We revisit gravitational wave echoes as a pathway to measuring quantization effects of black hole horizons. In particular, we construct a robust and phenomenologically motivated toy model for black hole reflectivity, enabling us to estimate the detectability of waveform echoes in gravitational wave interferometer data. We demonstrate how the frequency information of echoes, upon detection, can be leveraged to probe and constrain the quantum scale associated with the underlying theory of quantum gravity. Our analysis shows that echoes may manifest in LISA data with an estimated signal-to-noise ratio of up to $\mathcal{O}({10}^2)$. For such signals, the frequency features indicative of horizon quantization can be determined with subpercent precision.

It has recently been revealed that, in curved black-hole spacetimes, nonminimally coupled massive Proca fields may be characterized by the existence of poles in their linearized perturbation equations and may therefore develop exponentially growing instabilities. Interestingly, recent numerical computations [H. W. Chiang, S. Garcia-Saenz, and A. Sang, arXiv:2504.04779] have provided compelling evidence that the onset of monopole instabilities in the composed black-hole-field system is controlled by the dimensionless physical parameter $\mu r_{-}$, where $\mu$ is the proper mass of the nonminimally coupled Proca field and $r_{-}\equiv (-2\alpha {)}^{1/3}{r}_{\mathrm{H}}$ is the radial location of the pole [here $\alpha$ is the nonminimal coupling parameter of the Einstein-Proca theory and $r_{\mathrm{H}}$ is the radius of the black-hole horizon]. In the present paper we use analytical techniques in order to explore the physical properties of critical (marginally-stable) composed Schwarzschild-black-hole nonminimally-coupled monopole-Proca-field configurations. In particular, we derive a remarkably compact analytical formula for the discrete spectrum $\{\mu (r_{\mathrm{H}},r_{-};n){\}}_{n=1}^{n=\infty }$ of Proca field masses which characterize the critical black-hole monopole-Proca-field configurations in the dimensionless regime $\frac{r_{-}-r_{\mathrm{H}}}{r_{\mathrm{H}}}\ll 1$ of near-horizon poles. The physical significance of the analytically derived resonance spectrum stems from the fact that the critical field mass ${\mu }_{\mathrm{c}}\equiv \mu (r_{\mathrm{H}},r_{-};n=1)$ marks the onset of instabilities in the Schwarzschild-black-hole nonminimally-coupled monopole-Proca-field system. In particular, composed black-hole linearized-Proca-field configurations in the small-mass regime $\mu \le {\mu }_{\mathrm{c}}$ of the Proca field are stable.

Electrical resistivity plays a fundamental role in many astrophysical systems, influencing the evolution of the magnetic field and energy dissipation processes. During the coalescence of two neutron stars, resistive effects can significantly affect the dynamics and gravitational and electromagnetic signatures associated with these events, such as gamma-ray bursts and kilonova emission. Here, we present our new code, named mir (an acronym for magnetoidrodinamica resistiva, i.e., resistive magnetohydrodynamics in Italian). Developed within the Einstein Toolkit framework, mir solves the general relativistic magnetohydrodynamic equations in 3D Cartesian coordinates and on a dynamical spacetime using the $3+1$ Eulerian formalism, in both the ideal and resistive regimes, filling a crucial gap in the toolkit’s capabilities.

Spherically symmetric Einstein-æther (EÆ) theory with a Maxwell-like kinetic term is revisited. We consider a general choice of the metric and the æther field, finding that (i) there is a gauge freedom allowing one always to use a diagonal metric; and (ii) the nature of the Maxwell equation forces the æther field to be timelike in the coordinate basis. We derive the vacuum solution and confirm that the innermost stable circular orbit and photon ring are enlarged relative to general relativity (GR). Buchdahl’s theorem in EÆ theory is derived. For a uniform physical density, we find that the upper bound on compactness is always lower than in GR. Additionally, we observe that the Newtonian and EÆ radial acceleration relations run parallel in the low pressure limit. Our analysis of EÆ theory may offer novel insights into its interesting phenomenological generalization: Æther-scalar-tensor theory.

