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The other issue is one of being slowed down. You could say, well the matter is slowing down, but how much is it slowing down? Will it slow down and eventually fall back in? Interesting questions.

We can measure how much it is slowing down. That tells us how much mass is here, that tells us how much it will slow down in the future. The question of whether it will fall back down has some very interesting consequences. If it all comes back we can refer to the final event as "The Big Crunch" (in the same spirit of humor that Hoyle used). We know the "Big Crunch" will not occur for at least another 50 or 60 billion years, just from our present measurements. Will it occur ever? We don't know, yet. We may know in a year or two.

If the "Big Crunch" does occur, then there is an interesting consequence. If you work out the theory as to what happens to the "Big Crunch", you find out that we have not been able to work out the theory of what follows the crunch. We don't understand how to work out the math or whether the math is right at the time of the "Big Crunch". Maybe theoretically we can figure this out some day but we don't know what the consequences of the "Big Crunch" are. How do we measure this?

SLIDE15 Well, what you have to do is look at very distance galaxies and measure their distance and measure how much they are slowing down. This is something, this is another program that I began at Berkeley. It is one of these wonderful programs where after working on it for 10 years, you discover you have a graduate student (he is now a post-doc) and he is better than you are, and so he is now running the program. Saul Perlmutter, over at Berkeley, has some results that were just released this week and they will be published in Nature shortly. What Saul is doing, and this is also being done by a group at Harvard, is that you look at these, these are Hubble photographs with images reversed because the black is bright. You find these little exploding stars in these distant galaxies and they are supernovas and they proved to be a wonderful, so called, "standard candle". These supernovas, when they go off, are all essentially the same brightness and so by measuring how bright they appear you can tell how far away they are. SLIDE16 By measuring the Doppler shift you get the distance, by looking at the variation with distance, which is done here, you begin to look for the slow down and so this is what is being done.


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