 Good afternoon. My name's Grace Julie, and I'm an associate professor at the State Department. I would like to thank you all for coming today. I would like to thank Marisha Black for agreeing to take off our celebration of International Dark Matter Day. October 31 has been designated Dark Matter Day to call attention to the importance of the performed cosmic mystery that is dark matter. This is the first of three events the Physics Department, SMU, has planned to mark the location. Tomorrow, I will be getting a technical clock at our weekly Physics Department secret series on recent events and results from the Supercedon experiment. My research for that SMU is a leader in this effort. On Tuesday, we'll be hosting a campus-wide dark matter rock hunt on campus. We will be finding 26 dark matter rocks on campus. These rocks represent the 26% of the universe that is composed of dark matter. Throughout the day, people on campus will be searching for the rocks, and you are invited to join us. If you find a rock, you should take a selfie with your rock in the location that you find it, post it on social media with the hashtag SMU Dark Matter, and then turn the rock in to the main office in 500 Science Room 102 that day between hours of 9 a.m. and 4 a.m. or 4 p.m. 4 a.m. I'll put it down right here at 4 a.m. to collect the prize. Now for the main event. Dr. Berisha Roddick is a Professor of Physics at UC Davis. Urgently from Slovenia, Dr. Roddick earned her PhD from the University of Bonn in Germany in 2004. Following her PhD, she went to work as a postdoc at the Catholic Institute for Particle Astrophysics and Cosmology at the Slack National Laboratory, which is where I first met her. She won a prestigious Hubble Fellowship in 2007. In 2009, she accepted a faculty position at the University of California in Davis, which is where she currently holds the title of Chancellor Fellow. Her research includes studying the composition of the universe professionally being properties of dark matter. Dr. Roddick also studies first galaxies that form in the cosmos. The tools of her trade are telescopes, both in space and on the ground, including the Hubble Space Telescope, the Spitzer Telescope, and the CAC Telescope in Hawaii. Dr. Roddick uniquely combines her passion for the universe with her outdoor love of skiing, surfing, and motor biking. She is passionate about her research and is committed to provide the best possible education for her students. One thing that excites her even more than her own research in the outdoors is seeing young and bright minds excited about the world around them. Please join me in welcoming to SMU Dr. Roushav Raj. Thank you so much. Can you guys hear me? No? I can yell if you want, although you might not want that. It's a great pleasure to be here. Thank you so much for the invitation. I spent a lovely morning in Dallas. It's a beautiful place. And I wish I could stay here until Tuesday and collect those rocks. I would love to go on a treasure hunt. Unfortunately, my job calls me back, and I'll be teaching a class on Tuesday. But today I'm here to tell you something about the dark side of the universe. So this is the light side of the universe. You've been seeing this movie now go around a couple of times. It starts with the Orion, the constellation. Some of you might have recognized it. And it flies out all the way out. You can see now our own galaxy, the Milky Way. And at the end, the fly goes through the nearest cluster of galaxies, the Virgo cluster. So it's the clusters of galaxies we'll be talking about, but really we'll be talking about how we use those clusters of galaxies, which is basically, as the name suggests, just thousands of galaxies piled together to study the dark side of the universe. So let's start at the very beginning. Let's start with the Big Bang. If you Google Big Bang, you've probably seen one of those pictures. Everybody thinks of Big Bang as a big explosion or a TV show, depending who you are. Well, what we'll follow is an actual image of the universe 13.8 billion years ago. This is when the Big Bang happened. And that's the time of the Big Bang. And it looks like this. Very boring. Universe was tiny, tiny, tiny at that point. And there was nothing like an explosion. In fact, what happened after the Big Bang is the universe started expanding. And it was expanding anywhere, everywhere, not just from a single point. And the stuff that governed that expansion were dark matter, light matter, or us and you, and all those stars you've seen in that movie, and the dark energy. And let's talk about them in turn. So you think we know by now what 95% of the universe is made of? Well, tough luck. We actually have no idea what 95% of the universe is made of. For all those high school students in the audience, this is your chance. There is so much left to be discovered. We only know this 5% of the stuff that you, me, all the particles ever detected here on Earth, our Earth, this lecture hall, planets, stars, galaxies, anything you can see, it's all in those 5%. In fact, the human beings are way less than 1%. We are all less than 1%. And so the question today is, what is the rest of the universe? What is it made of? And as Professor Cooley already