We study linear perturbations around a static and spherically symmetric black hole solution in spatially covariant gravity with just two tensorial degrees of freedom. In this theory, gravity modification is characterized by a single time-dependent function that appears in the coefficient of $K^2$ in the action, where $K$ is the trace of the extrinsic curvature. The background black hole solution is given by the Schwarzschild solution foliated by the maximal slices and has a universal horizon at which the lapse function vanishes. We show that the quadratic action for the odd-parity perturbations is identical to that in general relativity upon performing an appropriate coordinate transformation. This in particular implies that the odd-parity perturbations propagate at the speed of light, with the inner boundary being the usual event horizon. We also derive the quadratic action for even-parity perturbations. In the even-parity sector, one of the two tensorial degrees of freedom is mixed with an instantaneous scalar mode, rendering the system distinct from that in general relativity. We find that monopole and dipole perturbations, which are composed solely of the instantaneous scalar mode, have no solutions regular both at the universal horizon and infinity (except for the trivial one corresponding to the constant shift of the mass parameter). We also consider stationary perturbations with higher multipoles. By carefully treating the locations of the inner boundary, we show that also in this case there are no solutions regular both at the inner boundary and infinity. Thus, the black hole solution we consider is shown to be perturbatively unique.

Finsler spacetime geometry is a canonical extension of Riemannian spacetime geometry. It is based on a general length measure for curves (which does not necessarily arise from a spacetime metric) and it is used as an effective description of spacetime in quantum gravity phenomenology as well as in extensions of general relativity aiming to provide a geometric explanation of dark energy. A particular interesting subclass of Finsler spacetimes are those of Berwald type, for which the geometry is defined in terms of a canonical affine connection that uniquely generalizes the Levi-Civita connection of a spacetime metric. In this sense, Berwald Finsler spacetimes are Finsler spacetimes closest to pseudo-Riemannian ones. We prove that all Ricci-flat, spatially spherically symmetric Berwald spacetime structures are either pseudo-Riemannian (Lorentzian), or flat. This insight enables us to generalize the Jebsen-Birkhoff theorem to Berwald spacetimes.

The Bardeen and Hayward spacetimes are here considered as standard configurations of spherically symmetric regular black holes. Assuming the thermodynamics of such objects to be analogous to standard black holes, we compute the island formula in the regime of small topological charge and large vacuum energy, respectively for Bardeen and Hayward spacetimes. Late and early time domains are separately discussed, with particular emphasis on the island formations. We single out conditions under which it is not possible to find out islands at early times and how our findings depart from the standard Schwarzschild case. Motivated by the fact that those configurations extend Reissner-Nordström and Schwarzschild-de Sitter metrics through the inclusion of regularity behavior at $r=0$, we show how the effect of regularity induces modifications on the overall entanglement entropy. Finally, the Page time is also computed, showing its asymptotic behavior. Specifically, the Page time exhibits slight departures with respect to the Schwarzschild case, more evident in the Hayward metric only. Conversely, our findings on Page time show that the Bardeen regular black hole is quite indistinguishable from the Schwarzschild case.

Generalized BMS (gBMS) is the Lie group of the asymptotic symmetries at null infinity, and is proposed to be a symmetry of the quantum S-matrix. Despite much progress in understanding the symplectic structure at null infinity consistent with the gBMS symmetries, the construction of a radiative phase space where all the physical soft modes and their conjugate partners are identified remains elusive. We construct just such a radiative phase space for linearized gravity by a systematic constraint analysis. In addition, we highlight the difficulties that arise in extending this analysis to the nonlinear case. In order to analyze the difficulties we face in extending these ideas to the nonlinear setting, we consider a toy model in which we gauge the action of the Weyl scaling in the Weyl Bondi-Metzner-Sachs (BMS) group. We find that supertranslations are no longer well-defined symmetries on the reduced phase space of the gauged Weyl, as Weyl scalings do not commute with supertranslations. In this restricted case we obtain the symplectic form and derive the reduced phase space.

We consider a test charged particle falling onto a Schwarzschild black hole and evaluate its electromagnetic field. The Regge-Wheeler equation is solved analytically by approximating the potential barrier with Dirac delta function and rectangular barrier. We show that for asymptotically large times measured by a distant observer the electromagnetic field approaches the spherically symmetric electrostatic field. This implies that in the region accessible to a distant observer the initial state of separated charge and the electromagnetic field outside the event horizon of Schwarzschild black hole becomes asymptotically indistinguishable from the Reisnner-Nordström solution. The implications of this result for some astrophysical models of black holes accreting charged particles are discussed.

