We present a class of relativistic fluid models for cold and dense matter with bulk viscosity, whose equilibrium equation of state is polytropic. These models reduce to Israel-Stewart theory for small values of the viscous stress $\mathrm{\Pi }$. However, when $\mathrm{\Pi }$ becomes comparable to the equilibrium pressure $P$, the evolution equations “adjust” to prevent the onset of far-from-equilibrium pathologies that would otherwise plague Israel-Stewart. Specifically, the equations of motion remain symmetric hyperbolic and causal at all times along any continuously differentiable flow, and across the whole thermodynamic state space. This means that, no matter how fast the fluid expands or contracts, the hydrodynamic equations are always well behaved (away from singularities). The second law of thermodynamics is enforced exactly. Near equilibrium, these models can accommodate an arbitrarily complicated dependence of the bulk viscosity coefficient $\zeta$ on both density and temperature.

Model misspecification analysis strategies, such as anomaly detection, model validation, and model comparison are a key component of scientific model development. Over the last few years, there has been a rapid rise in the use of simulation-based inference (SBI) techniques for Bayesian parameter estimation, applied to increasingly complex forward models. To move toward fully simulation-based analysis pipelines, however, there is an urgent need for a comprehensive simulation-based framework for model misspecification analysis. In this work, we provide a solid and flexible foundation for a wide range of model discrepancy analysis tasks, using distortion-driven model misspecification tests. From a theoretical perspective, we introduce the statistical framework built around performing many hypothesis tests for distortions of the simulation model. We also make explicit analytic connections to classical techniques: anomaly detection, model validation, and goodness-of-fit residual analysis. Furthermore, we introduce an efficient self-calibrating training algorithm that is useful for practitioners. We demonstrate the performance of the framework in multiple scenarios, making the connection to classical results where they are valid. Finally, we show how to conduct such a distortion-driven model misspecification test for real gravitational wave data, specifically on the event GW150914.

A search is performed for continuous gravitational waves emitted by unknown neutron stars in five nearby globular clusters using data from the third Laser Interferometer Gravitational Wave Observatory (LIGO) observing run, over the frequency range 100–800 Hz. The search uses a hidden Markov model to track both the frequency and phase of the continuous wave signal from one coherent segment to the next. It represents the first time that a phase-tracking hidden Markov model has been used in a LIGO search. After applying vetoes to reject candidates consistent with non-Gaussian artifacts, no significant candidates are detected. Estimates of the strain sensitivity at 95% confidence $h_{0,\mathrm{eff}}^{95\%}$ and corresponding neutron star ellipticity ${\epsilon }^{95\%}$ are presented. The best strain sensitivity, $h_{0,\mathrm{eff}}^{95\%}=2.7\times {10}^{-26}$ at 211 Hz, is achieved for the cluster NGC6544.

Ultralight bosons, proposed as candidates for dark matter (DM), are predicted by various new physics models. In the presence of bosons with suitable masses, superradiant (SR) instability can naturally transform a spinning black hole (BH) into a gravitational atom (GA). Here we study the dynamics of intermediate mass-ratio inspirals (IMRIs) around a GA formed by ultralight vector field saturated in its SR ground state. We employ a perturbative model at the leading Newtonian order to consistently account for both the conservative effect of cloud gravity and the dissipative effect of cloud ionization. We find the cloud can make a sizable negative contribution to the secular periastron precession at binary separations comparable to the gravitational Bohr radius. Meanwhile, the backreaction of ionization could significantly accelerate the process of orbital decay and circularization. Considering reasonably small vector boson masses, we examine the adiabatic orbital evolution and gravitational waveforms of eccentric inspirals. The results indicate that vector GAs might be detectable through observations of low-frequency IMRIs by the future space-based gravitational-wave detectors, such as LISA and Taiji.

Extreme mass-ratio hyperbolic encounters (EMRHEs) around the supermassive black holes will be observable at the future gravitational-wave (GW) detectors in space, such as LISA and Taiji. Here we consider such EMRHEs in the presence of surrounding matter distribution including baryonic accretion disk and dark matter (DM) spike, and estimate their effects on the orbital evolution and GW waveforms. We find that large possible impacts come from the gravitational potential of the accretion disk, while the influence of the DM spike is small. We also illustrate that environments can leave distinctive imprints on the GW waveforms, but resolving such modifications is found to be challenging for LISA-like detectors in the near future.

