Tracing Light to Map the Cosmic Darkness

IRA FLATOW: This summer, I hope you all get a chance to get out of town to go stargazing– one of my favorite things. Get the old telescope out and stare and wonder at those millions of silent worlds out there so far away. But as you look up at all that stuff– the planets, the stars, the streaks of the Milky Way, keep in mind that everything we see, including every single element in the periodic table– and yes that includes us– everything we see makes up, oh, just about 4% of the total matter and energy in the universe– 4%. The other 96%? It’s dark to us– dark matter, dark energy. Dark, because one, we can’t see.

And as my next guest says, when astronomers don’t understand something they just add dark to it. But she is trying to shed some light on that mysterious dark matter by mapping it– how it spread around the universe. Is it smooth and streaky, clumpy and bunched together, or thin and wispy like spider webs? As you can imagine, you’ve got to get a little creative to map something you can’t see. And one of the things she writes about in her new book, Mapping the Heavens is how to do that.

Priyamvada Natarajan is a theoretical physicist and professor in the departments of physics and astronomy at Yale University– author of, Mapping the Heavens, the Radical Scientific Ideas that Reveal the Cosmos. Welcome to Science Friday.

PRIYAMVADA NATARAJAN: Delighted to be here, Ira.

IRA FLATOW: How do you map something we can’t even see like the dark matter?

PRIYAMVADA NATARAJAN: So we exploit the bending of light that mass generates. And so what is done is use this effect called gravitational lensing, which is predicted by Einstein’s theory of general relativity. So if you have distant galaxies whose light is propagating towards us, and along the way you encounter a clump of dark matter, that causes deflections and therefore distortions in the shapes of galaxies. And because dark matter is smeared everywhere in the universe, but quite lightly in most places, and it’s clumped in rare locations, by looking at regions where there is very little distortion or no distortion, we know what the shape distributions of galaxies are on average, statistically.

So then when we look through these regions where there’s a lot of distortion compared to what is the norm, we can back out the distribution of matter which is mostly unseen that is causing these deviations in shapes. So essentially what happens is a galaxy, in sort of looking somewhat– if you started with circular galaxies, would get stretched out and they would look like little ellipses. And they can be quite dramatically stretched out if there’s a lot of dark matter along the line of sight to us.

IRA FLATOW: What does the map tell you? I mean, what can we learn from mapping about the true nature of dark matter?

PRIYAMVADA NATARAJAN: So some of the regions where there’s a lot of dark matter are called clusters of galaxies. So these are the largest repositories of dark matter. And since, with data from the Hubble Space Telescope where we get these very exquisite images, you can actually map in the granularity of dark matter– so how it’s clumped. And there’s information in the clumping of dark matter on what its potential nature might be.

So for example, if dark matter is what we believe it to be– these cold collision-less particles, probably created at some point in the early universe, then they tend to clump on all scales. So you should see a whole range, a hierarchy of clumps– some large ones, some small ones, with no real end. I mean there’s no cut-off. You see all kinds of clumps.

But if dark matter was a warm dark matter particle, something that was moving much faster from the early universe, formed then, then there’s sort of a scale beyond which there would be no clumps. It would be a lot smoother, the distribution. So the goal, since we haven’t detected the dark matter particle directly yet– we’re really looking for indirect clues on what it might be. And I think that’s what gravitational lensing can provide.

IRA FLATOW: Talking with the Priyamvada Natarajan, author of, Mapping the Heavens, the Radical Scientific Ideas that Reveal the Cosmos, on Science Friday from PRI, Public Radio International. Let’s talk more about it. You write in your book that you first got interested in this stuff by mapping the night sky over Delhi using a Commodore 64. I had one of those. It couldn’t have been easy. Tell us about that.

PRIYAMVADA NATARAJAN: That’s right. And it actually dates me, doesn’t it?

IRA FLATOW: I’m in that boat even worse than you are. But go ahead.

PRIYAMVADA NATARAJAN: Right. So I have this computer and I was itching to do something. I learned programming on my own, and I went to the director of the Nehru Planetarium, and I asked for a project to do. So she mushed me off and said, OK, you have a computer, why don’t you map the night sky and which planets and constellations you’d be able to see above the night sky of Delhi.

