Unknown "Structures" Tugging at Universe, Study Says

[Junghalli]

Accounting for the redshifting of light that we see, the OU is roughly 45 Gly (45 billion light years) across at present; the true universe is anywhere from several hundred to several million or billion times that size, although we really don't know.

Right now I'm looking at the observable universe page on Atlas of the Universe, and contemplating the fact that whole thing is at least hundreds of times that size, maybe millions or billions ... I think my brain just exploded.

Holy guacamole that means there must be at least hundreds of trillions of galaxies and trillions of billions of stars in the whole universe ... and that's at the low end.

[Kuroneko]

As for the dark flows, there's no way this is gravitational if real, because gravitational interactions travel at c in both GR and any proposed theory of quantum gravity. Gravity is bound by the same rules as light with regard to light cones and absolute elsewheres.

The gravitational effects of perturbations should travel at c, but it doesn't imply anything of the sort. Things beyond a horizon today were not necessarily beyond it in the past.

Relating to dark matter, I've read that if you add a fudge factor to gravity that changes how it affects objects at truly large scales then you can explain and predict the observed motions of the universe without needed to postulate the existence of dark matter. I think that hypothesis (theory?) hasn't gained much traction because dark matter is more testable.

It's a hypothesis (I've never heard this one before; where'd you read it?).

He's probably referring to MOND, the "modified Newtonian dynamics." Or some other iteration of modifying gravity--there are several.

The reason it hasn't gained much traction is that GR as is has passed every test posed to it, and there is no indication that it needs to be modified on extremely large scales (though I admit I'm don't know the ins and outs of GR).

True, although there was at one point an interesting proposal that has since then failed. The galactic rotation curve is the main piece of evidence for dark matter, but the inference was initially based on the argument that low-velocity weak-field GTR is identical to Newtonian gravity, so that one can apply Kepler's laws to measure the deviation. That's fine for a test particle in an external matter distribution (i.e., a particular that does not itself contribute to the field), but the stars are also themselves sources, invalidating one of the assumptions. Now, that doesn't mean that there's no dark matter (indeed, that would be a problem, since there is now further evidence for it from gravitational microlensing), but there was a proposal to do away with it purely in GTR. That didn't work out, although if IIRC, a more precise analysis showed that the amount of dark matter should be less than predicted by Kepler's laws by a fair amount.

The most obvious implication is that our universe is not a closed system. This can be filed under "good news", I think, because it posits the existence of extra-universal energy sources.

That doesn't follow at all. The observation was only about the velocity observed today, after all.

Back to topic, and blatantly displaying my ignorance, are there any current theories that our universe is actually larger than we can see? That is, are there any theories about the size of the universe being larger than the visible volume that are consistent WRT Relativity and Quantum Theory?

Our observed universe is approximately flat, in fact so close to zero curvature we aren't even sure whether it's positive or negative. Thus, we don't know whether the universe is finite (positive curvature) or infinite (negative or zero curvature), but we can say that the universe must be significantly larger than our observed one, since otherwise we would have unambiguously observed a positive curvature.