We study the electromagnetic radiation by a fermion carrying an electric charge $q$ embedded in a medium rotating with constant angular velocity $\mathbf{\Omega }$ parallel or antiparallel to an external constant magnetic field $\mathit{B}$. We assume that the rotation is “relatively slow”; namely, that the angular velocity $\mathrm{\Omega }$ is much smaller than the inverse magnetic length $\sqrt{qB}$. In practice, such angular velocity can be extremely high. The fermion motion is a superposition of two circular motions: one due to its rigid rotation caused by forces exerted by the medium, another due to the external magnetic field. We derive an exact analytical expression for the spectral rate and the total intensity of this type of synchrotron radiation. Our numerical calculations indicate very high sensitivity of the radiation to the angular velocity of rotation. We investigate its dependence on the sign of electric charge, direction of the magnetic field and the sense of rotation and show that the radiation intensity is strongly enhanced if $q\mathit{B}$ and $\mathbf{\Omega }$ point in the opposite direction and is suppressed otherwise.

We report calculations of a wave-packet amplitude of the two-body scattering $\varphi \varphi \to \mathrm{\Phi }\to \varphi \varphi$, which leads to the measured probability in realistic experiments. We elucidate the decay amplitude of $\mathrm{\Phi }\to \varphi \varphi$ from this. In such an amplitude of wave packets, there are in and out time boundaries for the initial $\mathrm{\Phi }$ and final $\varphi \varphi$ configurations, respectively. In this paper, we prove that the effect of the in time boundary of $\mathrm{\Phi }\to \varphi \varphi$ emerges from $\varphi \varphi \to \mathrm{\Phi }\to \varphi \varphi$ without assuming any time boundary a priori. This effect has been overlooked in the standard plane-wave formulation and can exhibit distinct phenomena in wide areas of science. We confirm the result in different integration orders. The result is also interpreted as a Stokes phenomenon in the Lefschetz-thimble decomposition.

We consider the flavor structure of the $S_1$ leptoquark model and derive conservative constraints on the elements of the left- and right-handed coupling matrices in a number of representative scenarios. We focus on the cases where the muon $g-2$ deviation is explained by real muon couplings to either the top-quark or to the charm-quark or to all up-type quarks. The most significant constraints arise from charged lepton flavor-violating decays of the muon and the $\tau$ lepton and from the $\mu –e$ conversion process. Kaon decays and perturbativity provide further constraints. We find strong constraints on almost all coupling matrix elements, implying a very hierarchical matrix structure, where individual entries must differ by at least four orders of magnitude. The flexiblesusy program was used with appropriate model files incorporating the parametrization of the couplings in the up-type mass diagonal basis. The expressions for the leptonic observables were generated and cross-checked with the help of the NPointFunctions extension of the flexiblesusy program.

We present a detailed theoretical study of nonfactorizable contributions of the charm-quark loop to the amplitude of the $B_s\to \gamma \gamma$ decay. This contribution involves the $B$-meson three-particle Bethe-Salpeter amplitude, $⟨0|\overline{s}(y)G_{\mu \nu }(x)b(0)|{\overline{B}}_s(p)⟩$, for which we take into account constraints from analyticity and continuity. The charming-loop contribution of interest may be described as a correction to the Wilson coefficient $C_{7\gamma }$, $C_{7\gamma }\to C_{7\gamma }(1+\delta C_{7\gamma })$. We calculate an explicit dependence of $\delta C_{7\gamma }$ on the parameter ${\lambda }_{B_s}$. Taking into account all theoretical uncertainties, $\delta C_{7\gamma }$ may be predicted with better than 10% accuracy for any given value of ${\lambda }_{B_s}$. For our benchmark point ${\lambda }_{B_s}=0.45\mathrm{GeV}$, we obtain $\delta C_{7\gamma }=0.045\pm 0.004$. Presently, ${\lambda }_{B_s}$ is not known with high accuracy, but its value is expected to lie in the range $0.3\le {\lambda }_{B_s}(\mathrm{GeV})\le 0.6$. The corresponding range of $\delta C_{7\gamma }$ is found to be $0.02\le \delta C_{7\gamma }\le 0.1$. One therefore expects the correction given by charming loops at the level of at least a few percent.

Fitting the recent $e^{+}{e}^{-}\to \varphi \eta$ data by the Belle Collaboration with a theoretical formula reveals, besides the dominant $\varphi (1680)$ resonance, two narrow resonances: expected $\varphi (2170)$ resonance and an unexpected resonance with the mass of about 1851 MeV. Close proximity to the $X(1835)$ resonance suggests that the new resonance may be interpreted as the $p\overline{p}$ baryonium in an excited state. Follow-up analysis found the same resonance also in $e^{+}{e}^{-}\to \omega \eta$ data by the CMD-3 experiment.

