We study first-order scalar and tensor perturbations of Jordan-frame scalar-tensor gravity about a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background using the Einstein-like effective-fluid decomposition of the scalar sector. In the scalar-gradient frame, we derive the perturbed effective density, pressure, heat flux, and anisotropic stress, and show that they admit an exact Eckart-type constitutive identification at linear order. We then show that these same quantities appear explicitly and exhaustively in the linearized field equations: the scalar Hamiltonian, momentum, trace, and traceless Einstein-like equations are governed, respectively, by the effective density, heat-flux, pressure, and anisotropic-stress channels, while the tensor propagation equation is governed by the transverse-traceless anisotropic-stress channel. In particular, the Jordan-frame modification of gravitational-wave damping is identified with the effective transverse-traceless anisotropic stress of the scalar sector. We also derive the perturbed evolution equation for the invariant product $\kappa T$, clarify its gauge behavior, and show that flux matching on FLRW fixes only the background value $\overline{\kappa T}$, not its perturbation. These results leave open the possibility that gravitational waves in scalar-tensor gravity admit a deeper thermodynamic characterization, perhaps even an intrinsic one, although the present analysis establishes this only at the level of an effective constitutive description.

We investigate quartically self-interacting massive boson stars by constructing equilibrium sequences and performing dynamical evolutions. The mass curve $M(|{\varphi }_c|)$ along these sequences develops multiple extrema, yet stability changes only at the first maximum; configurations beyond it become highly compact and collapse under numerically induced perturbations, with near-critical models displaying a short-lived double-dive behavior. Head-on collisions of equal-mass stars yield three distinct outcomes—boson star remnants, black hole formation at contact, and collapse of each star to a black hole prior to contact. The associated gravitational-wave energies reflect a competition between increasing compactness, which enhances the efficiency of gravitational-wave emission, and decreasing tidal deformability, which suppresses merger asymmetries, and at large self-interaction strengths the collapse-before-contact branch exhibits a pronounced nonmonotonic structure. The simulations reported here constitute a substantial catalog of initial conditions and waveforms, providing a natural basis for constructing surrogate models capable of rapidly predicting gravitational-wave signals across an extended parameter space.

Using matching properties of the gravitational field at timelike and spatial infinity, together with universal formulas for gravitational wave tails, we obtain exact expressions for the peeling-violating components of the Weyl tensor at future and past null infinity. The coefficients depend solely on incoming scattering data and reduce, in the Newtonian limit, to those found by Damour long ago. The computation is facilitated by the observation that the tail and peeling-violating coefficients organize into a “celestial diamond” introduced in the context of celestial holography. The analysis suggests a new matching condition at spatial infinity which, together with previously found matching at timelike infinity, capture both tail and peeling-violation formulas. We conclude by pointing out that stronger peeling violations occur in perturbative quantum gravity, in agreement with a recently reported amplitude-based result.

We study the bound state solutions for the polar perturbations in the interior of the Schwarzschild black hole. It is shown that for a given value of the spherical harmonic index $\ell$, there are a total of $\ell -1$ bound states for polar perturbations. We show both analytically and numerically that the spectrum of $\ell -2$ of these perturbations coincides exactly with the spectrum of axial perturbations. Consequently, the isospectrality between the bound states of axial and polar perturbations in the interior of the black hole is preserved. Furthermore, the additional mode found in the spectrum of polar perturbations is the algebraically special mode, which also furnishes the ground state of polar perturbations. It is shown that the spectrum of the highly excited states is equally spaced, which, in the semiclassical approximation, yields the black hole area quantization $\mathrm{\Delta }A=16\pi l_{\mathrm{Pl}}^2$.

Future gravitational wave detections of merging binary neutron star systems have the possibility to tightly constrain the equation of state of dense nuclear matter. In order to extract such constraints, gravitational waveform models need to be calibrated to accurate numerical relativity simulations of the late inspiral and merger. In this work, we take an essential step toward classifying the error and potential systematics in current generation numerical relativity simulations of merging binary neutron stars. To this end, we perform a direct comparison of two codes (fil, spec), which differ in many aspects, including the numerical methods and discretizations used and equations solved. We find that despite these different approaches, the codes are—within current numerical resolution bounds—fully consistent, and broadly comparable in cost for a given accuracy level. Our results indicate that the error in the waveforms is primarily dominated by the hydrodynamic evolution, consistent with earlier findings in the literature. We also discuss current limitations and cost estimates for numerical relativity simulations to reach the accuracies required in the era of next-generation gravitational detectors.

