Physics in the Dark: Searching for the Universe’s Missing Matter

[Music] thank you all for coming out tonight you know it is a it is a special night tonight for science do you who knows what I'm referring to the anniversary do you know what I'm talking about somebody what is it well yes but light Falls is celebrating a particular event in the history of science which was confirmation of Einstein's general theory of relativity 100 years ago today Eclipse observations were used to confirm the prediction that Einstein made in summon made reference indeed we do have a program on tonight on PBS so after this program if you want more you can run home turn on channel 13 from 10 to 11:30 you have a program that was filmed on this very stage in February but tonight our focus is upon a related idea that has a deep connection to theories of gravity our understanding of the force of gravity because we're going to be discussing the possibility that many scientists have bought into over the course of many decades that there's much more to the universe out there that meets the eye and the reason that we've come to this conclusion is by virtue of studying the gravitational poles that happen out there in the cosmos and coming to the conclusion that the stuff that we see is unable to give rise to the gravitational pull that we observe and one way of thinking about this just to kind of get into the subject is to imagine that you know you're flying into New York City at night many of us perhaps all of us have done this and as you fly in at night of course all that you really can see are the lights in the buildings now we all know when we see an image like this that there is more structure to the image to the reality than the lights themselves how do we know that well because we all have an intuitive understanding of gravity and if there were no buildings if there were no structure out there these lights they'd fall down to the ground and because they don't fall we know that there is something out there that's holding them and of course by the light of day we can begin to see what that structure actually is and of course we all know what the structure is it is the architecture the physical makeup of the buildings themselves but without being able to see that structure itself if we only have access to the light which is all that we have access to when we look out to the universe we have to infer the existence of the dark stuff that's not visible at the moment and that's how we come to this conclusion that when we're talking about not a cityscape but the universe that there might be more out there than meets the eye so this is an idea actually it has a long history and I won't take you through the full history it's a fascinating story but I'll just give you some highlights of it toward the end of the 1800s as we began to have ever greater ability to observe the night sky scientists began to try to really understand the motion of stars out there and one scientists in particular Lord Kelvin he had this idea to model the stars out there in the galaxy as if they were perhaps molecules in a box of gas you know those scales are completely different but the idea was to apply the understanding of how particles move inside of a box of gas apply the same ideas now to stars moving within a galaxy and when he did that analysis he came to the conclusion that there might be stuff out there that we can't see in fact in his own words he described it like this it is nevertheless probable that there may be as many as a billion stars but many of them may be extinct and dark and nine-tenths of them though not all dark may not be bright enough to be seen by us at their actual distances many of our stars perhaps a great majority of them may be dark bodies so the possibility that perhaps the majority of stuff that's out there might be dark this is an idea now the going way way back late 1800s and then another great mathematician scientists on Ray Poincare a he was inspired by kelvins analysis to do his own version of that analysis and he came to a different conclusion but he did introduce an important phrase that's yes obscure he did it in French of course but of course that is dark matter so this idea of dark matter goes all the way back to the late 1800s Early 1900s and as people began to think about this idea over the ensuing decades it's this fella right here do you know who this is anybody know this is Ricky yeah Fritz Zwicky who was a Swiss American astronomer kind of a wonderful character but he began to study the motion of galaxies in the Coma Cluster this is a few hundred light years away and again he found that the motion of the galaxies was such that it couldn't be solely due to the ingredients that he could see by virtue of their light he concluded that there had to be additional dark stuff that was out there that would be responsible for the gravity that was pushing and pulling these galaxies around but it's really the work of this person right here Vera Rubin who really cinched the case in the minds of many physicists many astronomers for the existence of dark stuff that's out there because what she did was she studied the motion of stars in swirling galaxies and found that the galaxies are swirling too quickly the stars should be sort of ejected out right now one way of thinking about this we can do a little quick demonstration of this you know if you have for instance a very simple pedestrian situation where you have a a wheel right and you have water on the wheel and we all know we don't have to do this but it's sort of fun to see it if you take this wheel and it's spinning slowly enough very little water will fly off but of course you give it a real spin and the water droplets fly right off thank you very much and the idea the idea was that the galaxies were spinning as fast as I was spinning the wheel said that the Stars should be flying off as the water did but Vera Rubin found that the stars are not flying off and therefore there had to be something else that was pulling them in something else that's dark because we don't see it giving rise to the gravitational pull that was holding the galaxies together and of course it wasn't just pictures there was mathematical analysis behind this when you do the mathematics which we won't go into any detail on but the expectation was that the farther out you go from the center of the galaxy the slower the Stars should be moving but in fact her observations and analysis showed that that wasn't seeming the case the speed of the stars out on the edge of the galaxy was too fast relative to what we thought it should be they should be flying off but they weren't and therefore this notion that there should be some dark stuff that would be out there and when you put the numbers into it you