So I went off and I did it. It took me a good part of six to eight weeks. And then I went back and she said, oh. She thought she’d gotten rid of me, because I was this pesky little kid, right. And so then she said, OK. I said I’ve done it. So I showed her the map and then she sort of said, well, you live in Delhi now. But maybe you live in Boston or Brisbane. What about the night sky? So I said, oh, I’ve done that already. You just have to put in the latitude and longitude of any place on earth and I can pull up the star map.

So at that point, finally she sort of took me seriously and then gave me a little research project. And before that, I had actually looked through a telescope and I fell in love anyway with the cosmos. So that sort of really got me started on this sort of real love affair with astrophysics.

IRA FLATOW: So how did you follow that up to become an astrophysicist?

PRIYAMVADA NATARAJAN: Well, I was really fortunate. I got a full scholarship to come to college at MIT. So I did my undergraduate there. And I had some fabulous professors, great courses. And then I went on after a slight detour because I’m interested in the history of ideas. So I did some graduate work in history and philosophy of science, also at MIT. Then I went to Cambridge, England for my Ph.D. And I had the great fortune of working with one of the greatest living cosmologists, Martin Reese. I did my thesis there. I was a member of Trinity College and a fellow there. And then I got a job at Yale. And I’ve been there since.

IRA FLATOW: You mention about studying philosophy. Do you need to be philosophical? Have a little bent there to work on this kind of stuff you can’t see?

PRIYAMVADA NATARAJAN: Well, I don’t know about philosophic. I mean, human beings by nature have always been intrigued by the invisible. Right.

IRA FLATOW: Yeah.

PRIYAMVADA NATARAJAN: And I think the philosophical angle helps to sort of keep everything in perspective. Because you know, in a way, I find it much more comfortable to deal with these sort of cosmic scales– talk about 3.5 billion years, a process taking 100 million years. All of that is somehow much more comprehensible to me than day to day life stuff. So I think being philosophical has been very helpful.

IRA FLATOW: Yeah. Well, we’ll talk more. We have to take a break. We’ll come back and talk lots more with the Priyamvada Natarajan– author of, Mapping the heavens. Stay with us. We’ll talk about more dark energy, dark matter, the dark side of the universe– all coming up after the break. We’ll take your calls. Our number 844-724-8255. You can also tweet us @SciFri. 844-724-8255. Tweet us @SciFri. Stay with us. We’ll be right back after this break.

IRA FLATOW: This is Science Friday. I’m Ira Flatow. We’re talking this hour about mapping the universe’s web of dark matter with Priya Natarajan. She’s a theoretical astrophysicist and professor in the Department of Physics and Astronomy at Yale, and author of, Mapping the Heavens– a really good book to read.

I’d like to bring on another astronomer, who’s also studying something notoriously hard to see, and that is a black hole. And he has a plan for snapping a closeup of it. He wants to use a telescope as big as the Earth. No big deal. Shep Doeleman is director of the Event Horizon Telescope project. He’s also an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. Welcome back to Science Friday.

SHEP DOELEMAN: Thanks, Ira.

IRA FLATOW: So what’s so hard about looking at a black hole that you need a telescope as big as the Earth to take a picture of it?

SHEP DOELEMAN: Right. So first of all, black holes are by definition, something you can’t see. I like to imagine it as trying to take a picture of a dinosaur. We know dinosaurs exist. We see their footprints in clay. We see their bones, but no one’s ever seen one. And it’s the same with black holes.

As Priya was saying, you can see light bend around them, but you can see them themselves. So you have to see find a way to build a telescope that has about 2,000 times the magnifying power of the Hubble Space Telescope, because these are the smallest things in the heavens that are predicted by Einstein’s Theory of General Relativity.

IRA FLATOW: You know, it’s kind of weird because we tend to think of black holes as these massive objects swallowing up everything around them, when in reality you’re saying that they are relatively small.

SHEP DOELEMAN: They are the tiniest things you can imagine. They are the end result of gravity going haywire and collapsing a bunch of matter into a point source. But around that point is this wonderful membrane called the event horizon, and that’s the point where the gravity is so intense that even light can’t escape. So– go ahead.

IRA FLATOW: No, go ahead. I’m sorry.

