Landscape of polymer quantum cosmology

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, as well as 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 types 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.