said, 26% is dark matter. And then there is this big swath on this graph that we have even less idea of what it really is. We'll call it dark energy for now, but it really isn't. We don't even know if it's energy. And so basically, you've learned about all the elements and everything in chemistry. You've been studying this, especially you in high school. You probably hate chemistry by now. We've been studying matter for a couple hundred years. And we have a fine understanding of chemicals, all those bonds, all those reactions they've taught us well. And so it was only in the recent years, in fact, after 1930s and even more so after 70s that we have discovered that all that work that we have been doing for all these 200 years, it's only a tiny fraction of what the universe is made of. So it's basically like somebody puts you on a task of studying an elephant, and all of a sudden you realize it's only its tail that you've been studying all those years. So exciting times are ahead in the studies of dark matter. But how do we study something that we don't see? Well, here we need something called general relativity. So here you go. Well, you saw my joke. Okay, general relativity. Okay, everybody take notes. There'll be a quiz after this presentation. You need to learn it. Bunch of equation. Albert Einstein is the one who figured it out. It's also only in German, okay? Rücksicht. It sounds so realistic and real. So as you know, I'm not going to make you study general relativity from scratch. Things are scary when you don't know them. As a little girl, I wanted to be an astronomer when I was seven years old. I watched the stars with my dad and I said, I wanted to be an astronomer. And my dad, who was a physicist, was like, okay, yeah. You know, first grow up a little and then we'll talk again. And then when I was 18, I said, I wanted to be a computer scientist because that brings way more money. But my passion for astrophysics remained and so I decided to be an astrophysicist. But as a little kid, I was scared of a drill. A drill like this, okay? It made a lot of noise. It was scary. I didn't know what it does. And so I was scared about the drill until Billy came along. Tall and handsome Billy from Sweden. You might have seen it before, that one. Okay? I know you have IKEA in Dallas. Anybody ever put an IKEA piece of furniture together? Do you remember how useful it is to have a drill that actually puts all these three million screws that those things come with together? So at that point, I fell in love with the drill. Okay, finally understood what it is about and I was not scared of it anymore. So where does this bring me? It brings me back to general relativity. When you understand what the things are good for, things are not scary anymore. So let's try this. Let me understand general relativity with a simple PowerPoint graphics. So imagine a galaxy in a background being observed with a Hubble Space Telescope. If there is nothing in between, the light from this galaxy simply passes on a straight line. The light that's going to hit the telescope is the one that comes from the same direction as the galaxy is actually located. Now by the magic of PowerPoint, I'm going to put a big galaxy cluster in the middle. And so what happens is according to general relativity, light will not go through the cluster and that's because the mass of this cluster bends space-time. Or in other words, it just creates a big giant indent. And so rather for the light, for the photons to go down in a hole and up, if you've ever been hiking, you know it's much better to go around the ridge and it's much easier to do it that way than going down the valley and back up the hill. So the light does the same thing. It doesn't go straight through the cluster but it rather avoids it and it's being bent like this. So where do you think we see this galaxy now? Anybody? Where does the light from this galaxy come from? Yes, you see high school students know it. All right? It's up there, okay? So you don't see the galaxy where it actually is but you see it over there. Any other solutions you can think of? High school students know it again. It goes down, okay? And so you see two images of the same source. No, it's not because you're drunk. You literally see two copies of the same source in the sky. But then there is another solution. Okay, somebody else but high school students? All right, yes. You can go outside of the blackboard and in and inside of the blackboard and out and if everything is perfectly symmetrical, you get a perfect ring. And so those multiple images would form further away the more mass there is. So you will get them further apart and if everything is perfectly symmetrical, you would get a ring that's bigger on the sky. And so the more mass you put inside and the lucky for us, general activity doesn't care whether that mass is visible or not. As long as something has mass, it will do this bending of spacetime. And the further apart those images are, the more mass is in there. So now we have a tool that allows us to study matter irrespective of its kind. Okay, you're still not convinced? Now this one is not for high school students. Although you can do it too. So if you're not convinced yet, I'm