There is no theoretical limit to the rate of metric expansion, but unfortunately metric expansion doesn’t allow for FTL communication. Let’s put aside issues of lightspeed in an expanding universe though because we don’t even need to go there to address this basic issue of why you can’t have a brain past certain limits.
At some point limited by the speed of light even in a static universe, your “thoughts“ would be incredibly slow. What would it mean to have a neuron the width of the galaxy? Would you be particularly intelligent just because you had a lot of individual parts from a certain distance? There’s probably some theoretical upper bound on the optimum size for a computing body in a universe bound by c, based on the total amount of entropy which could ever be contained in a given region and the rate of signal transmission.
As to adjacent dimensions and that kind of thing, who knows? In current physics that would be like magic, and magic can be whatever you want it to be after all.
An Alcubierre warp drive[1] would be capable of enacting causality faster than light, no?
Imagine that human civilization survives and progresses for 100 million years. Do you think that they'll be able to look back on any of our "laws" without laughing?
At that point we'd likely be far beyond Dyson spheres, harvesting large quantities of antimatter, solar system scale particle accelerators, artificial black holes, quantum supercomputers, etc., etc. Whatever would be discovered is almost guaranteed to be unimaginable by us.
For the artificial black holes, Dyson spheres, antimatter, quantum computers, and giant accelerators to actually work, the laws of physics they're based on can't be that wrong. They can be incomplete, requiring their place within a greater theory, but not simply wrong. There is observation consistent with the notion that the speed of light has been constant for the last ~14 billion years, everywhere we can see.
Well, if you're looking to claim that the universe used to be conscious prior to inflation, at which point it was torn to pieces in every direction possible simultaneously, sure. Probably not what you were fishing for, though.
"Not that any of this is particularly likely, but since we don't understand the core mechanisms of consciousness very well, it seems possible they could be operating on levels of physics that we also don't understand yet."
Well, if the universe is operating in that manner, it would have to be doing so with some rather tight bounds on it to ensure that we never ever have to take those things into consideration when plotting the behavior of a system. I would tend to say this falls into the "raises more questions than it answers" bucket, myself.
If consciousness is a complicated phenomenon that only manifests in some sort of very small way in certain interactions, then A: what are those interactions? and B: by what mechanism is the universe even discovering those interactions? It's not hard to imagine a universe that has physics just like ours, except that a particular combination of 10 to the 11th atoms of carbon in a very precise linked order will spontaneously emit very large quantities of blue light. But without a clue as to what that may be, how is anything in the universe ever supposed to find that pattern of atoms? That is, if this is how consciousness works and there is some sort of very ill-defined, handwavey "field" associated with it, by what mechanism did anything in the universe discover how to interact with it?
These are not necessarily forever unanswerable questions, but right now, they would in many ways be bigger questions than the question they are putatively being offered as the answer to.
Or even more theoretically, isn't it also possible that distant regions of our universe could be adjacent in other dimensions? Perhaps in some higher (or lower?) dimension, every particle in the universe is right next to each other, having never separated after the big bang.
Not that any of this is particularly likely, but since we don't understand the core mechanisms of consciousness very well, it seems possible they could be operating on levels of physics that we also don't understand yet.