The numerical calculation of gravitational self-force in extreme mass ratio inspiral systems is fundamentally challenging due to the singular nature of point-particle sources. To overcome these difficulties, the effective source method offers an innovative alternative by replacing traditional regularization techniques with a reformulation of the problem. In this paper, we present the first fully analytic framework for constructing effective sources to compute the gravitational self-force for generic orbits in Schwarzschild spacetime. By reformulating the singular field through angular modulation in terms of a tetrad decomposition, the effective source can be constructed with the linear combination of scalar modes. The derived effective source is continuous across the particle’s worldline, enabling efficient numerical implementation in $1+1$ dimensions.

Recent gravitational-wave (GW) observations offer a unique opportunity to probe the fundamental nature of compact objects. A growing body of research has focused on exploring the role of dark matter (DM) through the concept of DM-admixed neutron stars (NSs), where the presence of DM can significantly alter key physical properties of NSs, such as their mass, radius, and tidal deformability, ultimately affecting the predicted GW waveform emitted during binary coalescences. In this work, we present a novel observational test that, for the first time, uses GW inspiral waveforms in terms of DM parameters to place constraints on the influence of DM inside NSs using real GW data. By reanalyzing signals from events such as GW230529, GW200115, and GW200105, we derive new upper bounds on the DM fraction, $F_{\chi }$, and particle mass, $m_{\chi }$, under the assumption that DM is described by a scalar field with a self-interaction potential. We find that the upper bound on $F_{\chi }$ depends on the specific binary system under analysis, indicating that different DM configurations can be consistent with observations in different ways. In particular, the event GW190814 may be compatible with a DM halo configuration. In contrast, the other events analyzed (GW230529, GW200105, and GW200115) are consistent with DM forming a core inside the NS, yielding strong upper bounds on $F_{\chi }$. The corresponding values for the mass scale $m_{\chi }$ are also discussed in the text. This work offers a new approach to probing DM in the context of compact NS objects through GW observations.

Ultralight bosonic fields around a rotating black hole can extract energy and angular momentum through the superradiant instability and form a dense cloud. We investigate the scenario involving two scalar fields, $\varphi$ and $\chi$, with a coupling term $\frac{1}{2}\lambda \varphi {\chi }^2$, which is motivated by the multiple-axion framework. The ultralight scalar $\varphi$ forms a cloud that efficiently produces $\chi$ particles non-perturbatively via parametric resonance, with a large mass hierarchy, ${\mu }_{\chi }\gg {\mu }_{\varphi }$. Rather than escaping the system as investigated by previous studies, these $\chi$ particles remain bound, orbiting the black hole. Moreover, the particle production occurs primarily at the peak of the cloud’s profile, allowing $\chi$ particles in quasi-circular orbits to pass repeatedly through resonant regions, leading to cumulative amplification. This selective process naturally forms a dense ring of $\chi$ particles, with a mass ratio to the cloud fixed by $({\mu }_{\varphi }/{\mu }_{\chi }{)}^2$. Our findings reveal a novel mechanism for generating bound-state particles via parametric resonance, which also impacts the evolution of the cloud. This process can be probed through its imprint on binary dynamics and its gravitational-wave signatures.

Neutral hydrogen hyperfine splitting of the atomic energy levels originates from the interaction between the electron and proton spins. Such a splitting is responsible for the 21-cm emission line of hydrogen during a transition between the hyperfine states. The 21-cm emission line represents a fundamental tool in astrophysics for studying the distribution of neutral hydrogen gas in galaxies and in the early Universe, e.g., during the Dark Ages and Cosmic Dawn. In the present paper we investigate the influence of gravity on the 21-cm emission line considering, in particular, the case of a cloud of hydrogen orbiting in the equatorial plane of a Kerr black hole. Using a Fermi-Walker tetrad, adapted to an orbiting hydrogen atom, we solve the related two-body Dirac equation in the Kerr background, analyzing the resulting hyperfine structure, as regards the energy spectrum and the polarization of the photons resulting from the allowed transitions. We show that the combined gravito-inertial effects of the orbital motion and of the background geometry, due to the Kerr black hole, cooperate to remove the residual three-fold degeneracy of the higher energy level in the hyperfine splitting. The studied effect can be considered as the gravitational analog of the anomalous Zeeman effect, with respect to which a peculiar property of innermost stable circular orbits is also highlighted.

