We review some classic works on ground state entanglement entropy in ($1+1$)-dimensional free scalar field theory. We point out identifications between the methods for the calculation of entanglement entropy and we show how the formalism developed for the discretized theory can be utilized in order to obtain results in the continuous theory. We specify the entanglement spectrum and we calculate the entanglement entropy for the theory defined on an interval of finite length $L$. Finally, we derive the modular Hamiltonian directly, without using the modular flow, via the continuum limit of the expressions obtained in the discretized theory. In a specific coordinate system, the modular Hamiltonian assumes the form of a free field Hamiltonian on the Rindler wedge.

In this paper charged black holes are obtained assuming that a Born-Infeld electrodynamics may arise from an interaction between the electromagnetic field and a vacuum component. In this context, Cauchy horizons do not appear in the maximal analytical extension once an event horizon is formed so that the interior spacetime does not suffer from any sorts of instabilities which are well known in the literature. On the contrary, the causal structure exhibits an event horizon—encapsulating a spacelike singularity—and a cosmological horizon. We show that the strong cosmic censorship is then restored for a wide range of the parameters including configurations in which the black hole charge is much larger than its mass. We also show that the black hole thus formed described by our solution exhibits an unstable photon sphere analogous to that of the Schwarzschild metric.

We discuss the motion of electrically and magnetically charged particles in the electromagnetic swirling universe. We show that the equations of motion can be decoupled in the Hamilton-Jacobi formalism, revealing the existence of a fourth constant of motion. The equations of motion can be analytically integrated. The solutions are presented in terms of elementary and elliptic functions. In addition, we discuss the possible orbits for both uncharged particles (in which case the motion is geodesic) and charged particles, respectively. A typical orbit is bounded in the radial direction and escapes to infinity in the $z$ direction. However, the presence of the electromagnetic fields also leads to the existence of planar orbits.

We discuss the dynamics of a (neutral) test particle in topological star spacetime undergoing scattering processes by a superposed test radiation field, a situation that in a 4D black hole spacetime is known as relativistic Poynting-Robertson effect, paving the way for future studies involving radiation-reaction effects. Furthermore, we study self-force-driven evolution of a scalar field, perturbing the top-star spacetime with a scalar charge current. The latter for simplicity is taken to be circular, equatorial and geodetic. To perform this study, besides solving all the self-force related problem (regularization of all divergences due to the self-field, mode sum regularization, etc.), we had to adapt the 4D Mano-Suzuki-Takasugi formalism to the present 5D situation. Finally, we have compared this formalism with the (quantum) Seiberg-Witten formalism, both of which are related to the solutions of a Heun confluent equation but appear in different contexts in the literature: the first in black hole perturbation theory and the second in quantum curves in super-Yang-Mills theories.

In this paper, we investigate spacetime characterized by a hidden symmetry defined by a given Killing tensor. To exhibit this hidden symmetry, the inverse metric must commute with the Killing tensor under the Schouten-Nijenhuis bracket, which translates into a system of partial differential equations (PDEs) for the inverse metric. For some significant examples, we solve these PDEs directly, deriving spacetimes with prescribed Killing tensor symmetries, including those specified by higher-rank Killing tensors. Utilizing these symmetries, we study related problems such as null geodesics, photon region, and separation of variables of wave equations. Through this work, we aim to demonstrate that hidden symmetry is more accessible than previously believed.

Eikonal quasinormal modes (QNMs) of black holes (BHs) and parameters of null geodesics, ultimately tied to the appearance of BHs to external observers, are known to be related, and the eikonal QNM-BH shadow radii correspondence has been extensively studied for spherically symmetric BHs. The extension to rotating BHs is nontrivial, and has been worked out only for equatorial ($m=\pm \ell$) QNMs, or for general modes but limited to the Kerr metric. We extend the QNM-shadow radius correspondence to more general rotating space-times, and argue that the requirements for it to hold amount to conditions on the separability of the Hamilton-Jacobi equation for null geodesics and the Klein-Gordon equation. Metrics obtained by the Newman-Janis algorithm enjoy these conditions, provided certain mathematical requirements are imposed on the line element. We explicitly verify the correspondence for the rotating Bardeen and Hayward regular BHs, both of which satisfy the separability requirements. Our findings show that the QNM-shadow radius correspondence holds for a wide range of axisymmetric space-times beyond Kerr. This paves the way to potential strong-field multi-messenger tests of fundamental physics by hearing (via gravitational wave spectroscopy) and seeing (via VLBI imaging) BHs, although substantial improvements relative to the current observational sensitivity are required to make this possible.

