Revealing the Nature of Dark Matter

good evening welcome to Fermilab I'm Tom Carter your host for this evening I'd like to tell you about some upcoming shows we have we have one that's going to be a very exciting that's not in the regular schedule on Sunday March the 8th at 2:30 we're going to have the ad the army jazz ambassadors the concert is free it's going to be great but you're going to have to get tickets so if you'd like to come to the army jazz ambassadors it's going to be here in the auditorium on March the 8th just call and get your tickets so the next next show after after this evening is Saturday February 7th it's going to be cirque du mazuma which is going to be an interesting combination of african african traditions and modern-day circus right the next talk is going to be on the understandings of cancer on March the 13th and that's going to be dr. Mindy Bessel from Lawrence Livermore so that brings us to this evening speaker everybody pumped up okay Dan Hooper okay so Dan completed his PhD in 2003 at the University of Wisconsin he was a postdoc at Oxford and then the Dave David Schramm fellow here at here at Fermilab he's currently he's an associate scientist with the theoretical astrophysics group here an assistant professor at University of Chicago he's written more papers you know can't can't cut him up on cosmology and and dark dark matter he's the author of two books two books available on Amazon Dark Cosmo in search of our universe is missing mass and energy and nature's blueprint okay so if you're impressed so far hang in there gets better okay dan is actually okay an accomplished guitarist right okay when he was a postdoc at Oxford he actually played in a band with Jim Viner of The Pogues how cool is that right okay but here's okay here's the here the coolest part okay Dan is actually in a blues band the congregation who played here at fermilab on this stage so in the history of the lab you are about to hear the only person that has both spoken on dark energy and play blues guitar on this stage so please welcome Dan Hooper that was ridiculous Wow quite an introduction I don't think I can live up to this so I just want to start by saying how cool it is to have this room filled we live in a time in place where the common narrative you always hear is that the public's not really interested in science and that you have to somehow trick or you know otherwise convince them to pay a little attention to it but yet here we have a 900 or so people selling out the the joint that's great so especially all the young people in the room I'm just thrilled to have this this sort of interest in the science we're doing here at Fermilab so I'm going to start by posing a pretty simple question and then we'll get a little more complicated as it goes on the question I want you to think about and everybody think about is just what is in our universe sounds like a very simple question we all think we have some intuitive ideas about this we might start by just looking around the room and seeing okay there's a stuff in the chair I'm sitting on there's this this wood that makes up this paneling there's a clothing there's a stuff that makes up my body all this stuff but that's just taxonomy and what physicists are really interested in is trying to understand what all those things have in common what are their smallest constituents why do they come in the patterns they come in and how do they work this is not a new question this is an ancient effort to try to answer that question so this is actually a diagram from the 17th century but it depicts um a much older idea than that this is the classical Greek elements we can see the Oh in the corners there's the air earth fire and water this is alchemy of as of the 17th century and this was at that time the best answer anyone had come up with to the question I'm posing here tonight this is the elements as we understand them in the 20th century this is a periodic table that you learn about in chemistry and all that stuff I listed a couple minutes ago that share your clothes the wood your body is all made up of stuff on this table that's a remarkable accomplishment all the stuff you have first-hand experience with we can put on one chart what's an even cooler accomplishment is the fact that everything on this chart is really just a combination of three things protons neutrons and electrons if you take those three things and combine them in different ways you can make all this stuff we have first-hand experience with and if you asked an astrophysicist or a cosmologists I don't know 30 years ago what was this stuff in the universe they would have pointed to this and said probably it's mostly made of this as far as we know this is the answer mostly hydrogen and helium in fact a little smattering of other stuff but if you ask the same collection of scientists today they'll show you this plot and this is staggering this is crazy okay all that stuff in the periodic table makes up about 4% of the matter and energy in the universe today okay so most of that is hydrogen and helium out in intergalactic space just in the form of free gas a smaller piece about 0.4% of everything is in stars planets and other concentrated objects okay the other 96% we don't know what it is okay we use words like dark energy and dark matter but the honest fact is we don't have much more than an educated guess about what this stuff is or why it's there how it works I'm not going to talk about dark energy today except for a few seconds now we don't know what it is but we know that the universe is growing or expanding at an accelerating rate whatever causes that we call dark energy we don't know why it's there we don't know how it works but it seems to be doing its job and then the part that we're really the part I'm going to talk about for the next hour or so is dark matter which is kind of my area of expertise these days or the investigation so turning I'm going to frame this whole talk around questions so the next question isn't what's in the universe but now that I've told you that cosmologists think there's a lot of dark matter in the universe I'm going to try to explain to you some of the read why we've come to that conclusion some of the earliest and most compelling evidence we had have for Dark Matters existence comes from the ways that galaxies rotate or orbit so here's a picture of a galaxy kind of like the Milky Way this is what the Milky Way would look like if you got up above and look down and each of these little dots are just a star or group of stars and just like planets orbit around the star the star they are nearby stars orbit very slowly around their galaxy it takes about a 250 million years but the Sun will go all the way around the Milky Way and that sort of time aggghhhhh lactic year is a much longer than the solar year so this picture on the left is what a galaxy would look like and what it what the Stars would do if there was no extra matter so all the matter is the stars gas and dust that we see with telescopes and then here's the same galaxy with a large amount of dark matter in it about the same amount that we think the Milky Way has at first glance they look really similar these look about the same unless you start paying