In light of recent interpretations attributing pulsar timing array (PTA) signal to second-order gravitational waves induced by linear cosmological curvature perturbations in the early Universe, the overproduction of primordial black holes (PBHs) poses a theoretical tension. In this work, we address this issue through extending such a scalar-induced gravitational wave (SIGW) framework to include third-order gravitational waves, which allow for a substantial enhancement in the spectral amplitude of SIGWs. Analyzing a combined dataset from cosmic microwave background and baryon acoustic oscillations, we derive cosmological constraints on the physical energy-density fraction of cosmological gravitational waves. Further incorporating PTA data, we obtain constraints on the spectral amplitude and peak frequency of SIGWs. Our results indicate that the parameter region favored by the data combination can to some extent alleviate the PBH overproduction problem, thereby supporting the theoretical consistency of our model. Furthermore, we demonstrate the robustness of our SIGW interpretation for the PTA signal by extending the analysis to include a gravitational wave background from supermassive black hole binaries. These findings are poised for further scrutiny with future high-precision observations.

We investigate how interactions affect the quantum state of scalar perturbations during inflation and the quantum correlations they may exhibit. Focusing on the case of scalar perturbations in single-field inflation, we model interactions using a Lindblad equation with a nonunitary contribution quadratic in the scalar perturbations and of parametrizable amplitude and time dependence. We compute the quantum state of these interacting perturbations, which is fully described by its purity and squeezing parameters. First, we show that, on top of the well-studied decrease of purity, interactions asymptotically decrease the squeezing parameter by up to 75%. Second, we show that this desqueezing induced by the interactions, in addition to the purity loss, causes a further suppression of quantum correlations. We thus emphasize that the quantum or classical character of the correlations exhibited by the perturbations cannot be correctly determined by computing the effect of interactions on the purity alone. Since the phenomenological framework adopted in this paper encompasses a wide class of possible interactions, our results provide general insights into the nature of decoherence processes in primordial fluctuations.

In this work, we perform the first numerical study of strong scalar backreaction in spectator chromo-natural inflation in the case where the spectator sector decays during inflation. The tachyonic instability in scalar fluctuations, activated as the system crosses the $m_Q=\sqrt{2}$ threshold, amplifies perturbations and may significantly alter the background dynamics. The strong scalar backreaction regime introduces an effective quartic term in the potential for the gauge field background that rapidly drives it to zero, accelerating the axion-gauge system decay. We take a first step toward characterizing the dynamics of this scenario by studying the fluctuations at the linearized level, thus providing a necessary benchmark for future nonperturbative analyses. We describe the dynamics of such decay and derive the gravitational wave spectrum for a set of benchmark parameters. Interestingly, the signal may peak at interferometer scales and lie within Laser Interferometer Space Antenna’s projected sensitivity.

We extend a unified theoretical framework for induced (stimulated) scattering—parametric instabilities of electromagnetic waves, including induced Compton, stimulated Brillouin, and stimulated Raman scattering (SRS)—in strongly magnetized electron-positron ($e^{\pm }$) pair plasma. By solving the dispersion relations derived from kinetic theory, taking into account the ponderomotive force due to the beat of incident and scattered waves, we obtain analytical expressions for the linear growth rates of the ordinary, neutral, and charged modes of density fluctuations. Our results clarify which type of scattering dominates under different thermal coupling, resonance, and density conditions. In strong magnetic fields, scattering of perpendicularly polarized waves is generally suppressed, but by different powers of the cyclotron frequency. Moreover, SRS, which is forbidden in unmagnetized $e^{\pm }$ pair plasma, becomes possible in the charged mode. This framework enables a comprehensive evaluation of induced scattering in extreme astrophysical and laboratory plasma, such as fast radio burst emission and propagation in magnetar magnetospheres.

