Abstract
A successful baryogenesis mechanism is proposed in the cosmological framework of Brans-Dicke modified gravity. Primordial black holes with small mass are produced at the end of the Brans-Dicke field domination era. The Hawking radiation reheats a spherical region around every black hole to a high temperature and the electroweak symmetry is restored there. A domain wall is formed separating the region with the symmetric vacuum from the asymmetric region where electroweak baryogenesis takes place. First-order phase transition is not needed. In Brans-Dicke cosmologies, black hole accretion can be strong enough to result in cosmic black hole domination, an extension of the lifetime of black holes and enhanced baryogenesis. The analysis of the whole scenario provides very easily and without fine tuning the observed baryon number asymmetry for either small or big $CP$-violating angles in the finite temperature corrected effective potential of two-Higgs-doublet models. The advantage of our proposed scenario with Brans-Dicke modified gravity is that it naturally provides both black hole domination and efficient baryogenesis for smaller $CP$-violating angles compared to the same mechanism applied in a Friedmann-Robertson-Walker cosmological background.