We investigate the sensitivity of a search for the oscillating current induced by axion dark matter in an external magnetic field using optically pumped magnetometers. This experiment is based upon the LC circuit (circuit with inductor and capacitor) axion detection concept of Sikivie et al. [Phys. Rev. Lett. 112, 131301 (2014)]. The modification of Maxwell’s equations caused by the axion-photon coupling results in a minute magnetic field oscillating at a frequency equal to the axion mass, in the presence of an external magnetic field. The axion-induced magnetic field could be searched for using a LC circuit amplifier with an optically pumped magnetometer, the most sensitive cryogen-free magnetic-field sensor, in a room-temperature experiment, avoiding the need for a complicated and expensive cryogenic system. We discuss how an existing magnetic resonance imaging experiment can be modified to search for axions in a previously unexplored part of the parameter space. Our existing detection setup, optimized for magnetic resonance imagining, is already sensitive to an axion-photon coupling of ${10}^{-7}{\mathrm{GeV}}^{-1}$ for an axion mass near $3\times {10}^{-10}\mathrm{eV}$, which is already limited by astrophysical processes and solar axion searches. We show that realistic modifications, and optimization of the experiment for axion detection, can probe the axion-photon coupling up to 4 orders of magnitude beyond the current best limit, for axion masses between ${10}^{-11}$ and ${10}^{-7}\mathrm{eV}$.

An analysis of the bottom baryon decay ${\mathrm{\Lambda }}_b\to J/\psi (\to {\mu }^{+}{\mu }^{-})\mathrm{\Lambda }(\to p{\pi }^{-})$ is performed to measure the ${\mathrm{\Lambda }}_b$ polarization and three angular parameters in data from $pp$ collisions at $\sqrt{s}=7$ and 8 TeV, collected by the CMS experiment at the Large Hadron Collider. The ${\mathrm{\Lambda }}_b$ polarization is measured to be $0.00\pm 0.06(\mathrm{stat})\pm 0.06(\mathrm{syst})$ and the parity-violating asymmetry parameter is determined to be $0.14\pm 0.14(\mathrm{stat})\pm 0.10(\mathrm{syst})$. The measurements are compared to various theoretical predictions, including those from perturbative quantum chromodynamics.

We consider a chain of Abelian Klebanov-Tarnopolsky fermionic tensor models coupled through quartic nearest-neighbor interactions. We characterize the gauge-singlet spectrum for small chains ($L=2,3,4,5$) and observe that the spectral statistics exhibits strong evidence in favor of quasi-many-body localization.

We present $\mathcal{N}=4$ supersymmetric mechanics on $n$-dimensional Riemannian manifolds constructed within the Hamiltonian approach. The structure functions entering the supercharges and the Hamiltonian obey modified covariant constancy equations as well as modified Witten–Dijkgraaf–Verlinde–Verlinde equations specified by the presence of the manifold’s curvature tensor. Solutions of original Witten–Dijkgraaf–Verlinde–Verlinde equations and related prepotentials defining $\mathcal{N}=4$ superconformal mechanics in flat space can be lifted to $so(n)$-invariant Riemannian manifolds. For the Hamiltonian this lift generates an additional potential term which, on spheres and (two-sheeted) hyperboloids, becomes a Higgs-oscillator potential. In particular, the sum of $n$ copies of one-dimensional conformal mechanics results in a specific superintegrable deformation of the Higgs oscillator.

In the present work we study the propagation of a probe minimally coupled scalar field in Einstein-power-Maxwell charged black hole background in ($1+2$) dimensions. We find analytical expressions for the reflection coefficient as well as for the absorption cross section in the low energy regime, and we show graphically their behavior as functions of the frequency for several values of the free parameters of the theory.

We address the problem of thermodynamic equilibrium with constant acceleration along the velocity field lines in a quantum relativistic statistical mechanics framework. We show that for a free scalar quantum field, after vacuum subtraction, all mean values vanish when the local temperature $T$ is as low as the Unruh temperature $T_U=A/2\pi$ where $A$ is the magnitude of the acceleration four-vector. We argue that the Unruh temperature is an absolute lower bound for the temperature of any accelerated fluid at global thermodynamic equilibrium. We discuss the conditions of this bound to be applicable in a local thermodynamic equilibrium situation.

