We investigate the scalar sector of linear cosmological perturbations in quadratic gravity. Working in the Einstein frame, we derive the equations of motion in a gauge-independent manner and express them in terms of three sets of gauge-invariant variables. This approach allows us to distinguish genuine physical effects from gauge artifacts, which is particularly relevant for assessing the stability of perturbations in this theory. In the superhorizon limit, we obtain the leading-order behavior of the relevant gauge-invariant variables and analyze the perturbations in commonly used gauges. We find that the Newtonian gauge exhibits an apparent instability, characterized by the exponential growth of the metric perturbations. However, this growth is nongeneric and gauge dependent; in the other gauges analyzed in this work, the perturbations remain well-behaved within the perturbative regime. Our analysis also demonstrates how the evolution behavior of a gauge-invariant variable changes under the frame transformation and clarifies the relation between results obtained in the Jordan and Einstein frames.

Cosmological data provide us two key constraints on dark matter: it must have a particular abundance, and it must have an adiabatic spectrum of density perturbations in the early Universe. Many different cosmological scenarios have been proposed that establish the abundance of axion dark matter in qualitatively different ways. In this paper we emphasize that, despite this variety of backgrounds, the perturbations in axion dark matter can be understood from universal principles. How does a feebly interacting axion field acquire perturbations proportional to those of photons? How do the isocurvature power spectrum and non-Gaussianity depend on the background evolution of the Universe? We answer these questions for a completely general choice of cosmological background and temperature-dependent axion potential. We show that the most general solution to the axion field equation on superhorizon scales is entirely determined by the family of background solutions for different initial field values ${\theta }_{\mathrm{ini}}$. This holds for both the component in the field perturbation solution contributing to the dark matter isocurvature perturbation (enhanced at late times by the sensitivity of the dark matter abundance to the initial condition, $\partial {\mathrm{\Omega }}_a/\partial {\theta }_{\mathrm{ini}}$, which can be large for initial conditions near the hilltop), and the other component that contributes to the dark matter curvature perturbation. In particular, we explain that an unperturbed axion field in the early Universe evolving into one with nontrivial adiabatic perturbations is guaranteed by Weinberg’s theorem on adiabatic modes. These results have been derived before with various assumptions, such as a radiation-dominated background or a quadratic potential. Our aim is to give a clear, simple derivation that is manifestly independent of those assumptions, and thus can be applied to any cosmological axion scenario.

We study the neutrino distributions that arise in a simulation of a neutron star merger that uses a Monte Carlo (MC) neutrino transport scheme. In a snapshot taken 1 ms after merger, we calculate relevant observables to test when neutrinos behave like a thermalized gas and when a free-streaming picture is more appropriate. We find that in hot, dense regions where neutrino-matter interactions are frequent, MC neutrino and antineutrino distributions are consistent with thermalized neutrinos. In moderately warm regions, where neither approximation is expected to hold, we find significant departures from the predictions of the thermalized-neutrino approximation, particularly for the (anti)neutrino average opacity and net rate of absorption per baryon, even when average energies appear approximately thermal. At lower temperatures, MC results approach the free-streaming limit. Our results demonstrate that energy-averaged agreement with thermalized-neutrino assumptions does not guarantee accurate weak interaction rates. Nonequilibrium aspects of the neutrino distribution are therefore crucial for neutrino-mediated microphysics such as composition evolution in the early postmerger phase.

Recent measurements of the compact star XTE J1814-338, with a mass of $M={1.2}_{-0.05}^{+0.05}{M}_{\odot }$ and a radius of $R=7_{-0.4}^{+0.4}\mathrm{km}$, alongside those of HESS J1731-347, which has a mass of $M=0.77_{-0.17}^{+0.20}{M}_{\odot }$ and a radius of $R={10.4}_{-0.78}^{+0.86}\mathrm{km}$, provide indicative evidence for the potential existence of exotic matter in neutron star cores. These observations offer important insights into the equation of state of dense nuclear matter. Recently, Fornal and Grinstein, in order to overcome the discrepancy between the neutron lifetime measured in beam and bottle experiments, proposed the existence of neutron dark decay. In the present work, an effort is made to connect the interpretation of the above exotic compact objects with the possible existence of dark particles, assumed to be products of neutron dark decay. Our hypothesis offers an advantage over comparable proposals, as the coexistence of dark matter and hadronic matter within neutron stars emerges from an intrinsic mechanism, thereby obviating the need to invoke external merger-related processes. It is still unclear to what extent the proposed dark decay of the neutron is affected by the extreme environment within neutron stars. Within this framework, we examined the case in which a mechanism suppressing the dark neutron decay becomes operative at densities few times above nuclear saturation density. We found that the proposed alternative explanation accommodates the simultaneous existence of neutron dark decay while consistently predicting both the two-solar-mass limit and the presence of compact objects extending into the low-mass and subsolar-mass regime.

