 Ah, there we are. Hi everybody. My name's Bill Lamar, and I'd like to welcome you to NASA's Ask the Astronomers Live, where you, the audience, can ask, in one of the astronomers, your questions, as I chat with them, are just amazing from this moment today. I'm going to show we're going to be delving into, oh, looking at invisible stars that can tell us about any visitor in NASA. And I just want to point out for anyone joining us live on Facebook or YouTube, you can ask questions, and this is going to raise a lot of questions. Go ahead and put your questions in the comments section of the screen, and I'll pass them on. We'll ask our astronomers. Also, after the show, make sure to visit our website at UniverseUnplugged.org. There, you can find out about our next show, other upcoming shows, and see a lot of really fun astronomy things. All right, let's meet our guests. Today, we have two astronomers who've been studying the observable consequences of the, as yet, unobserved dark matter. See, it's all just going to raise some questions. We're going to be joined today by Dr. Charlie Conroy from Harvard University and Dr. Gretina Besla from the University of Arizona. Hello, Charlie. Hello, Gretina. Hello. Hello. Charlie, can you tell the folks a little bit about yourself and or the work that you do? Sure. So I am a professor of astronomy at Harvard University. I was born and raised in Sacramento, California, and got my undergraduate degree at the University of California at Berkeley and my doctorate at Princeton. Here, my group studies questions related to how galaxies like our Milky Way and the many other galaxies that are in the universe, how those galaxies grow over time. We do that using both observations with telescopes and other techniques as well. Okay, excellent. The growth of galaxies. Gretina. Hi. Can you give us a little background? Sure. So I grew up in Mississauga, which is a city just outside of Toronto, Canada. My parents originally from Punjab, India. And I did my undergraduate studies at the University of Toronto, my PhD at Harvard, and I started as a professor of astronomy at the University of Arizona in 2014. My research team creates detailed computer simulations of our Milky Way galaxy and its nearest galaxy neighbors to constrain their properties over time, including their dark matter content. Oh, interesting. So you're doing simulations. That's amazing. All right. Today's topic is how the distribution of stars around our galaxy is helping us, you guys, learn about the dark matter that is in and around our galaxy. What did I just say? No, Gretina, can you tell us about dark matter? I mean, it sounds so science fiction-y that it's amazing that you brilliant PhDs are doing it and not just like some dude in his mother's basement. But tell us about dark matter and how someone studies it. Yeah, so we don't actually know what dark matter is. This is one of the most important questions in fundamental physics. We do know that it primarily interacts with normal matter, stars, and gas through gravity. And so we have inferred that dark matter exists based on the motions of stars. In fact, Dr. Vera Rubin made some of the earliest measurements that stars within our galaxy are actually moving faster than we would expect if the gravitational forces are coming only from the matter that we can see, like stars and gas. This is actually true in our own Milky Way as well. This faster motion can be explained if there is more material in our galaxy than we can see, and we call this unseen material dark matter. We don't expect dark matter to radiate in, for example, visible or infrared light. So our telescopes won't be able to directly see dark matter. Instead, astronomers can use contests for dark matter's existence through the motions of stars that orbit our galaxy. So my team is creating models of our Milky Way, including its dark matter, to predict how stars should orbit around our Milky Way if dark matter is there. So making accurate models of the Milky Way is critical to interpret the data that is measured and analyzed by observers like Charlie. So were the predictions that we were doing based on what we thought before we had the idea for dark matter, were they a little off and going, hey, something's not right here. We're missing something. And then that's where it's like, okay, we've got to fill in the gap. You know, conceptually, that's fascinating. And Charlie, obviously the dark matter is everywhere including our galaxy, the Milky Way? Yeah, that's right. So we know a lot about our galaxy because we can study the individual stars in our galaxy with much greater detail than more distant galaxies. So our galaxy has several important components. Probably the most well-known is the disk of stars. It's where most of the stars in our galaxy lie. It's what comprises that spectacular Milky Way that you can see in the night sky in a dark site. Our sun is within that disk of stars. But in fact, the Milky Way system extends far beyond that disk of stars about 10 times further away. And that much larger region of space is called the halo of our galaxy. And in that halo, that's where most of the dark matter in our galaxy we think resides in this more spherical region that sort of surrounds the disk. In addition to where most of the dark matter resides, this halo also contains stars as well. A very small number of stars, but as we'll see later, a very important subset of stars. And then finally, out in this halo, in these outer regions of the Milky Way, there's also a handful of smaller so-called satellite galaxies that are orbiting around the Milky Way. And the largest of those is known as the Large Magellanic Cloud. So if we can bring up image number six, it gives it just kind of a nice picture. So that small disk in the