Going back to address some of the other questions that cropped up....
But how about being able to observe the past (i.e. without being able to change anything)?
Is there any argument against that being possible?
There is at least one, and it is subtle...
It is thermodynamically impossible to transmit information without also transmitting energy. Therefore, to get information out of the past, you also take energy out of the past. That energy was in the past before you observed, and now it isn't - the past is thus changed (perhaps a miniscule amount, perhaps not in a way you intend to control, but change nonetheless) by your observation. Ergo, there is no such thing as "observe, but not change".
I have this hypothesis, that I'd love to talk to a qualified physicist/cosmologist about someday, regarding a possible explanation of the increasing rate of the expansion of the universe.
The current wisdom is that this is explained by the presence of sufficient "dark energy" (which, I know, sounds like, "It's magic!!!1!"). This happens to be pretty consistent with General Relativity, and doesn't have profound implications for time travel.
The problem with that is that every single event (on a quantum level, no less) creates an entirely new universe - duplicating everything that exists. Where does all the energy come from?
There are a couple of ways to think of that answer, but here's a simple one: Let us consider the case where there are not Many Worlds. Where does the energy for the one single Universe come from?
The energy for the Many Worlds ensemble of universes comes from the exact same place. Of course, "place" is not the right word, and here we get into one of the places where cosmology and quantum mechanics give people headaches...
I suppose it's possible that there could be a finite (and fixed) number of parallel timelines, constantly splitting from and converging with one another, in some sort of ball of wibbly-wobbly timey-wimey stuff. But that does seem rather unlikely.
It does seem unlikely, but probably not for the reason you think it does. Recall that the ideas of "where" and "when" (as in, "
Where does the energy come from
when the universe splits?") are *internal* to the universe. The idea of conservation of energy (even the phenomenon of "energy") is likewise internal to the universe, and only holds within our spacetime. Our clocks and rulers (by which we define energy) do not function, apply, or exist outside our spacetime.
Basically asking where the energy comes from is rather like asking where time comes from, or where space comes from. In a sense, the energy does not come from anywhere - it is a function of the mere existence of the universe.