We explore dynamical features of the maximally symmetric nonlinear extension of classical electromagnetism, recently proposed in the literature as “ModMax” electrodynamics. This family of theories is the only one that preserves all the symmetries of Maxwell’s theory, having applications in the study of regular black hole solutions and supersymmetry. The purpose of this article is threefold. Firstly, we study the initial-value problem of ModMax and show, by means of a simple geometric criterion, that such a theory admits a well-posed formulation. Secondly, we prove a series of geometric inequalities relating energy, charge, angular momentum, and size in ModMax. The validity of these bounds gives strong evidence of an universal inequality conjectured by Bekenstein for macroscopic systems. Finally, we perform the first stable numerical simulations of ModMax in the highly nonlinear regime. We apply our simulations for verifying an inequality between energy, size and angular momentum in bounded domains, and for showing birefringence as a function of the nonlinear parameter, comparing with other nonlinear extensions.

We study the evolution of $Q$-balls under a spontaneously broken global U(1) symmetry. $Q$-balls are stabilized by the conservation of U(1) charge, but when the symmetry is spontaneously broken, the resulting Nambu-Goldstone boson can carry charge away from the $Q$-ball, potentially leading to charge leakage. To study this process in a controlled setting, we consider a scenario where $Q$-balls first form under an unbroken U(1) symmetry, which is then spontaneously broken. We introduce two complex scalar fields: one responsible for forming the $Q$-ball, and the other for spontaneously breaking the U(1) symmetry, allowing us to clearly separate the formation and symmetry-breaking phases. Using numerical simulations in a spherically symmetric system, we find that the evolution of $Q$-balls depends sensitively on the structure of the interaction between the two fields and the magnitude of symmetry breaking. Depending on parameters, $Q$-balls can completely decay, evaporate into smaller, stable $Q$-balls, or transition into oscillons/I-balls. In particular, we find that stable, localized remnants often survive the evolution over long timescales, especially when the symmetry-breaking scale is small. These results demonstrate that, even though spontaneous U(1) breaking can lead to significant energy and charge loss from $Q$-balls, stable localized objects with reduced or no charge can frequently survive and potentially contribute to cosmological relics.

Light primordial black holes (LPBHs) with masses in the range $10\mathrm{g}\le M_{\mathrm{BH}}\le {10}^9\mathrm{g}$, although they evaporate before Big Bang Nucleosynthesis, can play a significant role in the production of both dark matter and dark radiation. In particular, LPBHs can evaporate into light axions or axionlike particles (ALPs) with masses $m_a\lesssim \mathrm{MeV}$, contributing to the effective number of neutrino species, $\mathrm{\Delta }{N}_{\mathrm{eff}}$. Additionally, heavy scalar particles known as moduli, predicted by string theory, can be produced both via Hawking evaporation and through amplification by a mechanism called superradiant instability in the case of spinning primoridal black holes (PBHs). These moduli can subsequently decay into ALPs, further amplifying their abundance. In this work, we calculate the number density of ALPs in the presence of moduli enhanced by superradiance for Kerr PBHs. Using current limits on $\mathrm{\Delta }{N}_{\mathrm{eff}}$ from Planck satellite observations, we derive updated constraints on this scenario.

We show how certain types of magnetic asymptotically anti–de Sitter Euclidean wormholes can catalyze the onset of inflation. These wormholes can be embedded as saddle point solutions of general relativity coupled to the Standard Model, the inflaton being identified with the Higgs particle. Our scenario is based on the assumption that the quantum effective potential for the Higgs turns negative at a certain high energy window, in line with current measured values for the Higgs and top quark masses. Within our proposal, we can estimate various parameters and physical quantities of interest, in consistency with current observational bounds.

In light of recent observations by the Dark Energy Spectroscopic Instrument (DESI), we study evidence for thawing quintessence over a cosmological constant as dark energy, with emphasis on the effect of the choice of priors. Working with a parametrization for the equation of state parameter motivated by the theory, we analyze the DESI baryon acoustic oscillation data jointly with Planck 2018 and $\mathrm{Pantheon}+\text{or}$ Dark Energy Survey supernovae data and find a preference for thawing quintessence compared to a bare cosmological constant only if we use priors which are heavily informed by the data itself. If we extend the priors to physically better-motivated ranges, the evidence for thawing quintessence disappears.

