Critical Evaluation of Common Claims in Loop Quantum Cosmology

Abstract

A large number of models have been analyzed in loop quantum cosmology, using mainly minisuperspace constructions and perturbations. At the same time, general physics principles from effective field theory and covariance have often been ignored. A consistent introduction of these ingredients requires substantial modifications of existing scenarios. As a consequence, none of the broader claims made mainly by the Ashtekar school -such as the genericness of bounces with astonishingly semiclassical dynamics, robustness with respect to quantization ambiguities, the realization of covariance, and the relevance of certain technical results for potential observations -hold up to scrutiny. Several useful lessons for a sustainable version of quantum cosmology can be drawn from this outcome. * e-mail address: bojowald@gravity.psu.edu 1 All quotations from [1] given in this paper refer to the second preprint version.

Cosmic Tangle: Loop Quantum Cosmology and CMB Anomalies

“…It is instructive to analyze the brief, but rapid-fire response given in [2] to the previous criticism of certain claims in loop quantum cosmology [3]. As already mentioned in the Introduction, the authors stated that "Many of the specific technical points were already addressed, e.g., in [42,64,65] and in the Appendix of [66]."…”

“…As already mentioned in the Introduction, the authors stated that "Many of the specific technical points were already addressed, e.g., in [42,64,65] and in the Appendix of [66]." However, the references provided tried (but failed [28,29]) to address an older issue, cosmic forgetfulness [30,31], that did not play a major role in the recent discussion of [3] (since the publication of these older papers, cosmic forgetfulness has been strengthened to signature change).…”

“…(Note that the authors were hedging their statement by correctly saying that "these equations carry some of the leading-order information" (emphasis added). The fact that they carry only some, but not all of the leading-order information is a problem in itself that will not be discussed here; see [3,32] for details. ).…”

“…It therefore serves as a case study of the complicated interplay between quantum cosmological modeling and observations. A detailed technical discussion of some of the underlying problems of current versions of loop quantum cosmology was already given in [3]. The analysis here is held at a non-technical level and is more broadly accessible.…”

“…The analysis here is held at a non-technical level and is more broadly accessible. It highlights conceptual problems that are relevant for [2], but were not considered in [3], for instance concerning suitable justifications of initial states in a bouncing cosmological model.…”