A question to the theory of multiverse

Question

Guys I couldn't catch a point of multiverse theory..

Theory: If space-time goes on forever, then it must start repeating at some point, because there are a finite number of ways particles can be arranged in space and time..

Question: It seems like as if multiverse theory stands on a optical argument. We call there is a next universe because we are not seeing that since there is a speed limit of light and our optical universe is 14 billions light yrs in diameter(or whatever it be..since we can not assume our space time a sphere or 2dimensional plane).Doesn't it sound weird? If there is a space between two universe then what is the value of calling it a system of two universe, Isn't it just a single universe?

Question:Is it a good idea to call there is a next universe because it is exactly alike?

Question: Another question is How does the multiverse theory provides solution to the grandfather paradox? Isn't it like we fold the two dimensional spacetime graph where universe exactly alike to our universe exits??If my above argument is correct then how is it valid?? Note that I am not questioning multiverse theory this time..So please Don't give answer to this question proving my above arguments wrong..Let this be independent question..

I know I am wrong But please Make me clear.

Answer 1

So first of all, there are several multiverse theories. As a cosmologist, I personally don't know anyone that subscribes to the one you've written here.

The standard multiverse theory comes from quantum mechanics. In it, we say that every time an action is taken that (more or less) collapses a wavefunction in our universe, it simply means that other universes exists in which all other possibilities occurred. Simply put, there are an infinite number of universes, in which all possible events occur.

This pretty much renders your first question moot. However, yes, if we were to hypothesize about a region separated from us by only distance, it would make sense to refer to it as part of our universe.

Your second question is easy to answer. In multiverse theory there are an infinite number of universes exactly like ours, but they are still different universes. They only exists because the one outcome that differs between our universes hasn't happened yet.

As for the third question, the grandfather paradox is that if you went back in time and killed your grandfather, you would cease to exist and so noone would have killed your grandfather. The multiverse theory attempts to solve this paradox by stating that when you go back and kill your grandfather, you have gone back to another universe, not your own. So the person you killed is not actually your grandfather, but the grandfather of your parallel self. Your grandfather is still safe and sound in your universe and you will continue to exist.

You ask if your above argument is correct then how is it valid? If your argument were correct, it wouldn't be valid.

Answer 2

As I mentioned in the comments, your first two questions are based on an assumption about repeats that isn't justified. see Multiverse theory and infinite individuals for more details.

Re the last question: the grandfather paradox is based on the existance of closed time-like curves, and the existance or otherwise of these is unrelated to the concept of a multiverse. There are various suggestions for how CTCs might exist, but so far none of them have been physically reasonable. None of the multiverse ideas I know of have any bearing on the existance of CTCs.