The invisible low-frequency gravitons and the audio band

TL;DR

This paper explores the potential detectability of low-frequency gravitons at higher frequencies, analyzing how post-inflationary expansion affects tensor-to-scalar-ratio bounds and the implications for gravitational wave observations.

Contribution

It introduces a framework linking low-frequency graviton invisibility with high-frequency detection prospects, deriving bounds on the tensor-to-scalar-ratio based on post-inflationary expansion.

Findings

Low-frequency gravitons may be undetectable in the aHz region.

High-frequency observations can provide constraints on low-frequency gravitons.

Bounds on tensor-to-scalar-ratio depend on post-inflationary expansion rate.

Abstract

The low-frequency gravitons correspond to typical wavelengths that left the Hubble radius during the early inflationary stages of expansion and reentered after matter radiation equality. Consequently the temperature and the polarization anisotropies of the cosmic microwave background constrain the tensor-to-scalar-ratio in the aHz region but since the audio band and the MHz domain are sensitive to the post-inflationary expansion rate, the low-frequency determinations of the tensor-to-scalar-ratio can be combined with the high-frequency constraints. In this framework we examine the possibility that the low-frequency gravitons remain invisible in the aHz region but are still potentially detectable at much higher frequencies. Because the number of $e$-folds associated with the exit of the cosmic microwave background wavelengths depends both on the slow-roll parameters and on the total expansion rate after inflation, this approach leads to a set of lower bounds on the tensor-to-scalar-ratio.