In this work, we propose a new $2+1$ theory of gravity. We start with a modification of Chern-Simons $2+1$ gravity. By introducing a vector field $\mathrm{\Phi }$ to the usual composition of the vierbein and connection, we derive a new action that includes the Einstein-Hilbert term, a cosmological constant, and a polynomial term for the vector field. This allows us to write a Chern-Simons-inspired massive gravity that generalizes new massive gravity. After solving the polynomial function, the action can be rewritten as a Born-Infeld-type action. Moreover, we show that the second-order expansion of this action is equivalent to new massive gravity. We explore solutions to the theory and show that it has a Bañados-Teitelboim-Zanelli solution with an effective cosmological constant in terms of the parameters of the theory, global Lifshitz space-times as well as regular Lifshitz black holes, and asymptotically locally flat black holes. Furthermore, we study the decoupling limit of the theory, showing that it contains the quartic Galileon in addition to the cubic Galileon that is characteristic of new massive gravity.

The spontaneous breaking of a discrete symmetry can lead to the formation of domain walls in the early Universe. In this work, we explore the impact of bias directions on the dynamics of $Z_N$ domain walls, mainly focusing on the $N=3$ model with a biased potential. Utilizing the Press-Ryden-Spergel method, we numerically investigate the dynamics of domain walls with lattice simulations. We find notable differences in the dynamics of domain walls due to bias directions. Our results indicate that the annihilation time depends not only on the vacuum energy difference $\delta V$ but also on bias directions described by the relative potential difference $\zeta$.

We clarify that chemical and kinetic equilibration in the early Universe are distinct: neither implies the other, and the ordering of their decouplings need not be universal. We illustrate this with Standard Model neutrino decoupling, strong-washout leptogenesis, dark–matter scenarios where kinetic decoupling precedes chemical freeze-out (resonant/forbidden, conversion/coannihilation, coscattering), and dark sectors at with temperatures distinct from the visible-sector temperature, with semiannihilation or $3\to 2$ cannibal dynamics. The moral of the story is simple: Chemical equilibrium governs numbers, kinetic equilibrium governs shapes. In an expanding Universe the operators that control them rarely fade at the same time, and when they do not, the order of decoupling is model dependent. Turning to phase-space evolution whenever momentum selectivity matters is the surest way to obtain robust cosmological predictions.

We study the sensitivity of nEXO to solar neutrino charged-current interactions, ${\nu }_e+{}^{136}\mathrm{Xe}\to {{}^{136}\mathrm{Cs}}^{*}+e^{-}$, as well as analogous interactions predicted by models of fermionic dark matter. Due to the recently observed low-lying isomeric states of ${}^{136}\mathrm{Cs}$, these interactions will create a time-delayed coincident signal observable in the scintillation channel. Here we develop a detailed Monte Carlo simulation of scintillation emission, propagation, and detection in the nEXO detector to model these signals under different assumptions about the timing resolution of the photosensor readout. We show this correlated signal can be used to achieve background discrimination on the order of ${10}^{-9}$, enabling nEXO to make background-free measurements of solar neutrinos above the reaction threshold of 0.668 MeV. We project that nEXO could measure the flux of neutrinos from the carbon-nitrogen-oxygen cycle with a statistical uncertainty of 25%, thus contributing a novel and competitive measurement toward addressing the solar metallicity problem. Additionally, nEXO could measure the mean energy of the ${}^7\mathrm{Be}$ neutrinos with a precision of $\sigma \le 1.5\mathrm{keV}$ and could determine the survival probability of ${}^7\mathrm{Be}$ and pep solar ${\nu }_e$ with precision comparable to the state of the art. These quantities are sensitive to the Sun’s core temperature and to nonstandard neutrino interactions, respectively. Furthermore, the strong background suppression would allow nEXO to search for charged-current interactions of fermionic dark matter in the mass range $m_{\chi }=0.668–7\mathrm{MeV}$ with a sensitivity up to three orders of magnitude better than current limits.

