Brad Messenger

I am not expecting this thread to be long nor controversial. It is actually just a question I have regarding what it means to speak of the "Big Bang."

Should it be regarded as an actual event in which an explosion occurred? It seems for this to be true, there would need to exist a state of nothingness and then an expansion occurs. Since time "began" with the Big Bang, does it make sense to say there was once a pure state of nothingness?

Or should it be regarded as a description of the universe's rapid expansion and even continued expansion that humans can only compare to an explosion? In this case, the universe has always been in a state of expansion and it seems unnecessary to postulate a cause for something that did not really occur.

RealityCheck

The Big Bang is not an explosion. It is the rapid expansion of the universe itself in a very
short time.

The state of nothingness you refer to is actually a sort of ground state of the vacuum, which is not nothing. At the quantum level, there is a lot of energy and activity in pure vacuum. It is not a state of nothingness.

Alan Guth's book, <a href="http://www.amazon.com/exec/obidos/ASIN/0201328402/qid=1020682818/sr=2-1/ref=sr_2_1/103-2847656-4096614" target="_blank">The Inflationary Universe</a> is one of many that can answer your questions, but as far as I know, no one has been able to explain or prove the absolute
cause or beginning of the universe. IMO Alan Guth is one of many who have the best possible answer to how the universe began. There are quite a few other authors who explain the process well. John Wheeler, Steven Weinberg, Gerard t'Hooft, Richard Feymann, Paul Davies, to name a few.

Late_Cretaceous

"a state of nothingness" is a contradictory term. THere has to be something for it to be in a state, in this case a vacuum

Tim Thompson

Brad: Or should it be regarded as a description of the universe's rapid expansion and even continued expansion that humans can only compare to an explosion?

That's how I see it. Analogies are where we always wind up in science, on way or another. Is a classical electron really a "particle", or do we just use the "particle model" to describe it, because it's the best analogy we can come up with? Extend the problem to a more modern, quantum mechanical electron, and the "wave-particle duality" problem exposes the analogous nature of the explanation. We don't know what an electron (or anything else) really is, but "wave" and "particle" are the best physical & mathematical analogies we can come uo with, so we use the one that suits the purpose of the moment. But it remains evident that the reality of the situation is something we haven't hit on yet.

Likewise, we know that the "big bang' is not really an explosion. But what other analogy can we use, ans still preserve the maximum useful information about what the "bang" might really have been?

The best explanation that I can give, and avoid the "explosion" route, looks like this. We are in a universe of what we call space & time, which has some fundamental properties that we can measure physically, and describe mathematically. One of them is an apparent global radius of spatial curvature, a geometric property. That apparent global radius of spatial curvature is time variable. The analogy of an "expanding" universe, is a way of saying that the apparent global radius of spatial curvature evolves with time, in the direction of small (highly curved) to large (nearly flat).

That strikes me as the kind of explanation that somebody with a reasonable mathematical vocabulary could probably understand, though it is not without its own weaknesses (such as ignoring the curvature of time). But it's pretty hard to get that kind of explanation through to somebody who doesn't really grasp the concept of curvature. But it has the advantage of concentrating on what one would see & measure from a vantage point inside the universe. The idea of a vantage point "outside" the universe simply cannot be treated as anything that it is possible to understand physically, but can be seen only as a mathematical speculation (or "recreation"), at least for the time being.

Brad: In this case, the universe has always been in a state of expansion and it seems unnecessary to postulate a cause for something that did not really occur.

But I'm not so ready to go along with this. Even if you use a more complete explanation than mine, and include the curvature of space-time together, I can't see how to construct a classical theory that avoids the initial condition of zero curvature, which is about as close to "nothing" as I can think of off hand. That initial condition of zero curvature corresponsd to the physical initial condition of the "bang". If you stick to classical physics (i.e., general relativity), it's a real issue. Indeed, Stephen Hawking's PhD thesis was a formal proof that the initial condition of zero curvature (i.e., "singularity") is unavoidable in general relativity. So there is something that needs explaining, or the simple admission that it is unexplainable, in a strictly classical cosmology based on general relativity theory.

However, we now know that classical physics has to supplemented by quantum physics, and that a mathematically complete cosmology requires both classical & quantum elements. Quantum gravity & string theory are the two most common current efforts to combine general relativity & quantum theory. The gross details are worth a few hundred pages at least, and are far beyond any ability to settle here. Suffice to say that you can construct cosmologies in which the "bang" does not happen, in the classical sense of "first there was nothing" (where "nothing" is the real thing, and not some quantum version of "vacuum"). Maybe the "bang" was a quantum fluctuation, or something analagous to that, in a super vacuum. Or maybe it was the visual manifestation of colliding branes in string theory, or something more exotic but not yet mentioned.

Maybe the universe is infinite, but I don't think it can be infinite in any classical theory. Only quantum physics opens the door to an temporally and/or spatially infinite universe. At least, that's how I read the cosmic tea leaves.