Is the Hypothesis About a Low Entropy Initial State of the Universe Necessary for Explaining the Arrow of Time?

TL;DR

This paper explores how the thermodynamic arrow of time can emerge in cosmological models without assuming a low entropy initial state, challenging traditional explanations based on initial conditions.

Contribution

It demonstrates that models not relying on a low entropy initial universe can still produce a consistent thermodynamic arrow of time.

Findings

Thermodynamic arrow can emerge without low entropy initial state

Recent cosmological models avoid initial state hypotheses

Statistical mechanics principles can still apply in these models

Abstract

According to statistical mechanics, micro-states of an isolated physical system (say, a gas in a box) at time $t_0$ in a given macro-state of less-than-maximal entropy typically evolve in such a way that the entropy at time $t$ increases with $|t-t_0|$ in both time directions. In order to account for the observed entropy increase in only one time direction, the thermodynamic arrow of time, one usually appeals to the hypothesis that the initial state of the universe was one of very low entropy. In certain recent models of cosmology, however, no hypothesis about the initial state of the universe is invoked. We discuss how the emergence of a thermodynamic arrow of time in such models can nevertheless be compatible with the above-mentioned consequence of statistical mechanics, appearances to the contrary notwithstanding.