The causal structure of the recent loop quantum gravity black hole collapse model [J. G. Kelly et al., Classical Quant. Grav. 38, 04LT01 (2021)] is analyzed. As the spacetime is only approximately diffeomorphism invariant up to powers of $\hslash$, it is not straightforwardly possible to find global conformally compactified coordinates or to construct the Penrose diagram. Therefore, radial ingoing and outgoing light rays are studied to extract the causal features and sketch a causal diagram. It was found that the eternal metric [J. G. Kelly et al., Phys. Rev. D 102, 106024 (2020)], which is the vacuum solution of the collapse model, has a causal horizon. However, in the collapsing case, light rays travel through matter to causally connect the regions inside and outside the horizon—the causal horizon is not present in the collapsing scenario. It is worked out that this is related to the shock wave and spacetime discontinuity, which allows matter traveling superluminally along a spacelike trajectory from the vacuum point of view, but remaining timelike from the matter perspective. The final causal diagram is a compact patch of a Reissner-Nordström causal diagram. Further, possibilities of a continuous matter collapse with only timelike evolution are studied. It is found that the time-reversed vacuum metric is also a solution of the dynamical equations, and a once continuously differentiable matching of the vacuum spacetime across the minimal radius is possible. This allows an everywhere continuous and timelike collapse process at the cost of an infinite extended causal diagram. This solution is part of an infinitely extended eternal black hole solution with a bounce, whose global extension is constructed. Due to the analysis of radial light rays, it is possible to sketch causal diagrams of these spacetimes.

Recently, the image of a Schwarzschild black hole with an accretion disk has been revisited, and it showed that the “photon ring,” defined as highly bent light rays that intersect the disk plane more than twice, is extremely narrow and makes a negligible contribution to the total brightness. In this paper, we investigate the observational appearance of an optically and geometrically thin accretion disk around a hairy black hole in an Einstein-Maxwell-scalar model. Intriguingly, we find that in a certain parameter regime, due to an extra maximum or an “anklelike” structure in the effective potential for photons, the photon ring can be remarkably wide, thus making a notable contribution to the flux of the observed image. In particular, there appears a wide and bright annulus, which comprises multiple concentric bright thin rings with different luminosity, in the high resolution image.

We use covariant phase space methods to study the metric and tetrad formulations of general relativity in a manifold with boundary and compare the results obtained in both approaches. Proving their equivalence has been a long-lasting problem that we solve here by using the cohomological approach provided by the relative bicomplex framework. This setting provides a clean and ambiguity-free way to describe the solution spaces and associated symplectic structures. We also compute several relevant charges in both schemes and show that they are equivalent, as expected.

Cosmic strings may have formed in the early Universe due to the Kibble mechanism. While string networks are usually modeled as being of Nambu-Goto type, this description is understood to be a convenient approximation, which neglects the typically expected presence of additional degrees of freedom on the string world sheet. Previous simulations of cosmic strings in expanding universes have established beyond doubt the existence of a significant amount of short-wavelength propagation modes (commonly called wiggles) on the strings, and a wiggly string extension of the canonical velocity-dependent one-scale model has been recently developed. Here we improve the physical interpretation of this model, by studying the possible asymptotic scaling solutions of this model, and in particular how they are affected by the expansion of the Universe and the available energy loss or transfer mechanisms—e.g., the production of loops and wiggles. In addition to the Nambu-Goto solution, to which the wiggly model reduces in the appropriate limit, we find that there are also solutions where the amount of wiggliness can grow as the network evolves or, for specific expansion rates, become a constant. Our results show that full scaling of the network, including the wiggliness, is much more likely in the matter era than in the radiation era, which is in agreement with numerical simulation results.

We study the entanglement production for Dirac and Klein-Gordon fields in an expanding spacetime characterized by the presence of torsion. Torsion is here considered according to the Einstein-Cartan theory with a conformally flat Friedmann-Robertson-Walker spacetime. In this framework, torsion is seen as an external field, fulfilling precise constraints gotten directly from the cosmological principle. For Dirac field, we find that torsion increases the amount of entanglement. This turns out to be particularly evident for small values of particle momentum. We discuss the role of Pauli exclusion principle in view of our results, and, in particular, we propose an interpretation of the two maxima that occur for the entanglement entropy in the presence of torsion. For Klein-Gordon field, and differently from the Dirac case, the model can be exactly solved in some cases. We discuss, in particular, conformal coupling to the scalar curvature and the special case of antisymmetric torsion. Again, we show how torsion affects the amount of entanglement, providing a robust physical motivation behind the increase or decrease of entanglement entropy. A direct comparison of our findings is also discussed in view of previous results derived in absence of torsion. To this end, we give prominence on how our expectations would change in terms of the coupling between torsion and the scale factor for both Dirac and Klein-Gordon fields.

