Primordial black holes from temporally enhanced curvature perturbation

Scalar field with generalized kinetic interactions metamorphoses depending on its field value, ranging from a cosmological constant to stiff matter. We show that such a scalar field can give rise to temporal enhancement of the curvature perturbation in the primordial universe, leading to the efficient production of primordial black holes (PBHs) while the enhancement persists. If the inflation energy scale is high, those mini--black holes evaporate by the Hawking radiation much before big bang nucleosynthesis, and the effective reheating of the Universe is achieved by the black hole evaporation. The dominance of PBHs and the reheating by their evaporation modify the expansion history of the primordial universe. This results in a characteristic feature of the spectrum of primordial tensor modes in the DECIGO frequency band, opening an interesting possibility of testing the PBH reheating scenario by measuring the primordial tensor modes. If the inflation energy scale is low, the PBH mass can be much larger than the solar mass. In this case, the PBH is an interesting candidate for seeds for supermassive black holes residing in present galaxies.