Particle horizon

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

How Do Nonlinear Voids Affect Light Propagation

Abstract: Propagation of light in a clumpy universe is examined. As an inhomogeneous matter distribution, we take a spherical void surrounded by a dust shell, where the “lost mass” in the void is compensated by the shell. We study how the angular-diameter distance behaves when such a structure exists. The angular-diameter distance is calculated by integrating the Raychaudhuri equation including the shear. An explicit expression for the junction condition for the massive thin shell is calculated. We apply these results to a dust shell embedded in a Friedmann universe and determine how the distance-redshift relation is modified compared with that in the purely Friedmann universe. We also study the distribution of distances in a universe filled with voids. We show that the void-filled universe gives a larger distance than the FRW universe by ∼ 5% at z ∼ 1 if the size of the void is ∼ 5% of the Horizon radius.

Model universe with variable space dimension: Its dynamics and wave function

Abstract: Assuming the space dimension is not constant, but varies with the expansion of the universe, a Lagrangian formulation of a toy universe model is given. After a critical review of previous works, the field equations are derived and discussed. It is shown that this generalization of the FRW cosmology is not unique. There is a free parameter in the theory, C, with which we can fix the dimension of space say at the Planck time. Different possibilities for this dimension are discussed. The standard FRW model corresponds to the limiting case C → +∞. Depending on the free parameter of the theory, C, the expansion of the model can behave differently to the standard cosmological models with constant dimension. This is explicitly studied in the framework of quantum cosmology. The Wheeler–De Witt equation is written down. It turns out that in our model universe, the potential of the Wheeler–DeWitt equation has different characteristics relative to the potential of the de Sitter minisuperspace. Using the appropriate boundary conditions and the semiclassical approximation, we calculate the wave function of our model universe. In the limit of C → +∞, corresponding to the case of constant space dimension, our wave function has not a unique behavior. It can either leads to the Hartle–Hawking wave function or to a modified Linde wave function, or to a more general one, but not to that of Vilenkin. We also calculate the probability density in our model universe. It is always more than the probability density of the de Sitter minisuperspace in 3–space as suggested by Vilenkin, Linde, and others. In the limit of constant space dimension, the probability density of our model universe approaches to Vilenkin and Linde probability density being exp(−2|SE|), where SE is the Euclidean action. Our model universe indicates therefore that the Vilenkin wave function is not stable with respect to the variation of space dimension.

The Apparent Universe

Abstract: We exploit the parallel between dynamical black holes and cosmological spacetimes to describe the evolution of Friedmann-Lemaitre-Robertson-Walker universes from the point of view of an observer in terms of the dynamics of the apparent horizon. Using the Hayward-Kodama formalism of dynamical black holes, we clarify the role of the Clausius relation to derive the Friedmann equations for a universe, in the spirit of Jacobson's work on the thermodynamics of spacetime. We also show how dynamics at the horizon naturally leads to the quantum-mechanical process of Hawking radiation. We comment on the connection of this work with recent ideas to consider our observable Universe as a Bose-Einstein condensate and on the corresponding role of vacuum energy.

General relativistic cosmological models and the cosmic microwave background radiation

Abstract: We survey recent theoretical ideas regarding the possible value of the present total density of the Universe and show how physically realistic cosmological models containing small deviations from isotropy allow observations of intrinsically general relativistic effects on the microwave background to determine whether or not the Universe is open or closed no matter how close the total density lies to the critical value. Applications of these results to inhomogeneous cosmological models, cosmic vorticity, and observational studies of the microwave-background-radiation isotropy are also discussed.

Warm-Logamediate inflationary universe model

Abstract: Warm inflationary universe models in the context of logamediate expansion are studied. General conditions required for these models to be realizable and discussed. This study is done in the weak and strong dissipative regimes. The parameters of our models are constrained from the observational data.