The Rindler horizon in Minkowski spacetime can be implanted with supertranslation hair by a matter shock wave without planar symmetry, and the hair is observable as a supertranslation memory on the Rindler family of uniformly linearly accelerated observers. We show that this classical memory is accompanied by a supertranslation quantum memory that modulates the entanglement between the opposing Rindler wedges in quantum field theory. A corresponding phenomenon across a black hole horizon may play a role in Hawking, Perry, and Strominger’s proposal for supertranslations to provide a solution to the black hole information paradox.

Superradiant scattering is a radiation enhancement process that takes place in many contexts, and which has recently found exciting applications in astrophysics and particle physics. In the framework of curved spacetime physics, it has been associated with the classical Penrose process for particles. Superradiance is usually also associated with bosonic fields around geometries with ergoregions and horizons. These notions are in clear tension however: the Penrose process occurs for horizonless geometries, and particles are composed of fermions. Here, we resolve the tension in its different aspects, by showing that (i) superradiance occurs for self-interacting fermions on flat spacetime. (ii) Superradiance occurs also for horizonless geometries, where it leads to an ergoregion instability. Ultracompact, horizonless geometries will usually respond with echoes of growing amplitude, until rotational (or electrostatic) energy is extracted from the object. (iii) The Fourier-domain analysis leads to absence of superradiance when horizons are not present. We elucidate why this analysis fails to give meaningful results. (iv) Finally, we show that superradiant, ergoregion instabilities have a particle analog of similar growth timescales and which can power the formation of a structure outside a compact, rotating star.

We study strange stars in the framework of $f(R,\mathcal{T})$ theory of gravity. To provide exact solutions of the field equations it is considered that the gravitational Lagrangian can be expressed as the linear function of the Ricci scalar $R$ and the trace of the stress-energy tensor $\mathcal{T}$, i.e. $f(R,\mathcal{T})=R+2\chi \mathcal{T}$, where $\chi$ is a constant. We also consider that the strange quark matter (SQM) distribution inside the stellar system is governed by the phenomenological MIT bag model equation of state (EOS), given as $p_r=\frac{1}{3}(\rho -4B)$, where $B$ is the bag constant. Further, for a specific value of $B$ and observed values of mass of the strange star candidates we obtain the exact solution of the modified Tolman-Oppenheimer-Volkoff (TOV) equation in the framework of $f(R,\mathcal{T})$ gravity and have studied in detail the dependence of the different physical parameters, like the metric potentials, energy density, radial and tangential pressures and anisotropy etc., due to the chosen different values of $\chi$. Likewise in GR, as have been shown in our previous work [Deb et al., Ann. Phys. (Amsterdam) 387, 239 (2017)] in the present work also we find maximum anisotropy at the surface which seems an inherent property of the strange stars in modified $f(R,\mathcal{T})$ theory of gravity. To check the physical acceptability and stability of the stellar system based on the obtained solutions we have performed different physical tests, viz., the energy conditions, Herrera cracking concept, adiabatic index etc. In this work, we also have explained the effects, those are arising due to the interaction between the matter and the curvature terms in $f(R,\mathcal{T})$ gravity, on the anisotropic compact stellar system. It is interesting to note that as the values of $\chi$ increase the strange stars become more massive and their radius increase gradually so that eventually they gradually turn into less dense compact objects. The present study reveals that the modified $f(R,\mathcal{T})$ gravity is a suitable theory to explain massive stellar systems like recent magnetars, massive pulsars and super-Chandrasekhar stars, which cannot be explained in the framework of GR. However, for $\chi =0$ the standard results of Einsteinian gravity are retrieved.

We study the local shadow of the Schwarzschild black hole with a quadrupole distortion and the influence of the external gravitational field on the photon dynamics. The external matter sources modify the light ring structure and lead to the appearance of multiple shadow images. In the case of negative quadrupole moments we identify the most prominent mechanism causing multiple shadow formation. Furthermore, we obtain a condition under which this mechanism can be realized. This condition depends on the quadrupole moment, but also on the position of the observer and the celestial sphere.

