Quantum Transitions Between Classical Histories: Bouncing Cosmologies

TL;DR

This paper explores how quantum effects influence classical spacetime histories, especially in bouncing cosmologies, revealing that classical predictability can break down at scales below the Planck length and connecting different classical histories through quantum transitions.

Contribution

It introduces a quantum framework for understanding transitions between classical cosmological histories, including bouncing scenarios, and calculates probabilities for such transitions in a no-boundary state.

Findings

Classical histories are limited to specific patches in configuration space.

Quantum evolution connects classical histories across patches.

Classical predictability can break down below the Planck scale.

Abstract

In a quantum theory of gravity spacetime behaves classically when quantum probabilities are high for histories of geometry and field that are correlated in time by the Einstein equation. Probabilities follow from the quantum state. This quantum perspective on classicality has important implications: (a) Classical histories are generally available only in limited patches of the configuration space on which the state lives. (b) In a given patch states generally predict relative probabilities for an ensemble of possible classical histories. (c) In between patches classical predictability breaks down and is replaced by quantum evolution connecting classical histories in different patches. (d) Classical predictability can break down on scales well below the Planck scale, and with no breakdown in the classical equations of motion. We support and illustrate (a)-(d) by calculating the quantum transition across the de Sitter like throat connecting asymptotically classical, inflating histories in the no-boundary quantum state. This supplies probabilities for how a classical history on one side transitions and branches into a range of classical histories on the opposite side. We also comment on the implications of (a)-(d) for the dynamics of black holes and eternal inflation.