We employ scattering amplitudes in Einstein-Maxwell theory to compute the classical Hamiltonian of a binary system of two charged, nonspinning compact objects. The effective Hamiltonian is valid to all orders in velocity and up to second post-Minkowskian (2PM) order, i.e., $\mathcal{O}(G^2)$. The classical interaction potential is extracted via matching the four-point tree and one-loop amplitudes to those of a suitably chosen classical effective field theory. We derive the 2PM scattering angle from the Hamiltonian and find complete agreement with known results. Using also the third-order result for the scattering angle computed by Wilson-Gerow, we expand the scattering angle at second post-Newtonian (2PN) order. Additionally, we provide expressions for the binding energy and periastron advance at 2PN order via the boundary-to-bound dictionary introduced by Kälin and Porto and find agreement with previous findings.

It is well known that the most reliable way to reveal the quantum nature of light is through photon number statistics, since photons exhibiting sub-Poissonian statistics unambiguously demonstrate their quantum behavior. In this paper, we show that gravitons emitted by binary systems can, in principle, exhibit analogous sub-Poissonian statistics. The key idea is that the vacuum state of gravitons may not be the standard Minkowski vacuum but rather a nonclassical state imprinted with the physics of the early Universe, such as inflation. Accordingly, gravitational waves from binary systems provide a means to probe the graviton states generated in the early Universe. As a concrete example, we show that squeezed graviton states originating from inflation can, in principle, imprint nonclassical graviton number statistics on gravitational waves from binary systems. In particular, we identify the frequency range in which the resulting coherent-squeezed graviton state can exhibit sub-Poissonian statistics. A realistic assessment of observational feasibility is left for future work.

We discuss the amplitude and phase fluctuations of gravitational waves due to wave optics lensing in the presence of both a strong lens and cosmological weak lenses. By applying the geometric optics approximation to the strong lens and treating the weak lensing potential perturbatively, we obtain the amplification factor up to the second order in the weak lensing potential. Additionally, we establish a methodology to systematically evaluate the weak lensing effects based on diagrammatic rules. Based on the derived amplification factor, we evaluate the statistics of the fluctuations and demonstrate that the consistency relations originally established in the absence of a strong lens still hold in exactly the same form when a strong lens is present. The physical origin of these relations is also discussed. Furthermore, we demonstrate that for the mean of the weak lensing signal, both the magnification of the signal and the shift of the Fresnel scale to larger scales occur, consistent with the behavior observed in the variance.

We discuss key aspects of the nature of radiation from global strings and its impact on the relic axion density. Using a simple model we demonstrate the dependence on the spectrum of radiation emitted by strings. We then study the radiation emitted by perturbed straight strings paying particular attention to the difference between the overall phase of the field and the small perturbations about the string solution which are the axions. We find that a significant correction is required to be sure that one is analyzing the axions and not the self-field of the string. Typically this requires one to excise a sizeable region around the string—something which is not usually done in the case of numerical field theory simulations of string networks. We have measured the spectrum of radiation from these strings and find that it is compatible with an exponential, as predicted by the Nambu-like Kalb-Ramond action, and in particular is not a “hard” spectrum often found in string network simulations. We conclude by attempting to assess the uncertainties on relic density and find that this leads to a wide range of possible axion masses when compared to the measured density from the cosmic microwave background, albeit that they are typically higher than what is predicted by the initial misalignment mechanism. If the primary mode of decay is via a “soft spectrum” from loops produced close to the backreaction scale we find that $m_{\mathrm{a}}\approx 160\mathrm{\mu }\mathrm{eV}$ and a detection frequency $f\approx 38\mathrm{GHz}$. If axions are primarily emitted directly by the string network, and we use emission spectra reported in typical field theory simulations, then $m_{\mathrm{a}}\approx 4\mathrm{\mu }\mathrm{eV}$ and $f\approx 1\mathrm{GHz}$, however this increases to $m_a\approx 125\mathrm{\mu }\mathrm{eV}$ and $f\approx 30\mathrm{GHz}$ using our spectra for the case of an oscillating string. In all scenarios there are significant remaining uncertainties that we delineate.

