Risa Wechsler
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Do you ever think about what would happen if the world were a little bit different? How your life would be different if you were born 5,000 years from now instead of today? How history would be different if the continents were at different latitudes or how life in the Solar system would have developed if the Sun were 10 percent larger.

Well, playing with these kinds of possibilities is what I get to do for a living but with the entire universe. I make model universes in a computer. Digital universes that have different starting points and are made of different amounts of different kinds of material. And then I compare these universes to our own to see what it is made of and how it evolved.

This process of testing models with measurements of the sky has taught us a huge amount about our universe so far. One of the strangest things we have learned is that most of the material in the universe is made of something entirely different than you and me. But without it, the universe as we know it wouldn't exist. Everything we can see with telescopes makes up just about 15 percent of the total mass in the universe. Everything else, 85 percent of it, doesn't emit or absorb light. We can't see it with our eyes, we can't detect it with radio waves or microwaves or any other kind of light. But we know it is there because of its influence on what we can see.

It's a little bit like, if you wanted to map the surface of our planet and everything on it using this picture of the Earth from space at night. You get some clues from where the light is, but there's a lot that you can't see, everything from people to mountain ranges. And you have to infer what is there from these limited clues. We call this unseen stuff "dark matter."

Now, a lot of people have heard of dark matter, but even if you have heard of it, it probably seems abstract, far away, probably even irrelevant. Well, the interesting thing is, dark matter is all around us and probably right here. In fact, dark matter particles are probably going through your body right now as you sit in this room. Because we are on Earth and Earth is spinning around the Sun, and the Sun is hurtling through our galaxy at about half a million miles per hour. But dark matter doesn't bump into us, it just goes right through us.

So how do we figure out more about this? What is it, and what does it have to do with our existence? Well, in order to figure out how we came to be, we first need to understand how our galaxy came to be. This is a picture of our galaxy, the Milky Way, today. What did it look like 10 billion years in the past or what would it look like 10 billion years in the future? What about the stories of the hundreds of millions of other galaxies that we've already mapped out with large surveys of the sky? How would their histories be different if the universe was made of something else or if there was more or less matter in it? So the interesting thing about these model universes is that they allow us to test these possibilities.

Let's go back to the first moment of the universe — just a fraction of a second after the big bang. In this first moment, there was no matter at all. The universe was expanding very fast. And quantum mechanics tells us that matter is being created and destroyed all the time, in every moment. At this time, the universe was expanding so fast that the matter that got created couldn't get destroyed. And thus we think that all of the matter was created during this time. Both the dark matter and the regular matter that makes up you and me.

Now, let's go a little bit further to a time after the matter was created, after protons and neutrons formed, after hydrogen formed, about 400,000 years after the big bang. The universe was hot and dense and really smooth but not perfectly smooth. This image, taken with a space telescope called the Planck satellite, shows us the temperature of the universe in all directions. And what we see is that there were places that were a little bit hotter and denser than others. The spots in this image represent places where there was more or less mass in the early universe.

Those spots got big because of gravity. The universe was expanding and getting less dense overall over the last 13.8 billion years. But gravity worked hard in those spots where there was a little bit more mass and pulled more and more mass into those regions.

Now, all of this is a little hard to imagine, so let me just show you what I am talking about. Those computer models I mentioned allow us to test these ideas, so let's take a look at one of them. This movie, made by my research group, shows us what happened to the universe after its earliest moments. You see the universe started out pretty smooth, but there were some regions where there was a little bit more material. Gravity turned on and brought more and more mass into those spots that started out with a little bit extra. Over time, you get enough stuff in one place that the hydrogen gas, which was initially well mixed with the dark matter, starts to separate from it, cool down, form stars, and you get a small galaxy. Over time, over billions and billions of years, those small galaxies crash into each other and merge and grow to become larger galaxies, like our own galaxy, the Milky Way.

Now, what happens if you don't have dark matter? If you don't have dark matter, those spots never get clumpy enough. It turns out, you need at least a million times the mass of the Sun in one dense region, before you can start forming stars. And without dark matter, you never get enough stuff in one place.

So here, we're looking at two universes, side by side. In one of them you can see that things get clumpy quickly. In that universe, it's really easy to form galaxies. In the other universe, the things that start out like small clumps, they just stay really small. Not very much happens. In that universe, you wouldn't get our galaxy. Or any other galaxy. You wouldn't get the Milky Way, you wouldn't get the Sun, you wouldn't get us. We just couldn't exist in that universe.

