We investigate the structural, dynamical, and oscillatory properties of neutron stars admixed with dark matter, modeled via a single-fluid formalism where dark matter interacts with nuclear matter through an effective Higgs-portal coupling. Employing three relativistic mean-field nuclear matter equations of state—IOPB-I, BigApple, and NL3—we incorporate a physically motivated dark matter number density profile that scales with baryon density and is controlled by two parameters: a scaling factor $\alpha M_{\chi }$ ($M_{\chi }$ being the mass of dark matter particle) and a steepness index $\beta$. We construct equilibrium configurations and analyze their stability via radial oscillations, finding that dark-matter-induced gravitational compression lowers the maximum mass and alters the radial mode spectrum in a nontrivial, $\beta$-dependent fashion. We also compute the frequencies of nonradial fluid oscillations under the relativistic Cowling approximation and analyze the persistence of universal relations in the presence of dark matter. While deviations appear under extreme configurations, the overall structure of these relations remains robust. Our findings offer a consistent framework to probe dark matter effects on neutron star dynamics across a range of realistic models.

We derive scaling laws that connect certain macroscopic observables of strange quark stars with key microscopic properties of self-bound quark matter, such as the energy per baryon at zero pressure and the strength of repulsive interactions. We also identify universal relations linking global properties of strange quark stars; specifically, their moment of inertia, tidal deformability, and both gravitational and baryonic compactness. Remarkably, these relations hold for two substantially different microscopic models—the quark-mass density-dependent model with excluded-volume corrections and the vector MIT bag model—underscoring their robust, model-independent nature. We demonstrate that the universal relations for strange quark stars differ significantly from those previously established for neutron stars composed of hadronic matter, thus enabling discrimination between the two types of objects without requiring detailed knowledge of their equations of state. Moreover, observational constraints on the maximum mass of compact stars could place bounds on both the depth of quark-matter self-binding and the strength of quark repulsive interactions.

Quantum entanglement is a cornerstone of quantum mechanics. While the entanglement of confined electron pairs has been established early on, the entanglement of free-traveling electron pairs, particularly at high energies, remains largely unexplored due to the substantial challenges involved in measuring the spins of free-traveling electrons. In this study, we investigate the entanglement and the Bell inequality violation of free-traveling electron-positron pairs generated in a fixed-target experiment. This experimental setup facilitates the creation of a controllable source of entangled electron-positron pairs, where entangled events are produced in specific phase spaces. Based on this source and the prior knowledge of the entangled state, we demonstrate the feasibility of measuring the polarization correlations of the entangled $e^{+}{e}^{-}$ pairs through their individual secondary scatterings off two separate additional targets.

In loop quantum cosmology, the quantization of the Hamiltonian constraint involves a regularization procedure which is affected by certain ambiguities. Moreover, different regularizations lead to distinct mathematical formulations and, consequently, to different physical predictions. In this work, we explore the impact of this regularization on the primordial power spectrum of cosmological perturbations. More specifically, we study this power spectrum for the conventional regularization used in loop quantum cosmology and for two alternative prescriptions suggested in the literature. We set initial conditions for the perturbations at the bounce corresponding to a recently proposed vacuum state, optimally adapted to the background dynamics. This choice of vacuum is based on an asymptotic Hamiltonian diagonalization in the ultraviolet sector of the perturbations which provides a nonoscillating power spectrum. Employing a suitable approximation to the propagation equations of the perturbations around the bounce, we are able to obtain an analytic expression for the primordial power spectrum of this vacuum for all the discussed regularizations. We compare the results and prove that the main relevant distinction between the corresponding spectra is the scale where power suppression occurs. The associated wave number scale is proportional to the square root of the critical density in loop quantum cosmology, which is different for each of the studied regularizations.

We point out that the odd-in-velocity contribution to the $\mathcal{O}(G^4)$ radiated angular momentum for two-body scattering is determined by the radiation-reaction (RR) term in the one-loop waveform. This RR term is actually proportional to the tree-level waveform, and this reduces the calculation of the odd-in-velocity contribution to the $\mathcal{O}(G^4)$ angular momentum loss, $J_{2\mathrm{rad}}$, to two loops, instead of three loops as one would expect by power counting. We exploit this simplification, which follows from unitarity, to obtain a closed-form expression for $J_{2\mathrm{rad}}$ for generic velocities, which resums all fractional post-Newtonian (PN) corrections to the $\mathcal{O}(G^4)$ angular momentum loss starting at 1.5PN.