Cosmic ray (CR) feedback in galaxy evolution has seen a theoretical resurgence in the past decade, but significant uncertainties remain in CR transport through the interstellar and circum-galactic media (ISM and CGM). While several works indicate CR effects may be notable in both star-forming and quenched massive galaxies, modeling the vast CR transport parameter space currently allowed by observations is computationally restrictive to survey. Analytic treatments of CR feedback have provided useful insights to potential ramifications in different regimes but have relied on time-steady assumptions which may not well characterize CR effects at different cosmic epochs and galaxy mass scales. We present semianalytic approximations and numerical solutions describing the time-dependent evolution of CR pressure in the CGM under simplified assumptions, which allow for quick evaluation of the vast allowable CR transport parameter space. We demonstrate that time-dependent injection from bursty star formation and/or episodic black hole accretion can substantially alter CR pressure profiles, particularly in the outer halos of massive galaxies ($\gtrsim R_{\mathrm{vir}}$). Finally, we benchmark the approximate solutions from our semianalytic formalism against a cosmic ray-magnetohydrodynamic (CR-MHD) cosmological zoom-in galaxy simulation directly modeling the CR scattering rate and emergent transport in full generality, highlighting the validity of our approach. We conclude by motivating careful consideration of time-dependent “softening” effects in subgrid routines for CR feedback, particularly for use in large cosmological volumes.

This work presents a new numerical code for solving the time-dependent Gross-Pitaevskii-Poisson (GPP) system using adaptive mesh refinement (AMR). The code is designed to study the nonlinear dynamics of self-gravitating bosonic matter in three spatial dimensions under periodic boundary conditions. It combines high-order spatial discretization, explicit time integration, and dynamic refinement driven by the magnitude of the gravitational potential. The implementation is validated through a set of test problems in the nonlinear regime. These benchmarks demonstrate that the solver accurately preserves global conservation laws, resolves strong wave interference and phase singularities, and maintains consistency across refinement levels in highly dynamical scenarios.

We investigate the feasibility of detecting extensive air showers via their geo-synchrotron x-ray emission from high-altitude platforms. Starting from first principles, we derive a differential expression for the number of emitted photons per unit grammage and photon energy for an ensemble of gyrating shower electrons. The calculation uses noted parametrizations of the electron state variable distributions in the shower to establish a scale for the photon footprint and, further, takes into account the propagation of emitted photons in the atmosphere. The computed fluxes at the position of the detector are used to estimate the detector acceptance and event rate using a bootstrap Monte-Carlo procedure. For a 1 m-radius, 70° half-aperture circular detector at an altitude between 20 km to 30 km viewing the Earth’s limb, we find acceptances at the $\mathcal{O}(1{\mathrm{m}}^2\mathrm{sr})$ level and integral event rates of $\mathcal{O}(10)$ per month. These results indicate that x-ray geo-synchrotron emission is a promising, complimentary channel for high-altitude indirect cosmic ray detection in the PeV regime.

We introduce a universal evolution equation for elastic scattering of hadrons, derived from Reggeon field theory and solved in closed analytical form. The equation has a complex logistic structure and evolves initial amplitude profiles from existing models at a fixed energy, reproducing both differential cross sections and integrated quantities over a broad energy range. It admits a unique solution for each initial condition and rigorously satisfies unitarity, the Froissart–Martin bound, and dispersion relations. The dynamics are governed by two physically meaningful parameters: the effective Pomeron mass ${\epsilon }_{\mathcal{P}}$ and the nonlinear coupling $\lambda$, both fitted at a single energy. By decoupling the nonperturbative input from the universal energy evolution, the framework enables model-independent extrapolations and provides a minimal predictive alternative to eikonal resummation. Moreover, the structure of the equation—featuring rapidity evolution, saturation, and impact-parameter dependence—shares qualitative features with nonlinear QCD equations at small-$x$, such as Balitsky-Kovchegov and Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, and Kovner, suggesting a possible bridge between Regge-based and QCD-based approaches to high-energy scattering.

We introduce a new method that exploits the combination of the heat kernel (HK) and background field method to compute gauge-invariant and gauge parameter-independent quantities such as the effective potential, anomalous dimensions, and renormalization group equations. In contrast to currently employed techniques, these results are obtained exclusively from the dynamics of the background fields, without relying on supplementary input from, e.g., traditional diagrammatic calculations. This is achieved by a consistent treatment of open and closed derivatives in the HK expansions. In this way, we compute the standard quantities such as $\beta$ functions and their gauge-parameter independence when background fields are on-shell. We demonstrate this formalism for instructive examples such as scalar QED and Yukawa theory. Full results for the bosonic part of the Standard Model provide further validation of our approach.