We explore the possibility of parity-violating, nonminimally coupled two-form field theories that retain the same dynamical degrees of freedom as a massive two-form and thus are ghost-free. Starting from the most general kinetic terms and dimension-four couplings between the two-form field and the curvature tensors, we find a two-parameter family of such theories. However, we also find that parity-violating terms involving the dual two-form field can be absorbed into a field redefinition, leaving a theory with essentially the same structure as that obtained by Heisenberg and Trenkler (i.e., the parity-preserving coupling to the double dual Riemann tensor). After the field redefinition, the only place where parity-violating terms can appear is in the potential. We then consider a homogeneous and isotropic cosmological model with an isotropic configuration of a triplet of two-form fields, and study tensor perturbations in this setup. There are three types of tensor perturbations, two of which are dynamical. We show that chiral gravitational waves can be generated in the presence of parity-violating terms in the potential.

We assess dataset agreement and late-time predictive adequacy in $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) and its sign-switching extension, ${\mathrm{\Lambda }}_{\mathrm{s}}\mathrm{CDM}$, using a suite of Gaussian and exact non-Gaussian consistency diagnostics. Both models are constrained with cosmic microwave background (CMB) measurements from Planck, Atacama Cosmology Telescope, and South Pole Telescope, baryon acoustic oscillation (BAO) data from Dark Energy Spectroscopic Instrument Data Release 2, and low-redshift Type Ia supernova data from Pantheon Plus + SH0ES. We find that commonly used Gaussian tension metrics can significantly overstate inconsistencies when broad, non-Gaussian posteriors are combined with tightly constrained datasets. In contrast, the exact non-Gaussian parameter shift indicates excellent consistency between the full CMB and BAO observations in both models. The ${\mathrm{\Lambda }}_{\mathrm{s}}\mathrm{CDM}$ extension modestly improves geometric compatibility at intermediate redshifts, although reductions in parameter-level tension do not necessarily imply improved predictive consistency. These results highlight the importance of exact, non-Gaussian, and predictive diagnostics for robust assessments of cosmological model consistency.

Wavelike dark matter composed of spin-1 particles, known as dark photons, is theorized to form clumps called “vector solitons.” These solitons are compact astrophysical objects that exhibit coherent oscillations and a high concentration relative to the local dark matter density. A significant portion of dark matter in galactic halos today may consist of these solitons. This study explores how photons can be produced from these vector solitons by the influence of external electromagnetic fields or charge densities, via a dimension-6 dark photon-photon coupling and a kinetic mixing, respectively. We further explore the astrophysical implications of these phenomena, highlighting a novel avenue for dark matter discovery that our research provides.

The $\gamma$-ray emission from active galactic nuclei (AGN), including both beamed blazars and misaligned-AGN, dominates the extragalactic $\gamma$-ray point-source population count and flux. While multiwavelength studies have detected an increasing number of AGN within dwarf galaxies in the local Universe, $\gamma$-ray emission has so far only been associated with systems hosting supermassive black holes (SMBHs). Dwarf-galaxy AGN are of particular interest because their central black holes fall in the intermediate-mass black hole regime, offering insight into the early evolution of SMBHs. Using 15 years of Fermi-LAT data, we present the first search for $\gamma$-ray emission from dwarf-galaxy AGN. In the sample of 67 x-ray-selected dwarf-galaxy AGN, we find no sources that exceed the Fermi-LAT detection threshold. However, a joint-likelihood analysis reveals a modest, trials-corrected population-level excess ($\sim 2\sigma$) above blank-field expectations at very soft photon indices $\mathrm{\Gamma }\gtrsim 3.8$ above 500 MeV. This hint is most pronounced when source contributions are weighed by $M_{\mathrm{IMBH},i}^{\alpha }/d_i^2$, with $\alpha \simeq 1–1.5$, suggesting—but not confirming—that $\gamma$-ray emission could scale with the central black hole mass or a property correlated with it (e.g., accretion rate), but with a markedly softer spectrum than in SMBH-hosted AGN.

