Brane-Universes with Variable G and Lambda

TL;DR

This paper explores a brane-world cosmological model with time-varying G and Lambda, analyzing its implications for universe expansion, recollapse, and compatibility with observations of acceleration and density parameters.

Contribution

It introduces a model where G and Lambda vary with time, constrained by observations, and examines its cosmological consequences including universe recollapse and acceleration.

Findings

Models with zero or negative bulk cosmological constant fit observed acceleration.

Universe's age in these models exceeds that of standard FRW universe.

Recollapse is inevitable if embedded in Anti-de Sitter bulk.

Abstract

We investigate the cosmological consequences of a brane-world theory which incorporates time variations in the gravitational coupling G and the cosmological term Lambda. We analyze in detail the model where (dG/dt)/G ~ H and Lambda ~ H^2, which seems to be favored by observations. We show that these conditions single out models with flat space sections. We determine the behavior of the expansion scale factor, as well as, the variation of G, Lambda and H for different possible scenarios where the bulk cosmological constant 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 constant 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 FRW universe.