Trouble for Loop Quantum Gravity?

[ohwilleke]

TL;DR Summary

A new pre-print identifies a quite general problem with the vast majority of loop quantum gravity models - specifically a general covariance violation. How big a deal is this?

A new preprint establishes that almost all models implementing loop quantum gravity violate the principle of general covariance, and can only satisfy a more limited condition. Is this the huge problem for loop quantum gravity theory that it appears to be? Or is the less strict condition that can sometimes be satisfied merely less elegant but perfectly serviceable to explain all gravitational observations?

Given the great emphasis that loop quantum gravity gives to background independence, which is, more or less, what general covariance implies, this would seem to be a fatal flaw.

Also, is there any reason to doubt the credibility of the author, who appears on 161 preprints at arXiv, most published eventually in peer reviewed journals, and most apparently in this relevant subfield?

The paper and its abstract are as follows:

[Submitted on 31 Jul 2020]
A no-go result for covariance in models of loop quantum gravity
Martin Bojowald

Based on the observation that the exterior space-times of Schwarzschild-type solutions allow two symmetric slicings, a static spherically symmetric one and a timelike homogeneous one, modifications of gravitational dynamics suggested by symmetry-reduced models of quantum cosmology can be used to derive corresponding modified spherically symmetric equations. Generally covariant theories are much more restricted in spherical symmetry compared with homogeneous slicings, given by 1+1-dimensional dilaton models if they are local. As shown here, modifications used in loop quantum cosmology do not have a corresponding covariant spherically symmetric theory. Models of loop quantum cosmology therefore violate general covariance in the form of slicing independence. Only a generalized form of covariance with a non-Riemannian geometry could consistently describe space-time in models of loop quantum gravity.

Comments:	14 pages
Subjects:	General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Cite as:	arXiv:2007.16066 [gr-qc]
	(or arXiv:2007.16066v1 [gr-qc] for this version)

 

[Auto-Didact]

Also, is there any reason to doubt the credibility of the author, who appears on 161 preprints at arXiv, most published eventually in peer reviewed journals, and most apparently in this relevant subfield?

Bojowald is a big name in LQG and IIRC one of the - if not the - founder(s) of loop quantum cosmology.

 

[George Jones]

Summary:: A new pre-print identifies a quite general problem with the vast majority of loop quantum gravity models - specifically a general covariance violation. How big a deal is this?

A new preprint establishes that almost all models implementing loop quantum gravity violate the principle of general covariance, and can only satisfy a more limited condition. Is this the huge problem for loop quantum gravity theory that it appears to be? Or is the less strict condition that can sometimes be satisfied merely less elegant but perfectly serviceable to explain all gravitational observations?

Given the great emphasis that loop quantum gravity gives to background independence, which is, more or less, what general covariance implies, this would seem to be a fatal flaw.

Also, is there any reason to doubt the credibility of the author, who appears on 161 preprints at arXiv, most published eventually in peer reviewed journals, and most apparently in this relevant subfield?

Martin Bojowald is credible, but "credible doesn't always translate to "correct". (I am not saying that I think that Bojowald's result is incorrect, I am just urging caution.) Also, the preprint actually is a Phys. Rev. D publication. It will be interesting to see how this plays out over the next few years.

It seems that Bojowald has become a vocal critic of loop quantum gravity. A longer review paper outlining this and other criticisms of loop quantum gravity:

https://arxiv.org/abs/2002.05703

 

[ohwilleke]

A longer review paper outlining this and other criticisms of loop quantum gravity:

https://arxiv.org/abs/2002.05703

[Submitted on 13 Feb 2020]
Critical evaluation of common claims in loop quantum cosmology
Martin Bojowald

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.

Comments:	32 pages, invited contribution to Special Issue "Quantum Models for Cosmology"
Subjects:	General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Journal reference:	Universe 6 (2020) 36
DOI:	10.3390/universe6030036
Cite as:	arXiv:2002.05703 [gr-qc]
	(or arXiv:2002.05703v1 [gr-qc] for this version)