Global structure of the multiverse and the measure problem

TL;DR

This paper investigates how non-standard high-curvature dynamics, like bounces at big crunch singularities, could resolve the measure problem in eternal inflation by proposing a geodesic-based measure that is independent of initial conditions.

Contribution

It introduces a novel measure based on a probe geodesic that traverses crunches and bounces, offering a potential solution to the measure problem in multiverse cosmology.

Findings

The proposed measure is independent of initial conditions.

It avoids ambiguities related to cut-off surface choices.

The measure accounts for bounces at singularities, altering multiverse structure.

Abstract

An unresolved question in inflationary cosmology is the assignment of probabilities to different types of events that can occur in the eternally inflating multiverse. We explore the possibility that the resolution of this "measure problem" may rely on non-standard dynamics in regions of high curvature. In particular, "big crunch" singularities that develop in bubbles with negative vacuum energy density may lead to bounces, where contraction is replaced by inflationary expansion. Similarly, singularities inside of black holes might be gateways to other inflating vacua. This would drastically affect the global structure of the multiverse. We consider a measure based on a probe geodesic which undergoes an infinite number of passages through crunches. This can be thought of as the world-line of an eternal "watcher", collecting data in an orderly fashion. This watcher's measure is independent of initial conditions and does not suffer from ambiguities associated with the choice of a cut-off surface.