In this paper we describe a model for the $(\ell =2,m=0)$ spherical harmonic mode of the gravitational wave signal emitted by the coalescence of binary black holes, in particular, spin-aligned systems. This mode can be viewed as consisting of two components, gravitational wave memory and quasinormal ringdown, which are both included in our model. Depending on the parameters of the binary and the sensitivity curve of the detector, but in particular for high masses, the ringdown part can contribute significantly to the signal-to-noise ratio. The model is constructed using the methods of the phenomenological waveforms program, and is calibrated to public numerical relativity data from the Simulating eXtreme Spacetimes (SXS) waveforms catalog, with the analytical results derived from the Bondi-Metzner-Sachs (BMS) balance laws. The code has been implemented as an extension to the computationally efficient IMRPhenomTHM model, it can therefore be used for computationally expensive applications such as Bayesian parameter estimation. The region of validity of our model in the parameter space is given by: $q\le 10$ and ${\chi }_1,{\chi }_2\in [-1,1]$, and no restrictions apply in terms of the length of the waveforms.

In this paper, we investigate the gravitational collapse to form the black hole in the accelerating, expanding Universe in the frame of Einstein-Gauss-Bonnet theory having two scalar fields and we study the propagation of the gravitational wave (GW). This may describe the creation of the primordial black holes in the early stages of the Universe and the impact of their creation on the propagation of primordial gravitational waves. The collapsing spacetime can be obtained by using the formulation of the “reconstruction,” that is, we find a model that realizes the desired or given geometry. In the reconstructed models, ghosts often appear, which could be eliminated by imposing constraints. We show that the standard cosmological solutions or self-gravitating objects such as a planet, the Sun, various types of stars, etc., in Einstein’s gravity, are also solutions in this model. Using the dynamical value of the Gauss-Bonnet coupling, the propagation of the high-frequency GW is investigated. The propagating speed changes due to the coupling during the period of the black hole formation. The speed of the GW propagation going into the black hole is different from that of the wave going out.

We revisit the three black hole scenario with numerical relativity techniques to study hierarchical configurations where the inner binary contains highly spinning black holes. We find that the merger time of the binary gets a delay (with a number of orbits to merger increase), depending strongly on the distance to the orbiting third hole $D$ as $\sim 1/D^{1.6\pm 0.1}$. Notably, this is a different dependence from what we had found in the nonspinning case, $\sim 1/D^{2.5}$. We interpret this effect as mostly due to a spin-orbit coupling between the third hole and the closest member of the binary in the successive approaches. This leads us next to study scattering configurations of the third hole with the binary in order to evaluate the extent of these “sudden” interactions, finding also a correlation of the delay in merger times with the closest distance, even for the nonspinning cases. We then explore the mass ratio dependence of the triple system by modeling binaries orbiting a larger black hole bearing mass ratios $8:1:1$ and $18:1:1$ in co-orbiting or counterorbiting configurations, finding merger times increasing with increasing mass ratios and for the counterorbiting cases.

In this study, we investigate the quasinormal modes (QNMs), pseudospectrum and time evolution of a massive vector field around a quantum corrected black hole in de-Sitter spacetime. We start by parametrization and using orthonormal tetrads to get the effective potential. Methodologically we use the hyperboloidal framework together with discretizing the nonself-adjoint operator through Chebyshev-Gauss-Labatto grid to attain the QNMs. We explore the parametric instability of QNMs caused by quantum correction, cosmological constant and Proca mass, and these three factors show very different influences on the QNMs’ migration flow. On the other hand, we discuss the instability of QNMs with arbitrary-shape perturbation and the effectiveness of numerical results through pseudospectrum. We use high frequency approximation to attain the expression of the time domain Green function and clarify the origin of two different stages in time evolution. Through numerical methods we confirm that no power-law late time tail is expected, and the possible impact on time evolution caused by quantum correction is discussed.

Recently, it has been proposed that the black hole greybody factors can be important to model ringdown spectral amplitudes. We study the stability of greybody factors against a small-bump correction in the perturbation equation. We find (I) that the greybody factor is stable in the frequency region relevant to ringdown and (II) that it is destabilized at higher frequencies, especially for a sharper bump correction. This behavior is similar to the case of higher overtones, which is also very sensitive to a small correction. We clarify this (in)stability with the WKB analysis. As the greybody factor is stable at the frequency region relevant to the main part of ringdown, we conclude that the greybody factor is suitable to model ringdown amplitude. In order to investigate a bump correction in a self-consistent manner, we consider the small-bump correction that can be realized in the general framework of effective field theory of black hole perturbations.

