Covariant Effective Action for Loop Quantum Cosmology from Order Reduction

TL;DR

This paper explores the possibility of deriving a covariant effective action for loop quantum cosmology, showing that higher order gravity theories can reproduce modified Friedmann equations in isotropic models, but challenges remain for anisotropic cases.

Contribution

It demonstrates that a covariant effective action can be constructed for isotropic LQC models using higher order gravity theories and effective field theory methods.

Findings

Covariant action exists for isotropic models using higher order gravity.

Doubts are raised about covariant descriptions for anisotropic models.

Effective field theory techniques are crucial in the construction.

Abstract

Loop quantum cosmology (LQC) seems to be predicting modified effective Friedmann equations without extra degrees of freedom. A puzzle arises if one decides to seek for a covariant effective action which would lead to the given Friedmann equation: The Einstein--Hilbert action is the only action that leads to second order field equations and, hence, there exists no covariant action which, under metric variation, leads to modified Friedmann equation without extra degrees of freedom. It is shown that, at least for isotropic models in LQC, this issue is naturally resolved and a covariant effective action can be found if one considers higher order theories of gravity but faithfully follows effective field theory techniques. However, our analysis also raises doubts on whether a covariant description without background structures can be found for anisotropic models.