An isolated critical point is a peculiar thermodynamic critical point that occurs in the phase diagram of hyperbolic black holes in $K$th-order Lovelock gravity in higher dimensions (with $K$ odd) for special tuned Lovelock coupling constants. It corresponds to a “merger” of two swallowtails and is characterized by nonstandard critical exponents. Upon employing a recent proposal for assigning a topological charge to thermodynamic critical points, we argue that the isolated critical point offers an interpretation corresponding to the onset of a topological phase transition of a vortex-antivortex pair.

An alternative quantization of the gravitational Hamiltonian constraint of the $k=-1$ Friedmann-Robertson-Walker model is proposed by treating the Euclidean term and the Lorentzian term independently, mimicking the treatment of full-loop quantum gravity. The resulting Hamiltonian constraint operator for the $k=-1$ model with a massless scalar field is successfully constructed, and it is shown to have the corrected classical limit. Compared to the former quantization schemes in the literature where only the Euclidean term is quantized, the new quantum dynamics of the $k=-1$ model with a massless scalar field indicates that the classical big bang singularity is replaced by an asymmetric quantum bounce.

We give from first principles the nonrelativistic limit of scalar and Dirac fields in curved spacetime. We aim to find general relativistic corrections to the quantum theory of particles affected by Newtonian gravity, a regime nowadays experimentally accessible. We believe that the ever-improving measurement accuracy and the theoretical interest in finding general relativistic effects in quantum systems require the introduction of corrections to the Schrödinger-Newtonian theory. We rigorously determine these corrections by the nonrelativistic limit of fully relativistic quantum theories in curved spacetime. For curved static spacetimes, we show how a noninertial observer (equivalently, an observer in the presence of a gravitational field) can distinguish a scalar field from a Dirac field by particle-gravity interaction. We study the Rindler spacetime and discuss the difference between the resulting nonrelativistic Hamiltonians. We find that for sufficiently large acceleration, the gravity-spin coupling dominates over the corrections for scalar fields, promoting Dirac particles as the best candidates for observing non-Newtonian gravity in quantum particle phenomenology.

We analyzed how entanglement harvesting is affected by the performance of a measurement on the quantum field. The measurement on the field is modeled as the coupling of a particle detector to the field, followed by a projective measurement performed on the detector. In our analysis, we considered different arrangements for two detectors harvesting entanglement and an ancillary detector used to perform a measurement. We found different regimes for how performing measurements on the field affects the entanglement harvested, depending on the initial and final states of the detector used to measure the field, as well as its coupling strength. We identified the regimes where it is possible to measure the field during the preparation of entanglement harvesting protocols without significantly interfering in the entanglement harvested. We also identify in what regimes the field measurement can degrade or slightly enhance the ability of particle detectors to harvest entanglement.

We make a detailed comparison between entanglement harvesting for uniformly accelerated detectors in vacuum and static ones in a thermal bath at the Unruh temperature and find that, for a small energy gap relative to the Heisenberg energy of the detectors, static detectors in the thermal bath can harvest more entanglement and possess a comparatively larger harvesting-achievable range than the uniformly accelerated ones; however, as the energy gap grows sufficiently large, the uniformly accelerated detectors are instead likely to harvest more entanglement and possess a relatively larger harvesting-achievable range than inertial ones in the thermal bath. In comparison with static detectors in vacuum, there exist phenomena of acceleration-assisted entanglement harvesting but never that of thermal-noise-assisted one. A notably interesting feature is that, although both the amount of entanglement harvested and the harvesting-achievable interdetector separation for static detectors in a thermal bath are always a decreasing function of temperature, they are not always so for uniformly accelerated detectors as acceleration (Unruh temperature) varies, suggesting the existence of the anti-Unruh effect in the entanglement harvesting phenomena of the accelerated detectors.