find something rather remarkable if you calculate the amount of dark matter that must be out there to hold these galaxies together you find that it's four five times as much as the amount of matter the protons neutrons electrons that makes us up so you're talking the majority of the matter in the universe might be dark stuff and indeed we'll even talk about a different kind but perhaps related dark entity called dark energy later in the program as well bottom line is the stuff that makes up you me and everybody else maybe as tiny sliver of the mass energy budget of the entire universe and that's a remarkable pie chart they're showing us we know a lot about reality but it might be that a lot of our focus has been on the tiny piece of the full story now before bringing out the participants who will discuss this further with us I want to mention one point that's actually related to the anniversary the hundredth anniversary of the general theory of relativity which is this when Einstein was doing his calculations on relativity in November of 1915 he focuses attention on a particular puzzle particular problem that had to do with the motion of the planet Mercury it had been known for a long time that Mercury's orbit wasn't doing what the gravitational equations said that it should instead the orbit was kind of shifting a little bit each year and to explain this shifting orbit some astronomers introduced the possibility that maybe there is a dark planet out there called Vulcan a hidden planet undetected planet that was tugging on Mercury and that's what was causing the orbit to shift this was the going idea until Einstein came along and with a deeper understanding of the force of gravity and his general theory of relativity was able to fully explain the data without any dark stuff which is just to say that you have to have an open mind as we'll see in the discussion going forward maybe there's dark stuff out there but maybe our understanding of gravity needs to be deepened as this historical example shows us and perhaps a deeper understanding of the force of gravity might explain the anomalous observation so this is a point that we will come back to but the bottom-line question that we will be discussing is it the case that much like this cityscape we see the light stuff the Stars the lights in the buildings is there additional dark stuff out there and if there is what is it made of those are the questions for tonight and we are fortunate to have some of the world's experts to discuss these questions with us to illuminate these deep puzzles so let's now bring them out our first participant is the director of the Kavli Institute for particle astrophysics and cosmology he's playing a leading role in modeling and mapping out tens of billions of galaxies to understand the evolution of the universe and the nature of dark matter and dark energy please welcome Risa Wexler [Music] our next participant is a professor of physics and astronomy at Johns Hopkins University and a science writer and author his work on the cosmic microwave background galaxy formation exploration of nature dark matter please welcome Joe silk all right also joining us tonight is an associate professor of physics at Princeton University whose research focuses on the nature of dark matter she has tested theories of dark matter using data from a wide range of experiments please welcome Maria Angela Santi all right our final guest tonight is a professor of theoretical physics at the University of Amsterdam his research deals with string theory quantum gravity black holes and cosmology he's a recipient of the Spinoza prize the highest award available to Dutch scientists please welcome Eric Berlin day [Music] all right thank you all for being here for this discussion of dark stuff out there in the universe and I just want to begin by amplifying the point that I was making at the very end so when we're thinking about this puzzle that there's motion in the universe that seems that we can't explain it using the stuff that we can see using light there seems to be two general ways of addressing this problem it could be that there's more stuff out there than meets the eye there could be dark stuff you can actually bring this up on the screen if you with these two possibilities but it could also be that our understanding of the forces and particularly force of gravity needs sort of a deeper explanation a more full understanding much as Einstein's general theory of relativity did that for the motion of the planet Mercury so part of our discussion tonight will be these two possibilities and I'll just sort of lay out the the teams if you will so these three folks over here I think it's fair to say you're more on the dark stuff side of the explanatory possibilities and over there on the far left side or far right side from your perspective is Erik for Linda one of the most thoughtful creative insightful physicists that I have ever met in my entire life so not to bias the conversation but he has a very interesting but I think it's fair to say speculative idea which is more on the right side of the explanation there so just just quickly where where do you see your ideas fitting in this did you feel that there developed to the point where there a competitor to the dark stuff that will be some of the focus or it's still very much a work in progress so busy understanding gravity in a more fundamental way I mean many colleagues I do this with who are trying to combine general relativity with quantum mechanics with using string theory and this has developed into a new framework in which we can explain actually where gravity comes from from a more microscopic picture and I indeed believe that we can then explain these phenomena without the need for dark matter but this is a theory that well has sort of started to become developed but we need to do more work to be ventually get the complete thing out there so it's not like I can explain everything yet but there's a first hint of a new theory and then indeed you don't need the dark matter particles to explain this extra gravity that we have been observing and just to give a snapshot so that folks who aren't following the details of this in the research literature is this a minority opinion equal of page how would you sort of frame the number of people who are thinking this way versus the dark stuff certainly a minority and and of course I'd like to encourage more people to think about it this way and it's I think that once we start trying to understand gravity better and have developed that theory this will come out automatically but this will take some time and so it's fair to say that I I'm not certainly in the majority but there's a well hope hopefully a growing number of people taking my point of view and so