going to convince you you can do general relativity with a glass of wine. We won't be drinking any wine and you won't be using the glass the way you're normally using, but rather you will be using the stem of the glass. So here it is. Next time you're at a party, take a candle and then tilt the glass and look at the image of the candle through the stem of the glass. And you can see here was your big ring, right? If everything is symmetric, if you put the glass symmetric, you will get a full ring. If you tilt it a little bit, you get those two images just like we had at the slide at the beginning, right? The two images of the same source. This is the two images of the same candle. Or you can get four images, one, two, three, four. And if you still think that I'm just making this up, here the actual, here is an image of our neighboring galaxy, Andromeda galaxy. But these kind of galaxies, there is billions of them in the universe. And occasionally they are placed behind another galaxy in this case. And you can see this is an actual image of a galaxy that's doing the lensing. And in the background, this kind of galaxy is being lensed and you have one, two, three, four images of the same source. Here you have the full ring, right? And here you have sort of a double image. So this kind of configuration happened in nature. And you can use the separation of these images to tell how much matter is in there. And yes, there is visible matter in there. But if there is more matter than just the visible matter, you will see that because the ring will be bigger than what you usually know it is. So now we know general relativity. Not so scary, isn't it? And to be honest, even before this lecture, I can assure you that 99% of the people in this room, or maybe even a hundred, have been using general relativity every single day. Because it turns out that your little phone uses general relativity to correctly predict your position. The GPS signal would have been several tens kilometers, or several tens miles, if you want to think in miles, off in your location. So you think you're in Dallas, in SMU, but you're really in Ikea, okay? If they would switch off the correction for the general relativity. So your phone does general relativity, and so do you. The other place where general relativity is really useful is have somebody heard about gravitational waves recently? Yeah, a couple of people. What happened with gravitational waves? The neutron stars, they saw them when they merged and they were ejecting gravitational waves. What else about gravitational waves in the news? Anybody heard of that guy named Noble? Yeah? Noble prize for physics went to the discovery of gravitational waves this year. So, we know general relativity works, and it's incredibly useful. Now back to what happened to our universe after the Big Bang. So as I said, we are less than 1%. The total matter that you've ever known off is only about 4%. What is the rest of the universe doing? So the majority of the matter and energy in the universe is in form of something else, and that's something else governs what is happening to our universe. So the matter, just like when you throw a rock up in the sky, it will get attracted back to Earth. That's a gravity, right? So the matter in Earth is keeping the rock tied to the Earth. In the same way, the matter in the universe is trying to make the universe collapse, to make the universe stay together, okay? Matter is responsible for us to be together, whereas dark energy is trying to push everything apart. And so if you have more dark energy than dark matter, then the universe is going to be expanding forever. If you have more dark matter plus normal matter is trying to do the same, is doing the same, is just way less of normal matter than dark matter, then the universe will stick together. And so we want to know the fate of our lives, right? And so that's why we want to study how much dark matter there is, how much dark energy there is, and what is going to happen with our universe. So today we are going to be talking about mostly dark matter. If I haven't convinced you that this is an interesting thing to study, well, maybe eight million Swedish Kronas could. So here's a chart of Nobel prizes. The gravitational waves, as I just mentioned, recently got a Nobel prize, and they kind of did it a little bit from an economic standpoint in a bit of a, basically they could do it better because they've done proof of existence and detection all at the same time, so they just won one Nobel prize. They could actually squeeze two out of that, but everything happened way too close to each other. So they have their Nobel prize for both existence and detection. Whereas for dark matter and dark energy, there was a Nobel prize in 2011 where these three physicists have realized that the universe is expanding, and it's expanding whilst accelerating. So something is pushing the universe apart. So that was the first time that we really knew that dark energy exists. So proof of existence is there, but we have no clue what it is, okay? For dark matter, the proof of existence, if you were following the slideshow before, Fritz Wickey in 1939 and Vera Rubin in the 1970s were both already showing the existence