The GeV-scale gamma-ray excess observed from the region surrounding the Galactic Center has been interpreted as either the products of annihilating dark matter particles, or as the emission from a large population of faint and centrally located millisecond pulsars. If pulsars are responsible for this signal, then they should also produce detectable levels of TeV-scale emission. In this study, we employ a template-based analysis of simulated data in an effort to assess the ability of the Cherenkov Telescope Array to detect or constrain the presence of this emission, providing a new and powerful means of testing whether millisecond pulsars are responsible for the observed excess. We find that after even a relatively brief observation of the inner galaxy, the Cherenkov Telescope Array will be able to definitively detect this TeV-scale emission, or rule out pulsars as the source of the Galactic Center gamma-ray excess.

The addition of a Ricci coupling to Einstein-scalar-Gauss-Bonnet theories makes general relativity a cosmological attractor. Previous work considered a quadratic coupling function with two independent coupling constants in such theories and showed that static, spherically symmetric, spontaneously scalarized black holes are radially stable beyond a critical value of the Ricci coupling constant. Here we demonstrate that these black holes are affected by a quadrupole instability which leads to two new branches of static, axially symmetric scalarized black holes. We discuss the properties of these solutions and provide embedding diagrams.

We study various holographic pure and mixed-state entanglement measures in the confined/deconfined phases of a bottom-up $\mathrm{AdS}/\mathrm{QCD}$ model in the presence of a background magnetic field. We analyze the entanglement entropy, entanglement wedge cross section, mutual information, and entanglement negativity and investigate how a background magnetic field leaves its imprints on the entanglement structure of these measures. Due to the anisotropy introduced by the magnetic field, we find that the behavior of these measures depends nontrivially on the relative orientation of the strip with respect to the field. In the confining phase, the entanglement entropy and negativity undergo a phase transition at the same critical strip length, the magnitude of which increases/decreases for parallel/perpendicular orientation of the magnetic field. The entanglement wedge cross section similarly displays discontinuous behavior each time a phase transition between different entangling surfaces occurs, while further exhibiting anisotropic features with a magnetic field. We further find that the magnetic field also introduces substantial changes in the entanglement measures of the deconfined phase; however, these changes remain qualitatively similar for all orientations of the magnetic field. We further study the inequality involving the entanglement wedge and mutual information and find that the former always exceeds half of the latter everywhere in the parameter space of the confined/deconfined phases.

We explore static and spherically symmetric solutions of the 4-dimensional semiclassical Einstein’s equations using the quantum vacuum polarization of a conformal field as a source. These solutions may be of interest for the study of exotic compact objects (ECOs). The full backreaction problem is addressed by solving the semiclassical Tolman-Oppenheimer-Volkoff (TOV) equations making use of effective equations of state inspired by the trace anomaly and an extra simplifying and reasonable assumption. We combine analytical and numerical techniques to solve the resulting differential equations, both perturbatively and nonperturbatively in $\hslash$. In all cases the solution is similar to the Schwarzschild metric up to the vicinity of the classical horizon $r=2M$. However, at $r=2M+ϵ$, with $ϵ\sim O(\sqrt{\hslash })$, we find a coordinate singularity. In the case of matching with a static star, this leads to an upper bound in the compactness, and sets a constraint on the family of stable ECOs. We also study the corrections that the quantum-vacuum polarization induces on the propagation of waves, and discuss the implications. For the pure vacuum case, we can further extend the solution by using appropriate coordinates until we reach another singular point, where this time a null curvature singularity arises and prevents extending beyond. This picture qualitatively agrees with the results obtained in the effective two-dimensional approach, and reinforces the latter as a reasonable method.

We show that tunneling between two degenerate minima, as allowed in a finite volume, leads to a nonextensive symmetric ground state. This results in a null energy condition violation for sufficiently low temperatures, when a continuous set of momenta in the box containing the field is assumed. Taking into account discrete momenta can modify this picture and is achieved via the addition of the Casimir energy to the tunneling-induced ground state energy. Focusing on zero-temperature, these nontrivial effects are found to compete, depending on the typical length scales involved.

