With the multitude of gravitational wave observations that have been made in the past ten years, probing the dynamical and nonlinear nature of strong gravity is becoming more and more feasible. One promising way to test the nonlinear nature of Einstein’s theory of general relativity (GR) is through the gravitational wave null memory effect: a nonlinear prediction of GR which corresponds to initially comoving observers being permanently displaced due to a burst of gravitational radiation. Previous studies have shown that, while it is unlikely that the memory effect will be observed in a single event by the LIGO-Virgo-KAGRA (LVK) detectors, evidence for memory in the population of LVK events should be attainable after $\sim 2000$ gravitational wave detections. Many of these works, however, largely relied on Bayes factors to perform their memory analyses: an approach that can depend sensitively on the analysis priors and, when multiplied across many events, can even favor incorrect conclusions. In this work, using the GWTC-5.0 catalog of binary black hole observations, we instead perform hierarchical Bayesian inference—which is not subject to the issues associated with Bayes factors—to measure the evidence for memory in current LVK observations. We find that we can constrain what we call the memory enhancement factor—the constant appearing in front of the contribution to the strain from the supermomentum flux—to ${0.26}_{-4.08}^{+4.09}$ (with $\pm$ values denoting the 68% credible interval), consistent with its GR value of 1. We also forecast that $\sim 2000$ detections will be needed to constrain the memory enhancement factor away from zero at the $1\sigma$ level.

We present a complete pipeline for detecting and identifying gravitational waves (GWs) produced by the inspiral of stellar-mass binary black holes in data from the Laser Interferometer Space Antenna (LISA). The analysis framework relies on an efficient time-frequency implementation of an adaptive semicoherent detection statistic, which we show to be robust against nonstationary noise and the presence of gaps of varying duration and cadence. The search is able to detect signals with coherent signal-to-noise ratios as low as $\approx 11–14$ over the full parameter space of spin-aligned binaries (using nonspinning templates) with orbital eccentricity $\le 0.01$ when deployed on the 2-year-long LISA Data Challenge Yorsh. The search can be run within a day using $\approx 40$ GPUs. The techniques presented here have wider applications in GW astronomy, in particular the search for extreme-mass-ratio inspirals in LISA data.

Quasinormal modes (QNMs) of black holes can exhibit avoided crossings (ACs), in which specific QNM frequencies approach each other while their amplitudes are enhanced and acquire nearly opposite phases, leading to characteristic interference. Resolving such closely spaced modes through black hole spectroscopy is observationally challenging. In this paper, we investigate the detectability of nearly degenerate QNMs in the presence of an AC within a Bayesian framework using three waveform models. We examine how the inference of the complex frequencies and amplitudes depends on the separation between the two QNM frequencies and on the choice of template waveform. We find that resolving the individual QNM frequencies is difficult even under optimistic conditions. On the other hand, collective waveform signatures associated with ACs may still be identified through complementary waveform descriptions, provided that the AC-related modes dominate the observed ringdown signal and contamination from more slowly damped modes is negligible or can be removed.

This work presents a new class of exact analytic rotating black hole solutions within five-dimensional generalized Proca theories. Through a Kerr-Schild ansatz where the Proca field is set along a null geodesic congruence, the nonlinear field equations reduce to a consistent set of three master equations. This geometric configuration ensures that the vector field remains lightlike on-shell, effectively restricting the theory’s functional couplings to discrete constants and allowing for a fully analytic treatment. The resulting solutions, incorporating a cosmological constant and two independent angular momenta, exhibit primary hair given by an arbitrary function of the non-Killing angular coordinate. We identify several solution branches defined by specific algebraic relations between the Proca coupling constants, providing a significant generalization of the Myers-Perry family. Notably, the metric retains a Kerr-Schild form identical to the Myers-Perry representation, with an additional contribution constructed from the tensor product of the Proca one-form with itself.

