coktail said:
Oookay.
So is it fair to say that multiple universes, each with their own big bang or big bounce or whatever, and far enough apart that they don't influence each other, do NOT fit into the current mainstream thinking?
As I understand it, this scenario is impossible (according to the mainstream) because it implies that something can exist outside of the universe, which in this case would be another universe.
Maybe I've veered way off track here. Thanks for following me.
Well, the big bang isn't something that can 'happen'. Remember that the big bang is the term referring to the
moment in time at which the
entire universe was filled with a hot dense plasma. So, saying that multiples big bangs happened is a meaningless statement, since it refers to one moment in the history of the universe.
So yes, the scenario is impossible because you need to specify what you mean by several big bangs happening. There is one very viable scenario called 'eternal inflation'. I'll explain firstly what normal inflation is.
There are a few problems with the vanilla big bang model - first is the flatness problem. We observe that the universe is very, very close to being spatially flat. Since expansion would cause the universe to deviate away from flatness, it must have been even flatter at the time of the big bang. Ridiculously flat. How did it get this way? Second is the horizon problem. We observe that the universe is homogeneous on large scales, that is, it's pretty much the same everywhere. This means that primordial plasma must also have been perfectly homogeneous, which is confirmed by observations of the cosmic microwave background. However, if the expansion of the universe was extremely rapid from time zero onward, how did this plasma come to equilibrium? It certainly wouldn't have the time to do this. And third is the monopole problem. Grand Unified Theories, or GUTs, are theories that unify the electroweak interaction with the strong nuclear force. They have the unfortunate feature of predicting that hot temperatures of the early universe should have produced an abundance of heavy magnetic monopoles, which we certainly don't observe. Fourth is the homogeneity problem - why are there no inhomogeneities besides galaxies? What made the early plasma so 'smooth'?
Inflation fixes all of these problems. Inflation proposes that the very early universe underwent an enormous expansion, growing the universe by at least 60 e-folds. This expansion would be driven by the inflaton field. This field would reach an undesirable energy value, called a false vacuum. When it's in this false vacuum, it has the property that it exerts an enormous negative pressure (somewhat similar to dark energy). This drives inflation. After a very short period of time, the inflaton field reaches it's true vacuum (through normal quantum effects such as tunneling). When this happens, it decays into a bath of radiation, heating the universe so that the big bang model can go from there.
So, how does this solve the problems of the big bang model? Well, the enormous expansion would eliminate any curvature, making the universe extremely flat. This solves the flatness problem. Second, it would allow the universe to expand very slowly before inflation, allowing it to come to equilibrium. This solves the horizon problem. Any monopoles produced in the early universe would have been spread out so that we would only see about one in the entire observable universe, so the monopole problem is solved. And finally, inflation would 'iron out' any large scale inhomogeneities with the rapid expansion.
So, what is eternal inflation? The idea is that inflation ends only in certain places, so that these slowly expanding regions are separated by the eternally inflating region in between them. So, each of these slowly-expanding would develop independently, forever separated by the continually inflating region.
That is the closest thing I can think of to what you are asking about. Since the big bang is the time when the universe is filled with a hot, dense, plasma, these separate slowly expanding regions would all be undergoing this process separately, and so we could see that separate big bangs are occurring. But it's a bit of an abuse of terminology.
