The Steep Price of No Hair in a Modified Loop Quantum Cosmology

TL;DR

This paper investigates the effects of a modified loop quantum cosmology model (mLQC-I) on anisotropic Bianchi-I spacetimes, revealing an emergent de Sitter phase that dampens anisotropy but prevents the universe from becoming truly classical, with the mechanism being non-generic.

Contribution

It extends the analysis of mLQC-I to anisotropic spacetimes, clarifying the origin and limitations of quantum shear damping mechanisms in this model.

Findings

Emergent de Sitter phase dampens anisotropic shear.

Universe remains non-classical post-bounce.

Isotropization mechanism is non-generic.

Abstract

A specific loop quantum cosmology model, mLQC-I, motivated by Thiemann's regularization of the Hamiltonian constraint, leads to the resolution of the big bang singularity and bounce in the isotropic setting, in which either the pre-bounce or post-bounce epoch is necessarily characterized by an emergent Planckian de Sitter phase. In this work we explore the Planckian physics of this mLQC-I prescription for the Bianchi-I spacetimes. We show that as in the isotropic model, there exists an emergent de Sitter phase which naturally dampens anisotropic shear and removes cosmic hair. However, this isotropization comes at a steep price: although a macroscopic post-bounce regime is achieved, the universe never becomes truly classical. We further demonstrate that this isotropization mechanism is non-generic. These results help clarify and reinterpret recent results by Gan et al. [1] that, in anisotropic mLQC-I, quantum gravity effects generically damp anisotropic shear in a way that is independent of initial conditions and the matter content, and that this anisotropic shear damping mechanism arises from a novel quantum gravity effect. Our work explains the origin of this mechanism and its limitations in the mLQC-I model.