We study electromagnetic and gravitational properties of anti–de Seitter (AdS) black shells (also referred to as AdS black bubbles)—a class of quantum gravity motivated black hole mimickers, that in the classical limit are described as ultracompact shells of matter. We find that their electromagnetic properties are remarkably similar to black holes. We then discuss the extent to which these objects are distinguishable from black holes, both for intrinsic interest within the black shell model, and as a guide for similar efforts in other subclasses of exotic compact objects (ECOs). We study photon rings and lensing band characteristics, relevant for very large baseline interferometry (VLBI) observations, as well as gravitational wave observables—quasinormal modes in the eikonal limit and the static tidal Love number for nonspinning shells—relevant for ongoing and upcoming gravitational wave observations.

We study equivariant wave maps from the $2+1$ dimensional wormhole to the 2-sphere. This model has explicit harmonic map solutions which, in suitable coordinates, have the form of the sine-Gordon kinks/antikinks. We conjecture that there exist asymptotically static chains of $N\ge 2$ alternating kinks and antikinks whose subsequent rates of expansion increase in geometric progression as $t\to \infty$. Our argument employs the method of collective coordinates to derive effective finite-dimensional ODE models for the asymptotic dynamics of $N$-chains. For $N=2$, 3 the predictions of these effective models are verified by direct PDE computations which demonstrate that the $N$-chains lie at the threshold of kink-antikink annihilation.

Gravitational wave detection has opened up new avenues for exploring and understanding some of the fundamental principles of the Universe. The optimal method for detecting modeled gravitational-wave events involves template-based matched filtering and performing a multidetector coincidence search in the resulting signal-to-noise ratio time series. In recent years, advancements in machine learning and deep learning have led to a flurry of research into using these techniques to replace matched filtering searches and for efficient and robust parameter estimation of the gravitational wave sources. This paper presents a feasibility study for a novel approach to detecting binary black hole gravitational wave signals, which utilizes deep learning techniques on the signal-to-noise ratio time series produced from matched filtering. We show that a deep-learning search can efficiently detect binary black hole gravitational waves from the signal-to-noise ratio time series in simulated Gaussian noise with simulated transient glitches. Furthermore, our search method can outperform a maximum SNR-based matched filtering search on simulated data of the Hanford and Livingston LIGO detectors in the presence of glitches. Lastly, since we are building upon the foundations of a matched filtering search pipeline, we can extract estimates for the signal-to-noise ratio and detector frame chirp mass of a gravitational wave event with similar accuracy as existing pipelines.

Gravitational wave (GW) observations from extreme-mass-ratio inspirals (EMRIs) are powerful tools for testing general relativity (GR). However, systematic errors arising from waveform models could potentially lead to incorrect scientific conclusions. These errors can be divided into two main categories: fundamental bias (due to limitations in the validity of the Einstein field equations) and modeling error (due to inaccuracies in waveform templates). Using Bayesian inference, we investigate the impact of these systematic errors on tests of GR. Regarding fundamental bias, we find that at low signal-to-noise ratios (SNR), there is a risk of misidentifying a non-GR EMRI signal as a GR-EMRI one, and vice versa. However, this risk diminishes as the SNR increases to around 40 or higher. Additionally, modeling errors might reduce the SNR of detected EMRI signals and could be misinterpreted as deviations from GR, leading Bayesian inference to favor non-GR scenarios, especially at high SNR. We emphasize the importance of developing sufficiently accurate waveform templates based on alternative gravity theories for testing GR.

Among the models addressing the Hubble tension, those introducing a dynamical dark component around recombination have been the most promising thus far. Their study has highlighted that, in fact, cosmic microwave background (CMB) and baryon acoustic oscillation observations can allow for such components before and near recombination. The new dynamical degree of freedom can be early dark energy (EDE) or early modified gravity depending on its coupling to gravity. We study a new model, $\mathcal{G}\mathrm{EDE}$, featuring the cubic Galileon operator $X\square \varphi$ and test it against the most recent Planck public release 4 CMB and Cepheid calibrated $\mathrm{Pantheon}+$ type Ia supernovae data. Thanks to the kinetic braiding effects, $\mathcal{G}\mathrm{EDE}$ gives a better fit to the data, with a higher $H_0$, and is preferred over the canonical EDE with a Bayes factor $\ln B=0.9$, despite introducing one more parameter. This calls for further explorations of modified gravity near and before last scattering. To facilitate these, we introduce a substantial extension of the cosmological code eftcamb that allows one to fully evolve the background and linear dynamics of any covariant theory, oscillatory or not, belonging to the Horndeski class.

