The first observations of wide-band interferometers and the spectra of relic gravitons

TL;DR

This paper discusses the detection prospects of relic gravitational waves across a broad frequency spectrum, highlighting the potential for high-frequency detectors to observe signals uncontaminated by astrophysical foregrounds.

Contribution

It provides the first observational insights into wide-band interferometers' capabilities to detect relic gravitons, emphasizing the importance of high-frequency ranges for future gravitational wave astronomy.

Findings

Relic graviton spectra extend from aHz to GHz frequencies.

Strain amplitude at interferometer frequencies is much smaller than recent black hole merger signals.

High-frequency detectors may observe relic gravitons with minimal astrophysical contamination.

Abstract

Stochastic backgrounds of relic gravitons of cosmological origin extend from frequencies of the order of the aHz up to the GHz range. Since the temperature and polarization anisotropies constrain the low frequency normalization of the spectra, in the concordance paradigm the strain amplitude corresponding to the frequency window of wide-band interferometers turns out to be, approximately, nine orders of magnitude smaller than the astounding signal recently reported and attributed to a binary black hole merger. The backgrounds of relic gravitons expected from the early Universe are compared with the stochastic foregrounds stemming from the estimated multiplicity of the astrophysical sources. It is suggested that while the astrophysical foregrounds are likely to dominate between few Hz and 10 kHz, relic gravitons with frequencies exceeding 100 kHz represent a potentially uncontaminated signal for the next generation of high-frequency detectors currently under scrutiny.