Primordial black holes (PBHs) forming prior to big bang nucleosynthesis evolve during the reheating epoch, an environment dominated by an oscillating inflaton field decaying into a relativistic thermal bath. In this work, we track the complete lifecycle of PBHs within this coupled inflaton-radiation background. Utilizing $\alpha$-attractor E models, we analytically anchor the reheating initial conditions directly to cosmic microwave background observations. By matching exact scalar field solutions in a Schwarzschild spacetime to the cosmological far zone, we derive the cycle-averaged mass accretion rate and couple it to the growing radiation bath. We find that this combined accretion induces a highly nonlinear enhancement of the final PBH mass. Because the Hawking evaporation timescale scales cubically with mass, PBHs forming near their critical runaway limits experience a massive extension of their lifespans. Surviving deeper into the radiation-dominated era triggers a multi-order-of-magnitude amplification in their emitted stochastic gravitational wave background.

We analyze $F$-term hybrid inflation (FHI) within various grand unified theories in the presence of a Kähler potential for the inflaton field which parametrizes the Kähler manifolds $SU(1,1)/U(1)$ or $SU(2)/U(1)$. We take into account supergravity, radiative, and soft supersymmetry-breaking corrections to the tree-level potential and find that viable FHI can be realized without extrema along the inflationary trajectory for a broad region of the parameter space. For selected superpotential parameters, the models’ predictions are largely influenced by the curvature of the internal space and the magnitude of the tadpole parameter which are constrained so as to achieve compatibility with the current Atacama Cosmology Telescope and the South Pole Telescope data. We also discuss the formation of cosmic strings and their associated gravitational wave signals, potentially detectable by current and upcoming experiments.

A puzzling implication of current data interpreted in the $\mathrm{\Lambda }$ cold dark matter cosmology is the preference for a negative sum of neutrino masses. Moving to $w_0{w}_a\mathrm{CDM}$ brings an appreciable fraction of the neutrino mass posterior back to positive values, while the constant equation-of-state dark energy case $w\mathrm{CDM}$ does not. We investigate a variety of one-parameter dynamical dark energy equations of state (DE EOS) whose redshift evolution is controlled by a single free parameter, each representing distinct physical properties, to understand whether a two-parameter DE EOS is required to bring the neutrino mass into the positive region. The conclusion is that certain one-parameter DE EOS can suffice, implying that the data are pointing toward physical characteristics rather than a broad degeneracy. This behavior effectively lowers the dark energy density at high redshift, allowing the sum of neutrino masses to shift toward the physical region. The required characteristics are identified as phantom dark energy at high redshift, crossing $w=-1$ at lower redshift.

We perform a comprehensive hierarchical, multiscale morphological analysis to search for anomalous behavior in the large-scale matter distribution, using the convergence map from the Atacama Cosmology Telescope Data Release 6 (ACT DR6). Our analysis employs a suite of morphological statistics, including Minkowski functionals, contour Minkowski tensors, and Betti numbers, and measures their deviations from ensemble expectations and median values obtained from isotropic $\mathrm{\Lambda }\mathrm{CDM}$ (cold dark matter) simulations provided by ACT. To assess the statistical significance of these deviations, we develop a general methodology based on the persistence of anomalies across threshold ranges and spatial resolutions, while accounting for correlations among the statistics. Analyzing both the full dataset and hemispherical subregions, we find overall consistency with isotropic $\mathrm{\Lambda }\mathrm{CDM}$ expectations. Recognizing that potential deviations in smaller sky regions may be diluted when averaged over larger areas, we further examine localized sky patches. This patchwise analysis identifies several regions exhibiting statistically significant deviations, which we designate as anomalous. In particular, we find that positive density fluctuations near the CMB Cold Spot are anomalous, exceeding 99% confidence level. This region also overlaps with a previously reported anomalous southern spot in the Planck CMB temperature data. Analysis of the corresponding null map indicates potential contamination from residual foregrounds or instrumental systematics. We also compare the anomalous patches identified in ACT data with those from a previous analysis of the Planck convergence map. No overlapping anomalous regions are found between the two datasets, suggesting that the anomalies in the Planck data are likely attributable to noise. Further investigation of these atypical patches, considering the role of residual foregrounds, instrumental systematics, reconstruction noise, and independent large-scale structure surveys, is warranted to determine their physical origin.