Pulsar Timing Array (PTA) projects have reported various lines of evidence suggesting the presence of a stochastic gravitational wave (GW) background in their data. One key line of evidence involves a detection statistic sensitive to interpulsar correlations, such as those induced by GWs. A $p$-value is then calculated to assess how unlikely it is for the observed signal to arise under the null hypothesis $H_0$, purely by chance. However, PTAs cannot empirically draw samples from $H_0$. As a workaround, various techniques are used in the literature to approximate $p$-values under $H_0$. One such technique, which has been characterized as a model-independent method, is the use of “scrambling” transformations that modify the data to cancel out pulsar correlations, thereby simulating realizations from $H_0$. In this work, scrambling methods and the detection statistic are investigated from first principles. The $p$-value methodology that is discussed is general, but the discussions regarding a specific detection statistic apply to the detection of a stochastic background of gravitational waves with PTAs. All methods in the literature to calculate $p$-values for such a detection statistic are rigorously analyzed, and analytical expressions are derived for the distribution of the detection statistic and the corresponding $p$-values. All this leads to the conclusion that scrambling methods are not model independent and thus not completely empirical. With a single realization of data our results are necessarily always model dependent, which any analysis will need to accept. Instead of scrambling approaches, rigorous Bayesian and Frequentist $p$-value calculation methods are advocated, the evaluation of which depend on the generalized ${\chi }^2$ distribution. This view is consistent with the posterior predictive $p$-value approach that is already in the literature. Efficient expressions are derived to evaluate the generalized ${\chi }^2$ distribution of the detection statistic on real data. It is highlighted that no Frequentist $p$-values have been calculated correctly in the PTA literature to date.

The displacement-noise-free interferometer (DFI) is designed to eliminate all displacement-induced noise while retaining sensitivity to gravitational wave (GW) signals. Ground-based DFIs suffer from physical arm-length limitations, resulting in poor sensitivity at frequencies below 1 kHz. To address this, previous research introduced a neutron-based DFI, which replaces laser light with neutrons and achieves exceptional sensitivity down to a few hertz. In this study, we conducted a proof-of-principle experiment using a pulsed neutron source at the Japan Proton Accelerator Research Complex. Despite practical constraints that led to deviations from the ideal experimental design, we optimized the setup and developed a novel analysis method that successfully cancels displacement noise while preserving simulated GW signals. This work presents the first successful demonstration of a neutron DFI and a neutron interferometer for GW detection.

We face here the problem of uncertainties in correlation functions due to the freedom in the off-shell dependence of the wave functions or, equivalently, off-shell ambiguities in the scattering matrices. We make the study for the case of meson baryon interaction, choosing the $K\mathrm{\Lambda }$, $K\mathrm{\Sigma }$, and $\eta N$ coupled channels, which are related to the $N^{*}(1535)$ resonance, and the $K^{+}{\overline{K}}^0$ channel related to the $a_0(980)$ resonance. We find that, by using realistic interactions based on chiral dynamics, the uncertainties are small, of the order of 2–3%, but could be much bigger if other methods are used. However, our study shows the way to optimally overcome these uncertainties in the analysis of correlation functions, and we provide a series of recommendations for any general analysis.

We extend our recent analytic study of the strong coupling ${\alpha }_{\mathrm{eff}}$ in the nonperturbative and near-perturbative regimes [Phys. Rev. Lett. 133, 181901 (2024)] by imposing rigorous renormalization-group constraints from asymptotically free gauge theories at $Q^2\to \infty$. The asymptotic boundary conditions modify the scaling properties of ${\alpha }_{\mathrm{eff}}$ at large values of the momentum transfer $Q^2$ and lead to a scale-dependent confinement strength $\kappa (Q^2)$. This requires that both $\kappa (Q^2)$ and ${\alpha }_{\mathrm{eff}}(Q^2,\kappa (Q^2))$ remain holomorphic in the complex $Q^2$ plane, except at the physical cuts associated with the heavy-quark thresholds and the singularity flow trajectory studied in our previous Letter. For color SU(3), a precise connection is found between the scaling exponent of $\kappa (Q^2)$ in the ultraviolet, the value of the infrared fixed point of the strong coupling, and the number of flavors in agreement with observations. The nonperturbative analytic model gives an accurate description of the strong coupling across all scales, up to the highest available data.

We determine the possible trajectories the Universe may have followed in the quantum chromodynamics (QCD) phase diagram during the QCD epoch. We focus on the roles of chiral symmetry breaking and pion condensation under high imbalances in lepton asymmetry. Adopting the quark-meson model as an effective description of QCD at finite temperature, charge and baryon chemical potentials we show that, for sufficiently large but physically motivated asymmetries, the Universe may have entered the pion condensation phase through a first-order phase transition, followed by a second-order phase transition when exiting it. Such a first-order phase transition represents a new possible source of primordial gravitational waves during the QCD epoch.