When cosmic strings are formed during inflation, they regrow to reach a scaling regime, leaving distinct imprints on the stochastic gravitational wave background (SGWB). Such signatures, associated with specific primordial features, can be detected by upcoming gravitational wave observatories, such as the LISA and Einstein Telescope (ET). Our analysis explores scenarios in which cosmic strings form either before or during inflation. We examine how the number of e-folds experienced by cosmic strings during inflation correlates with the predictions of inflationary models observable in cosmic microwave background (CMB) measurements. This correlation provides a testable link between inflationary physics and the associated gravitational wave signals in a complementary manner. Focusing on $\alpha$-attractor models of inflation, with the Polynomial $\alpha$-attractor serving as an illustrative example, we find constraints, for instance, on the spectral index $n_s$ to $0.962\lesssim n_s\lesssim 0.972$ for polynomial exponent $n=1$, $0.956\lesssim n_s\lesssim 0.968$ for $n=2$, $0.954\lesssim n_s\lesssim 0.965$ for $n=3$, and $0.963\lesssim n_s\lesssim 0.964$ for $n=4$, which along with the GW signals from LISA, are capable of detecting local cosmic strings that have experienced $\sim 34–47$ e-folds of inflation consistent with current Planck data and are also testable in upcoming CMB experiments such as LiteBIRD and CMB-S4.

Tests of cosmological models via measurements of galaxy correlations will require increasing modeling accuracy, given the high precision of measurements promised by forthcoming galaxy surveys. In this work we investigate the biases introduced in parameter estimation when using different approximations in the modeling of the galaxy two point correlation function. We study this for two example surveys, with different binning strategies, for measurements of the primordial non-Gaussianity parameter $f_{\mathrm{NL}}$ and the growth rate of structures $\gamma$. We then investigate the same issue for the nDGP model, to see if results will change for a different cosmological model. Our results show that failing to properly account for radial and angular separation between galaxies will induce a considerable shift in parameters best fit estimates, the bias being larger for thicker redshift bins. When accounting for radial evolution within the bins by integrating over $z$, such shifts are reduced but still present. We then introduce a new hybrid model, which we call 3D radial, where we neglect the purely wide angle terms, but include a proper 3D modeling of the system by including radial modes. Using this model, we show that biases are greatly reduced, making it an accurate formalism to be safely used for forthcoming galaxy surveys. This moreover confirms other recent findings on the importance of including radial modes to accurately model the galaxy correlation function.

The path integral of 4D Einstein-Hilbert gravity for the de Sitter–like Universe with fluctuations is investigated, and the transition amplitude from one boundary configuration to another is computed. The gravitational system is described by lapse, scale factor, and metric-fluctuation field. Variational consistency demands augmenting the bulk theory with suitable boundary action. A given boundary choice on scale factor is seen to be achievable via an infinite family of covariant boundary actions, each restricting the boundary choices for the fluctuation field. General covariance intimately ties the two boundary choices, which no longer can be chosen independently. For vanishing metric fluctuations at the boundaries, the gauge-fixed gravitational path integral disintegrates into path integral over scale factor and metric-fluctuation field, connected via only lapse integration. While the former is exactly doable, the latter is computed up to one loop, leading to one-loop corrected lapse action. Ultraviolet (UV) divergences are systematically extracted and removed by the addition of suitable counterterms, leading to finite effective action for the lapse. The lapse effective action is then utilized for computing finite transition amplitude. Contributions from virtual gravitons are seen to be secularly growing with Universe size, leading to an infrared divergent transition amplitude. The presence of nonvanishing metric fluctuation at the boundaries implies that the “no-boundary” saddles of the theory without metric fluctuations are no longer the saddles of the one-loop corrected action. The corrected saddles have the Universe starting from a nonzero size.