This paper extends the de Sitter/anti–de Sitter (dS/AdS) background method based on the optical metric for gravitational lens [K. Takizawa and H. Asada, Phys. Rev. D 105, 084022 (2022).] to a static optical constant-curvature (SOCC) background. It is shown that the exact lens equation on the SOCC background can be written in the same form as that for the Minkowski, dS, or AdS background in terms of flat, spherical, or hyperbolic trigonometry, depending on the Gaussian curvature of the equatorial plane in the SOCC background. To exemplify the SOCC method, we consider the gravitational lens in the Mannheim-Kazanas (MK) solution of Weyl gravity, which includes Rindler and de Sitter terms. In the zero mass limit, the deflection angle of light for the MK solution in the literature diverges to infinity. This is because there is a self-contradiction in their perturbative approximations of the MK metric and the orbit equation. The SOCC method incorporates the long-distance curvature effect into the background. Thereby the SOCC expression for the deflection angle of light in the MK solution is finite also in the zero mass limit.

The Bardeen black hole holds historical significance as the first model of a regular black hole. Recently, there have been proposed interpretations of the Bardeen spacetime as quantum corrections to the Schwarzschild solution. Our study focuses on investigating the quasinormal modes and Hawking radiation of the Bardeen black hole. We have observed that previous studies on the quasinormal modes for the Bardeen black hole suffer from inaccuracies that cannot be neglected. Therefore, we propose accurate calculations of the quasinormal modes for scalar, electromagnetic, and neutrino fields in the Bardeen spacetime. Additionally, we have computed the gray-body factors and analyzed the emission rates of Hawking radiation. Even when the quantum correction is small and the fundamental mode only slightly differs from its Schwarzschild value, the first several overtones deviate at an increasingly stronger rate. This deviation leads to the appearance of overtones with very small real oscillation frequencies. This outburst of overtones is closely linked to the fact that the quantum-corrected black hole differs from its classical limit primarily near the event horizon. Moreover, the intensity of the Hawking radiation is significantly suppressed (up to 3 orders of magnitude) by the quantum correction.

Lorentz symmetry is a fundamental property of Einstein’s theory of general relativity that one may wish to test with gravitational wave observations. Einstein-æther theory is a model that introduces Lorentz-symmetry breaking in the gravitational sector through an æther vector field, while still leading to second-order field equations. This well-posed theory passes particle physics constraints because it modifies directly only the gravitational sector, yet it predicts deviations in the inspiral and coalescence of compact objects. We here, for the first time, put this theory to the test by comparing its gravitational wave predictions directly against LIGO/Virgo gravitational wave data. We first construct a waveform model for Einstein-æther theory, EA_IMRPhenomD_NRT, through modifications of the general relativity IMRPhenomD_NRTidalv2 model (used by the LIGO/VIRGO collaboration). This model constructs a response function that not only contains the transverse-traceless polarization but also additional Einstein-æther (scalar and vectorial) polarizations simultaneously. We then use the many current constraints on the theory to construct nontrivial priors for the Einstein-æther coupling constants. After testing the waveform model, we conduct parameter estimation studies on two gravitational wave events: GW170817 and GW190425. We find that these data are not sufficiently informative to place constraints on the theory that are stronger than current bounds from binary pulsar, Solar System, and cosmological observations. This is because, although Einstein-æther modifications include additional polarizations and have been computed beyond leading post-Newtonian order, these modifications are dominated by (already-constrained) dipole effects. These difficulties make it unclear whether future gravitational wave observations will be able to improve on current constraints on Einstein-æther theory.

