gugglebum said:
We have all heard the theory that the universe is expanding - which has always made me think: Expanding into what?
If the universe is supposedly all there is, how could it be expanding? Something without a border can't really expand, can it?
Also, if it's expanding, into what is it expanding? There must be something on the other side into which the universe is expanding.
Sorry I know this isn't very scientific and probably very misinformed but I've always wondered what the latest research and theories on this are? Anyone care to enlighten me? In layman's terms if possible, I'm not much of a scientist!
The confusion comes from two different definitions of the word 'expanding.'
Usually when you think of something 'expanding,' you think of looking at something and seeing it
growing larger. Obviously this definition only works for finite objects which you can imagine being 'outside' of. By definition you can't be outside the universe, and it may not even be finite.
The proper way to define 'expanding' here is to mean that
distances between things are getting bigger. This is what scientists mean when we say the universe is expanding. Pick any two galaxies, and they're moving further apart. And the greater the distance between them now, the faster they're moving apart. It's sort of like the picture below:
You can see how this picture could extend forever.
akoto said:
If gravity could be strong enough to overcome expansion and maintain our supercluster, why not hold superclusters together?
Excellent question! There's a short answer and a long answer. The short answer is: Because gravity is stronger at smaller distances. Imagine a universe filled with clumps of matter flying apart; whether or not the universe sticks together depends on how far apart the clumps are. But whether or not each clump stays in one piece just depends on its size & mass. So make each clump small enough and it'll hold itself together, but spread the clumps out far enough apart, and their mutual attraction is too weak to hold them together.
The long answer: Well, if you start with a perfectly homogenous expanding universe -- that is, a universe which is exactly the same everywhere, kind of a uniform fluid or plasma -- then yes, in fact, either everything will stick together, or everything will get pulled apart. The only way you can get things like clusters which individually hold together but separate from each other is if you have some kind of unevenness. I mean, some kind of perturbation which makes
this part of universe slightly more dense than
that part of the universe. Our universe had such tiny, tiny perturbations within the first fraction of a second after the Big Bang.
If you use Einstein's theory of gravity (general relativity, a.k.a. GR) to figure out what would happen to such a perturbation, you find the following: For a given environment (the density of the universe, the speed of sound in it, its rate of expansion, etc) there is a length called the
Jeans length. If a region
smaller than the Jeans length is slightly more dense than its surroundings, it will undergo a chain reaction: since it has more mass than the surrounding part of the universe, its gravitational pull is slightly stronger. Since its gravitational pull is stronger, it pulls more mass towards it, making it even more dense and its mass even greater, thus increasing its gravitiational pull even more.... You get the idea. This process is what causes the collapse of matter into things like clusters, galaxies, stars, and planets.
However, if a region
bigger than the Jeans length becomes slightly-more-dense than its surroundings, nothing happens -- the expansion (and pressure) of the universe is too large and overpowers the slightly bigger gravitiational pull of the region, smoothing things out. So there's a maximum size of the bodies that collapse -- no objects bigger than the Jeans mass can form in a perpetually-expanding universe. That's why all superclusters are destined to pull apart forever (or at least as long as the universe continues expanding).