Particle horizon

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

Brane-Universes with Variable G and (4)

Abstract: We investigate the cosmological consequences of a braneworld theory which incorporates time variations in the gravitational coupling G and the cosmological term Λ(4). We analyse in detail the model where Gdot/G ~ H and Λ(4) ~ H2, which seems to be favoured by observations. We show that these conditions single out models with flat space sections. We determine the behaviour of the expansion scale factor, as well as the variation of G, Λ(4) and H, for different possible scenarios where the bulk cosmological constant, Λ(5), can be zero, positive or negative. We demonstrate that the universe must recollapse, if it is embedded in an anti-de Sitter five-dimensional bulk, which is the usual case in brane models. We evaluate the cosmological parameters, using some observational data, and show that we are nowhere near the time of recollapse. We conclude that the models with zero and negative bulk cosmological constants agree with the observed accelerating universe, while fitting simultaneously the observational data for the density and deceleration parameters. The age of the universe, even in the recollapsing case, is much larger than in the Friedmann–Robertson–Walker universe.

Constraining cosmological ultralarge scale structure using numerical relativity

Abstract: Cosmic inflation, a period of accelerated expansion in the early universe, can give rise to large amplitude ultra-large scale inhomogeneities on distance scales comparable to or larger than the observable universe. The cosmic microwave background (CMB) anisotropy on the largest angular scales is sensitive to such inhomogeneities and can be used to constrain the presence of ultra-large scale structure (ULSS). We numerically evolve nonlinear inhomogeneities present at the beginning of inflation in full General Relativity to assess the CMB quadrupole constraint on the amplitude of the initial fluctuations and the size of the observable universe relative to a length scale characterizing the ULSS. To obtain a statistically meaningful ensemble of simulations, we adopt a toy model in which inhomogeneities are injected along a preferred direction. We compute the likelihood function for the CMB quadrupole including both ULSS and the standard quantum fluctuations produced during inflation. We compute the posterior given the observed CMB quadrupole, finding that when including gravitational nonlinearities, ULSS curvature perturbations of order unity are allowed by the data, even on length scales not too much larger than the size of the observable universe. Our results illustrate the utility and importance of numerical relativity for constraining early universe cosmology.

Will the universe expand forever

Abstract: Plausible cosmological models are examined with respect to measurements of deceleration, the age of the universe, the average density of matter, and the abundance of deuterium. These factors together indicate that the expansion of the universe cannot be halted or reversed, that the universe is open. (BJG)

A General Relativistic Rotating Evolutionary Universe

Abstract: We show that when we work with coordinate cosmic time, which is not proper time, Robertson-Walker's metric, includes a possible rotational state of the Universe. An exact formula for the angular speed and the temporal metric coefficient, is found.