Fate of inflation and the natural reduction of vacuum energy

In the standard cosmology, an artificial fine tuning of the potential is in- evitable for vanishing cosmological constant, though slow-rolling uniform scalar field easily causes cosmic inflation. We focus on the general fact that any potential with neg- ative region can temporally halt the cosmic expansion at the end of inflation, where the field tends to diverge. This violent evolution naturally causes particle production and strong instability of the uniform configuration of the fields. Decaying of this uniform scalar field would leave vanishing cosmological constant as well as locally collapsed ob- jects. The universe then continues to evolve into the standard Freedman model. We study the detail of the instability, based on the linear analysis, and the subsequent fate of the scalar field, based on the non-linear numerical analysis. The collapsed scalar field would easily exceed the Kaup limiting mass and forms primordial black holes, which may play an important role in galaxy formation in later stages of cosmic expansion. We systematically describe the above scenario by identifying the scalar field as the boson field condensation (BEC) and the inflation as the process of phase transition of them.