Causal structures in cosmology

TL;DR

This paper examines the limitations of observational cosmology imposed by horizons, especially in inflationary universes, and argues that event horizons do not impact what we can empirically test about the universe or multiverse.

Contribution

It clarifies the roles of different horizons in cosmology and demonstrates that visual horizons limit our ability to test multiverse hypotheses, while event horizons are observationally irrelevant.

Findings

Visual horizons constrain observable regions in cosmology.

Event horizons do not affect observational cosmology.

Limits on probing beyond the visual horizon hinder multiverse verification.

Abstract

This article reviews the properties and limitations associated with the existence of particle, visual, and event horizons in cosmology in general and in inflationary universes in particular, carefully distinguishing them from `Hubble horizons'. It explores to what extent one might be able to probe conditions beyond the visual horizon (which is close in size to the present Hubble radius), thereby showing that visual horizons place major limits on what are observationally testable aspects of a multiverse, if such exists. Indeed these limits largely prevent us from observationally proving a multiverse either does or does not exist. We emphasize that event horizons play no role at all in observational cosmology, even in the multiverse context, despite some claims to the contrary in the literature.