Implications of quantum ambiguities in k=1 loop quantum cosmology: distinct quantum turnarounds and the super-Planckian regime

TL;DR

This paper compares two quantization methods in loop quantum cosmology, revealing differences in quantum turnarounds, energy densities, and the potential for cyclic universes, with implications for understanding quantum gravitational effects in cosmology.

Contribution

It demonstrates the phenomenological differences between holonomy and connection quantizations in closed FLRW models, including the existence of distinct quantum turnarounds and super-Planckian regimes.

Findings

Connection quantization yields two distinct quantum turnarounds.

Holonomy quantization generally limits energy density, but extreme values can occur.

Pure quantum cyclic universes are possible under certain initial conditions.

Abstract

The spatially closed Friedmann-Lema\^{i}tre-Robertson-Walker model in loop quantum cosmology admits two inequivalent consistent quantizations: one based on expressing the field strength in terms of the holonomies over closed loops, and, another using a connection operator and open holonomies. Using the effective dynamics, we investigate the phenomenological differences between the two quantizations for the single fluid and the two fluid scenarios with various equations of state, including the phantom matter. We show that a striking difference between the two quantizations is the existence of two distinct quantum turnarounds, either bounces or recollapses, in the connection quantization, in contrast to a single distinct quantum bounce or a recollapse in the holonomy quantization. These results generalize an earlier result on the existence of two distinct quantum bounces for stiff matter by Corichi and Karami. However, we find that in certain situations two distinct quantum turnarounds can become virtually indistinguishable. And depending on the initial conditions, a pure quantum cyclic universe can also exist undergoing a quantum bounce and a quantum recollapse. We show that for various equations of states, connection based quantization leads to super-Planckian values of the energy density and the expansion scalar at quantum turnarounds. Interestingly, we find that very extreme energy densities can also occur for the holonomy quantization, breaching the maximum allowed density in the spatially flat loop quantized model. However, the expansion scalar in all these cases is bounded by a universal value.