I think that the the reason for just two universes is because there appears to be only one single dimension of time, in which "Movement" can only occur within (forward and backwards). This is analogous to spacial dimensions, of which we can detect only three of, where an objects position or movement is defined with respect to each of those dimensions. For example, if we call each of those directions the x-direction, y-direction, and z-direction, then a movement in 3D space will be defined as perhaps, 3 spaces forward along x, 4 spaces backwards along y, and 5 spaces forward along z. Thus the movement in 3D space is broken down into three individual movements confined to a single dimension, but has the overall effect of moving through a 3D volume.

It gets more complicated when you add in extra spatial dimensions. Physically it is hard to picture, but mathematically movement through extra spatial dimensions is treated just the same as moving through 3D space. So imagine walking around a rectangular garden (i.e. a 2D space). If you go 30 feet along x, and 40 feet along y, then it is the same as going 50 feet along direction R from one corner of the garden to the opposite corner. This is simply Pythagoras where R = square root of the sum of the squares of x and y. This can be expanded to 3D space (x, y, z), where R will be equal to the square root of the sum of the squares of distance moved along the x, y and z dimensions. For a 4D space, R would be the square root of the sum of the squares of all the distances moved.. and so on for N-dimensional space.

If it ever turns out that our universe has more than one temporal dimension, then we introduce a more complicated time-scape, where you can have multiple time directions that one could "move" through and thus a potential of more universes pivoted on either side of the Big Bang. However, i think that if the properties of the multiple time dirmensions are identical, but mirrored, then we would still only see two universes, on either side of the Big Bang.

Another aspect of why we would only have two universes lies with the properties of matter and anti-matter. If we look at it from a Feynman diagram, then a a particle of matter moving forward in time is identical to its anti-particle moving backwards in time. In our universe, the one that we see today, we see very little anti-matter, meaning that our universe is predominately normal matter, and thus moves forward through time. If there was a mirror universe created at the same time as our universe, but filled with anti-matter, it would be identical to ours, but would appear to move backwards with respect to our arrow of time. Note that people in the mirror universe would state that they were moving forward in time (as they would measure their universe made up of normal matter), and it would be our universe they would see being made up of anti-matter and running backwards in time (from their perspective of the arrow of time).

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup