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Building a Story Line for the Universe Interview with Dr. Priya Natarajan
May 1, 2011



Human beings have wrestled with questions about the origin of our existence and the fabric of our universe for thousands of years. This questioning formed the discipline of cosmology. Dr. Natarajan explores the nature of our world and shares her perspectives about how new scientific discoveries are reshaping our understanding of the universe and our place in it. Until very recently, the universe had been considered a vast empty space. But the Dark matter theory states that the universe is not empty, but rather filled with an invisible matter. As new mysteries unfold, Dr. Natarajan points out that it is our fundamental human curiosity that leaves us no choice but to explore the universe and how we fit into its grand picture.

M&B: Dr. Natarajan, you work on dark matter in the universe. What is the theory?

It turns out that the bulk of the matter in the universe is not made of ordinary atoms that you and I or the universe that we know and experience is made of, but instead made of this mysterious particle. Ninety percent of all the matter in the universe is dark matter, and we believe this is a set of particles that was created very early in the universe. These particles do not have charge, but they have mass. Since they dominate the mass of the universe, it turns out that they really form the scaffolding in the universe around which all galaxies, stars, and so on, form. They are sort of the basis or the framework within which normal atoms actually cool to form star galaxies, and then have generated us.

M&B: What is the evidence for their existence?

The reason dark matter remains a mystery, – although there is incontrovertible evidence for the existence of dark matter – is the fact that you only detect it indirectly. What I mean by that is you detect its presence because it has mass, and since it doesn’t have any charge it doesn’t couple to any radiation. So you don’t see radiation in any wavelength in the electromagnetic spectrum: no X-rays, no gamma rays, no visible light, nothing. But since dark matter has mass, it aggregates gravitationally, so they feel gravitational pull towards each other and they cluster. It’s the clustering of the dark matter, the fact that it gets compact, that we detect the effects of dark matter.

M&B: What happens near the regions where dark matter clusters? In your research papers you use the bending of the light rays. How do you observe it?

The primary evidence is the gravitational bending of light produced by these aggregates of dark matter. The universe is really composed of a smooth distribution of dark matter, with a lot of clump regions where this dark matter has aggregated, and then enabled the formation of galaxies. The presence of these large amounts of dark matter causes light from background galaxies coming towards us to bend. The actual shapes of galaxies that are behind a big clump of dark matter, which is along our line of sight, are actually distorted when we see it. The dark matter in general is smoothly distributed everywhere in the universe. But there are these particular regions that are denser. So we can look at regions in which the dark matter is not so densely distributed, look at the shapes of undistorted shapes of galaxies, and then use that to infer how much distortion we’re actually seeing when we see a lump, and infer its existence from the distortion. Because the strength of the distortion is directly proportional to the mass, you can directly infer how much mass there is between us and those objects.

M&B: Is there evidence other than the bending of the light rays?

The other evidence eludes to the fact that dark matter actually gives the basis for the formation of stars and galaxies. If you look at the motions of stars and galaxies, you find that, unlike the solar system, if you look at the velocities of the planets in the solar system as a function of distance from the sun you find that it’s falling. So the planets in the inner regions are moving very fast; they feel the gravity of the sun much more strongly than the outer planets, which are not actually moving as fast. So if you plot the velocity from the center or from the sun outwards in the solar system, the velocities are falling. Whereas if you go to a galaxy and you do the same experiment, you look at stars at different radii and you try to see what their speeds look like. They are speeding up as you go outwards. And the only way they can do that is there is something that is sitting outside the galaxy that holds it up as it were, and has gravity. And it turns out that our current picture is that most galaxies have very extended dark matter halos. The key point is that there’s a lot of dark matter sitting outside the galaxy well beyond where we see the stars.

The other compelling lines of evidence for the existence of dark matter come from larger scale observations of the universe. One of the leftovers of the big bang or relic of the big bang is this microwave background radiation that is detected today in the universe. It was a very hot radiation that has cooled with the expansion of the universe, and we are bathed in it. I mean it’s everywhere, in every direction, and measurements of the directional dependence of the microwave background shows that it’s very uniform. But in one part it’s anisotropic, so there are cool zones and hot zones in the sky. On very small scales we see these cold and hot spots. And these and isotropy’s in the microwave background are exactly predicted by this model, the cold dark matter model, which I said you know is postulated on the very early generation of the universe and these dark matter particles.

M&B: So we know that they exist. But what is their essence? What are their properties?

This is still a mysterious kind of beast. In fact, we don’t know the nature, but it appears that it’s collisionless. It’s very counterintuitive. That’s one of the things I find fascinating about the fact that it is so counterintuitive. These particles are actually collisionless. What that means is that when two dark matter particles approach each other, they pass through each other, they don’t actually bounce off of each other, so these particles don’t collide, which means they don’t have pressure, because pressure is generated by collision. So they are pressureless particles that have mass. Gravity holds them together, but they don’t actually collide.

So a current understanding of dark matter is that it’s definitely generated very early in the universe and that it clusters very strongly and it’s distributed on very different scales in the universe, so there’s like a smooth background and there are lots of clumps on top of it and so on.

M&B: As far as the mass of Dark Matter how much mass are we talking about? If there is no clustering, something like in the volume of the earth, for example, how much total dark matter mass do we have?

Well I guess the thing is at the position of the earth and at the position of the sun from the center of our galaxy. We are not really dominated by dark matter anymore, so dark matter at that radius doesn’t constitute significant portion of the mass. We are bathed in it, but the density is quite low. It’s at the innermost part of our galaxy where the density is very, very high, so we actually expect that the inner most regions of our galaxy is a very complicated, violent place where not only do you have a black hole which we know exists. This is a black hole that is not made of dark matter. This is a black hole that has gobbled gas and grown to about a million times the mass of the sun, and which is sitting at the center and it creates a very violent place for stars because it can rip them apart and so on. And this whole system is sort of embedded in this very, very dense region full of dark matter.