of dark matter. But they didn't win the Nobel prize. Fritz Wickey, because he was really annoying personality, he said, all my colleagues, he was doing this all the time, and people were asking him, what is, people were asking him, what is this? He's like, this is my colleagues. They are spherical bastards. No matter which side I look at them, they still look like bastards. So of course, Wickey didn't like anybody, and nobody liked Wickey. So nobody proposed him for the Nobel prize. Whereas Vera Rubin really deserved the Nobel prize. She was the first one who realized that the galaxies are spinning too fast, and so if there is only visible matter in galaxies, they would have to fly apart. So something is keeping them together. Remember, dark matter is the one that keeps things together. And so she coined the existence of dark matter. Why do you think she didn't win the Nobel prize? She was a woman. Yes, she wanted to study in Princeton. Princeton Graduate School said, women are not allowed to study physics. Lucky for all of you high school students, times are changing. And hopefully women will soon win the Nobel prizes. You can definitely come study at any university, okay? And it's definitely okay to do physics, and it's exciting. And so for that very reason, this might be you, or maybe this might be you. And especially this might be you. Because here at SMU, there is some research going on that is tackling directly this question. And I'm going to talk a little bit more about it. So how do we figure out, how do we detect dark matter? And how do we figure out its properties? Okay, as little kids, what did you used to do to figure out things? Anybody? What do you do in chemistry? Try out, experiment, pour through things together, smash things together, you know, and see what happens. And that's exactly what we are doing. So here at SMU, there is dark matter interacting with normal matter. So here we have dark matter, smash it with normal matter and see what happens. And so that's the kind of research you'll hear a lot about in tomorrow's talk by Professor Cooley. And you could see the students working on it. They are mostly sent underground. And there will be more on this tomorrow. It's a dark matter day. But today I'm going to tell you about our research in which what we do is we smash these two together. Dark matter with dark matter, not dark matter with good matter. I'm kind of jealous of that research, you know. We just do dark matter with dark matter and we'll see what happens. Now, how do you smash? You don't even know what dark matter is. You've never detected it. How do you make it smash with each other? Well, you have to be a little bit clever about this and you have to go look up into space and you have to find places where nature has done that for you. We are not, we at UC Davis are not building a billion dollar particle detectors and billion dollar particle colliders are actually colliding normal matter with normal matter. So nobody can build yet a dark matter, dark matter collider. So for that reason, we go to space and here is what I call the cluster of galaxies. So all these orange galaxies like the ones you've seen before belong to a cluster. And we looked at this galaxy cluster with Hubble Space Telescope. Here is Hubble Space Telescope in space. You know, you would think Hubble is some ginormous telescope. It's super powerful because it's up in space but it's only two and a half meters in diameter. So it's not that huge, but it does amazing things. And this is an image of visible matter in that galaxy cluster. And very quickly we figure out that it's not just one galaxy cluster here, but it's two galaxy clusters, okay? So we have two lumps that we see in here of galaxies. Now, Tzviki was the one who for the first time realized that galaxy clusters also have dark matter because he looked at galaxies themselves and they were also moving too fast, just like very Rubin confirmed later in galaxies themselves. However, what Tzviki didn't know is that there is an extra material in galaxy clusters which is not part of the dark matter story. But so when we looked at these clusters, we see they shine very brightly in X-rays. And this is the image that we got from Chandra Space Telescope overlaid. And what you see there glowing is the hot plasma, 10 million degrees Kelvin, really, really, really hot, okay? Plasma that is shining in X-rays. Why do you think Tzviki didn't know about it? Any guesses? It needs a space telescope. Why does it need a space telescope? What's about X-rays? What happens to X-rays from space, you think? They get absorbed by the atmosphere. Unlucky for the astronomers, very lucky for the public, okay? We would all be dead if that wasn't the case. There's a lot of X-rays coming from space and X-rays can kill you. So astronomers can deal with it too, so don't feel too sad for us because we just send a telescope up in space and we can observe. And so when we first saw that image, it was a huge surprise because what we would normally expect, so you have, I told you, this is going to be a lecture. Is there something that writes? Oh, that one. So we have galaxies and so we have two clusters of galaxies and what would you normally expect for the