The interior volume of black holes, as defined by Christodoulou and Rovelli, exhibits many surprising features. For example, it increases with time, even under Hawking evaporation. For some black holes, the interior volume is not even a monotonic increasing function of its area, which means one cannot infer how large a black hole is by just looking from the outside. Such a notion of volume, however, turns out to be useful in the context of holography, as it seems to be dual to the complexity of the boundary field theory. In this study, we investigate the properties of the interior volume of four-dimensional Kerr-AdS black holes, fixing either the mass parameter $M$ or the physical mass $E$, whilst varying the values of the cosmological constant. We found that the volume as a function of the radial coordinate features a “double lobe” while fixing $M$, whereas fixing $E$ yields behaviors that are qualitatively similar to the asymptotically flat case. We briefly comment on the holographic complexity of Kerr-AdS black holes.

Recently, a novel model for a regular black hole was advocated which possesses an asymptotically Minkowski core implemented via an exponential suppression (in the core region) of the Misner-Sharp quasilocal mass. Using this regular black hole as a template, we construct a spherically symmetric thin-shell traversable wormhole using the “cut-and-paste” technique, thereby constructing yet another black hole mimicker. The surface stress energy at the wormhole throat is calculated, and the stability of the wormhole is analyzed. An important result is that (as compared to their Schwarzschild thin-shell counterparts) increasing the exponential suppression of the Misner-Sharp quasilocal mass by increasing the suppression parameter $a$ also considerably increases the stability regions for these thin-shell wormholes, and furthermore minimizes the amount of energy condition violating exotic matter required to keep the wormhole throat open.

We explore the prospects of detecting Galactic double white dwarf (DWD) binaries with the space-based gravitational wave (GW) observatory TianQin. In this work, we analyze both a sample of currently known DWDs and a realistic synthetic population of DWDs to assess the number of guaranteed detections and the full capacity of the mission. We find that TianQin can detect 12 out of $\sim 100$ known DWDs; GW signals of these binaries can be modeled in detail ahead of the mission launch, and therefore they can be used as verification sources. Besides, we estimate that TianQin has a potential to detect as many as ${10}^4$ DWDs in the Milky Way. TianQin is expected to measure their orbital periods and amplitudes with accuracies of $\sim {10}^{-7}$ and $\sim 0.2$, respectively, and to localize on the sky a large fraction (39%) of the detected population to better than $1{\mathrm{deg}}^2$. We conclude that TianQin has the potential to significantly advance our knowledge on Galactic DWDs by increasing the sample up to 2 orders of magnitude, and will allow their multimessenger studies in combination with electromagnetic telescopes. We also test the possibilities of different configurations of TianQin: (1) the same mission with a different orientation, (2) two perpendicular constellations combined into a network, and (3) the combination of the network with the ESA-led Laser Interferometer Space Antenna. We find that the network of detectors boosts the accuracy on the measurement of source parameters by 1–2 orders of magnitude, with the improvement on sky localization being the most significant.

Ultralight bosons can form large clouds around stellar-mass black holes via the superradiance instability. Through processes such as annihilation, these bosons can source continuous gravitational-wave signals with frequencies within the range of LIGO and Virgo. If boson annihilation occurs, then the Galactic black hole population will give rise to many gravitational signals; we refer to this as the ensemble signal. We characterize the ensemble signal as observed by the gravitational-wave detectors; this is important because the ensemble signal carries the primary signature that a continuous wave signal has a boson annihilation origin. We explore how a broad set of black hole population parameters affects the resulting spin-0 boson annihilation signal and consider its detectability by recent searches for continuous gravitational waves. A population of ${10}^8$ black holes with masses up to $30{\mathrm{M}}_{\odot }$ and a flat dimensionless initial spin distribution between zero and unity produces up to 1000 signals loud enough in principle to be detected by these searches. For a more moderately spinning population, the number of signals drops by about an order of magnitude, still yielding up to 100 detectable signals for some boson masses. A nondetection of annihilation signals at frequencies between 100 and 1200 Hz disfavors the existence of scalar bosons with rest energies between $2\times {10}^{-13}$ and $2.5\times {10}^{-12}\mathrm{eV}$. Finally, we show that, depending on the black hole population parameters, care must be taken in assuming that the continuous wave upper limits from searches for isolated signals are still valid for signals that are part of a dense ensemble: Between 200 and 300 Hz, we urge caution when interpreting a null result for bosons between $4\times {10}^{-13}$ and $6\times {10}^{-13}\mathrm{eV}$.