Left-right (LR) symmetric theories predict right-handed charged currents whose flavor structure encodes the realization of parity. While the right-handed quark mixing matrix closely tracks its left-handed counterpart, the leptonic sector with purely Dirac neutrinos has remained structurally unclear. We show that, in contrast to the quark case, parity in the Dirac leptonic sector admits a localized, branch-dependent enhancement band in which right-handed–left-handed (RH-LH) misalignment becomes parametrically large despite small parity breaking. We derive analytic solutions of the LR consistency equation and demonstrate that the interplay between spontaneous parity violation and spectral near degeneracies leads to a qualitatively new pattern of right-handed mixing. This establishes the Dirac leptonic sector of the minimal LR model as a predictive and structurally distinct regime.

We compute the electromagnetic form factors of the proton and neutron using lattice QCD. We employ ${\mathrm{N}}_{\mathrm{f}}=2+1+1$ twisted mass clover-improved fermions with quark masses tuned to their physical values. Three ensembles with lattice spacings of $a=0.080\mathrm{fm}$, 0.068 fm, and 0.057 fm and approximately the same physical volume allow us to obtain the continuum limit directly at the physical pion mass. For each ensemble, we use several values of the sink-source time separation, ranging from 0.5 to 1.5 fm, to allow for a thorough analysis of excited-state effects via multistate fits. The disconnected contributions are also analyzed using high statistics combined with techniques to mitigate stochastic noise in the estimation of the fermion loop. These techniques include low-mode deflation, dilution in the color and spin components, and hierarchical probing. We study the momentum-transfer dependence of the form factors using the $z$-expansion and dipole Ansätze, thereby enabling the extraction of the electric and magnetic radii and the magnetic moments, as well as the Zemach and Friar radii in the continuum limit. Results for the proton and neutron electric and magnetic mean square radii are $\sqrt{⟨r_E^2{⟩}^p}=0.860(38)(23)\mathrm{fm}$, $⟨r_E^2{⟩}^n=-0.147(48){\mathrm{fm}}^2$, $\sqrt{⟨r_M^2{⟩}^p}=0.870(53)(15)\mathrm{fm}$, and $\sqrt{⟨r_M^2{⟩}^n}=0.913(67)(19)\mathrm{fm}$, and for the proton and neutron magnetic moments, ${\mu }^p=2.849(92)(52)$ and ${\mu }^n=-1.819(76)(29)$, respectively. In all cases, the first error is statistical, and the second is systematic, where the latter includes an estimate of the error from the fits to the momentum dependence of the form factors and from the continuum extrapolation.

We extend our dispersive analyses on meson static properties to those of light baryons. The formalism treats the dispersion relation, which a baryonic correlation function obeys, as an inverse problem, solves for the involved spectral density with available operator-product-expansion (OPE) inputs directly, and extracts baryon static properties from the spectral density. We observe that the simultaneous implementation of the chiral-even and chiral-odd dispersive constraints unambiguously determines baryon masses and pole residues. A common set of quark and gluon condensates, which appear in OPE factorization and are universal, is found to accommodate the masses of a $\rho$ meson, a proton, and a $\mathrm{\Delta }(1232)$ baryon. The advantage of our approach over the conventional handling of QCD sum rules is advocated. This work encourages broad applications of our nonperturbative analytical method to baryon systems.

