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

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.

14 pages LaTeX, 3 figures; v2: first paragraph added, section 8 added, 9 references added; v3: further references added

References in corpus (4)

• Identification of a gravitational arrow of time
• Spontaneous Inflation and the Origin of the Arrow of Time
• From Time-symmetric Microscopic Dynamics to Time-asymmetric Macroscopic Behavior: An Overview
• Entropy and the Typicality of Universes

Cited by in corpus (11)

• CPT-Symmetric Universe
• Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State
• From Time Asymmetry to Quantum Entanglement: The Humean Unification
• Understanding Gravitational Entropy of Black Holes: A New Proposal via Curvature Invariants
• Thermodynamics of Shearing Massless Scalar Field Spacetimes is Inconsistent With the Weyl Curvature Hypothesis
• In Praise of Clausius Entropy: Reassessing the Foundations of Boltzmannian Statistical Mechanics
• Time's Arrow and Self-Locating Probability
• Classical Fractons: Local chaos, global broken ergodicity and an arrow of time
• Alternative interpretation of relativistic time-reversal and the time arrow
• Under an Iron Sky: On the Entropy at the Start of the Universe
• Inflation and Dark Matter from The Low Entropy Hypothesis and Modeling Mechanism of Modified Gravity