People tend to say that the universe is infinitely large. However they also tend to say that the universe is expanding. If you know that something is expanding, you know it is increasing in size. As infinity has no end and cannot therefore expand, you know that it is not infinitely large, just very very big. Nearly infinitely big.
However, the theory goes, you can travel in any one direction for infinity, even if the universe was not expanding (or contracting).
This probably seems mutually exclusive. While it is possible to travel around the planet in the same direction forever, you can only do so because it is not in a straight line, but rather a curve that brings you back to your starting point (more or less, it's not a perfect sphere). But the universe is not unrelated to the concept.
Imagine you are a one dimensional being. Everything is either in front of you or behind you. There are no angels, there is no up, down, left, right, or catty-corner. Just one line extending forever. Let us pretend that there are such things as landmarks in this universe, so that you can know where you are. You go forward for a very, very long time, and suddenly you find yourself back where you started. But you went in the only direction possible other than backwards: forwards. This would be impossible for you to comprehend.
The reason, of course, would be that your universe is a circle, a two dimensional ring. A very very big circle, but a circle nonetheless. It would seem infinite to a one dimensional being (especially without the possibility of landmarks as there is only the line), but it isn't, it is a contained universe. It could get bigger or smaller, but it would stay a circle without end.
Now, let's step it up a bit. You're a two dimensional being. You can go any direction that you could draw on a piece of paper, all 360 degrees. Much more freedom than the one dimensional universe. But, the same basic concept applies, with the circle having an extra dimension: the third. It is now a sphere. You can travel forever in any direction or combination of directions, but it is not infinite. It is simply beyond comprehension for someone with no sense of up or down.
So that's the theory on our universe, that it is, essentially, a four dimensional circle: a hypersphere. We could go forever in one direction, and eventually end up right back where we started (of course our planet, star, and galaxy are always moving, so there probably won't be much there). The universe is not infinite, but it is infinitely confined. You just can't quite picture it because two of the directions have no meaning. Call them in and out.
Here is were we bring in mass, and gravity. Most people believe gravity is an active force, and for all practical purposes it is. However, in reality (according to the theory), it is a curvature in the "fabric" of space; sort of a dimple on the sphere if you will. The more mass, the more the space around it curves "in" drawing you in like a ball on a slope. Theoretical antigravity, if it exists, would be the opposite, a bulge "outwards" in space, pushing matter away, like a ball going up a slope and rolling back down.
However, space at least in our dimension is not simply up down, left right, forwards and backwards. It also incorporates time. Some people call time the fourth dimension, which works well enough for some models, but that is not suitable for this discussion.
What it is is an aspect of space, and the reason it is called space-time (in sci-fi there are countless references to the space-time continuum or however you spell it). This is closely tied in with the theory of relativity. Basically, the easy way to look at it is that space and time are linked, as are the way we move through them. However, rather than moving through one faster and thus the other, they are inversely proportionate.
I'm sure that sounds confusing, but it's not. Look at it like you have a pair of gauges. When one if full, the other is empty, and as you fill the other, it lowers the first. If you are not moving through space at all (let us ignore the movement of the planet, sun, and galaxy for this, and say you're siting at your computer not moving), then time is at its fastest. If you get up and walk around the room, you're moving through space at a very slow rate, and time slows by the tiniest fraction. If you run, it slows a tiny bit more, and so on.
Mind you, this part is not theoretical, it actually has been proven. If you want to know the details, you can look it up, but basically they took two identical clocks, the most precise they could find (and that are not updated automatically like most fancy clocks these days), and set them to the precise same time. Then they took one and put it on, let's say, a Concord, a faster than sound aircraft. They flew it around for a while, then brought the two together. Each clock is extremely precise and regular, mind you. The one that went for the ride was slightly behind the one who stayed still. The barest of differences, but it was there.
At the other end of the extreme is the speed of light, the fastest it is possible to move in the universe around us. At this point, if you were able to match it (which is impossible, but let's ignore that for a second), time would stop. You would have emptied the gauge for time and put it all into spatial movement. Just for yourself, mind you, as time is relative.
If you were going just under light speed, say 99.99% of it, time would all but stop, it would move at a crawl. You could go incredible distances away, and come back to find everyone you know either dead or extremely aged, while you're just a touch older than when you left. Now, beyond that I don't know very much about the actual mechanics of how it all works on traveling up close to the speed of light. What I do know is that is the basis of the concept for traveling into the past, by going faster than light and thus reversing time. There are some theories, but they are uncertain at best and the rest of this post is already too heavy without adding that.
One last bit to throw in.
We have said that mass curves the fabric of space inwards. However, since space and time are connected, this is also something of a curve in time as well. Due to this, being on a planet, our time goes a bit slower than someone outside the curve, due to the mass itself. Meanwhile, black holes, the densest objects in the universe, curve the universe so much that time virtually stops, in effect simulating incredibly high speeds by virtue of curving time and space. While objects take very long times to fall into a black hole, the time would speed by for someone right up in there, drawing it out far longer than those outside the curve in space. It could even be said to tear the fabric, and what not, but I've gone as far as I think I should with all of this. Let's just say that theories involving wormholes involve fourth dimensional cuts across the universe (cutting across the circle to the other side to save some time), and other science fiction stories involve "sub-space" which is similar in ways.
Anyway, sorry if I've given you a headache, and congrats if you understand what the hell I just said. It's weird and gave me many headaches before I understood it even this much.
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