The interior crust and much of the liquid core of neutron stars is believed to be a quantum liquid mixture of neutron and proton superfluids and a relativistic electron liquid. Quantized vortices in the neutron superfluid and quantized flux lines in the proton superconductor are topological defects of these hadronic condensates. I consider the formation of the superfluid state in young neutron stars under nonequilibrium conditions imposed by the neutrino cooling rate. The nonequilibrium phase transition implies that the onset of superfluidity is accompanied by the generation of quantized vortices based on the mechanism envisioned by Kibble in the context cosmic string formation in evolutionary models of an expanding universe, and further developed by Zurek for nonequilibrium phase transitions in quantum liquids such as ${}^4\mathrm{He}$. I discuss the Kibble-Zurek mechanism (KZM) and scaling relations for topological defect formation starting from the Cooper pair fluctuation propagator for temperatures approaching $T_c$. I then calculate the predicted vortex densities based on Urca and modified Urca cooling mechanisms in the cores of neutron stars for several models of the superfluid gap and transition temperature of the interior neutron superfluid. In all cases studied the KZM leads to a large density of topological defects in the condensate phase, which in 3D form a random network of vortex lines and loops, i.e. the generation of quantum turbulence.

Maximizing premerger warning times in gravitational-wave searches requires minimizing algorithmic latency. While current pipelines typically rely on truncated linear-phase filters, minimum-phase whitening offers a zero-latency alternative that eliminates the acausal look-ahead buffer. However, this causal approach exposes the analysis to “spectral drift,” where the whitening operator applied to live data diverges from the static template bank, creating a functional perturbation of the matched-filter metric. We develop a perturbative framework generalizing the Cutler-Vallisneri formalism to address these metric errors, deriving analytic expressions for the resulting timing, phase, and SNR biases. Validated against exact stationary-phase models and numerical injections, the corrections achieve $<1\%$ fractional error at typical catalog drift amplitudes ($\epsilon \sim 0.1$), remaining controlled out to $\epsilon \lesssim 0.5$. Applied to GWTC-4.0 events under realistic 1-wk power spectral density lags, the framework reveals uncorrected timing biases exceeding $200\mathrm{\mu s}$, phase shifts up to 0.2 rad, sky-localization centroid shifts of 5°–10° in a timing-only triangulation model and a median SNR loss of 3%–5% ($\sim 15\%$ reduction in detection volume). These results demonstrate that while minimum-phase whitening maximizes the early-warning window, analytic drift corrections are essential to maintain detection sensitivity in future observing runs.

We investigate whether retarded scalar propagators on causal sets can be expressed in terms of the link matrix $\mathbf{L}$. For Poisson sprinklings into $1+1$ dimensional Minkowski spacetime, we show by asymptotic analysis and supporting numerical simulations that the averaged massless retarded propagator is naturally associated with a normalized exponential $\exp (\mathbf{L})$. We then extend the construction to the massive case via the usual mass-scattering series and obtain good agreement with the continuum propagator after averaging. Finally, we discuss the inverse kernel $\exp (-\mathbf{L})$ as a possible candidate for a discrete d’Alembertian.

We propose a quantum-enhanced sensing scheme for the detection of wavelike dark matter and high-frequency gravitational waves using two-dimensional ion crystals in a Penning trap. The protocol employs spin-motion squeezed states to improve the signal-to-noise ratio and enable a super-Heisenberg scaling with respect to the number of ions over a broad parameter range. We analyze the sensitivity of the protocol to representative wavelike dark matter candidates, including the axionlike particle and the dark photon, as well as to high-frequency gravitational waves, taking into account the decoherence and dephasing of the ion spins. Our results indicate that two-dimensional ion crystals and this new protocol provide a promising platform for probing previously unexplored parameter space in searches for light dark matter and high-frequency gravitational waves.

Nucleon-nucleon systems in the ${}^3{S}_1$ and the ${}^1{S}_0$ channels are studied in lattice quantum chromodynamics at a pion mass of approximately $m_{\pi }\approx 292\mathrm{MeV}$, employing three $N_f=2+1$ ensembles with the same pion mass and lattice spacing $a=0.10530(18)\mathrm{fm}$ but different spatial volumes. Finite-volume energies of the nucleon-nucleon systems are determined in both the rest frame and a moving frame. The distillation quark smearing method is applied to improve the precision and to ensure the symmetric correlators by using the same interpolating operators at sink and source. The scattering amplitudes are extracted from the finite-volume spectra using the Lüscher’s finite-volume method. At the studied pion mass, both the ${}^3{S}_1$ (deuteron) and ${}^1{S}_0$ (dineutron) channels exhibit a virtual state pole, with binding energies of $6_{-3}^{+5}\mathrm{MeV}$ and ${11}_{-5}^{+6}\mathrm{MeV}$, respectively. To investigate the effects of the left-hand cut, an alternative method—the nonperturbative Hamiltonian framework—is used for the scattering analysis and yields consistent results with those from the Lüscher method.

