Time (a-)symmetry in a recollapsing quantum universe

Abstract

It is argued that Hawking's `greatest mistake' may not have been a mistake at all. According to the canonical quantum theory of gravity for Friedmann type universes, any time arrows of general nature can only be correlated with that of the expansion. For recollapsing universes this seems to be facilitated in part by quantum effects close to their maximum size. Because of the resulting thermodynamical symmetry between expansion and (formal) collapse, black holes must formally become `white' during the collapse phase (while physically only expansion of the universe and black holes can be observed). It is conjectured that the quantum universe remains completely singularity-free in this way (except for the homogeneous singularity) if an appropriate boundary condition for the wave function is able to exclude {\it past} singularities (as is often assumed).