Sterile neutrinos in the keV mass range have long been considered a well-motivated dark matter (DM) candidate. In this work, we explore a sterile neutrino production mechanism through active neutrino self-interactions in the early universe, assuming that they constitute the full DM abundance. We implement a self-consistent treatment of the sterile-neutrino free streaming and the active-neutrino self-interactions on structure formation, which yield a unique scale-dependent modification to the linear matter power spectrum. We then set bounds on this scenario using a combination of the cosmic microwave background and $\mathrm{Ly}\alpha$ forest constraints. Specifically, we utilize the two recent likelihoods derived from eBOSS data: (i) an effective field theory (EFT) based full-shape likelihood and (ii) a compressed likelihood obtained from the PRIYA-simulation emulator. We produce some of the most stringent observational constraints to date on sterile neutrino DM, comparable to the bounds from the most stringent laboratory constraints.

We identify a geometric obstruction to resolving the Hubble tension by combining early-time sound-horizon reduction with late-time smooth dark energy. The baryon acoustic oscillation (BAO)-supernova (SN) matter-density gap, $\mathrm{\Delta }{\mathrm{\Omega }}_m=0.037$ within Lambda cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$), is exactly invariant under the sound-horizon rescaling $\alpha \equiv r_s^{\mathrm{mod}}/r_s^{\mathrm{\Lambda }\mathrm{CDM}}$, and late-time $w(z)$ deformations cannot eliminate it either; reconciling the two datasets requires opposite deformations; phantom ($w<-1$) for BAO, quintessence ($w>-1$) for SN at $z<0.5$; an antialignment quantified by $\cos \theta =-0.98$ in $w(z)$ space. A full Markov chain Monte Carlo analysis of DESI DR2 BAO, Planck plik_lite, and Pantheon+ bears this out; the optimal ${\alpha }^{*}=0.992$ ($0.8\%r_s$ reduction) brings the joint fit to $H_0=70.3\pm 0.3\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$, still $3.2\sigma$ below SH0ES, with the interdataset tension reduced but not removed. The obstruction reflects not a shortage of model freedom but an irreducible disagreement between probes. The deformation space $\{\alpha ,{\beta }_{\mathrm{damp}},w(z)\}$ already spans 94% of the ${\mathrm{\Omega }}_m$ response direction; nonetheless BAO and SN constrain ${\mathrm{\Omega }}_m$ through independent channels and disagree, while the residual $H_0$ deficit, anchored by the local distance ladder, resides in the absolute distance scale, which $w(z)$ reshapes but cannot rescale.

Systems that feature a scalar field $\varphi$ with a quasi scale invariant potential, metastable at $\varphi =0$, can remain trapped, during cosmic evolution, in the ‘wrong’ vacuum because the process of bubble nucleation to the true vacuum is inefficient. If $\varphi$ carries a conserved charge $Q$, the presence in the system of a nonzero chemical potential for $Q$ offers the possibility of escaping eternal supercooling; when a species decouples from the plasma the balance among the stabilizing effect of temperature and the destabilizing effect of chemical potential can change in favor of the latter, so that $\varphi$ condenses and triggers the transition to the stable phase.

In the excursion-set formalism, the mass distribution of primordial black holes (PBHs) is derived from the first-passage time of a random walk describing the density contrast as the coarse-graining scale varies. We address two recent concerns that have been raised about this approach. First, it was argued that the random walks are subject to colored (i.e. correlated over time) noise, making the first-passage-time problem cumbersome. We show that this arises from an incorrect separation of drift and noise when sampling on the Hubble-crossing surface: if Fourier modes are uncorrelated, the noise is strictly white. Moreover, sampling along the Hubble-crossing surface precludes using the density dispersion as a time variable, explaining some pathologies. Sampling instead on a synchronous surface removes both issues. This requires solving a first-passage-time problem with a moving barrier, for which we provide an efficient numerical framework. Second, it was suggested that cloud-in-cloud (i.e. that large black holes may engulf smaller ones) is irrelevant for PBHs and that the excursion set is therefore not needed. While valid for widely separated scales, this statement fails for broad power spectra with enhanced continua of modes. We further show that Press-Schechter estimates neglecting boundary evolution can break down even without cloud-in-cloud effects.