really close attention look out here see how slowly these stars are meandering and then look out here you can see there's ipping around a lot faster so the Galactic rotation curves as we call it look really different when you put dark matter in a galaxy and we see this in hundreds and hundreds of galaxies we study in detail we also see evidence of dark matter and bigger systems that we call galaxy clusters a galaxy clusters just a bunch of galaxies gravitationally bound to each other we the Milky Way is in part of a cluster we call the local group it's not a very creative name but that's the one we're stuck with so this is actually a very special system it's not just a galaxy cluster or a cluster of galaxies but it's a pair of them one here and one here and about a hundred million years ago they went underwent a collision and pass through each other that gives us a very special opportunity to study the dynamics of these systems I'm going to tear this this image apart and show you one piece at a time now so first the is what the the direction of the sky in question looks like in an optical wavelength so this is what it would look like if you just could look at it with your eyes so an invisible light these bright things they kind of look like stars but they're actually individual galaxies so these are all galaxies then this red or pink stuff that's where an x-ray telescope detects light so if you if your eyes could see an x-rays that you would be able to see this part of the image this part this arc er this this bullet-like shape that comes from the collision so when these two clusters pass through each other it created a shock or a wake and that's the shape of this this bullet and then this part these big blue blobs are where we can tell most of the masses so basically this is where the gravity seems to be located you can see this by looking at how light is deflected around the system as it passes by the cluster so let's put it all together the interesting thing is the pink or the red the hot gas and the gravity are in different places so we're looking at an example where the gas collided with each other slowed down and the dark matter just kept going so you could pull apart the atoms from the dark matter isolating them and this is the first time anyone saw something like this and it convinced a lot of people who had been skeptical of Dark Matters existence that no this stuff's real and is abundant in our universe the third line of arguments I want to present is a an explanation that Dark Matter offers for why galaxies and clusters of galaxies come in two patterns we observe so the standard picture of the Big Bang which is empirically confirmed in lots and lots of different ways that I won't be able to go into today says that when the universe was young and hot everything and it was pretty uniformly distributed there was about the same amount of stuff everywhere this is a dark matter simulation I'm about to run this is what the universe looked like in the early universe all these dots are a chunk of matter in particular dark matter and we're going to let that dark better evolved through the force of gravity and here we go you can see slowly the dark matter collapses under gravity forming dense clumps of stuff we call halos and that's what it looks like now these sorts of dots are all individual galaxies or halos around galaxies and then the big clumps of clumps those are clusters of galaxies or super clusters the really remarkable thing is if I showed you this map and then I showed you a picture of a map of galaxies that were actually observed in a in cosmology in space you wouldn't be able to tell them apart these simulations predict what the large-scale structure of our universe should look like and we don't have an explanation for that unless there's dark matter and then the fourth and final argument I'm going to give as to why cosmologists become convinced that Dark Matter exists comes from this map give me a show of hands if you've seen this map before ok yes a lot of you but not all of you that's kind of what I expected so this is something we call the Cosmic Microwave Background so three or four hundred thousand years after the Big Bang the universe was several thousand degrees in temperature and I don't mean some places where I mean the whole thing all of space permeating the entire universe was full of 5000 degree plasma it turns out that that's about the temperature where you can melt atoms this is a phrase that's not used much I don't know why but I think that's exactly what's going on if you heat up an atom to about 5000 degrees the electrons and the protons don't stick together anymore and they become free okay so before that point when the universe was hot you had electrons and you had protons but you didn't have any bound atoms and then you reached a point where it cooled enough and they became complete atoms or they they became solid atoms if you will and suddenly the universe changed and it released a whole lot of light and that light has been traveling through space cooling ever since and we can't acted with radio telescopes and microwave telescopes and this is that map the I think they're red I'm colorblind the red orange points are the hottest parts of the Cosmic Microwave Background and the blue are the coldest parts they're just slightly hotter and colder than other parts of the sky so cosmologists can take this map and use it to learn not only what the universe was like a few hundred thousand years at the Big Bang but how its evolved and changed ever since this is the sort of plot we make with that data and don't don't let it intimidate you it's actually really simple this bump here just means that most of the little fluctuations on here are about one degree in size so something about this twice the size of the moon okay that's like a typical hot spot or cold spot is about that size there's some stuff at smaller angular scales and larger angular scales most of its here the line through these points these measurements are our best fit cosmological model and that includes some dark matter and dark energy and all this stuff if we changed the amount of dark energy in the universe or the amount of atoms in the universe or the amount of total matter in the universe or whatever you change that line so we can turn all of these knobs at our model until we get the best possible fit and when we do that we reach this conclusion about 85% of all the matter in the universe isn't atoms it's this other stuff we call dark matter cool huh all right going back to the theme of questions I hope I've convinced you that well you shouldn't really be convinced that Dark Matter exists but you can take my word at it that cosmologists are pretty convinced at this point if anyone can convince you of something that complicated in five minutes you're too too gullible but we're convinced and you should take our word so the thing we'd really like to know and the thing that I would really like to know and I've spent the last 15 years or so thinking about is what is this stuff that we call Dark Matter a core a kind of a companion question is if all this dark matter is out there we can't see it why can't we see it why is it so dark or if you turn the question on its