 As we mentioned previously, there are reasons to suspect the invalidity of classical general relativity in regions near a singularity—most importantly, for times very close to the big bang. In particular, when lengths are very small, and curvature and temperatures are very high, then—if the gravitational force behaves like all other known forces of nature—quantum effects will take over, and we should accordingly expect different outcomes. This observation is itself sufficient to destroy the aspirations of big-bang theology—unless there are good reasons to think that the finite-age prediction of relativistic cosmology will be preserved in a quantum gravity or in string theory. In this section, we briefly review the known data about singularities in theories that attempt to unify gravity and quantum mechanics. Our review supports two conclusions: (1) We do not know yet if the best model will predict a finitely old universe, but (2) there are good reasons to think that the big bang is not necessarily an absolute beginning.