degree of pressure anisotropy as well for some anisotropy models. In contrast, the relation between the imaginary part of the $w$-mode frequency and the stellar compactness depends on the degree of anisotropy but remains equation-of-state universal when the anisotropy is fixed. Finally, we discuss potential mode crossings and the validity of certain approximations that have been shown to work well for $w$-mode calculations in the isotropic case.

Primordial black holes (PBHs) provide a well-motivated nonparticle candidate for dark matter, requiring an enhancement of curvature perturbations on small inflationary scales consistent with observational constraints. In this work we study PBH production within Horndeski gravity, accounting for compatibility with the GW170817 constraint on the gravitational-wave (GW) speed and imposing a constant coupling to the Ricci scalar. Under these conditions, and assuming an inflaton field characterized by a canonical kinetic term and a smooth potential, the inflationary dynamics is controlled by the cubic Horndeski interaction. By investigating standard functional forms of the latter we identify the specific kinetic structure that allows enhancement of the effective friction on the inflaton, thereby inducing a transient ultraslow-roll phase embedded within a standard slow-roll evolution. For representative parameter choices we find that pronounced amplifications in the scalar power spectrum are generated, leading to the formation of asteroid-mass PBHs with masses of order $\mathcal{O}({10}^{-16})M_{\odot }$, which can account for a substantial fraction of the dark matter abundance, reaching $f_{\mathrm{PBH}}\simeq 0.9$, while satisfying current observational constraints. The resulting characteristic features in the scalar power spectrum also imply potentially observable scalar-induced gravitational-wave signatures.

We present an improved subdominant-mode amplitude (SMA) test of general relativity (GR), which probes amplitude-level deviations in the higher-order modes of gravitational-wave (GW) signals from binary black hole mergers while keeping the dominant quadrupole mode fixed. Using a comprehensive parameter-estimation campaign, we benchmark the test against Gaussian noise fluctuations, waveform modeling systematics, and physical effects such as spin precession and orbital eccentricity. When applied to numerical-relativity simulations, the SMA test performs reliably for aligned-spin and mildly precessing systems but exhibits measurable biases for strongly precessing or eccentric binaries. Although designed to detect amplitude deviations, the test also responds coherently to phase perturbations, yielding apparent GR violations when applied to phase-modified waveforms. Applied to recent GW detections, we report the strongest constraint on the hexadecapolar (4, 4) mode amplitude deviation, $\delta A_{44}=-0.30_{-3.45}^{+1.16}$, consistent with GR. With these results, this work establishes the SMA test as a robust and broadly sensitive null test of general relativity and demonstrates a systematic approach for assessing the robustness of GW tests of GR.

The energy extraction from a magnetized Reissner-Nordström black hole is analyzed within the framework of the electric Penrose mechanism. The presence of an external magnetic field induces an axisymmetric configuration and an ergosphere (the region where energy extraction is possible) arises, allowing for negative energy states even in an otherwise static spacetime. By analyzing the decay of particles at turning points of the radial motion, we derive the general expression for the efficiency of the process in terms of the metric coefficients and the electromagnetic potential. The resulting efficiencies are interpreted as Killing energy efficiencies associated with the local splitting process, while the escape of the positive energy fragment is treated as an additional effective-potential requirement. This formulation provides a direct criterion for identifying the ergoregions and we show that the magnetic field acts as a control parameter that governs both the configuration of the ergosphere and the efficiency of the process. In particular, analytical expressions for the critical magnetic fields that determine the onset and suppression of energy extraction are determined. Our results extend previous analysis of the electric Penrose process for magnetized configurations and clarify the role of the external field in enhancing or inhibiting energy extraction from charged black holes.

We investigate the properties of charged black hole geometries in nonlinear electrodynamics. We focus on the recently reported analytic charged black hole solutions to illustrate the consequences of a nonmonotonic lapse function that exists for a wide range of black hole solutions. The spacetime admits stable light-rings, static near-horizon observers, and trapped near horizon photon orbits. We also show that although these modifications near the horizon are screened from afar, they nonetheless lead to additional branches of quasinormal modes for the black hole that are longer lived than the canonical Einstein branches.

