Symmetric-Bounce Quantum State of the Universe

TL;DR

This paper proposes a symmetric-bounce quantum state for the universe, characterized by a time-symmetric initial condition at a bounce, with entropy growth during inflation, avoiding issues like Boltzmann brains present in other models.

Contribution

It introduces a novel quantum state model with a symmetric bounce and minimal entropy at the bounce, differing from traditional no-boundary proposals.

Findings

The quantum state is macroscopically time symmetric about a bounce.

Entropy is minimized at the bounce and increases during inflation.

The model avoids Boltzmann brain dominance issues.

Abstract

A proposal is made for the quantum state of the universe that has an initial state that is macroscopically time symmetric about a homogeneous, isotropic bounce of extremal volume and that at that bounce is microscopically in the ground state for inhomogeneous and/or anisotropic perturbation modes. The coarse-grained entropy is minimum at the bounce and then grows during inflation as the modes become excited away from the bounce and interact (assuming the presence of an inflaton, and in the part of the quantum state in which the inflaton is initially large enough to drive inflation). The part of this pure quantum state that dominates for observations is well approximated by quantum processes occurring within a Lorentzian expanding macroscopic universe. Because this part of the quantum state has no negative Euclidean action, one can avoid the early-time Boltzmann brains and Boltzmann solar systems that appear to dominate observations in the Hartle-Hawking no-boundary wavefunction.