Frontiers and Controversies in Astrophysics: Lecture 16 Transcript
March 29, 2007
Professor Charles Bailyn:
Welcome to part three of the course. This is going to be about cosmology. One of
the most amazing things that's happened over the past half century or so is that cosmology, which is the study
of the Universe as a whole, has become a scientific subject, and something that one can say something about
in scientific terms, rather than merely philosophical terms.
In recent years, in the past ten years or so, there has been kind of a revolution in cosmology, which has come
about because of the discovery that the vast majority of the Universe is made out of stuff that we have no idea
what it is. So, the discovery is, we have no idea what the Universe is made of. And that actually, you know, it
doesn't sound so good. It sounds like this is a big failure of science, somehow.
But scientists are clever, so we describe it differently. What we claim is that what has happened is that we
have discovered dark energy. Dark energy, what is that? We don't have a clue, but it's most of the Universe.
And we've discovered that it exists. So, that's kind of where we are right now.
What I want to do over the next four or five weeks is talk about the discovery of dark energy--how that was
done. And now that we know that it's there, but we don't what it is, what people intend to do about it. And
this is, I should say, only a relatively small fraction of modern cosmology. If you want to know about many
of the other interesting things that are going in cosmology, you have to take a whole course, or several
courses, on this topic. Such courses exist. I recommend Astro 170 or 220 if you find yourself interested in
this kind of thing.
But nevertheless, this one particular discovery--and, in fact, what I'm going to be focusing on is not just dark
energy, but one particular way that dark energy--the first way. There are now other indications that exist. One
particular way in which dark energy has been discovered. And so, it's a fairly narrow focus that I'll be taking
here, but that will enable us to get into some depth about how this discovery was made.
So, let me let you in on the secret before we even begin. Dark energy is something that you can't see. That's
sort of its name. We've done this twice already, right? We've discovered planets around other stars--that you
couldn't see the planets. How do you do that? You look at the star--the motion of the star. We've discovered
the existence of black holes which, of course, you can't see. How do you do that? You look at the motion of
some star that is influenced by the existence of the black hole. So, what are we going to do about dark
energy? Well, this isn't hard to extrapolate. What we're going to do is we're going to look at the motion of
things that we can see, and infer the presence of dark energy.