The Measure Problem in Eternal Inflation/Multiverse Cosmology

Develop a well‑defined and physically justified measure for models of eternal inflation that enables calculation of the probabilities of different pocket universes, thereby allowing quantitative assessment of how typical our observed universe is within the multiverse.

Background

In eternal inflation, an ensemble of pocket universes can realize different parameter values, potentially explaining fine‑tuning anthropically. Assessing how typical our universe is requires computing probabilities across this ensemble.

The paper emphasizes that this is obstructed by the measure problem: without a well‑defined measure over the multiverse, probability assignments—and hence claims about typicality—are ill‑posed.