An upper limit on cosmological chiral gravitational wave background

TL;DR

This paper establishes a model-independent upper limit on the amplitude of primordial chiral gravitational waves, especially constraining high-frequency modes and offering a new probe of parity-violating physics in the early Universe.

Contribution

It derives a novel upper bound on chiral gravitational wave amplitude that is independent of production time for superhorizon modes and more stringent than BBN constraints at high frequencies.

Findings

Upper limit on chiral GWs amplitude derived

Bound is independent of production time for superhorizon modes

More restrictive than BBN constraints at MHz frequencies

Abstract

Within the standard framework in which electroweak sphaleron processes relate lepton and baryon number, we derive an upper limit on the amplitude of a chiral gravitational wave background produced prior to the electroweak epoch. This bound is independent of the production time of chiral GWs for superhorizon modes, while it becomes sensitive to the production time for subhorizon modes. For sufficiently high reheating temperatures, the bound becomes significantly more stringent than the conventional big bang nucleosynthesis constraints at frequencies above the MHz scale, thereby providing a powerful and \emph{model-independent} probe of parity-violating physics in the early Universe.