Read with Caution: I'm a theist so you may not like me ;)
I think that time is a solid. Just like you can have a point, line, or 3d object...I think time in its entirety is a solid, but we can only see it like a line. We see in 2d like a hammerhead shark sees in 2d, but the world exists in 3d, which we can't see, because we don't have stereoscopic time vision with overlapping fields of view if you will.
To answer your question: I think that our universe has a definite structure kind of like stories have a definite structure, but with the same number of uncertain things as stories have uncertain elements. We have free will, but certain things are bound to happen if we make various choices. Not every decision is determined, but "important" ones are already "determined". So I don't think there's such a thing as going back in time.
I also think that there are many worlds, but not multiple realities. Worlds being defined as places that you couldn't get to, even if you traveled for an infinite amount of time. There are different sets of physical laws for every universe, but there is another reality - unbound by the physical laws of our particular (or any) universe that contains all of these.
Sort of like you can have a game of chess - that is contained by all its particular rules - and checkers - which is bound by a whole other set of particular rules, some of which are similar, but others different...they both exist and were created in OUR world.
Each game is like a universe. The rules to our universe are our laws of physics. But, just like we can sometimes allow an opponent to take back an occasional move in a game of chess, you could theoretically have something like miracles...occasional breaking of the laws of physics. However, the rules of physics must be obeyed almost always (just like in the game), or else, there would be no way to play and it would just be random pieces moving around randomly on a board. That's why we have physics and why cheating rules in general is inherently "bad conscience" to us.
Thus, I believe that there could have been something like the big bang theory. There could be string theory, relativity, or any of these things...but at the end of the day, it's irrelevant. I think it's more important to learn about the rules of the "outside world" if you will - the place where all the games/universes are contained. I think that there is a God that made our world and could have made many others. The Big Bang only explains how something happened...once there was something to be exploded, but it does not explain why or where that infinitesimally small piece of something came from in the first place.
Idk, obviously we can't KNOW any of this...but for me personally, it seems that the theistic point of view makes a whole lot more rational sense.
So yes! I believe in multiple universes, multiple laws of physics, etc -- just not multiple realities.