as we focus our attention for the first part on the dark stuff you're gonna be able to withstand this obviously good no I I mean I can understand even the logic for it and yeah so it's certainly something that's well motivated so I can even great executive all right just so by chapter three of this discussion we're gonna come to a focus but let's for the first two parts focus on the the left-hand side and when we're talking about dark stuff the natural thought that might come to mind it's the kind of stuff that we know about maybe it's planets maybe it's stars that have burned out maybe it's black holes that are out there in the universe that's sort of one collection of possibilities the other collection is fundamental ingredients that make up those entities particles perhaps the ones we know about or perhaps hypothetical particles so let's just start on the on the left-hand column here Joe could it be that the dark stuff is just things that we know about that are hard to see because they don't give off light we know about ordinary stars and we infer from things we're going to talk about there's lots of dark matter so could there not be dark stars now we have extremely tight constraints on how much ordinary matter there could be in the universe even in dark form because long ago in the first minutes of the Big Bang this stuff was just ordinary matter although it's now a Dark Star and we'd participate in thermonuclear reactions that made all the helium in the universe a wonderful prediction of The Big Bang Theory the lightest element the second lightest of mother hydrogen and very very abundant and that great success tells us that the amount of this dot baryonic doc stellar stuff it's it's a small fraction it can't be the dark matter which then leads us to consider more exotic things and so one intriguing possibility is the possibility of black holes that were formed very early in the universe and so these would be ideal dark candidates because we simply can't see them and the interesting thing is you can decide how many might be left over because they do affect light transmission from stars in nearby galaxies they deflect the light slightly as as they might pass by in front of the background star and so this is like just landing which again just for the historical fact is again the very experiment that was done a hundred years that's right it's a version of Einstein's original prediction and and it's a way of indirectly inferring how many dark stars there can be and so we're now at the point where we can set limits on the possibilities of these dark stars over a huge range in masses and it turns out that you can people have studied for example the the nearest galaxies they have looked at stars that would be slightly affected by a passing black hole and they can say that at most 10% of the dark matter could be in massive stars or small stars or but what is left over very exciting possibility are really really tiny black holes essentially the masses of asteroids but black holes and so incredibly tiny but there are there could be a lot of them and enough of them to essentially evade all possible constraints from this twinkling effect on stars lensing of stars and the exciting thing about the newest day that we have which consists of staring at largest nearby galaxy the Andromeda galaxy for a long time and looking again for slight variations in star patterns and we're beginning you know there are even reports that they've found the first event that might be what they call a primordial black hole from the beginning roughly asteroid mass we have to take a lot more data there could be more but the beauty of this hypothesis is that we're not inventing any new physics right not inventing new particles we're taking known physics we know that black holes can form they do form massive ones from dead stars and now we actually have a photograph of a black hole so I think it's a it's a wonderful hypothesis that again has enormous potential we're just beginning to explore but if you wanted my vote on what the dark matter could be I will put this high on the list of possible candidates the alternative being elementary particles that will be discussed in a moment left over also from the Big Bang gotcha so let's hold that as a possibility one of the candidates for the dark stuff might be these black holes that were formed the very early universe but now let's turn to the other possibility that you mentioned which is elementary particles the stuff that makes up familiar matter from stars to people to rocks and so forth now there are a number of possibilities so let's start real simple could the dark matter just be an electron could it be a lot of electrons out there could it be neutrons could it be protons could that kind of stuff well we're pretty sure that all this extra matter has got to be charged neutral because if it were electrons would be have electric force on one side protons on another side will be a disaster we could rule that out completely yeah something neutral works and something stable works neutrons decay they're unstable so it couldn't be neutrons although they're neutrals at 10-15 minutes they're there that's right and we also need a particle that interacts very weakly with other particles unlike electrons and protons which make up the Stars the earth etc this dark stuff is outside the boundaries of where we see the stars so it's a particle that interacts weakly and basically we have to invent a new class of particles to be candidates for the dark matter and so we we do have a wonderful theory which predicts such particles it's called supersymmetry it postulates that very early on there were equal numbers of all types of particles partners of each other and the heavier partner which had a slightly different spin would be unstable but the lightest of those will be left behind and could be the dark matter that's a prediction of this very elegant theory which hasn't been actually verified yet but it argues for perfect symmetry and the very early stages of the universe so it's compelling for that reason so you mentioned supersymmetry yes and so this is a theory that particle physics folks come up with not to solve the dark matter problem they the introduce it for differently so part of the the attractive feature of black holes was that you didn't have to invent something new is already within the equations so Eric just very briefly why do physicists particle physicists in particular introduce this idea of supersymmetry and just to lead you in the direction I have you know we we have another program in the first about the Higgs particle so what's the relevant to this for that may indeed I mean it's a beautiful idea for support theoretically that we can relate the particles in the standard model to