We explore the modifications of the hadron structure in a nuclear medium, focusing on the spacelike electromagnetic form factors (EMFFs) of light and heavy-light pseudoscalar mesons. By combining the light-front quark model with the quark-meson coupling model, which reasonably reproduces EMFFs in free space and the saturation properties of nuclear matter, respectively, we systematically analyze the in-medium EMFFs and charge radii of mesons with various quark flavors. Our findings show that the EMFFs of charged (neutral) mesons exhibit a faster falloff (increase) with increasing four-momentum transfer squared and nuclear density. Consequently, the absolute value of the charge radii of mesons increases with nuclear density, where the rate of increase depends on their quark flavor contents. We observe that the EMFFs of pions and kaons undergo significant modifications in the nuclear medium, while heavy-light mesons are only slightly modified. By decomposing the quark flavor contributions to EMFFs, we show that the medium effects primarily impact the light-quark sector, leaving the heavy-quark sector nearly unaffected. The results of this study further suggest the importance of the medium effects at the quark level.

We present the first example of an interacting Carroll supersymmetric field theory with both temporal and spatial derivatives, belonging to the Galileon class, where the nonlinear field equation remains second order in derivative. To achieve this, we introduce two novel tools. First, we generalize the bosonic map between the Galilei and Carroll algebras to include supersymmetry by using a spinor basis for the symmetry generators. We then show that Carroll superalgebras are naturally connected to Euclidean, rather than Poincaré, superalgebras. Using the real multiplet of the three-dimensional $\mathcal{N}=1$ Euclidean supersymmetry, we construct the scalar multiplet for $\mathcal{N}=2$ Carroll supersymmetry and develop a tensor calculus to realize the aforementioned model. These results offer new insights into the structure of genuine higher-dimensional Carroll field theories and Carroll supersymmetry. While these tools are utilized to build a specific model, we anticipate that they possess broader applications in Carrollian physics.

We investigate the mass superposition of Bañados-Teitelboim-Zanelli (BTZ) black holes using the velocity correlation function. Interestingly, the quantum signatures of BTZ black hole mass superposition can be revealed by the velocity correlation peaks. Additionally, different mass superpositions for the same mass ratio can be distinguished, a phenomenon not previously documented. We also find that the correlation function method can pinpoint the location where Hawking radiations are generated, even for mass-superposed BTZ black holes. This supports the concept of a quantum atmosphere as the origin locus of Hawking radiation, even in quantum-superposed spacetime.

We investigate the pseudospectrum of the Kerr black hole, which indicates the instability of the spectrum of quasinormal modes (QNMs) of the Kerr black hole. Methodologically, we use the hyperboloidal framework to cast the QNM problem into a two-dimensional eigenvalue problem associated with a nonself-adjoint operator, and then the spectrum and pseudospectrum are solved by imposing the two-dimensional Chebyshev collocation method. The (energy) norm is constructed by using the conserved current method for the spin $s=0$ case. For the finite rank approximation of the operator, we discuss the convergence of pseudospectra using various norms, each involving different orders of derivatives. The convergence of the pseudospectrum improves as the order of the derivatives increases. We find that an increase in the imaginary part of complex frequency can deteriorate the convergence of the pseudospectrum under the condition of the same norms.

Type N spacetimes of the Kundt class are presented as solutions to Einstein’s equations sourced by a real scalar field whose equation of motion is conformally invariant and that generalizes the standard conformal scalar field. The specific spacetimes considered model gravitational waves with uniform and totally geodesic wave fronts, propagating in a maximally symmetric background, and are characterized by the value of their constant scalar curvature and by the so-called wave profile function. All subclasses of such spacetimes are analyzed. It is shown that the scalar field solutions generically divide into two branches, of which one has a strikingly different behavior from that of the standard conformal case. The scalar field contributes to the equation satisfied by the wave profile function by adding new singular terms. Closed form and mode solutions for the wave profile function are found for different scenarios. The resulting energy-momentum tensor has a null eigenvector, but is more general than the pure radiation type usually coupled with this kind of spacetimes.

The gravitational deflection effect of relativistic massive and massless particles up to the first post-Minkowskian order caused by a moving Schwarzschild black hole with a two-dimensional equatorial velocity, which contains the radial and transversal components, is studied analytically, and a new unified formula for the deflection angle is achieved. The expression of the angle matches well with the results of the weak deflection of relativistic particles induced by a radially moving Schwarzschild source given in the literature, when the transversal component of the lens velocity vanishes. The joint velocity effect, which consists of the influences of the transversal and radial motions of the lens on the leading-order Schwarzschild deflection of the massive particles and light, is then discussed in the context of general relativity. We analyze the order of magnitude of this kinematical effect and evaluate the possibility of its astronomical detection subsequently.

