Particle horizon

About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.

Phase transition in cosmology with magnetic field

Abstract: In this paper we have investigated the Bianchi type-II cosmological model with variable parameters in the frame work of modified f(R, T) gravity theory as suggested by Harko et al. (Phys. Rev. D, 84:024020). As we know that the effect of space-time curvature upon phase transition is an expanding universe. In this communication we have constructed a cosmological model of the universe by taking suitable assumptions along with string in presence of magnetic field. It is to be noted that our procedure for solving the field equations is different from other authors as we have consider the time dependent deceleration parameter (DP), it means that the universe which was decelerating in the past is accelerating at present time. We found that the universe is decelerating for q > 0 and accelerating for −1 ≤ q < 0, which shown signature flipping.

K-matter as Mach's principle realization

Abstract: It is shown that if one takes into account Mach's principle in the form which follows from quantum theory and considers it as a complementary constraint between the parameters which characterize the energy density and geometry of the universe in addition to Einstein equations for a FRW universe, non-relativistic matter transforms into an analogue of K-matter. The exact solutions of the Einstein equations for the universe with such matter and cosmological constant are found. It is demonstrated that the Machian universe under consideration with a nonzero cosmological constant is equivalent to the open de Sitter universe. In the limit of zero cosmological constant such a universe evolves as a Milne universe, but in contrast to it, it contains matter with nonzero energy density. The possible application of proposed approach to the description of the present cosmological data is discussed. The problem of the age of the universe is considered as an example.

The Hyperboloidal Universe

Abstract: This paper investigates a relativistic model of the Universe in which the geometry describes a 4D version of the 2-sheeted hyperboloid that is isotropic, homogeneous in space at a given time and inhomogeneous in time. The internal Schwarzschild metric is used for this model, which is justified by the fact that spherically-symmetric empty spaces in the Universe are effectively surrounded by a shell of infinite mass (the surrounding Universe and in particular, the infinitely dense mass at the Big Bang). Thus the metric for the empty spaces must be described by the Schwarzschild metric according to Birkhoff&rsquo;s theorem. Since the shell&rsquo;s mass is infinite, the external solution cannot describe this spacetime and therefore the internal Schwarzschild solution must be the correct metric for this spacetime. The important insight here is that the source of the metric is not at r = 0, but that the event horizon is the metric source in both cases, representing a location/time of infinite density. This is supported by looking at the internal geometry in Kruskal coordinates where the event horizon surrounds the vacuum at an infinite distance, meaning the Schwarzschild radius, and therefore mass, of the source shell is infinite. The full spatial homogeneity of the internal metric is also demonstrated by visualizing at the Schwarzschild geometry (with 2 spatial dimensions and the time dimension) in Kruskal coordinates. The model predicts both a Universe and Anti-Universe moving in opposite directions of time undergoing an expansion phase, followed by a collapsing phase. Using only the current coordinate age of the Universe and transition redshift, it predicts the accelerated expansion and it is shown that its Hubble diagram fits currently available supernova and quasar data as well as predicting a Hubble constant H0 &asymp; 71.6km/s/M pc. The angular term of the metric describes time dilation caused by the relativistic kinematic precession effect known as Thomas Precession which can be interpreted as spin about the time dimension. The model also makes two novel predictions: that the early Universe should have structures older than expected due to an increased amount of proper time relative to coordinate time in that era and that the background Universe should appear brighter than current models predict.

Evolution of thermodynamic quantities on cosmological horizon in $\Lambda(t)$ model

Abstract: The horizon of a flat Friedmann--Robertson--Walker (FRW) universe is considered to be dynamic when the Hubble parameter $H$ and the Hubble radius $r_{H}$ vary with time, unlike for de Sitter universes. To clarify the thermodynamics on a dynamic horizon, the evolution of a dynamical Kodama--Hayward temperature and Bekenstein--Hawking entropy on the horizon of a flat FRW universe is examined in a $\Lambda(t)$ model similar to time-varying $\Lambda(t)$ cosmologies. The $\Lambda(t)$ model includes both a power-law term proportional to $H^{\alpha}$ (where $\alpha$ is a free variable) and the equation of state parameter $w$, extending a previous analysis [Phys. Rev. D 100, 123545 (2019) (arXiv:1911.08306)]. Using the present model, a matter-dominated universe ($w=0$) and a radiation-dominated universe ($w=1/3$) are examined, setting $\alpha <2$. Both universes tend to approach de Sitter universes and satisfy the maximization of entropy in the last stage. The evolution of several parameters (such as the Bekenstein--Hawking entropy) is similar for both $w=0$ and $w=1/3$, though the dynamical temperature $T_{H}$ is different. In particular, $T_{H}$ is found to be constant when $w=1/3$ with $\alpha=1$, although $H$ and $r_{H}$ vary with time. To discuss this case, the specific conditions required for constant $T_{H}$ are examined. Applying the specific condition to the present model gives a cosmological model that can describe a universe at constant $T_{H}$, as if the dynamic horizon is in contact with a heat bath. The relaxation processes for the universe are also discussed.