First of all, the theories shown here are either something I have personally thought or imagined, or my interpretation of a previous theory. I will try to cite these as much as possible, and refer you to more official documents as I update this page. Yes, I know these theories are more philosophical rather than scientific.
Closed Universe
How a universe restarts after ending. Not uniquely mine
This is a fairly straight-forward and well-known hypothesis. The universe is created with a Big Bang, which then expands from the size of a pea outwards until gravitation brings it all back to the center. Here, it is compressed back down until the pressure is so bad, that another big bang occurs.
Multiple Universes
Crash rundown on several theories. Not uniquely mine
Some physicists believe there are 11 dimensions: Nine Spatial, One Time and The Eleventh. (Some believe 10, but they're less fun.)
The eleventh dimension (D11) is the dimension we would have to travel through to get to other universes.
Physicists noticed that gravity is weakest of all forces. The conclusion they came to was that gravity leaks out of our universe.
When scientists try to locate the precise position of atoms and their particles, they can't. It is as if atoms don't have an exact point; they keep jumping all over the place.
The theory began with, that the particles were in more than one place at a time - Along the 11th Dimension. Gravity is so much weaker because it is spread out over all the universes - or at least the ones our atoms are involved with (our Multiverse).
Travelling Back In Time - Splitting
What happens when someone time travels to the past. Not uniquely mine
The theory starts when one considers what would happen if someone (or something) travels back through time [Assuming instantanious travel, ie. "Back to the Future", not Wells' "The Time Machine"]. Many previous theories have come up with Grandfather Paradoxes. I cannot accept such a easy way out of a situation such as this.
If you think of time as a row of dominoes, the present is the domino currently falling, the past consists of all fallen dominoes and the future is dominoes still standing. If you arrange the dominoes in a loop, having a future domino run into a previous domino, it won't keep going in a circle. I believe what has happened has happened; it can't change. But if you go back in time, where do you go?
When you travel to a previous moment, another version of it is 'created'; the original still exists, plus one you've infected. The universe has split. If you were to travel to the future you would be travelling up a new time-line, parallel to the original.
If you in some way change the course of time so that you will not be born, nothing will happen to you, because you are from another universe.
Theoretically, now that there is another universe in our multiverse, gravity would become weaker again. But that is something you're going to have to ask the physicists, not me.
Exponential Divergence
An alternate theory. Not Mine
Another theory is that the multiverse is always expanding. Whenever (for instance) an electron makes a seemingly random movement, actually it took every movement possible, each one playing out in a different universe. I choose to ignore this hypo since it is not fun. Nothing else on this page assumes this hypo is correct.
Not Just Time Travel
How travelling to another universe, without any time travel could split it. Mine
According to the previous theory, a universe splits because another instance of that point in time already exists, when someone time travels to that moment. What if a parallel universe's instant at our current time has already occured? Someone who travels to this pre-occured instant will split it. The reasons why that moment has already existed are explored later, but the same rule should apply in this situation.
Maybe Just Energy Matters
Considers if actual matter is the difference between two universes, or just energy is. Mine
A universe splits because two versions are forced to exist. But twin universes might only be different by the location of a piece of dust or a stray light ray.
Example: Universe A has (only) Particle1 at position 1 and Particle2 at position 2. An Extrauniversal Alien travels to an instant already occured in this universe (A), swaps where the particles are, and leaves this universe (B) . Newly created Universe B has (only) Particle1 at position 2 and Particle2 at position 1. Are universes A and B the same universe? For the rest of my theories, it is assumed that they are.
What Is The Present?
Is perception of time relative, continuous/quantised? How 2D time has some complications. Mine
Suppose you travel to a previous time or another universe. In your new environment you travel forwards through time at your regular rate, but do all your friends at home experience normal time, or don't they exist yet? According to my domino analogy, two dominos must be falling at the 'same time'. Therefore, if you ever travel to another universe, you can't come back the moment you left (unless you plan to split your home universe); you must travel to the 'Active' Present (the domino currently falling).
This sparks new questions; what happens if you travel to the future, to a domino not yet falling? Do you exist in void, until the universe builds itself around you, or do you travel forwards in time at the same rate as the universe, forever just in front of it? Does this void consist of vacuum? Is a universeless void's vacuum stronger than a universe's vacuum? Is there any way to survive a universeless void's vacuum...?
Closed Multiverse?
Do the two hypotheses fit? Mine
If twin universes converge into a singularity, are these two universes now equal again (as in Maybe Just Energy Matters)? I believe they are (or are made to be, as discussed later) and therefore the two universes fuse back together; their time-lines converge.
A problem showing itself here is what happens if someone travels back 1000 years. The two Active Presents are then timeA = t and timeB = t - 1000y. Billions of years later, the original Universe-A dies (swallows itself into a singularity), yet Universe-B wants to collapse in 1000 years and converge with Universe-A at the same time.
I assume in this case that the rate of time is stretched to allow the two to converge at the same time. The forces involved with pulling these parallel presents together will be discussed soon.
Forces, Energy and Momentum in 11D
A 5th Fundamental Force Mine
Anything that travels to another universe clearly breaks the laws of Conservation of Energy and Conservation of Momentum. A popular solution is to rewrite the scope of the laws to encompass the multiverse, rather than each universe individually.
Looking back at the given reason for D11 in Multiple Universes, there are one universe's worth of particles, which are jumping, or vibrating, in the 11th dimension. I hypothesise, with only linear motion in mind, that these particles don't like this very much. Suppose there is no friction against these particles from moving in D11. It is the moving backwards and forwards within the other 10 dimensions that mustn't be comfortable.
Eg. In two different dimensions, you might be standing in a slightly different place. The atoms of your body don't like having to jump back and forwards like that, [similar to how atoms swap gravitons] which imposes an attractive force between your bodies. This will lead to all universes eventually converging.
See wiki: Time Travel
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