Theoretical analysis of the interaction between superfluid dark matter and rotating supermassive black holes offers a promising framework for probing quantum effects in ultralight dark matter and its role in galactic structure. We study how black hole rotation influences the state of ultralight bosonic dark matter, focusing on the stability and dynamics of vortex lines. The gravitational effects of both dark matter and the black hole on the physical properties of these vortex lines, including their precession around the black hole, are analyzed.

WIMP-type dark matter may have additional interactions that break baryon number, leading to induced nucleon decays which are subject to direct experimental constraints from proton decay experiments. In this work, we analyze the possibility of continuous baryon destruction, deriving strong limits from the dark matter accumulating inside old neutron stars, as such a process leads to excess heat generation. We construct the simplest particle dark matter model that breaks the baryon and lepton numbers separately but conserves $B-L$. Virtual exchange by DM particles in this model results in dinucleon decay via $nn\to n\overline{\nu }$ and $np\to n{e}^{+}$ processes.

In this study, we calculate the $CP$-averaged branching ratios and the direct $CP$-violating asymmetries of the three body decays $B^0\to T(\pi \pi ,K\overline{K},\pi \eta )$ in the perturbative QCD approach, where $T$ denotes tensor mesons $a_2(1320)$, $K_2^{*}(1430)$, $f_2(1270)$, and $f_2^{\prime }(1525)$, and the daughter branching is $f_0(980)\to \pi \pi ,K\overline{K}$,$f_0(500)\to \pi \pi$, $a_0(980)\to K\overline{K},\pi \eta$. By introducing two-meson distribution amplitudes parametrized by the timelike form factors, the three-body decay is simplified to quasi-two-body decay. The scalar mesons $a_0(980)$, $f_0(980)$, and $f_0(500)$ are regarded as the lowest-lying $q\overline{q}$ state, considering the effect of mixing angle $\theta$ between $f_0(980)$ and $f_0(500)$, $\varphi$ between $f_2(1270)$ and $f_2^{\prime }(1525)$ on our calculations. We found the following. (a) With $\theta =135°$, we evaluate the branching fractions of $B^0\to K_2^{*}(1430{)}^0{f}_0(980)[\to {\pi }^{+}{\pi }^{-}]$ to be $4.30\times {10}^{-6}$,then under the narrow-width approximation we extract the branching fraction of the decay$B^0\to K_2^{*}(1430{)}^0{f}_0(980)$ to be $8.78\times {10}^{-6}$, which is consistent with the current experimental data well. (b) The decay rates for the considered decay modes are generally in the order of ${10}^{-8}$ to ${10}^{-5}$. (c) The branching fractions are sensitive to the $\theta$, and the opposite is true for $\varphi$. The $\varphi$ is really small, so the decay branching ratio has only little change, except for some decays involving $f_2^{\prime }(1525)$. (d) The $\theta$ and $\varphi$ can bring remarkable change to the direct $CP$ asymmetries of pure penguin processes so that it is not 0. (e) We calculate the relative partial decay widths $\mathrm{\Gamma }(a_0\to K\overline{K})/\mathrm{\Gamma }(a_0\to \pi \eta )$ and the ratio $\mathcal{B}(f_0\to K^{+}{K}^{-})/\mathcal{B}(f_0\to {\pi }^{+}{\pi }^{-})$, which are in agreement with the existing experimental values. Our results can help one to understand the internal structure of scalar mesons and the nature of tensor mesons and can be tested by future experiments.