middle there is where most of the stars, including the sun, resides. That blob at the lower left there is the Large Magellanic Cloud. And the halo is all of that sort of glowing stuff. It's not actually glowing like that, but it surrounds that system. Okay. Fascinating. Wow. So the halo is the entire area, the large, large area around what we saw there as the disk, which is obviously the common image that we have of the Milky Way of the galaxy, is the disk with the bright, you know, sort of circular thing in the middle. Okay, great. And does the Large Magellanic Cloud have the same shape? Is it also disk-like? Yes, it looks a little bit different, but it does have a similar disk-like behavior. But it is quite a bit smaller. And the Large Magellanic Cloud plays a key part of the story in the study that we are going to talk about later. Okay. Fascinating. Now, Gartina, you said that your group works on computer simulations. Yes. Right, so my team creates these detailed computer simulations of the Milky Way, accounting for all the things that Charlie just described. So this dark matter distribution that we think is in sort of like a halo around our Milky Way. And these models include surrounding galaxies like the Large Magellanic Cloud, which I referred to here as the LMC. So astronomers have actually used the Hubble Space Telescope to clock exactly how fast the LMC is moving around the Milky Way. And if we can bring up graphic number, image number two, we can see that this is sort of a super zoomed-out image and our Milky Way is all the way in the center. And there's a little red dot just to the bottom and to the left. And that is the LMC. And what we've been able to do with this new speed measurements is actually track the past orbit of the LMC around our Milky Way. And that's indicated by this red line and these red arrows. So based on these calculations, we actually know that the LMC has arrived in our neighborhood very recently, only within the past one billion years. And it made its closest approach to our Milky Way only 50 million years ago, which is actually a timescale that is relevant for life on the Earth. Now, given that we know where the LMC has been in the past, my graduate student Nico Garavito-Camargo at the University of Arizona, who just defended his PhD, he was able to make a simulation that accurately captured the gravitational forces the LMC exerts on the dark matter that surrounds our Milky Way. So as the LMC moves through the Milky Way's dark matter, its gravitational force pulls dark matter towards it. So if you go back to image number two, you'll see that as the LMC moves along this red orbit, it's actually pulling dark matter towards it. And this causes this pile up of dark matter in these contours, these blue sort of shapes that you see. And there's these blue shapes are shown on top of this red line. So the LMC is pulling dark matter along with it. And we call this pile up of dark matter, the LMC's dark matter wake. And excitingly, stars should also be pulled along this wake. And this is where Charlie's team comes in. Oh, fascinating. Okay, Charlie. I love that the path of the LMC. Can we call that the run LMC? Charlie, can you tell us more about that? Yeah. So what we want to go out and then look for is evidence or any sign that the stars in the halo are being affected by the orbit of the large Magellanic cloud. So the challenge here is that when you look out at the night sky, you see, of course, many stars, but you can't tell just by looking at them which stars are intrinsically faint and nearby. Versus ones that are very luminous and very far away. And what this really is is a needle in the haystack problem. You've got only one in a thousand stars in the galaxy are out in this halo region. So you have to figure out a way to sift all of the stars in the foreground away to find those rare, very valuable stars that are very, very far away. And so what we did was we combined data from two space satellite telescopes, one built by the Europeans called Gaia and the other built by NASA called WISE. And when we combined data from both of those satellites, we were able to identify special fingerprints that allowed us to say this star must be in the halo very far away, whereas this star must be in the disc of our galaxy very close by. And once we were able to do that, we were able to pretty quickly see the structures that Gratina's simulation had predicted. Okay. Okay. Yeah, I was going to ask about, you know, the ever increasing technology, but that needle in the haystack thing that you brought up, the fact that, you know, actually so much of these discussions with astronomy is about seeing, but of course that raises the problem of optical illusions. What is the difference between a star that is faint and close and bright and far? But as you're saying, the Gaia and the WISE, the telescopes are able to now make a distinction that our eyes would not be able to. Wow. That's amazing. Yes. Well, let me quickly for anybody who missed the beginning of the show and missed the incredible introduction of our incredible astronomers, University of Arizona's Gratina Besla and Harvard University's Dr. Charlie Conroy. This is Ask the Astronomers Live and you're watching us live with astronomers. And we are talking about dark matter, but not in that voice. No, really what we're talking about is how observations of the faintest stars of the Milky Way, who use those to trace out where the dark matter is because dark matter cannot be seen. And hence the name, right? It's dark is sort of a metaphorical name in the sense of like it's too dark for it to be seen as opposed to it's not darker than something else. Right. So it's just, it's not radiating in any way. So because of that, you know, or at least not in optical