The properties of quantum chromodynamics (QCD) matter at finite isospin densities are investigated employing holographic hard-wall and soft-wall anti–de Sitter (AdS)/QCD models. It is confirmed that at high enough isospin densities, charged pions start to condense and the pion superfluid phase appears in the system. It is shown that the chiral condensate and the pion condensate can be transformed to each other and form a chiral circle in the superfluid phase. We derived the equation of state (EoS) for pionic matter, calculated the normalized trace anomaly $\mathrm{\Delta }$ and $(\epsilon -3p)/m_{\pi }^4$, and analyzed the sound speed and adiabatic index. Additionally, we provided data on the mass-radius relation and tidal deformability of pion stars. The results indicate that the holographic models align well with lattice QCD concerning isospin density, axial-vector condensation, EoS, and trace anomaly, though discrepancies in sound speed and adiabatic index emerge at higher isospin chemical potentials. The holographic models closely match the leading-order results from chiral perturbation theory ($\chi \mathrm{PT}$) in certain observables, suggesting that they may effectively capture aspects of $\chi \mathrm{PT}$ dynamics within a five-dimensional framework under the probe approximation.

In quantum mechanics, the measurable quantities of a given theory are predicted by performing a weighted sum over possibilities. We show how to arrange the possibilities into bundles such that the associated subsums can be viewed as well-defined theories on their own right. These bundles are submanifolds of possibilities which we call possifolds. We collect and prove some basic facts about possifolds. Especially, we show that possifolds are ensembles of what in a certain broadly defined sense that we explain can be regarded as soliton excitations (soliton-possifold correspondence). We provide an outlook on some applications. Among other things, we illustrate the use of the developed framework for the example of the Lieb-Liniger model. It describes nonrelativistic bosons with an attractive interaction. We derive a dual theory describing the lowest-lying energy excitation modes. While the standard Bogoliubov-approximation breaks down at the critical point, our derived summation prescription stays regular. In the Bogoliubov-limit we observe the summation to possess an enhanced symmetry at this point while the summation cannot be ignored there. We finally provide a glimpse on the restrictions black hole physics implies in this context for the gravitational path integral.

Gravitational waveforms capturing binary evolution through the early-inspiral phase play a critical role in extracting orbital features that nearly disappear during the late-inspiral and subsequent merger phase due to radiation reaction forces; for instance, the effect of orbital eccentricity. Phenomenological approaches that model compact binary mergers rely heavily on combining inputs from both analytical and numerical approaches to reduce the computational cost of generating templates for data analysis purposes. In a recent work, Chattaraj et al. [Phys. Rev. D. 106, 124008 (2022)] constructed a dominant ($\ell =2$, $|m|=2$) mode model for nonspinning binary black holes (BBHs) on elliptical orbits. The model was constructed in time domain and is fully analytical. The current work is an attempt to improve this model by making a few important changes in our approach. The most significant of those involves identifying initial values of orbital parameters with which the inspiral part of the model is evolved. While the ingredients remain the same as in the previous work, the resulting (new) model, when compared against a set of target waveforms constructed here, produces match values better than 96.5% for systems heavier than $80M_{\odot }$, while with the old model this limit on the total mass is $115M_{\odot }$. The updated model is validated against an independent eccentric waveform family (TEOBResumS-Dali) for an initial eccentricity ($e_0$), mass ratio ($q$), and mean anomaly ($l_0$) in the range $0\lesssim e_0\lesssim 0.3$, $1\lesssim q\lesssim 3$, and $-\pi \le l_0\le \pi$, respectively. Further, an alternate model including the effect of higher order modes is also provided. Finally, while our model assumes nonspinning components, we show that it could also be used for systems with component spin vectors (anti)aligned with respect to the orbital angular momentum and small spin magnitudes.