We seek the inverse formulas for the cosmological unifying relation between gluons and conformally coupled scalars. We demonstrate that the weight-shifting operators derived from the conformal symmetry at the de Sitter (dS) late-time boundary can serve as the inverse operators for the 3-point cosmological correlators. However, in the case of the 4-point cosmological correlator, we observe that the inverse of the unifying relation cannot be constructed from the weight-shifting operators. Despite this failure, we are inspired to propose a “weight-shifting uplifting” method for the 4-point gluon correlator.

We compute the relative entropy between the vacuum and a coherent state for a massive scalar field in de Sitter spacetime, using Tomita-Takesaki modular theory and the Araki-Uhlmann formula for the relative entropy. Embedding de Sitter spacetime as a hyperboloid in the ambient Minkowski space, we can restrict the Minkowski wedge and the corresponding modular operator to de Sitter, and we verify that this construction gives the correct modular flow. We check that the relative entropy is positive and jointly convex, relate it to the Noether charge of translations along the trajectories of the modular flow, and determine the local temperature as seen by an observer that moves along these trajectories.

The theory of entropic gravity conjectures that gravity emerges thermodynamically rather than being a fundamental force. One of the main criticisms of entropic gravity is that it would lead to quantum massive particles losing coherence in free fall, which is not observed experimentally. This criticism was refuted in Schimmoller et al. [Phys. Rev. Res. 3, 033065 (2021)], where a nonrelativistic master equation modeling gravity as an open quantum system interaction demonstrated that in the strong coupling limit, coherence could be maintained and reproduce conventional free-fall dynamics. Moreover, the nonrelativistic master equation was shown to be fully compatible with the qbounce experiment for ultracold neutrons. Motivated by this, we extend these results to gravitationally accelerating Dirac fermions. We achieve this by using the Dirac equation in Rindler space and modeling entropic gravity as a thermal bath thus adopting the open quantum systems approach as well. We demonstrate that in the strong coupling limit, our entropic gravity model maintains quantum coherence for Dirac fermions. In addition, we demonstrate that spin is not affected by entropic gravity. We use the Foldy-Wouthysen transformation to demonstrate that it reduces to the nonrelativistic master equation, supporting the entropic gravity hypothesis for Dirac fermions. Also, we demonstrate how antigravity seemingly arises from the Dirac equation for free-falling antiparticles but use numerical simulations to show that this phenomenon originates from zitterbewegung thus not violating the equivalence principle.

We investigate the conserved quantities associated to Killing isometries for asymptotically AdS spacetimes within the framework of quadratic-curvature gravity. By constructing a rank-4 tensor possessing the same index symmetries as the ones of the Riemann tensor, we propose a 2-form potential resembling the Noether one for quadratic-curvature gravity. Such a potential is compared with the results via other methods existing in the literature to establish the equivalence. Then this potential is adopted to define conserved quantities of asymptotically AdS spacetimes. As applications, we explicitly compute the mass of static spherically symmetric spacetimes, as well as the mass and the angular momentum for rotating spacetimes, such as the four(higher)-dimensional Kerr-AdS black holes and black strings embedded in quadratic-curvature gravities. Particularly, we emphasize the conserved charges of Einstein-Gauss-Bonnet, Weyl, and critical gravities, together with the ones for the asymptotically AdS solutions satisfying vacuum Einstein field equations.

Various astrophysical and cosmological observations today serve as indirect evidence of the existence of dark matter in the Universe. In the present work, we propose a class of spacetimes that show some important characteristics relevant to the spacetime of a dark matter halo. These spacetimes are static and spherically symmetric solutions of the Einstein field equations. The proposed spacetimes satisfy the flat velocity profile of a galactic object far away from the center, they give good agreement to the astrometric data of the S2 star, and these also cast a central shadow. Using the Penrose diagram, we show that the causal nature of the central singularity here is null. This single spacetime model for galactic dark matter may therefore be used to explain some of the properties of galactic dynamics at different length scales.