Energetics of the ultrahigh-energy cosmic rays (UHECRs) generated in the Universe is crucial for pinning down their candidate sources. Using the recent Auger data on UHECR spectra, we calculate the UHECR energy generation rate density for different species of nuclei at the injection, considering intermediate and heavy nuclei as well as protons, through scanning over source parameters on the spectral index, maximum energy and redshift evolution. We find the resulting UHECR energy generation rate density to be $\approx (0.2–2)\times {10}^{44}\mathrm{erg}{\mathrm{Mpc}}^{-3}{\mathrm{yr}}^{-1}$ at ${10}^{19.5}\mathrm{eV}$ with a nontrivial dependence on the spectral index. Nuclei other than protons and irons favor hard spectral indices at the injection, and the required value of energy budget is smaller for intermediate nuclei. Given significant uncertainties in hadronic interaction models and astrophysical models for the Galactic-extragalactic transition, our results can be regarded as conservative. The composition data on $X_{\max }$ give additional constraints, but the results are consistent within the model uncertainties.

A hidden Markov model (HMM) solved recursively by the Viterbi algorithm can be configured to search for persistent, quasimonochromatic gravitational radiation from an isolated or accreting neutron star, whose rotational frequency is unknown and wanders stochastically. Here an existing HMM analysis pipeline is generalized to track rotational phase and frequency simultaneously, by modeling the intrastep rotational evolution according to a phase-wrapped Ornstein-Uhlenbeck process, and by calculating the emission probability using a phase-sensitive version of the Bayesian matched filter known as the $\mathcal{B}$-statistic, which is more sensitive than its predecessors. The generalized algorithm tracks signals from isolated and binary sources with characteristic wave strain $h_0\ge 1.3\times {10}^{-26}$ in Gaussian noise with amplitude spectral density $4\times {10}^{-24}{\mathrm{Hz}}^{-1/2}$, for a simulated observation composed of $N_T=37$ data segments, each $T_{\text{drift}}=10\mathrm{days}$ long, the typical duration of a search for the low-mass x-ray binary (LMXB) Sco $\mathrm{X}-1$ with the Laser Interferometer Gravitational Wave Observatory (LIGO). It is equally sensitive to isolated and binary sources and $\approx 1.5$ times more sensitive than the previous pipeline, which achieves $h_0\ge 2.0\times {10}^{-26}$ for a comparable search. Receiver operating characteristic curves (to demonstrate a recipe for setting detection thresholds) and errors in the recovered parameters are presented for a range of practical $h_0$ and $N_T$ values. The generalized algorithm successfully detects every available synthetic signal in Stage I of the Sco $\mathrm{X}-1$ Mock Data Challenge convened by the LIGO Scientific Collaboration, recovering the frequency and orbital semimajor axis with accuracies of better than $9.5\times {10}^{-7}\mathrm{Hz}$ (one part in $\sim {10}^8$) and $1.6\times {10}^{-3}\text{lt}\mathrm{s}$ (one part in $\sim {10}^3$) respectively. The Viterbi solver runs in $\approx 2\times {10}^3$ CPU-hr for an isolated source and $\sim {10}^5$ CPU-hr for a LMXB source in a typical, broadband (0.5-kHz) search, i.e., $\lesssim 10$ times slower than the previous pipeline.

We investigate the $\sin ({\varphi }_{\mathrm{\Lambda }}-{\varphi }_S)$ single-spin asymmetry in the transversely polarized $\mathrm{\Lambda }$ production in semi-inclusive deeply inelastic scattering process within the transverse momentum dependent (TMD) factorization. The asymmetry is contributed by the convolution of the polarizing TMD fragmentation function $D_{1T}^{\perp }$ of the $\mathrm{\Lambda }$ hyperon and the unpolarized TMD distribution function $f_1$ of the proton target. We adopt the spectator diquark model result and the available parametrization for $D_{1T}^{\perp ,{\mathrm{\Lambda }}^{\uparrow }/q}$ to numerically estimate the $\sin ({\varphi }_{\mathrm{\Lambda }}-{\varphi }_S)$ asymmetry at the kinematical region of the electron ion collider (EIC). To implement the TMD evolution formalism, we apply two different parametrizations on the nonperturbative Sudakov form factors associated with the distribution function of the proton and the fragmentation function of the $\mathrm{\Lambda }$. It is found that the two sets of $D_{1T}^{\perp ,{\mathrm{\Lambda }}^{\uparrow }/q}$ lead to different $\sin ({\varphi }_{\mathrm{\Lambda }}-{\varphi }_S)$ asymmetry, particularly in sign. We also discuss the impact of the assumptions and approximations applied in the calculations, which may bring large uncertainties to the results in the EIC. Future measurements on the $\sin ({\varphi }_{\mathrm{\Lambda }}-{\varphi }_S)$ asymmetry with high precision at the EIC can provide important cross checks on the available $\mathrm{\Lambda }$ polarizing fragmentation functions, as well as constrain them more stringently.

