The landscape of polymer quantum cosmology

TL;DR

This paper explores the broad implications of quantization ambiguities in polymer quantum cosmology, revealing generic features like a well-defined big bang dynamics, quantum superpositions of universes, and additional degrees of freedom relevant to quantum gravity.

Contribution

It demonstrates that many qualitative features of loop quantum cosmology are robust against quantization ambiguities, providing insights into the fundamental nature of the big bang and quantum universe branching.

Findings

Existence of well-defined dynamics across the big bang.

Presence of tunneling solutions with spacetime inversion.

Quantum superpositions of universes emerging at the big bang.

Abstract

We show that the quantization ambiguities of loop quantum cosmology, when considered in wider generality, can be used to produce discretionary dynamical behavior. There is an infinite dimensional space of ambiguities which parallels the infinite list of higher curvature corrections in perturbative quantum gravity. There is however an ensemble of qualitative consequences which are generic in the sense that they are independent of these ambiguities. Among these, one has well defined fundamental dynamics across the big bang, and the existence of extra microscopic quantum degrees of freedom that might be relevant in discussions about unitarity in quantum gravity. We show that (in addition to the well known bouncing solutions of the effective equations) there are other generic type of solutions for sufficiently soft initial conditions in the matter sector (tunneling solutions) where the scale factor goes through zero and the spacetime orientation is inverted. We also show that generically, a contracting semiclassical universe branches off at the big bang into a quantum superposition of universes with different quantum numbers. Despite their lack of quantitative predictive power these models offer a fertile playground for the discussion of qualitative and conceptual issues in quantum gravity.