The neutrino telescopes have firmly established the existence of ultrahigh-energy neutrinos. Observations of these neutrinos offer a unique probe of neutrino self-interactions. This work investigates how the self-interacting neutrinos, mediated by scalar bosons, inject energy into the medium through radiative scattering with the cosmic neutrino background, leaving an imprint on the cosmic microwave background (CMB) spectrum. The energy injection into plasma in redshift ranges, $5\times {10}^4\lesssim z\lesssim 2\times {10}^6$ and $z\lesssim 5\times {10}^4$, leads to $\mu$-type and $y$-type CMB spectral distortions, respectively. Using observational constraints from Cosmic Background Explorer/Far Infrared Absolute Spectrophotometer and projected sensitivities from Primordial Inflation Explorer (PIXIE) experiments for $\mu$-type and $y$-type CMB distortions, we derive the stringent upper bounds on the self-interaction coupling strength as a function of mediator mass for neutrino interactions. We focus on flavor-specific self-interaction related to muon neutrinos and sub-GeV mass mediators ($m_{\varphi }$). We find the upper bound on the self-interaction coupling strength to be $\sim 2.8\times {10}^{-4}$ for the muon neutrino, considering ultrahigh-energy muon neutrino energy to be 1 PeV and PIXIE projected upper bounds on $y$-type CMB spectral distortion. The bound remains constant till the mediator mass reaches the center-of-mass energy, and after that, it gets relaxed and becomes proportional to the mediator mass. We have also compared our results with existing bounds in the literature. Our findings indicate that CMB spectral distortion could play a decisive role in exploring neutrino physics beyond the standard model of particle physics, and future missions like PIXIE can provide valuable insights.

We propose a novel self-interacting dark matter (SIDM) model mediated by a light leptophilic scalar boson to solve long-standing small-scale structure problems within the Lambda cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) framework. Small-scale anomalies such as the core-cusp problem challenge the traditional CDM framework. Therefore, we introduce a scalar-mediated SIDM model as an alternative to CDM that naturally addresses these problems. It also exhibits $p$-wave-suppressed annihilation and avoids constraints from the cosmic microwave background. In our model, we assume pseudo-Dirac dark matter with a small mass splitting of order ${10}^2\mathrm{eV}$ to ensure kinematic scattering suppression in satellites. We also introduce a dimension-five transition magnetic dipole operator to satisfy big bang nucleosynthesis requirements. It allows the decay ${\chi }_2\to {\chi }_1\gamma$, so the excited-state abundance is primarily depleted by exothermic self-interactions (${\chi }_2{\chi }_2\to {\chi }_1{\chi }_1$) in the early Universe. This ensures that the excited-state population is negligible for structure formation at the dipole scale ${\mathrm{\Lambda }}_{\mathrm{eff}}\gtrsim {10}^7\mathrm{GeV}$. The model is made inelastic by a dark discrete ${\mathbb{Z}}_2$ symmetry that prevents tree-level elastic scattering. Treating the scattering dynamics nonperturbatively in a Schrödinger framework yields a relatively small residual resonant benchmark window ($m_{\chi }\approx 40\mathrm{GeV}$, $\mathrm{\Delta }m\approx 100\mathrm{eV}$, $m_{\varphi }\approx 20\mathrm{MeV}$). In such a system, the kinematic threshold minimizes interactions in ultrafaint Milky Way satellites, while resonant effects produce large cross sections, $\sigma /m\sim \mathcal{O}(10){\mathrm{cm}}^2/\mathrm{g}$, in field dwarf galaxies. Current direct-detection prospects are weak, and the leptophilic scalar mediator keeps the model away from existing nuclear recoil constraints. However, the low-threshold dipole operator provides a potential discovery channel for future xenon-based experiments.