Virial-like identities obtained through Derrick’s scaling argument are powerful, multipurpose tools to study general relativistic models. Applications comprise establishing no-go/hair theorems and numerical accuracy tests. In the presence of a horizon (also known as a boundary), the spacetime can be divided into regions, each with its own identity. So far, such identities have only been computed in the region outside the event horizon; however, adding a positive cosmological constant endows an additional boundary (the cosmological horizon), with the region between the latter and the former being of particular interest. In this letter, by performing a radial coordinate transformation, we generalize Derrick’s scaling argument to compute virial identities across the whole nonasymptotically flat spacetimes. The developed method is applied to the entire Reissner-Nordstrom-de Sitter spacetime. A convenient gauge that trivializes the gravitational contribution to the identity between horizons is also found.

The NANOGrav, Parkes and European Pulsar Timing Array (PTA) experiments have collected strong evidence for a stochastic gravitational wave background in the nHz-frequency band. In this work we perform a detailed statistical analysis of the signal in order to elucidate its physical origin. Specifically, we test the standard explanation in terms of supermassive black hole mergers against the prominent alternative explanation in terms of a first-order phase transition. By means of a frequentist hypothesis test we find that the observed gravitational wave spectrum prefers a first-order phase transition at $2-3\sigma$ significance compared to black hole mergers (depending on the underlying black hole model). This mild preference is linked to the relatively large amplitude of the observed gravitational wave signal (above the typical expectation of black hole models) and to its spectral shape (which slightly favors the phase-transition spectrum over the predominantly single power-law spectrum predicted in black hole models). The best fit to the combined PTA dataset is obtained for a phase transition which dominantly produces the gravitational wave signal by bubble collisions (rather than by sound waves). The best-fit (energy-density) spectrum features, within the frequency band of the PTA experiments, a crossover from a steeply rising power law (causality tail) to a softly rising power law; the peak frequency then falls slightly above the PTA-measured range. Such a spectrum can be obtained for a strong first-order phase transition in the thick-wall regime of vacuum tunneling which reheats the Universe to a temperature of $T_{*}\sim \mathrm{GeV}$. A dark sector phase transition at the GeV-scale provides a comparably good fit.

We investigate the anisotropic parametrization of the dark energy equation of state within the framework of an axisymmetric (planar) Bianchi-I universe. Using the latest $\mathrm{Pantheon}+$ Type Ia Supernova dataset, augmented by SH0ES Cepheid distance calibrators, we constrain both the equation of state for anisotropic dark energy and other standard cosmological parameters. Additionally, we examine the presence of an underlying anisotropic axis. Our analysis yields a mean anisotropic dark energy equation of state as $\overline{w}=-0.86_{-0.11}^{+0.15}$ and a difference in the equation of states in and perpendicular to the plane of the axisymmetric Bianchi-I space-time as ${\delta }_w=-0.129_{-0.064}^{+0.090}$. We also identify an axis of anisotropy at approximately $(272°,21°)$ in galactic coordinates. Through a comparative study of different cosmological models, we find that the data favor a Bianchi-I universe with anisotropic dark energy, where the equation of state deviates from “$-1$” along the axis of anisotropy (the $w_b\mathrm{CDM}$ model), over other anisotropic models considered and the standard flat $\mathrm{\Lambda }\mathrm{CDM}$ or $w\mathrm{CDM}$ models.

We perform fully general relativistic simulations of binary strange star mergers considering two different approaches for thermal effects. The first uses a cold equation of state derived from a modified version of the MIT bag model which is then supplemented by a $\mathrm{\Gamma }$-law correction. The second approach employs a microphysical description of the finite temperature effects. We describe the results obtained with the two treatments, highlighting the influence of thermal effects. We find that the postmerger dynamics differs significantly in the two cases, leading to quantitative differences in the postmerger gravitational wave spectrum and ejecta mass. The peak frequency of the postmerger gravitational wave emission is consistent with the established quasiuniversal relations for binary neutron star mergers and as a result, our simulations cannot distinguish between mergers of neutron stars and those of strange stars. Our models with realistic treatment of finite temperature effects produce a significant amount of ejecta $\gtrsim 0.02M_{\odot }$. The resulting flux of strangelets near the Earth, computed assuming that all neutron star mergers are in fact strange-stars mergers, that the average mass of each strangelet is $A\sim 100$, and that the binary considered here is representative, is in tension with experimental upper limits.

