Gravitational waves and dark radiation from dark phase transition: Connecting NANOGrav pulsar timing data and hubble tension
Yuichiro Nakai (Tsung-Dao Lee Institute, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China); Motoo Suzuki (Tsung-Dao Lee Institute, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China)
; Fuminobu Takahashi (Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Japan, Department of Physics, Tohoku University, Sendai, Japan); Masaki Yamada (Department of Physics, Tohoku University, Sendai, Japan, Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan)
Recent pulsar timing data reported by the NANOGrav collaboration may indicate the existence of a stochastic gravitational wave background around 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 - 100) MeV.