In this paper, we discuss various possible schemes for the perturbative generation of the axion-photon interaction term in different Lorentz-breaking extensions of QED, involving operators with mass dimensions up to five. We demonstrate explicitly that there are only a few schemes allowing one to generate a finite axion-photon interaction term from one-loop radiative corrections, and within all these schemes, the generated term turns out to be ambiguous. Also, through the current experimental limits on the axion-photon coupling, we extract some present constraints on the Lorentz violation parameters.

We discuss thermodynamical stability for hairy black hole spacetimes, viewed as defects in the thermodynamical parameter space, taking into account the backreaction of a secondary hair onto the spacetime geometry, which is modified nontrivially. We derive, in a model independent way, the conditions for the hairy black hole with the secondary hair to reach a stable thermal equilibrium with the heat bath. Specifically, if the scalar hair, induced by interactions of the matter fields with quadratic-curvature corrections, produces an inner horizon in the deformed geometry, a thermodynamically stable configuration will be reached with the black hole becoming extremal in its final stage. We also attempt to make some conjectures concerning the implications of this thermal stability for the existence of a minimum length in a quantum spacetime.

Adopting the manifestation of low frequency and low mass for the scalar perturbation, we perform a semiclassical analysis of the superradiance phenomenon for a rotating hairy Horndeski black hole (BH). For the spacetime under study enriched by the hairy Horndeski parameter $h$, in addition to the mass $M$ and spin $a$, we compute the amplification factor of scalar wave scattering indicating the energy extraction from the BH. We find that due to the addition of the hairy parameter $h$ in the geometry, the superradiance scattering and its frequency range enhance compared to the Kerr BH. This implies that Horndeski’s gravity belongs to those alternative theories of gravity that make the amplification factor larger than the Kerr BH so that the energy extraction in its framework is more efficient than general relativity. Calculating the outgoing energy flux measured by an observer at infinity verifies the role of the hairy parameter $h$ in the increase in energy extraction efficiency from the rotating BH. By implementing the BH bomb mechanism, we present an analysis of the superradiant instability of the underlying BH spacetime against massive scalar fields. Our analysis indicates that the hairy Horndeski parameter leaves no imprint on the standard superradiant instability regime.

The LIGO-Virgo-KAGRA (LVK) Collaboration has detected gravitational waves (GWs) from 90 compact binary coalescences. In addition to fortifying the linearized theory of general relativity (GR), the statistical ensemble of detections also provides prospects of detecting nonlinear effects predicted by GR, one such prediction being the nonlinear gravitational memory effect. For detected stellar and intermediate mass compact binaries, the induced strain from the memory effect is 1 or 2 orders below the detector noise background. Additionally, since most of the energy is radiated at merger the strain induced by the memory effect resembles a step function at the merger time. These facts motivate the idea of coherently stacking up data streams from recorded GW events at these merger times so that the cumulative memory strain is detected with a sufficient signal to noise ratio (SNR). GW detectors essentially record the integrated strain response at timescales of the round-trip light travel time, making future space-based long arm interferometers like the Laser Interferometer Space Antenna (LISA) ideal for detecting the memory effect at low frequencies. In this paper, we propose a method that uses the event catalog of ground-based detectors and searches for corresponding memory strains in the LISA data stream. Given LVK’s O3 science run catalog, we use scaling arguments and assumptions on the source population models to estimate the run-time required for LISA to accumulate a memory SNR of 5, using triggers from current and future ground-based detectors. Finally, we extend these calculations for using beyond LISA missions like Advanced Laser Interferometer Antenna (ALIA), Advanced Millihertz Gravitational-wave Observatory (AMIGO), and Folkner to detect the gravitational memory effect. The results for LISA in conjunction with Einstein Telescope or a combination of Einstein Telescope and Cosmic Explorer indicate a possible detection of the memory effect within the 10 year LISA mission lifetime. The corresponding results for beyond LISA missions are even more promising.