The Mathisson-Papapetrou-Dixon (MPD) equations, providing the “pole-dipole” description of spinning test particles in general relativity, have to be supplemented by a condition specifying the worldline that will represent the history of the studied body. It has long been thought that the Mathisson-Pirani (MP) spin condition—unlike other major choices made in the literature—does not yield an explicit momentum-velocity relation. We derive here the desired (and very simple) relation and show that it is in fact equivalent to the MP condition. We clarify the apparent paradox between the existence of such a definite relation and the known fact that the MP condition is degenerate (does not specify a unique worldline), thus shedding light on some conflicting statements made in the literature. We then show how, for a given body, this spin condition yields infinitely many possible representative worldlines, and derive a detailed method how to switch between them in a curved spacetime. The MP condition is a convenient choice in situations when it is easy to recognize its “nonhelical” solution, as exemplified here by bodies in circular orbits and in radial fall in the Schwarzschild spacetime.

We explore spacetime symmetries and topologies of the two metric sectors in Hassan–Rosen bimetric theory. We show that, in vacuum, the two sectors can either share or have separate spacetime symmetries. If stress–energy tensors are present, a third case can arise, with different spacetime symmetries within the same sector. This raises the question of the best definition of spacetime symmetry in Hassan–Rosen bimetric theory. We emphasize the possibility of imposing ansatzes and looking for solutions having different Killing vector fields or different isometries in the two sectors, which has gained little attention so far. We also point out that the topology of spacetime imposes a constraint on possible metric combinations.

We study the gravitational field of static $p$-branes in $D$-dimensional Minkowski space in the framework of linearized ghost-free (GF) gravity. The concrete models of GF gravity we consider are parametrized by the nonlocal form factors $\exp (-\square /{\mu }^2)$ and $\exp ({\square }^2/{\mu }^4)$, where ${\mu }^{-1}$ is the scale of nonlocality. We show that the singular behavior of the gravitational field of $p$-branes in general relativity is cured by short-range modifications introduced by the nonlocalities, and we derive exact expressions of the regularized gravitational fields, whose geometry can be written as a warped metric. For large distances compared to the scale of nonlocality, $\mu r\to \infty$, our solutions approach those found in linearized general relativity.

There is an exciting prospect of obtaining the shadow of astrophysical black holes (BHs) in the near future with the Event Horizon Telescope. As a matter of principle, this justifies asking how much one can learn about the BH horizon itself from such a measurement. Since the shadow is determined by a set of special photon orbits, rather than horizon properties, it is possible that different horizon geometries yield similar shadows. One may then ask how sensitive is the shadow to details of the horizon geometry? As a case study, we consider the double Schwarzschild BH and analyze the impact on the lensing and shadows of the conical singularity that holds the two BHs in equilibrium—herein taken to be a strut along the symmetry axis in between the two BHs. Whereas the conical singularity induces a discontinuity of the scattering angle of photons, clearly visible in the lensing patterns along the direction of the strut’s location, it produces no observable effect on the shadows, whose edges remain everywhere smooth. The latter feature is illustrated by examples including both equal and unequal mass BHs. This smoothness contrasts with the intrinsic geometry of the (spatial sections of the) horizon of these BHs, which is not smooth, and provides a sharp example on how BH shadows are insensitive to some horizon geometry details. This observation, moreover, suggests that for the study of their shadows, this static double BH system may be an informative proxy for a dynamical binary.

Higher-derivative gravity theories, such as Lovelock theories, generalize Einstein’s general relativity (GR). Modifications to GR are expected when curvatures are near Planckian and appear in string theory or supergravity. But can such theories describe gravity on length scales much larger than the Planck cutoff length scale? Here we find causality constraints on Lovelock theories that arise from the requirement that the equations of motion (EOM) of perturbations be hyperbolic. We find a general expression for the “effective metric” in field space when Lovelock theories are perturbed around some symmetric background solution. In particular, we calculate explicitly the effective metric for a general Lovelock theory perturbed around cosmological Friedman-Robertson-Walker backgrounds and for some specific cases when perturbed around Schwarzschild-like solutions. For the EOM to be hyperbolic, the effective metric needs to be Lorentzian. We find that, unlike for GR, the effective metric is generically not Lorentzian when the Lovelock modifications are significant. So, we conclude that Lovelock theories can only be considered as perturbative extensions of GR and not as truly modified theories of gravity. We compare our results to those in the literature and find that they agree with and reproduce the results of previous studies.

