> pretty much all of current cosmology rules out FTL
Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.
Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves.
Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.
Redshift also occurs to light emitted from within a gravity well. Obviously that isn't what's happening at a cosmic scale (our light isn't blueshifted to other observers) but it's at least conceivable that some third phenomenon could be the cause.
> but what the heck else are you expecting to pop up to explain redshift
For example, some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform and we're in an area that has properties slightly different from the surrounding space.
What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?
Anisotropy of space is (a) again adding complications and (b) unlikely to produce such a spherically symmetric redshift. The CMB is infamous for how smooth it is. The different region of space would have to be perfectly spherical with us in the dead center. May as well revive geocentrism.
Look, if we did find evidence like that from JWST or whatever, we'd re-evaluate what best meets Occam's razor for the totality of the evidence. But that doesn't change the fact that with our current evidence, that explanation is expansion of space. Other stuff is sci-fi at best. Fun to think about, but no reason to actually believe it.
The story for dark matter is more complicated, but similar in broad outline. The more the alternative theories try to fit all the evidence, the closer they get to reinventing dark matter. You have something that walks like matter and quacks like matter, so the smart money says it's matter.
> some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform
I think people have explored the photons thing with regard to “solving” dark matter, and found the evidence lacking. As for spacetime… trivially yes, but I don’t think this is what you meant? I mean, spacetime being non-uniformly curved just means there’s stuff in it, and that is literally one of the things we can directly observe, and we see that in e.g. the Bullet Cluster the dark matter kept moving when the visible matter collided and slowed down.
> What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?
If, after careful work to exclude all other possibilities, then a party and a Nobel prize.
Similarly, for dark matter it’s not just galaxy rotation, it’s also observations of gravitational lensing and again the CMB, formation and evolution of galaxies, mass location during galaxy collisions, and movement of galaxies within galaxy clusters. Again, the observational evidence today is overwhelming: https://en.m.wikipedia.org/wiki/Dark_matter#Observational_ev...
If you're not convinced, you should look in to the evidence and the deductions that lead to what is now presented as fact. Very few popular presentations cover the reasons behind the wacky stuff scientists say, which is to blame for you evaluating that stuff on the basis of whether it rings true - because you've been given no other way to evaluate it.
Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.