Stochastic gravitational-wave backgrounds (SGWBs) of primordial origin offer a powerful probe of early-Universe physics and possible dark-sector dynamics. While most searches focus on the GW power spectrum, additional information is encoded in higher-order correlators that characterize the statistical properties of the signal. In this work we study non-Gaussian features of a cosmological SGWB generated at second order by vector fluctuations, a class of sources well motivated in early-Universe scenarios. Within this framework we develop tools to characterize higher-order GW correlators and compute representative four-point functions that generate a connected contribution to the GW trispectrum. We show that the trispectrum amplitude scales as the square of the GW power spectrum and peaks in characteristic folded momentum configurations, reflecting the structure of the nonlinear source. We then explore the observational implications. First, we demonstrate that the connected trispectrum contributes to the variance of two-point overlap reduction functions, including the Hellings-Downs curve relevant for pulsar timing arrays. We then construct the optimal estimator to measure the connected trispectrum with ground-based interferometers. Our results highlight how non-Gaussian SGWB statistics provide a complementary observable to probe the origin of GW backgrounds and to distinguish cosmological from astrophysical sources.

We explore the sensitivity of weak lensing surveys to gravitational waves (GWs) emitted by inspiraling supermassive black hole binaries (SMBHBs) in the nanohertz to microhertz frequency band, bridging the gap between pulsar timing arrays and space-based interferometers. Building on the formalism for GW-induced shear distortions, we develop a signal-to-noise framework that incorporates survey characteristics such as cadence, angular resolution, and depth. We model the effective galaxy population to evaluate the noise power spectral density and derive characteristic strain sensitivity curves. Applying this framework to both LSST-like and idealized survey configurations, we show that current surveys are limited by angular resolution and measurement noise, while an idealized, cosmic-variance-limited survey could in principle probe this frequency range. We emphasize that such sensitivity requires observational capabilities far beyond those of existing or planned facilities, and our results should be interpreted as an ultimate limit on the information accessible through weak lensing measurements.

Gravitational lensing observables, including anomalies in image positions, flux ratios, and time delays, serve as usual probes of dark matter substructure. When dark matter substructure possesses sufficient perturbations, it may lead to the formation of extra images in otherwise canonical doubly or quadruply imaged systems. With the advent of increasingly precise observational instruments, previously undetectable images may become measurable and image number anomalies therefore could be an increasingly viable method. In this paper, we utilize the gravitational lensing phenomenon of image number anomaly to derive constraints on dark matter substructure. We present the extra images induced by distinct forms of dark matter substructure, specifically primordial black holes (PBHs) and fuzzy dark matter and show that higher angular resolution observations increase the probability of detecting additional lensed images. Based on a null detection of image number anomalies in a sample of 3500 lens systems generated from the strong lensing halo model-based mock catalogs, we derive upper limits on the abundance of PBHs. At the 95% confidence level, the PBH abundance is constrained to $\lesssim 0.125\%$, 0.08%, and 0.04% for PBH masses in the range $\sim {10}^7–{10}^9{M}_{\odot }$, corresponding to angular resolutions of ${0.1}^{\prime \prime }$, $0.05^{\prime \prime }$, and $0.01^{\prime \prime }$, respectively. Similarly, we exclude particle masses below 0.4, 0.6, and $3.5\times {10}^{-22}\mathrm{eV}$ for fuzzy dark matter at the same confidence level for the respective resolutions. Furthermore, the abundance of $\mathrm{PBHs}\lesssim 0.9\%$ could be constrained at an angular resolution of ${0.5}^{\prime \prime }$ for the Legacy Survey of Space and Time observations. Finally, we discuss methodologies for identifying image number anomalies in special cases and demonstrate feasibility using a fitting procedure.

