Quantum decay of a de Sitter universe and inflation without fine-tuning in 1 + 1 dimensions

The problem of back reaction is investigated in a model of evolving de Sitter spacetime. It is shown in the leading semiclassical approximation to a dilaton gravity theory that a quantum correction gives rise to a decay of a de Sitter spacetime without encountering a curvature singularity. As an application this model may serve as a prototype of the inflationary scenario followed by a hot big bang free from the fine-tuning problem of a cosmological constant.