hot gas is to stick with where the galaxies are, is to stick with the clusters and we see thousands of galaxy clusters and they would all have hot gas sort of lining up with the galaxies. So you have mass, so the hot gas naturally falls inside the massive part just like drawn here. But what about this picture? Where do you see the hot gas? Is it lining up with galaxies? Not really, right? So what you see up there is actually more galaxies on the left and on the right. And what does the gas look like? Is it nicely spherically around those galaxies? No, you have a pancake here and something that looks like, can somebody guess? It looks a little bit like bullet flying through the air. Okay? I didn't like the military term, the bullet, but people were stuck with it. So you can see this kind of, it's a Mach cone. Okay, for those of you who know aeroplanes and who know supersonic velocity, this is a Mach 3 shock. So this thing, actually, what happened is it smashed together. 4,000 kilometers a second. Mach 3, the most energetic event since the Big Bang. This is what we have here. 10 million miles an hour. Super fast. And because it smashed together, so now you can see this guy went over here and this guy went over there. So the galaxies themselves, I draw them incredibly big on this picture and they're distributed in space. Even though there's thousands of galaxies, they're tiny. So when the two things smash together in terms of galaxies, they literally go straight through. Just like Harry Potter going through the wall because it's too few of them. They never really do this head on collisions because there's enough space everywhere and so they don't actually hit each other. Maybe one does, but that is not going to slow anybody down. Okay, so it's like having a big football field and you send two people from each side going over the chances that they're actually going to have a head on collision. You blindfold them, it's very tiny. Okay, so that's why the galaxies went straight through. But what about the gas? So the gas is distributed everywhere and it turns out there's a lot more of the gas that there are in galaxies in those galaxy clusters. So there's about 10 times more mass and six times more mass in gas than there is in galaxies. Just like regular matter. We are still with a regular matter. The gas, of course, smashed into each other. It's like bullets flying through the air. You get a math cone. The other one, so the gas from this bigger galaxy cluster is sort of pancake together because the two collide it. And so this picture alone allowed us to say this cluster collided with, these two clusters collided. They collided with a mass speed of three super fast. And this is approximate trajectory of that collision. So sort of the little one flying through the big one. And so after that, the cluster got the name, the bullet cluster. As I said, I tried to call it the pancake cluster, but I didn't quite fly. So now the question is, what did dark matter do in this whole case? So how would you figure this out? Anybody? Can you remember how we measure the mass distribution in the universe? How can we tell there is mass in these clusters? Which is the magic word, general relativity. That wine glass. And so if you look really closely, you can see here some arcs. And there is a lot of lens galaxies in that cluster that we can use to trace where the mass is. So this will give us the total mass. If there is only galaxies and gas in the system, where would you expect the gravitational lensing effect to show up? Remember what is the most massive part of this cluster? Anybody remembers? The gas, right? The galaxies are tiny bit. So if there is only gas and galaxies, you would have the total mass distribution pick up here and here. If there's something else, it might go somewhere else. And so we did the experiment. We measured, we done all our general relativity calculations, we measured the effect of what is called gravitational lensing. So that's exactly where you look at those images and you figure out where the mass distribution is. And this is what we got. In blue now, you see where the total matter is located. And you can see it's clearly located above the galaxies. So this told us that dark matter exist. Because if there was no dark matter, we would have to see those total matter peaks above the gas. But now we see them above next to galaxies. And so for that very reason, we figure out that dark matter exist. And so in addition, we made simulations of this. These are the results and then we try to model this. So in blue is dark matter and in red is gas. And when you see those two smashed together, the gas does the pancake just like we noticed. And then there is the bullet-like shape that you see here. But the matter went straight through. And so when you compare the results of these computer simulations, weight observations, you will see that they match pretty well. So that gave us confidence that we are indeed seeing dark matter. So we know the galaxies are only a tiny bit of the whole cluster, but total mass distribution lines up with galaxies. And that told us that the dark matter really needs to be there. Because something needs to make up for that difference between galaxies and