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Solutions to the Cosmic Initial Entropy Problem without Equilibrium Initial Conditions

Research School of Astronomy and Astrophysics, Australian National University, Canberra 2600, Australia
Planetary Science Institute, Australian National University, Canberra 2600, Australia
Research School of Earth Sciences, Australian National University, Canberra 2600, Australia
Author to whom correspondence should be addressed.
Entropy 2017, 19(8), 411;
Received: 30 June 2017 / Revised: 31 July 2017 / Accepted: 7 August 2017 / Published: 10 August 2017
(This article belongs to the Special Issue Entropy, Time and Evolution)
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. View Full-Text
Keywords: entropy; gravity; inflation; initial conditions entropy; gravity; inflation; initial conditions
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Patel, V.M.; Lineweaver, C.H. Solutions to the Cosmic Initial Entropy Problem without Equilibrium Initial Conditions. Entropy 2017, 19, 411.

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