Jesse

You could say the area of each square approaches zero as time approaches zero, the moment of the Big Bang itself. But general relativity is a continuous theory, so there's no problem talking about points (with zero size) in spacetime, just like we can talk about real numbers as points on the number line. Singularities are points where a finite amount of mass/energy becomes compressed into a point of zero volume, resulting in infinite density--this would be true of the point that's the Big Bang along with the point that's the center of a black hole. But quantum gravity may show that this prediction of general relativity is wrong, and that singularities won't actually turn out to have real physical existence in our final theory of gravity.

Kat_Somm_Faen

Jesse,

I was under the impression that "closed" reffered to the system as in the physical definition of a system i.e. nothing is coming into the system through the system boundary?

Jesse

That's the meaning of "closed" in thermodynamics, but it means something different in the context of cosmology: there, "closed" refers to overall positive spatial curvature, as opposed to zero curvature (flat) or negative curvature (open). See this page for some more details.

schu

If the universe is expanding, what is it expanding into?

If the galaxies we see are 14 billion light years away are we not seeing them as they were and where they were 14 billion years ago? Can we know where they are now and what they look like?

Wiploc

First, let me establish my credentials, so you'll know how seriously to take me:

1. I've read some Asimov.
2. Some of my friends know something about physics.

In other words: no credentials. As Bokonon said, "All the sweet truths I am about to tell you are lies."


It seems to me that you are mixing metaphors here, or mixing paradigms. If you think of the Bang as happening everywhere, then you don't want to think of things as coming from it. If you think of stuff as coming from the Bang, then you want to think of the Bang as tiny and in a specific place and time. Both ways of thinking about it are legitimate.



I don't think anybody believes this anymore. It is an old relativist model. Quantum mechanics rules the small stuff, and the Bang started small. If we "play the film backward," and watch the universe shrink towards the singularity, we see that it doesn't actually get there. Either it is unbounded in some sense (curving off into imaginary time; or like a circle not quite touching a point; or does a Zeno's Paradox because time is a ray with a circle on one end rather than a dot; or whatever) or it just stops. If it stops, it stops when it is small enough for everything to everything jump to the singularity in a single quantum leap.

So, playing the film forward again, the universe starts (if it starts at all) just "after" the singularity. The first frame of the film shows a finite universe. There is no frame showing a singularity.


In what sense are you thinking it's infinite?



All of those formulations seem somehow misleading. I'm tempted to say that spacetime _is_ the big bang.

How about, the Big Bang is an explosion _of_ spacetime?

Picture a very small (point sized) firecracker. Now remove the rest of the universe so that nothing but the firecracker exists. Set off the firecracker; see the universe expand. Is there time? Yes, because there is change. Is there space? Yes, because there is matter. But, did the spacetime "come from" the firecracker? No, because, uh, you know, it's like spacetime is a description of matter in motion, or an aspect of it, or a parameter of it. Spacetime doesn't come from matter in motion in the same sense that acceleration doesn't "come from" a sports car.

The Big Bang isn't an explosion _in_ spacetime because there is no spacetime without the explosion. The explosion fills every bit of space and time, and the spacetime exists only because of the explosion. It is tempting to say that the explosion _is_ spacetime.

Spacetime isn't "within" the Big Bang any more than the Big Bang is within spacetime. Neither can exist without the other.

Your question is something like asking whether circles are caused by radii or radii are caused by circles. Spacetime and matter-in-motion are more closely related than Siamese twins.

I hope this helps. And I hope someone corrects it if it's wrong.

crc