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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, an independent constraint on the lower bound to the mass density was derived based on deviations of galaxy velocities from a smooth universal expansion, and together with a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable.
Abstract: Recent observations1,2 of high-redshift supernovae seem to suggest that the global geometry of the Universe may be affected by a ‘cosmological constant’, which acts to accelerate the expansion rate with time. But these data by themselves still permit an open universe of low mass density and no cosmological constant. Here we derive an independent constraint on the lower bound to the mass density, based on deviations of galaxy velocities from a smooth universal expansion3,4,5,6,7. This constraint rules out a low-density open universe with a vanishing cosmological constant, and together the two favour a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable. This type of universe, however, seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with ‘common wisdom’.
66 citations
TL;DR: If the speed of sound were vastly larger in the early Universe, a near scale-invariant spectrum of density fluctuations could have been produced even if the Universe did not submit to conventional solutions to the horizon problem.
Abstract: If the speed of sound were vastly larger in the early Universe, a near scale-invariant spectrum of density fluctuations could have been produced even if the Universe did not submit to conventional solutions to the horizon problem. We examine how the mechanism works, presenting full mathematical solutions and their heuristics. We then discuss several concrete models based on scalar fields and hydrodynamical matter that realize this mechanism, but stress that the proposed mechanism is more fundamental and general.
65 citations
TL;DR: It is shown that one can use the no-boundary wave function to calculate small departures from homogeneity within the authors' past light cone despite the possibility of much larger fluctuations on super horizon scales.
Abstract: We consider landscape models that admit several regions where the conditions for eternal inflation hold. It is shown that one can use the no-boundary wave function to calculate small departures from homogeneity within our past light cone despite the possibility of much larger fluctuations on super horizon scales. The dominant contribution comes from the history exiting eternal inflation at the lowest value of the potential. In a class of landscape models this predicts a tensor to scalar ratio of about 10%. In this way the no-boundary wave function defines a measure for the prediction of local cosmological observations.
65 citations
TL;DR: The consequences of taking the total active gravitational mass of the universe phasewise constant together with a decaying vacuum energy in the background of Robertson-Walker space-time are investigated in this paper.
Abstract: The consequences of taking the total active gravitational mass of the universe phasewise constant together with a decaying vacuum energy in the background of Robertson-Walker space-time are investigated. The model so determined admits a contracted Ricci-collineation along the fluid flow vectorν
i. It is geometrically closed but ever-expanding and does not possess the initial singularity, horizon, entropy, monopole or cosmological constant problems of the standard big bang cosmology. Estimates of the present matter; radiation and vacuum energy densities, the age of the universe and the present values of the deceleration parameter and the scale factor are also obtained.
65 citations
TL;DR: The existence of a thermodynamic arrow of time in the present universe implies that the initial state of the observable portion of our universe at (or near) the "big bang" must have been very special as discussed by the authors.
Abstract: The existence of a thermodynamic arrow of time in the present universe implies that the initial state of the observable portion of our universe at (or near) the “big bang” must have been very “special”. We argue that it is not plausible that these special initial conditions have a dynamical origin.
65 citations