Quasinormal modes are characteristic signatures of compact objects. Here, we consider rotating regular black holes, representing rotating generalizations of the Simpson and Visser metric. We present the spectrum of scalar quasinormal modes and compare it with the spectrum of Kerr black holes. The calculations are done using a spectral decomposition method. The scalar modes smoothly connect to the Kerr limit. Interestingly, for particularly low scaled Hawking temperatures, the dependence of the modes changes as rotation increases. A further investigation shows that the real part of the fundamental and first excited modes of the static and slowly rotating black holes (about 13% of the extremal angular momentum) progresses in an opposite behavior in this low temperature region. Meanwhile, the imaginary part of the modes crosses where the excited modes become longer lived than the fundamental modes. Rapid rotation, however, suppresses such a tendency.

gwnrhme is a recently developed framework that seamlessly converts a multimodal (i.e., with several spherical harmonic modes) quasicircular waveform into a multimodal eccentric waveform if the quadrupolar eccentric waveform is known. Here, we employ the gwnrhme framework to combine a multimodal quasicircular numerical relativity surrogate waveform model nrhybsur3dq8 and quadrupolar nonspinning post-Newtonian eccentric waveform model eccentricimr to construct multimodal nonspinning eccentric model nrhybsur3dq8-gwnrhme. Using a total of 35 eccentric numerical relativity (NR) simulations obtained from the SXS and RIT catalogs, we demonstrate that nrhybsur3dq8-gwnrhme model predictions agree well with NR (with typical relative $L_2$ errors of $\sim 0.01$ for the dominant quadrupolar mode) for mass ratios $1\le q\le 4$ and eccentricities up to $\sim 0.2$ measured about 10 cycles before the merger. Our frequency-domain mismatches (calculated assuming advanced LIGO design sensitivity curve) are mostly below 0.01. To demonstrate the modularity of the gwnrhme framework, we further combine eccentricimr with the bhptnrsur1dq1e4bhptnrsur1dq1e4 model and develop a nonspinning eccentric model named bhptnrsur1dq1e4-gwnrhme. Finally, we develop a different variant of these models by replacing eccentricimr with eccentrictd. Both the gwnrhme framework and associated models are available through the gwmodels package.

Pulsar timing arrays are playing a crucial role in the ongoing gravitational wave astronomy revolution. The current evidence for a stochastic gravitational wave background at nHz frequencies offers an opportunity to discover cosmological signals and threatens the observability of other subdominant gravitational waves (GWs). We explore prospects to constrain second-order scalar-induced GWs (SIGWs) associated with enhanced curvature perturbations in the primordial universe, forecasting realistic future pulsar timing array datasets. We assess how the currently observed signal could eventually limit future capabilities to search for GW relics of primordial phenomena and associated phenomenological consequences such as primordial black hole (PBH) formation. Given the sensitivity of PBH abundance to spectral parameters, measuring it remains a challenge for realistic signals. However, future observation could still rule out nearly subsolar mass PBHs formed through standard formation scenarios in some cases. Future progress in constraining PBH models is expected to stem from theoretical advancements in PBH computations, which should help resolve the tension between different computational methods. The analysis is based on and extends the python code fastpta.

We investigate the capability of constraining the mass and redshift distributions of binary black hole systems jointly with the underlying cosmological model using one year of observations of the Einstein Telescope. To this aim, we fixed the underlying cosmological model to a flat $\mathrm{\Lambda }$ Cold Dark Matter model, then we considered the mass distribution given by a smoothed power law, and the redshift distributions given by the Madau-Dickinson model. We built mock catalogs with different signal-to-noise ratio (SNR) thresholds, and finally inferred astrophysical and cosmological parameters jointly adopting a hierarchical Bayesian framework. We found that as the SNR threshold decreases, the precision on the matter density parameter ${\mathrm{\Omega }}_{m,0}$ and the Hubble constant $H_0$, improves significantly due to the increased number of detectable events at high redshift. However, degeneracies between cosmological and astrophysical parameters exist and evolve with the SNR threshold. Finally, we showed that one year of observations will serve to reconstruct the mass distribution with its features. Conversely, the redshift distribution will be poorly constrained and will need more observations to improve.

