Density is important because entanglements run away at light speed: if you can't get those entanglements back then you can't reverse the past, you can only reverse back to the point where the superintelligence originated, which isn't that neat. The only way to recohere the system is if there's a boundary condition or, more understandably, if a superintelligence catches your lost information and rushes it back to you (and presumably you would do the same for it, so it's a very clear-cut trade scenario). Problem is that it might take a very high density of superintelligences throughout the universe to pull this off all the way back to the big bang: even so it seems likely that you can get perfect information for at least the last few thousand years, enough to, say, ressurect every human who'd ever died. Even cooler than cryonics. (ETA: Insofar that these ideas are correct they're Steve Rayhawk's, insofar as they're retarded they're mine.)
I don't follow what consciousness has to do with it? I think I see what you're getting at but your use of the word "consciousness" kinda threw me off, and I'd rather not misinterpret you.
The only way to recohere the system is if there's a boundary condition
Or if spacetime is compact in any way, or in fact if there is only finitely much negentropy. In any of these cases, your abstraction of a bunch of distinct but potentially interfering branches will break down, and you can pick up all of your old waste heat even if it is "running away" at light speed.
In the other universes, where somehow things can continue expanding at light speed indefinitely, you can recover perfect info by exploring possible physical theories until you f...
This post may be interesting to some LWers.
In summary: it looks like our universe can support reversible computers which don't create entropy. Reversible computers can simulate irreversible computers, with pretty mild time and space blowup. So if moral value comes from computation, negentropy probably won't be such an important resource for distant future folks, and if the universe lasts a long time we may be able to simulate astronomically long-lived civilizations (easily 10^(10^25) clock cycles, using current estimates and neglecting other obstructions).
Has this been discussed before, and/or is there some reason that it doesn't work or isn't relevant? I suspect that this consideration won't matter in the long run, but it is at least interesting and seems to significantly deflate (long-run) concerns about entropy.