Expanding universe

[Policenaut]

I am interested in space but have no real knowledge about it besides what I hear on tv shows and from Mr. Tyson. The universe is expanding. That seems to be a fact. What is it expanding into? I have read that it depends on if the universe is finite or infinite and there seems to be no answer as to which it is right now. But pretend it's finite. If so is there something outside the universe that the universe is expanding into or is there something beyond space that the universe can't expand into? I can't really wrap my head around it. If the universe is infinite then there is nothing but the universe but if it is finite then from what I have read people still say that there is nothing else but the universe.

[edd]

Broadly speaking we don't know. It might not be a meaningful question, or it might turn out that there is something. What we can say is we don't have any way of telling, unless perhaps some theory which makes predictions about it has other effects that we can see.

http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

I think that link puts it well when it says:
"Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about"

Currently, our standard theories basically don't tell us. So we'll have to wait and see if someone has a better idea that turns out to be favoured by our experiments.

I don't think it particularly depends on whether the universe is infinite or not though.

[fuelair]

The really bad news on this is that there is zero chance that any of us alive now will ever know (assuming we are right about no afterlife - and if wrong, the afterlife has has no inquiry functions).

[Ziggurat]

What is it expanding into?

In mathematics, people sometimes distinguish between what is known as extrinsic geometry and intrinsic geometry (http://en.wikipedia.org/wiki/Differential_geometry#Intrinsic_versus_extrinsic).

Let's consider a simple example: the surface of a sphere. This is a 2D space, but it's not Euclidean. Parallel lines intersect and the angles of a triangle add up to something more than 180 degrees, for example. You can think of this 2D non-Euclidean space as being embedded in a 3D Euclidean space, and in this higher dimensional space, everything remains Euclidean. So you can figure out everything about your 2D space from the way in which it's embedded in your 3D space. Distort the 2D surface within your 3D space, and you change the curvature properties. This is extrinsic geometry.

But it turns out, you don't actually need to even consider the 3D space you've embedded your 2D surface into. The curvature properties of your 2D space can be fully specified on their own, without reference to any higher dimensional space. This is intrinsic geometry. For someone confined to such a space, there is no difference between the space having only these intrinsic properties and the space being embedded in a higher-dimensional space with extrinsic properties.

In regards to our universe, your question assumes an extrinsic geometry perspective. And that's a fairly natural way to think about things since the world seems Euclidean and 3D so that's the easiest way to think about other dimensions and non-Euclidean spaces. But the thing is, it's not a necessary perspective. Intrinsic geometry is the only thing that's actually required. It also happens to be the only thing we can actually measure.

So given that we can't measure any embedding, and given that we don't actually need any embedding, can we even say meaningfully that anything outside our universe exists? No, we cannot.

[Aitch]

Are you sure the universe is expanding? Maybe it stays the same size, but the contents are shrinking... :boggled:

[fuelair]

Are you sure the universe is expanding? Maybe it stays the same size, but the contents are shrinking... :boggled:

And, how could you tell the difference?:confused:

[JoeTheJuggler]

I am interested in space but have no real knowledge about it besides what I hear on tv shows and from Mr. Tyson. The universe is expanding. That seems to be a fact. What is it expanding into?

Space itself is expanding. It's not expanding into some other external space.

Think of it instead as more space coming into being between the galaxies (or clusters).

People use the idea of putting dots on a rubber balloon, and inflating it, but that analogy only works if you treat the surface of the balloon as a 2-d space (and ignore the fact that the balloon rally is expanding into existing space). So thinking of the 2-d surface, just imagine that more space is coming into existence between the dots.

Imagine more vacant farmland between St. Louis and Kansas City rather than pushing KC west or STL east.

When thinking of the Big Bang, it's misleading to imagine yourself outside the universe watching it expand toward you.

[JoeTheJuggler]

But it turns out, you don't actually need to even consider the 3D space you've embedded your 2D surface into.

That's what I meant when I said to ignore that an expanding rubber balloon really is expanding into existing space around it. Instead just think of the 2D surface.

Zig said it more betterly than I did.

[Policenaut]

Ok so I think I have a better idea about this now. Thank you all. Science is fascinating but I need things put more simply (especially regarding space) to understand them. I read another simplification where the example was the universe as an unknown size of dough and the galaxies as raisins in the dough. That also helped a lot. Now to all you smarties I have another question. According to what I have been reading the expansion is accelerating. Since it is accelerating does that necessarily mean this force going to get exponentially stronger over time?

[Aitch]

Are you sure the universe is expanding? Maybe it stays the same size, but the contents are shrinking... :boggled:
And, how could you tell the difference?:confused:

At the risk of being serious, would you be able to? Would the maths work in both cases?

[KingMerv00]

At the risk of being serious, would you be able to? Would the maths work in both cases?

I don't think so.

In order for it to make sense the forces of the universe would have to scale in a proportionate way along with the mass. If matter was shrinking but the forces didn't, you'd notice.

Also it would have to explain why matter farther away is shrinking faster than matter close to us. If we are shrinking, everything would recede at the same rate.