We investigate energy-momentum tensor (EMT) form factors and the spatial spin density distribution in the nucleon within a framework of the quantized Skyrme model with vector mesons. We construct both the canonical and Belinfante improved EMTs and analyze how pseudogauge uncertainty influences local spin and momentum densities while leaving the global nucleon properties unchanged. Using the inversion formulas from nucleon matrix elements in the forward limit, we extract the form factors, $A(t)$, $D(t)$, and $J(t)$, in both pseudogauges and the additional antisymmetric form factor, $\mathcal{G}(t)$, associated with the canonical EMT. We find that the pseudogauge choice leads to sizable differences in the local spin and momentum densities. In particular, the canonical EMT naturally encodes spin density through the antisymmetric tensor structure, while the Belinfante EMT is sensitive to the total angular momentum only. Our results illustrate explicitly how different pseudogauges correspond to different spatial interpretations of nucleon spin structure within the same underlying dynamics. These findings provide a concrete model realization of the pseudogauge ambiguity in QCD-inspired nucleon structure and offer useful intuition for interpreting spatial distributions.

Characterizing low-energy, keV-range nuclear recoils near the detector threshold is one of the major challenges for large direct dark matter detectors. To that end, we have successfully used an Yttrium-Beryllium photoneutron source that emits 152 keV neutrons for the calibration of the light and charge yields of the XENONnT experiment for the first time. After data selection, we accumulated 474 events from 183 hours of exposure with this source. The expected background was $55\pm 12$ accidental coincidence events, estimated using a dedicated 152 hour background calibration run with a Yttrium-PVC gamma-only source and data-driven modeling. From these calibrations, we extracted the light (charge) yield for liquid xenon at our field strength of $23\mathrm{V}/\mathrm{cm}$ between 0.3 (0.7) ${\mathrm{keV}}_{\mathrm{NR}}$ and $5.0{\mathrm{keV}}_{\mathrm{NR}}$. This calibration is crucial for accurately measuring the solar ${}^8\mathrm{B}$ neutrino coherent elastic neutrino-nucleus scattering and searching for light dark matter particles with masses below $12\mathrm{GeV}/{\mathrm{c}}^2$.

We report a search for highly ionizing magnetic monopoles in the cosmic-ray flux using a 2713-day dataset collected during 2015–2025 with the NOvA Far Detector, a 14-kt segmented detector located on Earth’s surface in Minnesota, United States. The search is sensitive to monopoles across a wide range of speeds, $7\times {10}^{-4}<\beta <0.995$, and is sensitive to masses as low as $2\times {10}^5\mathrm{GeV}$ for the fastest monopoles. No signal was observed. With the detector’s large surface area and minimal overburden, we achieve the strongest flux limits reported to date in several regions of speed and mass. For heavy monopoles with masses above ${10}^{13}\mathrm{GeV}$ that are able to reach the detector from above or—crossing Earth—from below, we find a flux limit ${\varphi }_{90\%}<2\times {10}^{-16}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ (90% confidence level) for monopoles with $0.005<\beta <0.8$. Across the same range of speeds, we report a limit ${\varphi }_{90\%}<8\times {10}^{-16}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ for light monopoles with masses above ${10}^8\mathrm{GeV}$ that can reach the detector from above.

We present a new analytic solution to the Einstein-Dirac equations formulated by Finster et al. [Phys. Rev. D 59, 104020 (1999)] to describe the stationary states of a pair of gravitationally interacting neutral fermions. The fermions’ wave function in our analytic solution, as in their numerical ones, is both exponentially localized and normalizable. However, our solution differs from theirs in two key respects: it features a naked spacetime singularity at the origin, and the gravitational (Arnowitt-Deser-Misner) mass of the localized object is zero, making it gravitationally undetectable to an external observer. This is despite the arbitrarily large mass of the constituent fermions. This unexpected result may have significant implications for astronomy and cosmology, as it gives a mechanism by which mass could become “hidden” during the Universe’s evolution.

We perform canonical quantization of general relativity, as an effective quantum field theory below the Planck scale, within the Becchi-Rouet-Stora-Tyutin (BRST)-invariant framework. We show that the promotion of constraints to dynamical equations of motion for auxiliary fields leads to the healthy Hamiltonian flow. In particular, we show that the classical properties of Einstein’s gravity, such as vanishing Hamiltonian modulo boundary contribution, are realized merely as an expectation value in appropriate physical states. Most importantly, the physicality is shown not to entail trivial time evolution for correlation functions. In the present approach, we quantize the theory once and for all around the Minkowski vacuum and treat other would-be classical backgrounds as BRST-invariant coherent states. This is especially important for cosmological spacetimes as it uncovers features that are not visible in ordinary semiclassical treatment. The Poincaré invariance of the vacuum, essential for our quantization, provides strong motivation for spontaneously broken supersymmetry.