light. So you're not going to be able to see it, you know, with telescopes that we normally use for optical images. Do we think that perhaps there will be, there's some other technological advance that will allow us to detect dark matter on levels that we don't even know about yet? Yeah. I mean, so one idea is that dark matter could actually self annihilate, in which case they would actually emit radiation in gamma rays through that process. But you need a really high density or a large number of dark matter particles in a really small volume to do that. And it can get confused with other things that emit gamma rays. So it's super tricky to make the measurements, but people have been trying that way. Also, there are detectors on the surface of the earth. This is a huge area of study in particle physics to try and detect a dark matter particles directly because there should be dark matter particles zooming around in our own solar neighborhood and we try to detect them on the surface of the earth. Wow. Fascinating. Oh, wait, we have some actual audience questions. This one's from Mark Davidge. At this point, isn't it likely that dark matter is a property of higher dimensional space and therefore unobservable? Did we just answer that? Who are you feeling this question to? Who wants to take it? Well, I'll give it a shot. I don't exactly know exactly what the question is referring to, but what I can say is that one of the unique things about the measurements that we've made and the simulations that Gertina and her students have run is that this wake that's trailing behind the large Magellanic cloud is a very different signature of dark matter than many other observational signatures in the past. And although we can't really completely rule out other models yet, it gets harder to describe dark matter as something that's not kind of a standard model. The particle that's acting in many ways normally through gravity, but just not interacting by matter. So that wake is really a pretty strong indication that our basic models of dark matter are probably on the right track. I don't know if you want to add to that. I think that's a great answer. I think so far we're still within standard models and nothing has completely fallen apart yet. So we're still pushing that direction. Well, and to Mark's question, if it were a property of higher-dimensional space, then the fact that it has impact and effect on things in our space, doesn't that mean it's part of? Right. So all of the models that we've created have assumed that we're occupying basically the same volume in the same space. So there's nothing fancier than Newton's laws of gravity that are being included in these simulations. So if we can explain it with these models, then we don't need to turn to more exotic options. It's not that I think what's cool about astronomy is that we are in a very creative space. We're open to all sorts of suggestions, which is why we would never dismiss any sorts of comments or questions out of hand because we don't know all the details. But we are rooted within physics, and so we're pushing the boundaries of what is physically possible that is also in order to stay consistent with the other physical laws we know operate on Earth, for example. There you go. Thank you. I appreciate having physical laws rather than just it's the God of gravity who is moving things, which is, you know. Oh, we have another question from Earthskan Bruce Kaldron. How close is the nearest dark matter that has been detected slash derived? So we haven't actually detected dark matter yet. So people are trying, and there actually are detectors on the Earth. So that will be the closest detection point. And so what they have are these detectors with kind of heavy atoms, and what they're hoping for is that a dark matter particle will come and actually collide with one of the atoms in their detector. And so what you would actually detect in the detector is the recoil of the atom that's in your detector itself. So it's direct in the sense that a dark matter particle hit your actual detector, but you're still not seeing it necessarily. You're seeing the reaction of it. So yeah, you know, I mean, it's a big effort again, across like an international effort. There are these detectors all over the world and we are hoping that at some point we'll be able to get to a sensitivity level where Rashi detecting it. It's super exciting. Is there, I mean, as you all know, I am not at your level intellectually and in terms of study, but is there any sort of correlation between the study and observation of dark matter via its effect? And for those of us down here on the earth who don't have your degrees, wind, because I feel like a lot of times people think of wind as a thing, but it is not something that can be seen. All that can be observed is its effect. Yeah, I mean, that's great. It kind of gets into a little bit of a philosophical point about what is the nature of observation and when do we decide that we've detected something? I mean, you know, the nature of the electron and other subatomic particles like quarks, you know, we don't have a good photograph of it that we can point to like other things, but we, through the process of deduction, have figured out that we were pretty sure that things exist. And so I think dark matter is in the same category, although not, you know, if you rank something from being very hypothetical to very solidly existing, you know, dark matter somewhere in the middle, and with all of these observations, whether on earth from the experiments that Kirtina mentioned or from the observations in space, we're kind of trying to get outlines of what we think is going on. And over time, you know, the outline becomes clearer, even if we don't really have that