The Decoherent Arrow of Time and the Entanglement Past Hypothesis

TL;DR

This paper explores how the quantum state of the universe's initial low entanglement entropy could explain the decoherent arrow of time, offering an alternative boundary condition to the thermodynamic approach.

Contribution

It introduces the Entanglement Past Hypothesis as a new boundary condition for understanding the arrow of time, contrasting it with the traditional thermodynamic hypothesis.

Findings

The initial quantum state of the universe likely had very low entanglement entropy.

The Entanglement Past Hypothesis provides a different perspective on the origin of temporal asymmetry.

Comparison with the Thermodynamic Past Hypothesis highlights unique challenges and open questions.

Abstract

If an asymmetry in time does not arise from the fundamental dynamical laws of physics, it may be found in special boundary conditions. The argument normally goes that since thermodynamic entropy in the past is lower than in the future according to the Second Law of Thermodynamics, then tracing this back to the time around the Big Bang means the universe must have started off in a state of very low thermodynamic entropy: the Thermodynamic Past Hypothesis. In this paper, we consider another boundary condition that plays a similar role, but for the decoherent arrow of time, i.e. the quantum state of the universe is more mixed in the future than in the past. According to what we call the Entanglement Past Hypothesis, the initial quantum state of the universe had very low entanglement entropy. We clarify the content of the Entanglement Past Hypothesis, compare it with the Thermodynamic Past Hypothesis, and identify some challenges and open questions for future research.