Singularities in loop quantum cosmology

TL;DR

This paper examines singularities in loop quantum cosmology, revealing that some models have physical singularities despite quantum effects, but a new perturbative approach can maintain a consistent space-time structure.

Contribution

It introduces a new effective Friedmann equation showing a bounce at sub-Planckian densities and a novel perturbative inhomogeneity method that avoids singularities.

Findings

A bounce occurs at sub-Planckian densities.

Some models have physical singularities despite quantum effects.

A new perturbative inhomogeneity maintains a consistent space-time structure.

Abstract

Quantum effects are expected to modify the cosmological dynamics of the early universe while maintaining some (potentially discrete) notion of space-time structure. In one approach, loop quantum cosmology, current models are shown here to either be incompatible with a consistent space-time structure, or to have physical singularities. The latter happens in spite of a non-zero scale factor in the isotropic background dynamics. A new effective Friedmann equation shows that a bounce is obtained at sub-Planckian densities, preceded by a physical singularity at infinite scale factor that resembles a time-reversed big rip. The entire phase is accompanied by rapid changes of the Hubble radius. In addition, a new version of perturbative inhomogeneity in loop quantum cosmology is introduced that maintains a consistent space-time structure and has a non-singular background dynamics.