We consider the energetics and thermodynamics of spacetimes with no horizons, but endowed with a preferred timelike junction surface. They could arise as a limiting case of the gravastar and other constructions regularizing the interior of the horizon of a black hole, or from the conceptual cutting of a portion of a nonasymptotically flat space and gluing it with flat space. We find that such surfaces can be made to have zero energy, so that the energetics of such spaces is not encumbered by them. They do have a transverse pressure, fixed by the jump in surface gravity. A peculiar matter thermodynamics then follows, with well-defined entropy, temperature, and surface pressure, constrained by specific relations arising from the zero energy condition. This is confirmed by the Euclidean path integral, with the proviso that the Tolman-Ehrenfest temperature should be used. The entropy of the space is then the area of the timelike surface in units of a fundamental area, and the matter temperature is proportional to the transverse pressure and so the jump in surface gravity. However, when the gravitational action is added, the free energy of the surface is also zero. The fact that for some such spaces the temperature comes out negative raises interesting questions regarding the third “law” of thermodynamics.

Neutrino decay to a lighter neutrino and a massless or almost massless (pseudo)scalar Goldstone boson remains of wide interest, as in the search for ultralight dark matter or neutrinoless double beta decay, and for its implications in astrophysics and cosmology. Neutrino interactions with Majorons can affect the dynamics of supernovae and impact the emitted neutrino flux. Using a three-neutrino framework and detailed supernova simulations, we perform the first likelihood analysis of the 24 neutrino events from SN1987A, including nonradiative decay in matter to a massless (pseudo)scalar boson like a Majoron. Focusing on the induced spectral distortions, we present bounds on the neutrino-Majoron couplings, as a function of the lightest neutrino mass, that are either complementary or competitive with current ones.

We compute the next-to-leading-order radiation-reaction modification to the harmonic coordinate quasi-Keplerian parametrization of the binary dynamics, the two bodies undergoing a scattering process. The solution for the radiation-reaction corrections to the orbital parameters is examined both in the time domain (exact results) and in the frequency domain (results presented in the limit of large angular momentum, i.e., as a post-Minkowskian series expansion). The knowledge of the radiation-reaction corrected orbit is a key ingredient for the calculation of the fractional 3.5 post-Newtonian corrections to the radiative losses as well as to the radiative multipole moments needed to build up the waveform at the same accuracy.

Observations of microlensed gravitational waves (GWs) emanated by compact binary coalescences are essential for studying the mass density distribution in the Universe, including black holes and dark matter halos. However, no confident detection of microlensed GWs have been reported to date. There are two important challenges in the identification of microlensed GWs. The first is that the source waveform and lens structure models are not known a priori. The second is that certain classes of unlensed GWs could mimic microlensed GWs, resulting in undesirable false alarms. In this work, we propose to use the Kramers-Kronig (KK) relation for gravitational lensing systems. We argue that such systems are essentially linear response systems obeying causality, where KK relation must hold. The power of this method lies in the fact that microlensed GWs, regardless of the lens structure, must obey KK relation, while unlensed GW events are not in general expected to obey it. This, in principle, allows us to identify microlensed GWs while dismissing microlensing mimickers. We provide the first important steps towards a methodology that exploits KK relation and test its usefulness under idealized conditions.

We investigate the breakdown of the separate-universe approximation during transitions in transient ultraslow-roll inflation by analyzing the evolution of the comoving curvature perturbation $\mathcal{R}$ and its conjugate momentum $\mathrm{\Pi }$. It is well known that spatial gradient terms lead to a failure of this approximation, particularly at the transition from slow- to ultraslow-roll phase. We show that a similar breakdown also occurs during the second transition back to slow roll when considering the evolution of $\mathrm{\Pi }$. Interestingly, while the homogeneous solution for $\mathrm{\Pi }$ accurately captures the dynamics across the first transition, it is the homogeneous solution for $\mathcal{R}$ that becomes valid across the second. Furthermore, we demonstrate that the spatial curvature term introduced in the extended $\delta N$ formalism of Artigas et al. [Phys. Rev. Lett. 134, 221001 (2025).] can be interpreted as arising from the contribution of $\mathrm{\Pi }$ to the energy density perturbation. Importantly, this modification of the local Hubble parameter is valid only when the first slow-roll parameter is both small and strictly constant.

