Flat spectra of cosmic gravitons in the nHz and audio bands

TL;DR

This paper proposes a new method for normalizing the spectrum of relic gravitons across a wide frequency range, challenging conventional approaches that focus on low frequencies, and applies it to various inflationary models.

Contribution

It introduces a novel normalization strategy for relic graviton spectra applicable at high frequencies, extending beyond traditional low-frequency assumptions.

Findings

New normalization approach for relic gravitons across frequencies

Application to conventional and unconventional inflationary models

Implications for gravitational wave background measurements

Abstract

The spectra of the relic gravitons are customarily normalized in the low-frequency domain where the signal of the concordance paradigm is expected to peak and this is why their contribution to the temperature and polarization anisotropies of the microwave background is only described by the tensor to scalar ratio. If the consistency relations are broken, the same strategy is accomplished by introducing the tensor spectral index as a further independent parameter. When the dominant component of the spectral energy density is distributed for frequencies much larger than the aHz, the logic behind this conventional approach is much less compelling. The improved bounds in the audio band and the current data from the pulsar timing arrays in the nHz region motivate a new strategy for the absolute normalization of the cosmic background of relic gravitons. After introducing a general four-dimensional action for the analysis of the relic gravitons the new approach is illustrated in the case of conventional and unconventional inflationary models.