Condition for Primordial Black Hole Formation in Randall-Sundrum Cosmology

We consider spherical collapse in the Randall-Sundrum type II model and estimate the critical over density for black hole formation in the radiation dominated era. It is found that when (density)${}^2$-term is dominant in the modified Friedmann equation the critical density is smaller than in the standard cosmology, which implies that PBHs are more easily produced in the Randall-Sundrum model.