In this study, we employ a two-step method to analyze models of holographic dark energy (HDE) and interacting holographic dark energy (IHDE), incorporating three distinct dark energy (DE)-dark matter (DM) interaction terms. First, using the latest background dataset, we conduct a Markov chain Monte Carlo analysis to constrain the free parameters of the models. Then, we assess the models against each other using the key background parameters and compare them to the $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) standard model. Our results show that at high redshifts, the equation of state parameter related to the models for both homogeneous and clustered DE cases falls within the quintessence region. However, as we approach the present time, all models except HDE transition into the phantom region, and two models cross the phantom line earlier than others. In the next step, we focus on the evolution of perturbations in DE and DM. Using background and growth rate data, we constrain parameters including ${\sigma }_8$. We then investigate the evolution of the growth rate of matter perturbations, $f{\sigma }_8(z)$, and its deviation, $\mathrm{\Delta }f{\sigma }_8$, from the $\mathrm{\Lambda }\mathrm{CDM}$ model. The HDE model shows the best agreement with observational data, while other models predict varying growth rates compared to $\mathrm{\Lambda }\mathrm{CDM}$. Finally, we demonstrate through Akaike and Bayesian information criteria analysis that the compatibility of models with observational data depends on the type of data used, the DE-DM interaction term, and the assumptions regarding DE homogeneity and clustering. Our results suggest that homogeneous DE models yield more agreement with observational data than clustered DE models.

To maximize the amount of information extracted from cosmological datasets, simulations that accurately represent these observations are necessary. However, traditional simulations that evolve particles under gravity by estimating particle-particle interactions ($N$-body simulations) are computationally expensive and prohibitive to scale to the large volumes and resolutions necessary for the upcoming datasets. Moreover, modeling the distribution of galaxies typically involves identifying virialized dark matter halos, which is also a time- and memory-consuming process for large $N$-body simulations, further exacerbating the computational cost. In this study, we introduce CHARM, a novel method for creating mock halo catalogs by matching the spatial, mass, and velocity statistics of halos directly from the large-scale distribution of the dark matter density field. We develop multistage neural spline flow-based networks to learn this mapping at redshift $z=0.5$ directly with computationally cheaper low-resolution particle mesh simulations instead of relying on the high-resolution $N$-body simulations. We show that the mock halo catalogs and painted galaxy catalogs have the same statistical properties as obtained from $N$-body simulations in both real space and redshift space. Finally, we use these mock catalogs for cosmological inference using redshift-space galaxy power spectrum, bispectrum, and wavelet-based statistics using simulation-based inference, performing the first inference with accelerated forward model simulations and finding unbiased cosmological constraints with well-calibrated posteriors.

Significant uncertainties persist in describing the internal structure and equation of state of hyperon stars due to the limited understanding of the mechanisms underlying hyperon interactions. Constraining the interaction parameter space through a combination of the latest astronomical observations and hypernuclear physics experiments is therefore essential. In this study, we incorporate experimental constraints from $\mathrm{\Xi }$ hypernuclear physics on top of $\mathrm{\Lambda }$ hyperons considered in [Astrophys. J. 942, 55 (2023)]. Specifically, based on updated measurements of hyperon separation energies from $\mathrm{\Xi }$ hypernuclear experiments, three sets of $\mathrm{\Xi }N$ effective interactions are constructed and a linear correlation between their scalar ($\sigma$) and vector ($\omega$) coupling strength ratios is proposed as a constraint derived from $\mathrm{\Xi }$ hypernuclear physics. Together with experimental correlations and astronomical observational data, four types of analyses are performed to constrain hyperon-nucleon interactions and the properties of hyperon stars. Compared to $\mathrm{\Lambda }$ hyperons, the shallower potential and larger rest mass of $\mathrm{\Xi }$ hyperons lead to a higher threshold for their appearance in hyperon stars. As a result, the parameter space for the hyperon-nucleon interaction of $\mathrm{\Xi }$ hyperons remains more loosely constrained. In particular, compared to the vector $\omega$ meson-hyperon coupling, the introduction of linear correlations in hypernuclear physics imposes a more substantial constraint on the scalar $\sigma$ meson-hyperon coupling, significantly enhancing its coupling strength and ensuring the stiffness of the equation of state, highlighting the crucial role of hypernuclear studies in solving the hyperon puzzle. Consequently, a maximum mass of around $2M_{\odot }$ can be achieved with all four interactions considered in this study under the combined constraints from astronomical observations and nuclear physics. Moreover, the uncertainties in both the fractions and the threshold densities at which hyperons appear inside neutron stars are notably reduced, along with those in the mass-radius predictions. The corresponding uncertainties associated with hyperon-nucleon contributions and the impact on structural properties of hypernuclei are also reliably estimated.

