If there is nothing beyond the universe ....

[MrFun]

what is the universe expanding into??

[Aeson]

Nothing?

[marylove]

thank you very much

[Lorizael]

This is a misunderstanding of what it means for the universe to be expanding. The current understanding is that the universe is infinite in extent (because it is, by definition, everything) and has been for all time. The universe "expanding" simply means that the distances between two points in space are growing with time. You can think of this distance as just a property of the universe, like its shape or its density or anything else. The dimensions that we experience the expansion of space through only have meaning within the universe, so it doesn't really make sense to talk about the universe's dimensional extent (size).

Think about it this way. Take a bowl of water. If you want to measure the volume of an object, you can put it in the bowl and measure how much water the object displaces. But what do you do if you want to measure the volume of the water itself? You cannot ask how much water the water displaces, because the water in the bowl is what you use to measure the displacement of water. The measurement system of "water displacement" only has meaning in the context of objects within the water. Similarly, the measurement system of "distance" only has meaning in the context of objects in the universe.

[MrFun]

This is a misunderstanding of what it means for the universe to be expanding. The current understanding is that the universe is infinite in extent (because it is, by definition, everything) and has been for all time. The universe "expanding" simply means that the distances between two points in space are growing with time. You can think of this distance as just a property of the universe, like its shape or its density or anything else. The dimensions that we experience the expansion of space through only have meaning within the universe, so it doesn't really make sense to talk about the universe's dimensional extent (size).

Think about it this way. Take a bowl of water. If you want to measure the volume of an object, you can put it in the bowl and measure how much water the object displaces. But what do you do if you want to measure the volume of the water itself? You cannot ask how much water the water displaces, because the water in the bowl is what you use to measure the displacement of water. The measurement system of "water displacement" only has meaning in the context of objects within the water. Similarly, the measurement system of "distance" only has meaning in the context of objects in the universe.

Interesting.

I'm not sure what you mean by the universe having a shape, though. If the universe is everything and infinite, then there is no edge or boundary of the universe which would define its shape.

[Lorizael]

Well, that's only tangentially related to the discussion, but the universe does have a shape. The usual example here is a sphere. If you're a two dimensional bug living on the surface of a sphere, then the sphere has no boundaries or edge. You can pick any direction and just keep on going forever. Yet a sphere most certainly has a shape. It's just a shape that can only be fully realized in three dimensions. But even thinking only in two dimensions, the surface of a sphere has geometric properties. It has a certain amount of curvature, which you can measure by finding the distance to the horizon.

Similarly, the universe has a degree of curvature which can be measured (in a much more complicated fashion). All measurements to date show the universe having essentially no cosmic curvature to it. And similarly, you can think of the universe's "true" shape as being something embedded in a higher dimension, such that it can have no boundaries but still have geometric properties. It's important to remember, however, that thinking of the universe as being embedded in another dimension is only a useful visual aid; the universe simply exists and is embedded in nothing.

[self biased]

http://youtu.be/XBr4GkRnY04

:wiglaf: VIDEO EMBEDDING IS NOT MY FORTE.

[Lorizael]

Veritasium is rad. Also, Minute Physics.

[Lorizael]

Here, btw, is MinutePhysics' explanation of the expansion of the universe:

[Dr Strangelove]

I recall it being said last year that the data which confirmed the existance of the Higgs-Bosun particle means that in several billion years our universe will terminate. Does this happen everywhere all at once? Does it mean a new universe will be born in its place?

[Lorizael]

The only thing that's going to happen in the next several billion years is that our sun will die.

As far as the universe ending, the medium answer is: if there are no new physics and we've basically figured everything out (a very bad assumption), then current theories predict that the universe is intrinsically unstable and could be destroyed by the sudden appearance of a new universe within our own... or something. And this would be bad.

However, even if that's the case, our universe is still mostly stable (metastable) and is expected to live for a period of time that is significantly longer than its present age. (If you google this, you'll see the phrase "billions of eons" tossed around without eon being defined.) Regardless, this is only a possibility if the current model of particle physics is exactly correct and the presently known values for the masses of various particles are exactly correct. Basically any change in the theories or numbers (which is expected in both cases) means the universe is stable rather than metastable and there's no possibility of it suddenly ending.

[MrFun]

That video was interesting.

I always had the understanding that our universe began as something smaller than an atom and that in billionths of one second, it expanded larger, into something of the size of a baseball, and so on and so on.

So documentaries that try to explain the Big Bang, such as How the Universe works, with one of its entire episodes on just the Big Bang, are entirely false, according to the narrator of the above video.

[Lorizael]

So, I'm no expert when it comes to this stuff, but my understanding is that the observable universe--with all the mass and energy we can account for--was condensed to a singularity-ish point, but the observable universe is not the whole universe. Don't take my word as gospel, though.

[Lorizael]

Don't take my word as gospel, indeed. I've done a little more reading on the subject and what I said above is almost certainly wrong.

So before the Big Bang was cosmic inflation, when the universe expanded exponentially. What inflation suggests is that the universe didn't need to begin as a singularity. Think about an exponential graph. As you go off to the left, the graph asymptotically approaches zero but only gets there at negative infinity. This holds for the universe. At a time infinitely before the Big Bang, the universe may have been a singularity, but we can't know because we don't yet (and possibly won't ever) have the tools to look at what happened during/before inflation.

But the point is this: if inflation is correct (and some form of it looks to be), then ~13.8 billion years ago, when the Big Bang occurred, the universe was decidedly not a singularity. It was some fraction of the size it is today, and it was quite hot and dense, but it wasn't infinitely dense or hot or small.

[Dr Strangelove]

I thought the apearence of a new universe sometime within several billion years was predicted by the results of the LHC experiments that proved the existence of the Higgs Bosun? Would this new universe be like ours? Would it simply wipe out ours all at once or would it start somewhere and expand outwards over a period of time?