A measurement of the time-dependent ratio of the $D^0\to K^{+}{\pi }^{-}$ to ${\overline{D}}^0\to K^{+}{\pi }^{-}$ decay rates is reported. The analysis uses a sample of proton-proton collisions corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$ recorded by the LHCb experiment from 2015 through 2018 at a center-of-mass energy of 13 TeV. The $D^0$ meson is required to originate from a $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$ decay, such that its flavor at production is inferred from the charge of the accompanying pion. The measurement is performed simultaneously for the $K^{+}$ ${\pi }^{-}$ and $K^{-}$ ${\pi }^{+}$ final states, allowing both mixing and $CP$-violation parameters to be determined. The value of the ratio of the decay rates at production is determined to be $R_{K\pi }=(343.1\pm 2.0)\times {10}^{-5}$. The mixing parameters are measured to be $c_{K\pi }=(51.4\pm 3.5)\times {10}^{-4}$ and $c_{K\pi }^{\prime }=(13\pm 4)\times {10}^{-6}$, where $\sqrt{R_{K\pi }}{c}_{K\pi }$ is the linear coefficient of the expansion of the ratio as a function of decay time in units of the $D^0$ lifetime, and $c_{K\pi }^{\prime }$ is the quadratic coefficient, both averaged between the $K^{+}$ ${\pi }^{-}$ and $K^{-}$ ${\pi }^{+}$ final states. The precision is improved relative to the previous best measurement by approximately 60%. No evidence for $CP$ violation is found.

In gauge theories, the running of the anomalous dimension of a fermion bilinear operator is believed to lead to chiral symmetry breaking when $\gamma =1$. Naïvely using perturbative results to judge when $\gamma =1$ leads to the possibility of large dynamically generated mass hierarchies in models with both two-index symmetric representation and fundamental representation fermions. In this paper we study a holographic model of this physics to predict the separation in scales in the meson spectrum. We study $\mathrm{SU}(N_c)$ theories at different $N_c$ with one flavor of two-index symmetric representation as a function of the number of fundamental fermion flavors $N_f^F$. The largest hierarchy we find is for $N_c=7$ and $N_f^F=22$ where the rho mesons made of the two different representations are separated in scale by a factor of 13. For general $N_c$ the hierarchy can be made greater than 7 by tuning $N_f^F$. We display the hierarchy as a function of $N_f^F$ and investigate the quark mass dependence. These predictions do depend on the extrapolation of $\gamma$ from the perturbative regime—even in a pessimistic scenario, a distinct gap can be achieved.

Numerical relativity simulations of merging black holes provide the most accurate description of the binary dynamics and the emitted gravitational wave signal. However, practical considerations such as imperfect initial data and initial parameters mean that achieving target parameters, such as the orbital eccentricity or the black hole spin directions, at the beginning of the usable part of the simulation is challenging. In this paper, we devise a method to produce simulations with specific target parameters, namely the Keplerian orbital parameters—eccentricity, semimajor axis, mean anomaly—and the black hole spin vectors using SpEC. The method is an extension of the current process for achieving vanishing eccentricity and it is based on a parameter control loop that iteratively numerically evolves the system, fits the orbit with analytical post-Newtonian equations, and calculates updated input parameters. Through SpEC numerical simulations, we demonstrate $\lesssim {10}^{-3}$ and $\mathcal{O}(\mathrm{degree})$ convergence for the orbital eccentricity and the spin directions respectively in $\le 7$ iterations. These tests extend to binaries with mass ratios $q\le 3$, eccentricities $e\le 0.65$, and spin magnitudes $|\chi |\le 0.75$. Our method for controlling the orbital and spin parameters of numerical simulations can be used to produce targeted simulations in sparsely covered regions of the parameter space or study the dynamics of relativistic binaries.

