Rapid and reliable detection and dissemination of source parameter estimation data products from gravitational-wave events, especially sky localization, is critical for maximizing the potential of multimessenger astronomy. Machine learning based detection and parameter estimation algorithms are emerging as production ready alternatives to traditional approaches. Here, we report validation studies of AMPLFI, a likelihood-free inference solution to low-latency parameter estimation of binary black holes. We use simulated signals added into data from the LIGO-Virgo-KAGRA’s (LVK’s) third observing run (O3) to compare sky localization performance with BAYESTAR, the algorithm currently in production for rapid sky localization of candidates from matched-filter pipelines. We demonstrate sky localization performance, measured by searched area and volume, to be equivalent with BAYESTAR. We show accurate reconstruction of source parameters with uncertainties for use in distributing low-latency coarse-grained chirp mass information. In addition, we analyze several candidate events reported by the LVK in the third gravitational-wave transient catalog (GWTC-3) and show consistency with the LVK’s analysis. Altogether, we demonstrate AMPLFI’s ability to produce data products for low-latency public alerts.

We introduce a computational framework, Bayesian evidence calculation for model selection (BEOMS) to evaluate multiple Bayesian model selection methods in the context of determining the equation of state (EOS) for cold neutron star, focusing on their performance with current and next-generation gravitational wave (GW) observatories. We conduct a systematic comparison of various EOS models by using posterior distributions obtained from EOS-agnostic Bayesian inference of binary parameters applied to GWs from a population of binary neutron star (BNS) mergers. The cumulative evidence for each model is calculated in a multi-dimensional parameter space characterized by neutron star masses and tidal deformabilities. Our findings indicate that Bayesian model selection is most effective when performed in the two-dimensional subspace of component mass and tidal deformability, requiring fewer events to distinguish between EOS models with high confidence. Furthermore, we establish a relationship between the precision of tidal deformability measurements and the accuracy of model selection, taking into account the evolving sensitivities of current and planned GW observatories. BEOMS offers computational efficiency and can be adapted to execute model selection for gravitational wave data from other sources.

Ultrahigh-energy cosmic-ray (UHECR) observatories require unbiased direction reconstruction to enable multimessenger astronomy with sparse, nanosecond-scale radio pulses. Explicit likelihood methods often rely on simplified models, which may bias results and understate uncertainties. We introduce a simulation-based inference pipeline that couples a physics-informed graph neural network (GNN) to a normalizing-flow posterior within the Learning the Universe Implicit Likelihood Inference framework. Each event is seeded by an analytic plane-wavefront fit; the GNN refines this estimate by learning spatiotemporal correlations among antenna signals, and its frozen embedding conditions an eight-block autoregressive flow that returns the full Bayesian posterior. Trained on about 8,000 realistic UHECR air-shower simulations generated with the zhaires code, the posteriors are temperature-calibrated to meet empirical coverage targets. We demonstrate a subdegree median angular resolution on test UHECR events, and find that the nominal 68% highest-posterior-density contours capture $71\%\pm 2\%$ of true arrival directions, indicating a mildly conservative uncertainty calibration. This approach provides physically interpretable reconstructions, well-calibrated uncertainties, and rapid inference, making it ideally suited for upcoming experiments targeting highly inclined events, such as GRAND, AugerPrime Radio, and BEACON.

There is great interest within the particle physics community for building a ${\mu }^{+}{\mu }^{-}$ collider with center-of-mass (CoM) energies ranging from $\sqrt{s}\sim 1–14\mathrm{TeV}$. For beyond-the-Standard-Model (BSM) physics, natural supersymmetry seems perhaps the most motivated, plausible extension of the Standard Model. Here, we examine what can be accomplished by a muon collider with regards to natural SUSY at various muon collider CoM energies. In natural SUSY—especially in the guise that would emerge from the string landscape—one expects sparticles to be spread over two orders of magnitude in mass values. A muon collider with highly variable beam energies would be most useful for targeting 2-body reaction thresholds and Higgs boson resonances.

Algorithms for computing neutrino oscillation probabilities in sharply varying matter potentials such as the Earth are becoming increasingly important. As the next generation of experiments, DUNE and HyperK as well as the IceCube upgrade and KM3NeT, come online, the computational cost for atmospheric and solar neutrinos will continue to increase. To address these issues, we expand upon our previous algorithm for long-baseline calculations to efficiently handle probabilities through the Earth for atmospheric, nighttime solar, and supernova neutrinos. The algorithm is fast, flexible, and accurate. It can handle arbitrary Earth models with two different schemes for varying density profiles. We also provide a c++ implementation of the code called nufast-earth along with a detailed user manual. The code intelligently keeps track of repeated calculations and only recalculates what is needed on each successive call which can also help provide significant speed-ups.