We explore the conditions for chiral symmetry breaking in reduced (or pseudo)quantum electrodynamics at finite temperature in connection with graphene and other 2D materials with an underlying Dirac behavior of the charge carriers. By solving the corresponding Schwinger-Dyson equation in a suitable truncation (either the nonlocal Nash gauge including vacuum polarization effects in the large fermion family number $n_f$ limit or the quenched rainbow approximation, in a Landau-like gauge) and neglecting wave function renormalization effects, we find the need of the effective coupling to exceed a critical value ${\alpha }_c$ in order for chiral symmetry to be broken, in agreement with known results from other groups. In this supercritical regime, we add the effects of a thermal bath at temperature $T$ and find the critical values of this parameter that lead to chiral symmetry restoration.

We have performed the first $n_f=2+1+1$ lattice QCD computations of the properties (masses and decay constants) of ground-state charmonium mesons. Our calculation uses the Highly Improved Staggered Quark (HISQ) action to generate quark-line connected two-point correlation functions on MILC gluon field configurations that include $u/d$ quark masses going down to the physical point, tuning the $c$-quark mass from $M_{J/\psi }$ and including the effect of the $c$ quark’s electric charge through quenched QED. We obtain $M_{J/\psi }-M_{{\eta }_c}$ $(\text{connected})=120.3(1.1)\mathrm{MeV}$ and interpret the difference with experiment as the impact on $M_{{\eta }_c}$ of the ${\eta }_c$ decay to gluons, missing from the lattice calculation. This allows us to determine $\mathrm{\Delta }{M}_{{\eta }_c}^{\text{annihiln}}=+7.3(1.2)\mathrm{MeV}$, giving its value for the first time. Our result of $f_{J/\psi }=0.4104(17)\mathrm{GeV}$ gives $\mathrm{\Gamma }(J/\psi \to e^{+}{e}^{-})=5.637(49)\mathrm{keV}$, in agreement with, but now more accurate than, experiment. At the same time we have improved the determination of the $c$-quark mass, including the impact of quenched QED to give ${\overline{m}}_c(3\mathrm{GeV})=0.9841(51)\mathrm{GeV}$. We have also used the time moments of the vector charmonium current-current correlators to improve the lattice QCD result for the $c$ quark hadronic vacuum polarization (HVP) contribution to the anomalous magnetic moment of the muon. We obtain $a_{\mu }^c=14.638(47)\times {10}^{-10}$, which is $2.5\sigma$ higher than the value derived using moments extracted from some sets of experimental data on $R(e^{+}{e}^{-}\to \text{hadrons})$. This value for $a_{\mu }^c$ includes our determination of the effect of QED on this quantity, $\delta a_{\mu }^c=0.0313(28)\times {10}^{-10}$.

We investigate a nonrelativistic model of tetraquarks, which are assumed to be compact and to consist of diquark-antidiquark pairs. We fit, for the first time, basically all currently known values for the measured masses of 45 mesons, including both charmed and bottom mesons, to the model and predict masses of tetraquarks as well as diquarks. In particular, we find masses of four axial-vector diquarks, i.e., $qc$, $cc$, $qb$, and $bb$, where $q=u$, $d$, and 24 ground-state tetraquarks, including both heavy-light tetraquarks ($qc\overline{qc}$ and $qb\overline{qb}$) and heavy tetraquarks ($cc\overline{cc}$ and $bb\overline{bb}$). In general, our results for the masses of $qb\overline{qb}$, $cc\overline{cc}$, and $bb\overline{bb}$ are largely comparable with other reported results, whereas our results for the masses of $qc\overline{qc}$ are slightly larger than what has been found earlier. Finally, we identify some of the obtained predictions for masses of tetraquarks with masses of experimental tetraquark candidates, and especially, we find that $\psi (4660)$, $Z_b(10610)$, and $Z_b(10650)$ could be described by the model.

We consider charmless weak decays of beauty baryons into decuplet baryons and pseudoscalar mesons in a general framework based on the $SU(3)$-flavor decomposition of the decay amplitudes. The dynamical assumption independent $SU(3)$ analysis accounts for the effects of an arbitrarily broken $SU(3)$ symmetry in these decays. An alternative approach in terms of quark diagrams is also provided and compared with the $SU(3)$ decomposition in the limit of an exact $SU(3)$-flavor symmetry. Furthermore, the symmetries of the effective Hamiltonian is used to relate or neglect reduced $SU(3)$ amplitudes to derive several sum rule relations between amplitudes and relations between $CP$ asymmetries in these decays and identify those that hold even if $SU(3)$ is broken.