Gravitational waves provide a powerful enhancement to our understanding of fundamental physics. To make the most of their detection we need to accurately model the entire process of their emission and propagation toward interferometers. Cauchy-characteristic extraction and matching are methods to compute gravitational waves at null infinity, a mathematical idealization of detector location, from numerical relativity simulations. Both methods can in principle contribute to modeling by providing highly accurate gravitational waveforms. An underappreciated subtlety in realizing this potential is posed by the (mere) weak hyperbolicity of the particular partial differential equation (PDE) systems solved in the characteristic formulation of the Einstein field equations. This shortcoming results from the popular choice of Bondi-like coordinates. So motivated, we construct toy models that capture that PDE structure and study Cauchy-characteristic extraction and matching with them. Where possible we provide energy estimates for their solutions and perform careful numerical norm convergence tests to demonstrate the effect of weak hyperbolicity on Cauchy-characteristic extraction and matching. Our findings strongly indicate that, as currently formulated, Cauchy-characteristic matching for the Einstein field equations would provide solutions that are, at best, convergent at an order lower than expected for the numerical method, and may be unstable. In contrast, under certain conditions, the extraction method can provide properly convergent solutions. Establishing however that these conditions hold for the aforementioned characteristic formulations is still an open problem.

We consider static and spherically symmetric wormhole solutions in extended metric-affine theories of gravity supposing that stability and traversability of these objects can be achieved by means of the more geometric degrees of freedom with respect to general relativity. In particular, we concentrate on $f(R)$ metric, $f(T)$ teleparallel, and $f(Q)$ symmetric teleparallel models, where $f$ is a smooth function of curvature, torsion, and nonmetricity, respectively. In these extended frameworks, $R$, $T$, and $Q$ rule entirely the background geometry without the need to invoke any exotic energy-momentum tensor as matter field source. Assuming that the solution does not present any kind of singularities, we start from the flaring out and null energy conditions to gather together a series of constraints. They allow us to have general indications, which are only necessary but not sufficient and concern only the existence of a throat rather than a global spacetime configuration, to build up then traversable wormholes through a purely geometric approach. The stability cannot be simply assessed via inequalities, because it requires the explicit solution for a detailed analysis.

The motion of a radiating point particle can be represented by a series of geodesics whose “constants” of motion evolve slowly with time. The evolution of these constants of motion can be determined directly from the self-force equations of motion. In the presence of spacetime symmetries, the situation simplifies; there exist not only constants of motion conjugate to these symmetries, but also conserved currents whose fluxes can be used to determine their evolution. Such a relationship between point-particle motion and fluxes of conserved currents is a flux-balance law. However, there exist constants of motion that are not related to spacetime symmetries, the most notable example of which is the Carter constant in the Kerr spacetime. In this paper, we first present a new approach to flux-balance laws for spacetime symmetries, using the techniques of symplectic currents and symmetry operators, which can also generate more general conserved currents. We then derive flux-balance laws for all constants of motion in the Kerr spacetime, using the fact that the background, geodesic motion is integrable. For simplicity, we restrict derivations in this paper to the scalar self-force problem. While generalizing the discussion in this paper to the gravitational case will be straightforward, there will be additional complications in turning these results into a practical flux-balance law in this case.

We show that the dynamics of Schwarzschild-(anti–)de Sitter [(A)dS] black holes admits a symmetry under the 2D Schrödinger group, whatever the sign or value of the cosmological constant. This is achieved by reformulating the spherically symmetric reduction of general relativity as a 2D mechanical system with a nontrivial potential controlled by the cosmological constant, and explicitly identifying the conserved charges for black hole mechanics. We expect the Schrödinger symmetry to drive the dynamics of quantum Schwarzschild-(A)dS black holes. This suggests that Schrödinger-preserving nonlinear deformations (of the Gross-Piteavskii type) should capture universal quantum gravity corrections to the black hole geometry. Such a scenario could be realized in condensed matter analog models.

Recent studies have shown that quasinormal modes suffer from spectral instabilities, a frailty of black holes that leads to disproportional migration of their spectra in the complex plane when black-hole effective potentials are modified by minuscule perturbations. Similar results have been found with the mathematical notion of the pseudospectrum which was recently introduced in gravitational physics. Environmental effects, such as the addition of a thin accretion disk or a matter shell, lead to a secondary bump that appears in the effective potential of black hole perturbations. Regardless of the environment’s small contribution to the effective potential, its presence can completely destabilize the fundamental quasinormal mode and may potentially affect black hole spectroscopy. Here, we perform a comprehensive analysis of such phenomenon for Schwarzschild, Reissner-Nordström, Schwarzschild-de Sitter, and Reissner-Nordström-de Sitter black holes by considering the potential for a test scalar field with the addition of a tiny bump sufficiently away from the photon sphere. We find a qualitatively similar destabilization pattern for photon sphere, complex, scalar quasinormal modes in all cases, and a surprising spectral stability for dominant scalar, purely imaginary, de Sitter and near-extremal modes that belong to different families of the spectrum. For Reissner-Nordström-de Sitter black holes, we reevaluate the validity of the strong cosmic censorship and find that the addition of a realistic bump in the effective potential cannot prevent its violation due to a combination of the spectral stability of dominant de Sitter and near-extremal modes for small cosmological constants and an ineffective migration of the photon sphere modes that dominate the late-time ringdown signal for sufficiently large cosmological constants.