We investigate a scenario in which the recently observed ultra-high-energy neutrino event KM3-230213A, with a median energy of approximately 220 PeV, as well as the high-energy neutrinos detected by IceCube Observatory, originate from the decay of superheavy dark matter (DM) particles produced through primordial black hole (PBH) evaporation. To establish this connection, we derive constraints on the PBH abundance parameter $\beta$ as a function of the initial PBH mass $M_{{\mathrm{BH}}_0}$ and DM mass $m_{\mathrm{DM}}$, by considering the bound from the observed relic DM abundance. Using these constraints, we compute the resulting neutrino flux and show that DM masses in the PeV-EeV range can yield neutrinos of comparable energies, capable of accounting for both the KM3-230213A and IceCube events while remaining consistent with the relic abundance constraint. Interestingly, the scenario remains viable over a broad region of parameter space while satisfying existing cosmological and astrophysical bounds. Overall, our results demonstrate that PBH evaporation followed by DM decay provides a consistent and natural explanation for the observed ultra-high-energy neutrino events in the absence of accompanying multimessenger signatures.

A new chapter is opening in the theory of core-collapse supernovae and neutron star mergers as simulations of these events begin to incorporate fast flavor conversion (FFC) and other forms of neutrino flavor mixing. Using numerical experiments, we show that the approximations of FFC that have been implemented so far are limited by at least two of three factors: (1) approximating continuous evolution as a discrete sequence of instabilities, (2) using spatially homogeneous asymptotic states, and (3) assuming that FFC must be accompanied by instability. The factors we identify in this work will be important considerations as the research area progresses from initial exploratory studies to more quantitatively precise assessments.

So far a major source of uncertainty in the study of heavy-ion collisions arises from the early time dynamics which includes initial state and preequilibrium dynamics. The state-of-the-art framework, KøMPøST [Kurkela et al. Phys. Rev. C 99, 034910 (2019); Kurkela et al. Phys. Rev. Lett. 122, 122302 (2019)], employs nonequilibrium Green’s functions to propagate the initial energy-momentum tensor to the hydrodynamic phase, yet currently only treats transverse plane dynamics under boost-invariant conditions. In this work, we extend KøMPøST to include nonboost-invariant responses to initial conditions, essential for accurately capturing the longitudinal structures observed in heavy-ion collisions. Nonboost-invariant fluctuations on top of a homogeneous background are evolved using $(3+1)\mathrm{D}$ response functions calculated in kinetic theory. To assess kinetic theory’s transition towards hydrodynamic evolution, we systematically compare the out-of-equilibrium shear-stress tensor from KøMPøST-3D with estimates based on Navier-Stokes hydrodynamics. Subsequently, a comprehensive $(3+1)\mathrm{D}$ framework, mcdipper + KøMPøST-3D + CLVisc + SMASH, is utilized to simulate the complete spacetime evolution of heavy-ion collisions. The sensitivity of key observables, including longitudinal structure of anisotropic flow, to variations in the hydrodynamic initialization time is thoroughly investigated.

Tensor form factors encode essential information about the internal spin structure and tensor dynamics of baryons. In this work, we investigate the tensor form factors of the baryon hyperons ${\mathrm{\Omega }}^{-}$, ${\mathrm{\Sigma }}^{*+}$, and ${\mathrm{\Xi }}^{*-}$ within the framework of QCD sum rules. The complete set of tensor form factors is numerically evaluated in the momentum transfer region $0<Q^2<10{\mathrm{GeV}}^2$. In addition, the quark tensor charges of the considered hyperons are extracted in the forward limit. The results provide new nonperturbative insight into the tensor structure and spin content of spin-$3/2$ baryons and offer valuable theoretical input for future phenomenological analyses and experimental studies.

The polarized light of the cosmic microwave background is sensitive to new physics that violates parity symmetry. For example, the interaction of photons with the fields of elusive dark matter and dark energy could cause a uniform rotation of the plane of linear polarization across the sky, an effect known as cosmic birefringence. We extract the cosmological rotation angle, $\beta$, using Bayesian analysis of parity-violating correlations, $EB$ and $TB$, of polarization data from the Atacama Cosmology Telescope (ACT) Data Release 6. We use prior probabilities for instrumental miscalibration angles derived from the optics model for the ACT telescope and instruments, and marginalize over a residual intensity-to-polarization leakage. We measure $\beta =0.215°\pm 0.074°$ (68% confidence level), which excludes $\beta =0$ with a statistical significance of $2.9\sigma$. Although there remain systematics in the ACT data that are not understood and do not allow us to draw strong cosmological conclusions, this result is consistent with previous independent results from the WMAP and Planck missions. It is suggestive that independent data sets and analyses using different methodologies have yielded the same sign and comparable magnitudes for $\beta$.

