Astrometric measurements provide a unique avenue for constraining the stochastic gravitational wave background (SGWB). In this work, we investigate the application of two neural network architectures, a fully connected network and a graph neural network, for analyzing astrometric data to detect the SGWB. Specifically, we generate mock Gaia astrometric measurements of the proper motions of sources and train two networks to predict the energy density of the SGWB, ${\mathrm{\Omega }}_{\mathrm{GW}}$. We evaluate the performance of both models under varying input datasets to assess their robustness across different configurations. We also perform a direct comparison with a likelihood-based approach using Markov chain Monte Carlo (MCMC) methods, finding out that the neural-network-based approach is significantly faster, taking on the order of minutes, compared to MCMC’s order of days, while still capturing the same features in the data. Our results demonstrate that neural networks can effectively constrain the SGWB, showing promise as tools for addressing systematic uncertainties and modeling limitations that pose challenges for traditional likelihood-based methods.

Big bang nucleosynthesis (BBN), as one of the earliest processes in the Universe accessible to direct observation, offers a powerful and independent probe of the cosmic expansion history. With recent advances in both theory and observation, including efficient and flexible BBN codes, percent-level measurements of primordial deuterium and helium-4 abundances, refined measurements of nuclear reaction rates, and precise determinations of the baryon density from the cosmic microwave background, particularly keen insights can be gained from BBN. In this work, we leverage these developments to place model-independent constraints on deviations from the Standard Model expansion history during BBN. Using the latest abundance data, we apply principal component analysis to identify the most constrained and physically meaningful modes of expansion history variation. This approach allows us to impose the most general constraints on early dark energy during the epoch of BBN. We further examine whether general modifications to the expansion rate could alleviate the long-standing lithium problem. Our results demonstrate that BBN, sharpened by modern data and statistical techniques, remains an indispensable probe of dark energy and new physics in the early Universe.

Simulation-based inference (SBI) has emerged as a powerful tool for extracting cosmological information from galaxy surveys deep into the non-linear regime. Despite its great promise, its application is limited by the computational cost of running simulations that can describe the increasingly-large cosmological datasets. Recent work proposed a hybrid SBI framework (HySBI), which combines SBI on small-scales with perturbation theory (PT) on large-scales, allowing information to be extracted from high-resolution observations without large-volume simulations. In this work, we lay out the HySBI framework for galaxy clustering, a key step toward its application to next-generation datasets. We study the choice of priors on the parameters for modeling galaxies in PT analysis and in simulation-based analyses, as well as investigate their cosmology dependence. By jointly modeling large- and small-scale statistics and their associated nuisance parameters, we show that HySBI can obtain 20% and 60% tighter constraints on ${\mathrm{\Omega }}_m$ and ${\sigma }_8$, respectively, compared to traditional PT analyses, thus demonstrating the efficacy of this approach to maximally extract information from upcoming spectroscopic datasets.

Recent observations of the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) show some tension with a $\mathrm{\Lambda }\mathrm{CDM}$ cosmology. For one, the cosmological parameters determined by the CMB are at odds with the expansion history determined by latest BAO measurements. In addition, the combined data has placed uncomfortably strong constraints on neutrino mass. Both effects can be interpreted as negative neutrino mass, one describing the change to the expansion history and the other one describing enhanced lensing. In this paper, we show the current tensions can be solved with a single change either to the lensing of the CMB or the expansion of the universe. We show additional lensing could arise from a variety of models with new light fields. However, these models rarely give the same signal in temperature and polarization, giving a concrete test of the scenario. Alternatively, dark sector models can explain the changes to the expansion by changing the evolution of the matter density. These models introduce new forces, giving rise to long-range signals in the three-point statistics of galaxies. We discuss a range of other examples which all illustrate the pattern that additional signals should appear if these tensions are explained by beyond the Standard Model physics.

