Accelerated expansion is a built-in property of the Boltzmann universe

Abstract

Large cosmological redshifts show that the expansion of the universe is accelerating, whereas a deceleration was expected under the influence of gravity. Since this discovery, the search for a repulsive force has gone in several directions. The main candidate is the postulated dark energy whose physical support is the subject of intense research, but alternative ideas such as a different physics of gravity on a large scale or the backreaction effect, are not ruled out. However, all of these lines of research fall within the same framework which was established by the pioneers of modern cosmology in the early 20th century. This note proposes to look back at an earlier view of the universe by Boltzmann. Based on his thermodynamic intuition, Boltzmann proposed that our visible universe could be part of a fluctuation within a global universe in equilibrium. This seemingly far-fetched vision is supported here by a minimalist Newtonian modeling which predicts that an acceleration of expansion would be initiated by a fluctuation in density, thus eliminating the objections to the Boltzmann universe regarding its size and maintenance. The strengths and weaknesses of this amusing hypothesis are discussed.