Problems with Tachyon Inflation

We consider cosmological consequences of string theory tachyon condensation.We show that it is very difficult to obtain inflation in the simplest versionsof this theory. Typically, inflation in these theories could occur only atsuper-Planckian densities, where the effective 4D field theory is inapplicable.Reheating and creation of matter in models where the tachyon potential V(T) hasa minimum at infinitely large T is problematic because the tachyon field insuch theories does not oscillate. If the universe after inflation is dominatedby the energy density of the tachyon condensate, it will always remaindominated by the tachyons. It might happen that string condensation isresponsible for a short stage of inflation at a nearly Planckian density, butone would need to have a second stage of inflation after that. This would implythat the tachyon played no role in the post-inflationary universe until thevery late stages of its evolution. These problems do not appear in the recentlyproposed models of hybrid inflation where the complex tachyon field has aminimum at T << M_p.Comment: 13 page