Density Perturbations and Black Hole Formation in Hybrid Inflation

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Abstract
We investigate the recently proposed hybrid inflation models with two stages of inflation. In order to study density perturbations in these models we develop a new method to calculate density perturbations in a system of two scalar fields. We show that density perturbations in hybrid inflation models of the new type can be very large on the scale corresponding to the phase transition between the two inflationary stages. Moreover, quantum fluctuations at that time can trigger the formation of a large number of inflating topological defects. The resulting density inhomogeneities lead to a copious production of black holes. This could be an argument against hybrid inflation models with two stages of inflation. However, we find a class of models where this problem can be easily avoided. The number of black holes produced in these models typically is very small, but in general it could be made sufficiently large to have interesting cosmological and astrophysical implications. It may be even more interesting to have hybrid models with two stages of inflation where the black hole production is not suppressed, but the typical masses of the black holes are very small. Such models lead to a completely different thermal history of the universe, where post-inflationary reheating occurs via black hole evaporation.

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