Quantum cosmology at the turn of Millennium

TL;DR

This paper reviews modern quantum cosmology, focusing on initial conditions for inflation, discussing no-boundary and tunneling states, and highlighting the need for quantum gravitational effects to reconcile probability peaks with slow roll inflation.

Contribution

It introduces a quantum gravitational mechanism for the origin of inflationary universes with observational parameters, addressing limitations of classical approximations.

Findings

Probability peaks in initial data distributions are incompatible with slow roll in tree-level approximation.

Quantum gravitational effects are likely necessary to resolve this contradiction.

A low-energy quantum origin mechanism for inflation with non-minimal coupling is proposed.

Abstract

A brief review of the modern state of quantum cosmology is presented as a theory of quantum initial conditions for inflationary scenario. The no-boundary and tunneling states of the Universe are discussed as a possible source of probability peaks in the distribution of initial data for inflation. It is emphasized that in the tree-level approximation the existence of such peaks is in irreconcilable contradiction with the slow roll regime -- the difficulty that is likely to be solved only on account of quantum gravitational effects. The low-energy (typically GUT scale) mechanism of quantum origin of the inflationary Universe with observationally justified parameters is presented for closed and open inflation models with a strong non-minimal coupling.