the total matter. But it also tells us something about the properties of dark matter. So now we figure out how much dark matter there is in this cluster and it's about the same as in the whole universe in fraction. And also we could figure out the properties of those dark matter particles. So we have some dark matter, but once again not enough to slow down the expansion of the universe. So that's why the research was really important. As I said, in addition, we figured out what's the density of dark matter. It's all around us as well. If you think that dark matter is something that only exists in space, well, you're out of luck. Exists probably here as well. Depending on the mass of the particle, there might be one or two dark matter particles in this room. Can I do that? Why haven't we seen it then if it's here on Earth? High school students, how many particles of normal matter is there in this room? Anybody can give me a guess? Okay, no? How many? Give me rough guess. 10 to the? 25. 25, 27 probably, right? Avogadro number, just the mole of gas has the Avogadro number of particles. There's a lot of particles in this room and only two of them are dark matter. And even that we don't know because we don't know what's the mass. But we know that matter is here but it's very low density. So 10 to the minus 27 kilograms per cubic meters. Now these kind of tiny numbers, you don't really know what they tell you until you start comparing them with something. Okay, how dense is 10 to the minus 27 kilograms? Is it less dense than water? Sure, of course. Because water is about 1,000 kilograms per cubic meter. Remember cubic meter is about this and this and this, okay? And 1,000 kilograms is about 2,000 pounds, I hope. I come from Europe, I do metric. But it's about 2,000 pounds, right? So that's water. We are talking here about 10 to the minus 27 kilograms per cubic meter. Is it less dense than air? Sure. Is it less dense than the best vacuum we reach today on earth? The answer to this is yes. 10 to the minus 17 is what we've achieved. Is it less dense than interstellar material? Anybody? Yes, no? Maybe. Maybe, that's a good answer. It actually is. It's less dense than interstellar material. So you're like, wait a second. You told us there is so much dark matter it makes up six times more than anything else. Matter-wise in the universe, yet it's less dense than everything. Well, turns out regular matter is governed by electromagnetic forces. So that's why this clusters together. That's why this is hard. And everything else, including the interstellar material, it's kind of clumps together. So it makes it more dense. But dark matter is everywhere else as well. So while interstellar material only exists in a tiny piece of, say this is our galaxy, there is a big halo of dark matter everywhere, not everywhere, but surrounding our own galaxy. And a big halo of dark matter surrounding our clusters. So if I were to draw dark matter on here, I would have to go outside of this building and on the other side, the outside of this building. It's spread out, but it's there and it's a lot of it. So yeah, dark matter exists. And on top of that, what we figured out, it doesn't really interact. Now, as people here at SMU will tell you, this has been very difficult. They're trying to detect dark matter interacting with good matter, with normal matter. And it's not happening yet. They can put limits on it, but they haven't seen this interaction taking place yet directly. We are trying to do that and we see almost no interaction. The thing is weird. You would expect when the two clouds of dark matter collide, just like when gas collides, if dark matter was anything like anything we ever detected, no, not if the two collide, they would have to interact somehow because there's so many dark matter particles in there that they should have interacted. Okay, but they don't. What happens is the dark matter goes straight through, right? It lines up with those two clumps of galaxies. Goes straight through, like Harry Potter through the wall. Doesn't interact. They don't even look at each other straight through. So, I'd like to conclude by saying that dark matter is very, very antisocial. Okay, it doesn't do anything. It doesn't interact much, at least not from what we see right now. So, while the quest for discovering dark matter is ongoing, because it's one of the most important questions of modern day physics, it's been difficult. But it's going to happen, I think, sooner rather than later. We are on the way to make these discoveries happen and we are on the way to make those Nobel Prize for dark matter finally happen. Because we think we are going to be able to finally get our limits low enough that we'll be detecting either the interaction or firmly conclude that dark matter does not interact. So, that will be the quest of our high school students to do that. It will take still a couple of years, maybe 10, maybe 20, but it will happen. So, I encourage you to start doing physics research. It's a lot of fun. And I'm going to conclude today's talk by saying there is a lot of dark matter, including dark matter around us. But most of the universe can't even be bothered to interact with you. Thank you very much.