M&B: How is your everyday work? Where do you get data about the light rays coming near the regions with high Dark matter density?

A: I’m a theoretical person and what that means is that I actually build models but models that are guided by observations. The theory has transformed in the past 10 to 15 years because of the amount of data that technological progress has given us.

For example, the Hubbell space telescope has really transformed the kind of modeling that can be done. So I actually use data that other people have procured. They’ve cleaned it up and they give it to me. These are very distorted images of background galaxies whose shape has been distorted because of the huge amounts of dark matter that is contained in a cluster of galaxies.

M&B: The bending of the light rays is predicted by the general relativity theory. Is it what you use for your calculations?

A: It’s a beautiful theory. I think that people really have understood the iconic status of Einstein. What is most fascinating about him for me is the profound insights that he had of such despaired phenomenon. There was a way in which he was able to see connections and synthesize. You know the whole understanding that the geometry, the fate and the contents of the universe ought to be linked is a profound insight. And I think you know it’s incredible that he enabled, you know, his mind enabled him to formulate it in the way that he did, which has allowed people like me to use general relativity and the sort of elegance of general relativity because of how simple it is in fact to apply and test.

M&B: This looks like an intriguing line of work?

I want to stress that this is a particularly wonderful time for cosmology. It’s a particularly special time. I don’t want to use the sort of often abused golden age as it were, but it’s the confluence of technological progress with the kind of data that we can obtain and the level of understanding that we have built up. This is a very special time to be doing this kind of work. And while the mystery of what dark matter might be made of, you know, it may or may not get solved in my lifetime.

M&B: If you want to describe your work, what term would you use?

In a more descriptive way, I guess. What I really do is I build a story line. I build a story line for the universe. I mean, I build a story of the sequence of events of how a structure forms and assembles. The key there is that you are guided, you have a few snapshots, so you have a bit of data, but then you have to extrapolate and you make some predictions. The goal of the kind of work that I do is to make predictions, and have them be taken seriously, to either be falsified or shown to be true, and what’s exciting about this particular time is that people can falsify your theories or your models in a very short time.

M&B: Isn’t it interesting that we can actually make a story out of universe?

Well, I mean, I think that what is fascinating, and this is a personal view, what is fascinating about cosmology is the… you know it’s mystical and it’s mysterious at the same time and it’s highly abstract. It’s not intuitive because the scale that I’m working with in terms of distances, masses, and energy are just unfathomable. So there’s a real irresistible pull for certain kinds of people to do this kind of work. And I think the universe is a pretty good subject for a narrative because of the range of phenomena that occur in it. This is possibly why in all ancient civilizations there is some notion of cosmogony and this idea of fascination with where we came from. I don’t want to sound arrogant by saying that it’s the fundamental question, but you know it’s an inescapable question for anybody to ponder where we came from.

M&B: Is there a sense of awe that motivates you in your studies?

There is a sense of wonder that the universe generates and I think that you know personally for me I’m a bit of an adventurist, and I think if I can imagine myself, if I had been born 200, 300 years ago I would have been one of those explorers. And I think there are people like that amongst us always, who want to explore in different ways. I mean, there are some people who want to explore via music. And there are frontiers that they want to break through in music, and they have the creativity to do that. And I think that for a lot of us who are doing science, and cosmology in particular, there is a sense of exploration, there’s a sense of examining or grappling with things that are really at the limits of our capabilities in a way.

M&B: In some ways one of the most important fruits of the process and coming along it’s our imagination. And the human imagination is also a part of the universe.

Absolutely. It is an inescapable part of the universe and what is fascinating is the idea that we have the capability to even contemplate the origin of the universe given that we are a part of the universe.

M&B: Many people approach science in a utilitarian way and this is not what you are doing.

I think that the fact that it is not utilitarian is precisely what attracts me to it. I think it is the other side of the coin of this, which is why we probably aren’t as well funded and we ought to be better funded than we are, because this is such a fundamental human curiosity. There is enormous public interest in what we do and the level of funding that we have does not reflect that. For instance, obviously medicine is very critical. It’s critical to our existence. But the disparity in how they are funded and how the pure sciences are funded is sort of disturbing. Because I think as a culture, as a world, we can afford to indulge in understanding basic sciences partly because there are spinoffs from all the work that we do. It is unpredictable, and I think that is what’s fascinating. There are no guaranteed benefits to human kind and society today that cosmologists can bring. However, they satisfy this hunger for knowing and going beyond, you know, satisfying your hunger and thirst on a day to day basis.

Mustafa Tabanli is a producer at Ebru TV. He conducted this interview for Emmy Award winning television series Matter&Beyond.


Dr. Priya Natarajan is a Professor of Astronomy and Physics at Yale University, and an Associate at the Dark Cosmology Center, which is part of the Niels Bohr Institute at the University of Copenhagen, Denmark. Her research interests include cosmology, gravitational lensing, and black hole physics. She earned a B.A. degree in physics and mathematics at M.I.T. and her doctorate at the Institute of Astronomy, University of Cambridge in England, where she was a member of Trinity College and elected to a Title A Research Fellowship that she held from 1997 to 2003. She is currently on leave from Yale to take up her Guggenheim Fellowship. She is deeply invested in the public dissemination of science and is currently a member of the Science Advisory Board for the public television series NOVA.