Gravitational Waves and Dark Radiation from Dark Phase Transition: Connecting NANOGrav Pulsar Timing Data and Hubble Tension

TL;DR

This paper proposes that a dark sector phase transition could produce low-frequency gravitational waves and dark radiation, potentially explaining NANOGrav signals and alleviating the Hubble tension, with testable implications for future experiments.

Contribution

It introduces a novel connection between dark sector phase transitions, gravitational waves, and dark radiation, linking astrophysical observations to particle physics.

Findings

Dark radiation near current upper bounds from NANOGrav data

Dark sector phase transition can generate observable gravitational waves

Future CMB-S4 could confirm or refute dark radiation presence

Abstract

Recent pulsar timing data reported by the NANOGrav collaboration may indicate the existence of a stochastic gravitational wave background around $f \sim 10^{-8}$ Hz. We explore a possibility to generate such low-frequency gravitational waves from a dark sector phase transition. Assuming that the dark sector is completely decoupled from the visible sector except via the gravitational interaction, we find that some amount of dark radiation should remain until present. The NANOGrav data implies that the amount of dark radiation is close to the current upper bound, which may help mitigate the so-called Hubble tension. If the existence of dark radiation is not confirmed in the future CMB-S4 experiment, it would imply the existence of new particles feebly interacting with the standard model sector at an energy scale of O(1-100) MeV.