A warm neutron star is expected to have a fluid “ocean” of heavy elements at its outermost part of the outer crust. As is on the terrestrial ocean, the neutron star ocean also has surface gravity waves. Around a magnetic pole of a star, the ocean may have a dip due to the strong magnetic pressure coming into play in the hydrostatic balance of the ocean. The dip enables the surface gravity wave to be trapped around the magnetic pole to form eigenmodes. The frequency of the mode is much lower than the dynamical frequency at the stellar surface, owing to the weak Coriolis force and the gradient in the ocean’s depth that makes the eigenmodes present. We solve the equation of surface gravity waves in the local $\beta$-plane approximation and obtain the spectrum of discrete eigenmodes. We see that there are no axisymmetric modes and that the mode frequency decreases and asymptotes to zero as the number of nodes of the corresponding eigenfunction increases. This is reminiscent of the g-modes in the context of asteroseismology. Observations of x-ray binaries containing neutron stars reveal that some of the systems exhibit low-frequency quasiperiodic oscillations (QPOs) whose frequency is $1-{10}^3\mathrm{mHz}$. We investigate whether the eigenmodes considered here may explain the low-frequency QPO spectrum. It is suggested that some of the QPOs in the system whose neutron star spins at the period less than 10s may be consistent with the model. As far as the spin period is larger than 10s, the eigenmode frequencies are too low to explain the observed QPOs.

The detectability of neutrinos from past Galactic core-collapse supernovae (SNe) is investigated in terms of nuclear recoil tracks in ancient minerals, known as paleodetectors. To account for the diversity of core-collapse outcomes, variations in neutron star (NS) masses as well as failed SNe leading to black hole formation are taken into account. The role of the nuclear equation of state is also considered, as it determines NS radii and maximum masses. The enhancement of sensitivity is quantified for models with larger neutrino emission. This emission reflects the gravitational energy released during core collapse and is larger for more compact remnants in the NS-forming case and for equation of state with larger maximum masses in the black-hole-forming case. Furthermore, motivated by the hypothesis that nearby SN activity may have contributed to the snowball Earth events, the sensitivity to SNe occurring in a burstlike manner is also investigated. The results indicate that burstlike SN activity involving several tens of events at a distance of 10 pc would be within the reach of paleodetectors.

Neutron stars are very dense media in which axions may be produced. This has been used to set a limit on the QCD axion mass, usually under the assumption that only neutrons, protons, electrons, and muons appear in the star core. Given the extreme conditions reached within neutron stars, it is reasonable to consider that other particles, such as hyperons and $\mathrm{\Delta }$ resonances, may appear in the neutron star core. In this work, we study how the limit on the mass of QCD axions, namely Kim-Shifman-Vainshtein-Zakharov and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) invisible axions, is altered when different equations of state are used, allowing for heavier particles to appear in the neutron star core. We find that this dependence is in general mild and thus reinforces the reliability of the known limit. Additionally, in the DFSZ scenario, it may drive the limit within the sensitivity window for IAXO. This would allow this experiment to discern the composition of neutron star cores if an axion were to be observed within that window.

The paper revisits the role of hyperon bulk viscosity during neutron-star inspiral. We find that hyperon bulk viscosity exerts only a minor influence on the gravitational-wave phase, causing a phase shift of ${\sim 10}^{-3}\mathrm{rad}$. This shift is too small to be detected by existing gravitational-wave observatories, and it seems unlikely that next-generation detectors will be able to isolate this effect from other factors. However, our analysis indicates that hyperon bulk viscosity can significantly heat the hyperon core of a neutron star, raising its temperature to approximately ${5\times 10}^8\mathrm{K}$.

Dwarf spheroidal galaxies provide some of the most sensitive astrophysical probes of ultralight dark matter (ULDM), but the inferred constraints can be affected by two important systematics: tidal interactions with the Milky Way, which reduce ULDM-induced dynamical heating, and stellar self-gravity, which can become relevant if the stellar component was more compact at earlier times. In this work, we attempt to estimate both effects by reconstructing dwarf-galaxy orbital histories in a Milky Way potential, adopting a simple and approximate tidal-susceptibility diagnostic that we argue provides a conservative description of tidal stripping, and explicitly including stellar self-gravity in our numerical simulations. Within our framework, which we apply to five different dwarf galaxies, we find that ULDM with masses $5\times {10}^{-22}\lesssim m/\mathrm{eV}\lesssim 5\times {10}^{-21}$ remains in tension with current data.

