Expansion Explained please

We cannot know, what was before the "big bang" as the "big bang" created there and then (space and time). Before, there might have been something or nothing at all or ...[This message has been edited by Alex, 11-13-2003]

ok I understand there was nothing before the big bang, but how do you get something from nothing?

The model does not predict that there was anything before the big bang. With some assumptions, GR when applied to the the early universe does predict a singularity, ie. a beginning of spacetime. However, the statement, "there was nothing before the big bang" is logically equivalent to "there was no before the big bang". So to avoid confusion, there is no mystical state called nothingness in which the universe emerged from. It's just a negative of the english language.I should point that few cosmologists would be confident in claiming the universe truely begins with a singularity, since GR is not valid at the quantum level involved in such conditions.

This is how I conceptualize it. Keep in mind that these are just analogies, and not literal descriptions.If you look at this ordinary x/y/z graph, imagine a sphere like this one drawn on it with the center of the sphere at the origin. The origin in this analogy represents the Big Bang singularity, and the surface of the sphere represents spacetime in the present moment. Now, imagine the sphere inflating along all the axes simultaneously and that gives you time relative to the Big Bang. In other words the radius of the sphere from the origin to the surface would be the measure of time from the Big Bang to that point. Notice that no matter what point you reference on the surface of the sphere, that point is always a positive distance from the origin or Big Bang. That is to say, every point in spacetime lies in the future of the Big Bang. Therefore there is nothing "before" the origin.Alternatively you can conceptualize it this way: Imagine now a different sphere like this one. Notice that this sphere has "latitude" and "longitude" sort of mapped out on it. Let's call the hindmost convergence of the longitudinal lines the Big Bang. It's the very faint one in the background of the sphere. Time relative to the Big Bang will thus advance from the rear of the sphere forward. You'll notice how the latitudinal circles get larger as they move away from the Big Bang. This is analagous to expanding spacetime. Now, once those circles reach the middle of the sphere, they begin to get smaller until they reach another singularity at the convergence in the foreground. This is sort of how the universe is conceptualized in a Big Bang-Big Crunch cosmology. It may be, however, that spacetime will not converge at the Big Crunch but instead continue to expand forever. If that is the case, then the universe might be more like a cone.Remember that each of these is just an analogy and must be in some sense incomplete since they are illustrated in three dimensions and spacetime has at least four dimensions, and it may be that the universe has many many more than those.Hope that helped.[This message has been edited by ::, 11-13-2003][This message has been edited by ::, 11-13-2003]

How you get "something from nothing" is a question being asked by physicists and cosmologists right now. There are a number of ideas.For one thing "something from nothing" happens all the time. There are "virtual particles" appearing in matched pairs from the "vacuum". This is supported by the "casimir effect" (google that).The idea of "branes" is a new speculation about a "universe" (or whatever you would call it) of 11 dimensions in which 4 dimensional objects move forever (no beginning, no end). When these things ("branes") collide the energy of this "collision" shows up in the brane as a "big bang". Note this is a speculation. There is no experimental evidence for it and I don't think there is any complete math either (as if I would be able to understand it anyway).To summarize:
We don't know. There are lots of different ideas. Some may actually lead to testable predictions. It is mind warping. It is fun!

Thank you for your replies I am currently reading about Branes in Stephen Hawkings, The Big Bang for Dummies (actually The Universe in a Nutshell)

Well... I would say that the appearance of matched pairs of virtual particles in a vacuum is evidence tthat there really is no such thing as "nothing" in reality. When you think about it on a universal scale, the notion of there existing a state of complete "nothing"-ness is really nonsensical. What I mean is that is doesn't make any sense to try to conceive of a state of being that is a state of total non-being (i.e nothing). If it truly were a state of total non-being, then it couldn't exist by definition! If it did exist, then it would no longer be total nonexistence.Keep in mind that I mean this in the case of actual reality. "Nothing" is still a meaningful abstract concept. It's just that the idea of "something from nothing" supposes that there is a state of actual pure non-existence in reality. There is no "something from nothing" in reality because if there were "nothing" there would be no reality. "Nothing" is non-reality. "Nothing" is not real. Instead there is just something, always.

A quantum vacuum is something in the ontological sense. So is the pre big bang void proposed by folks like Alan Guth, where spacetime doesn't exist but the laws of physics do. If the big bang was caused some pre existing state, it's much easier to reply to the question with "there was something else before our universe". Calling such states "nothing" is silly, and will result in creationists being more confused than they already are.

Ok, I'll go along with you that one definition of "nothing" doesn't make sense. That is a bit of why I had it in quotes in the first place. I think one problem is that English is not prepared to talk about this very well. I sure don't know the right words.