Model #1: The Universe out of Nothing
Vilenkin noted that if we were to somehow start with a small bubble of a universe, two things could happen. If it were large enough, it would undergo exponential growth — just like our universe did in the first instants. If it were small, it would collapse.
What happened before the Big Bang?Here's where things get weird. Quantum mechanics predicts all sorts of strange things, including half-dead/half-alive cats, or the possibility of teleportation. It also predicts the possibility that apparently impossible things are really just improbable. Image by CottonIJoe/Flickr.
For instance, it's possible (but brain-bendingly unlikely) that you could spontaneously find yourself teleported to Alpha Centauri (readers: please insert obligatory Hitchiker's reference here). More commonly radioactive decay can be thought of as a small piece of an atomic nucleus that shouldn't really be able to escape from the rest somehow randomly tunneling away. The universe is just like that sometimes.
In the same way, a small universe can randomly tunnel into a larger one. The amazing thing about Vilenkin's model is that even if you make the "little" universe as small as you like, this tunneling still can occur. It even works if the little universe has no size at all. You know what we call something with no size?
Nothing.
Prior to the Big Bang, the state of the universe was something that possessed (no fooling) zero size and for which time was essentially undefined. The universe then tunneled out of nothing into the expanding universe we know and love.
The problem is that the "nothing" that the universe popped out of wasn't really nothing. It had to know about quantum mechanics somehow, and we've always been taught to think that the physics is a property of the universe. It's troubling to think that the physics existed before the universe did, or, for that matter, before time did.
Of course, this is the basic problem with any definite origin for the universe. Somehow all of the complexity had to be created from nothing, and it's difficult to reconcile that.
The other possibility seems equally troubling. The universe might literally be eternal — or at least have an infinite history. While it's not clear what the theological implications of an infinite universe, we can at least try to figure out how an infinite universe might work.