head why is ordinary matter so bright or why can we perceive ordinary matter when I look at that podium why is it giving off light that my eyes can detect and for that matter when I put my hand into this wall why does it stop why do I feel this why doesn't my hand just go straight through that wall after all my hand is basically empty space the fraction of the volume of my hand that atoms are actively occupying is minuscule and that same thing is true about the wall so when I put my hand into the wall you might have expected that my hand should have passed straight through it but it doesn't does anyone know why unless you're in the first ten rows I can't see your hand so here what's it electromagnetism so it turns out that there's a lot of stuff with electric charge in my hand the protons and electrons and in that wall and when I push my hand towards that wall those electrically charged particles push against each other and that prevents my hand from going into the wall so it turns out that the reason light is emitted reflected or absorbed by stuff is because stuff in it has electric charge something without electric charge will be dark an ordinary matter is bright because it has electric charge to kind of generalize this argument think about the four forces that physicists have identified in nature the by far the weakest of these forces is gravity it's basically not measurable in the laboratory we can barely tell there's gravity between atoms like it's extremely feeble and then there's the electromagnetic force that we're talking about there's also the strong force and weak force these you only notice in subatomic physics for the most part the strong force though as its name implies is very strong if dark matter interacted through the strong force with with atoms we would have detected it a long time ago so what we learn from this is whatever dark matter is made of whatever kind of substance it is it better not feel either of these two forces it can't have electric charge and it can't have this stuff we call color it's not the kind of color that you know about but it's the same word it's confusing it's this thing that makes you feel the strong force whatever the dark matter is made of can't feel these two forces this chart shows every kind of matter we understand in the universe okay so all the stuff on the periodic table is made up of these guys basically along with the electron these six quarks are all the things that are strongly interacting in nature these six leptons include the well known electron the neutrinos that you've probably heard of and a couple of other particles and then these are the particles that carry forces the photon is why there's electromagnetism the gluon is why there's a strong force the WZ is why there's a weak force and then there's a Higgs boson that's been all over the news since it's been discovered a couple years ago at the Large Hadron Collider let's contemplate for a second whether any of these known types of matter or energy might make up the dark matter well the first thing we can do is rule out anything that feels the electromagnetic force so I've crossed out everything on the chart that has electric charge that's a lot of it right off the bat that simplifies the problem now let's carry one step further rule out everything that feels the strong force and again the quarks get ruled out the gluons gone now and we're left with a few options the Z the Higgs and the neutrinos but now we also have to require that the dark matter is stable and that rules out these guys the Z and the Higgs what I mean by that is if all the dark matter were made up of Higgs bosons today tomorrow there wouldn't be any dark matter the stuff decays in a fraction of a second it's it just it doesn't play out and play the kind of role we're hoping to play and we're left with only the three particles that are called neutrinos and they look like the kind of thing you'd want to make up the dark matter except it turns out that they're really fast moving or we like to call it hot and that just doesn't look like the stuff we call dark matter and out and out in the universe so none of this stuff seems to do a good job of explaining the phenomena that we observe in dark matter this might sound like a catastrophe but where some people might have seen a catastrophe particle theorists see a playground I spend like several hours a day just brainstorming new crazy ideas of what dark matter might be it's great all right this is a plot that a friend of mine Tim Tate a UC Irvine made it's his artistic impression of some of our ideas about what dark matter might be made up of some of these ideas are really well motivated elegant particle physics frameworks like supersymmetry you might have heard of some of them involve extra dimensions of space some of them involve neutrino like particles with even weaker than weak interactions and many many other things the point is we have all sorts of ideas for what the dark matter might be and there's even a good chance that whatever it really is isn't on this chart or anyone's mind yet we just don't know what it is but we have lots of ideas and what we really want to do is to find a way to test different ideas and eventually figure out what the characteristics of dark matter are and maybe where and this or whatever plot it might might fall we have three different classes of ways that we do experiments trying to test these different different theories trying to figure out what kind of matter the dark matter consists of what kind of particles there's the using big machines like the Large Hadron Collider they're big underground detectors we call them direct detection experiments and then telescopes let me go through each of these briefly this is a it's not the Tevatron it kind of looks like Fermilab but this is the Large Hadron Collider in Geneva Switzerland it's been in the news a lot it's currently shut down but it had been running for the last few years and it will be running again at a higher energy later this year it's a 17 mile underground tunnel around that tunnel particle beams are accelerated by powerful magnets - I think it's 99.999999 seven percent of the speed of light so virtually the speed of light and then those beams are collided head-on inside of these giant machines we call them particle detectors hundreds of millions of times a second when you collide these protons together at such high speeds you put so much energy one place at one time that you can manufacture entirely new kinds of matter so this all this comes down to the simple formula equals MC squared e for energy M for mass if you want to create something with a lot of mass you put a lot of energy in one place at one time you can create new things with a lot of mass so this is how you discovered the Higgs boson a couple of years ago this is how we discovered the top quark back at Fermilab back in the 90s and all of those other particles we see on these charts it's possible in the next run or other future experiments at the Large Hadron Collider that we could be manufacturing Dark Matter in a way that we could observe and detect the second class of experiments are what we call direct detection and it takes advantage of a pretty simple observation so do