The doubly charmed baryon, ${\mathrm{\Xi }}_{cc}^{++}$, was first observed by LHCb through the nonleptonic decay modes of ${\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+}$ and ${\mathrm{\Xi }}_c^{+}{\pi }^{+}$. Following this discovery, researchers shifted their focus to identifying other doubly charmed baryons, specifically ${\mathrm{\Xi }}_{cc}^{+}$ and ${\mathrm{\Omega }}_{cc}^{+}$. In this study, we examine the nonleptonic weak decays of doubly charmed baryons, denoted as ${\mathcal{B}}_{cc}\to {\mathcal{B}}_{c}P$, where ${\mathcal{B}}_{cc}$ represents the doubly charmed baryons, specifically $({\mathrm{\Xi }}_{cc}^{++},{\mathrm{\Xi }}_{cc}^{+},{\mathrm{\Omega }}_{cc}^{+})$. The notation ${\mathcal{B}}_c$ denotes the singly charmed baryons, specifically $({\mathcal{B}}_{\overline{3}},{\mathcal{B}}_6)$, while $P$ signifies the light pseudoscalar. These terms are pertinent to the nonleptonic decay modes under discussion. While the short-distance contributions can be precisely estimated through theoretical calculations, addressing the long-distance contributions for final-state-interaction effects presents a significant challenge. In order to address this issue, we utilize the rescattering mechanism of final state interaction effects to compute the long-distance contributions. We initially derive the entire hadronic loop contributions for these two-body nonleptonic decays of doubly charmed baryons. In subsequent analyses, we are able to calculate relative strong phases. As a result, we can provide predictions for their decay asymmetry parameters and $CP$ violations. Furthermore, we employ experimental data from the LHCb collaboration, specifically the ratio $\mathrm{Br}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{\prime +}{\pi }^{+})/\mathrm{Br}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{+}{\pi }^{+})=(1.41\pm 0.17\pm 0.10)$, to ascertain the model parameters $\eta =0.9\pm 0.2$. Consequently, we present the predictions of branching ratios and decay asymmetry parameters for 67 distinct decay processes and $CP$ violations for the singly Cabibbo suppressed channels. This not only strengthens the validity of our theoretical predictions, but also provides a more comprehensive theoretical framework for the future identification of other doubly charmed baryons.

A higgsino could be some or all of the dark matter, with a mass bounded from above by about 1.1 TeV assuming a thermal freeze-out density, and from below by collider searches. Direct detection experiments imply purity constraints on a dark matter higgsino, limiting the mixing with the electroweak gauginos. Using the new strong limits available as of the end of 2024 from the LUX-ZEPLIN experiment, I quantify the resulting lower bounds on gaugino masses and upper bounds on higgsino mass splittings, assuming that the scalar superpartners and Higgs bosons of minimal supersymmetry are in the decoupling limit. Similar bounds are projected for the critical future scenario that direct detection experiments reach the neutrino fog that hampers discovery prospects.

It has long been pointed out that there are seven different two-zero texture neutrino mass matrices compatible with neutrino oscillation data. We perform an updated analysis with the recently published Nu-Fit 6.0 results. We also subject the seven two-zero textures to constraints on neutrino mass from cosmology, end point of beta decay spectrum, and neutrinoless double beta decay experiments. We find that all seven textures are compatible with the new oscillation parameters. However, we find five textures, whose 1-1 entry is nonvanishing, are in severe tension with the constraints from cosmology and neutrinoless double beta decay. With the next generation experiments, these five textures could be decisively ruled out. For the remaining two textures, one of them could be ruled out, or severely constrained, if the octant of ${\theta }_{23}$ is determined.

Same-sign lepton colliders offer a promising environment to probe lepton number violation. We study processes that change lepton number by two units in the context of Majorana heavy neutral leptons and neutrinophilic scalars at $\mu \mathrm{TRISTAN}$, a proposed same-sign muon collider. Our work shows that such colliders, with modest energy and luminosity requirements, can either reveal direct evidence of lepton number violation or significantly constrain unexplored regions of parameter space, especially in the case of a neutrinophilic scalar.