We study the properties of chiral anomalies in a wide class of spacetimes that possess a principal Killing-Yano tensor. This class includes metrics of charged rotating black holes as a special physically important case. The spacetimes that admit a principal Killing-Yano tensor possess a number of remarkable properties. In particular, such spacetimes have two commuting Killing vectors and a Killing tensor responsible for their hidden symmetries. We calculate the gravitational and electromagnetic contributions to the axial anomaly currents in the spacetime of a charged rotating black hole, and we demonstrate that the equation for the chiral anomaly current has special solutions which respect both explicit and hidden symmetries. Two of these solutions have the form of currents propagating along two principal null directions, which are null eigenvectors of the Riemann tensor. These solutions describe chiral currents for the incoming and outgoing polarization fluxes. It is demonstrated that these principal chiral currents can be written explicitly in the form which contains the off-shell metric coefficients and their derivatives. We discuss conditions where the principle chiral anomaly current is regular at the horizon and the axes of symmetry. We demonstrate that for states where the current vanishes at the past horizon and at the past null infinity, there exist chirality fluxes at both the future horizon and future infinity. The latter is directly related to the polarization asymmetry of Hawking radiation for massless spinning particles. We also calculate the Chern-Simons currents for both gravitational and electromagnetic chiral anomalies in the black hole spacetime, and we discuss the properties of the chirality fluxes associated with these currents.

We obtain the Arnowitt-Deser-Misner formulation of general relativity in $n$ dimensions ($n\ge 3$) from its either $SO$($n-1$, 1) [$SO(n)$] or $SO(n-1)$ Palatini Hamiltonian formulations and vice versa [we recall that $SO$($n-1$, 1) [$SO(n)$] requires no gauge fixing whereas $SO(n-1)$ involves the time gauge]. Similarly, the Hamiltonian formulation of general relativity in terms of Ashtekar-Barbero variables can also be directly obtained from the Arnowitt-Deser-Misner Hamiltonian formulation and vice versa, which is an alternative approach to the way followed by Barbero. We give the relevant maps among the phase-space variables and relate the corresponding symplectic structures and the first-class constraints.

The direct discovery of gravitational waves (GWs) from the coalescence of compact binary components by the LIGO/Virgo/KAGRA Collaboration provides an unprecedented opportunity for exploring the underlying theory of gravity that drives the coalescence process in the strong and highly dynamical field regime of gravity. In this paper, we consider the observational effects of spatial covariant gravities on the propagation of GWs in the cosmological background and obtain the observational constraints on coupling coefficients in the action of spatial covariant gravities from GW observations. We first decompose the GWs into the left- and right-hand circular polarization modes and derive the effects of the spatial covariant gravities on the propagation equation of GWs. We find that these effects can be divided into three classes: 1) frequency-independent effects on GW speed and friction, 2) a parity-violating amplitude and velocity birefringences, and 3) a Lorentz-violating damping rate and dispersion of GWs. With these effects, we calculate the corresponding modified waveform of GWs generated by the coalescence of compact binaries. By comparing these new effects with the publicly available posterior samples or results from various tests of gravities with LIGO/Virgo/KAGRA data in the literature, we derive the observational constraints on coupling coefficients of the spatial covariant gravities. These results represent the most comprehensive constraints on the spatial covariant gravities in the literature.

We describe a shielding mechanism for a charged black hole immersed in a background involving charged matter fields, solely arising from the Einstein-Maxwell field equations. In particular, we consider a charged generalization of the Einstein cluster, that is a charged black hole surrounded by an effective fluid model for a partially charged dust cloud. We show that the shielding mechanism, arising thereof, is generic and appears in a different parametrization of the problem as well. In this process, we provide the most general electrovacuum solution in a spacetime region devoid of charges, but in the presence of a static and spherically symmetric charge distribution elsewhere. Side by side, we also introduce a convenient parametrization, providing the global solution of the Einstein-Maxwell’s field equations in the presence of a charged black hole within the environment of charged fluid. We also comment on the nature of the photon sphere, shadow radius and the eikonal quasinormal modes in the Einstein-Maxwell cluster.

We investigate the scalar field system over a charged Weyl black hole, depicted by a parameter $\lambda$. It is found that the imaginary part of the quasinormal mode spectra is always negative and the perturbation does not increase with the time, indicating that the system is stable under scalar field perturbation. Furthermore, the quasinormal mode spectra and Hawking radiation exhibit a qualitatively similar characteristic in that they both rise with rising $\lambda$ and approach a constant when $\lambda$ is large enough. Especially, we would like to emphasize that an exponential decay obviously emerges in the phase of the ringing tail as $\lambda$ increases. It indicates that the characteristic parameter $\lambda$ has an obvious imprint on the ringing tail, which is expected to be detected by future observations.

