Astrophys.J. 979, 178 (2025)
Constraints on Primordial Black Holes from Neff: Scalar-induced Gravitational Waves as an Extra Radiation Component
Jing-Zhi Zhou1, Yu-Ting Kuang2,3, Zhe Chang2,3 and Hong Lu1,4
1 Center for Joint Quantum Studies, Department of Physics, School of Science, Tianjin University, Tianjin 300350, China.
2 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
3 University of Chinese Academy of Sciences, Beijing 100049, China.
4 Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, China.
* zhoujingzhi@tju.edu.cn
Abstract
In 2023 June, multiple pulsar timing array collaborations provided evidence for the existence of a stochastic gravitational-wave background. Scalar-induced gravitational waves (SIGWs), as one of the most likely sources of stochastic gravitational waves, have received widespread attention. When primordial curvature perturbations on small scales are sufficiently large, primordial black holes (PBHs) inevitably form, concurrently producing SIGWs with significant observable effects. These SIGWs can serve as an additional radiation component, influencing the relativistic degrees of freedom Neff. Taking into account primordial non-Gaussianity, we study the energy density spectrum of SIGWs up to the third order and use the current observational data of Neff to constrain small-scale primordial curvature perturbations and the abundance of PBHs.
