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Mapping dark matter may help solve a cosmic mystery

April 14, 2015 at 6:15 PM EDT
We can't see dark matter, but scientists have made the largest map yet of the invisible material that helps make up the universe. Researchers used a dark energy camera and a large telescope to create a color-coded chart of just a small part of the cosmos. Jeffrey Brown talks to Sean Carroll of the California Institute of Technology about how they did it and why it matters.

JUDY WOODRUFF: Scientists have announced the creation of the largest map yet of the invisible material that helps make up the universe, what’s known as dark matter.

Jeffrey Brown explores some of the very cosmic questions around this story.

JEFFREY BROWN: That’s worth saying again: We can’t see it, but we can apparently map it. What’s called dark matter is, in fact, everywhere, and it’s believed to play a crucial role in forming and holding together galaxies with its gravitational pull.

In findings announced Monday, researchers used a dark energy camera and a large telescope in Northern Chile to create this color-coded map, showing a small piece of the visible sky. Orange and red areas represent denser concentrations of dark matter. Blue areas are less dense.

And Sean Carroll joins us now to tell us about it. He’s a cosmologist and theoretical physicist at the California Institute of Technology.

Thanks so much for joining us and helping us here.

Can we start with a basic question?  What is dark matter?

SEAN CARROLL, California Institute of Technology: Sure.

Dark matter is some kind of particle. It’s just some — like — just like ordinary matter. You and I are made of atoms. There’s some other kind of particle, not anything we find in atoms, not anything we have ever found here on Earth. It’s dark, it’s invisible, but it’s most of the matter in the universe.

JEFFREY BROWN: So, how do we know it exists?

SEAN CARROLL: Well, because of gravity. Gravity is universal. Everything that exists creates gravity and is affected by gravity.

So the dark matter, which is most of the matter in the universe, creates a lot of gravity, and then it pushes around the things in the universe, including light from distant galaxies as they pass by the dark matter concentrations.

JEFFREY BROWN: And I referred to it as somehow holding together galaxies. That’s through its gravitational pull?

SEAN CARROLL: Well, that’s right.

Even without dark matter, the galaxies would still be held together, but they would be moving much more slowly. A stronger gravitational field together by the dark matter is what dominates the gravity inside the galaxy and sort of sets it spinning with the speed that it has.

JEFFREY BROWN: All right. We’re talking here about the latest thing, which is mapping this stuff that we can’t see. Sounds strange. How do you map it?  What are they actually looking at?

SEAN CARROLL: Well, what they’re looking at is actually the part that we can see.

You can see the light from galaxies that fill the universe. Our universe has over 100 billion galaxies. And so this new image has looked at about two million of those galaxies. And they have looked for slight distortions in the images caused by the fact that the light from those galaxies passed through more or less dark matter on its way to us.

JEFFREY BROWN: And that’s done through this — describe — the camera that I was referring to, a dark — dark energy matter — dark energy camera — excuse me — right?

SEAN CARROLL: It’s confusingly called the dark energy camera.


SEAN CARROLL: It’s not made of dark energy.

JEFFREY BROWN: It’s not the first — perhaps the first confusion here, but let’s stay with that one.


SEAN CARROLL: It’s one of the confusions that we can clear up.


SEAN CARROLL: There is this thing called dark energy. It’s not matter, as you might guess.

Dark matter, like we said, is actually kind of understandable. It’s just some particle that we can’t see that’s invisible, but nevertheless gives rise to gravity. Dark energy is something that isn’t even a particle. It’s something that’s intrinsic to space itself. It’s some field of energy that’s smoothly distributed throughout the universe and is pushing it apart.

So, the dark energy camera, its whole design purpose is to measure properties of the dark energy. But, as a bonus, along the way, we get an unprecedentedly good map of where the dark matter is.

JEFFREY BROWN: I like how you took us from the really understandable stuff to the completely, completely obscure stuff, right?

SEAN CARROLL: That’s my job description. That’s my expertise right there.


JEFFREY BROWN: Yes. I appreciate that.

All right, why is this important?  I mean, why should the rest of us care about this?

SEAN CARROLL: Well, you know, the 1990s will go down in human history as the decade in which we figured out the inventory of the stuff from which the universe is made, 70 percent dark energy, 25 percent dark matter.

Only 5 percent of the universe, by mass, is the ordinary stuff out of which you and I are made. So, if you care about understanding the universe, 95 percent of it is dark matter and dark energy. If you want to know how the universe works, you have to understand that stuff.

JEFFREY BROWN: And in terms of what’s next, I know this map was just a very small — it’s huge. It covers a lot of ground, a lot of space, but still very small.

SEAN CARROLL: Well, what we’re trying to do is to figure out more about the physics of the dark matter.

It’s very annoying to us, as scientists, because we know it’s there. We know how much of it is there. We know where it is. But we don’t know what it is. We don’t know what is actually making up the dark matter. So the more we can study its properties, how it collects, how it evolves over time, the more of a hope we get to understand what it is made out of and why there is dark matter at all.

JEFFREY BROWN: All right. Sean Carroll, thank you so much.

SEAN CARROLL: Sure. My pleasure.