Alex Filippenko – TRANSCRIPT
Is everyone having a good time? All right! What an amazing set of speakers, ha? TEDxBerkeley go! All right. Great to see you all here, hope you’re having a great day, and a wonderful Cal Day outside as well.
I want to tell you today about the accelerating expansion of the Universe, a revolutionary discovery that definitely has catalyzed change, a discovery that was honored with the Nobel Prize in Physics to the team leaders: Saul Perlmutter, here at Cal, professor of Physics, in the middle, Brian Schmidt on the right, at the Australian National University, and Adam Ries at the left, now at Johns Hopkins University. He was a postdoctoral scholar working with me at Berkeley in the mid to late 1990s when we made this discovery.
There were 48 other people associated with the discovery, but the rules of the Nobel are that at most three can get the prize, other than a Peace Prize, which can go to organizations. Fortunately, these gentlemen understand that without the rest of us working in the trenches, they wouldn’t have been so honored. So they spent part of their prize money flying the rest of us out to participate in Nobel Week in Stockholm in December of 2011.
We went to the most of the parties, and the celebrations, and lunch and dinner at the embassy, and all-night things put on by the students; it was just a wild time. About the only thing we didn’t get was the gold medal and part of the million bucks, or the chauffeured limousine. But, oh well, here we are.
Here’s one of the two teams on which I participated. I had the interesting distinction of being the only person who was, at one time or another, a member of both teams, and if you get me sufficiently drunk, I’ll tell you the inside story behind that.
Anyway, the story starts with Edwin Hubble, who nearly a century ago studied these gigantic collections of stars, called spiral nebulae, and their true nature was not yet known. It was thought by some that they are clouds of gas in our own galaxy. Others thought that they are other galaxies. He studied certain types of stars within them and showed that these are gigantic systems far, far away, millions of light years away, consisting of hundreds of billions of stars; island universes, if you will. They come in many shapes and forms, mostly these beautiful spiral ones, like the one we live in, and elliptical ones.
He noticed something very interesting: they’re all going away from us, or almost all of them are going away from us. This had been discovered actually by some other astronomers. By looking at the spectra, you can tell that they are moving away because all those dark lines, which are produced by chemical elements in the atmospheres of stars, showed the same pattern, but that pattern was shifted to longer wavelengths, redder wavelengths, as though the things were moving away from us.
But Hubble noticed that the nearby galaxies are moving away only pretty fast and the more distant galaxies, which generally looked smaller and fainter in the sky, are moving away faster, and the more distant galaxies are moving away really fast. So at a given time, right now, the Universe is expanding, but the more distant galaxies are moving away from us faster than the nearby galaxies.
And we now know it is space itself that it’s expanding Hubble resisted this interpretation for a while, but we can now tell that it’s not that the galaxies are moving through some pre-existing space, rather the space itself is expanding, and the waves of light stretch with it. This is the cause of the redshift. It’s not that they’re moving away, not the classic Doppler effect, but rather that the waves themselves are stretching. You’re not stretching because you’re bound by electromagnetic forces.
You may think you stretch after a big lunch, but that’s your fault, not the Universe’s fault. The Earth isn’t stretching because it’s bound by gravity. Our galaxy isn’t stretching either, but it’s the space between the galaxies that is stretching. The Universe is expanding. So here’s, from our perspective, the view: we’re in the middle there, and all the galaxies are moving away from ours, and the more distant ones are moving faster than the near-by ones.
Now, it’s a bit strange that we’re at the center, right? Do all the other galaxies not like us? Is it something we said? Do we smell? Are all these other galaxies lactose-intolerant? Get it? Milky Way Galaxy, lactose-intolerant? Or, since this is a Cal-friendly crowd, what is it? Are we from Stanford or something? With due apologies to those of you who might be Stanford alumni, it’s an outstanding institution, just not quite as outstanding as Cal. Anyway, yeah, so… No, we’re not in any central position.
In fact, we think we would see the same thing no matter which galaxy we happen to be in. Imagine a one-dimensional expanding Universe where you have the galaxies – the ping pong balls, and the rubber – the space between them. From the perspective of the orange ball here, all the others are moving away, so it thinks it’s at the center, but the same can be said from the perspective of any other ball; all the balls are going away from it.
So there is no unique center, at least not in the dimensions to which we have physical access. With today’s great telescopes, we’ve measured the current expansion rate of the Universe, and it’s just some number; I won’t bore you with what it is. But in fact, the expansion rate should change, and that’s because the Universe isn’t empty; it has things in it. All those things will gravitationally pull on all other things; the galaxies pull on each other, just as the Earth pulls on the apple, and so, in its upward journey, the apple slows down; eventually it stops and comes back. So, if the Universe is dense, every little volume will pull on every other volume a lot, the Universe will slow down in its expansion, some day stop, and then reverse itself.
It’s hard to see with the lights here, but it goes – it starts with the Big Bang, ends with a Big Crunch, or you could say Big Bang, Gnab Gib, which is Big Bang backwards. In a similar way, though the Universe is expanding now, you could be lying on your back, looking at all these galaxies, saying, “Tam-ta-ram, the Universe is expanding.” Then you notice something strange, and right around now, you start getting a little bit nervous, then it’s sort of “Goodbye, cruel world!”, and the Universe implodes in on you.
But there’s another possibility. If the Universe isn’t very dense, then, although the galaxies are pulling on one another, slowing down the expansion, that expansion should never stop if the Universe has sufficiently low density. That would be like an apple that I heave up or above Earth’s escape speed; it keeps on going away from the Earth forever. So in that case, the Universe would expand eternally, becoming ever colder, darker, more dilute, and you’re sitting there on your back, looking at the galaxies living for billions of years, and all is fine, except that the Universe is going to become a very cold, and dark, and lonely place eventually.