Horizonless spacetimes describing spatially regular ultracompact objects which, like black-hole spacetimes, possess closed null circular geodesics (light rings) have recently attracted much attention from physicists and mathematicians. In the present paper we raise the following physically intriguing question: how compact is an ultracompact object? Using analytical techniques, we prove that ultracompact isotropic matter configurations with light rings are characterized by the dimensionless lower bound ${\max }_r\{2m(r)/r\}>7/12$ on their global compactness parameter.

The mass loss of an isolated gravitating system due to energy carried away by gravitational waves with a cosmological constant $\mathrm{\Lambda }\in \mathbb{R}$ was recently worked out, using the Newman-Penrose-Unti approach. In that same article, an expression for the Bondi mass of the isolated system, $M_{\mathrm{\Lambda }}$, for the $\mathrm{\Lambda }>0$ case was proposed. The stipulated mass $M_{\mathrm{\Lambda }}$ would ensure that in the absence of any incoming gravitational radiation from elsewhere the emitted gravitational waves must carry away a positive-definite energy. That suggested quantity, however, introduced a $\mathrm{\Lambda }$-correction term to the Bondi mass $M_B$ (where $M_B$ is the usual Bondi mass for asymptotically flat spacetimes), which would involve information not just on the state of the system at that moment but ostensibly also its past history. In this paper, we derive the identical mass-loss equation using an integral formula on a hypersurface formulated by Frauendiener based on the Nester-Witten identity and argue that one may adopt a generalization of the Bondi mass with $\mathrm{\Lambda }\in \mathbb{R}$ without any correction, viz., $M_{\mathrm{\Lambda }}=M_B$ for any $\mathrm{\Lambda }\in \mathbb{R}$. Furthermore, with $M_{\mathrm{\Lambda }}=M_B$, we show that for purely quadrupole gravitational waves given off by the isolated system (i.e., when the “Bondi news” ${\sigma }^o$ comprises only the $l=2$ components of the spherical harmonics with spin-weight 2) the energy carried away is manifestly positive definite for the $\mathrm{\Lambda }>0$ case. For a general ${\sigma }^o$ having higher multipole moments, this perspicuous property in the $\mathrm{\Lambda }>0$ case still holds if those $l>2$ contributions are weak—more precisely, if they satisfy any of the inequalities given in this paper.

We perform a model-independent analysis of kinetically driven inflation (KDI) which (partially) includes generalized G-inflation and ghost inflation. We evaluate the background evolution splitting into the inflationary attractor and the perturbation around it. We also consider the quantum fluctuation of the scalar mode with a usual scaling and derive the spectral index, ignoring the contribution from the second-order products of slow-roll parameters. Using these formalisms, we find that within our generic framework the models of KDI which possess the shift symmetry of scalar field cannot create the quantum fluctuation consistent with the observation. Breaking the shift symmetry, we obtain a few essential conditions for viable models of KDI associated with the graceful exit.

The observation of a special spectral feature in the gamma-ray data would be one of the best ways to identify dark matter (DM). The box-shaped gamma-ray spectra could be generated by the decay of intermediate particles produced by DM annihilation or decay. It provides another kind of signal that can be relatively easily distinguished from astrophysical backgrounds besides the linelike signals. Dwarf spheroidal galaxies are expected to be dominated by DM and may be one of the most promising targets for indirect DM searches. In this paper, we study the box-shaped DM signals with Fermi-LAT observations of dwarf spheroidal galaxies. We analyze 106 months of Fermi-LAT data to derive the upper limits on the annihilation cross section or the decay timescale of DM. In addition, we compare the results for different sample selections and DM density distributions. We expect that more dwarf spheroidal galaxies will be found and the sensitivity of box-shaped gamma-ray signal searches will be significantly improved in the future.