Phase transitions can play an important role in the cosmological constant problem, allowing the underlying vacuum energy, and therefore the value of the cosmological constant, to change. Deep within the core of neutron stars, the local pressure may be sufficiently high to trigger the QCD phase transition, thus generating a shift in the value of the cosmological constant. The gravitational effects of such a transition should then be imprinted on the properties of the star. Working in the framework of general relativity, we provide a new model of the stellar interior, allowing for a QCD and a vacuum energy phase transition. We determine the impact of a vacuum energy jump on mass-radius relations, tidal deformability-radius relations, I-Love-Q relations, and on the combined tidal deformability measured in neutron star binaries.

We constrain anisotropic cosmic birefringence generated at reionization using Planck PR4 polarization data for the first time. Several recent analyses of WMAP and Planck polarization data have found a tantalizing hint of isotropic cosmic birefringence. Ongoing and future cosmic microwave background (CMB) experiments will test isotropic cosmic birefringence by improving the absolute angle calibration and understanding the intrinsic parity-odd power spectrum of the galactic foregrounds. Alternatively, measuring anisotropies in cosmic birefringence and its time evolution is also a key observable to confirm the signal of cosmic birefringence and to investigate its origin. We discuss estimators of anisotropic cosmic birefringence generated at different redshifts. We then estimate anisotropic cosmic birefringence generated at reionization from the PR4 data, showing that the power spectrum is consistent with null. We find that the model proposed by Ferreira et al. is still consistent with the observation. Future full-sky CMB experiments such as LiteBIRD and PICO will help tighten the tomographic constraint to test models of cosmic birefringence.

Massive black-hole binaries will be the loudest sources detectable by the Laser Interferometer Space Antenna (LISA). These systems are predicted to form during the hierarchical assembly of cosmic structures and coalesce by interacting with the surrounding environment. The hardening phase of their orbit is driven by either stars or gas and encodes distinctive features into the binary black holes that can potentially be reconstructed with gravitational-wave observations. We present a Bayesian framework to assess the likelihood of massive mergers being hardened by either gaseous or stellar interactions. We use state-of-the-art astrophysical models tracking the cosmological evolution of massive black-hole binaries and construct a large number of simulated catalogs of sources detectable by LISA. From these, we select a representative catalog and run both a parameter estimation assuming a realistic LISA response as well as a model comparison capturing selection effects. Our results suggest that, at least within the context of the adopted models, future LISA observations can confidently constrain whether stars or gas are responsible for the binary hardening. We stress that accurate astrophysical modeling of the black-hole spins and the inclusion of subdominant emission modes in the adopted signal might be crucial to avoid systematic biases.

Works on the energy extraction from a rotating black hole via magnetic reconnection have attracted more attention in recent years. Discussions on this topic, however, are often based on many simplifications, such as assuming a circularly flowing bulk plasma and a fixed orientation angle. A significant gap remains between theoretical models and the magnetic reconnection occurring in real astrophysical scenarios. In our previous work, we investigated the influence of orientation angle on energy extraction and figured out the differences between the plunging and circularly flowing bulk plasma. We introduced the concept of covering factors to quantify the capability of an accretion system in extracting energy via magnetic reconnection from a rotating black hole. In this work, as an improvement, we extend our discussions by treating the parameters in reconnection models as free parameters, bringing the theoretical model closer to the situations in real astrophysical systems. We employ two reconnection models, in which the geometric index and the guide field fraction are respectively induced. We separately present the dependences of energy extraction on the geometric index and the guide field fraction. More importantly, we propose to define the averaged covering factor weighted by reconnection rate to quantify the overall capability of an accretion system in extracting energy via magnetic reconnection from a rotating black hole, under the assumption that the magnetic reconnection process occurs stochastically and randomly within the ergosphere.

