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So let me explain to you what the big bang really is. So forget that old picture which many of you might have brought with you. I want to take these galaxies, and you saw the galaxies, they are randomly spread around, but to make this more understandable, I am going to pretend that they are laid out in a regular array (that's a detail that doesn't really matter) but it will help you visualize what is going on.

grid1 So here is the universe, just laid out in regular array just for the purposes of teaching. Now, what Hubble observed was that the galaxies, let's pick our own galaxy, which one is us, it doesn't matter, let's pick this one. What he observed is that these galaxies are all moving away from us and that the ones that are further away are moving away twice as fast, so this is Hubble's law in pictures.

anim1 What will the galaxy look like 2 billion years from now? Well, just put those galaxies, I have marked them in red, just move them to their new locations and here is what it will look like. So we are going from this to this, the universe is getting bigger, all the galaxies are getting further apart, and yet it doesn't look so much like the Milky Way is special. In fact, here I put the Milky Way right there, put the Milky Way right there and then you drew these little arrows. But let's say someone is sitting out here on this one, well they say "oh I'm not moving, my galaxy is stationary, after all I'm still here", so you put that one as the one that is not moving, you move it over like that and guess what that person observes? You're right --- it's the same Hubble's Law. So there is no center to the universe. Or, you might say, everyone thinks they are in the center, regardless of where they are.

An analogy that I like to make and you may have seen this in books is that the universe is like raisin bread and here we are the raisins, and the raisin bread is rising and you are sitting on one raisin. There is your friend raisin over there; it is moving away from you and your friend over there is moving away from you and everywhere you look your friends are moving away from you.

anim1 It looks like you are at the center and everything is moving away from you, but to your friend, everything is moving away from them, everything is just getting further apart. When that happens you get Hubble's law because the raisin that is twice as far away will be moving away twice as fast. So this is the picture of the universe.

In fact, the picture of the universe that all people who work in this field hold is that the universe is more or less uniformly filled with galaxies. Not quite as regularly as we show here but it is filled everywhere with galaxies, that they are all moving apart from each other. "Oh, yeah," your student will ask you, "where is the crust?" At some point you must run out of space. The remarkable answer is "NO". You can have expansion of the universe without having additional space at the edges that you move into. In fact, as far as we know, there is no region of the universe that we know of that is different from what we see here today. The universe is full of matter everywhere and in every cosmological theory that is taken seriously. I'll show you the evidence for this; I'll show it to you in a moment. The evidence seems to be that the universe is completely filled with matter, everywhere, but that this matter is all moving apart from itself. Where is the space coming from?


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