We show that the predictions of commonly used spatially smeared particle detectors coupled to quantum fields are not generally covariant outside the pointlike limit. This lack of covariance manifests itself as an ambiguity in the time-ordering operation. We analyze how the breakdown of covariance affects typical detector models in quantum field theory such as the Unruh–DeWitt model. Specifically, we show how the violations of covariance depend on the state of the detectors-field system, the shape and state of motion of the detectors, and the spacetime geometry. Furthermore, we provide the tools to explicitly evaluate the magnitude of the violation and identify the regimes where the predictions of smeared detectors are either exactly or approximately covariant in perturbative analyses, thus providing limits of validity of smeared particle detector models.

We consider a quantum scalar field on the classical background of an asymptotically anti–de Sitter black hole and the backreaction the field’s stress-energy tensor induces on the black hole geometry. The backreaction is computed by solving the reduced-order semiclassical Einstein field equations sourced by simple analytical approximations for the renormalized expectation value of the scalar field stress-energy tensor. When the field is massless and conformally coupled, we adopt Page’s approximation to the renormalized stress-energy tensor, while for massive fields we adopt a modified version of the DeWitt-Schwinger approximation. The latter approximation must be modified so that it possesses the correct renormalization freedom required to ensure the semiclassical equations are consistent. Equipped with these approximations, the reduced-order field equations are easily integrated and the first-order (in $\hslash$) corrections to the metric are obtained. We also compute the corrections to the black hole event horizon, surface gravity, and minimum temperature as well as corrections to the photon sphere and quadratic curvature invariants. We pay particular attention to the temperature profiles of the semiclassical black holes compared with their classical counterparts, pointing out some interesting qualitative features produced by the backreaction. These results ought to provide reasonable approximations to the first-order (one-loop) quantum backreaction on the geometry of asymptotically anti–de Sitter black holes when the exact numerical stress-energy tensor sources the semiclassical equations.

It is a well-known fact that light rays do not follow the null geodesics of the space-time in nonlinear electrodynamics; instead, they follow the null geodesics of the so-called effective space-time. Taking this into account, in this paper, we aim to discuss the possibility of distinguishing the type of charge with which the black hole is endowed, via the motion of light rays. The results show that, for any black hole being a charged solution of the field equations of general relativity coupled to the nonlinear electrodynamics, one cannot distinguish the two types of charge (magnetic or electric) through the motion of light rays around it.

In this paper, we seek to answer the question “given a rotating core collapse gravitational wave signal, can we determine its nuclear equation of state?” To answer this question, we employ deep convolutional neural networks to learn visual and temporal patterns embedded within rotating core collapse gravitational wave (GW) signals in order to predict the nuclear equation of state (EOS). Using the 1824 rotating core collapse GW simulations by Richers et al. [Phys. Rev. D 95, 063019 (2017).], which have 18 different nuclear EOSs, we consider this to be a classic multiclass image classification and sequence classification problem. We attain up to 72% correct classifications in the test set, and if we consider the “top five” most probable labels, this increases to up to 97%, demonstrating that there is a moderate and measurable dependence of the rotating core collapse GW signal on the nuclear EOS.

In gravitational-wave observations of binary black holes (BBHs), theoretical waveform models are used to infer the black-hole properties. There are several sources of potential systematic errors in these measurements, including due to physical approximations in the models. One standard approximation is to neglect a small asymmetry between the $+m$ and $-m$ spherical-harmonic modes; this is the effect that leads to emission of linear momentum perpendicular to the orbital plane, and can result in large recoils of the final black hole. The asymmetry is determined by both the magnitude and direction of the spin components that lie in the orbital plane. We investigate the validity of this approximation by comparing numerical relativity (NR) simulations of single-spin NR systems with varying in-plane spin directions and magnitudes (including several “superkick” configurations). We find that the mode asymmetry will impact measurements at signal-to-noise ratios (SNRs) between 15 and 80, which is well within current observations. In particular, mode asymmetries are likely to impact measurements at comparable SNRs to those at which we might hope to make the first unambiguous measurements of orbital precession. We therefore expect that models will need to include mode-asymmetry effects to make unbiased precession measurements.

No. All theoretical predictions for the observational appearance of an accreting supermassive black hole, as measured interferometrically by a sparse Earth-sized array at current observation frequencies, are sensitive to many untested assumptions about accretion flow and emission physics. There is no way to distinguish a violation of general relativity from the much more likely scenario that the relevant “gastrophysical” assumptions simply do not hold. Tests of general relativity will become possible with longer interferometric baselines (likely requiring a space mission) that reach the resolution where astrophysics-independent predictions of the theory become observable.

The Jordan and Einstein frames are studied under the light of the Hamiltonian formalism. Dirac’s constraint theory for Hamiltonian systems is applied to Brans-Dicke theory in the Jordan frame. In both the Jordan and Einstein frames, Brans-Dicke theory has four secondary first class constraints and their constraint algebra is closed. We show, contrary to what is generally believed, the Weyl (conformal) transformation, between the two frames, is not a canonical transformation, in the sense of the Hamiltonian formalism. This addresses quantum mechanical inequivalence as well. A canonical transformation is shown.

