A critical note on time in the multiverse

TL;DR

This paper critically examines the concept of time within the multiverse, highlighting fundamental problems in defining a global time that affects our understanding of the multiverse's structure and history.

Contribution

It identifies three core issues—quantum, collision, and fractal problems—that challenge the notion of a well-defined global time in the inflationary multiverse.

Findings

Three main problems prevent a clear global time definition

Quantum issues may undermine causal and temporal precedence

Multiverse parts may not be meaningfully older or younger

Abstract

In recent analyses of standard, single-universe, cosmology, it was pointed out that specific assumptions regarding the distribution and motion of matter must be made in order to set up the cosmological standard model with a global time parameter. Relying on these results, we critically examine the notion of time in the multiverse, and in particular the idea that some parts of the multiverse are older than others. By focusing on the most elaborated multiverse proposal in cosmology, the inflationary multiverse, we identify three problems for establishing a physically well-defined notion of global time; a quantum problem, a collision problem and a fractal problem. The quantum problem, and the closely related "cosmic measurement problem", may even undermine the idea that parts of the multiverse causally and temporally precede our universe.