Gravitation and Cosmology最新文献

We study the thermodynamics of black holes (BHs) with a Gauss–Bonnet correction term in ((3+1))-dimensional AdS space-time. It is known that this term has no effect on the equations of motion, however, it modifies the entropy which is calculated using Wald’s formula. The corrections to the area law appear in the form of a term involving the Gauss–Bonnet parameter. We study charged BHs, namely, Reissner-Nordström and Born–Infeld, under this regime. The first thing we encounter are divergences in heat capacity. After eliminating the possibility of first-order phase transition, we apply two well trusted methods from standard thermodynamics, namely, the Ehrenfest scheme and the Ruppeiner state space geometry analysis to ensure the second-order nature of the phase transition points. Our main focus in this study is on the effects of the Gauss–Bonnet and Born–Infeld parameters on the phase transition points.

Against the background of insufficient information on the law of gravity in near space, a justification is proposed for conducting a high-precision artificial experiment to determine the law of gravity dominating the Solar System. It is proposed to use the Sun–Earth–Venus system, space probes, and observers as a “gravitational space laboratory.” The scheme of a “standard ballistic flight” is defined as a complex trajectory of the probe, comprising the Earth-Venus path, accelerating gravitational maneuver at Venus, and the Venus–Earth orbit path. The data at the end point of the trajectory provide a conclusion on the format of the law of gravity of the Sun. The key instruments of the experiment, the gravity assist maneuver and the function of its sensitivity to changes in the probe–planet impact parameter, are described in detail. Schemes and results of an analytical calculation and numerical construction of the probe trajectory are given. It is shown that this experiment provides a margin for successful observation of the probe positions in classical and relativistic gravity, which makes it possible to distinguish the gravity type. At the evaluation level, the issues of economics of the experiment are touched upon, and the provision of observational statistics and the possibility of obtaining additional scientific and practically significant information are discussed.

We have explored the observed matter-antimatter asymmetry in the Universe to constrain the model parameters in Extended Symmetric Teleparallel Gravity (STG) or (f(Q,T)) gravity, where (Q) is the nonmetricity and (T) is the trace of the energy momentum tensor. We have considered two functional forms of (f(Q,T)) to find the baryon asymmetry to entropy ratio calculated at a decoupling temperature. Two different data sets, namely, the Hubble data set and the (text{Hubble}+text{BAO}+text{Pantheon}) data set, are used to constrain the scale factor, and the constrained model is used to obtain the baryon asymmetry to entropy ratio. It is observed that the model constrained from the Hubble data set favors a narrow range of the (f(Q,T)) gravity parameters to reproduce the observed baryon asymmetry.

The Hořava–Lifshitz mixmaster cosmological model near the cosmological singularity is presented as a generalized Euclidean Toda chain. Restricting to dominant vectors of the spectrum, we get a truncated model that qualitatively well describes the mixmaster model. The truncated model is associated with an affine Kac–Moody Lie algebra (A_{2}^{+}). According to the Adler–van Moerbeke criterion, the truncated Hamiltonian system is algebraically completely integrable.

We investigate an exact Universe model which is observationally viable and filled with a binary mixture of a perfect fluid and the cosmological constant (Lambda). We consider the redshift drift (dot{z}=(1+z)H_{0}-H(z)) and perform statistical tests to obtain the best fit value of the model parameters of the derived Universe with its observed values. Here, (H_{0}) and (z) denote the present value of the Hubble constant and the redshift, respectively. We estimate the best fit values of the Hubble constant and the density parameters as (H_{0}=68.58pm 0.84) km/(s Mpc), ((Omega_{m})_{0}=0.26pm 0.010), and ((Omega_{Lambda})_{0}=0.71pm 0.025) by bounding the derived model with the latest observational Hubble data (OHD), while with joint Pantheon data and OHD, its values are (H_{0}=71.93pm 0.58) km/(s Mpc), ((Omega_{m})_{0}=0.272pm 0.06), and ((Omega_{Lambda})_{0}=0.74pm 0.09). The analysis of the deceleration and jerk parameters shows that the Universe in the derived model is compatible with the (Lambda)CDM model. We also investigate that the cosmological models owning a redshift drift minimize the (H_{0}) tension.

We study the dependence of the parameters of the evolution of scalarly charged black holes (BHs) in the early Universe on the parameters of field theories of interaction, and the influence of the geometric structure of the relative position of BHs on the limitation of their maximum mass, The problem of the metric of a scalarly charged BH in a medium of expanding scalarly charged matter is discussed, the expression is obtained for the macroscopic cosmological constant at late stages of expansion, generated by quadratic fluctuations of the metric, connecting the cosmological constant value with the BH masses and their concentration.

Various calculations carried out in the past to understand the propagation of light in a rotating gravitational field (viz., Kerr field) are examined. For a plane-polarized light, it is observed that due to the effect of rotational gravitational field, the polarization vector of light gets rotated, with the amount of rotation independent from the frequency of the light. In the present work, using the formulations of geometrical optics, I try to find the implications of such findings, which seem to be very strange and give rise to violation of Lorentz Invariance and the Equivalence Principle, which are mostly not accepted by present-day physics. The analysis involves splitting plane-polarized light into left and right circularly polarized components, and then one finds that these two components (with a given frequency) travel with two different velocities in the Kerr field. Also, for an individual circularly polarized component, the velocity of propagation depends on the frequency of light. Assuming the two opposite directions of circularly polarized light to represent two opposite photon spin states, the line element for circularly polarized light is found to depend on the photon spin in addition to frequency. Additional calculations are made to estimate the propagation time delay between two circularly polarized components (with given frequency) between the source and observer at finite distances from the Kerr mass. Some typical estimates of this time delay are made for the Sun and one pulsar, so that in the future one can experimentally verify these results. For an individual circularly polarized component, time delay expressions are also derived for the propagation of light at two different frequencies. It has been found that circularly polarized light with higher frequency (energy) travels faster in a rotating gravitational field as compared to its lower frequency counterpart.

A first introduction is presented to the comparison between classical and relativistic gravitational effects related to planetary shape characterization. The Earth and the giant planets are the examples considered. The analysis is mainly devoted to relativistic and classical predictions of periastron shifts for equatorial or almost equatorial orbits around the Earth and the giant planets, which can be used as tools for determinations of the shape and density distribution. The ratios between relativistic (up to the Lense–Thirring order correction) and classical (resulting from the harmonic expansion) effects and their dependence on the orbit parameters are analyzed in order to identify the conditions improving the possibility to resolve mixed effects. In a complementary approach, predictions for freely falling test particles from relativistic corrections and classical harmonic expansions of the Earth and other planets are compared within the same shape characterization framework.

In 1965, Penrose introduced the idea of a trapped surface which became an essential concept in proving the first singularity theorem. In this work, some neglected aspects of trapped surfaces are considered. Specifically, I discuss that the existence of such a surface in space-time may result in an undesirable consequence: either space-time is singular, or chronology violation arises. Although a trapping region often implies a singularity in space-time, but a singular space-time may not contain any trapped region. Finally, I show that noncosmological trapped surfaces are observer-dependent. It means that some observers in a trapped region may not observe trapped surfaces!

Acoustic space-times have been known to offer analogue models for black hole physics and cosmology. Within this context, aspects of analogue quantum field theories in curved space-time are studied. In particular, some new comments have been made on the analogue Hawking temperature including a quick derivation of the result. Further, analogue cosmology is explored, within which an acoustic version of the Parker–Toms model is proposed, and the corresponding quantities are calculated. The limits of the acoustic analogue are emphasized.