Mounting evidence suggests that the semiclassical description of a black hole breaks down at the latest after losing an $O(1)$ fraction of its mass. As a result, effects such as memory burden can slow down evaporation so that small primordial black holes (PBHs), in particular those in the mass range ${10}^6\mathrm{g}$ to ${10}^9\mathrm{g}$, become viable dark matter candidates. In this paper, we investigate the production of PBHs from a prototype model of polynomial inflation with a nonminimal coupling to gravity. We show that a sufficiently small PBH mass alleviates any tension with cosmic microwave background observations. Moreover, we develop efficient numerical procedures to identify model parameters and evolve Mukhanov-Sasaki modes to place bounds on the scalar-induced stochastic gravitational wave (GW) background. While we identify some prospects for observation with future GW detectors, our results highlight the need to develop new experiments for high-frequency GW detection in the $\sim \mathrm{kHz}$ to $\sim \mathrm{MHz}$ range. Finally, we demonstrate that previously used Ansätze for modeling the power spectrum only yield a reliable approximation for the GW signal if some input from inflation is used.

Orbital eccentricity in compact binary mergers carries crucial information about the binary’s formation and environment. There are emerging signs that some of the mergers detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational wave detectors could indeed be eccentric. Nevertheless, the identification of eccentricity via gravitational waves remains challenging, to a large extent because of the limited availability of eccentric gravitational waveforms. While multiple suites of eccentric waveforms have recently been developed, they each cover only a part of the binary parameter space. Here we evaluate the sensitivity of LIGO to eccentric waveforms from the Stimulating eXtreme Spacetimes and RIT numerical relativity catalogs and the teobresums-dali waveform model using data from LIGO-Virgo-KAGRA’s third observing run. The obtained sensitivities, as functions of eccentricity, mass, and mass ratio, are important inputs to understanding detection prospects and observational population constrains. In addition, our results enable the comparison of the waveforms to establish their compatibility and applicability for searches and parameter estimation.

Time-of-flight measurements of high-energy astrophysical neutrinos can be used to probe Lorentz invariance, a pillar of modern physics. If Lorentz-invariance violation (LIV) occurs, it could cause neutrinos to slow down, with the delay scaling linearly or quadratically with their energy. We introduce nonparametric statistical methods designed to detect LIV-induced distortions in the temporal structure of a high-energy neutrino flare as it travels to Earth from a distant astrophysical source, independently of the intrinsic timing properties of the source. Our approach, illustrated using the $2014/2015\mathrm{TeV}\text{−}\mathrm{PeV}$ neutrino flare from the blazar TXS $0506+056$ detected by IceCube, finds that the LIV energy scale must exceed ${10}^{14}\mathrm{GeV}$ (linear) or ${10}^9\mathrm{GeV}$ (quadratic). Our methods provide a robust means to investigate LIV by focusing solely on a neutrino flare without relying on electromagnetic counterparts and to account for realistic energy and directional uncertainties. For completeness, we compare our limits inferred from TXS $0506+056$ to the sensitivity inferred from multimessenger detection of tentative coincidences between neutrinos and electromagnetic emission from active galactic nuclei and tidal disruption events.

Black holes (BHs) with masses between $\sim 3–5M_{\odot }$, produced by a binary neutron star (BNS) merger, can further pair up with a neutron star or BH and merge again within a Hubble time. However, the astrophysical environments in which this can happen and the rate of such mergers are open questions in astrophysics. Gravitational waves may play an important role in answering these questions. In this context, we discuss the possibility that the primary of the recent LIGO-Virgo-KAGRA binary GW230529_181500 (GW230529, in short) is the product of a previous BNS merger. Invoking numerical relativity (NR)-based fitting formulas that map the binary constituents’ masses and tidal deformabilities to the mass, spin, and kick velocity of the remnant BH, we investigate the potential parents of GW230529’s primary. Our calculations using NR fits based on BNS simulations reveal that the remnant of a high-mass BNS merger similar to GW190425 is consistent with the primary of GW230529. This argument is further strengthened by the gravitational wave-based merger rate estimation of GW190425-like and GW230529-like populations. We show that around 18% (median) of the GW190425-like remnants could become the primary component in GW230529-like mergers. The dimensionless tidal deformability parameter of the heavier neutron star in the parent binary is constrained to ${67}_{-61}^{+163}$ at 90% credibility. Using estimates of the gravitational-wave kick imparted to the remnant, we also discuss the astrophysical environments in which these types of mergers can take place and the implications for their future observations.