Two puzzles continue to plague our understanding of angular momentum balance in the context of gravitational two-body scattering. First, because the standard definition of the Bondi angular momentum $J$ is subject to a supertranslation ambiguity, it has been shown that when the corresponding flux $F_J$ is expanded in powers of Newton’s constant $G$, it can start at either $O(G^2)$ or $O(G^3)$ depending on the choice of frame. This naturally raises the question as to whether the $O(G^2)$ part of the flux is physically meaningful. The second puzzle concerns a set of new methods for computing the flux that were recently developed using quantum field theory. Somewhat surprisingly, it was found that they generally do not agree with the standard formula for $F_J$, except in the binary’s center-of-mass frame. In this paper, we show that the resolution to both of these puzzles lies in the careful interpretation of $J$: Generically, the Bondi angular momentum $J$ is not equal to the mechanical angular momentum $\mathcal{J}$ of the binary, which is the actual quantity of interest. Rather, it is the sum of $\mathcal{J}$ and an extra piece involving the shear of the gravitational field. By separating these contributions, we obtain a new balance law, accurate to all orders in $G$, that equates the total loss in mechanical angular momentum ${\mathrm{\Delta }}_{\mathcal{J}}$ to the sum of a radiative term, which always starts at $O(G^3)$, and a static term, which always starts at $O(G^2)$. We show that each of these terms is invariant under supertranslations, and we find that ${\mathrm{\Delta }}_{\mathcal{J}}$ matches the result from quantum field theory at least up to $O(G^2)$ in all Bondi frames. The connection between our results and other proposals for supertranslation-invariant definitions of the angular momentum is also discussed.

This work investigates dark matter (DM) effects in compact objects in modified teleparallel gravity (MTG) in which a modification of teleparallel equivalent to general relativity is used. We applied a tetrad to the modified field equations where a set of relations is found. The conservation equation allows us to rewrite our Tolman-Oppenheimer-Volkoff equations with an effective gravitational coupling constant. As input to these new equations, we use a relativistic mean-field (RMF) model with dark matter content included, obtained from a Lagrangian density with both, hadronic and dark particle degrees of freedom, as well as the Higgs boson, used as a mediator in both sectors of the theory. Through numerical calculations, we analyze the mass-radius diagrams obtained from different parametrizations of the RMF-DM model, generated by assuming different values of the dark particle Fermi momentum and running the free parameter coming from the MTG. Our results show that it is possible for the system simultaneously support more DM content, and be compatible with recent astrophysical data provided by the LIGO and Virgo Collaborations, as well as by NASA’s Neutron star Interior Composition Explorer (NICER).

The scale-dependent galaxy bias is an important signal to be extracted in constraining local primordial non-Gaussianity ($f_{\mathrm{NL}}^{\text{local}}$) from observations of large-scale structure. Constraints so obtained rely on the assumption that horizon-scale features in the galaxy power spectrum are exclusively due to primordial physical mechanisms. Yet, postinflationary effects can induce modulations to the galaxy number density that appear as horizon-scale, scale-dependent galaxy bias. We investigate the effect of two such sources of scale-dependent galaxy bias—the free-streaming of light relics and fluctuations in the background of ionizing radiation—on precision measurements of local primordial non-Gaussianity $f_{\mathrm{NL}}^{\text{local}}$ from galaxy power spectrum measurements. Using the SPHEREx survey as a test case survey reaching $\sigma (f_{\mathrm{NL}}^{\text{local}})\lesssim 1$, we show that ignoring the scale-dependent galaxy bias induced by free-streaming particles can negatively bias the inferred value of $f_{\mathrm{NL}}^{\text{local}}$ by $\sim 0.1–0.3\sigma$. Ignoring the effect of ionizing radiation fluctuations can negatively bias the inferred value of $f_{\mathrm{NL}}^{\text{local}}$ by $\sim 1\sigma$. The extent of inaccuracies in parameter-inference so incurred depends on the source populations and the ranges of scales used in the analysis, as well as the value of the neutrino mass and the modeling of the impact of ionizing radiation. If these sources of scale-dependent galaxy bias are included in the analysis, forecasts for $f_{\mathrm{NL}}^{\text{local}}$ are unbiased but degraded.

We investigate the spectrum of gravitational waves arising from primordial inflation in the presence of a string-theoretical higher-curvature correction, specifically, the Gauss-Bonnet coupling term for the inflaton (modulus) field. We show that if the modulus field exhibits a wall-crossinglike behavior in the moduli space, there can be a period of Gauss-Bonnet coupling-term domination during the usual slow-roll. This phenomenon is potentially detectable as the gravitational wave spectrum exhibits a characteristic peak caused by the brief domination of the Gauss-Bonnet coupling term. We explore the possibility of measuring such gravitational waves with pulsar timing array experiments such as NANOGrav, and future space-borne interferometers such as LISA, DECIGO, and Taiji.

