Oscillations of Ultralight Dark Photon into Gravitational Waves

TL;DR

This paper explores how ultralight dark photons oscillate into gravitational waves within dark matter environments, producing detectable signals that could explain recent PTA observations.

Contribution

It introduces a novel mechanism for GW production via ULDP oscillations in PDMFs, linking dark matter physics to observable gravitational wave signals.

Findings

Stochastic GWs from ULDP oscillations can be detected by PTAs.

The model explains the low-frequency GW signals observed by NANOGrav.

GW spectrum depends on ULDP mass and primordial magnetic fields.

Abstract

The discovery of gravitational waves (GWs) opens a new window for exploring the physics of the early universe. Identifying the source of GWs and their spectra today turn out to be the important tasks so as to assist the experimental detection of stochastic GWs. In this paper, we investigate the oscillations of the ultralight dark photon (ULDP) into GWs in the dark halo. Assuming dark matter is composed of the ULDP and there are primordial dark magnetic fields (PDMFs) arising from the axion inflation and/or the dark phase transition, then the ULDP can oscillate into the GW when it passes through an environment of PDMFs. We derive the local energy density of GWs in the galaxy cluster induced by the instaneous oscillation of ULDP in the PDMFs. These stochastic local GWs exhibit a pulse-like spectrum, with frequency depending on the mass of the ULDP, and can be detected in Pulsar Timing Arrays (PTAs) or future space-based interferometers. We also find that the low-frequency GW signal observed by the NANOGrav collaboration and other PTA experiments can be explained by the oscillation of the ULDP in the PDMFs in the early universe.