Cosmology with primordial black holes

We study the effect of accretion of radiation in the early universe on primordial black holes in various modified gravity theories. We consider separately, a braneworld model, Brans-Dicke theory, and a more general scalar-tensor model. The rate of growth of a primordial black hole due to accretion of radiation in all these modified gravity models is smaller than the rate of growth of the cosmological horizon, thus making available sufficient radiation density for the black hole to accrete causally. We show that accretion of radiation overrides the effect of Hawking evaporation during the radiation dominated era. The subsequent evaporation of the black holes in later eras could further be modified due to either the modified braneworld geometry, or the variable gravitational coupling in scalar-tensor theories, and they could survive up to much longer times compared to the case of standard cosmology.