Cosmic backgrounds of relic gravitons and their absolute normalization

TL;DR

This paper analyzes the cosmic graviton background produced during inflation, using recent Bicep2 data to normalize spectra across a wide frequency range, exploring deviations from standard models and implications for future observations.

Contribution

It provides a comprehensive computation of the inflation-produced graviton spectrum from low to high frequencies, considering various physical deviations and their observational constraints.

Findings

Normalized spectra from Hubble scale to GHz frequencies.

Deviations like running spectral index and high-frequency spikes analyzed.

Future observations will constrain post-inflationary physics and alternative models.

Abstract

Provided the consistency relations are not violated, the recent Bicep2 observations pin down the absolute normalization, the spectral slope and the maximal frequency of the cosmic graviton background produced during inflation. The properly normalized spectra are hereby computed from the lowest frequencies (of the order of the present Hubble rate) up to the highest frequency range in the GHz region. Deviations from the conventional paradigm cannot be excluded and are examined by allowing for different physical possibilities including, in particular, a running of the tensor spectral index, an explicit breaking of the consistency relations and a spike in the high-frequency tail of the spectrum coming either from a post-inflationary phase dominated by a stiff fluid of from the contribution of waterfall fields in a hybrid inflationary context. The direct determinations of the tensor to scalar ratio at low frequencies, if confirmed by the forthcoming observations, will also affect and constrain the high-frequencies uncertainties. The limits on the cosmic graviton backgrounds coming from wide-band interferometers (such as Ligo/Virgo, Lisa and Bbo/Decigo) together with a more accurate scrutiny of the tensor B mode polarization at low frequencies will set direct bounds on the post-inflationary evolution and on other unconventional completions of the standard lore.