Here's my stab at a substitute for the balloon analogy:
Imagine you're in a room with two doors, one to the right, and one to the left. In fact, it's a chain of rooms, each one leading to another, like the cars of a train. Amazingly, the chain never seems to end. There may be an infinite number of rooms, or it may be that eventually the rooms "wrap" and you wind up back where you started -- that's still an open question.
Now, imagine that every room is slowly getting wider, all at the same time. If you look toward the door on the left, you'll see that doorway (and the room beyond it) gradually moving away from you, as the room you're standing in expands around you. Ditto the room on your right. On top of that, if you look through the room on your left, to the next room down, you'll see that it is receding even faster than the room next to you. It's pretty obvious that, if you try to look even further down, that more distant rooms will be receding even faster.
That's what the universe is like now. It's expanding, but it's not expanding into anything; the rooms are just getting bigger. Note that, from your vantage point, you appear to be in the center of the expansion. Rooms to your left are moving away to the left, and rooms to your right are moving to the right, and all the while you appear to be standing still. But you can also see that it would look the same from any of the rooms, and that there isn't really any special center to the expansion.
So what's the Big Bang, then? Well, it's easy to picture the Big Bang in reverse, using the room analogy. Imagine that time is going backwards, and the rooms are shrinking instead of growing. Now they're ten meters wide...now only five. It's starting to get crowded. One meter wide. You're elbow-to-elbow with the person in the next room. Now one centimeter wide -- you've been crushed out of existence...there are a hundred rooms per meter. Now there are a thousand...now a million...the matter that used to be your body is has been compressed to an unimaginable density. How far can the shrinking continue? What happens when the rooms shrink to zero size?
Well, picture that process in reverse, and you have a pretty good idea of the Big Bang. The rooms start out unbelievably compressed, but are rapidly expanding, getting bigger and bigger as time goes on. The matter in the rooms, incredibly dense to start with, gets more and more spread out, until eventually the rooms are so big that they're almost entirely filled with empty space.
The question of what happens at zero is the big mystery. What does it mean to have an infinite chain of zero-sized rooms? Our math can't answer that, so we throw up our arms and call it a singularity. It's the same with the Big Bang: we can envision what the universe was like earlier and earlier, back to just fractions of a second after the Bang, but our equations all fall apart at t=0.
Does that make any sense?
Jeremy
Imagine you're in a room with two doors, one to the right, and one to the left. In fact, it's a chain of rooms, each one leading to another, like the cars of a train. Amazingly, the chain never seems to end. There may be an infinite number of rooms, or it may be that eventually the rooms "wrap" and you wind up back where you started -- that's still an open question.
Now, imagine that every room is slowly getting wider, all at the same time. If you look toward the door on the left, you'll see that doorway (and the room beyond it) gradually moving away from you, as the room you're standing in expands around you. Ditto the room on your right. On top of that, if you look through the room on your left, to the next room down, you'll see that it is receding even faster than the room next to you. It's pretty obvious that, if you try to look even further down, that more distant rooms will be receding even faster.
That's what the universe is like now. It's expanding, but it's not expanding into anything; the rooms are just getting bigger. Note that, from your vantage point, you appear to be in the center of the expansion. Rooms to your left are moving away to the left, and rooms to your right are moving to the right, and all the while you appear to be standing still. But you can also see that it would look the same from any of the rooms, and that there isn't really any special center to the expansion.
So what's the Big Bang, then? Well, it's easy to picture the Big Bang in reverse, using the room analogy. Imagine that time is going backwards, and the rooms are shrinking instead of growing. Now they're ten meters wide...now only five. It's starting to get crowded. One meter wide. You're elbow-to-elbow with the person in the next room. Now one centimeter wide -- you've been crushed out of existence...there are a hundred rooms per meter. Now there are a thousand...now a million...the matter that used to be your body is has been compressed to an unimaginable density. How far can the shrinking continue? What happens when the rooms shrink to zero size?
Well, picture that process in reverse, and you have a pretty good idea of the Big Bang. The rooms start out unbelievably compressed, but are rapidly expanding, getting bigger and bigger as time goes on. The matter in the rooms, incredibly dense to start with, gets more and more spread out, until eventually the rooms are so big that they're almost entirely filled with empty space.
The question of what happens at zero is the big mystery. What does it mean to have an infinite chain of zero-sized rooms? Our math can't answer that, so we throw up our arms and call it a singularity. It's the same with the Big Bang: we can envision what the universe was like earlier and earlier, back to just fractions of a second after the Bang, but our equations all fall apart at t=0.
Does that make any sense?
Jeremy