We present a microscopic model of the dark sector that resolves the Hubble tension within standard current datasets (Planck 2018, $\mathrm{Pantheon}+$ and DESI DR2 BAO) based on well-known fundamental principles, gauge symmetry and spontaneous symmetry breaking. It builds on the hot new early dark energy (Hot NEDE) setup, featuring a dark $SU(N)$ gauge symmetry broken to $SU(N-1)$ in a supercooled phase transition that creates a thermal bath of self-interacting dark radiation in the epoch between big bang nucleosynthesis and recombination. Adding a fermion multiplet charged under the gauge symmetry provides a naturally stable component of dark matter that interacts with dark radiation. Spontaneous symmetry breaking predicts a decoupling of this interaction once the dark sector cools down, that we refer to as dark radiation matter decoupling (DRMD). We also provide a simplified DRMD model that captures the essential features of the full theory while retaining additional falsifiable predictions. Using the datasets stated above, we find agreement with the ${\mathrm{SH}}_0\mathrm{ES}$ determination of $H_0$ at the $1.4\sigma$ level, compared to a $5.7\sigma$ tension in $\mathrm{\Lambda }\mathrm{CDM}$, thereby providing a resolution of the Hubble tension.

We present a time-resolved digital quantum simulation of cosmological particle creation in a de Sitter–radiation transition in a Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. Instead of compiling only the final Bogoliubov transformation into a one-shot circuit, we discretize the conformal-time evolution and implement the dynamics as a Trotterized sequence of short-time circuit blocks. This formulation gives access not only to the late-time particle number, but also to the build-up of fixed-basis pair occupation during the nonadiabatic transition. Using a four-qubit single-excitation encoding for a momentum pair $(+\mathbf{k},-\mathbf{k})$, we compare matrix-Trotter evolution, noiseless statevector simulation, finite-shot Qiskit Aer simulation, and a shallow $N=1$ IBM hardware implementation. The simulator results are consistent with the analytic sudden-transition benchmark $n_k=1/[4(k{\eta }_e{)}^4]$ in the controlled single-excitation regime. The IBM experiment demonstrates execution of the shallow circuit block, but exhibits a residual hardware error of order ${10}^{-2}$, indicating that quantitative hardware reconstruction of the particle spectrum remains beyond current noisy intermediate-scale quantum (NISQ) performance.

We study stochastic gravitational waves (GWs) generated in a postinflationary magnetogenesis scenario with time-dependent gauge couplings during inflation and reheating. In this setup, magnetic anisotropic stress directly sources gravitational waves, while the induced curvature perturbations generate an additional scalar-induced GW component. We compare the spectral behavior of the two contributions and find that the magnetic-originated GW dominates the peak amplitude, whereas the scalar-induced contribution becomes important on larger scales. For $n_{\mathrm{b}}\ge 3/2$, both contributions exhibit the universal infrared scaling ${\mathrm{\Omega }}_{\mathrm{GW}}(f<f_{\mathrm{peak}})\propto f^3$, while their ultraviolet slopes differ, leading to distinct spectral signatures. For suitable reheating and magnetogenesis parameters, the resulting GW signal naturally extends into the nano-Hz range relevant for pulsar timing array observations, and it remains consistent with other bounds. The distinct spectral features of the two components may provide a useful probe of reheating dynamics and primordial magnetogenesis.

We present a comparative cosmological analysis of the Anton-Schmidt, $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$), and Chevallier-Polarski-Linder (CPL) models using baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument Data Release 2, combined with the Planck PR4 (NPIPE) CamSpec cosmic microwave background likelihoods and three type Ia supernova catalogs: Pantheon+, DES-Dovekie, and Union3. We use Markov Chain Monte Carlo analyses to constrain the parameters of the Anton-Schmidt model against each dataset combination. We find that the Anton-Schmidt model provides a good fit to current cosmological observations. The Anton-Schmidt model provides a good fit to the current cosmological observations but does not provide a significant alleviation of either the $H_0$ or the $S_8$ tensions. The Anton-Schmidt parameter is tightly constrained to $B\simeq -0.38$, indicating that the logarithmic correction becomes relevant only at late times. The Anton-Schmidt and CPL models predict quintessencelike present-day dark-energy equations of state and shows Quintom-B evolution, with their dark-energy equation-of-state parameters crossing the phantom divide at approximately $z\approx 0.5$. The characteristic density scale is constrained to be ${\rho }_{*}/{\rho }_{c,0}\sim 4-5$, substantially smaller than the large density scales typically assumed in logotropic-inspired scenarios, indicating that the Anton-Schmidt correction mainly affects the late-time expansion history. Finally, Bayesian evidence shows moderate-to-strong preference for the Anton-Schmidt model over the $\mathrm{\Lambda }\mathrm{CDM}$ model and strong-to-decisive preference over the CPL parametrization. Although the Anton-Schmidt model does not alleviate the current cosmological tensions, it emerges as a statistically favored dynamical dark-energy scenario whose cosmological implications deserve further tested with forthcoming stage-IV large-scale structure observations.