Using a data sample of $(2.712\pm 0.014)\times {10}^9\psi (3686)$ events collected at $\sqrt{s}=3.686\mathrm{GeV}$ by the BESIII detector, we search for the rare electromagnetic Dalitz decays ${\chi }_{cJ}\to e^{+}{e}^{-}\varphi (J=0,1,2)$ via the radiative transitions $\psi (3686)\to \gamma {\chi }_{cJ}$. No statistically significant ${\chi }_{cJ}\to e^{+}{e}^{-}\varphi$ signals are observed. The upper limits on the branching fractions of ${\chi }_{cJ}\to e^{+}{e}^{-}\varphi (J=0,1,2)$, excluding the $\varphi$ resonance to $e^{+}{e}^{-}$ final states, are set to be $2.4\times {10}^{-7},6.7\times {10}^{-7}$, and $4.1\times {10}^{-7}$ at the 90% confidence level, respectively. This is the first search for the electromagnetic Dalitz transition of P-wave charmonium ${\chi }_{cJ}$ states to a light vector meson.

We propose a new action for entanglement entropy in the framework of the ${\mathrm{AdS}}_3/{\mathrm{CFT}}_2$ correspondence. This action is constructed directly from the entanglement entropy of the ${\mathrm{CFT}}_2$, and we show that the Einstein equations of ${\mathrm{AdS}}_3$ gravity can be derived from it. In the near-coincidence limit, using Riemann normal coordinates, the action reduces to a string world-sheet action in a curved background that naturally includes the symmetric spacetime metric, an antisymmetric Kalb-Ramond field, and a dilaton. The Kalb-Ramond field gives rise to a string charge density, from which we demonstrate that bit threads can be exactly reproduced. This correspondence provides a clear physical interpretation of bit threads. Exploiting this correspondence, we establish explicit relations between the emergent string world sheet and the Ryu-Takayanagi (RT) surface, providing new insights into entanglement entropy. In particular, entanglement entropy can be computed from open string charge, while Bekenstein-Hawking entropy arises from closed string charge through open-closed string duality. These results suggest a unified picture in which the Susskind-Uglum conjecture, open-closed string duality, and the $\mathrm{ER}=\mathrm{EPR}$ proposal emerge as equivalent manifestations of the same underlying principle. Finally, we propose a quantization of the RT surface, pointing to a possible connection with loop quantum gravity that refines Wall’s conjecture.

We investigate the transition probability rate of a Unruh-DeWitt (UD) detector interacting with massless scalar field for a finite duration of proper time, $T$, of the detector. For a UD detector moving at a uniform acceleration, $a$, we explicitly show that the finite-time transition probability rate can be written as a sum of purely thermal terms, and nonthermal transient terms. While the thermal terms are independent of time, $T$, the nonthermal transient terms depend on $(\mathrm{\Delta }ET)$, $(aT)$, and $(\mathrm{\Delta }E/a)$, where $\mathrm{\Delta }E$ is the energy gap of the detector. Particularly, the nonthermal terms are oscillatory with respect to the variable $(\mathrm{\Delta }ET)$, so that they may be averaged out to be insignificant in the limit $\mathrm{\Delta }ET\gg 1$, irrespective of the values of $(aT)$ and $(\mathrm{\Delta }E/a)$. To quantify the contribution of nonthermal transient terms to the transition probability rate of a uniformly accelerating detector, we introduce a parameter, ${ϵ}_{\mathrm{nt}}$, called the nonthermal parameter. The smaller the nonthermal parameter is, the smaller the contribution due to nonthermal transient terms is, when compared with the purely thermal terms. Demanding the contribution of nonthermal terms in the finite-time transition probability rate to be negligibly small, i.e., ${ϵ}_{\mathrm{nt}}=\delta \ll 1$, we calculate the thermalization time—the time required for the detector to interact with the scalar field to arrive at the required nonthermality, ${ϵ}_{\mathrm{nt}}=\delta$, so that the nonthermal terms to be negligibly small, and the detector to be (almost) thermalized with the Unruh bath in its comoving frame. Specifically, for small accelerations, $a\ll \mathrm{\Delta }E$, we find the thermalization time, ${\tau }_{\mathrm{th}}$, to be ${\tau }_{\mathrm{th}}\sim (\mathrm{\Delta }E{)}^{-1}\times {\mathrm{e}}^{2\pi |\mathrm{\Delta }E|/a}/\delta$, and for large accelerations, $a\gg \mathrm{\Delta }E$, we find the thermalization time to be ${\tau }_{\mathrm{th}}\sim (\mathrm{\Delta }E{)}^{-1}/\delta$. We comment on the possibilities of bringing down the exponentially large thermalization time at small accelerations, $a\ll \mathrm{\Delta }E$.