We study the cosmology of a modified Majoron model motivated by the need to protect a global $U(1)$ symmetry from gravity-induced hard explicit breaking (by $d\le 4$ operators) at the Planck scale. The model extends the standard model by introducing a gauged $U(1{)}_{B-L}$ and an approximate global $U(1)$ symmetry, each spontaneously broken by a corresponding complex scalar singlet. This setup gives rise to a network of effectively global and local cosmic strings, whose stochastic gravitational wave signals can jointly account for the spectrum observed by NANOGrav collaboration, particularly for Majoron masses $m_{\chi }<{10}^{-23}\mathrm{eV}$. Although the fit is not as strong as that from supermassive black hole mergers, the model still provides an alternative explanation rooted in high-energy physics. The Majoron mass induces an infrared cutoff in the gravitational wave spectrum, and we examine how this effect shapes the fit to the NANOGrav data. The model also generates light neutrino masses via the seesaw mechanism and avoids cosmological constraints from $\mathrm{\Delta }{N}_{\mathrm{eff}}$, cosmic microwave background (CMB) anisotropies, and isocurvature fluctuations. Although the Majoron can contribute to dark matter through thermal, coherent oscillation, and string-induced production mechanisms, its relic abundance remains subdominant in the NANOGrav-compatible region. In contrast, the measured dark matter relic density is achievable at higher $m_{\chi }$, though at the cost of tension with cosmological bounds. If the NANOGrav fits are viewed as constraints, given their comparatively lower Bayes factors, they yield bounds that are significantly stronger than those imposed by the CMB and other cosmological data.

The evolution of old, compact stellar structures provides strong constraints on macroscopic dark matter candidates, such as primordial black holes (PBHs). In view of recent observational data for the Eridanus II dwarf galaxy, we perform the first $N$-body simulations of its central stellar cluster to model dynamical heating by PBHs. We find evidence that such candidates must be lighter than about one solar mass if they constitute the totality of the dark matter. We additionally derive constraints on the fraction of the dark matter in macroscopic objects as a function of mass, by modeling the remainder of the dark matter as standard, fluidlike cold dark matter.

The standard Hellings-Downs (HD) approach to searching for gravitational wave signatures in pulsar timing array (PTA) data has successfully produced evidence for the presence of nanohertz-wavelength gravitational waves. However, it does not, on its own, produce any directional information and is insensitive to polarization. An alternative approach is to construct maps of the polarized power on the sky. In this paper, we present a simple quadratic estimator of the gravitational wave power as a function of direction on the sky that is sensitive to the polarization state of the wave. We describe the full, $S_2\times S_2$, state space of a polarized gravitational wave background across the sky and the Poincaré sphere describing polarization. A natural question arises from this perspective: what is the resolution of a polarized sky map, i.e., effectively how many independent pixels can such a map contain? In other words, how many distinct gravitational waves can a PTA distinguish? It turns out the answer is finite, and is approximately $N_{\mathrm{res}}=16\times 2=32$ per frequency, where 16 is the number of resolvable sky positions and 2 is the number of distinct polarization states, corresponding to an angular resolution of 58°, achieved with a PTA with more than $N_{\text{pulsar}}\gtrsim 20$ pulsars. We demonstrate that the variance of the map is equivalent to the HD significance, while for a single point source, a $3\sigma$ HD signal corresponds to a $5.2\sigma$ map significance.

We extend the modified Nambu-Jona-Lasinio model—incorporating exchange interactions via the Fierz transformation—to finite temperatures in both two- and three-flavor scenarios, and investigate the properties of protoquark stars in $\beta$ equilibrium. Our results show that increasing the strength of exchange interactions, characterized by the parameter $\alpha$, changes the chiral phase transition from first order to crossover. We examine the effects of finite temperature, lepton fraction, and exchange interactions on the equation of state (EOS). We find that, in the crossover regime, the EOS is significantly stiffer than in the first-order case due to the substantial contribution of repulsive interactions in the exchange channels, while it remains relatively insensitive to variations in temperature and lepton fraction. Imposing the thermodynamic consistency, which requires the minimum of $f/{\rho }_B$ occurs at zero pressure, further constrains the minimum value of vacuum pressure.

We present hydrodynamic simulations with the moving-mesh code arepo of tidal disruption encores (TDEEs) in nuclear star clusters (NSCs). TDEEs arise when a stellar-mass black hole (sBH) disrupts a star within the NSC, producing debris that is unbound from the sBH but remains gravitationally bound to the central massive black hole (MBH), leading to a delayed secondary flare. We find that the morphology and thermodynamics of the fallback material depend sensitively on the disruption geometry, MBH mass, and sBH-MBH separation. We identify two distinct morphological outcomes: ring encores, where debris circularize into a torus, and direct encores, where streams plunge toward the MBH, with encore luminosities peaking at times corresponding to the freefall timescale and one orbital period, respectively. Across all simulated cases, we find these events exhibit luminosities of ${10}^{40}–{10}^{42}\mathrm{erg}/\mathrm{s}$ with lightcurves characteristic of their morphology. Our work greatly improves the predictions of TDEE lightcurves and empowers observations to probe into NSC dynamics and sBH population while providing possible explanations for anomalous TDE-like flares.

