Quantum cosmology from three different perspectives

TL;DR

This paper reviews three approaches in quantum cosmology, highlighting progress in nonperturbative renormalization, perturbative wave functions, and braneworld models, with implications for early universe physics and singularity avoidance.

Contribution

It provides a comparative overview of three distinct perspectives in quantum cosmology, emphasizing recent developments and open questions.

Findings

Progress in nonperturbative renormalization for gravity with variable G and Lambda

Diffeomorphism-invariant boundary conditions yield singularity-free wave functions

Braneworld models introduce a new concept of cosmological wave function of bulk space-time

Abstract

Our review is devoted to three promising research lines in quantum cosmology and the physics of the early universe. The nonperturbative renormalization programme is making encouraging progress that we here assess from the point of view of cosmological applications: Lagrangian and Hamiltonian form of pure gravity with variable G and Lambda; power-law inflation for pure gravity; an accelerating universe without dark energy. In perturbative quantum cosmology, on the other hand, diffeomorphism-invariant boundary conditions lead naturally to a singularity-free one-loop wave function of the universe. Last, but not least, in the braneworld picture one discovers the novel concept of cosmological wave function of the bulk space-time. Its impact on quantum cosmology and singularity avoidance is still, to a large extent, unexplored.