A Safe Beginning for the Universe?

TL;DR

This paper explores quadratic gravity's role in the early universe, showing that it naturally suppresses anisotropies and inhomogeneities near the big bang, favoring inflation and thermodynamic initial conditions.

Contribution

It demonstrates that quadratic gravity theories impose strong conditions on big-bang universes, promoting accelerated expansion and setting initial conditions compatible with inflation and thermodynamics.

Findings

Anisotropies and inhomogeneities are suppressed near zero volume.

Accelerated expansion is required as the universe approaches the big bang.

Provides initial conditions conducive to inflation and the second law of thermodynamics.

Abstract

When general relativity is augmented by quadratic gravity terms, it becomes a renormalisable theory of gravity. This theory may admit a non-Gaussian fixed point as envisaged in the asymptotic safety program, rendering the theory trustworthy to energies up to the Planck scale and even beyond. We show that requiring physical solutions to have a finite action imposes a strong selection on big-bang-type universes. More precisely we find that, in the approach to zero volume, both anisotropies and inhomogeneities are suppressed while the scale factor is required to undergo accelerated expansion. This provides initial conditions which are favourable to the onset of an inflationary phase while also providing a suitable starting point for the second law of thermodynamics in the spirit of the Weyl curvature hypothesis.