In arbitrary higher dimension, we consider the combination of Lovelock gravity alongside a scalar-tensor action built out of higher order operators and Euler densities. The latter action is constructed in such a way as to ensure conformal invariance for the scalar field. For the combined version of these theories, we show the existence of black hole solutions interpreted as stealth configurations within Lovelock gravity theory. The scalar field solutions are endowed with an integration constant that may be identified as a scalar charge. In particular, we show that these stealth solutions can be extended to include a time-dependent scalar field despite the underlying theory being non shift symmetric. Finally, we present a procedure to obtain a nonconformally invariant action in even dimensions from the considered theory. For the target theory, the scalar field is not conformally coupled to gravity although the scalar field equation itself is conformally invariant. By means of this procedure, the black hole stealth configurations are converted into nonstealth black hole solutions, as discovered recently in four dimensions.

We have studied analytically the approximate solutions to the gapped mode equations in the hydrodynamic regime for a class of binary Bose-Einstein condensate acoustic black holes. The horizon from the transonic flow is formed by manipulating the phonon sound speed and the flow velocity with the experimentally accessible parameters. The asymptotic modes of various scattering processes are constructed from which to obtain scattering coefficients and then to further decompose the field operator in terms of the asymptotic states. Also, the Unruh state is introduced to be the appropriate state for the description of gravitational collapse of the black hole. The particle densities of the outgoing modes are computed. The effective energy gap term in the dispersion relation of the gapped excitations introduces the threshold frequency ${\omega }_r$ in the subsonic regime, below which the propagating modes do not exist. Thus, the particle spectrum of the analogous Hawking modes in the exterior of the horizon of the subsonic region significantly deviates from that of the gapless cases near the threshold frequency due to the modified graybody factor, which vanishes as the mode frequency is below ${\omega }_r$. However, in the interior region of the horizon of the supersonic region, the spectrum of the particle production of the Hawking partner has the nonthermal feature. The correlators between the analog Hawking mode and its partner of relevance to the experimental observations are also investigated and show some peaks near the threshold frequency ${\omega }_r$ resulting from the gap energy term to be seen in future experiments.

In this work we analyze the observational properties of incompressible relativistic fluid spheres with and without thin-shells when surrounded by thin accretion disks. We consider a set of six configurations with different combinations of the star radius $R$ and the thin-shell radius $r_{\mathrm{\Sigma }}$ to produce solutions with neither thin-shells nor light-rings, with either of those features, and with both. Furthermore, we consider three different models for the intensity profile of the accretion disk, based on the Gralla-Lupsasca-Marrone (GLM) disk model, for which the peaks of intensity occur at the innermost stable circular orbit (ISCO), the light-ring (LR), and the center of the star. The observed images and intensity profiles for an asymptotic observer are produced using a Mathematica-based ray-tracing code. Our results indicate that, in the absence of a light-ring, the presence of a thin-shell produces a negligible effect in the observational properties of the stars. However, when the spacetime features a light-ring, the portion of the mass of the star that is stored in the thin-shell has a strong effect on its observational properties, particularly in the magnitude of the central gravitational redshift effect responsible for causing a central shadow-like dimming in the observed images. A comparison with the Schwarzschild spacetime is also provided and the most compact configurations are shown to produce observational imprints similar to those of black-hole solutions, with subtle qualitative differences, most notably extra secondary image components that decrease the radius of the shadow and are potentially observable.

We analyze numerically the existence of regular stationary rotating hairy black holes within the framework of general relativity, which are the result of solving the Einstein-Klein-Gordon system for a complex-valued scalar field under suitable boundary (regularity and asymptotically flat) conditions. To that aim we solve the corresponding system of elliptic partial differential equations using spectral methods which are specially suited for such a numerical task. In order to obtain such system of equations we employ a parametrization for the metric that corresponds to quasi-isotropic coordinates (QIC) that have been used in the past for analyzing different kinds of stationary rotating relativistic systems. Our findings are in agreement with those reported originally by Herdeiro and Radu. The method is submitted to several analytic and numerical tests, which include the recovery of the Kerr solution in QIC and the cloud solutions in the Kerr background. We report different global quantities that allow us to determine the contribution of the boson hair to the spacetime, as well as relevant quantities at the horizon, like the surface gravity. The latter indicates to what extent the hairy solutions approach the extremal limit, noting that for this kind of solutions the ratio of the angular momentum per squared mass $J_{\infty }/M_{\mathrm{ADM}}^2$ can be larger than unity due to the contribution of the scalar hair, a situation which differs from the Kerr metric where this parameter is bounded according to $0\le |J/M^2|\le 1$, with the upper bound corresponding to the extremal case.

