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Particle horizon
About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.
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TL;DR: In this article, the authors investigate the possibility of apparent acausality as seen by a brane-based observer resulting from signal propagation through the extra dimensions, and derive the equations of motion for the interbrane distance in a two-brane scenario.
Abstract: Models with extra dimensions are often invoked to resolve cosmological problems. We investigate the possibility of apparent acausality as seen by a brane-based observer resulting from signal propagation through the extra dimensions. Null geodesics are first computed in static and cosmological single-brane models, following which we derive the equations of motion for the inter-brane distance in a two-brane scenario, which we use to examine possible acausality in this more complex setup. Despite observing significant effective acausality in some situations there is no a priori solution to the horizon problem using this mechanism. In the two-brane scenario there can be significant late time violation of gravitational Lorentz invariance, resulting in the gravitational horizon being larger than the particle horizon, leading to potential signals in gravitational wave detectors.
37 citations
TL;DR: In this article, the cosmological equations are reduced to quadratures under the assumption of a power law between the expansion factor of the universe and the scalar field of the second self-creation theory proposed by G. A. Barber.
Abstract: Under the assumption of a power law between the expansion factor of the Universe and the scalar field of the second self-creation theory proposed by G. A. Barber, the cosmological equations are reduces to quadratures. Several exact solutions are obtained, among them linearly expanding and inflationary universes with a barotropic equation of state.
37 citations
TL;DR: In this article, the evolution of the observable part of the universe has been studied from a cosmological point of view, and a new concept, the Q-sphere (or Q-space, in the case of quintessence) is introduced.
Abstract: We consider ever-expanding big bang models with a cosmological constant, Λ, and investigate in detail the evolution of the observable part of the universe. We also discuss quintessence models from the same point of view. A new concept, the Λ-sphere (or Q-sphere, in the case of quintessence) is introduced. This is the surface in our visible universe that bounds the region where dark energy dominates the expansion, and within which the universe is accelerating. We follow the evolution of this surface as the universe expands, and we also investigate the evolution of the particle and event horizons as well as the Hubble surface. We calculate the extent of the observable universe and the portion of it that can be seen at different epochs. Furthermore, we trace the changes in redshift, apparent magnitude and apparent size of distant sources through cosmic history. Our approach is different from, but complementary to, most other contemporary investigations, which concentrate on the past light cone at the present epoch. When presenting numerical results we use the FRW world model with Ωm0 = 0.30 and ΩΛ0 = 0.70 as our standard cosmological model. In this model the Λ-sphere is at a redshift of 0.67, and within a few Hubble times the event horizon will be stationary at a fixed proper distance of 5.1 Gpc (assuming h0 = 0.7). All cosmological sources with present redshift larger than 1.7 have by now crossed the event horizon and are therefore completely out of causal contact.
37 citations
TL;DR: In this paper, the authors consider the case of a symmetric collision of infinitely thin branes having a hyperbolic or flat spatial geometry and show that such a collision results in a collapsing universe on the final brane unless the preexisting expansion rate in the bulk just prior to the collision is sufficiently large in comparison to the momentum transfer in the fifth dimension.
Abstract: It is intriguing to consider the possibility that the big bang of the standard (3+1)-dimensional cosmology originated from the collision of two branes within a higher dimensional spacetime, leading to the production of a large amount of entropy. In this paper we study, subject to certain well-defined assumptions, under what conditions such a collision leads to an expanding universe. We assume the absence of novel physics, so that ordinary (4+1)-dimensional Einstein gravity remains a valid approximation. It is necessary that the fifth dimension not become degenerate at the moment of collision. First the case of a symmetric collision of infinitely thin branes having a hyperbolic or flat spatial geometry is considered. We find that a symmetric collision results in a collapsing universe on the final brane unless the preexisting expansion rate in the bulk just prior to the collision is sufficiently large in comparison to the momentum transfer in the fifth dimension. Such prior expansion may either result from negative spatial curvature or from a positive five-dimensional cosmological constant. The relevance of these findings to the colliding bubble braneworld universe scenario is discussed. Finally, results from a numerical study of colliding thick-wall branes is presented, which confirm the results of the thin-wall approximation.
37 citations
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TL;DR: In this paper, the authors present a model of an inhomogeneous universe that leads to accelerated expansion after taking spatial averaging and show that after the region with positive spatial curvature begins to re-collapse, the deceleration parameter becomes negative and the averaged universe starts accelerated expansion.
Abstract: We present a model of an inhomogeneous universe that leads to accelerated expansion after taking spatial averaging. The model universe is the Tolman-Bondi solution of the Einstein equation and contains both a region with positive spatial curvature and a region with negative spatial curvature. We find that after the region with positive spatial curvature begins to re-collapse, the deceleration parameter of the spatially averaged universe becomes negative and the averaged universe starts accelerated expansion. We also discuss the generality of the condition for accelerated expansion of the spatially averaged universe.
36 citations