The Galactic Center excess (GCE) of GeV $\gamma$ rays may hint at dark matter (DM), yet its origin remains debated. Motivated by this, we survey weakly interacting massive particle (WIMP) models that can fit the GCE while satisfying relic-density, direct-detection (DD), and indirect-detection (ID) bounds. We group candidates into hadronic (Higgs portals; simplified scalar/vector mediators), leptonic [$U(1{)}_{L_i-L_j}$], and mixed [$U(1{)}_{B-L}$, $Z$-portal] classes. Across all cases, present DD and dwarf-spheroidal $\gamma$-ray limits exclude wide regions, leaving mainly narrow resonant funnels with $m_{\mathrm{DM}}\simeq m_{\mathrm{med}}/2$ and portal couplings $\ll 1$. In hadronic setups, scalar and vector Higgs portals survive only in a thin strip near $m_h/2\simeq 62.5\mathrm{GeV}$ with portal couplings $\sim {10}^{-4}$, while the Dirac Higgs and $Z$ portals are essentially excluded. The UV-complete vector Higgs portal retains resonant bands whose viable portal strength depends on the mixing angle. Simplified scalars allow small windows for complex-scalar or vector DM; Dirac DM is strongly disfavored, whereas a pseudoscalar with Dirac DM remains viable over a broader parameter range. For a simplified $Z^{\prime }$ mediator, a pure vector coupling leaves only a marginal region, while pure axial is excluded by DD/ID bounds. In leptonic scenarios, inverse-Compton emission is essential: $L_{\mu }-L_e$ (and, to a lesser extent, $B-L$) fits the GCE with near-thermal cross sections, while $L_{\mu }-L_{\tau }$ is disfavored. Overall, viable WIMP explanations are constrained to the finely tuned resonant regime, with leptophilic vectors and pseudoscalar portals emerging as the most robust options.

High-energy neutrino astronomy has advanced rapidly in recent years, with IceCube, KM3NeT, and Baikal-GVD establishing a diffuse astrophysical flux and pointing to promising source candidates. These achievements mark the transition from first detections to detailed source studies, motivating next-generation detectors with larger volumes, improved angular resolution, and full neutrino-flavor sensitivity. We present a performance study of large underwater neutrino telescopes, taking the proposed TRIDENT array in the South China Sea as a case study, with a focus on comparing the performance of various detector configurations against the TRIDENT baseline design. Both tracklike events primarily from muon neutrinos, which provide precise directional information, and cascade events from all flavors, which offer superior energy resolution, diffuse-source sensitivity, and all-sky flavor coverage, are included to achieve a balanced performance across source types. The time to discover potential astrophysical sources with both tracklike and cascadelike events is used as the figure of merit to compare a variety of detector design choices. Our results show that, for a fixed number of optical modules, simply enlarging the instrumented volume does not inherently lead to improved performance, while taller strings can provide modest gains across all detector channels, within engineering constraints. Distributing dense clusters of strings over a large volume is found to generally worsen discovery potential compared to the baseline layout. Finally, the optical properties of the seawater emerge as the key factor dictating the optimization of detector layout, highlighting the need for in situ measurements and early deployment of optical modules to guide the final array configuration.

The Schrödinger-Poisson (SP) equations predict fuzzy dark matter (FDM) solitons. Given the FDM mass $\sim {10}^{-20}\mathrm{eV}/{\mathrm{c}}^2$, the FDM soliton in the Milky Way is massive $\sim {10}^7{M}_{\odot }$ but diffuse $\sim 10\mathrm{pc}$. Therefore, such FDM soliton can serve as a gravitational lens for gravitational waves (GWs) with frequency $\sim {10}^{-8}\mathrm{Hz}$. In this paper, we investigate its gravitational lensing effects by numerical simulation of the propagation of GWs through it. We find that the maximum magnification factor of GWs is very small $\sim {10}^{-4}$, but the corresponding magnification zone is huge $\sim 6\mathrm{pc}$ for FDM with mass equal to $8\times {10}^{-21}\mathrm{eV}/{\mathrm{c}}^2$. Consequently, this small magnification factor compounding over such large magnification zone results in a small antisotropy of $\sim {10}^{-4}$ over a large solid angle in the GW background. That level of anisotropy is out of the sensitivity, $<20\%$, of the pulsar timing arrays today.

