Gravitation and Cosmology最新文献

An attempt is made to demystify loop quantum gravity (LQG) in a concise and lucid way. LQG is a background-independent as well as non-perturbative approach of the theory of quantum gravity. Since LQG is one of the supposed candidates of a theory of quantum gravity, firstly, prerequisite concepts that are needed for LQG are outlined. Since LQG belongs to the canonical quantization approach, the ADM formalism along with the metric formulation is introduced. Thereafter, other associated concepts regarding the connection formulation are given, such as tetrads, spin connection, and the Palatini action. Afterwards, a modification of the connection formulation, i.e., the Ashtekar formulation, a basis for the current framework of LQG, is presented. Thereafter, the kinematic and dynamical framework, i.e., spin network and spin foam, respectively, are explained; here, the geometrical observables such as area and volume are quantized. Applications of LQG, such as the black hole entropy problem and loop quantum cosmology, are also briefly introduced. This article targets on beginners and novice who wants to enter this research field.

We construct a chiral self-gravitating model corresponding to modified (f(R,square R)) gravity following by application of the Lagrange multipliers method and a conformal transformation to obtain the model in the Einstein frame. Killing symmetries of the target space are found. Using a special case of a scaling transformation, we find examples of exact solutions with zero and constant potentials. A linear dependence between the fields leads to new solutions for the model.

We consider pure radiation metrics which are conformal to a vacuum space-time, and we investigate whether there exist or not any pure radiation metrics which are Ricci soliton. In particular, we give a necessary and sufficient condition for such metrics to be Ricci solitons. As an application, we prove that they are gradient Ricci soliton only under a very restrictive condition for the defining function involved in those metrics.

In the framework of the Standard cosmological model, we study the “lookforward” history of the expanding universe, subject to certain rules, in order to calculate the kinetic  recession velocity of a luminous source along the line of sight of the observer in a unique way (a coordinate-independent definition), directly from the given cosmological redshift. In this case, we use the method of dividing the cosmological redshift into infinitesimally shifted “relative” spectral intervals between the neighboring emitter and absorber due to expansion of the universe, measured at infinitesimally separated points of space-time, and sum them over to overcome the ambiguity that represents the parallel transport of the four-velocity of the source to the observer in curved Robertson-Walker space-time. In the particular case of such a realization along a null geodesic, we show that the kinetic recession velocity is reduced to the Doppler global velocity. The relationship of the cosmological redshift and the kinetic recession velocity, which is completely different from the formula for the global Doppler shift, leads to important cosmological consequences that the kinetic recession velocity of a galaxy is always subluminal, even for large redshifts of order one or more, and thus it does not violate the fundamental physical principle of causality.

We compute the Hawking temperature of a regular self-gravitating ’t Hooft–Polyakov magnetic monopole in global and local monopole black holes. To this end, we apply two different methods: the tunneling method and the topological method, which both yield the standard Hawking temperatures of these two geometries. We then study a phase transition in the vicinity of the Planck scale. Based on the Hamilton–Jacobi (HJ) equation, by using the corrected classical action, the quantum tunneling method is applied to derive the corrected Hawking temperature within the framework of the generalized uncertainty principle. In the sequel, we check the validity of the first law of thermodynamics and test the thermodynamic instability for the global and local monopole black holes.

We present exact solutions of cosmological equations in elliptic Legendre integrals. Solutions of classical cosmology and conformal cosmology describe the modern Hubble diagram with the same accuracy. The Hubble curves are extrapolated to large redshift values.

The present work deals with the dynamical evolution of Friedmann-Robertson-Walker (FRW) Cosmologies with variable (Lambda) in Lyra geometry. We perform phase-plane analysis of the model with a time-dependent displacement vector, considering a variable (Lambda), i.e., (Lambdaproptobeta^{2}) for model I and (dot{Lambda}propto H^{3}) for model II. To analyze the evolution equations, we introduce a suitable transformation of variables. The results are presented by curves in the phase-plane diagram. The nature of critical points is analyzed, and stable attractors are examined for both cosmological models. We determine the classical stability of these cosmologies. We also examine the transition of an early decelerated stage of the Universe to the present accelerated stage for both models.

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.