Is there new physics hidden in the four-point function of the cosmic microwave background (CMB)? We conduct a detailed analysis of the Planck PR4 temperature and polarization trispectrum for $\ell \in [2,2048]$. Using the theoretical and computational tools developed by Philcox in Paper I [Phys. Rev. D 111, 123532 (2025).] and Paper II [Phys. Rev. D 111, 123533 (2025).], we search for 33 template amplitudes, encoding a variety of effects from inflationary self-interactions to particle exchange. We find no evidence for primordial non-Gaussianity and set stringent constraints on both phenomenological amplitudes and couplings in the inflationary Lagrangian. Because of the use of optimal estimators and polarization data, our constraints are highly competitive. For example, we find $\sigma (g_{\mathrm{NL}}^{\mathrm{loc}})=4.8\times {10}^4$ and ${\tau }_{\mathrm{NL}}^{\mathrm{loc}}<1500$ (95% CL), a factor of 2 improvement on effective field theory amplitudes, and a $43\sigma$ detection of gravitational lensing. Many templates are analyzed for the first time, such as direction-dependent trispectra and the collapsed limit of the “cosmological collider,” across a range of masses and spins. We perform a variety of validation tests; while our results are stable, the most relevant systematics are found to be lensing bias, residual foregrounds, and mismatch between simulations and data. The techniques discussed in this series can be extended to future datasets, allowing the primordial Universe to be probed at even higher sensitivity.

To unlock the vast potential of the cosmic microwave background (CMB) trispectrum, we require both robust estimators and efficient computational tools. In this work, we introduce the public code polyspec: a suite of quartic estimators designed to measure the amplitudes of a wide variety of inflationary templates, including local non-Gaussianity, effective field theory models, direction-dependent trispectra, spinning massive particle exchange, and weak gravitational lensing. polyspec includes a python/cython implementation of each estimator derived by Philcox [O. H. E. Philcox, companion paper, Phys. Rev. D 111, 123532 (2025).] and has been carefully optimized to ensure efficient use of computational resources. We perform a broad range of validation tests, which demonstrate that the estimator is unbiased and minimum variance, both in Gaussian and non-Gaussian regimes. In addition, we forecast constraints on various types of trispectra; this highlights the utility of CMB polarization and demonstrates that many models of primordial physics are poorly correlated with the simple templates considered in previous studies. This work lays the foundation for the Planck trispectrum analyses performed by Philcox [O. H. E. Philcox, companion paper, Phys. Rev. D 111, 123534 (2025).].

The primordial four-point function encodes a wealth of information about the inflationary Universe. Despite extensive theoretical work, most models of four-point physics have never been compared to data. In this series, we conduct a detailed analysis of cosmic microwave background temperature and polarization trispectra, searching for a wide variety of phenomena including local effects, self-interactions, curvatons, Dirac-Born-Infeld inflation, gauge fields, solid inflation, scalar field exchange, spinning massive field exchange, chiral physics, point sources, and gravitational lensing. After presenting a suite of separable primordial templates, we derive 13 quasioptimal estimators that directly estimate the underlying template amplitudes. These are unbiased, minimum variance, mask deconvolved, and account for correlations between templates (including with lensing). Each estimator can be efficiently implemented using spherical harmonic transforms, Monte Carlo methods, and optimization techniques, and asymptotes to standard forms in certain limits. In Paper II [O. H. E. Philcox, Phys. Rev. D 111, 123533 (2025)], we implement these estimators in public code, and in Paper III [O. H. E. Philcox, Phys. Rev. D 111, 123534 (2025)], use them to constrain primordial trispectra with Planck data. This enables a wide variety of tests of inflation, including some of the first direct constraints on cosmological collider physics.

Using a purely kinematical argument, the Borde-Guth-Vilenkin (BGV) theorem [Inflationary spacetimes are incomplete in past directions, Phys. Rev. Lett. 90, 151301 (2003)] states that any maximal space-time with average positive expansion is geodesically incomplete, hence past eternal inflation would be necessarily singular. Recently, discussions about the broadness of this theorem have been resurfaced by applying it to new models and/or challenging the space-time maximality hypothesis. In the present work, we use reference frames of noncomoving observers and their kinematical properties in order to inquire into the nature of such possible singular beginnings. Using the spatially flat de Sitter (dS) space-time as a laboratory, this approach allows us to exhaust all possibilities bounded by the BGV theorem in the case of general spatially flat Friedmann-Lemaître-Robertson-Walker geometries. We show that either there exists a scalar or parallelly propagated curvature singularity, or the space-time must be past asymptotically dS (with a definite nonzero limit of the Hubble parameter when the scale factor becomes null, hence excluding certain cyclic models) in order to be extensible. We are able to present this local extension without violating the null energy condition, and we show that this extension must contain a bounce. This is a mathematical result based on purely kinematical arguments and intuition. The possible physical realization of such extensions are also discussed. As a side product, we present a new chart that covers all de Sitter space-time.

