Gravitation and Cosmology最新文献

Mathematical cosmology is the branch of theoretical physics where some of the most intricate, complex, and deeply unresolved issues lie. Beginning with Einstein’s static universe in 1917, in this brief paper we freely float above all major developments that shaped the field until today. We discuss highlights that are further documented in the authors’ recent survey “100 years of mathematical cosmology” scheduled to appear in the Theme Issue “The Future of Mathematical Cosmology.” This Theme Issue is to be published in two parts by the Philosophical Transactions of the Royal Society A, and contain a number of important contributions by key researchers in the field.

We consider the metric-affine geometry whose intrinsic structure is defined in terms of two independent objects: the Riemannian metric and the general affine connection. By means of the metric tensor, for contraction of Riemannain curvature of the affine connection, we form an action density for gravity and matter. Variations of our natural Lagrangian give us two equations. The derived equations contain the Einstein field equation. The other equation describes matter in space-time. In this framework, the affine connection is related to the concept of matter in s*pace-time, so matter can be interpreted as a factor which leads curving and twirling of the space-time manifold.

Recently it was found from the Cassini data that the mean recession speed of Titan from Saturn is (v=11.3pm 2.0) cm/yr, which corresponds to a tidal quality factor of Saturn (Qcong 100) while the standard estimate yields (Qgeq 6times 10^{4}). It was assumed that such a large speed (v) is due to a resonance locking mechanism of five inner mid-sized moons of Saturn. In this paper, we show that an essential part of (v) may come from a local Hubble expansion, where the Hubble–Lemaître constant (H_{0}), recalculated to the Saturn–Titan distance (D), is 8.15 cm/(yr (D)). Our hypothesis is based on many other observations showing a slight expansion of the Solar system and also of our Galaxy at a rate comparable with (H_{0}). We demonstrate that the large disproportion in estimating the (Q) factor can be just caused by the local expansion effect.

In the context of (f(R,T)) modified gravity theory, we consider a homogeneous and anisotropic Bianchi type-I cosmological model which relies on the condition of a constant jerk parameter, (j=1), corresponding to a flat (Lambda)CDM model. Under this condition, we obtain two different solutions, one is power-law and the other one is exponential. The power-law solution gives a decelerating model, while the exponential one yields an accelerating cosmology. We discuss the physical and geometric properties of both models, validity of the solutions, and the significance of modified (f(R,T)) gravity for the models.

We study the evolution of cosmological density perturbations of cold dark matter (CDM) in an expanding matter-dominated universe with respect to a homogeneous spatially flat Friedman-Lemaître-Robertson-Walker (FLRW) background under a Newtonian gravity scenario after recombination. In particular, the density contrast equation for CDM is constructed from a two-fluids model. In deriving the equation, we use the well-known solution for the density contrast of baryonic matter presented in one of our recent papers. We employ a symmetry-based approach to study the CDM density contrast equation and find eight-parameter Lie group symmetries. We use these Lie symmetries to find group-invariant solutions of the density contrast equation for CDM governed by a Newtonian force law from the invariant curve condition and visualize the growth of density perturbation after recombination in the presence of baryonic matter. We also solve numerically the CDM density contrast equation in the presence of a baryonic component and check the evolution of the density contrast from recombination to the present age. In addition, considering only growing mode solutions, we compare the evolution of density contrasts of CDM (WIMPs) in the presence of a baryonic component and that of only baryons. We also compare the evolution of density contrasts of CDM in the presence of baryons under Newtonian gravity, baryons under Newtonian gravity, and baryons under MONDian gravity for the time period from recombination to the present age.

The current paper is concerned with the orbital precession of a satellite moving in an inclined plane to the equatorial plane, as defined in the Kerr–Newman field. Based on the angular momentum and the charge of the field generated by the Earth, a numerical value of the perigee advance of some artificial satellites is calculated. Due to the effect of the angular momentum and the Earth’s electromagnetic field, the stability of the satellite is examined. The achieved results are compared with those obtained by the Beacon Explorer C, LAGEOS, LAGEOS II, LARES, GPS, and GRACE A, B satellites.