We study primordial perturbations in Higgs–$R^2$ inflation in the presence of nonminimal kinetic mixing between the Higgs field and the scalaron. By numerically solving the multifield background and linear perturbation equations, we identify distinct dynamical regimes controlled by the Higgs nonminimal coupling ${\xi }_h$. For ${\xi }_h\sim \mathcal{O}(0.1)$, transient turning of the inflationary trajectory leads to a transfer between adiabatic and isocurvature modes, generating localized features in the primordial curvature power spectrum. In contrast, in the weak-coupling regime ${\xi }_h\ll 1$, the curvature spectrum remains nearly featureless while isocurvature perturbations do not fully decay, resulting in a residual isocurvature component at the end of inflation. We compute the associated cosmic microwave background angular power spectra and discuss the observational implications of these regimes. Our results highlight the role of multifield dynamics in shaping primordial perturbations and provide constraints on viable realizations of Higgs–$R^2$ inflation.

Dynamical tides can provide us vital information about the properties of neutron star (NS) matter. This is particularly true for $g$-modes, whose frequency and tidal coupling are highly sensitive to the composition of NSs, especially in their centers, where microphysical models are the least reliable. However, due to their weak coupling to external tidal fields, their effect on the gravitational-wave (GW) signal of binary inspirals can be difficult to observe. Here we show that the detectability of these tides can be significantly enhanced by binary NSs with moderate eccentricities. This is primarily due to higher eccentric harmonics in the early phase of the binary evolution experiencing larger phase shifts, which they transport to the sensitive band of GW detectors. In addition, $g$-mode tides in eccentric binaries undergo several epicyclic resonances, which also amplify the total phase shift. We demonstrate that these effects increase the detectability of $g$-mode dynamical tides by more than an order of magnitude for eccentricities of $e_{10\mathrm{Hz}}\sim 0.2–0.4$, making it possible to put robust constraints on $g$-mode properties using current GW detectors, while all relevant models could potentially be constrained with eccentric binary NSs with the Einstein Telescope.

The interiors of neutron stars enjoy ideal conditions for the conversion of hadrons to a strange quark phase, theorized to be the stablest form of matter. Though numerous astrophysical means to prompt such a deconfinement phase transition have been suggested, they may be preempted by a large energy barrier for nucleation of quark matter droplets. We will show that interactions of hidden sectors of particles with nucleons may surmount the barrier if it exceeds deca-GeV energies, and spark a phase transition. The neutron star would then, depending on the equation of state of QCD matter, convert to a black hole and/or set off a gamma-ray burst (GRB). Using the observed existence of ancient neutron stars and estimates of the GRB rate, we then set some of the strictest (albeit conditional) limits on dark matter scatters, annihilations, and decays that are tens of orders stronger than those from terrestrial searches. For smaller energy barriers, lower limits on nucleon decay lifetimes of the order of ${10}^{64}\mathrm{yr}$ may be obtained.

Two extreme events in the Universe, fast radio bursts (FRBs) and cosmic rays (CRs), could be correlated, where FRBs with extreme field strength near their sources may contribute to CRs. This study investigates localized particle acceleration driven by FRB-like ultra-relativistic electromagnetic pulses in an electron-positron-ion plasma system. It is found that ultrahigh energy neutral plasma sheets form constantly via the front erosion of an FRB pulse. There are two regimes of ion acceleration depending upon the field strength and the plasma density: the piston regime driven by the Lorentz force of the pulse, and the wakefield regime dominated by charge separation field. The predicted energy scalings align well with particle-in-cell simulations. A power-law energy spectrum with an index close to the CRs naturally emerges during FRBs expansion outward. Detecting high-energy particles possibly produced by FRBs enables deeper insights into their origins and promotes the development of multimessenger astronomy.

This study investigates the impact of self-interaction strength on the formation and scale of turbulent vortex structures during the merger of Bose stars, using numerical simulations of the Gross-Pitaevskii-Poisson equations. We find that vortex formation is a universal outcome of soliton mergers, with the vortex size strongly dependent on the self-interaction coupling parameter $g$. Through analysis of velocity correlations, kinetic energy spectra, and vorticity distributions, we conclude that, for repulsive self-interaction, the vortex region expands as self-interaction strength increases; conversely, for attractive self-interaction, the vortex region shrinks as self-interaction strength increases.

