General Relativity and Quantum Cosmology (gr-qc)

Abstract: The Vaidya metric serves as a useful model-building tool that captures many essential features of dynamical and/or evaporating black hole spacetimes. Working in a semiclassical setting, we show that in the limit of slow evaporation, a general spherically symmetric metric subject to certain regularity conditions is uniquely described by a linear ingoing Vaidya metric in the near-horizon region. This suggests a universal description of the near-horizon geometry of evaporating black holes in terms of the linear Vaidya metric. We also demonstrate that the linear Vaidya metric can be brought into manifestly conformally static form, allowing us to determine the Hawking temperature associated with the Vaidya background with respect to the conformal vacuum. Since back-reaction is implicitly accounted for, we conclude that slowly evaporating black holes are indeed accurately described by quasistatic sequences of Schwarzschild metrics even when dynamical effects are present.

Abstract: We present a numerical-relativity simulation of a black hole - neutron star merger in scalar-tensor (ST) gravity with binary parameters consistent with the gravitational wave event GW200115. In this exploratory simulation, we consider the Damour-Esposito-Farese extension to Brans-Dicke theory, and maximize the effect of spontaneous scalarization by choosing a soft equation of state and ST theory parameters at the edge of known constraints. We extrapolate the gravitational waves, including tensor and scalar (breathing) modes, to future null-infinity. The numerical waveforms undergo ~ 22 wave cycles before the merger, and are in good agreement with predictions from post-Newtonian theory during the inspiral. We find the ST system evolves faster than its general-relativity (GR) counterpart due to dipole radiation, merging a full gravitational-wave cycle before the GR counterpart. This enables easy differentiation between the ST waveforms and GR in the context of parameter estimation. However, we find that dipole radiation's effect may be partially degenerate with the NS tidal deformability during the late inspiral stage, and a full Bayesian analysis is necessary to fully understand the degeneracies between ST and binary parameters in GR.

Abstract: Using Israel-Stewart formalism for the description of thermodynamics of an arbitrary relativistic fluid we propose generalization of Tolman-Ehrenfest relation and Klein's law on a general background spacetime. The first relation is a consequence of thermal equilibrium only of a non-viscous fluid, while the latter one is reflection of only no-diffusion condition with or without viscosity. Interestingly, both the relations are obtained independently through the imposition of respective equilibrium conditions and also valid in presence of particular dissipative processes in the fluid. For a spacetime with a global timelike Killing vector, these two general relations boil down to standard forms.

Abstract: The NP constants of the spin-0 field propagating in Minkowski spacetime are computed close to spatial and null infinity by means of Friedrich's $i^0$-cylinder. Assuming certain regularity condition on the initial data ensuring that the field extends analytically to the critical sets, it is shown that the NP constants at future $\mathscr{I}^{+}$ and past null infinity $\mathscr{I}^{-}$ are independent of each other. In other words, the classical NP constants at $\mathscr{I}^{\pm}$ stem from different parts of the initial data given on a Cauchy hypersurface. In contrast, it is shown that, using a slight generalisation of the classical NP constants, the associated quantities ($i^0$-cylinder NP constants) do not require the regularity condition being satisfied and give rise to conserved quantities at $\mathscr{I}^{\pm}$ that are determined by the same piece of initial data which, in turn, correspond to the terms controlling the regularity of the field. Additionally, it is shown how the conservation laws associated to the NP constants can be exploited to construct, in flat space, heuristic asymptotic-system expansions which are sensitive to the logarithmic terms at the critical sets.

Abstract: We consider equilibrium models of spherical boson stars in Palatini $f(\mathcal{R})=\mathcal{R}+\xi \mathcal{R}^2$ gravity and study their collapse when perturbed. The Einstein-Klein-Gordon system is solved using a recently established correspondence in an Einstein frame representation. We find that, in that frame, the endpoint is a nonrotating black hole surrounded by a quasi-stationary cloud of scalar field. However, the dynamics in the $f(\mathcal{R})$ frame is dramatically different. The innermost region of the collapsing object exhibits the formation of a finite-size, exponentially-expanding $\textit{ baby universe}$ connected with the outer (parent) universe via a minimal area surface (a throat or umbilical cord). Our simulations indicate that this surface is at all times hidden inside a horizon, causally disconnecting the baby universe from observers above the horizon. The implications of our findings in other areas of gravitational physics are also discussed.