The deflection of light rays near gravitating objects can be influenced not only by gravity itself but also by the surrounding medium. Analytical studies of such effects are possible within the geometrical optics approximation, where the medium introduces additional light bending due to refraction. These studies typically assume a cold nonmagnetized plasma, for which light propagation is independent of the medium’s velocity. In this paper, we extend the analysis to the general case of dispersive refractive media in motion and study its influence on light deflection. We consider an axially symmetric stationary spacetime filled with a moving medium, motivated by the interplay between rotational effects originating from the spacetime and those induced by the medium’s motion. We begin by analyzing light deflection in the equatorial plane of a rotating object in the presence of a radially moving and rotating medium. Assuming a specific form of the refractive index enables a fully analytic treatment. In the particular cases of either pure radial or pure rotational motion, we obtain explicit expressions for the deflection angle. Next, we analyze the case of a slowly moving medium and identify two particularly interesting results. First, we show that to the first order in the medium’s velocity, the radial motion does not affect the light deflection. Second, assuming slow rotation of the gravitating object, we demonstrate that the black hole rotation and the medium motion can produce equivalent observational signatures. We find the quantitative condition under which these effects compensate each other. This relation becomes particularly clear for a Kerr black hole, discussed as an example.

Starobinsky inflation and nonminimally coupled (NMC) Higgs inflation have been among the most favored models of the early Universe, as their predictions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ fall comfortably within the constraints set by Planck and BICEP/Keck. However, new results from the Atacama Cosmology Telescope (ACT) suggest a preference for higher values of $n_s$, introducing tension with the simplest realizations of these models. In this work, being agnostic about the nature of the inflaton, we show that incorporating one-loop corrections to a quartic NMC inflationary scenario leads to a shift in the predicted value of $n_s$, which brings NMC inflation into better agreement with ACT observations. The effect is even more significant when the model is formulated in the Palatini approach, where the modified field-space structure naturally enhances deviations from the metric case. These findings highlight the importance of quantum corrections and gravitational degrees of freedom in refining inflationary predictions in light of new data.

Neutrino direct simulation Monte Carlo ($\nu \mathrm{DSMC}$) is a Monte Carlo method for solving the neutrino Boltzmann equation in the early Universe, designed to track the evolution of cosmic neutrinos across a wide range of cosmological scenarios. We develop a complete $\nu \mathrm{DSMC}$ solver that consistently incorporates the effects of the electron mass, three-flavor neutrino oscillations, and finite-temperature QED corrections to the thermodynamics of the electromagnetic plasma. As a first application, we perform a high-precision calculation of neutrino decoupling in the standard cosmological model and obtain $N_{\mathrm{eff}}=3.0439\pm 0.0006$, in excellent agreement with state-of-the-art results.