The high redshift merger rate and mass distribution of black hole binaries provide a direct probe to distinguish astrophysical black holes and primordial black holes (PBHs), which can be studied using the stochastic gravitational-wave background (SGWB). The conventional analyses solely based on the power spectrum are limited in constraining the properties of the underlying source population under the assumption of a nonsporadic Gaussian distribution. However, recent studies have shown that SGWB is expected to be sporadic and non-Gaussian in nature, which gives rise to nonzero spectral correlation that depends on the high redshift merger rate and mass distribution of the compact objects. In this work, we present the first spectral covariance analysis of the SGWB using data from the LIGO-Virgo-KAGRA collaboration during the third and the first part of the fourth observing runs. Our analysis indicates that the measured spectral correlation is consistent with noise, yielding no detection and providing only upper bounds over the frequency range of 20 Hz to 100 Hz. This upper bound on the spectral correlation translates into a mass-distribution-dependent upper bound on the merger rate of PBHs. This provides a stringent upper bound on the PBH merger rate at high redshift and hence puts constraints on the PBH formation scenarios. In the future, detection of this signal will provide a new avenue to probe the high-redshift black hole population using gravitational waves.

We systematically investigate the source confusion of massive black-hole binaries (MBHBs) for the Taiji space-based gravitational-wave mission. Source confusion arising from the overlap of signals in both time and frequency domains can degrade parameter recovery accuracy. To assess this effect, we simulate three representative MBHB population models to estimate the expected number of overlap events. Assuming 100 detections per year, only 0.31–4.2 overlaps are expected annually. Using the Fisher information matrix with the imrphenomd and imrphenomhm waveform models, we find that overlap significantly enlarges parameter uncertainties, while the inclusion of higher-order modes (HMs) effectively mitigates this effect. The cases most susceptible to severe confusion, characterized by $\mathrm{\Delta }{\mathcal{M}}_z/{\mathcal{M}}_z<0.2\%$, occur in fewer than 0.14% across the three population models. The full Bayesian analysis further corroborates the Fisher predictions and also reveals that HMs help break key parameter degeneracies, with or without signal overlap. These findings underscore the importance of incorporating HMs for accurate inference in future space-based observations.

We present the first systematic study of rigidly rotating protoquark stars based on isentropic equations of state (EOS) within the density-dependent quark mass framework. Using a quasistatic equilibrium approach, we follow the Kelvin-Helmholtz evolution from hot, lepton-rich matter to a cold, catalyzed quark star (QS). Rotation substantially enhances the maximum stable mass (by up to $\sim 40\%$), equatorial radius, and key rotational observables, with the ratio of rotational kinetic to gravitational potential energy, $T_{\mathrm{kin}}/|W|$, reaching 0.18–0.19 near the Keplerian limit, indicating a heightened susceptibility to gravitational-wave–emitting instabilities. Thermal evolution introduces a clear ordering: all stellar properties peak during the lepton-rich stages and decrease monotonically as the star cools. Compared to hadronic stars, rotating proto-QSs exhibit larger radii, higher moments of inertia, and stronger quadrupolar deformation, producing a distinct signature in the mass–radius–spin plane. The EOS parameters are constrained using current astrophysical observations, including mass-radius measurements from HESS J1731–347 and PSR $\mathrm{J}0030+0451$, the high-mass constraint from PSR $\mathrm{J}0740+6620$, and mass-radius constraints inferred from GW170817. The results demonstrate that future multimessenger observations must account for both thermal history and rotation to identify quark matter in compact stars robustly.

