Of course, I have no idea if this is true, or even the most popular theory. It's probably even dead wrong, but here's how I think the universe might be.
The universe is infinite in both space and time, but I'm not sure if time is infinite backwards. It's easier for me to imagine a starting point for time, but that doesn't say anything.
Space has three dimensions, time has one. Time, however, is not the same as space, because of its one-directional nature. What happens in the future can't affect the past, whereas every area of space potentially effects every other.
Timetravel. I don't know if it is possible, but if it is, there are the two possibilities, lets call them theory 1 and theory 2:
(1) Travel into the past will create an alternate reality. The traveler cannot go back to his own. If he travels back into the future, it will be the future that he's created.
(2) Travel into the past will be travel to your actual past. Same reality. This will create temporal paradoxes, and cause the end of time itself. You can't think it will only cause paradoxes if you kill your own grandfather. Limiting the danger to that sort of thing is just naive. At the very least you cause air molecules to move or something. Even in a vacuum you will probably effect some fotons or neutrino's. This will create an alternate future, from which the traveler will start his journey. This will effect the traveler, and the traveler will effect his surroundings differently when he goes back. Etc. Etc. There is no longer a fixed shape of the future, and thus there will be no future. This stopping of time will stretch throughout the entire infinity of the universe, since everything in space potentially effects everything else.
Travel into the future will be the same in both cases. It would be like spending time in suspended animation for a while, time not passing for yourself. No problems there. But, of course, you can't go back to your own time without one of the consequences mentioned before about traveling back. Traveling into the future with theory 1 cannot cause you to meet your older self, because if you ever travel back to be that self, it will be in another reality. In theory 2 it's not clear to me. Does time 'know' you will eventually travel back? Hm.. Tricky.
In any case, timetravel will cause mass from a certain stretch of time to move to another stretch of time. Moving mass to the past even enlarging the amount of mass overall. I don't know if that matters, though, the universe being infinitely large and all. Moving a finite amount of mass from now to then should not matter in the grand scheme, but of course, could have local consequences. Bringing enough mass from future earth to the past could mess with the earths orbit, for example.
Mass, of course, can be completely converted into energy, Star Trek style. This, combined with bringing mass to the past, however, cannot create an infinite local energy source in theory number 2. If you bring mass from the future and convert it into energy, the same amount of mass will be taken away in the future anyway. Not that this matters, time having stopped and all. In theory 1, I suppose it's possible to create an infinite local energy-source this way. Not that your fellow humans have any use for it, you bringing it into another reality.
Well, let's stop here for now.