Fast radio bursts (FRBs) are a unique probe of the cosmos, owing to dispersion caused by free electrons in the intergalactic medium. Two of the main quantities of interest are degenerate: the density of matter ${\mathrm{\Omega }}_{\mathrm{b}}{f}_{\mathrm{d}}$ outside of galaxies and the Hubble constant $H_0$. Here, we present a new possibility of breaking the degeneracy without invoking early-Universe priors on ${\mathrm{\Omega }}_{\mathrm{b}}$. Assuming some FRBs originate in compact object mergers, the combination of dispersion and luminosity distance from the gravitational wave (GW) can be used to measure ${\mathrm{\Omega }}_{\mathrm{b}}{h}^2{f}_{\mathrm{d}}$ (where $h$ is the dimensionless Hubble constant). We show that this measurement can be combined with the abundant FRBs that have a redshift measurement. This combination breaks the degeneracy with the Hubble constant. We develop a Bayesian framework and forecast that third-generation GW detectors are required to obtain meaningful constraints. We forecast that 1 year of Einstein Telescope operations can constrain $H_0$ to $\pm 6\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ and ${\mathrm{\Omega }}_{\mathrm{b}}{h}^2{f}_{\mathrm{d}}$ to $\genfrac{}{}{0ex}{}{+0.0015}{-0.0016}$ (68% credible interval). The method can also be used with luminosity distances obtained through other means than GWs.

We perform a thorough investigation of (ultra)relativistic freeze-out (UFO) during reheating. While the standard WIMP (nonrelativistic freeze-out) and FIMP (freeze-in) paradigms have been explored in detail during the reheating epoch, UFO has not been systematically studied, despite the fact that it is operative in a broad region of parameter space. Although dark matter (DM) is “hot” at the time of relativistic freeze-out, we show that it can easily undergo enough cooling by the time of structure formation to be compatible with $\mathrm{\Lambda }\mathrm{CDM}$. Unlike standard WIMP-like freeze-out, there can be significant out-of-equilibrium DM production after UFO, similar to the freeze-in mechanism. However, unlike freeze-in, UFO can accommodate much stronger couplings. The UFO parameter space consistent with ${\mathrm{\Omega }}_{\chi }{h}^2=0.12$ is quite large, with DM masses spanning about 13 orders of magnitude (${10}^{-7}\mathrm{GeV}\lesssim m_{\chi }\lesssim {10}^6\mathrm{GeV}$), reheating temperatures spanning 17 orders of magnitude (${10}^{-2}\mathrm{GeV}\lesssim T_{\mathrm{RH}}\lesssim {10}^{15}\mathrm{GeV}$), and beyond the Standard Model effective interaction scales spanning 11 orders of magnitude (${10}^3\mathrm{GeV}\lesssim \mathrm{\Lambda }\lesssim {10}^{14}\mathrm{GeV}$), where a thermally averaged cross section has been parametrized as $⟨\sigma v⟩=\frac{T^n}{{\mathrm{\Lambda }}^{n+2}}$. Interestingly, the most suitable range of couplings for UFO lies precisely between the typical couplings for WIMPs and FIMPs, rendering UFO quite attractive from the standpoint of detection. Particle physics models that are easily amenable to UFO include heavy vector or scalar portal interactions, along with nonrenormalizable effective interactions. Finally, we show there is a distinction between UV UFO and IR UFO, where the relic abundance for the former is sensitive to the freeze-out temperature, while the abundance for the latter is sensitive to the DM mass and the reheating temperature but insensitive to the freeze-out temperature.

Dark photons (Dphs) appear in theories beyond the Standard Model of particles (SM). Under certain conditions, it is possible to have a mixing between SM photons and Dphs that should be observed as anomalies in the spectrum of astrophysical sources. Our aim is to either find evidence of or set constraints on the existence of Dphs with masses in the range of $\mathrm{\mu }\mathrm{eV}$ using observations of two galactic sources observed at TeV energies. We use the flux of the Crab Nebula and MGRO $\mathrm{J}1908+06$ at TeV energies reported by High-Altitude Water Cherenkov observatory and Large High Altitude Air Shower Observatory and assuming a model where Dphs can mix with SM photons in the vacuum; we compute the test statistic to search for evidence of Dphs in the form of variations or attenuation in the observed spectrum. We do not find statistically significant evidence of the existence of $\mathrm{\mu }\mathrm{eV}$ Dphs. Then, we compute the 68% CL and 95% CL exclusion regions for Dphs with masses in the range from ${10}^{-8}$ to ${10}^{-5}\mathrm{eV}$ and mixing angles with values between 0.01 and 1.0.

