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

Abstract

A modified version of loop quantum cosmology model, the so-called mLQC-I, motivated by Thiemann's regularization of the Hamiltonian constraint, leads to the resolution of the big bang singularity and a bounce in the isotropic setting, where 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 does not become classical. As is well known from the Kasner solution, the classical evolution of a contracting Bianchi-I universe toward the singularity can in general be either point-like or cigar-like. However, cigar-like evolution is prevalent unless the matter content dominates over the anisotropic shear. For a class of physically admissible initial conditions corresponding to cigar-like evolution, we further demonstrate that this isotropization mechanism is non-generic. These results clarify and reinterpret recent claims by Gan et al. \cite{Gan:2025uvt} that, in anisotropic mLQC-I, quantum gravity effects generically damp anisotropic shear in a manner independent of initial conditions and matter content, and that this damping arises from a novel quantum gravity effect. Our work explains the origin of this mechanism and its limitations in the mLQC-I model.