Undergrad Loop Quantum Cosmology: Explained in Layman's Terms

[windy miller]

I have heard that one of the of the founders of LQC, Martin Bojowald is now rather critical of it. And sample of his criticism can be found here:
https://arxiv.org/pdf/1810.00238.pdf
I would be really grateful if someone could explain this in more laymen language . Any thoughts on the issues also welcome.

 

[kimbyd]

It sounds like what he's saying here is that inhomogeneities make the math break down at the "bounce" of loop quantum cosmology. Basically, before the bounce, when the universe is contracting, any small overdense regions will collapse, making the universe less and less homogeneous as the collapse continues. This makes the math at the "bounce" break down, requiring some really careful fine-tuning of the parameters to make the universe smooth at the bounce (as is required).

 

[windy miller]

Thanks KImbyd, much appreciated. He seems to refer to issues with infrared renormalisation and covariance. Am I right to interpret the former as saying that as one gets to different energy scales the maths breaks down and the latter as implying the solutions don't respect the symmetries of GR?

 

[kimbyd]

Thanks KImbyd, much appreciated. He seems to refer to issues with infrared renormalisation and covariance. Am I right to interpret the former as saying that as one gets to different energy scales the maths breaks down and the latter as implying the solutions don't respect the symmetries of GR?

My understanding is that a continuing problem with loop quantum gravity is that they don't know how to make it match General Relativity in the right limits. So there's uncertainty over whether or not it's even a viable theory of gravity, let alone quantum gravity.