We present constraints on models of cosmology and astrophysics using cosmic shear data vectors from three datasets: the northern and southern Galactic cap of the Dark Energy Camera All Data Everywhere (DECADE) project, and the Dark Energy Survey (DES) Year 3. These data vectors combined consist of 270 million galaxies spread across $13,000{\mathrm{deg}}^2$ of the sky. We first extract constraints for $\mathrm{\Lambda }\mathrm{CDM}$ cosmology and find $S_8=0.805_{-0.019}^{+0.019}$ and ${\mathrm{\Omega }}_{\mathrm{m}}=0.262_{-0.036}^{+0.023}$, which is consistent within $1.9\sigma$ of constraints from the Planck satellite. Extending our analysis to dynamical dark energy models shows that lensing provides some (but still minor) improvements to existing constraints from supernovae and baryon acoustic oscillations. Finally, we study six different models for the impact of baryons on the matter power spectrum. We show the different models provide consistent constraints on baryon suppression, and associated cosmology, once the astrophysical priors are sufficiently wide. Current scale-cut approaches for mitigating baryon contamination result in a residual bias of $\approx 0.3\sigma$ in the $S_8,{\mathrm{\Omega }}_{\mathrm{m}}$ posterior. Using all scales with dedicated baryon modeling leads to negligible improvement as the new information is used solely to self-calibrate the baryon model on small scales. Additional nonlensing datasets, and/or calibrations of the baryon model, will be required to access the full statistical power of the lensing measurements. The combined dataset in this work represents the largest lensing dataset to date (most galaxies, largest area) and provides an apt testing ground for analyses of upcoming datasets from stage IV surveys. The DECADE shear catalogs, data vectors, likelihoods, etc. are made publicly available.

Weak-interaction-mediated chiral imbalance generation in idealized massless electrons during core-collapse supernovae was once proposed to be the source of strong magnetic fields found in neutron stars. The effect goes by the name of chiral plasma instability. However, it was found that a finite electron mass damps out this process, inactivating the instability and preventing magnetic field growth. In this work, we show that the instability can survive in the presence of abrupt density and temperature perturbation that drives the system sufficiently far out of weak equilibrium. As an example, we work with such perturbations generated by shock waves which are common during both core collapse as well as neutron star mergers. We find that the chiral imbalance resulting from shock waves, under the right conditions of density and temperature, can sustain the chiral plasma instability despite the damping from the electron mass. Additionally, in an already magnetized medium, the chiral magnetic effect resulting from shock-wave density and temperature perturbation can generate substantial Ohmic heating. Our results imply that shock waves generated in core-collapse supernovae and merging neutron stars can act as a source of strong heating in a magnetized medium as well as chiral plasma instability.

In a dense star cluster, hierarchical mergers are among the most promising pathways to forming massive black holes such as GW231123. A key factor determining whether a merger-remnant black hole will be retained in these environments and thus participate in subsequent hierarchical mergers is the recoil kick velocity. Analytic models for the recoil velocity are currently employed in nearly all population-synthesis frameworks. We instead use a state-of-the-art recoil-kick model gwmodel_flow_prec [T. Islam and D. Wadekar, arXiv:2511.11536] developed from a combination of numerical-relativity and black-hole–perturbation-theory data, together with data-driven techniques such as normalizing flows and the post-Newtonian structure of the kick. Employing both back-of-the-envelope estimates and detailed $N$-body as well as semianalytical cluster simulations, we show that gwmodel_flow_prec leads to a noticeable increase in the retention probability of hierarchical-merger remnants compared to the previously used analytic model and changes the mass and spin distribution of the black holes formed through hierarchical mergers. Additionally, we discuss the implications of our results in the context of massive binaries such as GW231123.