Particle creation in a tunneling universe. I

An expanding closed universe filled with radiation can either recollapse or tunnel to the regime of unbounded expansion, if the cosmological constant is nonzero. We reexamine the question of particle creation during tunneling, with the purpose of resolving a long-standing controversy. Using a perturbative superspace model with a conformally coupled massless scalar field, which is known to give no particle production, we explicitly show that the breakdown of the semiclassical approximation and the “catastrophic particle production” claimed earlier in the literature are due to an inappropriate choice of the initial quantum state prior to the tunneling.