We study the relationship between mixed state entanglement and thermal phase transitions. As a typical example, we compute the holographic entanglement entropy (HEE), holographic mutual information (MI), and the holographic entanglement wedge minimum cross section (EWCS) over the superconductivity phase transition. We find that HEE, MI, and EWCS can all diagnose the superconducting phase transition. They are continuous at the critical point, but their first derivative with respect to temperature is discontinuous. MI decreases with increasing temperature and exhibits a convex behavior, while HEE increases with increasing temperature and exhibits a concave behavior. However, EWCS can exhibit either the same or the opposite behavior as MI, depending on the size of the specific configuration. These results show that EWCS captures more abundant information than HEE and MI. We also provide a new algorithm to compute the EWCS for general configurations.

We study the Schwinger process in a uniform non-Abelian electric field using a dynamical approach in which we evolve an initial quantum state for gluonic excitations. We evaluate the spectral energy density and number density in the excitations as functions of time. The total energy density has an ultraviolet divergence which we argue gets tamed due to asymptotic freedom, leading to $g^4{E}^4{t}^4$ growth, where $g$ is the coupling and $E$ the electric field strength. We also find an infrared divergence in the number density of excitations whose resolution requires an effect such as confinement.

We find the response function and the transition rate for an Unruh-DeWitt detector interacting with a conformal scalar field in global two-dimensional anti–de Sitter (AdS) spacetime with different boundary conditions at its conformal infinities. We calculate the particle energy spectrum as seen by subcritical accelerated detectors and discuss how it depends on the choice of the boundary condition. We show that, despite this nontrivial dependence on the boundary conditions, the limit when the AdS length scale tends to zero is well defined and leads to the well-known results of $1+1$ Minkowski space. One can thus interpret the AdS energy scale as a natural regulator for the well-known infrared ambiguity of massless scalar fields in $1+1$ Minkowski spacetime.

Although black holes are an integral part of the standard model of astrophysics and cosmology, their existence poses some serious fundamental problems. In recent years, several horizonless compact object models were proposed to address those issues. As gravitational-wave detectors observe more and more merger events with large signal-to-noise ratios, gravitational-wave spectroscopy could hold the key to uncovering the existence of these objects. This is because the late-time ringdown signals of horizonless compact objects differ from those of black holes. In this paper, we study the ringdown properties of charged compact objects and compare them with those obtained in the black hole scenario. Since the internal structure and equation of state of these compact objects are largely unknown, we employ the membrane paradigm to obtain appropriate boundary conditions for the perturbations of these objects. This model can describe the ringdown properties of a large variety of compact objects.

The event horizon of a black hole in general relativity absorbs all infalling radiation. Any observation of the contrary would immediately challenge the expectation that astrophysical black holes are described by the vacuum Kerr geometry. If a putative black hole does reflect part of the ingoing radiation, its quasinormal mode structure is drastically altered. Low frequency modes can be introduced that are resonantly excited during the inspiral of a binary system. We study the resulting phase shift of the gravitational wave signal. Building on neutron star results, we obtain a model-independent expression for the phase shift that depends only on quasinormal modes and Love numbers of the compact object. We find that the phase shift might be detectable with Einstein Telescope for asymmetric binaries in high signal-to-noise events ($\sim {10}^3$), but by far cannot explore the Planck scale.

There are three types of dynamical equations for a post-Newtonian Lagrangian formulation of three black holes. They are approximate post-Newtonian Lagrangian equations of motion, coherent post-Newtonian Lagrangian equations of motion, and post-Newtonian Hamiltonian canonical equations. The energy integral is conserved approximately in the first model, but exactly in the second and third models. The three formulations are not exactly equivalent, while they are approximately related in the orbital dynamical behavior. Figure-eight orbits in the three problems may have different initial conditions. Imai, Chiba, and Asada adjusted the initial velocities satisfying the figure-eight orbits for given initial positions in the first model. The initial velocities are still suitable for the figure-eight orbits of the latter two models when the initial separations of the first body are enough large. However, they are not generally applicable to relatively small initial separations in the three systems. We can obtain the desired initial velocities by scanning the initial velocities of the third body in the three systems. The figure-eight orbits in these models exhibit sensitive dependence on the scanned initial velocities for smaller initial separations. Although the differences in the scanned initial velocities among the three problems are minor for the small initial separations, the scanned initial velocities for any one of the three systems do not yield the figure-eight orbits for the other two systems.