Diffusion damping of the cosmic microwave background (CMB) power spectrum results from imperfect photon-baryon coupling in the pre-recombination plasma. Energy release at redshifts $5\times {10}^4<z<2\times {10}^6$ can create $\mu$-type spectral distortions of the CMB. These $\mu$ distortions trace the underlying photon density fluctuations, probing the primordial power spectrum in short-wavelength modes $k_{\mathrm{S}}$ over the range $50{\mathrm{Mpc}}^{-1}\lesssim k\lesssim {10}^4{\mathrm{Mpc}}^{-1}$. Small-scale power modulated by long-wavelength modes $k_{\mathrm{L}}$ from squeezed-limit non-Gaussianities introduces cross correlations between CMB temperature anisotropies and $\mu$ distortions. Under single-field inflation models, $\mu \times T$ correlations measured from an observer in an inertial frame should vanish up to a factor of $(k_{\mathrm{L}}/k_{\mathrm{S}}{)}^2\ll 1$. Thus, any measurable correlation rules out single-field inflation models. We forecast how well the next-generation ground-based CMB experiment CMB-S4 will be able to constrain primordial squeezed-limit non-Gaussianity, parametrized by $f_{\mathrm{NL}}$, using measurements of $C_{\ell }^{\mu T}$ as well as $C_{\ell }^{\mu E}$ from CMB $E$ modes. Using current experimental specifications and foreground modeling, we expect $\sigma (f_{\mathrm{NL}})\lesssim 1000$. This is roughly 4 times better than the current limit on $f_{\mathrm{NL}}$ using $\mu \times T$ and $\mu \times E$ correlations from Planck and is comparable to what is achievable with LiteBIRD, demonstrating the power of the CMB-S4 experiment. This measurement is at an effective scale of $k\approx 740{\mathrm{Mpc}}^{-1}$ and is thus highly complementary to measurements at larger scales from primary CMB and large-scale structure.

A cosmological scenario in which the onset of neutrino free streaming in the early Universe is delayed until close to the epoch of matter-radiation equality has been shown to provide a good fit to some cosmic microwave background (CMB) data, while being somewhat disfavored by Planck CMB polarization data. To clarify this situation, we investigate in this paper CMB-independent constraints on this scenario from the full shape of the galaxy power spectrum. Although this scenario predicts significant changes to the linear matter power spectrum, we find that it can provide a good fit to the galaxy power spectrum data. Interestingly, we show that the data display a modest preference for a delayed onset of neutrino free streaming over the standard model of cosmology, which is driven by the galaxy power spectrum data on mildly nonlinear scales. This conclusion is supported by both profile likelihood and Bayesian exploration analyses, showing robustness of the results. Compared to the standard cosmological paradigm, this scenario predicts a significant suppression of structure on sub-Galactic scales. While our analysis relies on the simplest cosmological representation of neutrino self-interactions, we argue that this persistent—and somehow consistent—picture in which neutrino free streaming is delayed motivates the exploration of particle models capable of reconciling all CMB, large-scale structure, and laboratory data.

We perform three-dimensional simulations of homogeneous and inhomogeneous cosmologies via the coupling of the einstein toolkit numerical relativity code for spacetime evolution to the phantom smoothed particle hydrodynamics code. Evolution of a flat dust and radiation-dominated Friedmann-Lemaître-Roberston-Walker (FLRW) spacetime shows an agreement of exact solutions with residuals on the order ${10}^{-6}$ and ${10}^{-3}$ respectively, even at low grid resolutions. We demonstrate evolution of linear perturbations of density, velocity and metric quantities to the FLRW with residuals of $\approx {10}^{-2}$ compared to exact solutions. Finally, we demonstrate the evolution of nonlinear density perturbations past shell crossing, such that dark matter halo formation is possible. We show that numerical relativistic smoothed particle hydrodynamics is a viable method for understanding nonlinear effects in cosmology.

We apply the full set of most updated dynamical and geometrical data in cosmology to the nonextensive Barrow entropic holographic dark energy. We show that the data point towards an extensive Gibbs-like entropic behavior for the cosmological horizons, which is the extreme case of the Barrow entropy, with the entropy parameter being $\mathrm{\Delta }>0.86$, close to the maximum threshold of $\mathrm{\Delta }=1$ where the fractal dimension of the area-horizon becomes almost or just the volume and the intensivity is recovered. Furthermore, we find that the standard Bekenstein area-entropy limit ($\mathrm{\Delta }=0$) is excluded by the set of our data. This contradicts the bounds obtained recently from early universe tests such as the baryon asymmetry, the big bang nucleosynthesis, and the inflation limiting $\mathrm{\Delta }<0.008$ at the most extreme case.