an experiment with me hold up your hand put it down if our leading particle physics theories are right about how heavy dark matter particles are in that second your hand was in the air about 10 million dark matter particles pass through it okay so if you can build a machine that can tell when a dark matter particle hits it you could maybe see these particles simply by just waiting long enough and being sensitive enough so here's an example of such an experiment we call it LZ or Luck Zeppelin is what it stands for it's in an underground former gold mine and Homestake or in South Dakota near the Black Hills it's Homestake mine this is for scale that's a person and this tank holds a substance called liquid xenon and it's super carefully instrumented to be ultra ultra sensitive and and if just one per week or one per month or one per year a Brock Meyer particle recoils off a little bit of that xenon they can detect it and an alert and study the particles that way and then the third strategy that I'm going to talk about I'm really going to talk about it for the rest of the rest of the talk is looking not for dark matter particles themself directly but instead looking for the kinds of energy or radiation that are produced when they interact with each other so this is a sort of depiction so we have two particles coming in maybe these are dark matter particles they interact with each other and become destroyed or annihilate one another and as in part of that reaction they give off other sorts of particles or energy and perhaps kinds that we can see ordinary kinds and you can try to detect this stuff with any variety of telescopes conceptually this is very similar to a process you may have heard about before if you take an electron and hold it in isolation its stable electrons will sit around forever but if you take an electron in something we call a positron the antimatter counterpart of an electron and put them together they will annihilate very quickly and when they do they produce two photons with a distinctive energy an energy of 511 Killah electron volts okay so every time you put an electron in an anti-electron together they make this distinctive kind of gamma ray and this is what's behind for example the technology we call positron emission tomography or a PET scan the idea is you take something like a radioactive nucleus like fluorine put it into your body let it produce occasional positrons those positrons annihilated mere by electrons and give off this distinctive gamma ray radiation and you detect those gamma rays and make them back of your brain like this okay so we're essentially going to try to do the same thing with dark matter so instead of looking for 511 kV gamma rays we're going to look for a different kind of gamma ray that comes from the annihilations of dark matter particles and this is the tool this is the PET scan for dark matter that I'm going to use okay it's called the Fermi gamma-ray Space Telescope it's a satellite in orbit presently it went it's been in orbit now for six years or so it is the the in my view the greatest gamma-ray telescope ever constructed by anyone it's an amazing instrument here's its view of the sky so this this is the plane of the Milky Way so that's where the middle the middle of our galaxy is this is straight and Worth and straight south of that Center and you can see the whole plane of the Milky Way's kind of lit up in gamma rays oops here we go kind of lit up in gamma rays if you could turn it 90 degrees you would see the spiral arms and everything but since we're looking straight into it you just can't tell that you see a number of features here like this guy this is a big bright pulsar that happens to be nearby this thing is something called a blazar which is really a supermassive black hole a very very long way away from our galaxy that happens to be shooting its particles in our direction and you can see any number of point sources and other Astrophysical objects in this map if many of our particle physics theories are right not all of them but if a large fraction of them are you should expect under this map if you could really dig deeply into it a pattern that looks a lot like this this is one in the lower right this is what Dark Matter annihilations should produce it's bright in the galactic center because that's where the most dark matter is and it should produce a roughly round or spherically symmetric pattern of gamma rays it gets much brighter as you move in towards the center this is the signal that we're going to be looking for in this data set all right so let me give you a timeline of how this story is played out so far I'm not I'm not going to give you the impression that this this story is a is has resolved is a long way from resolved it is still very much up in the air but it's developed a long long way since the story started back in 2008 2008 is when the telescope was first put into orbit this is a it's the vehicle that brought in an orbit is called a Delta rocket this is one of NASA's main large payload payload tools and just over a year later the data that had been collecting to that point became publicly available so let me explain what that means that means that if anyone of you had the wherewithal and ambition to study the scam array data you can go to the Fermi website and download it and start working with it yourself it's probably going to take a while to figure out it took me a long time to figure out how to use it but but it's there and it's it's publicly a part of the public domain sometimes I'm going to be talking about the fermi collaboration and by that I mean the people who run the experiment who built it and who who are kind of the caretaker of the data but just because they ran the experiment doesn't mean the rest of the scientific community can't use the data for that matter of the community at large so just to be clear so the data became public in August 2009 and then a couple of months later just just an August of the same month Lisa Goodenough who was a grad student at the time and I wrote our first paper on this data so Lisa was really important in this analysis because I'm not good at computing okay III write code in a language it was written in 1970s I in this this antiquated thing called top-drawer and people make fun of me yeah I'm very very old-school by computing and Lisa helped bridge that gap but we work together very well in essence she was a student at New York University at the time but she was spending a lot of time at Fermilab and we had a number of good collaborations around that time here's the paper that we put out that October possible evidence for dark matter annihilation in the inner Milky Way from the Fermi gamma-ray Space Telescope we saw that the okay just to kind of reiterate we studied these gamma rays and we found that the the distribution on the sky and the energy spectrum were both well described by a dark matter annihilating model okay so it looked kind of like the thing we are looking for and we thought that was pretty cool here's an example of one of the plots that appeared in that paper these are all the gamma rays from the inner 3° around the galactic center and we saw this bump and this was the sort of bump that these Dark Matter