A generic feature of nearly out-of-equilibrium dissipative systems is that they resonate through a set of quasinormal modes. Black holes—the absorbing objects par excellence—are no exception. When formed in a merger, black holes vibrate in a process called “ringdown,” which leaves the gravitational-wave footprint of the event horizon. In some models of quantum gravity which attempt to solve the information-loss paradox and the singularities of general relativity, black holes are replaced by regular, horizonless objects with a tiny effective reflectivity. Motivated by these scenarios, here we develop a generic framework to the study of the ringdown of a compact object with various shades of darkness. By extending the black-hole membrane paradigm, we map the interior of any compact object in terms of the bulk and shear viscosities of a fictitious fluid located at the surface, with the black-hole limit being a single point in a three-dimensional parameter space. We unveil some remarkable features of the ringdown and some universal properties of the light ring in this framework. We also identify the region of the parameter space which can be probed by current and future gravitational-wave detectors. A general feature is the appearance of mode doublets which are degenerate only in the black-hole limit. We argue that the merger event GW150914 already imposes a strong lower bound on the compactness of the merger remnant of approximately 99% of the black-hole compactness. This places model-independent constraints on black-hole alternatives such as diffuse “fuzzballs” and nonlocal stars.

In this paper, we explore the test particle motion around black hole in Einstein-Maxwell-scalar (EMS) theory using three different black hole solutions within this theory. We have first analyzed the spacetime curvature structure of these solutions and shown the existence of two singularities and the first one is at the center $r=0$. In black hole spacetime, there are two regions divided by the critical value of the cosmological parameter ${\lambda }_0$. The photon sphere around black hole in EMS theory has also been studied and found that it does not depend on cosmological parameter $\lambda$. We have analyzed the innermost stable circular orbits (ISCO) around black hole and shown that for all solutions ISCO radius for neutral particle decreases with the increase of black hole charge. We have also studied the charged particle motion around the black hole where charged particle motion is considered in the presence of gravitational field and the Coulomb potential. It is shown that ISCO radius for charged particles increases depending on the selected value of the coupling parameter which is in contradiction with observations of the inner edge of the accretion disks of the astrophysical black holes and can be used as powerful tool to rule out the EMS theory from consideration for the gravitational field theory. It also studied the fundamental frequencies governed by test particle orbiting around black hole in EMS theory. Finally, as test of black hole solution in EMS theory ISCO radii is compared with that in Kerr black hole and found that the spin parameter of Kerr can be mimic up to $a/M\simeq 0.936$.

We find the symmetry generators for the Friedman equations emanating from a perfect fluid source in the presence of a cosmological constant term. The relevant dynamics are shown to be governed by two coupled, first order ordinary differential equations, the continuity and the quadratic constraint equation. Arbitrary functions appear in the components of the symmetry vector, indicating the infinity of the group. When the equation of state is considered as arbitrary but ab initio given, previously known results are recovered and/or generalized. When the pressure is considered among the dynamical variables, solutions for models with different equations of state are mapped to each other, thus enabling the presentation of solutions to models with complicated equations of state starting from simple known cases.

We revisit the problem of generality of Starobinsky and Higgs inflation. The known results obtained in the Einstein frame are generalized for the case of an arbitrary initial energy of the scalar field. These results are compared with the results obtained directly in the Jordan frame, which, to our knowledge, has not been thoroughly explored in the literature previously. We demonstrate that the qualitative picture of initial conditions zone in the $(\varphi ,\dot{\varphi })$ plane, which leads to sufficient amount of inflation, is quite similar for both the frames in the case of Higgs inflation. For Starobinsky inflation, the conformal transformation between the frames relates the geometrical variables in the Jordan frame with the properties of an effective scalar field in the Einstein frame. We show that the transformation $(H,R)\to (\varphi ,\dot{\varphi })$ is not regular everywhere, leading to some peculiarities in the zone of good initial conditions in the $(H,R)$ plane.

We review of the comment on the rotating global monopole by Muhammed Haluk Seçuk [preceding Comment, Phys. Rev. D 102, 068501 (2020)] who affirms that it is not possible to obtain rotating global monopole systems in the Boyer-Lindquist coordinates using the Newman-Janis algorithm. In fact, this is true for the global monopole in $f(R)$, but not for the other two monopole systems.

We review the derivations made in Filho and Bezerra [Phys. Rev. D 64, 084009 (2001)], Barbosa and Bezerra [Gen. Relativ. Gravit. 48, 149 (2016)], and Graça and Bezerra [Mod. Phys. Lett. A 27, 1250178 (2012)] and show that the resulting line elements in Boyer-Lindquist coordinates are not valid.

A holographic model is used to investigate the thermodynamics and the phase diagram of a heavy quarks system. From such a model we obtain an equation of state and explore its applicability in astrophysical conditions. For this objective, we work in the context of the Einstein-Maxwell-Dilaton (EMD) holographic model for quantum chromodynamics (QCD). At first, we show the existence of a critical point where the first-order transitions line ends; later on, we calculated an analytic expression for the equation of state. Additionally, with the aim of investigating the global properties of compact stars, such as the total gravitational mass and radius, the equation of state is used to solve the Tolman-Oppenheimer-Volkov (TOV) equations for stellar structure. The numerical results show that our equation of state is able to reproduce the expected behavior of hybrid stars. Our main conclusion is that, by using an equation of state emerging in the framework of the EMD holographic model for QCD, it is possible to obtain quark matter properties and that it is also possible to extend the procedure to astrophysical applications.