Extreme mass-ratio inspirals (EMRIs) are one of the most sensitive probes of black hole spacetimes with gravitational-wave measurements. In this work, we systematically analyze the dynamics of an EMRI system near orbital resonances, assuming the background spacetime is weakly perturbed from Kerr. Using the action-angle formalism, we have derived an effective resonant Hamiltonian that describes the dynamics of the resonant degree of freedom, for the case that the EMRI motion across the resonance regime. This effective resonant Hamiltonian can also be used to derive the condition that the trajectory enters or exits a resonant island and the permanent change of action variables across the resonance with the gravitational-wave radiation turned on. The orbital chaos, on the other hand, generally leads to transitions between different branches of rotational orbits with finite changes of the action variables. These findings are demonstrated with numerical orbital evolutions that are mapped into representations using action-angle variables. This study is one part of the program of understanding EMRI dynamics in a generic perturbed Kerr spacetime, which paves the way of using EMRIs to precisely measure the black hole spacetime.

We present a numerically stable system of ($3+1$) evolution equations for the nonlinear gravitational dynamics of quadratic-curvature corrections to General Relativity (quadratic gravity). We also report on the numerical implementation of these evolution equations. We recover a well-known linear instability and gather evidence that—aside from said instability—quadratic gravity exhibits a physically stable Ricci-flat subsector. In particular, we demonstrate that Teukolsky-wave perturbations of a Schwarzschild black hole as well as a full binary inspiral (evolved up to merger) remain Ricci flat throughout evolution. This suggests that, at least in vacuum, classical quadratic gravity can mimic General Relativity, even in the fully nonlinear strong-gravity regime.

Bousso’s entropy bound is a conjecture that the entropy through a null hypersurface emanating from a two-dimensional surface with a nonpositive expansion is bounded by the area of that two-dimensional surface. We investigate the validity of Bousso’s entropy bound in the spatially flat, homogeneous, and isotropic universe with an adiabatic entropy current. We find that the bound is satisfied in the entire spacetime in which a cutoff time is introduced based on the entropy density and the energy density. Compared to the previously used prescription which puts a cutoff near the curvature singularity, our criterion for introducing the cutoff is applicable even to a nonsingular universe. Our analysis provides an interpretation of the incompleteness implied by the recently proposed singularity theorem based on the entropy bounds.

Black hole spectroscopy is the program to measure the complex gravitational wave frequencies of merger remnants, and to quantify their agreement with the characteristic frequencies of black holes computed at linear order in black hole perturbation theory. In a “weaker” (nonagnostic) version of this test, one assumes that the frequencies depend on the mass and spin of the final Kerr black hole as predicted in perturbation theory. Linear perturbation theory is expected to be a good approximation only at late times, when the remnant is close enough to a stationary Kerr black hole. However, it has been claimed that a superposition of overtones with frequencies fixed at their asymptotic values in linear perturbation theory can reproduce the waveform strain even at the peak. Is this overfitting, or are the overtones physically present in the signal? To answer this question, we fit toy models of increasing complexity, waveforms produced within linear perturbation theory, and full numerical relativity waveforms using both agnostic and nonagnostic ringdown models. We find that higher overtones are unphysical; their role is mainly to “fit away” features such as initial data effects, power-law tails, and (when present) nonlinearities. We then identify physical quasinormal modes by fitting numerical waveforms in the original, agnostic spirit of the no-hair test. We find that a physically meaningful ringdown model requires the inclusion of higher multipoles, quasinormal mode frequencies induced by spherical-spheroidal mode mixing, and nonlinear quasinormal modes. Even in this “infinite signal-to-noise ratio” version of the original spectroscopy test, there is convincing evidence for the first overtone of the dominant multipole only well after the peak of the radiation.

Deep surveys of the CMB polarization have more information on the lensing signal than the quadratic estimators (QE) can capture. We showed in a recent work that a CMB lensing power spectrum built from a single optimized CMB lensing mass map, working in close analogy to state-of-the-art QE techniques, can result in an essentially optimal spectrum estimator at reasonable numerical cost. We extend this analysis here to account for real-life nonidealities including masking and realistic instrumental noise maps. As in the QE case, it is necessary to include small corrections to account for the estimator response to these anisotropies, which we demonstrate can be estimated easily from simulations. The realization-dependent debiasing of the spectrum remains robust, allowing unbiased recovery of the band powers even in cases where the statistical model used for the lensing map reconstruction is grossly wrong. This allows robust and optimal CMB lensing constraints from CMB data, on all scales relevant for the inference of the neutrino mass, or other parameters of our cosmological model.