super clear photograph. And we may never have that clear photograph. That's also possible. And so it does beg the question of, you know, when you just pack up and go home. Well, but you guys are, you know, observing enough that you can make predictions about your future effects and when the predictions are correct, then you know you're correct. Oh, Erskine Kaldron wanted to rephrase, oh, they didn't mean an actual provable detection. I guess they meant the closest interaction, the closest dark matter effect that we've, yes. Yeah, thanks for the clarification, Erskine. Yeah. So, you know, I think the closest proof, sort of that they're, you know, that's motivating dark matter is just the motion of the sun. So we know that we're moving at around 240 kilometers per second around the center of the galaxy. And if you were to add up all the stars that are between, you know, us and the center of the galaxy and say that's all the mass that there is, then we shouldn't be moving that fast. So there has to be something else between us and the center of the Milky Way and even, you know, and this is also true for stars that are further away in the disk of the Milky Way than where we are. So there has to be some other matter, right, in our vicinity that is causing this. So that is really the sort of proof of concept of that there is actually dark matter and this is what really did motivate putting detectors on the earth to detect dark matter because we know the sun is moving faster which means there's other mass around which means there's some likelihood that a dark matter particle is passing by us. Okay, got it. So it's not just that we're observing other galaxies moving faster while ours is just moving at the speed limit. It's happening. So the closest is right here. Right, so inside our own Milky Way we know stars are actually orbiting around the center of our Milky Way. So all the stars are in motion in our own sun. Yes, wonderful, wonderful. Now, wait, there is an animation of what is it of? Oh, it's sort of is it the movie that sort of shows it's nice because it shows sort of the combined effort that we have here. Again, so this whole project was you know, so the theoretical basis that there should be a dark matter week the dark matter must exist in predictions and coming to teams like Charlie's and his graduate student Rohan Naidu and actually trying to figure out a way to observe this. So yeah, we could show the movie that sort of illustrates here the Milky Way and the LMC, the LMC being 160,000 light years away. And this is the region where Charlie's team maps stars in the outskirts of the Milky Way and this is over plotted the simulation created by my student Nico Garavito Camargo that illustrates the dark matter week that should be behind the LMC and the structure is just huge on the sky it should extend out to larger distances and as we had talked about you know, motivating future surveys we expect that we should see other structures at larger distances there are predictions for the speeds of these stars that are associated with this week so there's a lot more work to do that should be coming up. Okay, wow. That's amazing and it's great to see it in motion because we get a sense of and that red line path the what I call the run LMC that's the path that the LMC has taken over. You said it's been here relatively recently just in the last what billion years did you say? That's right so that red path would be the total amount of time it would take for the LMC to travel that path is about one billion years. Got it, got it. All right, wait we have one more we have another audience question from okay this is great we have a question about the unobservable from the unpronounceable nb8k asks will the understanding of dark matter change our theories about the big bang and the expansion of the universe or do existing models of physics already include the impacts of dark matter? Oh that's a good question who do you see a couple questions on that one? I can start so yeah so our standard cosmological model both for the big bang and the subsequent evolution expansion of the universe the growth of all the structure we observe all of those standard models include the effects of dark matter so yes it's already baked into all of those models and calculations and simulations we in fact some of those observations regarding the expansion of the universe and the nature of the big bang are further indirect evidence in support of dark matter which is to say if you tried to make a model of the universe which only had the stuff that we really know of the protons and you know atoms and stuff it wouldn't explain all of those observations from the so-called cosmic microwave background or the expansion of the universe so dark matter is actually an important part of that story as well got it oh that's great um and let's see oh here's another question um have we ever seen the Milky Way merge with another galaxy is the is the LMC gonna slam into us? so I mean I could talk about the LMC and then I think Charlie can mention about the past um so currently the LMC is 160,000 light years away from us and it is moving away but because of this dark matter weak that's created behind it that weak is actually building up all of this dark matter behind the LMC and that has a force of gravity of its own and it's pulling back on the LMC causing the LMC's orbit to slow down so its speed is decreasing because of this effect that we've just observed and so we expect that that orbit of the LMC will eventually it'll slow down and come back towards our Milky Way and eventually merge into our galaxy to form a slightly larger system and this is a normal process that we believe happens to all galaxies at some point in time but we do have evidence for past interactions which