We present a procedure for quantizing complex projective spaces $\mathbb{C}{\mathbb{P}}^{p,q}$, $q\ge 1$, as well as construct relevant star products on these spaces. The quantization is made unique with the demand that it preserves the full isometry algebra of the metric. Although the isometry algebra, namely, $su(p+1,q)$, is preserved by the quantization, the Killing vectors generating these isometries pick up quantum corrections. The quantization procedure is an extension of one applied recently to Euclidean two-dimensional anti–de Sitter space (${\mathrm{AdS}}_2$), where it was found that all quantum corrections to the Killing vectors vanish in the asymptotic limit, in addition to the result that the star product trivializes to pointwise product in the limit. In other words, the space is asymptotically anti–de Sitter, making it a possible candidate for the $\mathrm{AdS}/\mathrm{CFT}$ correspondence principle. In this article, we find indications that the results for quantized Euclidean ${\mathrm{AdS}}_2$ can be extended to quantized $\mathbb{C}{\mathbb{P}}^{p,q}$, i.e., noncommutativity is restricted to a limited neighborhood of some origin, and these quantum spaces approach $\mathbb{C}{\mathbb{P}}^{p,q}$ in the asymptotic limit.

General relativity with fermions has two independent symmetries: general coordinate invariance and local Lorentz invariance. General coordinate invariance is implemented by the Levi-Civita connection and by Cartan’s tetrads both of which have as their action the Einstein-Hilbert action. It is suggested here that local Lorentz invariance is implemented not by a combination of the Levi-Civita connection and Cartan’s tetrads known as the spin connection, but by independent Lorentz bosons ${L^{ab}}_i$ that gauge the Lorentz group, that couple to fermions like Yang-Mills fields, and that have their own Yang-Mills-like action. Because the Lorentz bosons couple to fermion number and not to mass, they generate a static potential that violates the weak equivalence principle. If a Higgs mechanism makes them massive, then the static potential also violates the inverse-square law. Experiments put upper bounds on the strength of such a potential for masses $m_L<20\mathrm{eV}$. These upper limits imply that Lorentz bosons, if they exist, are nearly stable and contribute to dark matter.

Pulsar timing arrays (PTA) have the promise to detect gravitational waves (GWs) from sources which are in a unique frequency range of ${10}^{-9}-{10}^{-6}\mathrm{Hz}$. This in turn also provides an opportunity to test the theory of general relativity in the low frequency regime. The central concept of the detection of GWs with PTA lies in measuring the time of arrival difference of the pulsar signal due to the passing of GWs; i.e., the pulses get redshifted. In this paper we provide a complete derivation of the redshift computation for all six possible polarizations of GWs which arise due to the modifications to general relativity. We discuss the smoothness of the redshift and related properties at the critical point, where the GW source lies directly behind the pulsar. From our mathematical discussion we conclude that the redshift has to be split differently into polarization part (pattern functions) and interference part, to avoid discontinuities and singularities in the pattern functions. This choice of pattern functions agrees with the formula one uses for interferometers with a single detector arm. Finally, we provide a general expression which can in principle be used for pulsars and GWs of any frequency without invoking the low frequency assumption and using said assumption we develop the expression up to first order in the strain and find correction terms to the canonical redshift formula.

We analyze the most general case of third-order Chern–Simons-like theories of massive 3D gravity. Results show the conditions for finding the unitary regions on the parameter space. There exists ($n-1$)th order theories on the boundary of a unitary $n$th order model on parameter space under certain conditions, as the first example recently demonstrated from the bimetric generalization of exotic massive gravity. We investigated the mechanism that causes this type of transition for third-order models. Hamiltonian analysis of the theory also presents that ghost and no-ghost regions can be separated by Chern–Simons theories.

Quasinormal modes of a scalar field perturbation are investigated in the background spacetime of Einstein-Yang-Mills black holes. The logarithmic term in the metric function of the five-dimensional Einstein-Yang-Mills black hole eliminates the divergence of the Hawking temperature but maintains the charge-dependent behavior of the quasinormal modes. The real and imaginary parts of quasinormal mode frequencies have the same charge-dependent behavior in different dimensions. Similar to the case of high dimensional Schwarzschild black holes, a compact approximate relation, ${\omega }_{\mathrm{R}}\sim D/r_{+}$, also exists between the real part of the quasinormal mode frequencies ${\omega }_{\mathrm{R}}$ and the number of dimensions $D$ in the higher than five-dimensional Einstein-Yang-Mills black holes.