Core Concepts
The existence of scalar-induced gravitational waves, generated alongside primordial black holes, could affect the effective number of relativistic species (Neff) in the early universe, providing a novel method to constrain the abundance of primordial black holes and the small-scale primordial curvature power spectrum.
Stats
The Planck + BAO + BBN data provides a 95% confidence level upper limit of ΔNeff (Neff - 3.046) < 0.175.
The total energy density spectrum of SIGWs is constrained to be less than 1.3 x 10^-6 ΔNeff/0.234.