General Relativity And Quantum Cosmology

In light of the growing interest in the Hayward black hole solution, a detailed study on the corresponding lapse function and its roots is presented. The lapse function is expressed in terms of the classical Schwarzschild radius r s and the Hayward's parameter l . Both of these quantities are used as thermodynamic variables to find related thermodynamic quantities. In this context, the variable l is associated with a canonical conjugate variable F H , and a free energy ? . Moreover, a second order phase transition is found to appears at l??.333 r s .

We introduce a gravitational waveform inversion strategy that discovers mechanical models of binary black hole (BBH) systems. We show that only a single time series of (possibly noisy) waveform data is necessary to construct the equations of motion for a BBH system. Starting with a class of universal differential equations parameterized by feed-forward neural networks, our strategy involves the construction of a space of plausible mechanical models and a physics-informed constrained optimization within that space to minimize the waveform error. We apply our method to various BBH systems including extreme and comparable mass ratio systems in eccentric and non-eccentric orbits. We show the resulting differential equations apply to time durations longer than the training interval, and relativistic effects, such as perihelion precession, radiation reaction, and orbital plunge, are automatically accounted for. The methods outlined here provide a new, data-driven approach to studying the dynamics of binary black hole systems.

Scalar-tensor theories are well studied extensions of general relativity that offer deviations which are yet within observational boundaries. We present the time evolution equations governing the perturbations of a nonrotating scalarized neutron star, including a dynamic spacetime as well as scalar field within the framework of such scalar-tensor theories. We employ a theory that allows for a massive scalar field or a self-interaction term and we study the impact of those parameters on the non-axisymmetric f -mode. The time evolution approach allows for a comparatively simple implementation of the boundary conditions. We find that the f -mode frequency is no longer a simple function of the star's average density when a scalar field is present. We also evaluate the accuracy of different variants of the Cowling approximation commonly used in previous studies of neutron star oscillation modes in alternative theories of gravity and demonstrate that it can give us not only qualitatively correct results, but in some cases also good quantitative estimates of the oscillations frequencies.

The Hong-Ou-Mandel (HOM) effect is analyzed for photons in a modified Mach-Zehnder setup with two particles experiencing different gravitational potentials, which are later recombined using a beam-splitter. It is found that the HOM effect depends directly on the relativistic time dilation between the arms of the setup. This temporal dilation can be used to estimate the γ and β parameters of the parameterized post-Newtonian formalism. The uncertainty in the parameters γ and β are of the order 10 ?? ??10 ??2 , depending on the quantum state employed.

We present the impact of non-minimal coupling ξ ϕ 2 R on the inflationary parameters by taking into account the models of single-field inflation with the inflaton that has a non-zero vacuum expectation value ( v ) after the period of inflation in Palatini gravity. We discuss the well-known symmetry breaking type potentials, namely the Higgs potential and Coleman-Weinberg potential. We show the inflationary predictions of these potentials, for both ϕ>v and ϕ<v inflation, the regions in the v−ξ plane for which the values of n s and r are in agreement with the recent measurements. We also show the linear inflation behavior as a solution of Coleman-Weinberg potential for ξ v 2 =1 limit. Finally, we take into account the inflationary predictions of Coleman-Weinberg potential for preferred ξ values as a function of v in Palatini formalism.

We introduce a method where particle physics processes in cosmology may be calculated by the usual perturbative flat space quantum field theory through an effective Minkowski space description at small time intervals provided that the running of the effective particle masses are sufficiently slow. We discuss the necessary conditions for the applicability of this method and illustrate the method through a simple example. This method has the advantage of avoiding the effects of gravitational particle creation in the calculation of rates and cross sections i.e. giving directly the rates and the cross sections due to the scatterings or the decay processes.

We study odd parity perturbations of spherically symmetric black holes with time-dependent scalar hair in shift-symmetric higher-order scalar-tensor theories. The analysis is performed in a general way without assuming the degeneracy conditions. Nevertheless, we end up with second-order equations for a single master variable, similarly to cosmological tensor modes. We thus identify the general form of the quadratic Lagrangian for the odd parity perturbations, leading to a generalization of the Regge-Wheeler equation. We also investigate the structure of the effective metric for the master variable and refine the stability conditions. As an application of our generalized Regge-Wheeler equation, we compute the quasi-normal modes of a certain nontrivial black hole solution. Finally, our result is extended to include the matter energy-momentum tensor as a source term.

This paper is devoted to analyzing the critical phenomenon and phase transition of quintessential Kerr-Newman-anti-de Sitter black hole in the framework of Maxwell equal-area law. For this purpose, we first derive thermodynamic quantities such as Hawking temperature, entropy and angular momentum in the context of extended phase space. These quantities satisfy Smarr-Gibbs-Dehum relation in the presence of quintessence matter. We then discuss the critical behavior of thermodynamic quantities through two approaches, i.e., van der Waals-like equation of state and Maxwell equal-area law. It is found that the latter approach is more effective to analyze the critical behavior of the complicated black holes. Using equal-area law, we also study phase diagram in T−S plane and find an isobar which shows the coexistence region of two phases. We conclude that below the critical temperature, black holes show a similar phase transition as that of van der Waals fluid. Finally, we study the effects of thermal fluctuations on the stability of this black hole.

Recently, a Friedmann-Robertson-Walker universe filled with various cosmological fluids has been considered by S.D. Odintsov et al. in [30] from phase space vantage point where various expressions for the Equation-of-State (EoS) parameter were studied. Since these types of EoS parameters are generative of appreciable results in the Hilbert-Einstein model, hence we intend to investigate all the cases in a homogeneous F(T) -gravity ( T is the torsion) through phase space analysis in precise detail. In short, three viable models of interaction between dark matter and dark energy, including usual-type, power-law type, and oscillating type, are investigated comprehensively. It is indicated that the power-law interaction in the related dynamical systems should be of increasing nature with time to get more critical points. Due to the failure of the oscillating model of ref. [30] in F(T) -gravity, four modified models are suggested and examined in both F(T) and Hilbert-Einstein models. As to be seen, the modified models not only are generative of critical points equivalent to that of ref. [30], but also give rise to further critical points covering crucial stages of the evolution of the universe. In the context of these four models, such as the old one, at early times the interactions are negligible and they commence to grow as the cosmic time approaches the late-time in which the unification of early inflation and late acceleration is obtained. Using an indirect method, it is shown that the oscillating models have substantial roles in transitions between eras.

We consider a situation where a light source orbiting the innermost stable circular orbit (ISCO) of the Kerr black hole is gently falling from the marginally stable orbit due to an infinitesimal perturbation. Assuming that the light source emits photons isotropically, we show that the last radius at which more than 50\% of emitted photons can escape to infinity is approximately halfway between the ISCO radius and the event horizon radius. To evaluate them, we determine emitter orbits from the vicinity of the ISCO, which are uniquely specified for each black hole spin, and identify the conditions for a photon to escape from any point on the equatorial plane of the Kerr spacetime to infinity by specifying regions in the two-dimensional photon impact parameter space completely. We further show that the proper motion of the emitter affects the photon escape probability and blueshifts the energy of emitted photons.