Previously, I wrote about there being two kinds of time travel. Â More specifically that there are only two kinds that work and make sense without leaving giant gaping holes in the stories. Â Now I’m going to spin-off into an examination of dimensions…
The Big Bang
If you ignore the faith-based beliefs that the Universe just sprang into existence when a deity willed it to be, then you pretty much have to accept the theory of the Big Bang, that everything exploded out of “something”. Â For a while, science and science fiction grasped on to the idea that eventually at some point down the line, the Universe would stop expanding and would begin to contract. Â People really like this idea because it lends itself into a nice loop. Â Everything racing back together, getting faster and faster, exceeding the speed of light, warping back in time billions and billions of years and then exploding again. Â The end is the beginning.
Assuming a warping of space and time, it isn’t hard to jump to the idea that when the Big Bang happens again that it doesn’t have to be exactly the same.  In one way of looking at it each version of the Universe is happening in sequence.  The end of Universe 1 is the beginning of Universe 2, and end of U 2 is the beginning of U 3, and U 3 to U 4 and so on.  Thanks to the warping of time and space, however, you can get to the idea that these Universes are also happening simultaneously but somehow out of phase with each other.  This conjures up the idea of wonderful strangeness, like in our Universe there are nine planets (Pluto, I’ll never let you go) but in one of our neighbors, X-1 or X+1, there are ten and due to some sort of anomaly, that tenth planet occasionally influences or even crosses over into our Universe.  Your keys weren’t sitting on the table the whole time you were looking, they had actually slipped into X-1 but found their way back eventually.
Now we have evidence that the expansion of the Universe isn’t slowing, and may even be speeding up. Â It’s hard to tell what is going on with all the dark matter out there and whatnot.
Echoes
But how does an examination of dimensions of this sort relate to time travel? Â You have to stop thinking so three dimensionally. Â What if the Big Bang wasn’t just a simple explosion, but instead ripped right through space and time. Â Now you have Universe 1 beginning, and then a tiny fraction of a second later Universe 1 sub 1 begins. Â A tiny fraction later Universe 1 sub 2 begins, and U 1 sub 3 shortly after, and U 1 sub 4, and so on. Â And infinite number of Universes trailing behind us through time. Â And since we are just as likely to not be the first Universe as we are to be it, there are an infinite number of Universes extending out in front of us as well.
Now, when you travel through time, you aren’t really. Â You are simply jumping to another copy of our own Universe. Â Jumping forward in time by one hour is actually sliding to a Universe that began exactly as ours did but an hour out of alignment.
The very first time that I thought about this was when I was watching the movie Bill & Ted’s Excellent Adventure.  In it, Rufus explains to them that no matter what they do or where they go, they have to keep an eye on their watch because the time in San Dimas is always moving forward.  This is because the phone booth isn’t actually travelling in time, it is sliding between dimensions where the intended time is happening “right now” and when they return to their own dimension if they’ve been gone for an hour it will be an hour since they left.  Time marches on, so to speak.  The only real problem with that theory as far as the movie is concerned is Rufus’ stated reason for being there.  His mission would appear to be a closed loop (he’s going back in time to help them with their report because he did go back in time to help them with their report that they almost failed), but then you get the paradox of how that loop got started.  It would have to be that a Rufus (or someone) from a Universe that didn’t have a successful Bill & Ted pinpointed the need for them to pass the history exam 700 years prior (or however long they needed) and traveled to the appropriate world to fix it, then returned home to live in his unacceptable world (or maybe he didn’t go home).  In that world, 700 years later, Rufus climbs into a time machine in a perfectly excellent world to go back 700 years to ensure it still happens… or rather, that it happens for someone else.  So either lots more cross Universe communication is happening, or only one world every 700 years gets to be excellent.  In a manner of speaking.
Of course, the second movie completely throws that out the window with all the jumping forward and backward and delivering items to themselves.
But assuming the Echoes theory is true, it means that you can’t change your past or future. Â You can only change the worlds offset from your own, and in order for your world to change an offset you needs to be the instigator of change.
No matter what, thinking about time travel this much will probably give me an aneurysm.