theories had long predicted and we thought huh maybe that bump comes from dark matter we were sure in fact we said we can't exclude the possibility these photons originated from some sort of Astrophysical source or source or sources that happen to look like dark matter and I think that was about about the the correct evaluation at the time nothing stronger than that could be said at the time and we were pretty enthusiastic and perhaps naive we thought the people in the fermi collaboration would like this paper and maybe it would spur further investigation into this topic that was not the reaction we got here are some quotes that appeared in the press shortly thereafter this was the most polite one Julie said the glycan Center is a very challenging region that was her way of saying these guys almost surely got it wrong but it's hard so you can forgive them for it yong konrad swedish guy in Fermi said good enough in Hooper's proposal is quote pretty shaky and then my very favorite was Troy Porter this is literally this is literally the very first thing Troy Porter ever said to me the first time I met him we are at a conference he looked at my nametag learned who I was and didn't say hello didn't introduce himself said what were you smoking when you wrote that paper that was the first thing he ever said to me so okay we were we were taken aback by our reaction okay but back to the timeline a few months later we decided we'd take take a take a look at this data once again and in fact in 2010 and 2011 I put out two new papers the first was with Lise again and then the second one with Tim Linden who at the time was a graduate student at University of California Santa Cruz and we put out two more papers going a little deeper into this data studying the backgrounds a little bit better and trying to really scrutinize the signal and trying to separate things that might be dark matter from things that probably weren't and and we thought we strengthened the case considerably and hindsight I think it's clear that we did it wasn't a bulletproof case yet but the sink the case did continue to get stronger as time went on and then starting in 2012 other groups began to look at this data and see the same thing and that started to get more people's attention it was it's one thing when one one lone scientist and his collaborators are saying this thing over and over again but when other groups including the University of california-irvine group and then the Canterbury group in New Zealand we're finding very very similar results I think that started to really get people to pay attention and then I decided you know this is important and more and also this is hard I'm going to need help so I called some favors and from some of the world's best gamma-ray analysts who I thought would work with me to really try to address this in some detail and here's the collaboration I put together so this is Tim Linden he would already came up he was originally a student at Santa Cruz but he's been a postdoc at the University of Chicago for some time now so he was quick to join the collaboration and then I had been working with Tracy Fletcher out at MIT and and she was seemed interested in helping and she brought in her grad student nick raj shown here and then we enlisted the help of doug Finkbeiner astronomer extraordinaire at harvard and his students tenza de lijn and stefan Portillo and then the the seven of us spent about a year working on a project that would culminate in this paper I think in the introduction they was mentioned that I had written a lot of papers and I have a lot of papers and none of them am I more proud of than this paper so this was a lot of work I think we did a really good job and it wasn't easy but it was certainly interesting we found the same excess of gamma rays in the galactic center that all the previous papers had found but we were able to say a lot more about the details of the signal than we could before we took a map that looked like this this is the raw map and we managed to separate the signal from background in such a way that we are left with just something like this a couple of things are really interesting about this for one this looks about circular or spherically symmetric it doesn't look elongated along the Galactic plane you don't see a bunch of extra point sources or other confusion it's a very very pristine signal and I think we managed to pull it apart pretty well and if you just take this part of the signal and look at this the shape of its spectrum so this is the energy of those photons and this is how bright they are you get this bump that looks just like a dark matter model that line is what a dark matter model would have produced so we were seeing something that both in terms of the distribution on the sky and in terms of the energy distribution looked like what these dark matter theories had long predicted it was also really extended on the sky it was big on the sky so here's a compilation of different papers so this is one of my early papers with Lisa here's a later paper with Tracy slasher and I here's the canterbury group here's the irvine group and here's our last paper and all the earlier papers were all the earlier papers were looking up here and you could see the signal but only in the inner few degrees a couple of degrees around the innermost part of the galaxy our more recent work showed that this extended way out here to ten degrees or more so instead of seeing something the size of the moon you were seeing something you know the size of this okay so here here's a make that point so you can kind of see the Galactic plane in the night sky here and this is the moon it's it's red because of the the atmospheric conditions but this is the size of the region that we could see the signal from now whereas before you were looking at something like this we could show that you could the excess photons extended at least to this bar and probably further and that circle corresponds to this so the the firmly collaboration got more favorable at this point which was was both refreshing and in an unnerving at the same time they put out a press release or I should say NASA did on their behalf saying that that among other things that is consistent when some swarms forms of dark matter and in the best evidence to data this sort of thing so not all the way to endorsing it but saying okay the signal seems to be there and it might be dark matter that's a great step in direction I was hoping they'd go and then even more interesting is just a few months ago for the first time in Japan a member of the Fermi collaboration said that they were finding the same thing in their own analysis so they find an enhancement approximately sent around the galactic center with a spectrum the peaks and GB energies which is what we find and persists across all different kinds of background models so basically whatever they tried to do with their background model the signal persisted and it looked like ours so those were both very exciting developments in our mind but just because the signals there and just because it looks like what you might expect dark matter to look like doesn't mean that it is dark matter the galactic center is a notoriously difficult and complicated