Primordial black holes and scalar-induced gravitational waves from the generalized Brans-Dicke theory

The power spectrum of the scalar-tensor inflation with a quadratic form Ricci scalar coupling function $\Omega(\phi)=1- 2\phi/\phi_c+(1+\delta^2)(\phi/\phi_c)^2$ can be enhanced enough to produce primordial black holes and generate scalar-induced gravitational waves. The masses of primordial black holes and the frequencies of scalar-induced gravitational waves are controlled by the parameter $\phi_c$, and their amplitudes are determined by the parameter $\delta$. Primordial black holes with stellar masses, planetary masses, and masses around $10^{-12} M_\odot$ are produced and their abundances are obtained from the peak theory. The frequencies of the corresponding scalar-induced gravitational waves are around $10^{-9}$ Hz, $10^{-6}$ Hz, and $10^{-3}$ Hz, respectively. The primordial black holes with masses around $10^{-12} M_\odot$ can account for almost all of the dark matter, and the scalar-induced gravitational waves with frequencies around $10^{-9}$ Hz can explain the NANOGrav 12.5yrs signal.