Gravitational waves and dark radiation from dark phase transition: Connecting NANOGrav pulsar timing data and hubble tension

Recent pulsar timing data reported by the NANOGrav collaboration may indicate the existence of a stochastic gravitational wave background around f similar to 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. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

Recent pulsar timing data from the NANOGrav collaboration suggests the presence of a stochastic gravitational wave background around a frequency of approximately 10(-8) Hz, possibly generated from a dark sector phase transition. The data implies that dark radiation, isolated from visible sector except via gravitational interaction, may have persisted until present, close to current upper bound, potentially helping alleviate the Hubble tension. Failure to confirm dark radiation in future CMB-S4 experiment may suggest existence of new particles weakly interacting with standard model sector at O(1-100) MeV energy scale.