The energy released from dark matter (DM) annihilation leads to additional ionization and heating of the intergalactic gas, impacting the hydrogen 21-cm signal during the cosmic dawn. The dark matter annihilation rate scales with its density squared and becomes inhomogeneously boosted with structure formation. This paper examines the inhomogeneity in DM annihilation rate induced by the growth of DM halo structures, and we show that this effect can significantly amplify the spatial fluctuations in temperature and ionization fraction of the gas. Consequently, the fluctuations in the 21-cm brightness temperature may also be enhanced. We showcase these effects for a DM mass of 100 MeV annihilating into ${\mathrm{e}}^{-}{\mathrm{e}}^{+}$ at a rate of $⟨\sigma v⟩/m_{\chi }\sim {10}^{-27}{\mathrm{cm}}^3{\mathrm{s}}^{-1}{\mathrm{GeV}}^{-1}$, which is consistent with current constraints set by the cosmic microwave background. We find that, compared to the homogeneous calculations, inhomogeneous annihilation can enhance the 21-cm power spectrum by up to a factor of 130 over the scales of $k\in [0.05,3]{\mathrm{Mpc}}^{-1}$ at redshifts 11–16. Such signatures could potentially be detected by upcoming radio observatories such as the Square Kilometer Array telescope.

Many astrophysical processes can produce gravitational wave (GW) sources with significant orbital eccentricity. These binaries emit bursts of gravitational radiation during each pericenter passage. In isolated systems, the intrinsic timing of these bursts is solely determined by the properties of the binary. The presence of a nearby third body perturbs the system and alters the burst timing. Accurately modeling such perturbations therefore offers a novel approach to detecting the presence of a nearby companion. Existing timing models account for Newtonian dynamics and leading order radiation reaction effects but neglect the higher order post-Newtonian (PN) contributions to the inner binary. In this paper, we present an improved timing model that incorporates conservative PN corrections that lead to the precession of the binary’s pericenter. We find that these PN corrections significantly impact the binary’s orbital evolution and the timing of the GW burst. In particular, 1PN precession gives rise to distinctive modulation features in the binary’s semilatus rectum and eccentricity. These modulations encode valuable information about the presence and properties of the third body, including its mass and distance. Furthermore, unmodeled 1PN effects significantly bias the tertiary’s mass and distance. Finally, we assess the detectability of GW bursts from such perturbed systems and demonstrate that the inclusion of PN corrections is crucial for accurately capturing the orbital dynamics of hierarchical triples.

We study the deflections of ultrahigh-energy cosmic rays in several widely used models of the coherent Galactic magnetic field (GMF), including PT11 [Pshirkov et al., Astrophys. J. 738, 192 (2011)], JF12 [Jansson and Farrar, Astrophys. J. 757, 14 (2012)], UF23 [Unger and Farrar, Astrophys. J. 970, 95 (2024)] and KST24 [Korochkin, Semikoz, and Tinyakov, Astron. Astrophys. 693, A284 (2025)]. We propagate particles with rigidities of 5, 10, and 20 EV and analyze the differences in deflection predictions across these GMF models. We identify the GMF components responsible for deflections in various regions of the sky and discuss the uncertainties in modeling these components, as well as potential future improvements.

