The Impossibility of Steady-State Inflation

TL;DR

This paper argues that inflation cannot be eternal in both the past and future, implying that the universe had a beginning, by analyzing conditions for past and future eternal inflation.

Contribution

It demonstrates that a steady-state, past-eternal inflationary universe is impossible under reasonable physical conditions.

Findings

Old inflation is not past-eternal.

Necessary conditions for past and future eternal inflation cannot both be satisfied.

Inflation must have had a beginning.

Abstract

Inflation is known to be generically eternal to the future: the false vacuum is thermalized in some regions of space, while inflation continues in other regions. Here, we address the question of whether inflation can also be eternal to the past. We argue that such a steady-state picture is impossible and, therefore, that inflation must have had a beginning. First, it is shown that the old inflationary model is not past-eternal. Next, some necessary conditions are formulated for inflationary spacetimes to be past-eternal and future-eternal. It is then shown that these conditions cannot simultaneously hold in physically reasonable open universes.