Phenomenological implications of modified loop cosmologies: an overview

TL;DR

This paper reviews two modified loop quantum cosmology models, highlighting their common resolution of the big bang as a quantum bounce, analyzing their perturbations, and identifying observable differences in primordial spectra and non-Gaussianities.

Contribution

It provides a comparative analysis of two regularizations of modified loop quantum cosmologies and clarifies the effects of different perturbation approaches and initial conditions on observable predictions.

Findings

Both models resolve the big bang with a quantum bounce.

Primordial power spectra are consistent with observations.

Differences in spectra and non-Gaussianities can distinguish models.

Abstract

In this paper, we first provide a brief review of the effective dynamics of two recently well-studied models of modified loop quantum cosmologies (mLQCs), which arise from different regularizations of the Hamiltonian constraint and show the robustness of a generic resolution of the big bang singularity, replaced by a quantum bounce due to non-perturbative Planck scale effects. As in loop quantum cosmology (LQC), in these modified models the slow-roll inflation happens generically. We consider the cosmological perturbations following the dressed and hybrid approaches and clarify some subtle issues regarding the ambiguity of the extension of the effective potential of the scalar perturbations across the quantum bounce, and the choice of initial conditions. Both of the modified regularizations yield primordial power spectra that are consistent with current observations for the Starobinsky potential within the framework of either the dressed or the hybrid approach. But differences in primordial power spectra are identified among the mLQCs and LQC. In addition, for mLQC-I, striking differences arise between the dressed and hybrid approaches in the infrared and oscillatory regimes. While the differences between the two modified models can be attributed to differences in the Planck scale physics, the permissible choices of the initial conditions and the differences between the two perturbation approaches have been reported for the first time. All these differences, due to either the different regularizations or the different perturbation approaches in principle can be observed in terms of non-Gaussianities.