Stochastic backgrounds of relic gravitons, T$\Lambda$CDM paradigm and the stiff ages

TL;DR

This paper explores a modified cosmological model called TΛCDM, where the early universe's plasma had a sound speed exceeding the standard value, leading to copious production of high-frequency relic gravitons potentially detectable by advanced interferometers.

Contribution

It introduces the TΛCDM scenario, extending the standard cosmological paradigm to include a stiff age phase, and analyzes its implications for relic graviton spectra within observational bounds.

Findings

High-frequency gravitons could be produced in the TΛCDM scenario.

Relic graviton spectra may be detectable by advanced interferometers.

The scenario remains consistent with current cosmological data.

Abstract

Absent any indirect tests on the thermal history of the Universe prior to the formation of light nuclear elements, it is legitimate to investigate situations where, before nucleosyntheis, the sound speed of the plasma was larger than $c/\sqrt{3}$, at most equalling the speed of light $c$. In this plausible extension of the current cosmological paradigm, hereby dubbed Tensor-$\Lambda$CDM (i.e. T$\Lambda$CDM) scenario, high-frequency gravitons are copiously produced. Without conflicting with the bounds on the tensor to scalar ratio stemming from the combined analysis of the three standard cosmological data sets (i.e. cosmic microwave background anisotropies, large-scale structure and supenovae), the spectral energy density of the relic gravitons in the T$\Lambda$CDM scenario can be potentially observable by wide-band interferometers (in their advanced version) operating in a frequency window which ranges between few Hz and few kHz.