In this paper, we investigate entanglement for causally connected subregions $A$ and $B$ in quantum field theory and holography. Recent developments have established that a transition operator $T_{AB}$ can be well-defined for such subregions, which is generally non-Hermitian. By employing the Schwinger-Keldysh formalism and the real-time replica method, we show how to construct $T_{AB}$ and compute associated entanglement measures. In certain configurations, this leads to a notion of timelike entanglement entropy, for which we provide explicit quantum field theory computations and propose a holographic dual via analytic continuation from the Euclidean setup. Both analytical and numerical results are compared and found consistent. If entanglement between causally connected subregions is to be meaningful, it should admit well-defined extensions of other entanglement measures beyond the spacelike setting. With this motivation we study the analytic continuation of the twist correlators for reflected-entropy and propose a holographic geometric analog in complexified bulk geometry, providing evidence that entanglement measures can be extended to timelike regions.

We prove that symmetry in the presence of gravity implies a version of the completeness hypothesis. For a broad class of theories, we demonstrate that the existence of finitely many charged particles logically necessitates the existence of infinitely many charged particles populating the entire charge lattice. Our conclusions follow from the consistency of perturbative gravitational scattering and require the following ingredients: (1) a weakly coupled ultraviolet completion of gravity, (2) a non-Abelian symmetry $G$, gauged or global, whose Cartan subgroup generates the Abelian charge lattice, and (3) a spectrum containing some finite set of charged representations, in the simplest cases taken to be a single particle in the fundamental. Under these conditions, the Abelian charge lattice is completely filled by single-particle states for $G=SO(N)$ with $N\ge 5$ and $G=SU(N)$ with $N\ge 3$, which in turn implies completeness for other symmetry groups such as $Spin(N)$, $Sp(N)$, and $E_8$. Curiously, a corollary of our results is that the $SU(5)$ and $SO(10)$ grand unified theories have precisely the minimal field content needed to derive completeness using our methodology.

Using ($2+1$)-flavor lattice QCD data, we refine the parameters of an analytical holographic model via gradient descent optimization to precisely locate the critical endpoint in the $T\text{−}\mu$ plane. Specifically, we calibrate the model using input data for the speed of sound at ${\mu }_B=0$, the second-order baryon number susceptibility ${\chi }_2^B$, and the baryon number density at ${\mu }_B/T=1$. With these parameters fixed, we calculate pressure and energy density versus temperature at small chemical potentials and compare the results with lattice QCD data using Taylor expansion techniques. This comparison validates the robustness of our model upon extension to finite chemical potentials, as the results show broad consistency with lattice QCD data in this regime. Finally, we employ the calibrated model to determine the coordinates of the critical endpoint in the $T\text{−}{\mu }_B$ plane, finding it located at $({\mu }_B=0.678\mathrm{GeV},T=0.110\mathrm{GeV})$.

We present the Batalin-Fradkin-Vilkovisky quantization of the quadratic gravity theory, which is the most general theory with terms up to quadratic order in curvature. This approach of quantization is based on the Hamiltonian formulation. In this sense, this study contributes to the consistency of the quantum formulation of the theory. With this scheme of quantization, we may introduce a broad class of additional conditions on the field variables, by including Lagrange multipliers and time derivatives. We find that a mandatory condition for the validity of the Hamiltonian formulation, previously known from classical analysis, can be incorporated consistently in this quantization. We obtain the propagators of the fields, including the propagators associated with the quantum states of negative norm. The spectrum of masses coincides with the results of Stelle [Renormalization of higher derivative quantum gravity, Phys. Rev. D 16, 953 (1977)], but distributed on a different way among the fields.