We explore the dynamics of Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies which consist of dark matter, radiation, and dark energy with a quadratic equation of state. Standard cosmological singularities arise due to energy conditions which are violated by dark energy, therefore we focus our analysis on nonsingular bouncing and cyclic cosmologies, in particular focusing on the possibility of closed FLRW models always having a bounce for any initial conditions. We analyze the range of dynamical behavior admitted by the system, and find a class of closed models that admit a nonsingular bounce, with early- and late-time accelerated expansion connected by a decelerating phase. In all cases, we find the bouncing models are only relevant when dark matter and radiation appear at a certain energy scale, and so require a period such as reheating. We then investigate imposing an upper bound on the dark matter and radiation, such that their energy densities cannot become infinite. In this case, we find that all closed models bounce, and a class of models exist with early- and late-time acceleration, connected by a decelerating phase. We also consider parameter values for the dark energy component, such that the discrepancy between the observed value of $\mathrm{\Lambda }$ and the theoretical estimates of the contributions to the effective cosmological constant expected from quantum field theory would be explained. However, we find that the class of models left does not allow for an early- and late-time accelerated expansion, connected by a decelerating period where large-scale structure could form. Nonetheless, our qualitative analysis serves as a basis for the construction of more realistic models with realistic quantitative behavior.

We explore a reparametrization of the lensing amplitude tension between weak lensing (WL) and cosmic microwave background (CMB) data and its implications for a joint resolution with the Hubble tension. Specifically, we focus on the lensing amplitude over a scale of 12 Mpc in absolute distance units using a derived parameter $S_{12}$ and show its constraints from recent surveys in comparison with Planck 2018. In WL alone, we find that the absolute distance convention correlates $S_{12}$ with $H_0$. Accounting for this correlation in the 3D space $S_{12}\times {\omega }_m\times h$ reproduces the usual levels of $2\sim 3\sigma$ tension inferred from $S_8\times {\mathrm{\Omega }}_m$. Additionally, we derive scaling relations in the $S_8\times h$ and $S_{12}\times h$ planes that are allowed by $\mathrm{\Lambda }\mathrm{CDM}$ and extrapolate target scalings needed to solve the $H_0$ and lensing-amplitude tensions jointly in a hypothetical beyond-$\mathrm{\Lambda }\mathrm{CDM}$ model. As a test example, we quantify how the early dark energy scenario compares with these target scalings. Useful fitting formulas for $S_8$ and $S_{12}$ as a function of other cosmological parameters in $\mathrm{\Lambda }\mathrm{CDM}$ are provided, with 1% precision.

We obtain the nonequilibrium condensate of the Chern Simons density induced by a misaligned homogeneous coherent axion field in linear response. The Chern-Simons dynamical susceptibility is simply related to the axion self-energy, a result that is valid to leading order in the axion coupling but to all orders in the couplings of the gauge fields to other fields within or beyond the standard model except the axion. The induced Chern-Simons density requires renormalization which is achieved by vacuum subtraction. For ultralight axions of mass $m_a$ coupled to electromagnetic fields with coupling $g$, the renormalized high temperature contribution postrecombination is $⟨\overrightarrow{E}·\overrightarrow{B}⟩(t)=-\frac{g{\pi }^2{T}^4}{15}\overline{a}(t)+\frac{g{m}_a^{2}T}{16\pi }\dot{\overline{a}}(t)$ with $\overline{a}(t)$ the dynamical homogeneous axion condensate. We conjecture that emergent axionlike quasiparticle excitations in condensed matter systems may be harnessed to probe cosmological axions and the Chern-Simons condensate. Furthermore, it is argued that a misaligned axion can also induce a non-Abelian Chern-Simons condensate of similar qualitative form, and can also “seed” chiral symmetry breaking and induce a neutral pion condensate after the QCD phase transition.