SHEP DOELEMAN: So all this gas and dust around the black hole is madly trying to get into a very small volume. And in a cosmic traffic jam it heats up to billions of degrees. So black holes can be some of the brightest things that we see in the sky.

IRA FLATOW: In the film Interstellar they use some equations by physicists to create what they thought was a real picture of a black hole. How close do you think they got to it?

SHEP DOELEMAN: So, they got really close. I mean, no one’s ever seen one, of course. But that is as close I think as we’re going to get. It turns out, speaking of Commodore 64s that Priya was, there was someone in 1979 that made a wonderful illustration of what a black hole might look like using primitive computers. And it looks very much like what the computer graphics designers did for the movie Interstellar. And that’s the kind of thing that we humbly hope to do with the Event Horizon Telescope.

IRA FLATOW: Can you see a black hole if you take a picture of it? Is that what it’s going to look like, a hole in space?

SHEP DOELEMAN: So, a black hole is surrounded by this– if you think of it as a three-dimensional flashlight of hot gas, that’s this radiating light all around the black hole. So what you wind up seeing is what’s called the shadow of the black hole. Light that normally would leave the black hole on the other side of the black hole, gets bent around in a u-turn towards you. So you wind up seeing a ring of light around a relatively dim interior. And that’s called the shadow. And that’s what we are trying to measure and image with the Event Horizon Telescope.

IRA FLATOW: Priya, what would you want to know from this very first picture of a black hole? What would most interest you to see?

PRIYAMVADA NATARAJAN: Well, I think the shape of the shadow is a very important test of the predictions of general relativity, and really of physics in the strong gravitational regime. So the effects that we expect theoretically on light, and light bending by black holes– this would be sort of one very important test at a very, sort of high-resolution small-scale test.

A slightly more compelling test I would say, then the bending that we see by larger more massive objects, because black holes are so compact. The clusters of galaxies that I was talking to you about are huge things in the sky, and they have a huge amount of matter. So the deflections that they generate, that matter generates, they are in consonance with Einstein’s theory.

The question is, when you have something that’s really that compact, how well does the theory still work, right? So, I think there’s no reason to believe that there’s any problem with the theory at the moment. But you know, who knows what the Event Horizon Telescope could tell us.

IRA FLATOW: You know, it’s interesting you say, who knows. One of the hottest topics about black holes in these last few decades has been a discussion started by Stephen Hawking and other people, about what happens to the stuff that falls into the black hole. What happens to the information that’s contained in all of that? Can you give us an idea of what that debate is all about?

PRIYAMVADA NATARAJAN: Sure. So I think, actually, Stephen Hawking has a beautiful analogy. So, one of the problems is that, as Shep mentioned, the event horizon is really seen as this point of no return. Because once you pass the event horizon which encases the singularity– and the singularity is the place where all the known physical laws that we know about and understand break down. So what really happens to information, to matter, when it crosses the event horizon is not very well understood. And we don’t even have a framework.

But Hawking had this interesting suggestion– and they’re working on it with Andrew Strominger and Malcolm Perry at Harvard and Cambridge– which is, suppose you had an encyclopedia that was encased in a glass case– a tight case. Right. And you want to look up the capital of Rwanda. So you just go there. You look it up. You look at the page. Now you burn the encyclopedia. And all the ashes of the encyclopedia are still in that box. Nothing’s left that book, right? It’s right in there.

So theoretically speaking, the information on the capital of Rwanda is still in there, it’s just that you don’t know how to access it and how to extract it any more. And I think that these are sort of the ways in which they’re starting to think about, sort of analogies for the event horizon to develop a quantum mechanical understanding, because that’s sort of what seems to be lacking.

And so string theory seems to be providing new insights. And so that’s what the buzz is about at the moment. But I love this analogy because it really drives home the point that the information is possibly there, we just don’t know how to extract it. We don’t have the language, mathematically, to describe it.

IRA FLATOW: Yeah. Shep, you agree with this analogy? Is it a good one?

SHEP DOELEMAN: Well, it is an interesting one, because the idea is, where does the information go when it falls into the black hole? And that’s why philosophers get very interested about black holes. If you talk to them about stars, they say they’re beautiful– even neutron stars, they say they’re wacky. But when you talk to them about black holes they get dreamy, and they get very interested. Because this is an area of space-time that is unaccessible to us. And that really is very, very startling and spooky.