Modeling the remnant recoil velocity (kick) distribution from binary black hole mergers is crucial for understanding hierarchical mergers in active galactic nuclei or globular clusters. Existing analytic models often show large discrepancies with numerical relativity (NR) data, while data-driven models are limited to mass ratios of $q\le 8$ (aligned spins) and $q\le 4$ (precessing spins) and break down when extrapolated outside their training ranges. Using $\sim 5000$ of NR simulations from the SXS and RIT catalogs up to $q=128$ and $\sim 100$ black hole perturbation theory simulations up to $q=200$, we present two classes of models: (i) gwModel_kick_q200 (gwModel_kick_q200_GPR), an analytic (Gaussian process regression) model for aligned-spin binaries. (ii) gwModel_kick_prec_flow, a normalizing-flow model for kick distribution from precessing binaries with isotropic spins. Our approach combines analytic insights from post-Newtonian theory with data-driven techniques to ensure correct limiting behavior and high accuracy across parameter space. Both gwModel_kick_q200 and gwModel_kick_prec_flow are valid from comparable to extreme mass ratios. Extensive validation shows all three models outperform existing ones within their respective domains. Finally, using both back-of-the-envelope estimates and 1404 detailed star cluster simulations incorporating our kick models, we find that the black hole retention probability in low mass globular clusters can vary noticeably when the gwModel_kick_prec_flow model is employed compared to previous analytic or data-driven models. The models are publicly available through the gwmodels package.

It is believed that rays bent inside and outside photon spheres could affect partially the black hole shadow images by the Event Horizon Telescope and the rays near photon spheres would be detected by near-future space observations. The investigation of the rays near the photon spheres in not only black hole spacetimes but also exotic spacetimes would be important since one will need them to exclude black hole mimickers. The deflection angles of the rays deflected by the photon spheres diverge logarithmically and we can treat them by a strong-deflection-limit analysis. The error of the strong-deflection-limit analysis becomes large if antiphoton spheres exist in the spacetimes and the analysis breaks down when the photon spheres and the antiphoton spheres degenerate to form a marginally unstable photon sphere. This is because the deflection angles of the rays bent by the marginally unstable photon sphere diverge in powers. In this paper, we extend Eiroa, Romero, and Torres’s method to gravitational lensing of rays inside and outside of the marginally unstable photon sphere in a general, static, spherically symmetric, and asymptotically-flat spacetime in strong deflection limits and we apply it to a Reissner–Nordström spacetime and a Hayward spacetime with the marginally unstable photon sphere. We have also confirmed that the deflection angles in the strong deflection limits by the method converge correctly to the deflection angle without approximations, while there are the mismatches of the coefficient of the power-divergent term of the deflection angles of the rays deflected just outside of the marginally unstable photon sphere in a semianalytic calculation by the author previously.

Nonlinearly scalarized black holes are investigated in Einstein-scalar-Gauss-Bonnet theory with polynomial coupling functions $\zeta (\varphi )$ satisfying ${\zeta }^{\prime \prime }(0)=0$, where ${\zeta }^{\prime }(\varphi )=0$ features besides $\varphi =0$ solutions with constant ${\varphi }_{\mathrm{s}}\ne 0$. We determine the threshold amplitudes for Gaussian pulses, above which Schwarzschild black holes (SBHs) transition to scalarized black holes for two coupling functions; $\zeta (\varphi )=\alpha {\varphi }^4-\beta {\varphi }^8$ and $\zeta (\varphi )=\alpha {\varphi }^4-\beta {\varphi }^6$. In contrast, for the quartic coupling function $\zeta (\varphi )=\alpha {\varphi }^4$ SBHs are stable. Treating $\zeta (\varphi )R_{\mathrm{GB}}^2$ as an effective potential $V_{\mathrm{eff}}$ provides an explanation for the “plateau” and the divergence observed in the time evolution. We then construct the branches of nonlinearly scalarized black holes in the probe limit and with backreaction. While the pattern of the solution branches in the probe limit exhibits universal features, the presence of backreaction reveals a distinct dependence on the coupling strength $\beta$.

