Re: Follow-up question to multiple universes and universe expansion

Hello Eliza,

So you have found yourself confused by multiple universes and expansion of the Universe. Join the crowd. This is abstract stuff and not at all easy to grasp.

Let me try and clarify your question over the previous MadScientist's answer to your question (http://w ww.madsci.org/posts/archives/sep2000/968454277.As.r.html). In his answer he remarks on the idea that parts of our Universe are disconnected from us and are completely unkown. The primary reason for this is the finite age of the Universe.

Imagine for a moment a Universe that is infinite in space, but completely empty of matter or energy. Suddenly, everywhere at the same time in this Universe matter and energy come into being. Light from each new galaxy begins heading out in all directions. From any location in this Universe there will be a spherical shell surrounding it beyond which no light has yet reached. The radius of this spherical shell is equal to the distance that light could have traveled since the moment of creation. Since nothing can travel faster than light, there can be no knowledge of anything beyond that shell. The shell is a horizon defining the known Universe. It grows with time. As the Universe gets older more of it comes into view. As you suggested the galaxies seen near the edge of the horizon will be seen as they were in the past. In this scenario the entire Universe could only be observed after an infinite amount of time. So essentially there would always be regions of it which are beyond knowledge (even the pasts of those regions). This illustrates how a Universe with a finite age has unobservable regions for any given observer.

Our Universe not only has a finite age, but it also is expanding. That throws in a whole new level of complicating factors. You could imagine that if the Universe had finite space and was decelerating at the right rate then it might be possible for the whole Universe to eventually become observable. However, we have recently discovered that the Universe is accelerating. This means that the farthest regions of the Universe which are currently not observable are likely to never be observable. The expansion will continue to carry them farther and farther away from us. The light from their past which is even now heading toward us will be outpaced by the expansion. As the previous MadScientist said, the expansion will become faster than the speed of light. The scary thing about an accelerating Universe is that the horizon of the observable Universe will eventually stop growing with time and will actually start to shrink again. In some very distant epoch of the Universe all galaxies will sit alone in their own Universes completely cut off from the rest of the larger Universe.

I know this is pretty mind blowing stuff. But that's what makes it so fun!

All the best,
Bryan Méndez, Space Sciences Lab, UC Berkeley