We derive the factorization theorem for the quasi–transverse-momentum-dependent (quasi-TMD) correlator, including kinematic power corrections to all orders. The resulting expression involves only twist-two TMD distributions and is frame invariant. As in the leading-power approximation, the reduced soft factor factorizes multiplicatively; however, in contrast, the TMD evolution factor is not multiplicative but enters as a convolution with the nonperturbative TMD distribution. We present a numerical estimate of the difference between the derived expression and the leading-power approximation, finding it to be of the order of tens of percentages for current lattice simulations. We also demonstrate that the inclusion of these corrections improves the agreement between phenomenological extractions of the Collins-Soper kernel and lattice results.

We investigate the extent of the critical scaling region of the chiral phase transition at finite chemical potential within the quark-meson model using the functional renormalization group approach. By analyzing the scaling behavior of the chiral order parameter and correlation length with respect to temperature and pion mass near the second-order phase transition, we extract critical exponents from the data and quantify the range over which the scaling relations remain valid. We find that both the leading-order and the next-to-leading-order scaling regions systematically shrink as the chemical potential increases. This behavior is observed in both the local potential approximation (LPA) and its extension including anomalous dimensions (${\mathrm{LPA}}^{\prime }$), with qualitatively consistent results, while the scaling region in ${\mathrm{LPA}}^{\prime }$ is slightly smaller than that in LPA.

Experimental searches for fully heavy flavored tetraquark states are actively pursued at the LHC. The LHCb, CMS, and ATLAS collaborations have investigated fully charmed $cc\overline{c}\overline{c}$, doubly charmed-bottom $bc\overline{b}\overline{c}$ and fully bottom $bb\overline{b}\overline{b}$ tetraquarks. Fully charmed states are probed as intermediate resonances in processes like $p+p\to J/\psi J/\psi$ and $p+p\to J/\psi {\mu }^{+}{\mu }^{-}$ at $\sqrt{s}=7,8$, and 13 TeV. Notably, narrow structures such as $X(6200)$, $X(6900)$, and $X(7300)$ observed in the di-$J/\psi$ mass spectrum have ignited theoretical interest. In this study, we use a nonrelativistic model to perform a detailed study of the mass spectra for the ground ($1S$) and excited ($1P$, $2S$, $1D$, $2P$, $3S$, and $4S$) states of fully heavy tetraquarks $Q_1{Q}_2{\overline{Q}}_1{\overline{Q}}_2$ $(Q_{1,2}\in \{b,c\})$ in the diquark-antiquark picture. Tetraquarks are modeled as color-singlet bound states build from an axial-vector diquark (${[Q_1{Q}_2]}_{s=1}$) and axial-vector antidiquark (${[{\overline{Q}}_1{\overline{Q}}_2]}_{s=1}$) in the $\overline{\mathbf{3}}$ and $\mathbf{3}$ color representations, respectively, bound together by color forces. With this setup, the Schrödinger equation is solved numerically for a modified version of the Cornell potential, using the three-point difference central method. The spin-dependent terms are dealt with nonperturbatively and are used to describe the splitting structure of the diquark. The tetraquark states are investigated for a wide range of $J^{PC}$ quantum numbers including $0^{++}$, $1^{+-}$, $2^{++}$ for $S$-waves, $0^{-+}$, $1^{-\pm }$, $2^{-\pm }$, $3^{--}$ for $P$-waves, and $0^{++}$, $1^{+\pm }$, $2^{+\pm }$, $3^{+\pm }$, $4^{++}$ for $D$-waves. Following the same line as many previous works, we attempt, through this study of tetraquark mass spectra, to provide their possible signatures in narrow structures recently discovered in the di-$J/\psi$ production spectrum by the LHCb, CMS, and ATLAS collaborations. Our results provide precise mass predictions, analyze stability against strong fall-apart decays, and are directly compared with recent LHC observations and other relevant theoretical models. Our findings supports the interpretation of these discovered exotic heavy resonances as the different excitations of the fully charmed tetraquark, offering a crucial guide for ongoing experimental identification.

