[photon]

Originally Posted by GoinAllTheWay

So basically, the universe is currently expanding due to the big bang.

Actually the big bang is the term used for the observation that the universe is expanding (or more precisely the big bang refers to the history of the universe, that it was once very dense and very hot, and now it's less dense and very cool).

The expansion is due to dark energy.

Originally Posted by GoinAllTheWay

I guess the jist of the show is they were trying to figure out if it's speeding up or slowing down and what would cause either effect. The got into the whole dark matter/dark energy thing and concluded that the rate of expansion in the universe is in fact accelerating. Which got me to thinking. Everything we see in the sky is basically a large debris field from the big bang which to me suggests the size of all this debris field could be somewhat estimated. Does that mean beyond all the stars/cosmic dust/planets is just really empty space?

The big bang is really a misnomer, it's not really an explosion in the sense that one would imagine an explosion. It's just something getting bigger.

But yeah you can look at the debris field (all the galaxies) and how fast they are moving, run everything backwards and come back to a point where everything was on top of each other.

But as far as we know there's no "beyond" all the galaxies and such. Think of the surface of a balloon, if you live on the surface of it there's no such thing as beyond the surface of it, but the surface can get bigger by expanding in all directions, there's no beyond.

Originally Posted by MarchHare

According to the theory of general relativity, gravity is a curvature of space-time caused by objects with mass. Because the universe is so massive, it actually curves back in upon itself, so there isn't an "edge" in the traditional sense. If you could fly a spaceship in a straight line forever, you'd eventually come back to your starting point, similar to an ocean liner or aircraft circumnavigating the Earth.

That's only one solution for GR though, there are other solutions which involve a flat universe or a universe curved the other way so that it's unbounded (you could fly in one direction forever).

There's dark matter and dark energy to consider now as well.

From experiments space appears quite flat, but it's unlikely it's actually flat since that would be a very specific solution to GR for Λ (lambda, the cosmological constant).

I think we know that the universe, if it is curved inwards, is bigger than the observable universe though since we don't see patterns being repeated in galaxies or the CMBR, which we would see if the universe was smaller than the observable universe.

But it could very well be infinite spatially.

I don't know if we'll ever know for sure because the speed of light puts constraints on how much we can observe.

You really want to make your brain melt? Try to imagine how a universe that is (and would have always been then) infinite spatially could once have been dense and has since expanded..

Hope that helps until the real astronomer shows up and corrects me.