The Unruh effect for a massive scalar field in ((3+1))D space-time is considered in the case of accelerated motion with a certain starting instant. A stationary energy-angular distribution of the corresponding Unruh radiation to be observed by a two-opposite-horn antenna is obtained. It is shown that this distribution is significantly anisotropic, and the Unruh radiation cannot be considered as thermal (equilibrium) radiation. It is also shown that Fulling modes are not completely relevant wave functions for the description of particles in semi-eternal Rindler space-time. More appropriate candidates are suggested for this aim.

In the recent years, a number of studies have been carried out to substantiate cosmological effects within the framework of the relational approach to the nature of space-time and physical interactions. It has been shown that the cosmological redshift and the cosmic microwave background can be a result of contributions from emitted but unabsorbed radiation. However, until now, Hubble’s law has not been entirely derived from the relational ideas themselves. In this paper, such a derivation is presented, and the Hubble parameter is calculated for the present epoch. To get this, the momentum contributions of emitted but unabsorbed radiation to the momentum of a distant astronomical object (a cluster of galaxies) are considered. It is shown that taking these contributions into account leads to the linear Hubble law.

The accelerated expansion of the Universe constitutes one of the biggest challenges in present-day cosmology. To understand and explain this phenomenon in the framework of general relativity, corrections and extensions to it are required, which make the so-called extended theories of gravity (ETGs). In these theories, the geometry of space-time that represents the gravitational sector at the left hand side of the Einstein field equation ({G_{munu}}=8pi{T_{munu}}) is necessarily modified. These theories have attracted much attention since the time the accelerated expansion was discovered. A class of these theories known as (f(R)) gravity, offers a potent candidacy for this purpose, in addition to matter content modifications. The gravitational sector depending on the Ricci scalar invariant (R) is basically replaced with some its general nonlinear function which consists of higher-order curvature terms. In this work, we attempt to realize the late-time accelerated expansion in the context of (f(R)) gravity using the dynamical system approach. Analyzing the dynamical system arising from a particular (f(R)) model, its stability is studied for the cosmological inferences. The particular model (f(R)={R^{p}}exp({qR})) with (m=frac{{R{f_{,RR}}}}{{{f_{,R}}}}=frac{{p(p-1)+2pqR+{q^{2}}{R^{2}}}}{{p+qR}}) and (r=-frac{{R{f_{,R}}}}{f}=-(p+qR)) and with the geometric curve (m(r)=-frac{{{r^{2}}-p}}{r}), is studied in this paper. We use the geometric approach for the curve (m(r)) in the plane ((r,m)) which provides some properties of the model. In the case of a matter-dominated era the viability conditions at (r=-1), (m(r)=0) and (dm/dr>-1) are investigated. On the other hand, for the late-time acceleration, however, at (r=-2), either of the two conditions (m(r)=-r-1) with (dm/dr<-1), (1geq m>(sqrt{3}-1)/2) and (1geq mgeq 0) are sought to fulfill. In the first place, the cosmic content is assumed to comprise matter and radiation only in the absence of a cosmological constant (Lambda). In this case, an interaction of any kind is disregarded. Afterwards, as the second consideration, an interaction term in the presence of a cosmological constant representing dark energy is taken into account. The effects of linear and nonlinear interactions between matter and dark energy are also taken into account orderly in this case. The results are presented for each case, along with a discussion of critical points, their eigenvalues, and the equation of state parameter.

Based on the previously formulated mathematical model of a statistical system with scalar interaction of fermions and the theory of gravitational-scalar instability of a cosmological model with a two-component statistical system of scalarly charged degenerate fermions, a numerical model of the cosmological evolution of gravitational-scalar perturbations is constructed, and specific examples of the development of instability are given. Some features of the instability development are investigated, depending on the behavior of the unperturbed cosmological model. It is shown that unstable modes can appear at very early stages of cosmological expansion or contraction, and the duration of the unstable phase is comparable to tens of Planck scales. In this case, however, a very significant increase in unstable modes is possible due to redistribution of energy between the components of the scalar doublet.