Primordial black holes (PBHs) arise from the collapse of density perturbations in the early Universe and serve as a dark matter (DM) candidate and a probe of fundamental physics. There remains an unconstrained “asteroid-mass” window where PBHs of masses ${10}^{17}\mathrm{g}\lesssim M\lesssim {10}^{23}\mathrm{g}$ could comprise up to 100% of the dark matter. Current $e^{\pm }$ Hawking radiation constraints on the DM fraction of PBHs are set by comparing observed spatial- and time-integrated cosmic ray flux measurements with predicted Hawking emission fluxes from the galactic DM halo. These constraints depend on cosmic ray production and propagation models, the galactic DM density distribution, and the PBH mass function. We propose to mitigate these model dependencies by developing a new local, time-dependent Hawking radiation signature to detect low-mass PBHs transiting through the inner Solar System. We calculate transit rates for PBHs that form with initial masses $M\lesssim 5\times {10}^{17}\mathrm{g}$. We then simulate time-dependent positron signals from individual PBH flybys as measured by the Alpha Magnetic Spectrometer (AMS) experiment in low-Earth orbit. We find that AMS is sensitive to PBHs with masses $M\lesssim 2\times {10}^{14}\mathrm{g}$ due to its lower energy threshold of 500 MeV. We demonstrate that a dataset of daily positron fluxes over the energy range 5–500 MeV, with similar levels of precision to the existing AMS data, would enable detection of PBHs drawn from present-day distributions that peak within the asteroid-mass window. Our simulations yield $\mathcal{O}(1)$ detectable PBH transits per year across wide regions of parameter space, which may be used to constrain PBH mass functions. This technique could be extended to detect $\gamma$-ray and x-ray Hawking emission to probe further into the asteroid-mass window.

Given the compact binary evolution problem of numerical relativity, in the finite-difference, block-based, adaptive mesh refinement context, choices must be made on how evolved fields are to be discretized. In gr-athena++, the space-time solver was previously fixed to be vertex centered. Here, our recent extensions to a cell-centered treatment are described. Simplifications in the handling of variables during the treatment of general relativistic magneto-hydrodynamical (GRMHD) evolution are found. A novelty is that performance comparison for the two choices of grid sampling is made within a single code base. In the case of a binary black hole inspiral-merger problem, by evolving geometric fields on vertex centers, an average $\sim 20\%$ speed increase is observed, when compared against cell-centered sampling. The opposite occurs in the GRMHD setting. A binary neutron star inspiral-merger-collapse problem, representative of typical production simulations, is considered. We find that cell-centered sampling for the space-time solver improves performance, by a similar factor.

Gravitational wave detectors are observing an increasing number of binary black hole (BBH) mergers, revealing a bimodal mass distribution of BBHs, which hints at diverse formation histories for these systems. Using the rapid binary population synthesis code mobse, we simulate a series of population synthesis models that include chemically homogeneous evolution (CHE). By considering metallicity-specific star formation and selection effects, we compare the intrinsic merger rates and detection rates of each model with observations. We find that the observed peaks in the mass distribution of merging BBHs at the low-mass end ($10M_{\odot }$) and the high-mass end ($35M_{\odot }$) are contributed by the common envelope channel or stable mass transfer channel (depending on the stability criteria for mass transfer) and the CHE channel, respectively, in our model. The merger rates and detection rates predicted by our model exhibit significant sensitivity to the choice of physical parameters. Different models predict merger rates ranging from 15.4 to $96.7{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$ at redshift $z=0.2$, and detection rates ranging from 22.2 to $148.3{\mathrm{yr}}^{-1}$ under the assumption of a detectable redshift range of $z\le 1.0$.