Ultra-high-energy cosmic rays (UHECR) scattering off the cosmic relic neutrino background have recently gained renewed interest in the literature. Current data suggest that (UHECR) are predominantly made of heavy nuclei. Similar to the coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\nu \mathrm{NS}$) observed at low-energy neutrino experiments, the cross section of heavy nucleus scattering off relic neutrinos will be coherently enhanced since the energy of relic neutrinos can reach $\sim \mathcal{O}(10)\mathrm{MeV}$ in the rest frame of the UHECR. We calculate the diffuse flux of relic neutrinos boosted by UHECR after taking into account the contributions from both coherent and incoherent scatterings. Using current data from IceCube and Pierre Auger Observatory, we place constraints on the overdensity of relic neutrinos down to $\sim {10}^8$. Since the flux of boosted relic neutrinos peaks at an energy of $\sim 200\mathrm{PeV}$, we also entertain the possibility to explain the recently observed KM3NeT event with boosted relic neutrinos from UHECR.

In the framework of Milgromian dynamics (MOND), galaxy clusters have been thought to have about a factor of two less baryonic mass than gravitational mass. One hypothesized source of this missing mass is undetected baryons. Extensive observations and studies indicate that the baryon content of galaxy clusters is primarily composed of the intracluster medium (ICM). In this work we reevaluate the overall stellar mass in galaxy clusters taking into account recent work on the galaxy-wide stellar initial mass function of stars (gwIMF) needed to synthesise the metals observed in galaxies. Given their supersolar metallicities and short formation timescales, massive elliptical galaxies are inferred to have formed with highly top-heavy gwIMFs, which in turn leave behind a substantial mass in stellar remnants. The dependency of the gwIMF on the properties and evolution of a galaxy is well encapsulated by the integrated galaxy-wide initial mass function (IGIMF) theory, developed independently of MOND. We utilize observational data at redshifts $\mathrm{z}<0.1$ from the Wide-field Nearby Galaxy-cluster Survey (WINGS) and the Two Micron All Sky Survey (2MASS). Masses of galaxies and intracluster light (ICL) are calculated for 46 galaxy clusters using the IGIMF theory. The resulting masses in stars and in remnants are combined with previously derived ICM masses to estimate the total baryonic masses of the clusters. These baryonic masses are then compared to the MOND dynamical masses of the clusters, which are derived from hydrostatic equilibrium of the ICM based on earlier studies. As a complement, we include a comparison with several weak/strong lensing masses in the MOND framework. Our results show that the stellar masses of galaxies and the ICL increase substantially when applying the galaxy-wide mass-to-light ratios derived from the IGIMF theory. This leads to a significant rise in the estimated baryonic masses of galaxy clusters. In the sample of 46 galaxy clusters, the baryonic component on average accounts for ${52}_{-3}^{+4}\%$ of the MOND dynamical mass when considering only the ICM contribution. The baryonic mass in stars, remnants and the ICM accounts for at least ${88}_{-4-1}^{+5+2}\%$ of the MOND dynamical mass. The contribution by stellar remnants that arises from nucleosynthesis constraints thus significantly alleviates the missing mass problem in MOND. Finally, we briefly discuss the compatibility of the IGIMF framework with the radial acceleration relation (RAR), and studies of MOND weak/strong lensing and related issues. A more comprehensive investigation will require future work that combines the IGIMF with self-consistent, spatially resolved formation, evolution, and resulting mass distribution models of galaxies.