We develop the equation of state (EOS) of protoneutron stars (PNSs) at various stages of evolution by varying entropy per baryon $S$, using the Korea-IBS-Daegu-SKKU density functional model. With finite values for both temperature and density, we systematically investigate the influence of nucleon effective mass on the EOS of PNSs, for different values of isoscalar effective mass ${\mu }_S^{*}$. For high entropy values, we aim to replicate conditions of failed core-collapse supernovae forming black holes. At each stage of evolution, structural and nonradial oscillation (fundamental $f$-mode and first pressure $p_1$-mode) properties are computed under isentropic conditions by varying ${\mu }_S^{*}$. We focus on the effects of $S$ and ${\mu }_S^{*}$ on the oscillation frequencies $f_f$ and $f_{p_1}$, adopting complete general relativistic formalism and Cowling approximation. Thermal effects reduce the values of $f_f$ and $f_{p_1}$ of PNSs compared to those of cold NSs; consequently the detection of the former gets facilitated. For high-mass PNSs, this reduction is more pronounced for $f_{p_1}$ than $f_f$. Moreover, lower values of ${\mu }_S^{*}$ reduce $f_f$ and $f_{p_1}$ further. Universality of mass-scaled angular frequency (${\omega }_{f}M$) with compactness ($C$) and tidal deformability ($\mathrm{\Lambda }$) are obtained as nonlinear fits that shift upwards (downwards) in the ${\omega }_{f}M-C$ (${\omega }_{f}M-\mathrm{\Lambda }$) plane for increasing values of $S$. For fixed $S$, the universality is also retained for variation of ${\mu }_S^{*}$. $S$ shows a stronger correlation than ${\mu }_S^{*}$ with the structural and oscillation properties of (P)NSs. The strength of the correlation of $S$ is more prominent with $f_{p_1}$ than $f_f$ while the trend is opposite for ${\mu }_S^{*}$. These findings suggest that detection of oscillation frequencies by upcoming GW detectors, could potentially indicate the evolutionary stage of a star during its transition from supernova to cold NS.

Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV $\gamma$-ray sources. In 2017, the VERITAS $\gamma$-ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength $\mathcal{O}(10\mathrm{\mu }\mathrm{G})$ through which escaping photons traverse, making it an excellent target to study ALPs. We analyze the VERITAS data of NGC 1275’s 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the ${\mathrm{CL}}_{\mathrm{s}}$ method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity. No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2\times {10}^{-7}\mathrm{eV}\lesssim m_a\lesssim 4\times {10}^{-7}\mathrm{eV}$ and at the coupling strength of $g_{a\gamma }\gtrsim 3\times {10}^{-11}{\mathrm{GeV}}^{-1}$ up to 80% confidence level, which are consistent with previous studies. We find the ${\mathrm{CL}}_{\mathrm{s}}$ to be a trustworthy approach, and advocate for its continued usage in future studies. We note that many of the limitations contributing to the limited sensitivity seen by VERITAS in this study will be improved with next-generation $\gamma$-ray instruments, such as the Cherenkov Telescope Array Observatory.

We perform a comprehensive investigation of the radiative decays of $P$-wave charmed baryons in the $SU(3)$ flavor ${\mathbf{6}}_F$ representation, employing the light-cone quantum chromodynamics (QCD) sum rule approach within the framework of heavy quark effective theory. We analyze their electromagnetic transitions into ground-state charmed baryons via photon emission. When combined with the mass spectra and strong decay properties previously studied in Yang and Chen [Phys. Rev. D 104, 034037 (2021)], our results constitute a systematic and complete QCD sum rule analysis of the $P$-wave singly charmed baryons within the framework of heavy quark effective theory. As summarized in Table V, several excited charmed baryons are found to possess suppressed strong decay widths, thereby rendering their radiative decay channels particularly significant for experimental identification and theoretical understanding.

We show quark transversity generalized parton distributions (GPDs) of the ${\mathrm{\Delta }}^{+}$ isobar by using the diquark spectator model for the first time. First, this model is tested by electric charge, magnetic-dipole, and axial charge form factors, and it is used for calculating the transversity GPDs $H_{1,3,5,7}^{qT}$ of ${\mathrm{\Delta }}^{+}$. The quark transversity distribution $h_1$ is then obtained from the transversity GPDs in the forward limit. Then, helicity-flip amplitudes are shown numerically by using relations between the helicity amplitudes and the GPDs. Finally, by taking first moments of the GPDs, tensor form factors are obtained and we predict the tensor charge. Experimentally, $N\text{−}\mathrm{\Delta }$ transition GPDs are investigated in deeply virtual Compton scattering and virtual meson-production processes, and generalized distribution amplitudes, which correspond to the $s$-channel GPDs, could be investigated by the two-photon processes ${\gamma }^{*}\gamma \to \mathrm{\Delta }\overline{\mathrm{\Delta }}$ at the electron-positron colliders. Therefore, the spin-$3/2$ $\mathrm{\Delta }$ GPDs could become interesting quantities experimentally in the future.