Light dark matter (DM) [$\mathrm{mass}\lesssim \mathcal{O}(100)\mathrm{keV}$] remains challenging to detect in several ongoing indirect detection experiments due to threshold limitations. Recent observations of diffuse x-ray photons from the NuSTAR stray-light (SL) data provide a powerful avenue to probe such light DM through its decay signatures in the galactic halo. This work explores the indirect detection prospects of decaying electrophilic scalar DM, electrophilic and photophilic axionlike particle DM, and dark photon DM using the recent NuSTAR SL data. We find that for DM scenarios producing monochromatic two-photon signals, NuSTAR SL data can yield the strongest indirect detection bound in the 7–36 keV mass range. In contrast, for dark photon (vector) DM featuring a continuous three-photon spectrum, the strongest indirect detection upper bound arises in the 22–65 keV mass range. Additionally, we discuss the detection prospects of inelastic DM where the heavier DM decays to a two or three-photon final state along with a massive lighter dark sector particle. By comparing the resulting continuous photon spectra with the NuSTAR SL data, we obtain the most stringent lower bound on the lifetime of such DM for the mass splitting $\mathrm{\Delta }m$ in the range 3 keV–100 keV.

Symbiotic recurrent novae provide a unique laboratory for studying thermonuclear explosions, shock evolution, and nonthermal particle acceleration in dense circumstellar environments. In this work, we develop a time-dependent, multienergy framework to describe neutrino emission from such systems, consistently incorporating both MeV neutrinos produced during thermonuclear runaway and GeV neutrinos generated through hadronic interactions in nova-driven shocks. Using RS Oph as a benchmark source, we model the evolution of the shock interacting with both the red giant wind and a dense accretion disk surrounding the white dwarf. We show that the resulting neutrino signal exhibits a characteristic two-component temporal structure: an early, rapidly rising MeV component tracing nuclear burning, followed by a delayed GeV component governed by shock propagation and particle acceleration. The presence of an accretion disk can significantly enhance the early-time GeV neutrino emission by providing a dense target for proton-proton interactions. This leads to a pronounced neutrino flux within the first few hours after eruption, a feature absent in wind-dominated scenarios. We further evaluate the detectability of these signals and find that while the MeV component remains below current detection thresholds, the GeV neutrino emission from nearby systems may become accessible to next-generation detectors. Our results highlight the critical role of the circumstellar structure in shaping nova neutrino emission and demonstrate that symbiotic recurrent novae are promising targets for future multimessenger observations.

We extend the concept study of a storage ring-based millihertz gravitational wave (GW) detector of our previous work by estimating the terrestrial gravity noise (TGN) for a singly charged di-uranium ion circulating in a model storage ring with the circumference of the Large Hadron Collider (LHC). In our estimate, we consider solely seismic activity originating from Rayleigh waves, which are one of the most dominant source of TGN. Our first result is an analytical calculation of the time delay caused by the gravitational pull of a mass fluctuation in the vicinity of such a storage ring that a particle builds up compared to the unperturbed circulation time. Furthermore we simulate a Rayleigh wave field in a homogeneous half space scaled with the Peterson low noise model and compute the gravitational forces acting on the particle stored in the model ring. We relate our analytical considerations to the numerical results for the time delay by solving the geodesic equations for a storage ring and a conservative background potential and analytically derive the function of the time delay from the result. By performing a Fourier transform we compute the characteristic noise strain numerically and analytically. Based on the results we conclude that the terrestrial gravity noise in a storage ring-based gravitational wave detector is weak compared to the GW signature. However, we expect that seismic perturbations of the storage ring structure leading to a shaking of the experimental apparatus can cause a very large noise contribution. This noise, however, is mitigable in principle—in contrast to the TGN.