I think Charlie can talk a little bit more about yes Charlie tell us about the history of galactic mergers in 60 seconds right so again in the model that we think reproduces the universe we believe that galaxies like our Milky Way grow from the assimilation of many smaller systems and we have evidence for several of those events and there's two large ones one that happened a long time ago about 10 billion years ago and they crashed into another galaxy and that actually created a lot of the stars in the halo that were an important part of the study and then there's another galaxy called the Sagittarius galaxy which is currently being disrupted and destroyed by our galaxy so those those two and the large Magellanic cloud are kind of the big three that have contributed to the growth of our Milky Way over the last 14 billion years of cosmic history I see and of course I'm guessing that we're looking at these which are the relatively closer examples of this but we're assuming that this is happening at galaxies far away as well yes that's right and we see some evidence for that when we look at more distant galaxies we see clues that they're undergoing similar processes ah so all we need is just to run one of those slow motion cameras for about 15-20 billion years and then boom we've got it exactly I mean this is actually what's pretty cool about setting the Milky Way and the local group is that we can actually watch stars inside these galaxies moving and so suddenly it's not just taking because you're exactly right it would be great to be able to see the whole process take place and it have a time lapse of the entire events but really we've been able we've had to piece it together by looking at different systems and different stages of these mergers and interactions but we're capturing the LMC coming in right now and we're actually able to walk the speeds of individual stars inside the LMC inside our Milky Way surrounding our Milky Way and actually get a dynamical picture of what's going on and that's a huge change that's only been possible over the past few years wow that's incredible wow and actually this whole discussion has been really incredible and but we're about at our time but before we finish up are there any final thoughts that you guys would like to share with our audience Charlie sure so I think pulling back I think one of the exciting things about this project for me is it is it really highlights the really collaborative nature of science so not only within my group where there's several people working on this project including a graduate student Rohan Naidu, postdocs and senior scientists but it was also a collaboration between institutions between Harvard and Arizona used data that was collected and carefully scrutinized by teams in Europe and in the US at NASA so it really is this example where you have to pull together and then the simulations that Gratina ran so everything kind of came together in this great, really international way just kind of the science at its best I think that's, yeah, that has actually such a great metaphor for when we all join together we can bring meaning to the universe Gratina, final thoughts? Yeah, you know there's something that you said earlier that's got me thinking where you're, you said that I'm not at your level intellectually and this is where I want to push back on I think that often people are sort of scared to pursue astronomy in particular theoretical work when I talk about theoretical astrophysics because it's sort of this perception that you need to be a genius to work on these things or that it'd be too hard, too complicated and really what it boils down to is just being willing to work hard and I really do want to think about when you're talking about these simulations that we've created the physics that goes into this is really just Newton's laws this is the physics you learned in high school and we turn to computer simulations because we have a billion to ten billion particles in our simulations and so you can't compute all of those forces by hand you need a computer simulation to do that and that's why we turn to those sorts of levels and we have teams of people this whole concept of this lone wolf genius sitting in a corner doing all the work by themselves is just totally obsolete this doesn't exist anymore so I do think that there is persistence that's necessary I also wanted to mention that getting involved in astronomy learning more about it can help to dispel a lot of these issues and so we have a talk series at the University of Arizona entirely in Spanish called Astrochialis that was started by my graduate student so you can do both astronomy and physics and also give back to your community is also a thing of being encouraged in the field and so to check out the website we have a great YouTube link and you can find some more talks there wonderful, wow that's amazing you guys you went from the unobservable you know billions of years away billions of light years away to our lives here today how to make them better that's some pretty great work thank you very much for joining us today and thank you all the audience members who were here for this conversation now you can see the whole 3D movie made from that image as well as all of our other videos at universeunplug.org or you can just click on the link if you're you know on Facebook or YouTube but yes make sure you subscribe to universeunplug.org on YouTube follow us on Facebook because even in the world of astronomy clicks and views matter a little bit and plus you don't want to miss the shows like this again thank you to Dr. Charlie Conroy and Dr. Katina Besla for taking the time and sharing your work and until next time I'm Phil Lamar and you've just asked the astronomers live see you next time