We demonstrate an extraordinary effect of energy gain by a single radiating charged particle inside the ergosphere of a Kerr black hole in presence of magnetic field. We solve numerically the covariant form of the Lorentz-Dirac equation reduced from the DeWitt-Brehme equation and analyze energy evolution of the radiating charged particle inside the ergosphere, where the energy of emitted radiation can be negative with respect to a distant observer in dependence on the relative orientation of the magnetic field, black hole spin and the direction of the charged particle motion. Consequently, the charged particle can leave the ergosphere with energy greater than initial in expense of black hole’s rotational energy. In contrast to the original Penrose process and its various modification, the new process does not require the interactions (collisions or decay) with other particles and consequent restrictions on the relative velocities between fragments. We show that such a radiative Penrose effect is potentially observable and discuss its possible relevance in formation of relativistic jets and in similar high-energy astrophysical settings.

We show that light scalars can form quasibound states around binaries. In the nonrelativistic regime, these states are formally described by the quantum-mechanical Schrödinger equation for a one-electron heteronuclear diatomic molecule. We performed extensive numerical simulations of scalar fields around black hole binaries, showing that a scalar structure condenses around the binary—we dub these states “gravitational molecules.” We further show that these are well described by the perturbative, nonrelativistic description.

It was pointed out that the black hole quasinormal modes resulting from a piecewise approximate potential are drastically distinct from those pertaining to the original black hole metric. In particular, instead of lining up parallel to the imaginary axis, the spectrum is found to stretch out along the real axis. In this work, we prove that if there is a single discontinuity in the effective potential, no matter how insignificant it is, the asymptotic behavior of the quasinormal modes will be appreciably modified. Besides showing numerical evidence, we give analytical derivations to support the above assertion even when the discontinuity is located significantly further away from the maximum of the potential and/or the size of the step is arbitrarily small. Moreover, we discuss the astrophysical significance of the potential implications in terms of the present findings.

We investigate gravitational lensing effects of spherically symmetric black holes in Einstein quartic gravity (EQG). Using an approximate analytic solution obtained by continued fraction methods we consider the predictions of EQG for lensing effects by supermassive black holes at the center of our galaxy and others in comparison with general relativity (GR). We numerically compute both time delays and angular positions of images and find that they can deviate from GR by as much as milliarcseconds, suggesting that observational tests of EQG are feasible in the near future. We discuss the challenges of distinguishing the predictions of EQG from those of Einstein cubic gravity.

If the symmetry breaking inducing the axion occurs after the inflation, the large axion isocurvature perturbations can arise due to a different axion amplitude in each causally disconnected patch. This causes the enhancement of the small-scale density fluctuations which can significantly affect the evolution of structure formation. The epoch of the small halo formation becomes earlier and we estimate the abundance of those minihalos which can host the neutral hydrogen atoms to result in the 21 cm fluctuation signals. We find that the future radio telescopes, such as the Square Kilometer Array, can put the axion mass bound of order $m_a\gtrsim {10}^{-13}\mathrm{eV}$ for the simple temperature-independent axion mass model, and the bound can be extended to of order $m_a\gtrsim {10}^{-8}\mathrm{eV}$ for a temperature-dependent axion mass.

We study the spectrum and anisotropies of ultrahigh energy cosmic ray transient sources, accounting for the effects of their propagation through the turbulent extragalactic magnetic fields. We consider either bursting sources or sources emitting since a given initial time. We analyze in detail the transition between the diffusive and the quasirectilinear regimes, describing some new features that could be present.

TeV-scale particles that couple to the standard model through the weak force represent a compelling class of dark matter candidates. The search for such weakly interacting massive particles has already spanned multiple decades, and whilst it has yet to provide any definitive evidence for their existence, viable parameter space remains. In this paper, we show that the upcoming Cherenkov Telescope Array (CTA) has significant sensitivity to uncharted parameter space at the TeV mass scale. To do so, we focus on two prototypical dark matter candidates, the Wino and Higgsino. Sensitivity forecasts for both models are performed including the irreducible background from misidentified cosmic rays, as well as a range of estimates for the Galactic emissions at TeV energies. For each candidate, we find substantial expected improvements over existing bounds from current imaging atmospheric Cherenkov telescopes. In detail, for the Wino we find a sensitivity improvement of roughly an order of magnitude in $⟨\sigma v⟩$, whereas for the Higgsino we demonstrate that CTA has the potential to become the first experiment that has sensitivity to the thermal candidate. Taken together, these enhanced sensitivities demonstrate the discovery potential for dark matter at CTA in the 1–100 TeV mass range.