We propose that the only possible realization of a magnetic pole is emergent, it being an artifact of a torsion defect in a curved spacetime. This special phase is built upon a class of degenerate metric spacetime solutions of first-order gravity in vacuum. The (apparent) magnetic charge is shown to have a topological origin, given by a lower-dimensional counterpart of the Nieh-Yan invariant. At the invertible metric phase at a distance, this topological charge gets reflected as the (magnetic) Reissner-Nordström charge to an asymptotic observer, even though the monopole defect itself remains hidden.

Future detectors such as the Laser Interferometer Space Antenna promise signal-to-noise ratios potentially in the thousands and data containing simultaneous signals. Accurate numerical relativity waveforms will be essential to maximizing the science return. A question of interest to the broad gravitational wave community is: Are the numerical relativity codes ready to face this challenge? Towards answering this question, we provide a new criteria to identify the minimum resolution a simulation must have as a function of signal-to-noise ratio in order for the numerical relativity waveform to be indistinguishable from a true signal. This criteria can be applied to any finite-differencing numerical relativity code with multiple simulations of differing resolutions for the desired binary parameters and waveform length. We apply this criteria to binary systems of interest with the fourth-order MAYA code to obtain the first estimate of the minimum resolution a simulation must have to be prepared for next generation detectors.

Using concomitantly the generalized second law of black hole thermodynamics and the holographic Bekenstein entropy bound embellished by loop quantum gravity corrections to quantum black hole entropy, we show that the boundary cross sectional area of the postmerger remnant formed from the compact binary merger in gravitational wave detection experiments like GW150914 et seq., by the LIGO-VIRGO collaboration, is bounded from below. This lower bound is more general than the bound obtained from application of Hawking’s classical area theorem for black holes, since it does not depend on whether the inspiralling compact binary pair or the postmerger remnant consists of black holes or other exotic compact objects. The derivation of the bound entails an estimate of the entropy of the gravitational waves emitted during the binary merger, which adapts to gravitational waves an extant formalism proposed originally for particle ensembles. The results for the minimal cross sectional area of the merger remnant due to binary compact mergers observed recently by the LIGO-VIRGO collaboration are discussed. While accurate measurement of the mass of the remnant for the binary neutron star merger GW170817 remains a challenge, we provide a proof of principle that for binary neutron star mergers our lower bound on the cross sectional area of the remnant provides an alternative approach to probe the validity of neutron star equations of state, independent of the tidal deformabilities of the components.

We perform the first test of dark matter (DM) stress-energy evolution through cosmic history, using cosmic microwave background measurements supplemented with baryon acoustic oscillation data and the Hubble Space Telescope key project data. We constrain the DM equation of state (EoS) in 8 redshift bins, and its sound speed and (shear) viscosity in 9 redshift bins, finding no convincing evidence for non-$\mathrm{\Lambda }\mathrm{CDM}$ values in any of the redshift bins. Despite this enlarged parameter space, the sound speed and viscosity are constrained relatively well at late times (due to the inclusion of CMB lensing), whereas the EoS is most strongly constrained around recombination. These results constrain for the first time the level of “coldness” required of DM across various cosmological epochs at both the background and perturbative levels. We show that simultaneously allowing time dependence for both the EoS and sound speed parameters shifts the posterior of the DM abundance before recombination to a higher value, while keeping the present day DM abundance similar to the $\mathrm{\Lambda }\mathrm{CDM}$ value. This shifts the posterior for the present day Hubble constant compared to $\mathrm{\Lambda }\mathrm{CDM}$, suggesting that DM with time-dependent parameters is well-suited to explore possible solutions to persistent tensions within the $\mathrm{\Lambda }\mathrm{CDM}$ model. We perform a detailed comparison with our previous study involving a vanishing sound speed and viscosity using the same datasets in order to explain the physical mechanism behind these shifts.

In standard stellar evolution, stars with masses ranging from approximately $150M_{\odot }$ to $240M_{\odot }$ are expected to evolve to a pair-instability supernova with no black hole (BH) remnant. This evolutionary behavior leads to a predicted gap in the black hole mass function from approximately $50M_{\odot }$ to $140M_{\odot }$. Yet the LIGO and Virgo collaborations [Phys. Rev. Lett. 125, 101102 (2020)] recently discovered black holes with masses of $66M_{\odot }$ and $85M_{\odot }$ in the gravitational-wave event GW190521. We propose a new method to populate the BH mass gap. If an energy source is added throughout the star in addition to nuclear fusion, it is possible for the altered evolution to avoid the complete destruction of a pair-instability supernova, and instead a BH remnant is left behind. An example of an extra energy source is dark matter annihilation within the star, but our results hold more generally. We show this phenomenon by exploring the effect of adding an energy source independent of temperature and density to a $180M_{\odot }$ star, using the mesa one-dimensional stellar evolution software. $\text{If}\sim 50\%$ of the star’s energy is due to this new source, the star is capable of avoiding the pair instability entirely, reaching a presupernova mass of $119M_{\odot }$ before evolving into a BH remnant in the mass gap.