We perform a comprehensive analysis of dark matter (DM)-admixed hybrid stars, combining a bosonic DM equation of state (EOS) with a well-constrained hybrid star matter EOS, in which the hadron phase and the quark phase are described by the improved isospin- and momentum-dependent interaction (ImMDI) model and the SU(3) Nambu-Jona-Lasinio (NJL) model, respectively. All EOSs considered satisfy the causality condition, ensuring the physical viability of our models and DM interacts with ordinary hybrid star matter only via gravity. Based on a wide parameter space of DM particle mass and DM fraction, we find that for heavy DM particles, DM remains concentrated in the stellar core, reducing the maximum mass, canonical radius $R_{1.4}$, and tidal deformability ${\mathrm{\Lambda }}_{1.4}$. For light DM particles, DM forms an extended halo, enhancing these quantities, particularly for $m_{\mathrm{DM}}\lesssim 0.1\mathrm{GeV}$. Within this scenario, several massive compact objects, including GW230529_18150, PSR J0952-0607, and the secondary component of PSR J0514-4002E may be naturally explained as DM-admixed stars. Furthermore, peculiar stars with normal optical radii but anomalously large tidal deformabilities could provide unequivocal signals for the presence of DM. Future gravitational-wave and multimessenger detections can constrain DM properties and identify its presence in compact stars.

In the gravity quantum theory, the quantization of spacetime may lead to the modification of the dispersion relation between the energy and the momentum and the Lorentz invariance violation (LIV). High energy and long-distance gamma-ray burst (GRB) observations in the Universe provide a unique opportunity to test the possibility of LIV. In this work, we use 88 time delays from GRBs ($0.117<z<6.29$), and provide a cosmological model-independent approach based on the luminosity distance data from 174 GRBs to test LIV. Combining the observation data from a multiband of GRBs provides us with an opportunity to mitigate the potential systematic errors arising from variations in the physical characteristics among diverse object populations, and to add a higher redshift dataset for testing the energy-dependent velocity caused by the corrected dispersion relationship of photons. These robust limits of the energy scale for the linear and quadratic LIV effects are $E_{\mathrm{QG},1}\ge 1.5\times {10}^{15}\mathrm{GeV}$, and $E_{\mathrm{QG},2}\ge 8.5\times {10}^9\mathrm{GeV}$, respectively. It exhibits a significantly reduced value compared to the energy scale of Planck in both scenarios of linear and quadratic LIV.

The advent of multimessenger astronomy has allowed for new types of source searches by neutrino detectors. We present the results of the search for 0.5–100 GeV astrophysical neutrinos detected with IceCube and emitted from compact binary mergers detected by the LIGO, Virgo, and KAGRA interferometers from their first run of observation (O1) to the end of the first part of the fourth (O4a). An innovative approach is used to lower the energy threshold to 0.5 GeV and to search for an excess of GeV neutrinos in time coincidence with astrophysical transient events. Furthermore, we use a statistical combination of all observations, a binomial test, to search for a subpopulation of neutrino emitters. No significant excess was found from the studied mergers, with a best post-trial $p$-value of 40%, and there is currently no hint of a population of GeV neutrino emitters found in the IceCube data (post-trial $p\text{−}\mathrm{value}=81\%$).

Four formulations of perfect spin hydrodynamics for spin-$1/2$ particles, distinguished by their treatment of spin (classical vs quantum) and by the underlying particle statistics (Boltzmann vs Fermi-Dirac), are analyzed and shown to satisfy the requirements of a divergence-type theory. Moreover, for all the formulations, we define the generating functions associated with the relevant thermodynamic currents and demonstrate that the constructed hydrodynamic theory is nonlinearly causal and stable. The latter is achieved by employing the exact expressions for the distribution functions, indicating a nonperturbative character of our approach.

As is well known, the chiral Gross-Neveu model at finite density can be solved semiclassically with the help of the chiral spiral mean field. The fermion spectrum has a single gap right at the Fermi energy, a reflection of the Peierls instability. Here, we divide the $N$ fermion flavors into two subsets to which we attribute two different densities. The Hartree-Fock ground state of such a system can again be found analytically, using as mean field the “twisted kink crystal” of Basar and Dunne. Its spectrum displays two gaps with lower edges coinciding with the two Fermi energies. This solution is favored over the homogeneous one, providing us with an explicit example of a multiple Peierls instability.

We investigate the Ward-Takahashi identity at one-loop in the light-front (LF) formalism for a bound state of fermions. We consider a spinless bound state made up of two fermions in which the Ward-Takahashi identity is satisfied in the covariant formulation. Considering the same system in the light-front formalism, we investigate the proof of Ward-Takahashi identity by integrating the light-front energy component through the identification of the relevant ranges of the longitudinal LF momentum. We elucidate that the pair production diagram plays a crucial role in establishing the Ward-Takahashi identity. We also point out the necessity of taking into account the corresponding zero modes for truly establishing the Ward-Takahashi identity in the LF formalism.