Hurdles for Recent Measures in Eternal Inflation

TL;DR

This paper reviews various measures used in eternal inflation to assign probabilities to different universes, highlighting their predictions, connections, and the challenges faced in making consistent cosmological predictions.

Contribution

It provides a comprehensive overview of existing measures in eternal inflation, analyzing their implications and identifying issues related to transition types and observer experiences.

Findings

Fast transitions between vacua are strongly favored by measures.

Connections between different measures reveal common predictions.

Challenges in defining observer-accessible transitions are highlighted.

Abstract

In recent literature on eternal inflation, a number of measures have been introduced which attempt to assign probabilities to different pocket universes by counting the number of each type of pocket according to a specific procedure. We give an overview of the existing measures, pointing out some interesting connections and generic predictions. For example, pairs of vacua that undergo fast transitions between themselves will be strongly favored. The resultant implications for making predictions in a generic potential landscape are discussed. We also raise a number of issues concerning the types of transitions that observers in eternal inflation are able to experience.