The recent astrophysical transient Swift $\mathrm{J}1913.1+1946$ may be associated with the $\gamma$-ray burst GRB 221009A. The redshift of this event is $z\simeq 0.151$. Very high-energy $\gamma$ rays (up to 18 TeV) followed the transient and were observed by the Large High Altitude Air Shower Observatory (LHAASO); additionally, Carpet-2 detected a photonlike air shower of 251 TeV. Photons of such high energy are expected to readily annihilate with the diffuse extragalactic background light (EBL) before reaching Earth. If the $\gamma$-ray identification and redshift measurements are correct, new physics could be necessary to explain these measurements. This paper provides the first $CP$-even scalar explanation of the most energetic 18 TeV event reported by LHAASO. In this minimal scenario, the light scalar singlet $S$ mixes with the Standard Model Higgs boson $h$. The highly boosted $S$ particles are produced in the GRB and then undergo the radiative decay diphoton $S\to \gamma \gamma$ while propagating to Earth. The resulting photons may thus be produced at a remote region without being nullified by the EBL. Hence, the usual exponential reduction of $\gamma$ rays is lifted due to an attenuation that is inverse in the optical depth, which becomes much larger due to the scalar carriers.

The accumulation of certain types of dark matter particles in neutron star cores due to accretion over long timescales can lead to the formation of a mini black hole. In this scenario, the neutron star is destabilized and implodes to form a black hole without significantly increasing its mass. When this process occurs in neutron stars in coalescing binaries, one or both stars might be converted to a black hole before they merge. Thus, in the mass range of $\sim 1–2M_{\odot }$, the Universe might contain three distinct populations of compact binaries: one containing only neutron stars, the second population of only black holes, and a third, mixed population consisting of a neutron star and a black hole. However, it is unlikely to have a mixed population as the various timescales allow for both neutron stars to remain or collapse within a short timescale. In this paper, we explore the capability of future gravitational-wave detector networks, including upgrades of Advanced LIGO and Virgo, and new facilities such as the Cosmic Explorer and Einstein Telescope (XG network), to discriminate between different populations by measuring the effective tidal deformability of the binary, which is zero for binary black holes but nonzero for binary neutron stars. Furthermore, we show that observing the relative abundances of the different populations can be used to infer the timescale for neutron stars to implode into black holes, and in turn, provide constraints on the particle nature of dark matter. The XG network will infer the implosion timescale to within an accuracy of 0.01 Gyr at 90% credible interval and determine the dark matter mass and interaction cross section to within a factor of 2 GeV and $10{\mathrm{cm}}^{-2}$, respectively.

We study a minimal model of pseudo-Dirac dark matter, interacting through transition electric and magnetic dipole moments. Motivated by the fact that xenon experiments can detect electrons down to $\sim \mathrm{keV}$ recoil energies, we consider $\mathcal{O}(\mathrm{keV})$ splittings between the mass eigenstates. We study the production of this dark matter candidate via the freeze-in mechanism. We discuss the direct detection signatures of the model arising from the down scattering of the heavier state, that are produced in solar upscattering, finding observable signatures at the current and near-future xenon based direct detection experiments. We also study complementary constraints on the model from fixed target experiments, lepton colliders, supernovae cooling and cosmology. We show that the latest XENONnT results rule out parts of the parameter space for this well motivated and minimal dark matter candidate. Next generation xenon experiments can either discover or further constrain how strongly inelastic dark matter can interact via the dipole moment operators.

In this work, we propose polarimetry experiments to search for low-mass (sub-eV) bosonic field dark matter, including axions and axionlike particles. We show that a polarimetry configuration consisting of a thick birefringent solid inside a Fabry-Pérot cavity is exceptionally sensitive to scalar field dark matter, which may cause oscillatory variations in the solid’s thickness and refractive index. In addition, we show that a reconfiguration of this polarimetry experiment, in which two quarter-wave plates are placed inside the Fabry-Pérot cavity instead of a thick birefringent solid, is very sensitive to axionlike particles. We investigate the possibility of using cross-correlation of twin polarimeters to increase the sensitivity of the experiment, which in turn could allow us to explore unexplored parts of the parameter space and potentially detect a signal in either dark matter scenario.