Primordial gravitational waves from the very early stages of the Universe, such as inflation or bounce processes, are an irreducible cosmological source of the stochastic gravitational wave background (SGWB). The recent detection of SGWB signals around the nano-Hertz frequency by pulsar timing arrays (PTAs), including NANOGrav, EPTA, PPTA, IPTA, and CPTA, opens a new window to explore these very early stages of the universe through these primordial gravitational waves. In this work, we investigate the generation and evolution of primordial gravitational waves in a generic big bounce cosmology by parametrizing its background evolution into four phases, where perturbation modes exit and reenter the horizon twice. By analytically solving the equation of motion for primordial gravitational waves and matching solutions at the boundaries, we obtain the explicit form of the primordial gravitational wave spectrum in a generic big bounce cosmology. We find that, according to the evolution of primordial gravitational waves, a generic scenario of big bounce cosmology can be categorized into four distinct types. We introduce four toy models for these categories, demonstrating that our analytical results can be straightforwardly applied to various bouncing universe models in which the equation of state of the background is constant in each phase. We also prospect future applications of our results in interpreting SGWB signals searched by PTAs and upcoming advanced gravitational wave detectors such as SKA, Taiji, Tianqin, LISA, DECIGO, and aLIGO/Virgo/KAGRA using Bayesian analysis.

We show that a rotating axion field that makes a transition from a matterlike equation of state to a kinationlike equation of state around the epoch of recombination can significantly ameliorate the Hubble tension, i.e., the discrepancy between the determinations of the present-day expansion rate $H_0$ from observations of the cosmic microwave background on one hand and type Ia supernovae on the other. We consider a specific, UV-complete model of such a rotating axion and find that it can relax the Hubble tension without exacerbating tensions in determinations of other cosmological parameters, in particular the amplitude of matter fluctuations $S_8$. We subsequently demonstrate how this rotating axion model can also generate the baryon asymmetry of our Universe, by introducing a coupling of the axion field to right-handed neutrinos. This baryogenesis model predicts heavy neutral leptons that are most naturally within reach of future lepton colliders, but in finely tuned regions of parameter space may also be accessible at the high-luminosity LHC and the beam dump experiment SHiP.

Clustering studies in current photometric galaxy surveys focus solely on autocorrelations, neglecting cross-correlations between redshift bins. We evaluate the potential advantages and drawbacks of incorporating cross-bin correlations in Fisher forecasts for the Dark Energy Survey (DES) and the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST). Our analysis considers the impact of including redshift space distortions (RSD) and magnification in model predictions, as well as systematic uncertainties in photometric redshift distributions (photo-$z$). While autocorrelations alone suffer from a degeneracy between the amplitude of matter fluctuations (${\sigma }_8$) and galaxy bias parameters, accounting for RSD and magnification in cross-correlations helps break this degeneracy—although more weakly than the degeneracy breaking expected from a combined analysis with other observables. Incorporating cross-bin correlations does not significantly increase sensitivity to photo-$z$ systematics, addressing previous concerns, and self-calibrates photo-$z$ systematics, reducing errors on photo-$z$ nuisance parameters. We suggest that the benefits of including cross-correlations in future photometric galaxy clustering analyses outweigh the risks, but caution that careful evaluation is necessary as more realistic pictures of surveys’ precision and systematic error budgets develop.

High-frequency gravitational waves ($f\gtrsim 1\mathrm{MHz}$) may provide a unique signature for the existence of exotic physics. The lack of current and future gravitational-wave experiments sensitive at those frequencies leads to the need of employing different indirect techniques. Notably, one of the most promising ones is constituted by graviton-photon conversions in magnetic fields. In this work, we focus on conversions of a gravitational-wave background into photons inside the Milky Way magnetic field, taking into account the state-of-the-art models for both regular and turbulent components. We discuss how graviton-to-photon conversions may lead to imprints in the cosmic photon background spectrum in the range of frequencies $f\sim {10}^9–{10}^{26}\mathrm{Hz}$, where the observed photon flux is widely explained by astrophysics emission models. Hence, the absence of any significant evidence for a diffuse photon flux induced by graviton-photon conversions allows us to set stringent constraints on the gravitational-wave strain $h_c$, strengthening current astrophysical bounds by $\sim 1–2$ orders of magnitude in the whole range of frequencies considered.

