General Relativity and Quantum Cosmology (gr-qc)

Abstract: The frozen star model provides a classical description of a regularized black hole and is based upon the idea that regularizing the singularity requires deviations from the Schwarzschild geometry which extend over horizon-sized scales, as well as maximally negative radial pressure as an equation of state. The frozen star has also been shown to be ultra-stable against perturbations; a feature that can be attributed to the equation of state and corresponds to this model mimicking a black hole in the limit $\hbar\to 0$ or, equivalently, the limit of infinite Newton's constant. Here, we ``defrost'' the frozen star by allowing its radial pressure to be perturbatively less negative than maximal. This modification to the equation of state is implemented by appropriately deforming the background metric so as to allow the frozen star to mimic a quantum black hole at finite $\hbar$ and Newton's constant. As a consequence, the defrosted star acquires a non-trivial spectrum of oscillatory perturbations. To show this, we first use the Cowling approximation to obtain generic equations for the energy density and pressure perturbations of a static, spherically symmetric background with an anisotropic fluid. The particular setting of a deformed frozen star is then considered, for which the dispersion relation is obtained to leading order in terms of the deviation from maximal pressure. The current results compare favorably with those obtained earlier for the collapsed polymer model, whose strongly non-classical interior is argued to provide a microscopic description of the frozen and defrosted star geometries.

Abstract: In this exclusive study of the modified $f(Q)$ theory of gravity in the open and closed type Friedmann-Lema\^itre-Robertson-Walker (FLRW) universe model, we impose some constraints from the classical energy conditions. The viable range of parameter $\beta$ for two different $f(Q)$ models, $f(Q)=Q+\beta Q^2$ and $f(Q)=Q+\beta\sqrt{-Q}$, are analyzed in details and the related cosmological implications are discussed. Violation of effective strong energy condition is resulting into late-time acceleration of the Universe. Present observational values of Hubble parameter and deceleration parameter are used to constrain the parameters.

Abstract: We consider a Weyl-Lorentz-U(1)-invariant gravity model written in terms of a scalar field, electromagnetic field and nonmetricity tensor in three dimensions. Firstly we obtain variational field equations from a Lagrangian. Then, we find some classes of circularly symmetric rotating solutions by exploiting the coincident gauge of symmetric teleparallel spacetime.

Abstract: Binary black hole simulations become increasingly more computationally expensive with smaller mass ratios, partly because of the longer evolution time, and partly because the lengthscale disparity dictates smaller time steps. The program initiated by Dhesi et al. (arXiv:2109.03531) explores a method for alleviating the scale disparity in simulations with mass ratios in the intermediate astrophysical range ($10^{-4} \lesssim q \lesssim 10^{-2}$), where purely perturbative methods may not be adequate. A region ("worldtube") much larger than the small black hole is excised from the numerical domain, and replaced with an analytical model approximating a tidally deformed black hole. Here we apply this idea to a toy model of a scalar charge in a fixed circular geodesic orbit around a Schwarzschild black hole, solving for the massless Klein-Gordon field. This is a first implementation of the worldtube excision method in full 3+1 dimensions. We demonstrate the accuracy and efficiency of the method, and discuss the steps towards applying it for evolving orbits and, ultimately, in the binary black-hole scenario. Our implementation is publicly accessible in the SpECTRE numerical relativity code.

Abstract: Gravitational radiation and some global properties of spacetimes can only be unambiguously measured at future null infinity. This motivates the interest in reaching it within simulations of coalescing compact objects, whose waveforms are extracted for gravitational wave modelling purposes. One promising method to include future null infinity in the numerical domain is the evolution on hyperboloidal slices: smooth spacelike slices that reach future null infinity. The main challenge in this approach is the treatment of the compactified asymptotic region at future null infinity. Evolution on a hyperboloidal slice of a spacetime including a BH entails an extra layer of difficulty, in part due to the finite coordinate distance between the BH and future null infinity. Spherical symmetry is considered here as simplest setup still encompassing the full complication of the treatment along the radial coordinate. First, the construction of constant-mean-curvature hyperboloidal trumpet slices for Schwarzschild and Reissner-Nordstroem BH spacetimes is reviewed from the point of view of the puncture approach. Then, the framework is set for solving hyperboloidal-adapted hyperbolic gauge conditions for stationary trumpet initial data, providing solutions for two specific sets of parameters. Finally, results of testing these initial data in evolution are presented.