part of the sky from an astrophysicists perspective so here's an image of that same part of the sky as seen in with a series of radio telescopes and you can see all these different labels and different things this is a complicated piece of the sky with a bunch of weird violent catastrophic events going on at all times here's a similar image but this happens to be a different set of wavelengths and you can see here's some star clusters here here and here we see a supernova remnant here we see something called a radio arc most notorious is this thing called Sagittarius a which is a the supermassive black hole that inhabits our galactic center and you worry that any number of these things might be faking the signal that we're seeing and making us think it's dark matter when in fact it's some number of Astrophysical things we just don't understand very well probably the leading hypothesis is that the signal might be made by a population of something like 10,000 millisecond pulsars so a pulsar is a neutron star okay let me I'm going to fast who knows what a neutron star is give me a show of hands okay all right so only about 20% of you guys say okay when a star explodes as a supernova if it's not big enough to form a black hole it will collapse down into something that's about the size of Fermilab but with about the mass of the Sun so wildly dense crazy dense amount of material basically made all of neutrons that's why we call it a neutron star all the protons and electrons have been destroyed you basically have Suns worth of neutrons crammed into something the size of Fermilab what do you think happens when you take a star that's slowly spinning and squeeze it into something the size of Fermilab it really starts to spin so picture a figure skater right and they're spinning they're spinning slowly and then they pull their arms in and they do this okay well imagine that but many many many many times more you take something that was spinning once a month and it's now spinning at a millisecond scale so every every millisecond it rotates on its axis as a result of that spinning powerful Jets of particles are accelerated along its axis so it's spinning and it really at one once per millisecond around this this orbit shooting out these Jets well if that were the end of the picture would be a little simpler but it's not those those configurations only last for like millions of years and then they basically fade away and we know there aren't millions of pulsars just you know there aren't uh there aren't ends of thousands of pulsars in the galactic center that are young and active like that so we need to invoke a different kind of pulsar to explain that and those are shown in this picture here so here's an example of a pulsar it's long since slowed down and kind of died away and then it started to suck material off of a in star so here's an ore like a big giant star next to it and this pulsar is gradually pulling matter from it and it's getting spun up in the process and when it's done doing that accretion it will be spinning once per millisecond and it will be in a stable configuration that can last for billions of years and it's possible that there are tens of thousands of these things crammed in the galactic center producing something a lot like the signal we're seeing in this telescope the other possibility that people talk about has to do with the black hole at the galactic center so this this is a computer simulation of the various stars that we observe right around the galactic center and you can see this is going into the future but you can tell by the way these things are orbiting that there's something with an awful lot of mass there it's a something in the ballpark of a few million times more massive than the Sun most galaxies are thought to Harbor a supermassive black hole ours is no exception and what we worry about is that if matter were falling into that black hole at times in the recent past it could have ejected very energetic particles that would have populated the inner galaxies in a way that might have faked the gamma rays we're seeing or might have produced the gamma rays we're seeing faking the dark matter like signal these are really the two ideas we worry the most about I think there are good reasons to think that pulsars aren't responsible and there are good reasons to think the supermassive black hole isn't responsible but we're not completely certain at this point and we need more kinds of observations to finally convince ourselves that we're really discovering something new so we have a puzzle that's half completed we hope what's going to fill in these missing pieces could be the Large Hadron Collider it wouldn't surprise me that much if in two or three years they start seeing the particles or start manufacturing the particles the Large Hadron Collider that we're seeing interacting in the galactic center it may be that one of these underground detectors like the one I told you about in South Dakota maybe that's this puzzle piece and maybe the those particles will show up there maybe with the same telescope the Fermi telescope will see a compatible signal from somewhere else in the sky maybe from something we call it worth galaxy or maybe will detect a new smaller halo of dark matter in our local neighborhood any of these things might help complete this puzzle and go from a really intriguing signal that very well might be evidence of dark matter to something where we can say we really may have made a discovery nailed it so in the last few minutes I want to talk about why this is so interesting because a discovery of this kind if it does turn out to be a discovery isn't the end but really just the beginning in one way you can imagine this could be the beginning of Dark Matter cartography we might be using gamma-ray telescopes in the future to map where the dark matter is including all of its little clumps and clusters and distributions and it may be that you know in any standard Atlas you buy in the 22nd century will have a detailed map of all the dark matter and how its distributed in the galaxy that's a real possibility if this turns out to be real we can also ask questions about how the dark matter these particles were talking about how they came into existence in the first place so in most of the theories I'm talking about at a point about a billionth of a second or even a tenth of a billionth of a second after the Big Bang these dark matter particles were being formed the universe was so hot and the conditions were kind of like they are in the Large Hadron Collider that you'd be producing copious amounts of this stuff in the the fraction of it that survived as the dark matter we have today but by studying what we have today and how it interacts we are effectively studying what the Big Bang was like when it created that stuff we can ask for a particle that behaves like this how much of it should have been produced in the Big Bang if the Big Bang was like we think it was a billionth of a second after the Big Bang I said Big Bang a lot of times in that sentence but I think you understand mean anyway so by studying these particles that is this exists today we can look at what the universe was like here which is something we presently can't do