I’ve often heard that there was no time before the big bang. Well what happens to this statement if you accept the idea of multiple universes? Is “time” localized to each universe?
That depends on the model you use to describe the “multiple universes”. There are many such models, with different implications.
Ok, (1) well then please explain a model where Multiple Universes are made at different times going farther and farther back in time. (2) Is the predominant view of multiple universes that they were all created at one time simultaneously?
The predominant view doesn’t include multiple universes at all. All of the models with something that might be described as “multiple universes” are variants off of the standard model.
I think the closest to what you’re looking for would probably be the eternal inflation models. The idea is that, prior to the Big Bang, the Universe spent an infinite amount of time in an inflationary state. This state is unstable, so pieces of it are constantly “falling over” into non-inflationary universes, but inflationary states also expand so rapidly that there’s new inflationary space being created just as quickly as it’s being lost. Every piece of primordial inflation that stopped inflating became a universe much like ours, and this can have happened an arbitrary amount of time ago. Our separation from these other universes could be described as spatial, but there’s no chance of ever having any interaction between universes, since the space between them is expanding faster than you could traverse it.
thanks 
One theoretical possibility is a partial ordering of time.
Think of a “tree of time”. The trunk of the tree is the “universe” from which other universes are spawned. The time in those other universes starts from their starting point on the trunk, but after that, it doesn’t mean anything to try to order time between the branch and the rest of the trunk above that point, or other branches.
As a software engineer, partial ordered sets show up in all kinds of solutions to ordinary problems, and we get used to them. It’s a difficult idea for some folks to grasp at first, though.
In partial-ordered sets (“PO sets”), you can have events a, b, and c, where you could say
a <= b
a <= c
but where none of the below are true:
b <= c
c <= b
(where <= means less than or equal.)
My unscientific wild-ass guess is that it’s quite possible that time is this kind of thing, in the multiverse. Once you’ve split off into your own “universe”, where nothing you can do will affect anything in another “universe” and vice versa, any comparisons of time between the two are effectively meaningless.
Note that this interpretation allows talking about “before the big bang”, which may or may not be meaningful. My understanding is that there are at least two good possibilities on this:
- it really doesn’t mean anything to say “before the big bang”. Time didn’t exist. (I can’t reconcile this with Multiverse, but I’m no expert.)
- time existed before the BB, but due to the rules, we know we can say very little about the conditions before the BB. So, something was there, but it’s not terribly meaningful to talk about it.
I bet there are more possibilities. Sherlock was oversimplifying when he implied that you can identify all the possibilities!
if time is a wholly internal property of universes, then multiple universes (if such exist) cannot be said to be either sequential to, or concurrent with one another, unless that sequence or simultaneity happens within some higher time-like dimension (but that’s still not the same as what we think of as ‘time’).
I think that actually implies that it’s meaningless to talk about other universes ‘existing’ - because if they are not before, after, or now, then what are they? They don’t exist to us.
The fact that events in spacetime are partially ordered is pretty well-known, although physicists don’t usually use that particular language. Basically, we say that two spacetime “events” (by which I mean specific locations in space and moment in time) are ordered if and only if a particle traveling at the speed of light (or slower) can get from one event to the other. It is quite possible to send out light rays in two different directions from one event A that are received at events B and C, but in such a way that no light ray could possibly get from B to C or vice versa. Thus, A <= B and A <= C but B and C are not ordered.
The separation of the Universes in the eternal inflation scenario that Chronos mentioned above is exactly of this sort: all of the universes were very very close together in the early stages of the Universe, but due to the rapid expansion of the multiverse, there’s no way for light to possibly travel between the various Universes now.
Actually, the separation between universes is a bit stronger than that: Perhaps the inhabitants of Arcturus have the championship for Arcturan Mega-Ball, and that championship is spatially separated from our Superbowl, so that no light from the Superbowl can reach the Mega-Ball Championship or vice-versa… but it’s still possible for light from the Superbowl to reach the Arcturan Mega-Arena, just years after the event. Likewise, light from the Arcturan championship could reach the Superbowl stadium, years later. Similarly, an observer between Earth and Arcturus could watch both events, possibly even at the same time. With the multiple universes, though, light from either will never reach the other, no matter how long you wait, nor will light from both ever reach any common point.
Defining the future and past of an event or events is the stuff of whole chapters in textbooks on relativity.
The language of posets isn’t used here because reflexivity is not really a desirable quality (when defining the set of events in the past of an event or the set of events in the future of an event you don’t want those sets to include the event themselves). However the ability to define such a poset wrt to a time-ordering would be the same as saying that the spacetime is causal: i.e. it doesn’t contain any closed causal curves (a causal curve is one that is either timelike or lightlike).
The idea of a Universe bifurcating into two separate universes in relativity probably most fits a spacetime where two subsets of the spacetime have different topologies for their Cauchy surfaces. This is the same as saying that the spacetime is not globally hyperbolic. All globally hyperbolic spacetimes are causal, but the reverse is not true.