FLRW metric challenged, Einstein-Euler equations as alternative?

[nomadreid]

TL;DR

An article (source given) presents an alternative to the FLRW metric. Is my short rough summary of it (given in the main text) correct?

The question concerns the article "The instability of critical and underdense Friedmann spacetimes at the Big Bang as an alternative to dark energy" by Alexander, Vogler & Temple in Volume 482, Issue 2338 (May 2026) of the Proceedings of the Royal Society A, found in the link:
https://royalsocietypublishing.org/...20/The-instability-of-critical-and-underdense

Its merits are beyond my ability to judge, but as the journal is respectable, I am presuming that it contains no glaring faults. I also presume that it has not had time to make a major impact.

My question is whether my limited understanding of the content is valid, roughly:

The FLRW metric is highly sensitive to small perturbations, hence not a proper explanation of the expansion of the universe. However, one can derive the expansion from the Einstein-Euler equations, and do away with the necessity of the cosmological constant. If one gives up the FLRW metric, then one perhaps does not need the assumption of isotropy.

Corrections are welcome. Thank you.

 

[PeterDonis]

I also presume that it has not had time to make a major impact.

Yes, that would be the case. The general idea, that deviations from the idealized FLRW solutions might be enough to account for observations without having to add a positive cosmological constant to the model, is not new; it's been around for quite some time, and we've had previous PF threads on other papers along these lines. This particular suggestion, however, appears to be new, so there might not be much that can be said about it at this point.

 

[Matterwave]

"If one gives up the FLRW metric, then one perhaps does not need the assumption of isotropy."

I haven't read the paper but would comment on the above statement^

If you say "the assumption of isotropy" I assume you mean "isotropy around every point" because isotropy *around us* is an observation not an assumption.

The FLRW metric arises precisely because of the assumption of everywhere isotropy. If we assume the universe looks the same in every direction at every point then we get the FLRW metric (people often also think we need to assume homogeneity but one can show that this is a consequence of everywhere isotropy). This is a purely geometric result, not depending on properties of the stress energy tensor except for that symmetry. So if you get rid of FLRW, you are rather *forced* to give up everywhere isotropy. Which usually means you give up the copernican principle (i.e. you say some other observers might not see isotropy) since, again, we see the universe as isotropic (to good approximation at least).

There are some technical ways you can avoid this -- some kind of global non simply connected topology for example (FLRW makes some global assumptions).