The cosmic gravitational wave background in a cyclic universe

TL;DR

This paper compares the primordial gravitational wave spectrum predicted by cyclic/ekpyrotic models to inflation, finding it to be blue and suppressed at long wavelengths, with implications for observational constraints.

Contribution

It provides the first detailed computation of the tensor spectrum in cyclic/ekpyrotic models and contrasts it with inflationary predictions.

Findings

Cyclic/ekpyrotic models predict a blue, suppressed gravitational wave spectrum.

The spectrum's amplitude is constrained by nucleosynthesis energy density limits.

Inflation predicts a nearly scale-invariant, red spectrum.

Abstract

Inflation predicts a primordial gravitational wave spectrum that is slightly ``red,'' i.e., nearly scale-invariant with slowly increasing power at longer wavelengths. In this paper, we compute both the amplitude and spectral form of the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The spectrum is blue and exponentially suppressed compared to inflation on long wavelengths. The strongest observational constraint emerges from the requirement that the energy density in gravitational waves should not exceed around 10 per cent of the energy density at the time of nucleosynthesis.