Our whole world view is based on Newtonian determinism. That if you know what is happening around you, you can propagate it forward in time and know where you’re going to be later. But what if you’re falling into a black hole, and you can’t tell somebody what happened to you? Is that determinism or not? Or does determinism break down? So black holes, in addition to the information theory problem, they strike at the core of Western thought.

IRA FLATOW: Shep, this is an anniversary of sorts, isn’t it, for the so-called Schwarzschild Radius? Describe what that is.

SHEP DOELEMAN: Right. So the event horizon occurs, for a non-spinning black hole, at the Schwarzschild Radius. And in 1915, Einstein came up with his field equations– his geometric interpretation of gravity. And it was communicated to Karl Schwarzschild who was in the army in the trenches of World War I. And unlike how I like to do my work with a cup of coffee and maybe some music, he was solving Einstein’s theoretical equations in the trenches.

And he came up with this solution called a point-source solution where he said, what if all of the mass is concentrated into a point? He didn’t think it was realistic, but he said, let’s focus everything into a point. And he found that, at a certain radius, called the Schwarzschild Radius, even light couldn’t escape because the gravitational field would be too great. And he wrote this down on a postcard, mailed it to Einstein, who very famously then presented it to the Prussian Academy of Sciences in 1916. So that’s the celebration and anniversary that we’re looking at today.

IRA FLATOW: The 100th anniversary of that.

PRIYAMVADA NATARAJAN: Right. And actually Einstein actually didn’t expect that there would be any exact solutions for his equations– field equations. So he was quite startled actually. He didn’t like the solution, but he was startled.

SHEP DOELEMAN: And he also did not accept black holes, as Priya said before, for many, many years.

PRIYAMVADA NATARAJAN: Yeah. I mean, Einstein is this really intriguing character, right? He comes up with all these incredible, radical ideas, and he is really unhappy and resists the implications of his own ideas. So this is something that I talk about in the book. And Einstein is a particularly interesting case.

IRA FLATOW: Yeah. He’s the father, so to speak, of all kinds of science that he didn’t agree with– was handled once he discovered it, especially quantum mechanics. Priya Natarajan is the author of, Mapping the Heavens. We’re also talking with Shep Doeleman, director of the Event Horizon Telescope Project.

We have a couple of phone calls. Let’s go to the phones. Let’s go to Philip in Portland, Oregon. Hi, Philip.

PHILIP: Hi, Ira. How are you?

IRA FLATOW: Hey there.

PHILIP: Thanks for taking my call.

IRA FLATOW: You’re welcome. Go ahead.

PHILIP: And I still have my Commodore 64 as well.

IRA FLATOW: Me too.

My question is this– it seems like the scientific community has basically concluded that dark matter must exist, because the equations that we have that predict gravitational lensing don’t seem to quite fit with the amount of matter that we’re able to directly observe. And I’m just wondering what your guests think about the alternative, which might be that the equations aren’t quite right?

PRIYAMVADA NATARAJAN: Can I take that?

IRA FLATOW: Please, Priya. Go right ahead.

PRIYAMVADA NATARAJAN: Yeah.

IRA FLATOW: I’m not touching that one.

PRIYAMVADA NATARAJAN: OK. So actually, Philip, there are many independent lines of evidence that point to the existence of dark matter, not just light bending. There’s the motions of stars in galaxies and galaxies and clusters, which is not commensurate with the matter that is seen. Right. So Einstein’s equations actually predict that the contents, the fate, and the geometry of the universe are interlinked.

And so now we have an inventory of all the matter. We also have an understanding of the geometry and the fate, independently. Right. And so they have to be sort of commensurate. So there’s room for dark matter. Although we haven’t found the particle, I understand that it might seem that scientists have evidence but they don’t have a direct detection yet. So we are awaiting the detection of a particle. Right.

And this is not like ether. Probably you’re wondering whether this is going to go away. The only the alternative that has been suggested, and that’s been worked on quite a lot, is sort of a modification of the equations of Newtonian dynamics. And it’s called MOND, this theory. And the interesting thing is that versions of this theory can explain the motions of stars, the evidence for dark matter from galaxy scales.

But this theory cannot really match up and give predictions for the light bending that is seen. So there is no real viable alternative theory at the moment. And there are independent lines of evidence that are very compelling for the existence of dark matter, although we are yet to detect the particle. But you know, that LIGO detection, remember?

