GUT Phase Transition and Hybrid Inflation
  The supersymmetric model of hybrid inflation is interesting not only because of its naturalness but also because of another reason. Its energy scale determined by the COBE normalization is 10^15 - 10^16 GeV. It happens to be the energy scale of interest in particle physics, namely, the mass scale of right-handed neutrinos or the energy scale of the gauge-coupling unification. It is true that topological defects are produced after the hybrid inflation if it is related to a U(1)_B-L or a GUT-symmetry breaking, and hence one cannot naively identify models of particle physics with that of inflation. But those defects are not necessarily found in modified models. We show in this article that quite a simple extension of the minimal supersymmetric hybrid inflation model is free from monopoles or cosmic strings. Moreover, it happens to be exactly the same as a well-motivated extension of SU(5)-unified theories. The vacuum energy is dominated by F-term. The \eta-problem is not necessarily serious when the model is realized by D-branes. Although it has been considered that a coupling constant has to be very small when the vacuum energy is dominated by F-term, this constraint is not applied either to the D-brane model. They are due to a particular form of the Kahler potential and interaction of the model. Reheating process is also discussed. 
Taizan Watari and T. Yanagida