Causal structures in cosmology

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.

21 pages, overview for the Académie des Sciences de Paris, published in the special volume of the Comptes Rendus de l'Académie "Inflation: theoretical and observational status"

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• Cosmological phase transitions: from perturbative particle physics to gravitational waves
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• Contracting Cosmologies and the Swampland
• Global Spacetime Structure of Compactified Inflationary Universe
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• Model of a multiverse providing the dark energy of our universe
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