Just like light, gravitational waves (GWs) are deflected and magnified by gravitational fields as they propagate through the Universe. However, their low frequency, phase coherence and feeble coupling to matter allow for distinct lensing phenomena, such as diffraction and central images, that are challenging to observe through electromagnetic sources. Here we explore how these phenomena can be used to probe features of gravitational lenses. We focus on two variants of the singular isothermal sphere, with (1) a variable slope of the matter density and (2) a central core. We describe the imprints of these features in the wave- and geometric-optics regimes, including the prospect of detecting central images. We forecast the capacity of LISA and advanced LIGO to study strongly lensed signals and measure the projected lens mass, impact parameter, and slope or core size. A broad range of lens masses allows all parameters to be measured with precision up to $\sim 1/\mathrm{SNR}$, despite large degeneracies. Thanks to wave-optics corrections, all parameters can be measured, even when no central image forms. Although GWs are sensitive to projected quantities, we compute the probability distribution of lens redshift, virial mass and projection scale given a cosmology. As an application, we consider the prospect of constraining self-interacting and ultralight dark matter, showing the regions of parameter space accessible to strongly lensed GWs. The distinct GW signatures will enable novel probes of fundamental physics and astrophysics, including the properties of dark matter and the central regions of galactic halos.

In this work, we propose a metamodeling technique to nuclear matter on the basis of a relativistic density functional with density-dependent couplings. Identical density dependence for the couplings in both the isoscalar and isovector sectors is employed. We vary the coupling parameters of the model to capture the uncertainties of the empirical nuclear matter parameters at saturation. Then, we construct a large ensemble of unified equations of state in a consistent manner for both clusterized and uniform matter in $\beta$ equilibrium at zero temperature. Finally, we calculate neutron star properties to check the consistency with astrophysical observations within a Bayesian framework. Out of the different sets of astrophysical data employed, the constraint on tidal deformability from the GW170817 event was found to be the most stringent in the posteriors of different neutron star properties explored in the present study. We demonstrate in detail the impact of the isovector incompressibility ($K_{\mathrm{sym}}$) on high-density matter that leads to a considerable variation in the composition of neutron star matter. A couple of selected models with extreme values of $K_{\mathrm{sym}}$, which satisfy various modern nuclear physics and neutron star astrophysics constraints, are uploaded in the CompOSE database [S. Typel et al., Phys. Part. Nucl. 46, 633 (2015).] for use by the community.

The properties of self-interacting boson stars with different scalar potentials going beyond the commonly used ${\varphi }^4$ ansatz are studied. The scalar potential is extended to different values of the exponent $n$ of the form $V\propto {\varphi }^n$. Two stability mechanism for boson stars are introduced, the first being a mass term and the second one a vacuum term. We present analytic scale-invariant expressions for these two classes of equations of state. The resulting properties of the boson star configurations differ considerably from previous calculations. We find three different categories of mass-radius relation: the first category resembles the mass-radius curve of self-bound stars, the second one those of neutron stars and the third one is the well-known constant radius case from the standard ${\varphi }^4$ potential. We demonstrate that the maximal compactness can reach extremely high values going to the limit of causality $C_{\max }=0.354$ asymptotically for $n\to \infty$. The maximal compactnesses exceed previously calculated values of $C_{\max }=0.16$ for the standard ${\varphi }^4$-theory and $C_{\max }=0.21$ for vectorlike interactions and is in line with previous results for solitonic boson stars. Hence, boson stars even described by a simple modified scalar potential in the form of $V\propto {\varphi }^n$ can be ultracompact black hole mimickers where the photon ring is located outside the radius of the star.

We consider constraints on $p$-wave dark matter in a dark matter spike surrounding the supermassive black hole at the center of M87. Owing to the large mass of the black hole, and resulting large velocity dispersion for the dark matter particles in the spike, it is possible for Fermi-LAT and MAGIC data to place tight constraints on $p$-wave annihilation, which would be far more stringent than those placed by observations of dwarf spheroidal galaxies. Indeed, for optimistic choices of the spike parameters, gamma-ray data would exclude thermal $p$-wave dark matter models with a particle mass $\lesssim 10\mathrm{TeV}$. But there is significant uncertainty in the properties and parameters of the spike, and for less optimistic scenarios, thermal dark matter candidates would be completely unconstrained. In addition to better understanding the spike parameters, a second key to improving constraints on dark matter annihilation is an accurate astrophysical background model.