In this work, we systematically treat the ambiguities that generically arise in the gradient expansion of any hydrodynamic theory. While these ambiguities do not affect the physical content of the equations, they induce two types of transformations in the space of transport coefficients. The first type is known as the “frame” transformations, and amounts to field redefinitions. The second type, which we introduce and formalize here, we term the “on-shell” transformations. This identifies equivalence classes of hydrodynamic theories that provide an equally valid low-energy description of the underlying microscopic theory. We show that in any (classical) theory of hydrodynamics (at arbitrary order in derivatives), the action of such transformations on the dispersion relations and two-point correlation functions is universal. We explicitly construct invariants which can then be matched to a microscopic theory. Among them are, expectedly, the low-momentum expansions of the hydrodynamic modes. The (unphysical) gapped modes can, however, be added or removed at will. Finally, we show that such transformations assign a nilpotent Lie group to every hydrodynamic theory, and discuss the related algebraic properties underlying classical hydrodynamics.

We investigate a two-qubit SWAP thermal machine—a streamlined analog of the four-stroke Otto cycle—whose working medium comprises inertially moving Unruh-DeWitt qubit detectors, each coupled to a thermal quantum field bath prepared at a different temperature. In the presence of relative motion between the working medium and the thermal baths, we derive thermodynamic uncertainty relations (TURs) that quantify the trade-off between performance, entropy production, and power fluctuations. Our analysis identifies regimes where relativistic motion leads to stronger violation of classical TURs, previously observed in static quantum setups. In addition, we establish generalized performance bounds for the thermal machine operating as either a heat engine or a refrigerator, and discuss how relativistic motion can enhance their performances beyond the standard Carnot limits defined by rest-frame temperatures.

The Benenti-Francaviglia (BF) family of metrics provides the most general form of a spacetime metric that admits two mutually commuting Killing vectors and an irreducible Killing tensor. The geodesic equations for the BF family are thus completely integrable by separation of variables. Within this broad class, we explore the Kerr-Schild transformation of a degenerate subclass distinguished by the existence of a shear-free null geodesic congruence. By requiring the deformed metric to preserve the Killing symmetry and circularity, we demonstrate that the deformed metric again falls into the degenerate BF family, modulo the replacement of a single structure function. We apply the present algorithm to $\mathcal{N}=2$ gauged supergravity and obtain a dyonic generalization of the Chong-Cvetič-Lü-Pope rotating black hole solution, by taking the background metric to be a solution of the Einstein-scalar gravity. The present prescription extends to five dimensions, provided that the constant of geodesic motion associated with the extra Killing direction vanishes. The same reasoning applies to the case where the background degenerate BF metric is distorted in a (non)conformal manner. Our formalism offers a unified perspective on the relation between seed and deformed metrics in the Kerr-Schild construction.

Covariant $f(Q)$ gravity is a viable extension of general relativity; however, its strong-field predictions remain largely untested. Using the static, spherically symmetric black-hole solutions of the theory, we confront it with the most stringent probes available: black-hole shadows, Event Horizon Telescope (EHT) measurements, S2-star precession, and strong gravitational lensing. We show that the two admissible solution branches behave very differently: Case I produces negligible deviations from Schwarzschild solution, whereas Case II yields significant, potentially observable corrections to the photon sphere and shadow size. From the EHT shadow diameters of M87* and Sgr A*, we obtain tight bounds, which are further strengthened by strong-lensing coefficients. These results provide the sharpest strong-field constraints on covariant $f(Q)$ gravity to date, and point toward future tests using next-generation horizon-scale imaging and precision Galactic Center astrometry.

Low-energy effective theories provide the natural description of four-dimensional physics in higher-dimensional geometries, where the imprint of the bulk appears as parameters of the lower-dimensional theory. Motivated by the recent progress in the first-order thermodynamic formulation of modified gravity theories, we investigate the thermodynamics of effective theories in braneworld scenarios and thereby the attractor mechanism toward general relativity in such theories. We consider the 2-brane Randall-Sundrum model, where the low-energy theory on either brane is of scalar-tensor nature with the extradimensional radion playing the role of the scalar. We study the thermodynamic implications of a nonvanishing gravitational contribution to the radion potential, and further explore the dynamics in the presence of a bulk stabilizing field.