Axion-like-particles (ALPs) emitted from the core of a magnetar can convert to photons in its magnetosphere. The resulting photon flux is sensitive to the product of (i) the ALP-nucleon coupling $G_{an}$ which controls the production cross section in the core and (ii) the ALP-photon coupling $g_{a\gamma \gamma }$ which controls the conversion in the magnetosphere. We study such emissions in the soft-gamma-ray range (300 keV to 1 MeV), where the ALP spectrum peaks and astrophysical backgrounds from resonant Compton up scattering are expected to be suppressed. Using published quiescent soft-gamma-ray flux upper limits in five magnetars obtained with CGRO COMPTEL and INTEGRAL SPI/IBIS/ISGRI, we put limits on the product of the ALP-nucleon and ALP-photon couplings. We also provide a detailed study of the dependence of our results on the magnetar core temperature. We further show projections of our result for future Fermi-GBM observations. Our results motivate a program of studying quiescent soft-gamma-ray emission from magnetars with the Fermi-GBM.

A possible setup for the experimental verification of linear Breit-Wheeler pair creation of electrons and positrons in photon-photon collisions is studied theoretically. It combines highly energetic bremsstrahlung photons, which are assumed to be generated by an incident beam of GeV electrons penetrating through a high-$Z$ target, with keV photons from an x-ray laser field, which is described as a focused Gaussian pulse. We discuss the dependencies of the pair yields on the incident electron energy, target thickness, laser parameters, and collision geometry. It is shown that, for suitable conditions that are nowadays in reach at x-ray laser facilities, the resulting number of created particles seems to be well accessible for enabling the first experimental observation of the linear Breit-Wheeler process $\gamma \gamma \to e^{+}{e}^{-}$.

We argue that the search for dark vector boson through $e^{+}{e}^{-}\to Z_d\gamma$ can determine the Lorentz structure of $Z_d{l}^{+}{l}^{-}$ couplings with the detection of leptonic decays $Z_d\to l^{+}{l}^{-}$. We assume a general framework that the dark vector boson interacts with ordinary fermions through vector and axial-vector couplings. As a consequence of Ward-Takahashi identity, $Z_d$ is transversely polarized in the limit $m_{Z_d}\ll \sqrt{s}$. On the other hand, the fraction of longitudinal $Z_d$ is non-negligible for $m_{Z_d}$ comparable to $\sqrt{s}$. Such polarization effects can be analyzed through angular distributions of final-state particles in $Z_d$ decays. Taking $l^{\pm }\equiv {\mu }^{\pm }$, we study the correlation between $Z_d$ angle relative to $e^{-}$ beam direction in $e^{+}{e}^{-}$ center of momentum frame and ${\mu }^{-}$ angle relative to the boost direction of $Z_d$ in $Z_d$ rest frame. This correlation is shown to be useful for probing the Lorentz structure of $Z_d{l}^{+}{l}^{-}$ couplings. We discuss the measurement of such correlation in Belle II detector, taking into account the detector acceptance and energy resolution.

In this work, we investigate the beyond standard model (BSM) impact of leptophilic U(1) models, namely $U(1{)}_{L_{\mu }-L_e}$, $U(1{)}_{L_e-L_{\tau }}$ and $U(1{)}_{L_{\mu }-L_{\tau }}$ on coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\nu \mathrm{NS}$) and hence its effect on dark matter (DM) direct detection experiments. Imposing the latest relevant experimental constraints on these models, we obtain $\mathcal{O}(50\%)$ enhancement for case of $U(1{)}_{L_{\mu }-L_{\tau }}$ in a region $m_Z^{\prime }\approx 20\mathrm{MeV}$. Subsequently, we observe that the enhancement seen in $\mathrm{CE}\nu \mathrm{NS}$ is roughly getting translated to enhancement by a factor of 2.7 (for germanium based detectors) and 1.8 (for xenon based detectors) in the neutrino scattering event rate which eventually enhances the neutrino floor by the same amount. This enhancement is more prominent in the region with DM masses less than 10 GeV. The model parameter space that leads to this enhancement, can simultaneously explain both anomalous magnetic moment of muon ($(g-2{)}_{\mu }$) and observed DM relic density, in a modified scenario. Enhancement of neutrino floor requires increased number of DM-nucleon scattering events in the future DM direct detection experiments, to establish themselves to be DM signal events. In absence of any DM signal, those experiments can directly be used to measure the neutrino rate, quantifying the BSM effects.

We present a possibility that the neutrinoless double beta decay can be hidden in the minimal seesaw mechanism where the standard model is extended by two right-handed neutrinos which have a hierarchical mass structure. In this framework, the lepton number is violated due to the massive Majorana neutrinos. Especially, we investigate the case that the heavier right-handed neutrino is sufficiently heavy to decouple from the decay while the lighter one is lighter enough than the typical Fermi-momentum scale of nuclei and gives a sizable contribution to the decay. Under the specific condition on mixing elements, the lighter right-handed neutrino can give a significant destructive contribution which suppresses or even hides to the effective mass of the neutrinoless double beta decay. In this case, the flavor structure of the mixing element of the lighter right-handed neutrino with ordinary neutrinos is predicted depending on the Majorana $CP$ violating phase of active neutrinos.

We discuss the possible origin of the Majorana mass scale(s) required for the “neutrino option” where the electroweak scale is generated simultaneously with light neutrino masses in a type-I seesaw model by common dimension four interactions. We establish no-go constraints on the perturbative generation of the Majorana masses required due to global symmetries of the seesaw Lagrangian.