Gravitational Waves in Bouncing Cosmologies from Gauge Field Production

TL;DR

This paper explores how gauge field production in bouncing cosmologies can generate observable gravitational waves with distinctive spectra, expanding the understanding beyond inflationary models and suggesting potential detectability in CMB observations.

Contribution

It generalizes the calculation of gauge field sourced gravitational waves from inflation to bouncing cosmologies, analyzing different couplings and their effects on the GW spectrum.

Findings

Gauge field sources can produce exponentially enhanced GW spectra.

The GW spectrum can be nearly scale invariant with specific couplings.

Observable gravitational waves are possible in bouncing cosmologies.

Abstract

We calculate the gravitational waves (GW) spectrum produced in various Early Universe scenarios from gauge field sources, thus generalizing earlier inflationary calculations to bouncing cosmologies. We consider generic couplings between the gauge fields and the scalar field dominating the energy density of the Universe. We analyze the requirements needed to avoid a backreaction that will spoil the background evolution. When the scalar is coupled only to $F \tilde F$ term, the sourced GW spectrum is exponentially enhanced and parametrically the square of the vacuum fluctuations spectrum, ${\cal P}^s_T\sim ({\cal P}^v_T)^2$, giving an even bluer spectrum than the standard vacuum one. When the scalar field is also coupled to $F^2$ term, the amplitude is still exponentially enhanced, but the spectrum can be arbitrarily close to scale invariant (still slightly blue), $n_T\gtrsim 0$, that is distinguishable form the slightly red inflationary one. Hence, we have a proof of concept of observable GW on CMB scales in a bouncing cosmology.