In our previous work [H.-S. Cho, Phys. Rev. D 105. 124022 (2022)], we studied the impact of eccentricity on gravitational-wave parameter estimation for a nonspinning binary neutron star (BNS) system. We here extend the work to a general binary system by including the spin parameter. As in the previous work, we employ the analytic Fisher-Cutler-Vallisneri method to calculate the systematic bias that can be produced by using noneccentric waveforms in parameter estimation, and we verify the reliability of the method by comparing it with numerical Bayesian parameter estimation results. We generate ${10}^4$ BNS sources randomly distributed in the parameter space $m_1–m_2–{\chi }_{\mathrm{eff}}–e_0$, where the neutron star mass is in the range of $1M_{\odot }\le m_{1,2}\le 2M_{\odot }(m_2\le m_1)$, the effective spin is $-0.2\le {\chi }_{\mathrm{eff}}\le 0.2$, and the eccentricity (at the reference frequency 10 Hz) is $0\le e_0\le 0.024$. For the true value of the tidal deformability ($\lambda$) of neutron stars, we assume the equation of state model APR4. For all gravitational-wave signals emitted from the sources, we calculate the systematic biases ($\mathrm{\Delta }\theta$) for the chirp mass ($M_c$), symmetric mass ratio ($\eta$), effective spin (${\chi }_{\mathrm{eff}}$), and effective tidal deformability ($\tilde{\lambda }$), and obtain generalized distributions of the biases. The distribution of biases in $M_c,\eta$, and ${\chi }_{\mathrm{eff}}$ shows narrow bands, and the median value of the bias increases or decreases quadratically with increasing $e_0$, indicating a weak dependence of biases on the three parameters. On the other hand, the biases of $\tilde{\lambda }$ are widely distributed depending on the values of the mass and spin parameters at a given $e_0$. We investigate the implications of biased parameters for the inference of neutron star properties by performing Bayesian parameter estimation for specific cases. We find that a BNS signal consisting of two neutron stars within the typical mass range $[1,2]M_{\odot }$ can be estimated to be a BNS signal whose component mass is much smaller or much larger than the typical mass range. In particular, by showing concrete examples, we demonstrate that parameter estimation using noneccentric waveforms for eccentric BNS signals can yield false predictions for the neutron star equation of state.

We identify a general class of spacetime metrics that mimic the properties of black holes without possessing a true event horizon. These metrics are constrained by the requirements of being singularity free and geodesically complete. Specifically, we study metrics that do not possess $Z_2$ symmetry and may deviate slightly or significantly from the symmetric case. Focusing on scalar perturbations propagating on such backgrounds, we analyze the resulting effective radial potentials and their dependence on different corners of the mimicker landscape. We further investigate the corresponding quasinormal modes and explore their characteristic features. Finally, we survey the landscape for potential observational signatures, including shadow properties and the possible presence or absence of echo effects.

Scattering black holes spin up and gain mass through the reabsorption of orbital angular momentum and energy radiated in gravitational waves during their encounter. In this work, we perform a series of numerical relativity simulations to investigate the spin-up and mass-gain for equal-mass black holes in a wide range of equal initial spins, ${\chi }_{\mathrm{i}}\in [-0.7,0.7]$, aligned (or antialigned) to the orbital angular momentum. We also consider a variety of initial momenta. Furthermore, we explore a range of incident angles and identify the threshold between scattering and merging configurations. The spin-up and mass-gain are typically largest in systems with incident angles close to the threshold value, large momenta, and negative (i.e., antialigned) initial spins. When evaluated at the threshold angle, we find that the spin-up decreases linearly with initial spin. Intriguingly, systems with initial spin ${\chi }_{\mathrm{i}}=0.7$ sometimes experience a spin-down, in spite of an increase in the black-hole angular momentum, due to a corresponding gain in the black-hole mass. Across the simulation suite, we find a maximum spin-up of 0.3 and a maximum increase in the black-hole mass of 15%.