OK, so this crazy stuff, dark matter, it's most of the mass in the universe, it's going through us right now, we wouldn't be here without it. What is it? Well, we have no idea.


But we have a lot of educated guesses, and a lot of ideas for how to find out more. So, most physicists think that dark matter is a particle, similar in many ways to the subatomic particles that we know of, like protons and neutrons and electrons. Whatever it is, it behaves very similarly with respect to gravity. But it doesn't emit or absorb light, and it goes right through normal matter, as if it wasn't even there. We'd like to know what particle it is. For example, how heavy is it? Or, does anything at all happen if it interacts with normal matter? Physicists have lots of great ideas for what it could be, they're very creative. But it's really hard, because those ideas span a huge range. It could be as small as the smallest subatomic particles, or it could be as large as the mass of 100 Suns.

So, how do we figure out what it is? Well, physicists and astronomers have a lot of ways to look for dark matter. One of the things we're doing is building sensitive detectors in deep underground mines, waiting for the possibility that a dark matter particle, which goes through us and the Earth, would hit a denser material and leave behind some trace of its passage. We're looking for dark matter in the sky, for the possibility that dark matter particles would crash into each other and create high-energy light that we could see with special gamma-ray telescopes. We're even trying to make dark matter here on Earth, by smashing particles together and looking for what happens, using the Large Hadron Collider in Switzerland.

Now, so far, all of these experiments have taught us a lot about what dark matter isn't


but not yet what it is. There were really good ideas that dark matter could have been, that these experiments would have seen. And they didn't see them yet, so we have to keep looking and thinking harder.

Now, another way to get a clue to what dark matter is is to study galaxies. We already talked about how our galaxy and many other galaxies wouldn't even be here without dark matter. Those models also make predictions for many other things about galaxies: How they're distributed in the universe, how they move, how they evolve over time. And we can test those predictions with observations of the sky.

So let me just give you two examples of these kinds of measurements we can make with galaxies. The first is that we can make maps of the universe with galaxies. I am part of a survey called the Dark Energy Survey, which has made the largest map of the universe so far. We measured the positions and shapes of 100 million galaxies over one-eighth of the sky. And this map is showing us all the matter in this region of the sky, which is inferred by the light distorted from these 100 million galaxies. The light distorted from all of the matter that was between those galaxies and us. The gravity of the matter is strong enough to bend the path of light. And it gives us this image. So these kinds of maps can tell us about how much dark matter there is, they also tell us where it is and how it changes over time.

So we're trying to learn about what the universe is made of on the very largest scales. It turns out that the tiniest galaxies in the universe provide some of the best clues. So why is that?

Here are two example simulated universes with two different kinds of dark matter. Both of these pictures are showing you a region around a galaxy like the Milky Way. And you can see that there's a lot of other material around it, little small clumps. Now, in the image on the right, dark matter particles are moving slower than they are in the one on the left. If those dark matter particles are moving really fast, then the gravity in small clumps is not strong enough to slow those fast particles down. And they keep going. They never collapse into these small clumps. So you end up with fewer of them than in the universe on the right. If you don't have those small clumps, then you get fewer small galaxies. If you look up at the southern sky, you can actually see two of these small galaxies, the largest of the small galaxies that are orbiting our Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud.

In the last several years, we have detected a whole bunch more even smaller galaxies. This is an example of one of them that we detected with the same dark energy survey that we used to make maps of the universe. These really small galaxies, some of them are extremely small. Some of them have as few as a few hundred stars, compared to the few hundred billion stars in our Milky Way. So that makes them really hard to find. But in the last decade, we've actually found a whole bunch more of these. We now know of 60 of these tiny galaxies that are orbiting our own Milky Way. And these little guys are a big clue to dark matter. Because just the existence of these galaxies tells us that dark matter can't be moving very fast, and not much can be happening when it runs into normal matter.

In the next several years, we're going to make much more precise maps of the sky. And those will help refine our movies of the whole universe and the entire galaxy. Physicists are also making new, more sensitive experiments to try to catch some sign of dark matter in their laboratories.

Dark matter is still a huge mystery. But it's a really exciting time to be working on it. We have really clear evidence it exists. From the scale of the smallest galaxies to the scale of the whole universe. Will we actually find it and figure out what it is? I have no idea. But it's going to be a lot of fun to find out. We have a lot of possibilities for discovery, and we definitely will learn more about what it is doing and about what it isn't. Regardless of whether we find that particle anytime soon, I hope I have convinced you that this mystery is actually really close to home. The search for dark matter may just be the key to a whole new understanding of physics and our place in the universe.

Thank you.