and then as a particle physicist and a particle theorist the questions I'm most interested in are not you know in about any one particular particle but about how all the particles fit into a broader theory so I want to know not just is what's the dark matter made of but how does it fit into the big picture is it part of a supersymmetric model or is it part of the model with extra dimensions is it a consequence of a some sort of grand unified theory that that explains all the batter and force of varieties of matter and force that we detect in the universe maybe that's something to do with quantum gravity in some way I can't currently understand or emit any number of other things the point is that every time you discover a new type of a matter or energy it gives you a better idea of how they all fit together just like any time you pick up one new puzzle piece it helps you see the image that might have been elusive otherwise so I see us right now at an important turning point it may be that a few years from now we'll all be convinced that this was really just some sort of mundane astrophysics maybe there are ten thousand pulsars that were behaving differently than than we expected and that provides a perfectly good explanation for the data but if I were a betting man I think that's probably unlikely I think it's pretty likely not a guarantee but pretty likely that we're really discovering a new fundamental type of subatomic particle in the galactic center and that this probably makes up a lot of the dark matter in our universe and that's the most exciting thing that's ever happened in my career and I hope you find it exciting too we may be finally sticking our head through the filament in seeing the universe a little bit closer to how it really is thanks for your attention I'd love to take some questions got the first one here for answering the question for the rotational velocities of the galaxies there are some alternate theories which are trying to answer that question like modified Newtonian dynamics and huh so what is your take on those theory whether it is I mean is too ridiculous to modify the classical mechanics or these theories are worth considering or they might another theories excellent question so so astronomers by the late 70s had found that galaxies were rotating in ways that couldn't be explained very easily and people are starting to talk about dark matter as an explanation and then in 1983 this guy morai Milgram and Israel came up with this theory called Mond or modified Newtonian dynamics and basically he said that this this thing you learn in your first week of physics F equals MA is not exactly right and if you have very very small accelerations it goes more like F equals MA squared okay perfectly self-consistent theory or at least a phenomenological theory it works is the point and you could explain why galaxies rotate the way they do without dark matter by changing this this simple law of physics and I think at that point it was a perfectly viable hypothesis but since then we've seen dark matter or the effects of dark matter in clusters where Mon doesn't seem to help we've seen the effects of dark matter in the Cosmic Microwave Background where Mon doesn't help and we've seen the effects of dark matter in how large-scale structure forms and it's not clear whether mod helps there so when you take that global piece of data that could all the data together it doesn't seem like Mondas isn't a fact of way to do this anymore if Hawking radiation is the only thing that can escape from a black hole why would it be making you think that it's dark matter so it's true that once something falls through the Schwarzschild radius or event horizon of a black hole unless it's radiated as Hawking radiation you know you won't you can't ever get out but we're when I talk about the black hole giving off particles I'm talking about stuff going on more around the black hole okay so you know here's my black hole and here's my Schwarzschild radius and this whole vicinity around it where the gravity is super strong but not quite black hole strong that can do a lot of violent stuff okay that makes and gamma rays so if you don't know what the dark matter is made of then how do you know at what frequency it what energy level the radiation created by the annihilation will have you don't that that's exactly right so what I can do is I can say hypothetically let's say the dark matter particles were 100 times heavier than a proton just a guess okay and let's say those when they when they when they interact with each other and annihilate they produce Jets of quarks that's stuff we find all the time in particle accelerators that seems plausible if that were true what would the gamma rays look like and then we test that theory so we write down any number of hypotheses for what the signal might look like and then we test them one by one so we have many guesses and we do experiments and we revise our guesses based on those experiments so we don't going into it know where exactly what we're looking for hi Dan excellent lecture Dean from Triton College just on the LZ detector why did we decide to use Eon and what kind of results from interacting with that atom would we be looking for so there are a pretty wide variety of experiments in this direct detection game some of them use liquid xenon some of them use germanium some of them use silicon some of them use the one one that's being done by some people at Fermilab here uses a c3 f8 so carbon and fluorine and such so there are lots of different options and they all have different advantages and disadvantages the xenon in particular has some advantages that it's really heavy and it's also a noble gas or noble liquid in this case and that that enables them to have a really stable configuration and because of its mass and it's is it's more sensitive to certain kinds of dark matter interactions so the short answers is complicated there are lots of advantages and disadvantages in xenon happens to be good for various various various reasons connected that hi thanks for election I was excellent don't see him yet raise your arm sorry good gotcha given that the universe is expanding at an ever-increasing rate yet at the same time the density of energy and matter of space seems to be constant what are the ideas about where all that additional matter and energy are coming from and where ever they're coming from why does that not violate the energy conservation law did you say that the density of matter and energy is constant of empty space ah oh you mean dark with dark energy yeah okay so yeah dark energy is is awfully weird for this reason right if I take a cubic meter of space it has a certain amount of dark energy in it and then I let the the the universe expand and still a cubic meter space has the same amount of dark energy in it that means that that expanding piece of space has more energy than it did when it was smaller so people always throw up their hands and say that violates the conservation of energy you can't do that but actually the law of conservation of energy isn't it doesn't really apply to that case what what the losslessly says is that in an interaction between objects the amount of energy is conserved not the