We present an interpretable emulator for the linear matter power spectrum (MPS) in the standard cosmological model $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$), constructed via a physics-informed symbolic regression framework. By combining domain knowledge with a machine learning technique known as genetic algorithms (GA), we explore the space of analytic expressions to derive closed-form, smooth, physically motivated approximations of the MPS that match the accuracy of standard broadband reconstruction methodologies such as the Savitzky-Golay filter. Building upon this baseline, we incorporate transparent oscillatory corrections informed by the physics of baryon acoustic oscillations (BAO). The resulting expression delivers mean sub-percent fractional errors across a broad range of scales ($k\in [{10}^{-5},1.5]h{\mathrm{Mpc}}^{-1}$) with an average deviation of $\sim 0.4\%$ when tested against spectra computed with a Boltzmann solver. Moreover, a comparable level of fractional deviation is maintained on smaller scales when the GA-derived formulation is used as input to the nonlinear emulator halofit. To illustrate the versatility of the framework beyond $\mathrm{\Lambda }\mathrm{CDM}$, we apply it to a representative $f(R)$ gravity model. Rather than training a general modified-gravity (MG) emulator, we compute the corresponding linear spectra with a Boltzmann solver and fit a parametric deformation of the $\mathrm{\Lambda }\mathrm{CDM}$ smoothed component. This procedure achieves average errors at the 1.5–1.8% level and captures the leading modulation of the MPS induced by modified gravity, enabling a controlled study of its impact on the BAO scale. Our results provide compact, accurate, and physically motivated fitting functions for the linear MPS in both standard and MG cosmologies, offering a fast and transparent alternative to existing emulators for parameter inference and theoretical modeling in large-scale structure analyses.

A massive vector field is a highly promising candidate for dark matter in the Universe. A salient property of dark matter is its negligible or null coupling to ordinary matter, with the exception of gravitational interaction. This poses a significant challenge in producing the requisite amount of dark particles through processes within the Standard Model. In this study, we examine the production of a vector field during inflation due to its direct interaction with the inflaton field through kinetic and axionlike couplings as well as the field-dependent mass. The gradient-expansion formalism, previously proposed for massless Abelian gauge fields, is extended to include the longitudinal polarization of a massive vector field. We derive a coupled system of equations of motion for a set of bilinear functions of the vector field. This enables us to address the nonlinear dynamics of inflationary vector field production, including backreaction on background evolution. To illustrate this point, we apply our general formalism to a low-mass vector field whose kinetic and mass terms are coupled to the inflaton via the Ratra-type exponential function. The present study investigates the production of its transverse and longitudinal polarization components in a benchmark inflationary model with a quadratic inflaton potential. It has been demonstrated that pure mass coupling is able to enhance only the longitudinal components. By turning on also the kinetic coupling, one can get different scenarios. As the coupling function decreases, the primary contribution to the energy density is derived from the transverse polarizations of the vector field. Conversely, for an increasing coupling function, the longitudinal component becomes increasingly significant and rapidly propels the system into the strong backreaction regime.

Parameter estimation from galaxy survey data from the full-shape method depends on scale cuts and priors on effective-field theory (EFT) parameters. The effects of priors, including the so-called “prior volume” phenomenon, have been originally studied in Ivanov et al. [J. Cosmol. Astropart. Phys. 05 (2020) 042] and subsequent works. In this note, we repeat and extend these tests and also apply them to other priors used in the literature. We point out that, in addition to the “prior volume” effect, there is a more important effect that is largely overlooked: a systematic bias on cosmological parameters due to overoptimistic scale cuts. Unlike the prior volume effect, this is a genuine systematic bias due to two-loop corrections that does not vanish with better priors or with larger data volumes. Our study is based on the high-fidelity BOSS-like PT challenge simulation data, which offer many advantages over analyses based on synthetic data generated with fitting pipelines. We show that some analysis choices associated with the pybird code, especially the scale cuts, significantly bias parameter recovery, overestimating ${\sigma }_8$ by 5% (equivalent to $1\sigma$). The bias on measured EFT parameters is even more significant. The analysis choices associated with the class-pt code lead to much smaller ($\lesssim 1\%$) shifts in cosmological parameters based on their best-fit values.

Caustic-crossing stars observed in giant arcs behind galaxy clusters provide a powerful probe of dark matter substructure. While previous work has focused on pointlike lenses such as primordial black holes, we extend this framework to extended dark objects (EDOs), including ultracompact minihalos formed from the collapse of primordial overdensities. We develop an analytic model of microlensing by EDOs embedded in a macrolensing cluster potential and derive the resulting caustics and light curves. Depending on the EDO size relative to the effective Einstein radius, we show that they may generate additional narrow caustics, leading to novel features in the light curve. Applying our framework to the MACS J1149 LS1 “Icarus” event, we constrain EDOs with radii up to ${10}^7{R}_{\odot }$. Our results demonstrate that caustic-crossing events complement galactic microlensing searches, as they can probe EDOs with larger physical size. We discuss the implications for current and future observations, which promise to deliver a statistical sample of caustic transients and correspondingly sharper constraints on dark objects.