Vebyast
Nascent Transhuman
Nnnnnnno, that's not quite right.IIRC, the size of the observable universe is actually constant-ish and is based on the fact that light gradually disperses as it hits other stuff, and that space isn't actually 100% empty.
Observable Universe is the max distance away from which light can reach us without getting completely dispersed first.
Light has a maximum speed. The universe is not infinite, and it's not constant in size. The universe - the whole thing, not just the observable part - is constantly expanding. However, it's not making more of itself at the edges like you might think; rather, every individual bit of space is pulling apart and more space is being added in between the bits that area pulling apart. Imagine drawing dots on the surface of a rubber balloon and then blowing the balloon up - more area, more universe. The speed of light never changes - balloon twice as big, light takes has to go twice as far, takes twice as long. The thing is, the rate of expansion is not constant. It's pretty slow right now, but for a few very very short bits of time at the very beginning of the universe, the expansion was actually happening far faster than the speed of light. The observable universe is the part of the universe from which light could have reached us by now. The thing is, because the universe expanded faster than the speed of light for a while, there are parts of the universe that receded from us faster than light could reach us from those parts, and those parts eventually got so far away that light can't have reached us from them simply because of the speed of light. The observable universe is the part of the universe that light could have reached us from. Imagine your balloon again, almost completely flat so it's not very big. Now imagine that there's an ant on it, crawling in a straight line as fast as it can. That's light. While the balloon is uninflated, the ant could reach almost any part of it in a matter of seconds. Now blow the balloon up really fast. Now the area that the ant could have reached is tiny, barely a tenth or a hundredth of the total surface area of the balloon. That's how the observable universe works.
But wait, you ask, what about the light that was there when the universe was inflating? What happened to it? Well, that's actually how we know all of this: the "cosmic microwave background" is all that primordial light from the big bang, just drifting around in space. We can infer the structure of the universe as it was before and during inflation by looking at the huge, scale-of-the-universe structures present in the cosmic microwave background. Like if you tie-died your balloon before blowing it up, you can look at the bit of dye and the crinkles in it in the little bit of the universe that's observable and figure out that the entire balloon was at one point all crunched up into a tiny bit and then tie-died.
The observable universe is expanding a very slightly less than a light-year per year. It has nothing to do with interstellar gas or dispersion or anything, and everything to do with the speed of inflation and the speed of light.
edit: augh, ninja edited.