Singularities in Inflationary Cosmology: A Review

TL;DR

This review discusses the inevitability of initial singularities in inflationary cosmology, introduces a new singularity theorem, and concludes that inflation cannot be extended infinitely into the past, implying a finite age of the Universe.

Contribution

The paper presents a new singularity theorem in inflationary cosmology and analyzes the impossibility of non-singular, infinite past inflationary models.

Findings

Inflationary models must contain initial singularities.

Inflation cannot be continued into the infinite past without singularities.

The Universe is likely not infinitely old.

Abstract

We review here some recent results that show that inflationary cosmological models must contain initial singularities. We also present a new singularity theorem. The question of the initial singularity re-emerges in inflationary cosmology because inflation is known to be generically future-eternal. It is natural to ask, therefore, if inflationary models can be continued into the infinite past in a non-singular way. The results that we discuss show that the answer to the question is ``no.'' This means that we cannot use inflation as a way of avoiding the question of the birth of the Universe. We also argue that our new theorem suggests - in a sense that we explain in the paper - that the Universe cannot be infinitely old.