The pair-instability supernova (PISN) mechanism predicts a mass gap in the black hole population, where no stellar-origin black holes are expected to form. However, several binary black hole merger events exhibit component masses that appear to lie within the PISN mass gap. If a gravitational-wave (GW) source is lensed, neglecting lensing effects leads to an underestimation of the luminosity distance and, hence, the redshift, resulting in an overestimation of the source-frame masses and potentially placing them within the mass-gap region. In this work, we analyze two representative events, GW190521 and GW231123. We establish a direct mapping between the lensing magnification and the fraction of posterior samples with source-frame masses below the mass-gap boundary. Adopting a lower bound of $65M_{\odot }$, we find that the magnifications required for 90% of the posterior samples to fall below this boundary are $\mu =444$ for GW231123 and $\mu =39$ for GW190521. At these magnifications, the corresponding source-frame masses of the primary black hole are ${52}_{-10}^{+10}{M}_{\odot }$ and ${42}_{-14}^{+19}{M}_{\odot }$, with lensed source redshifts of ${2.3}_{-0.5}^{+0.8}$ and ${2.7}_{-1.2}^{+1.5}$, respectively. These results provide a quantitative framework for assessing the lensing hypothesis as a possible explanation for binary black hole mergers observed within the PISN mass gap and highlight that the extreme magnifications required for GW231123 may challenge the astrophysical plausibility of simple strong-lensing interpretations.

The production of very-high-energy (VHE, $E_{\gamma }\gtrsim 100\mathrm{GeV}$) gamma rays resulting from the scattering of high-energy cosmic-ray protons off axionlike particles (ALPs) populating the dark matter halo of the Milky Way is investigated. By employing the latest instrument response functions for current and future facilities, we demonstrate that ground-based VHE gamma-ray observatories, such as H.E.S.S., CTAO, and SWGO, provide a promising and complementary avenue to probe the yet uncharted ALP-proton coupling $g_{ap}$. Assuming a pointlike proton, our results show that these experiments can reach sensitivity to couplings above ${10}^{-2}$ in the $1-{10}^8\mathrm{eV}$ ALP mass range, a region that remains largely unexplored by supernova and neutron star cooling observations. Interestingly, we demonstrate that this search channel is capable of probing effective ALP-proton couplings comparable to those predicted in benchmark quantum chromodynamics (QCD) axion scenarios, such as the Kim-Shifman-Vainshtein-Zakharov (KSVZ) and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) models. These findings highlight the potential of VHE gamma-ray astronomy to provide unique constraints on the interaction between ALPs and the baryonic sector and motivate the investigation of the impact of the proton structure on the predictions.

The interplay between quantum orientation entanglement and Wigner rotation plays a fundamental role in understanding the behavior of spin angular momentum in quantum states. To analyze the quantum orientation entanglement of the relativistic helicity states interpolating between the Jacob-Wick helicity and the light-front helicity, we examine the relative angle between the particle’s momentum direction and the spin orientation for the interpolating helicity states. For this analysis, we introduce a novel method for expanding the interpolating helicity states in terms of the Jacob-Wick helicity. The corresponding probabilistic coefficients follow the structure of the Wigner d-matrix elements, which we use for the interpretation of the quantum orientation entanglement manifested in the angular distributions of the interpolating scattering helicity amplitudes. As an explicit demonstration, we compute the interpolating helicity amplitudes for the pair production of spin-1 (vector) particles in the annihilation of two spin-0 (scalar) particles, focusing primarily on their contact interaction. In particular, we identify the critical interpolation angle that bifurcates the dynamical branches between the instant-form dynamics and the light-front dynamics and discuss the underlying orientation entanglement in the interpolating helicity amplitudes.

Studying phase transitions in interacting quantum field theories generally requires the numerical study of the dynamical system on a large lattice, which is, in most cases, computationally very challenging. In this work an alternative method is proposed to solve Euclidean path integrals in quantum field theories, using radial basis function-type neural networks. The method allows us to approximate observables in a very efficient manner, taking only seconds to do calculations that would otherwise take hours or even days with other existing methods. The model is used to describe phase transitions in the scalar ${\varphi }^4$ theory for a wide range of coupling strength. The obtained phase transition line is compared to previous lattice results, giving very good agreement between them.

