Decaying Lambda cosmology, varying G and holography

TL;DR

This paper explores decaying Lambda cosmologies with a variable gravitational constant G, highlighting a unique coasting expansion solution consistent with holographic and thermodynamic principles, and showing the cosmological term's proportionality to H^2.

Contribution

It introduces a class of isotropic, flat cosmological models with time-varying G and Lambda, identifying the coasting expansion as thermodynamically favored and linking Lambda to H^2.

Findings

The coasting expansion has a matter density parameter of 1/3.

The universe's age in this model satisfies Ht=1.

The cosmological term is proportional to the square of the Hubble parameter.

Abstract

We discuss a class of uniform and isotropic, spatially flat, decaying Lambda cosmologies, in the realm of a model where the gravitation constant G is a function of the cosmological time. Besides the usual de Sitter solution, the models at late times are characterized by a constant ratio between the matter and total energy densities. One of them presents a coasting expansion where the matter density parameter is equal to 1/3, and the age of the universe satisfies Ht = 1. From considerations in line with the holographic conjecture, it is argued that, among the non-decelerating solutions, the coasting expansion is the only acceptable from a thermodynamic viewpoint, and that the time varying cosmological term must be proportional to the square of the Hubble parameter, a result earlier obtained using different arguments.