Are there any scientific (non-religious, non-creationist, non-intelligent-design) theories explaining the development of the universe, that are alternatives to the Big Bang theory? Or has Big Bang made the transition from theory to established fact?
I’m not a cosmologist by any means, but I think the only alternative to the Big Bang was the continuous creation model which even it’s strongest proponent, Fred Hoyle, gave up on when quasars were finally identified. His reason for ceding was that continuous creation requires that the universe be uniform in composition to as far as our telescopes will take us. But the quasars are at great distances and thus existed at the earlier stages of the life of the universe and nothing like them is found existing at later times.
And I wouldn’t use the term “established fact.” All scientific theories, like Newton’s theory of gravitation, are subject to change without notice if new information requires it.
IANAC. I believe the basic tenets of the Big Bang theory are agreed to by the majority of the scientific community. There are disagreements about some aspects of it. There are observed phenomona that do not seem to fit very well. For these reasons, I would consider it by no means “established fact”, but it is the reigning consensus.
Dagnabit, David Simmons beat me to the punch while I’m Googling for citations. Here are two good links on the subject.
It’s also worth noting that the details of the Big Bang aren’t entirely worked out. We’re pretty sure that the Universe, at some finite time in the past, was a hell of a lot denser and hotter than it is now, but we’re pretty sure that we can’t extrapolate back in time beyond a certain point (about 10[sup]-42[\sup] seconds after the creation of the Universe), and as it stands if we extrapolate all the way back to the beginning of the Universe we get things — like infinite densities and temperatures — that even physicists find a little suspect.
There are also some modifications to the “Standard Big Bang” that have been proposed. Two such modifications that I can think of off the top of my head: Inflation says that some force (as yet unexplained) drove the Universe to expand incredibly fast in its early stages. This theory is gaining more & more acceptance (but there are a few disbelievers out there, and it generally isn’t regarded as completely established.) Another, the ekpyrotic scenario involves two three-dimensional Universes colliding in some fourth spatial dimension, thereby releasing the energies seen in the Big Bang. Needless to say, this is a theory only a string theorist could love, and is generally regarded as rather speculative.
What about Stephen Hawking?
The thesis of A Brief History of Time, assuming I understood it to a meaningful extent, was that when we conceptualize the Big Bang we continue to think of time as a linear phenomenon when we extrapolate backwards, thus ending up with a model where everything began with a singularity and instantaneously began rapidly expanding. Hawking says we should consider that time itself would be bent by the dynamics of so much matter in such a small space, so that as we extrapolate backwards, the functions that we overlay on the normally linear progression (in this case, reverse progression, since we’re looking backwards) need to be overlaid instead on an increasingly curved path that makes less and less progress towards the physical singularity, such that (let’s say) 12.5 billion years ago the universe was a hot and comparatively small place expanding rapidly, but as you “try to look before that”, the curvature of time simultaneously prevents there from having been such a time as you might be trying to look back to, and generates less and less regression towards a singularity which never at any point existed.
So instead of a Big Bang you get a Big Unfolding or something of the sort.
Wasn’t there something called Steady State theory at some point?
Yes there was, and was disproved via observations of the cosmic microwave background (CMB). Basically, in a steady state universe, perturbations that are seen in the CMB would not be seen. There was some thought that the pertubations could be caused by scattering off stars. However, this then poses the problem of homogeniety - why does the CMB look the same in all directions?
The big bang can explain the homogeniety and the perturbations in the CMB, but then we end up with all sorts of problems when we invoke “inflation”, faster than light expansion, and the laws of physics breaking down.