We utilize the now substantial amount of astrophysical observations of neutron stars (NSs), along with perturbative quantum chromodynamics (pQCD) calculations at high density, to directly constrain the NS equation of state (EOS). To this end, we construct nonparametric EOS priors by using Gaussian processes trained on 75 EOSs, which include models with either hadrons, hyperons, or quarks at high densities. We create a prior using the full EOS sample (model agnostic), and one prior for each EOS family to test model discrimination. We introduce a novel inference approach, which allows the simultaneous sampling of intrinsic and extrinsic parameters of binary NS mergers, as well as a nonparametric equation of state. We showcase this method in a Bayesian updating scheme by first performing a complete analysis of the binary NS merger event GW170817 with minimal assumptions, and sequentially adding information from x-ray and radio NS observations, along with pQCD calculations. Besides providing standard constraints, such as the pressure at twice nuclear saturation density $p(2{\rho }_{\mathrm{sat}})={4.3}_{-0.6}^{+0.6}\times {10}^{34}{\mathrm{dyne}/\mathrm{cm}}^2$, at 95% confidence level, for the model agnostic prior, our methodology shows how the choice of EOS families used in conditioning changes the inferred astrophysical properties of the EOS, namely tidal deformability and maximum supported NS mass. We find hyperonic priors predicting higher tidal deformabilities for a $1.4M_{\odot }$ NS, and hadronic priors being preferred by the considered astrophysical data.

We present a new survey for the satellites of the Milky Way, including nine known dwarf spheroidals galaxies (dSphs), supplemented with one ultrafaint dwarf Bootes I, based on Gaia DR3 data. Understanding their physical parameters enable the study of evolutionary processes of dwarf galaxies, significant differences between dark matter dominated objects and globular clusters. We identify the more complete and highly reliable star members of our target systems using a novel clustering algorithm; Forest Fire Clustering. We determine the physical parameters of these ten dSphs, including age, metallicity, distance modulus, reddening, structure parameters, and kinematics properties, by fitting observed data to theoretical models. For most systems, the cluster parameters are generally consistent with those presented in published literature. Similarly, our analysis reveal distinctly different half-light radii, with the ten objects having radii larger than $\sim 120\mathrm{pc}$, while the globular clusters exhibit radii smaller than $\sim 30\mathrm{pc}$. This study updates the census of star members for the dSphs in the Milky Way. With the physical parameters obtained from this work, it provides the groundwork for further investigation into dark matter on smaller scales.

We explore the potential for detecting the inelastic dark matter (DM) model with an additional $U(1{)}_D$ gauge symmetry at various types of lepton colliders. The new gauge boson $Z^{\prime }$ resulting from the spontaneous breaking of $U(1{)}_D$ can act as a portal connecting the Standard Model fermions and DM fermions $\chi$, thereby facilitating the production of $Z^{\prime }$ and its decay into DM fermions. The mass difference between the excited DM fermion ${\chi }_2$ and the ground DM fermion ${\chi }_1$ induces the subsequent decays ${\chi }_2\to {\chi }_1+\mathrm{vis}$, where ${\chi }_2$ and ${\chi }_1$ serve as long-lived particles and the DM candidate, respectively. For the inelastic DM with a mass $m_{{\chi }_1}$ in the range of (1–100) GeV, we find that the future lepton colliders can probe many regions not accessible by current experiments and provide greater projected sensitivity than the LHC and Belle in previous studies. This suggests that future colliders can provide complementary exploration of the inelastic dark matter model.

Though some LHC searches for new physics exceed the TeV scale, there may be discoveries waiting to be made at much lower masses. We outline a simple quirk model, motivated by models that address the hierarchy problem through neutral naturalness, in which new electroweakly charged states with masses as low as 100 GeV have not yet been probed by the LHC. We also describe a novel search strategy which is complementary to current search methods. In particular, we show its potential to discover natural quirks over regions of parameter space that present methods will leave unexplored, even after the LHC’s high-luminosity run.