total amount of energy and a volume so you can actually you can be you can basically violate that law without running into any problems with our understanding and say general relativity or quantum field theory or any of our ground level theories that's okay to do that in other words okay I was thinking maybe there's some evidence of subatomic activity like if we find out where all this dark energy and dark matter is being distributed maybe that's a sign that there's some type of atom in a larger universe just making itself known would that give us some clue about how string theory works maybe once we know where all this dark energy and dark matter is distributed so that's more or less what I was trying to convey with this slide and it's hard it's a hard sentiment to convey but my hope is that by understanding the particle nature or quantum nature of dark matter it will help guide us towards some sort of bigger theory so maybe it's a particle that fits cleanly in a theory that we call supersymmetry and then maybe that would help us understand how grand unification might work or maybe how quantum gravity might work or whatever else there are even theories where the dark matter is a particle traveling through extra dimensions of space that might have something to do with a string theory for example etc we're I'm only I'm only speculating but the hope is that the more we discover about the kinds of matter and energy that can take shape in our universe the closer we are the more tools we have to figuring out the underlying theory the grand unified theory or the theory of everything that underpins it all I was wondering when you're thinking about these kinds of radiation signals coming from near the galactic center are you presupposing that there's some kind of pair production of particle and antiparticle dark matter and if so why are you confident that the same kinds of fundamental conservation laws that rise two electrons and positrons also give rise to pairs of dark matter particles or am i incorrect that that's why you think they might be there so so you're sort of right but not completely so you can you can write down some theories in which there are dark matter particles and anti dark matter particles and they where were pair produced and that sort of thing and and that that that's very close to the analogy with electrons and positrons and then you could also write down theories where the dark matter and the anti dark matter are really the same thing we call them Ayana particles so there would only be one new kind of particle and it could be produced in the early universe without a Anti partner version either one of those can lead to the sort of annihilation signals we're talking about here but I do want to do want to say there's another point I want to drive home which is I wasn't confident when we looked for this that these particle theories were true they were only one logical possibility and we're trying to test that possibility so in until the only the only thing that makes me think something is true as a scientist is data that confirms it so I'm an empiricist I you know I don't have strongly held theoretical prejudices or at least I try to keep them on that on my sleeve so I wrote down a lot of theories some of them I liked more than others and then I went out to try to find ways to test them and I think that's that's the approach I would advocate in general you just maybe two more questions as uh you you just mentioned this briefly but in terms of an oscar-nominated film how does this impact or fit into the theory of everything in terms of an oscar-nominated film so I mean I can do some cheesy analogies but I don't think I'm going to give you a very good answer I mean if you're trying to put together a puzzle the more pieces you have the more you can get the underlying picture one more piece might be the what kind of particle makes up dark matter I could give you a different kind of example and this was supposed to be an illustration so in theories we call supersymmetry there are many many other kinds of particles that exist and if you were to start discover some of those and maybe dark matter might be one of those you might figure out that there's this new pattern in nature we call supersymmetry and the the space-time behaves in this particular way and then maybe you'll discover some more aspects of that and you'll see that it fits nicely into something we call a grand unified theory and that would get us a lot closer and understanding some of the some of the reasons for why things are behave the way they do and then maybe someday with that information you can build upon it and look for some sort of theory that ties that in with gravity something we call a theory of everything but you know who knows I mean going back to the history of science you know when any number of phenomena were observed for the first time when the electron was discovered did anyone know that it would lead the quantum field theory or when people are doing experiments on the speed of light did they know that this would lead to relativity or when you know Newton begin to contemplate gravity did anyone realize that it would somehow allow for the unification of the thing that pulls stuff towards the Earth and the thing that holds star stars and planets and orbits so I'm not going to presuppose that I can tell you how this is going to play out only that that's the way science works and the more you know the more likely you are to be able to build a bigger theory that explains more I was dislike really simple compared to all the other questions but um I wasn't quite sure what I understood like why the star spins when it or faster when it's like more condensed because okay I know like in it has to have like some force actually when ups like spins like in a motor electromagnetic ISM causes the thing to the axle to spin so yeah great so it's it's the idea is this quantity called angular momentum okay and this is something that's conserved so unless you unless you touch something you can't give it more or less angular momentum and if you have if you're big if you're wide and spinning slowly you have the same amount of angular momentum as if you're small in spinning quickly okay so you've seen the figure skater right so they pull in their arms to spin to speed up so by pulling their arms in or the star collapsing in order to keep the same amount of momentum they have to spin circling faster trying to think of other examples go to a merry-go-round take all of your friends and put them on this on the edge of the merry-go-round and then get it spinning okay and then all at the same time pull yourself to the middle the merry-go-round it will suddenly start go faster do that experiment and then you'll understand doc doctor Hooper will be up in the atrium to answer them any more informal questions there's a reception punch in cookies up there and thank you once again for coming to Fermilab

Channel: undefined

Views: 401,294

Rating: 4.7732882 out of 5

Keywords: Fermilab, Physics, Dark Matter, Dan Hooper, Dark Energy, Particle Physics, Astophysics

Id: aMWCkcn7TD4

Length: 65min 1sec (3901 seconds)

Published: Thu Feb 05 2015

Reddit Comments

[u/markseu]

Thanks, posted also to /r/nerdfun