The initial states of high frequency gravitons

TL;DR

This paper investigates the initial quantum states of relic gravitons in cosmology, constraining their properties at horizon crossing and analyzing their impact on the gravitational wave spectrum across different frequencies.

Contribution

It introduces a pragmatic approach to constrain initial graviton states at horizon crossing without relying on earlier timescales, highlighting the marginal allowance of non-vacuum states at low frequencies.

Findings

Low frequency initial states can differ from vacuum but are marginally permitted.

High and intermediate frequencies are dominated by vacuum-produced gravitons.

Quantum correlations are significant between kHz and THz frequencies.

Abstract

After distinguishing the role of classical and quantum inhomogeneities in cosmological backgrounds, we constrain the initial states of the relic gravitons as soon as the different wavelengths of the spectrum cross the comoving Hubble radius, without any reference to earlier timescales. According to this pragmatic perspective the quantum states with finite energy density at the crossing time consistently affect the two-point functions and the related power spectra. An initial state different from the vacuum turns out to be marginally permitted in the low frequency range (associated with the largest observable wavelengths that crossed the comoving Hubble radius) while the intermediate and high frequency domains of the spectrum are populated by the gravitons produced from the vacuum. The non classical correlations are expected to dominate between the kHz and the THz since in this region gravitons are produced quantum mechanically with a negligible contribution from the initial state.