Initial Condition Problem is Intractable in Cosmology

Abstract

Determining the initial state of the universe is a challenging problem in quantum cosmology and we argue that the issue is intractable if the basic postulates of quantum mechanics are not modified in a nontrivial way. Namely a "standard" quantum theory of gravity is expected to resolve the big-bang singularity either by yielding a regular past eternal evolution or by a smooth finite beginning; in both cases the initial state can in principle be totally arbitrary. We illustrate this point in a minisuperspace, gauge fixed, deparametrized toy model where there is a smooth beginning of the universe provided by the matter Hamiltonian degenerating to the zero operator. This arbitrariness is the source of several debates in the literature, especially in relation to inflation, which can only be solved by a new paradigm involving initial conditions that necessarily alters the usual quantum mechanical treatment, but we argue that this is highly improbable.