In this paper, a new family of Hawking-Page transitions, i.e., Hawking-Page transitions with reentrance and a triple point, is introduced for the first time by investigating the Hawking-Page transition of hyperbolic AdS black holes in the extended thermodynamics in Gauss-Bonnet gravity. The reentrant Hawking-Page transition is composed of two Hawking-Page transitions with a high and a low Hawking-Page temperature, and the triple point corresponds to small black hole, massless black hole, and large black hole phases all coexisting. We discuss the temperatures of two branches of Hawking-Page transitions, which both depend on the pressure (i.e., the cosmological constant) and the Gauss-Bonnet constant. The pressure and the Gauss-Bonnet constant both increase the high Hawking-Page temperature and diminish the low Hawking-Page temperature. We also show the coexistence lines in the $P-T$ phase diagrams. The triple point and critical point in the phase diagrams of the Gauss-Bonnet AdS black hole systems are given, together with some interesting universal relations that only depend on the dimensions of spacetime. The reentrant Hawking-Page transition and triple point may correspond to the phase transition and triple point in the QCD phase diagram, following the spirit of the $\mathrm{AdS}/\mathrm{CFT}$ correspondence. These results may improve the comprehension of the black hole thermodynamics in the quantum gravity framework and shed some light on the $\mathrm{AdS}/\mathrm{CFT}$ correspondence beyond the classical gravity limit.

Based on the effective dynamics in the $\overline{\mu }$ scheme of the spherical symmetry reduced model in the reduced phase space formulation of loop quantum gravity (LQG), we investigate the gravitational collapse of a homogeneous dust cloud, with Gaussian dust serving as both the reference field and the source of the gravitational collapse. The effective dynamics from the considered model for a homogeneous dust cloud reduces precisely to the effective dynamics of loop quantum cosmology (LQC) with extrinsic curvature based K-quantization, indicating that the LQC effective dynamics lives as a subsector of the model presented here. In both the marginally bound and the bound cases of the collapse in effective dynamics, the singularity is resolved and replaced by a bounce. Though quantum geometric modification from spatial curvature is not directly included in the K-quantization it does affect the qualitative dynamics of the collapsing dust cloud in the sense that on the one hand for the marginally bound case, the dust cloud bounces once at fixed maximum energy density and on the other hand for the bound case, the dust cloud undergoes infinite cycles of contraction and expansion at energy densities dependent on the dust mass. Finally, the mass threshold for the formation of a trapped surface in each case is found and the matching conditions between the interior collapsing spacetime and an effective exterior static solution are discussed.

In general relativity, isolated black holes obey the no-hair theorems, which fix the multipolar structure of their exterior spacetime. However, in modified gravity, or when the compact objects are not black holes, the exterior spacetime may have a different multipolar structure. When two black holes are in a binary, this multipolar structure determines the morphology of the dynamics of orbital and spin precession. In turn, the precession dynamics imprint onto the gravitational waves emitted by an inspiraling compact binary through specific amplitude and phase modulations. The detection and characterization of these amplitude and phase modulations can therefore lead to improved constraints on fundamental physics with gravitational waves. Recently, analytic precessing waveforms were calculated in two scenarios: (i) dynamical Chern-Simons gravity, where the no-hair theorems are violated, and (ii) deformed compact objects with generic mass quadrupole moments. In this work, we use these two examples to propose an extension of the parametrized post-Einsteinian (ppE) framework to include precession effects. The new framework contains $2n$ ppE parameters $({\mathcal{b}}_{(m^{\prime },n)}^{\text{ppE}},b_{(m^{\prime },n)}^{\text{ppE}})$ for the waveform phase, and $2n$ ppE parameters $({\mathcal{a}}_{(m^{\prime },n)}^{\text{ppE}},a_{(m^{\prime },n)}^{\text{ppE}})$ for the waveform amplitudes. The number of ppE corrections $n$ corresponds to the minimum number of harmonics necessary to achieve a given likelihood threshold when comparing the truncated ppE waveform with the exact one, and $(m^{\prime },n)$ corresponds to the harmonic numbers of the harmonics containing ppE parameters. We show explicitly how these ppE parameters map to the specific example waveforms discussed above. The proposed ppE framework can serve as a basis for future tests of general relativity with gravitational waves from precessing binaries.