We investigate the effect of higher-order curvature terms, specifically Gauss-Bonnet terms, on spacetime singularities in five dimensions. For Friedmann-Lemaître-Robertson-Walker cosmologies, we demonstrate that Gauss-Bonnet terms can replace the big bang/crunch with a “sudden” singularity characterized by a finite scale factor and Hubble rate but diverging higher-order derivatives. Investigating various branches of solutions shows the possibility of explicit extension of nonspacelike geodesics beyond the singular point. Furthermore, we employ the Gauss-Bonnet junction conditions to verify the consistency of the extension with the field equations. The whole solution describes a contracting phase prior to the expansion phase with a well-defined surface stress-energy tensor. Regarding the Boulware-Deser black hole, we find that Gauss-Bonnet terms soften the central singularity for radial geodesics—rendering them “weak” according to the Tipler and Królak criteria—whereas nonradial geodesics remain strongly singular. A junction condition analysis of this solution shows that although higher-curvature corrections alter the nature of the singularity, geodesics are still inextendible as a result of divergent extrinsic curvature. Our results are consistent with the Penrose-Hawking singularity theorems since in Gauss-Bonnet black holes, geodesics suffer from focusing (expansion parameter diverges), while in cosmology, there is no focusing since the expansion parameter remains finite at the singularity.

Balance flux laws of asymptotic symmetries in general relativity provide fully nonperturbative constraint equations on gravitational strain. They have proven useful for constructing numerical gravitational waveforms and for characterizing gravitational memory. As the precision of current and future detectors continues to improve, such constraints become increasingly important for high-precision tests of gravity, including searches for deviations from general relativity. This motivates a systematic understanding of analogous balance laws in theories beyond general relativity. In this work, we investigate the existence and structure of flux laws at null infinity in diffeomorphism-invariant extensions of general relativity. Our analysis is based on the covariant phase space formalism and the definition of conserved quantities, as presented by Wald and Zoupas [Phys. Rev. D 61, 084027 (2000)]. For a particularly relevant class of Horndeski theories, we derive a general expression for the flux and formulate the corresponding balance equation via the associated nonconserved charges. We cross-check our general results by comparing with previous studies of Brans–Dicke gravity. Furthermore, we demonstrate that the employed methods extend straightforwardly to scalar-Gauss–Bonnet gravity and provide a conjecture of the flux balance laws for full massless Horndeski theory. The null part of the resulting flux laws associated with null memory is compared with and validated against the alternative derivation based on the Isaacson approach to gravitational radiation. Beyond the specific results obtained, this work is intended to serve as a practical guide for computing balance laws in generic diffeomorphism-invariant theories of gravity and paves the way for an in-depth comparison between the Isaacson approach and the covariant phase space formalism.

We study the interaction of a stress-energy tensor describing a static point mass supported by a string outside a Schwarzschild black hole with the gravitons of the Hawking radiation. We derive a closed-form analytic expression for the total response rate of this stress-energy tensor to the thermal gravitons in the Unruh state, which models the quantum state in the spacetime of a spherically symmetric black hole formed by gravitational collapse. This response rate is finite in contrast with the infrared divergent response rate for a static point mass supported by a string in Rindler spacetime, i.e., a point mass accelerated uniformly by a string in Minkowski spacetime. By comparing the response rate near the black hole horizon with that in Rindler spacetime, we show that the size of the black hole acts as a natural infrared cutoff. We also find that the response rate of this stress-energy tensor to the thermal gravitons incoming from past null infinity in the Hartle-Hawking state vanishes. As a result, the total response rates of a static point mass (supported by a string) in the Unruh and Hartle-Hawking states for gravitons are identical. This is also the case for a static charge interacting with the electromagnetic field but not for a static source for a massless scalar field.