Time-delay interferometry (TDI) is a postprocessing technique used to suppress the laser phase noise for space-based gravitational wave detectors such as TianQin, by applying specific time-delay combinations to form a virtual equal-armlength interferometer. Recent studies have begun to focus on the onboard delay which is referred to as local delay in this article. Due to the stringent requirement for the laser interferometry in TianQin project, the impact of local delays should be carefully studied. We show the local delays will cause non-negligible effects on TianQin’s laser noise reduction, for which we propose two corrections before constructing TDI variables. While the long-arm delay correction has been discussed in previous studies, in this research we also suggest the correction to intermediary variables or original phase measurements. As a proof-of-principle, we present numerical simulation results that coincide with our analytical models, and show that these corrections are valid for different TDI combinations. We find that even if the second-generation TDI is executed perfectly, the residual laser noise is limited by the coupling of local delays and laser locking contents at high frequencies, instead of the previously believed armlength mismatch of the virtual interferometer. Moreover, we disucss the influence of local delays of the test mass interferometer and the reference interferometer, and suggest armlength differences of the two interferometers should be equal for a more effective laser noise elimination. In addition, the traditional expression of ranging bias impact should be rewritten to include additional terms for the local delays and the clock biases. Furthermore, we consider a set of scenarios including six independent lasers and different laser locking configurations, and find the suitable locking schemes for laser noise suppression, which is proved by the simulation results.

Electron-positron pair production is investigated in the multiphoton regime under polarized fields. Partial wave analysis can be applied to reveal the momentum spectral structure and identify the positions of valleys/peaks of the multiphoton rings by combining with $CP$ conservation. Moreover, considering the spin states of created pairs in partial wave analysis, the particle orbital angular momentum associated with the valleys/peaks can also be identified from multiphoton rings. It is found that the approximate analytical treatments for angular distribution by using partial wave analysis are consistent with those of the numerical calculation using the Dirac-Heisenberg-Wigner formalism. The results are helpful for understanding the angular momentum information in momentum spectra, and they also have an implication for revealing the vortex structure near the valleys where the amplitude nearly vanishes.

We reexamine the consequences of perturbative unitarity on dark matter freeze-out when both Sommerfeld enhancement and bound state formation affect dark matter annihilations. At leading order the annihilation cross section is infrared dominated and the connection between the unitarity bound and the upper bound on the dark matter mass depends only on how the different partial waves are populated. We compute how this picture is modified at next-to-leading order with the goal of assigning a reliable theory uncertainty to the freeze-out predictions. We explicitly compute next-to-leading order corrections in a simple model with Abelian gauge interactions and provide an estimate of the theoretical uncertainty for the thermal masses of heavy electroweak $n$-plets. Along the way, we clarify the regularization and matching procedure necessary to deal with singular potentials in quantum mechanics with a calculable, relativistic UV completion.

The current article presents an extension of the classical scotogenic neutrino mass paradigm, where the three issues in particle physics: dark matter, smallness of neutrino mass, and stability of the proton are interconnected. The scenario encompasses the neutrino mass as well as the proton decay as a consequence of an existence of the dark matter. The study successfully achieves the correlation between the naturally small neutrino masses and naturally long proton lifetime in the present paradigm. Furthermore, all relevant cosmological, collider, and flavor physics constraints are incorporated in the detailed analysis. The scotogenic fermionic dark matter with the mass in the range from 100 GeV to 10 TeV successfully satisfies all relevant constraints. The valid range of $2.51\times {10}^{-5}<\lambda <2\times {10}^{-3}$ is obtained for the 2HDM $\lambda$ coupling. We give a brief discussion, as well as outline some of the future prospects.

In the literature, motivated by the observed peculiar neutrino mixing pattern and a preliminary experimental hint for maximal Dirac $CP$ phase (i.e., $\delta \sim 3\pi /2$), a lot of flavor and $CP$ symmetries have been proposed to help us understand and explain these experimental results. However, for some flavor-symmetry scenarios (see section motivations of our study), the leptogenesis mechanism is prohibited from working as usual. To tackle this problem, in this paper we have made an exhaustive study of the possibility that the renormalization group evolution effect may induce a successful leptogenesis for these particular scenarios. Our study provides complementarities to the previous related studies in Xing and Zhang, [J. High Energy Phys. 04 (2020) 179]; Xing and Zhang, [Phys. Lett. B 804, 135397 (2020)]; and Cooper et al., [Nucl. Phys. B859, 159 (2012)] (see section complementarities of our study to previous related studies).