In this work, we investigate the electric, thermal, and thermoelectric response of a hot pion gas in the presence of a time-dependent background electromagnetic field. Using the relativistic Boltzmann equation within the relaxation time approximation, we evaluate the Ohmic and Hall conductivities, thermal and Hall-like thermal conductivities, and the magneto-Seebeck and Nernst coefficients, explicitly incorporating the effects of the magnetic field’s strength and decay dynamics. Unlike earlier studies restricted to static fields, our analysis accounts for the temporal evolution of the background magnetic field, capturing the nontrivial time dependence of transport coefficients and their sensitivity to the field profile. We further examine the elliptic flow coefficient, $v_2$, linked to the Knudsen number through the thermal conductivity, and demonstrate its dependence on both the magnitude and decay of the magnetic field. This work provides the first systematic study of pion transport in a realistic, time-varying electromagnetic background and underscores the importance of including such effects in modeling the late stages of heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and LHC.

We investigate the impact of the $CP$-violating $\theta$ term on isospin symmetry breaking in quark matter and compact star properties using a two-flavor Nambu-Jona-Lasinio model. By incorporating the $\theta$ parameter through the Kobayashi-Maskawa-’t Hooft determinant interaction, we derive the thermodynamic potential and gap equations under finite temperature, baryon chemical potential, and isospin chemical potential. At zero temperature and baryon density, $\theta$ suppresses conventional chiral ($\sigma$) and pion ($\pi$) condensates while promoting pseudoscalar ($\eta$) and scalar-isovector ($\delta$) condensates, thereby reducing the critical isospin chemical potential ${\mu }_I^{\mathrm{crit}}$ for spontaneous symmetry breaking. For $\theta =\pi$, a first-order phase transition emerges at ${\mu }_I^{\mathrm{crit}}=0.021\mathrm{GeV}$, accompanied by $CP$ symmetry restoration. Extending the investigation to finite temperature and baryon chemical potential reveals that these $\theta$-term-induced effects persist. Axion effects (modeled via $\theta \equiv a/f_a$) stiffen the equation of state of nonstrange quark stars, increasing their maximum mass and radii, in agreement with multimessenger constraints from pulsar observations and gravitational wave events. These results establish $\theta$ as a critical parameter modulating both the quantum chromodynamics phase structure and compact star observables.

We study low energy photons coupled to scalar and spinor matter in the presence of an arbitrary homogeneous electromagnetic field in a first-quantized (worldline) approach. Utilizing a Fock-Schwinger gauge for both the scattering photons and homogeneous background, simple compact expressions are found for both the photon- and background-dressed effective action and propagator in scalar and spinor quantum electrodynamics. The low-energy limit allows identification of the coupling of the scattering photons as one of an effective homogeneous superposition of their field strengths, with amplitudes following from application of a suitable linearization operator. To treat the linearization, several techniques are employed, including a functional expansion based on the proper time formalism and worldline Green’s functions, linearized vertex operators under a worldline path integral, and a matrix expansion in the field strengths. We find, in particular, that a replacement rule converting scalar amplitudes to spinor amplitudes at one-loop order can, surprisingly, be extended to tree level amplitudes in the low energy limit. Finally, we discuss a novel worldline representation of the momentum space matter propagators, obtaining a suitable worldline Green’s function for this path integral satisfying homogeneous Dirichlet boundary conditions and momentum space vertex operators representing the scattering photons already in momentum space.