Time, irreversibility and cosmological throttling

Abstract

In 2011, Perlmutter, Schmidt, and Reiss discovered that the expansion of the Universe is accelerating. This finding is contrary to the expectation that gravity should slow this expansion. This discovery raises fundamental questions about dark energy, which constitutes about three-fourths of the Universe's mass-energy and is responsible for driving this acceleration. In this paper, we propose a thermodynamic perspective based on a possible cosmological Joule-Thomson effect, supported by the significant entropy content of the Universe, primarily from black-body radiation. The evolution of entropy in an open system involves exchanges with the environment and is characterised by two components: entropy flow and internal entropy production. Understanding irreversible processes is essential to grasping these dynamics, as they link order and disorder. This approach shifts our understanding of thermodynamics, revealing that the creation of order is associated with non-equilibrium conditions, while disorder often corresponds to stable equilibrium. This perspective calls for a revaluation of how systems in Nature are analysed, emphasising the intricate relationship between order, disorder, and the definition of time.