IRA FLATOW: Yeah.

PRIYAMVADA NATARAJAN: It took 40 years.

IRA FLATOW: Yeah.

PRIYAMVADA NATARAJAN: So there are many dark matter experiments that are ongoing at the moment. And I’m actually sort of optimistic that we just might– in particular there’s one experiment called DAMA that claimed a detection more than 15 years ago. But the community was not persuaded. And only recently, a replication of that experiment in five different locations– on the South Pole, in Australia, South Korea, Spain– and so with a particular, a sodium iodide crystal as a detector. So within a couple of years we’ll know maybe that was a signal. And that maybe we should take that seriously. So I’m quite excited at the possibilities.

IRA FLATOW: I’m Ira Flatow, and this is Science Friday from PRI, Public Radio International. I’m talking about black holes and dark matter with Priya Natarajan, author of, Mapping the Heavens, the Radical Scientific Ideas that Revealed the Cosmos, and Shep Doeleman, director of the Event Horizon Telescope Project.

We have a phone call or two. Let’s go to the phones. Well, last time we took a tweet first because, you touched on this a little bit before. Martha Hussain says, is all dark matter the same? Is there anything to be inferred from the proportion of dark matter to non-dark matter?

PRIYAMVADA NATARAJAN: Yes. I mean, at the moment, the simplest assumption that we’re making is there’s only one kind of dark matter. But there’s no real reason to believe that there can’t be different kinds of matter. But, the dominant component appears to be cold that is moving very slowly and practically collision-less. One of my favorite candidates is all my unmatched socks. Every time I do laundry I miss socks. So, you know, that could be a component of dark matter.

IRA FLATOW: The greatest unsolved mystery of science. Where do the socks go in the laundry?

PRIYAMVADA NATARAJAN: That’s right.

[LAUGHING]

IRA FLATOW: Shep, how will we know if we see something truly shocking? And you take this image of the black hole and there’s something really shocking that the image itself is correct. Right? You’re expecting something. Are you ready to upset 100 years of black hole theory for what you see?

SHEP DOELEMAN: Well, as I like to say, it’s never a good idea to bet against Einstein. But even he, I think, would be really marvelously excited by what we’re about to do. So the idea is that, you should see this ring of light around the black hole. And the size and shape of that was predicted by Einstein.

If we see some deviations from that, it doesn’t look round, if it’s not as large or it’s smaller than we think it should be, that would be an indication that either we’re not looking at a black hole– it could be something weird and exotic, like a boson star or something that would be very difficult to think about how we could even construct it. But it’s potentially possible.

Or it would be, as Priya was discussing, a change in general relativity– a change in Einstein’s equations. So what we’re looking for is this ring of light. And if it’s the right size, let’s say around the 4 million solar mass black hole at the center of our galaxy, the Milky Way, then that would tighten the noose incredibly. So in the center of our galaxy, there’s this large black hole.

And some wonderful groups in Germany and UCLA have seen stars orbiting around this unseen mass. And that is very powerful evidence that it’s a black hole. But they only come within, let’s say, one or two thousand Schwarzschild radii of this black holes. So they’re very far away, and their motions are perfectly predicted by Newtonian dynamics. But we want to tighten the noose to within one Schwarzchild radii.

IRA FLATOW: How soon will we have a photo?

SHEP DOELEMAN: Oh, that’s the million dollar question. Our global team– and this really is a global team, Ira– are getting ready to take our first potentially imaging data set in the spring of 2017. That’s when we’ll add sites at the South Pole, in Chile, Hawaii, Arizona, Mexico, France, and Spain. Truly an Earth-sized array to look at this exotic object.

IRA FLATOW: Can’t wait till it gets back from the drugstore. Thank you very, very much for taking the time to be with us today. It sounds very exciting. Shep Doeleman, director of the Event Horizon Telescope Project. He’s also an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

Priya Natarajan is a theoretical astrophysicist, professor in the Department of Physics and Astronomy at Yale, author of, Mapping the Heavens, the Radical Scientific Ideas that Reveal the Cosmos– a great read. It’s a great wide story about the history of astronomy. And it’s something that explains stuff you’re always wondering about.

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