We show that the compactness gap between black holes and neutron stars witnessed in general relativity can disappear in Hořava-Lifshitz (HL) gravity. Assuming a fermionic equation-of-state and solving the Tolman-Oppenheimer-Volkoff equation within the HL gravity framework, we find that there exists a minimum fermion mass $m_f^{(\min )}(q,y)$, above which the gap of the compactness between black hole and fermionic compact object vanishes, for a given deformation parameter $q$ of HL and interaction strength $y$ between fermions. Thus, in HL gravity, the mass and radius of an object found in the lower mass gap by LIGO-Virgo-KAGRA observations might not be able to classify it as a black hole or a neutron star. It is interesting to note that a fermion of mass $\sim 40\mathrm{GeV}$ can form a highly compact object of mass $\sim {10}^{-4}{M}_{\odot }$ and radius $\sim 1\mathrm{m}$. Such configurations may be regarded as speculative candidates for a subcomponent of cold dark matter. In addition, we find evidence for the existence of another class of compact objects whose compactness approaches that of black holes.

We present a formulation of the spherically decomposed Green’s function for a Schwarzschild black hole, based on a decomposition into two components, $G^{+}$ and $G^{-}$, based on their large-frequency behavior. While similar decompositions have been considered previously, here we systematically apply it to Schwarzschild spacetime and analyze its implications for the analytic structure of the Green’s function in the complex-frequency plane. We show that both $G^{+}$ and $G^{-}$ possess branch cuts along the imaginary axis, which give rise to the direct part and the late-time tail, while the poles of $G^{+}$ correspond to the quasinormal mode spectrum. This allows us to identify a branch-cut direct part, a quasinormal-mode contribution, and a late-time tail through contours adapted to different causal spacetime regions. This is in sharp contrast to Leaver’s original formulation, where the prompt response is tied to a technically difficult large-arc contribution. We validate our decomposition with independent time-domain Regge-Wheeler simulations finding excellent agreement. Our results provide a practical and physically transparent framework for disentangling the distinct pieces of the Schwarzschild response, and offer a natural starting point for extensions to Kerr perturbations and nonlinear ringdown physics.

We perform a Bayesian inference of the dense-matter equation of state (EOS) within a unified framework that incorporates hadronic matter, quark matter, and a smooth hadron-quark crossover. The EOS is constrained using physical consistency conditions, gravitational wave data from GW170817, NICER mass-radius measurements, and hypothetical future high-precision radius observations. In contrast to most previous studies that assume a sharp first-order phase transition or fix part of the EOS, we simultaneously infer hadronic, quark, and crossover parameters within a single statistical framework. We find that current observations strongly constrain the density dependence of the nuclear symmetry energy, particularly its slope and curvature, while the highest-density hadronic parameters and quark matter properties remain only weakly constrained. The posterior distributions favor a crossover centered at an energy density $ϵ\sim (4–6){ϵ}_0$ with a width $\mathrm{\Gamma }\sim (0.5–1.0){ϵ}_0$. A pronounced peak in the speed of sound emerges naturally near the crossover region, typically around $4{ϵ}_0$, and often coincides with the central densities of $\sim 2M_{\odot }$ neutron stars. We further show that the trace anomaly exhibits a remarkably universal behavior across the accepted EOS ensemble and remains largely insensitive to current observational constraints. This indicates that present data primarily probe the low- to intermediate-density EOS, while robust inference of quark matter and genuinely high-density physics will require next-generation precision radius measurements or complementary observables.

Disturbances in gravitational wave (GW) observational data are often caused by nonstationary noise in the detector itself, such as backscattering of laser stray light into the signal field. Unlike GW signals, nonstationary noise can appear in both the GW-signal quadrature and the orthogonal quadrature, which is usually not measured. Simultaneous sensing of this orthogonal quadrature provides a witness channel that can be used to reconstruct the disturbance in the signal quadrature enabling subtraction of nonstationary noise. Here, we present the concept of quadrature witness that is compatible with frequency-dependent squeezing, which is already used to simultaneously reduce photon shot noise and photon radiation-pressure noise. We demonstrate that implementing this approach in a GW detector could reduce noise caused by loud backscatter events, thereby improving the overall sensitivity and robustness of GW observatories.