Maximal chirality transfer in the photon-graviton conversion in the early universe

TL;DR

This paper explores the conversion of chiral electromagnetic waves into chiral gravitational waves in the early universe, revealing that the process is unaffected by the magnetic field's chirality and peaks at high frequencies around 100 GHz.

Contribution

It demonstrates that the chirality of the background magnetic field does not influence the photon-graviton conversion probability in the early universe.

Findings

Conversion probability increases with magnetic field strength

Net chirality parameter is independent of magnetic field chirality

Chiral gravitational waves peak at ~100 GHz frequency

Abstract

While photons and gravitons do not interact significantly, photons can be converted to gravitons in a background magnetic field -- a phenomenon known as the Gertsenshtein effect. In this paper, we investigate whether chiral electromagnetic (EM) waves can be converted to chiral gravitational waves (GW) in the presence of primordial magnetic fields during the radiation-dominated epoch of the early universe. We consider two situations wherein chirality is either present in the propagating EM waves or it exists in the background magnetic field. Our analysis shows that while the conversion probability increases with stronger magnetic fields, it remains insensitive to the chiral nature of the background magnetic field. Consequently, the net chirality parameter is independent of the chirality of the background field in both cases. Finally, we demonstrate that the present-day energy density of the produced chiral GWs peaks at a frequency of $\sim 100$ GHz, and the corresponding characteristic strain can be sensitive to current and future missions designed to detect high-frequency GWs.