Abstract: In this paper, we provide the DeWitt propagator and its wave function in quantum cosmology using the path integral formulation of quantum gravity. The DeWitt boundary condition is introduced as a way of avoiding the Big Bang singularity by positing that the wave function of the universe vanishes near the Big Bang. However, there is currently no clear definition of the DeWitt boundary condition in the path integral formulation. To address this issue, we employ the image method, which eliminates singular paths in the forbidden region of an infinite potential and apply this method to quantum cosmology based on the Batalin-Fradkin-Vilkovisky formulation of the path integral and Picard-Lefschetz theory. We investigate the validity of the image method, and specifically, find that this method is appropriate only when the potential exhibits symmetry with respect to the boundary. Then, we show that the DeWitt propagator and the DeWitt wave function derived the image method are consistent with solutions of the Wheeler-DeWitt equation for specific models of quantum cosmology.

Abstract: We consider two accelerating Unruh-DeWitt detectors coupled linearly or quadratically with a scalar field. We show that entanglement can be created by acceleration, and is divergent only when the two detectors coincide. For linear coupling, entanglment decreases monotonically with the increase of acceleration. For quadratic coupling, entanglement behaves non-monotonically.

Abstract: General theory of relativity is the most popular theory to describe the dynamics of a system (especially the Universe) under gravity. In this framework, the solution of Einstein field equation under curved space-time yields the cosmic evolution equation. However the evolutionary dynamics of the Universe may also be obtained from the other aspects like thermodynamics, classical Lagrangian dynamics, symmetry analysis(Noether, Lie ) etc. This work provides an different approach to obtain the cosmic evolution equation from the quantization of the cosmic fluid under gravity.

Abstract: Owing to the forecasted improved sensitivity of ground-based gravitational-wave detectors, new research avenues will become accessible. This is the case for gravitational-wave strong lensing, predicted with a non-negligible observation rate in the coming years. However, because one needs to investigate all the event pairs in the data, searches for strongly-lensed gravitational waves are often computationally heavy, and one faces high false-alarm rates. In this paper, we present upgrades made to the \GOLUM software, making it more reliable while increasing its speed by re-casting the look-up table, imposing a sample control, and implementing symmetric runs on the two lensed images. We show how the recovered posteriors have improved coverage of the parameter space and how we increase the pipeline's stability. Finally, we show the results obtained by performing a joint analysis of all the events reported until the GWTC-3 catalog, finding similar conclusions to the ones presented in the literature.

Abstract: We present first results from a novel numerical relativity code based on a tetrad formulation of the Einstein-scalar field equations combined with recently introduced gauge/frame invariant diagnostics indicating that inflation does not solve the homogeneity and isotropy problem beginning from generic initial conditions following a big bang.

Abstract: We study the scattering of light-like geodesics and massless scalar waves by a static Konoplya-Zhidenko black hole, considering the case that the parametrized black hole solution contains a single deformation parameter. By performing a geodesic analysis, we compute the classical differential scattering cross section and probe the influence of the deformation parameter on null trajectories. Moreover, we investigate the propagation of a massless scalar field in the vicinity of the static Konoplya-Zhidenko black hole and use the plane waves formalism to compute the differential scattering cross section. We confront our numerical results in the backward direction with the glory approximation, finding excellent agreement. We compare the results for the deformed black hole with the Schwarzschild case, finding that the additional parameter has an important role in the behavior of the scattering process for moderate-to-high scattering angles.

Abstract: Black holes are a recently observed theoretical prediction of General Relativity, characterized by event horizons, from which information cannot escape. Examined through the lenses of quantum mechanics, they can radiate at a definite temperature inverse to their mass and horizon radius. Hawking radiation, whose spectrum was calculated considering particles scattering off black holes, is connected to the paradox of the loss of information falling into them. Information can become non-fungible, due to scrambling. We demonstrate this feature not to be restricted to curved space-times: soft radiation scattering in a flat space-time does scramble information as well. To this end, we compute the scrambling of information through the tripartite mutual information in a scattering process off a black hole and compare it with the flat space-time analog. We show that the scrambling power of the gravitational field of a black hole is negligible with respect to the scrambling power of flat space-time.