Covariant Effective Action for Loop Quantum Cosmology from Order Reduction

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.

minor changes to match published version

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• Modified Friedmann Equation from Nonminimally Coupled Theories of Gravity
• Primordial black holes in loop quantum cosmology: The effect on the threshold
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