We study self-gravitating bound states of a complex vector field, known as Proca stars, with a new type of quartic-order self-interaction, which does not exist in the case of either a complex scalar field or a real vector field. Depending on the sign of the coupling constant, this quartic self-interaction can yield a distinct feature of Proca stars from the previously investigated self-interaction of the vector field. We find that self-gravitating solutions can be so compact that the photon sphere could form. However, we also show that the self-interaction gives rise to a ghost instability for the stars whose compactness is close that for the formation of a photon sphere, which might invalidate the formation of the photon sphere.

We present a systematic exploration of the loss of predictivity in Einstein-scalar-Gauss-Bonnet (ESGB) gravity in spherical symmetry. We first formulate a gauge covariant method of characterizing the breakdown of the hyperbolicity of the equations of motion in the theory. With this formalism, we show that strong geodesic focusing leads to the breakdown of hyperbolicity, and the latter is unrelated to the violation of the null convergence condition. We then numerically study the hyperbolicity of the equations during gravitational collapse for two specific ESGB gravity theories: “shift symmetric Gauss-Bonnet gravity” and a version of the theory that admits “spontaneously scalarized” black holes. We devise a “phase space” model to describe the end states for a given class of initial data. Using our phase space picture, we demonstrate that the two theories we consider remain predictive (hyperbolic) for a range of GB couplings. The range of couplings, however, is small, and thus, the presence of spontaneously scalarized solutions requires fine-tuning of initial data. Our results, therefore, cast doubt as to whether scalarized black hole solutions can be realistically realized in nature even if ESGB gravity happened to be the correct gravitational description.

If the gauge fields are amplified from the inflationary vacuum, the quantum mechanical initial data correspond to travelling waves that turn asymptotically into standing waves whose phases only depend on the evolution of the gauge coupling. We point out that these gauge analogs of the Sakharov oscillations are exchanged by the duality symmetry and ultimately constrain both the relative scaling of the hypermagnetic power spectra and their final asymptotic values. Unlike the case of the density contrasts in a relativistic plasma, the standing oscillations never develop since they are eventually overdamped by the finite value of the conductivity as soon as the corresponding modes are comparable with the expansion rates after inflation. We show that the late-time value of the magnetic field is not determined at radiation dominance (and in spite of the value of the wave number) but it depends on the moment when the wavelengths (comparable with the Mpc) get of the order of the Hubble radius before equality. This means that the magnetogenesis requirements are only relaxed if the postinflationary expansion rate is slower than radiation but the opposite is true when the plasma expands faster than radiation and the corresponding power spectra are further suppressed. After combining the present findings with the evolution of the gauge coupling we show that these results are consistent with a magnetogenesis scenario where the gauge coupling is always perturbative during the inflationary stage while, in the dual case, the same requirements cannot be satisfied. This means, in practice, that large-scale magnetic fields can be successfully generated in a model that incorporates inflation and that is weakly coupled throughout all its stages without the need of an initial strongly coupled regime. We finally clarify that the late-time scaling of the gauge fields is not peculiar, as sometimes argued, but it follows without any ambiguity from the evolution equations of the model and it is determined by the rate of variation of the gauge coupling during inflation.

We revisit the quark-mass density-dependent model—a phenomenological equation of state for deconfined quark matter in the high-density low-temperature regime—and show that thermodynamic inconsistencies that have plagued the model for decades can be solved if the model is formulated in the canonical ensemble instead of the grand canonical one. Within the new formulation, the minimum of the energy per baryon occurs at zero pressure, and the Euler’s relation is verified. Adopting a typical mass-formula, we first analyze in detail a simple model with one particle species. We show that a “bag” term that produces quark confinement naturally appears in the pressure (and not in the energy density) due to density dependence of the quark masses. Additionally, the chemical potential gains a new term as in other models with quark repulsive interactions. Then, we extend the formalism to the astrophysically realistic case of charge-neutral three-flavor quark matter in equilibrium under weak interactions, focusing on two different mass formulas: a flavor dependent and a flavor blind one. For these two models, we derive the equation of state and analyze its behavior for several parameter choices. We systematically analyze the parameter space and identify the regions corresponding to self-bound 2-flavor and 3-flavor quark matter, hybrid matter and causal behavior.