New decay modes of $B$ meson into a baryon and invisible dark antibaryon $\mathrm{\Psi }$ are among the most distinctive signatures of the $B$-mesogenesis scenario. We concentrate on the proton mode and consider two versions of the underlying interaction of $\mathrm{\Psi }$ with quarks, the so-called models ($d$) and ($b$). To estimate the width of the $B^{+}\to p\mathrm{\Psi }$ decay, we obtain the $B^{+}\to p$ transition form factors, applying QCD light-cone sum rules with $B$-meson distribution amplitudes with an accuracy up to twist-5, while interpolating the proton with a current. This method is independent of the previously applied one, which was based on the nucleon distribution amplitudes. We estimate the partial width of the $B^{+}\to p\mathrm{\Psi }$ decay as a function of the dark antibaryon mass. Furthermore, we use the ratio of this width to the inclusive $B\to X_N\mathrm{\Psi }$ width, the latter predicted earlier using the heavy quark expansion method. This ratio, which is independent of the effective coupling, when combined with the minimal inclusive branching fraction of $O({10}^{-4})$, necessary for the feasibility of $B$-mesogenesis, yields lower limits on the $B^{+}\to p\mathrm{\Psi }$ branching fraction. We confront these limits with the most recent upper bounds obtained from BABAR and Belle/Belle II searches for the decays of the $B^{+}$ meson into a proton and missing energy. The comparison indicates that experimental upper bounds on the branching fraction of $B\to p\mathrm{\Psi }$ at the level of ${10}^{-8}–{10}^{-7}$ are needed for a decisive probe of this invisible mode of $B$ decays.

We present a next-to-leading-order (NLO) analysis of exclusive $J/\psi +\gamma$ production via photon-photon fusion in ultraperipheral collisions at the High-Luminosity Large Hadron Collider (HL-LHC) and the Future Circular Collider (FCC). The study is performed within the nonrelativistic quantum chromodynamics factorization framework for proton-proton, proton-nucleus, and nucleus-nucleus collisions, with nuclear species spanning a wide range of nuclear charges (O, Ca, Ar, Kr, Xe, and Pb), enabling a systematic investigation of nuclear effects on the production cross sections. The photon fluxes are modeled using an electric-dipole form factor, with the impact-parameter dependence strictly enforced to ensure the exclusivity of the process. We present predictions for total cross sections and kinematic distributions at leading order and NLO. With a transverse momentum cut of $p_T>2\mathrm{GeV}$, the NLO corrections reduce the cross section by approximately 35% at the central scale. The associated theoretical uncertainties are systematically estimated via renormalization scale variations. Despite this suppression, the cross sections remain sizable and indicate that exclusive $J/\psi +\gamma$ production serves as a sensitive probe of photon-induced quarkonium production mechanisms. We thus present the corresponding event yields predicted for the HL-LHC and the FCC.

In light of recent DAMIC-M results, we present the status of thermal-relic dark matter $\chi$ coupled to a kinetically mixed dark photon $A^{\prime }$. In the predictive “direct annihilation” regime, $m_{A^{\prime }}>m_{\chi }$, the relic abundance depends on the kinetic mixing parameter, and there is a minimum value compatible with thermal freeze-out. Using only electron- and nuclear-recoil direct-detection results, we find that, for complex scalar dark matter, the direct annihilation regime is now excluded for nearly all values of $m_{\chi }$; the only exception is the resonant annihilation regime where $m_{A^{\prime }}\approx 2m_{\chi }$. Direct annihilation relic targets for other representative models, including Majorana and pseudo-Dirac candidates, remain viable across a wide range of model parameters but will be tested with a combination of dedicated accelerator searches in the near future. In the opposite “secluded annihilation” regime, where $m_{\chi }>m_{A^{\prime }}$, this scenario is excluded by cosmic microwave background measurements for all $m_{\chi }\lesssim 30\mathrm{GeV}$. Similar conclusions in both the direct and secluded regimes hold for all anomaly-free vector mediators that couple to the first generation of electrically charged Standard Model particles.