We propose a cogenesis scenario in which both the baryon asymmetry and dark matter abundance originate from the kinetic misalignment of the QCD axion. The framework unifies the Peccei-Quinn (PQ) mechanism with a type I seesaw sector, where Hubble-induced masses and higher-dimensional PQ-violating operators drive early time axion rotation. Working within the Dine-Fischler-Srednicki-Zhitnitsky axion model augmented by heavy neutrinos, we identify the parametric window of right-handed neutrino masses, determined by its decay rate, and the range of Hubble scales compatible with successful cogenesis, while maintaining the axion solution to the strong $CP$ problem and satisfying current limits on axion isocurvature perturbations. Our results establish kinetic misalignment as a robust and predictive mechanism for the QCD-axion cogenesis, leading to correlated constraints on the right-handed neutrino mass and the inflation scale for an axion decay constant within the viable range $f_a\sim {10}^8–{10}^{12}\mathrm{GeV}$.

Experimental searches for neutron-antineutron ($n\to \overline{n}$) transitions can be considered via two approaches: conversion in free-neutron beams and intranuclear transformation leading to matter instability in large-mass detectors. Plans for next-generation searches make it timely to highlight the complementarity, necessity, and limitations of each method. Converting the bound neutron limit into one for free neutrons traditionally utilizes nucleus-specific estimates of the in-medium suppression of $n\to \overline{n}$, obtained within mean-field theory under a single-operator assumption. This paper highlights how this suppression can be scenario-dependent, which can lead to deviations from the standard approach that can span several orders of magnitude. A further goal of the paper is to point out the need for a broader phenomenology program for $n\to \overline{n}$ that is akin to those developed for electric dipole moments and other systems for which short-distance new physics must be studied in medium. We find that moderate deviations from the standard suppression factor arise generically via multioperator interference effects with the largest enhancements corresponding to regions of parameter space with partial phase alignment between operator contributions.

The complexity of highly excited eigenstates is a central theme in nonequilibrium many-body physics, underpining questions of thermalization, classical simulability, and quantum information structure. In this work, considering the paradigmatic Rokhsar-Kivelson model, we connect quantum many-body scarring in Abelian lattice gauge theories to an emergent stabilizer structure. We identify a distinct class of scarred eigenstates, termed sublattice scars, originating from gauge-invariant zero modes that form exact stabilizer states. Remarkably, although the underlying Hamiltonian is not a stabilizer Hamiltonian, its eigenspectrum intrinsically hosts exact stabilizer eigenstates. These sublattice scars exhibit vanishing stabilizer Rényi entropy together with finite, highly structured entanglement, enabling efficient classical simulation. Exploiting their stabilizer structure, we construct explicit Clifford circuits that prepare these states in a two-dimensional lattice gauge model. Our results demonstrate that the scarred subspace of the Rokhsar-Kivelson spectrum forms an intrinsic stabilizer manifold, revealing a direct connection between stabilizer quantum information, lattice gauge constraints, and quantum many-body scarring.

We develop the discrete set of Dyson-Schwinger equations for scalar fields and solve them for some cases. We show that their solutions are Gaussian in the continuum limit as expected from the theorems of Aizenman [Phys. Rev. Lett. 47, 886 (1981); Commun. Math. Phys. 86, 1 (1982)] and of Aizenman and Duminil-Copin [Ann. Math. 194, 163 (2021)] for $d\ge 4$. Extension to lower dimensionality fails, as it should, by observing that the triviality theorems used in our proof are not applicable in such cases.

In this work we investigate the influence of weak acceleration on the confinement-deconfinement phase transition in gluodynamics. Our study is carried out within lattice simulation in the comoving reference frame of accelerated observer which is parametrized by the Rindler coordinates. We find that finite temperature confinement-deconfinement phase transition turns into spatial crossover in the Rindler spacetime. In other words, spatially separated confinement and deconfinement phases can coexist in the Rindler spacetime within certain intervals of temperature and acceleration. We determine the position of the boundary between the phases as a function of temperature for several accelerations and find that it can be described by the Tolman-Ehrenfest law with rather good accuracy although a minor deviation takes place. Moreover, the critical temperature of the system in the weak acceleration regime is found to remain unchanged as that of the standard homogeneous gluodynamics. Our results imply that the spatial confinement-deconfinement transition might take place in the vicinity of the Schwarzschild black hole horizon.