We present a numerically efficient and accurate implementation of the Passaglia-Hu effective fluid approximation for ultralight axions within the Boltzmann code camb. This method is specifically designed to evolve the axion field accurately across cosmological timescales, mitigating the challenges associated with its rapid oscillations. Our implementation is based on the latest version of camb, ensuring compatibility with other cosmological codes., e.g., for calculating cosmological parameter constraints. Compared to exact solutions of the Klein-Gordon equation, our method achieves subpercent accuracy in the cosmic microwave background power spectrum across a broad range of axion masses, from ${10}^{-28}\mathrm{eV}$ to ${10}^{-24}\mathrm{eV}$. We perform Markov chain Monte Carlo analyses incorporating our implementation, and find improved constraints on the axion mass and abundance compared to previous, simpler fluid-based approximations. For example, using Planck Public Release 4 and Dark Energy Spectroscopic Instrument baryon acoustic oscillation data, we find $2\sigma$ upper limits on the axion fraction $f_{\mathrm{ax}}<0.0082$ and physical density ${\mathrm{\Omega }}_{\mathrm{ax}}{h}^2<0.0010$ for $m={10}^{-28}\mathrm{eV}$.

We investigate deviations from Lambda Cold Dark Matter ($\mathrm{\Lambda }\mathrm{CDM}$) model by independently parametrizing modifications in the background evolution and the growth of structures. The background is characterized by two parameters, $A$ and $B$, which reduce to $A={\mathrm{\Omega }}_{m0}$ and $B=2/3$ in the $\mathrm{\Lambda }\mathrm{CDM}$ limit, while deviations in the growth of structures are captured through a fitting function for $f{\sigma }_8$ involving the growth index $\gamma$. Using recent observational datasets involving background expansion and growth of structures (related to observations involving redshift space distortions), we find significant evidence for departures from $\mathrm{\Lambda }\mathrm{CDM}$ in the background expansion whereas there is no finite evidence for deviations from $\mathrm{\Lambda }\mathrm{CDM}$ behavior in the growth of structures. This suggests that with the current precision in observational data involving background and the perturbed Universe, a deviation from $\mathrm{\Lambda }\mathrm{CDM}$ behavior is confirmed (as shown in the recent DESI-DR2 results). But whether this deviation is due to an evolving dark energy or due to the modification of gravity at cosmological scales is still an open question, largely due to less precise data from the perturbed Universe. We further demonstrate how the future high-precision growth data (from Euclid, for example) can answer such a question using a forecast study.

We use machine learning methods to search for parity violations in the large-scale structure (LSS) of the Universe, motivated by recent claims of chirality detection using the 4-point correlation function (4PCF), which would suggest new physics during the epoch of inflation. This work seeks to reproduce these claims using methods originating from high energy collider analyses. Our machine learning methods optimize some underlying parity odd function of the data, and use it to evaluate the parity odd fraction. We demonstrate the effectiveness and suitability of these methods and then apply them to the Baryon Oscillation Spectroscopic Survey (BOSS) catalog. No strong evidence for parity violation is detected.

In this paper, we investigate the phenomenology of fermionic systems in uniform gravitational fields within the framework of very special relativity (VSR). We focus on the case of gravitational spectroscopy with ultracold neutrons, explored in experiments like qbounce. Calculating the leading ($c^0$) and next-to-leading ($c^{-1}$) order corrections to the nonrelativistic Hamiltonian in an accelerated frame, we obtain the perturbed fermionic energy spectrum. At leading order, we do not find any modifications except for a trivial mass shift, thus preserving the equivalence between inertial and gravitational mass and particle-antiparticle sectors. The next-to-leading order corrections, instead, introduce time-dependent anisotropic contributions depending on the preferred spatial direction in VSR, and can then be used to probe novel Lorentz-violating signatures. Taking qbounce sensitivity as a benchmark, we derive a first rough constraint for the neutron VSR parameter. Finally, we suggest alternative spin-flipping setups to better probe VSR effects and foresee potential future research directions.