Gravitational waves in the early universe from gauge field production

We demonstrate how observable gravitational waves (GW) signal on CMB scales can be produced in a bouncing cosmology from gauge field sources, thus generalizing earlier inflationary calculations. The GW spectrum is produced due to coupling between the scalar field dominating the energy density of the Universe and gauge fields, while not spoiling the background evolution. When the scalar is coupled only to FF term, one helicity of the sourced GW spectrum is exponentially enhanced. The sourced spectrum is parametrically the square of the vacuum fluctuations spectrum, P_T^s ∼ (P_T^v)², giving an even bluer spectrum than the standard vacuum one. In a companion paper we show that coupling the scalar also to F² term still enhances the spectrum and brings it arbitrarily close to scale invariance nT ≥ 0, allowing for GW observation on CMB scales, with distinct predictions compared to inflation.