Solutions to the Cosmic Initial Entropy Problem without Equilibrium Initial Conditions

TL;DR

This paper explores solutions to the cosmic initial entropy problem, contrasting inflationary explanations with equilibrium fluctuation models, and discusses their implications for the universe's low initial entropy state.

Contribution

It analyzes and compares two competing explanations for the universe's low initial gravitational entropy, providing insights into their plausibility and implications.

Findings

Inflation and baryogenesis can produce a low gravitational entropy universe.

Equilibrium fluctuation models face challenges explaining the observed universe.

The initial conditions of inflation are as natural as equilibrium models.

Abstract

The entropy of the observable universe is increasing. Thus, at earlier times the entropy was lower. However, the cosmic microwave background radiation reveals an apparently high entropy universe close to thermal and chemical equilibrium. A two-part solution to this cosmic initial entropy problem is proposed. Following Penrose, we argue that the evenly distributed matter of the early universe is equivalent to low gravitational entropy. There are two competing explanations for how this initial low gravitational entropy comes about. (1) Inflation and baryogenesis produce a virtually homogeneous distribution of matter with a low gravitational entropy. (2) Dissatisfied with explaining a low gravitational entropy as the product of a 'special' scalar field, some theorists argue (following Boltzmann) for a 'more natural' initial condition in which the entire universe is in an initial equilibrium state of maximum entropy. In this equilibrium model, our observable universe is an unusual low entropy fluctuation embedded in a high entropy universe. The anthropic principle and the fluctuation theorem suggest that this low entropy region should be as small as possible and have as large an entropy as possible, consistent with our existence. However, our low entropy universe is much larger than needed to produce observers, and we see no evidence for an embedding in a higher entropy background. The initial conditions of inflationary models are as natural as the equilibrium background favored by many theorists.