Is the squeezing of relic gravitational waves produced by inflation detectable?

TL;DR

This paper discusses the unique non-stationary, squeezed nature of inflation-produced gravitational wave backgrounds and concludes that direct detection of this signature with current detectors is infeasible, but indirect detection via CMBR remains possible.

Contribution

It analyzes the detectability of the non-stationary squeezing signature of inflationary gravitational waves, highlighting the observational challenges and potential for indirect detection.

Findings

Direct detection of the squeezing signature is impractical due to required observation times.

The background is non-stationary and Gaussian, with phase correlations from inflation.

Indirect detection via CMBR remains a feasible approach.

Abstract

Grishchuk has shown that the stochastic background of gravitational waves produced by an inflationary phase in the early Universe has an unusual property: it is not a stationary Gaussian random process. Due to squeezing, the phases of the different waves are correlated in a deterministic way, arising from the process of parametric amplification that created them. The resulting random process is Gaussian but non-stationary. This provides a unique signature that could in principle distinguish a background created by inflation from stationary stochastic backgrounds created by other types of processes. We address the question: could this signature be observed with a gravitational wave detector? Sadly, the answer appears to be "no": an experiment which could distinguish the non-stationary behavior would have to last approximately the age of the Universe at the time of measurement. This rules out direct detection by ground and space based gravitational wave detectors, but not indirect detections via the electromagnetic Cosmic Microwave Background Radiation (CMBR).