The flavor evolution of a neutrino gas can show “slow” or “fast” collective motion. In terms of the usual Bloch vectors to describe the mean-field density matrices of a homogeneous neutrino gas, the slow two-flavor equations of motion (EOMs) are ${\dot{\mathbf{P}}}_{\omega }=(\omega \mathbf{B}+\mu \mathbf{P})\times {\mathbf{P}}_{\omega }$, where $\omega =\mathrm{\Delta }{m}^2/2E$, $\mu =\sqrt{2}{G}_{\mathrm{F}}(n_{\nu }+n_{\overline{\nu }})$, $\mathbf{B}$ is a unit vector in the mass direction in flavor space, and $\mathbf{P}=\int d\omega {\mathbf{P}}_{\omega }$. For an axisymmetric angle distribution, the fast EOMs are ${\dot{\mathbf{D}}}_v=\mu ({\mathbf{D}}_0-v{\mathbf{D}}_1)\times {\mathbf{D}}_v$, where ${\mathbf{D}}_v$ is the Bloch vector for lepton number, $v=\cos \theta$ is the velocity along the symmetry axis, ${\mathbf{D}}_0=\int dv{\mathbf{D}}_v$, and ${\mathbf{D}}_1=\int dvv{\mathbf{D}}_v$. We discuss similarities and differences between these generic cases. Both systems can have pendulumlike instabilities (soliton solutions), both have similar Gaudin invariants, and both are integrable in the classical and quantum case. Describing fast oscillations in a frame comoving with ${\mathbf{D}}_1$ (which itself may execute pendulumlike motions) leads to transformed EOMs that are equivalent to an abstract slow system. These conclusions carry over to three flavors.

Nuclear reactions may affect gravitational-wave signals from neutron-star mergers, but the impact is uncertain. To indicate the significance of this effect, we compare two numerical simulations representing intuitive extremes. In one case, reactions happen instantaneously. In the other case, they occur on timescales much slower than the evolutionary timescale. We show that, while the differences in the two gravitational-wave signals are small, the mismatch between them satisfies the condition for distinguishability using the Einstein Telescope noise curve, assuming that the neutron-star equation of state can be well constrained by experiments or by the postmerger signal of the event. This suggests that, to avoid systematic errors in equation of state parameters inferred from observed signals, we need to accurately implement nuclear reactions in future simulations.

Even though the crystallized nature of the neutron star crust plays a pivotal role in describing various fascinating astrophysical observations, its microscopic structure is not fully understood in the presence of a colossal magnetic field. In the present work, we study the crustal properties of a neutron star within an effective relativistic mean field framework in the presence of magnetic field strength $\sim {10}^{17}\mathrm{G}$. We calculate the equilibrium composition of the outer crust by minimizing the Gibbs free energy using the most recent atomic mass evaluations. The magnetic field significantly affects the equation of state (EOS) and the properties of the outer crust, such as neutron drip density, pressure, and melting temperature. For the inner crust, we use the compressible liquid drop model for the first time to study the crustal properties in a magnetic environment. The inner crust properties, such as mass and charge number distribution, isospin asymmetry, cluster density, etc., show typical quantum oscillations (De Haas-van Alphen effect) sensitive to the magnetic field’s strength. The density-dependent symmetry energy influences the magnetic inner crust like the field-free case. We study the probable modifications in the pasta structures and it is observed that their mass and thickness changes by $\sim 10–15\%$ depending upon the magnetic field strength. The fundamental torsional oscillation mode frequency is investigated for the magnetized crust in the context of quasiperiodic oscillations in soft gamma repeaters. The magnetic field strengths considered in this work influences only the EOS of outer and shallow regions of the inner crust, which results in no significant change in global neutron star properties. However, the outer crust mass and its moment of inertia increase considerably with increase in magnetic field strength.