We present a new measurement of the electron neutrino charged current cross section on argon without pions in the final state. This measurement uses the full MicroBooNE booster neutrino beam dataset of $1.3\times {10}^{21}$ protons on target collected at Fermi National Accelerator Laboratory. Events are considered both with and without protons above the kinetic energy visibility threshold. Differential cross sections are extracted in proton and electron kinematics, including energy and angle relative to the neutrino beam direction. The relationship between the hadronic and leptonic systems is explored through the angle between the proton and electron directions. The resulting cross sections are compared to a variety of available generator predictions using different models of neutrino interactions. We find good agreement with most models in lepton kinematics and some discrepancies in the hadronic system modeling, particularly in proton angle.

The James Webb Space Telescope (JWST) revealed a large population of active galactic nuclei (AGN) with redshifts greater than five. We show that if they emit ultrahigh-energy protons with energies up to $\lesssim {10}^{19}\mathrm{eV}$, the cosmogenic neutrino production in the high-redshift CMB field yields a neutrino flux with a bump at around 50 PeV. This flux is potentially consistent with the current neutrino intensity estimate by the IceCube Neutrino Observatory within the parameter space allowed from the AGN luminosity and number density estimates by JWST. Future neutrino observations that confirm the 50-PeV bump and constrain the small-scale anisotropy will infer ultrahigh-energy cosmic-ray emissions in the early Universe.

Baryon acoustic oscillation (BAO) analyses usually report the anisotropic shift parameters ${\alpha }_{\perp }(z)$ and ${\alpha }_{\parallel }(z)$ relative to a fiducial cosmology, and these quantities are primarily used for cosmological parameter inference. Here we show that they can also be used to construct a direct internal consistency test of the background geometry. In particular, we derive a new null test of flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry written entirely in terms of the reported BAO shift parameters. The test is calibration free: the sound-horizon ratio $r_{\mathrm{d}}/r_{\mathrm{d}}^{\mathrm{fid}}$ cancels identically, so the relation is independent of the absolute BAO scale, and is also fiducial invariant, preserved under exact remapping between any two flat fiducial cosmologies, so the result is not an artifact of any particular survey choice. We also derive a calibration-free reconstruction of the deceleration parameter $q(z)$ from the radial BAO sector. Applying these results to anisotropic DESI DR2 BAO measurements, we find no evidence for a breakdown of the flat-FLRW distance relation within current uncertainties. Our results show that anisotropic BAO measurements already provide a nontrivial internal geometric consistency test before performing any model fit.

We generalize the equivalence between the off-equilibrium state and gravitational perturbation of equilibrium state from dynamics of macroscopic quantities to that of microscopic particles. The equivalence is achieved by mapping kinetic theories of spinless and spinning particles in curved space to flat space through suitable choice of inertial frame that eliminates geodesic forces on particles. The equivalence has been shown for hydrodynamic and elastic regimes. An exceptional case is the state with fluid vorticity, which requires torsion to simulate. We develop quantum kinetic theory in torsionful space and find that spin-torsion coupling cannot be eliminated but can be interpreted as spin coupling to axial gauge field. It leads to an axial torsion effect.

We analyze the emergence of nonlocal quantum correlations in Schwinger pair creation in strong non-Abelian (chromo)electric fields using holography. The produced quark-antiquark pair is entangled into a color singlet, yet accelerates into causally disconnected Rindler wedges. Using the Casini-Huerta-Myers conformal mapping and the probe-brane framework, we compute the refined Rényi entropy and its derivative, which captures the antiflatness of the entanglement spectrum for a spherical bipartition. We find that for boundary spacetime dimension $d>2$, the entanglement spectrum is nonflat, implying the dynamical generation of nonlocal nonstabilizerness in the pair creation process. Interestingly, the generation of nonlocal nonstabilizerness in the holographic dual is encoded in the free energy of the probe action.

The Kosower-Maybee-O’Connell (KMOC) formalism provides a systematic framework for extracting classical observables perturbatively from on-shell scattering amplitudes. In this work, we use this formalism to analyze electromagnetic infrared observables in four dimensions, focusing in particular on the linear memory and its tail contributions. Building on previous analysis of such observables using soft-photon theorems, we study the constraints implied by the logarithmic soft photon theorem on the corresponding soft radiation kernel. We show that demanding the expected nonperturbative properties of memory and tail effects imposes a nontrivial set of consistency conditions on the underlying S-matrix. We interpret these constraints as necessary consistency conditions, motivated by macroscopic causality, on the classical limit of the S-matrix as probed by inclusive observables.

Quantum field theory (QFT) on fractal spacetimes is a program aiming at quantizing the gravitational interaction consistently at all energy scales thanks to an intrinsically or dynamically induced multiscale or multifractal-like spacetime geometry that regularizes the infinities of standard QFT. We reach the goal of this program and formulate a field theory of quantum gravity which is shown to be super-renormalizable and unitary at all perturbative orders. Viable and unviable ways to test this proposal through black holes and gravitational waves are discussed.