We present a time-domain model for the gravitational waves emitted by equal-mass binary neutron star merger remnants for a fixed equation of state. We construct a large set of numerical relativity simulations for a single equation of state consistent with current constraints, totaling 157 equal-mass binary neutron star merger configurations. The gravitational-wave model is constructed using the supervised learning method of K-nearest neighbor regression. As a first step toward developing a general model with supervised learning methods that accounts for the dependencies on equation of state and the binary masses of the system, we explore the impact of the size of the dataset on the model. We assess the accuracy of the model for a varied dataset size and number density in total binary mass. Specifically, we consider five training sets of $\{20,40,60,80,100\}$ simulations uniformly distributed in total binary mass. We evaluate the resulting models in terms of faithfulness using a test set of 30 additional simulations that are not used during training and which are equidistantly spaced in total binary mass. The models achieve faithfulness with maximum values in the range of 0.980 to 0.995. We assess our models simulating signals observed by the three-detector network of Advanced LIGO-Virgo. We find that all models with training sets of size equal to or larger than 40 achieve an unbiased measurement of the main gravitational-wave frequency. We confirm that our results do not depend qualitatively on the choice of the (fixed) equation of state. We conclude that training sets, with a minimum size of 40 simulations, or a number density of approximately 11 simulations per $0.1M_{\odot }$ of total binary mass, suffice for the construction of faithful templates for the postmerger signal for a single equation of state and equal-mass binaries, and lead to mean faithfulness values of $\mathcal{F}\simeq 0.95$. Our model being based on only one fixed equation of state represents only a first step toward a method that is fully applicable for gravitational-wave parameter estimation. However, our findings are encouraging since we show that our supervised learning model built on a set of simulations for a fixed equation of state successfully recovers the main gravitational-wave features of a simulated signal obtained using another equation of state. This may indicate that the extension of this model to an arbitrary equation of state may actually be achieved with a manageable set of simulations.

Binary neutron star (BNS) mergers can result in the formation of a metastable hypermassive neutron star (HMNS), whose fate is determined primarily by the mass of the constituent neutron stars and the properties of matter at the highest densities reached in such events. Depending on the total mass and the equation of state (EOS), the HMNS can either survive for several milliseconds or promptly collapse into a black hole right after the collision. In this work, we carry out an in-depth study of BNS mergers near the prompt collapse regime. To this end, we perform a set of general-relativistic equal-mass BNS merger simulations using a family of realistic EOSs with different stiffness and that feature a first-order deconfinement phase transition between hadronic and quark matter. Motivated by the simulation results, we give a formal definition for the critical total mass of the binary $M_{\mathrm{crit}}$, above which the collapse happens promptly. We show that the critical mass obeys the relation $M_{\mathrm{crit}}/M_{\mathrm{TOV}}\approx 1.41\pm 0.06$ for the models we study, which links it to the maximum mass $M_{\mathrm{TOV}}$ of static neutron stars, and is in agreement with earlier results for related threshold masses. In addition, we introduce a novel gauge-independent definition for a sequence of threshold masses. These threshold masses are defined in terms of curvature invariants of the Riemann tensor and characterize the development toward a more rapid collapse with increasing binary mass. Using these diagnostics, we find that the amount of matter remaining outside the black hole sharply drops in supercritical equal-mass mergers compared to subcritical ones and is further reduced in mergers where the black hole collapse is induced by the formation of a quark matter core. Since the presence of a substantial amount of baryonic matter is required to drive an observable electromagnetic signal, it follows that $M_{\mathrm{crit}}$ imposes a strict upper limit for the total mass of equal-mass BNS mergers with any detectable electromagnetic counterpart.

We address the issue of cancellation of infrared (IR) divergences at the amplitude level in light front quantum chromodynamics (LFQCD) using the coherent state formalism. We consider the process $e^{+}{e}^{-}\to q\overline{q}g$ up to $\mathcal{O}(g^3)$ in light-cone-time-ordered Hamiltonian perturbation theory and show that IR divergences in S-matrix elements appear due to vanishing energy denominators. We construct the coherent state formalism for LFQCD and explicitly show that these divergences are canceled when a coherent state basis is used for calculating the S-matrix elements.

The relation between the Husain-Kuchař model and some extensions thereof that incorporate a dynamical time variable is explored. To this end, we rely on the geometric approach to the Hamiltonian dynamics of singular (constrained) systems provided by the Gotay-Nester-Hinds method. We also use recent results regarding the derivation of the Ashtekar formulation for Euclidean general relativity without using the time gauge. The insights provided by the geometric approach followed in the paper shed light on several issues related to the dynamics of general relativity and the role of time.