so there but something nice happens there namely there's a big puzzle in the standard model this Higgs particle and has been found we like to understand understand explain also why the mass that it has stays that value and it turns out that if you add these other mirror particles the supermassive metric partners that we can explain this in a very natural way and so the whole standard model works actually more beautifully if we add the supersymmetry I mean string theory actually also predicts the supersymmetry so also from string theory would have been very happy to find it and fortunately it has not been found yet but the idea is still alive and it's one of those more beautiful theoretical ideas that we hope is realized in nature so mariangela so up on up on the screen here we have the left hand side is the stuff that we've long known about the quarks that make up the protons and neutrons and so forth the right hand side are the particles that Joanne Derek referring to these these additional particles just point out the ones that we've we've actually seen just that we know which ones are actually real everything on the left we've seen with the Higgs being the most recent one in 2012 and actually this is coming back to the question you asked before which is of everything that we've seen can anything actually be the dark matter and it would be good to point out that the neutrinos which are the ones that look like a little squiggle v's yep are probably the only thing in the standard model that has the characteristics that we'd want to be dark matter in the sense that they're neutral and very weakly interacting so they're not emitting any kind of light otherwise we would have seen them yep and so the trouble with the neutrinos is that you can do a calculation and ask how much of the current amount of dark stuff that's here now can be comprised she knows it's a very tiny fraction small to small so that's how we know that the standard model on its own can't account for all of the dark matter so if we then hypothesize these Susy particles here on the right there are some candidates in this in this assortment that have the same kind of characteristics neutral give you the correct stability the stable so they give you the correct abundance today and interact very weakly but everything on the right we have intact observed yet yep and you know when I first started in graduate school there was a going lore which is if you wanted to write your first paper you just make up a dark matter a candidate you know motivated from these ideas you do sort of a calculations I don't know if you if you happen to have the first paper that I wrote out there I thought it would be kind of fun to to show it to you simply because human eyes will never look at this paper again because it's just not important it's just sort of there it is so it's seen the light of day now we can put it back into the journal and we'll never see it again but but this is the idea so there's the possibility of exotic particles that what presumably they be produced in the in shortly after the Big Bang and then and then how do you figure out how many of these particles would remain so imagine the Big Bang happens there's all this energy what's the what's the step is it is is there a well-defined procedure to figuring out how much of those dark particles will still be hanging around today yes indeed there is so and it's very predictive so what we need to know is how the dark matter particles interact with the standard model so that's where we have to make some kind of Theory assumption but if we do that then we can start modeling how these dark matter particles are interacting with all regular matter in the very early universe so you might have to dark matter particles come in interact give you some regular matter and vice versa and the whole system can be in equilibrium so the forward reaction happening with equal rate as the backward reaction but as a function of time what ends up happening is the universe is expanding and so if you have two dark matter particles at some point with the universe's expansion they'll no longer be able to find each other anymore and so finding is important because that's how they yes findings reporting is and that's how they can interact to give you the standard and when they interact they disappear by producing more exactly yeah so once they can no longer find each other then they're just this interaction stops and and then however many dark matter particles are around at that point in time stays essentially constant until today and that's the the amount that we measure so you can actually calculate what this interaction rate is accounting for the expansion of the universe and a specific prediction for the manager so we have a rough version of that calculation right here and of course you don't have to understand the mathematics behind this but the point is there is a well-defined rigorous mathematical procedure it allows us to figure out how much of this dark stuff would be hanging around and the wonderful beautiful perhaps unexpected fact is that when you do this calculation for supersymmetric particles the typical amount that will remain for the stable ones is pretty close to the amount of dark matter that observations suggest should be out there now when I learned that as a student I was bowled over by this I was like of course as ever even many were - it has to be the dark matter how else could it be the case that the amount that is left over is just what we need and then when you guys learn that did you have a similar reaction or you just sort of see this as a coincidence or or something else to me there only is new but I mean you're tweaking two numbers one that's in the numerator and the other one's in the denominator and you pick two values where it works but what happens if you change those ratios a little bit then you all of a sudden open up a much wider region of the parameter true but just to push back a little bit the natural values of those parameters like the mass of the particle being you know one to a hundred times the mass of a proton feels very natural the rate at which they annihilate that's not really put in by hand it's using the interaction rate of the weak interact you know so there's very natural choices that still didn't that was compelling but it also did the the numerology aspect of it always did make me feel a little uncomfortable is this point that mariangela made that if it's not that the parameter space opens up a lot and so that's an interesting situation we're in I mean I think it's interesting that the interesting paper that the original papers where where it was seem like wow that's amazing yep those models have actually already been ruled out and so now we have a much broader parameter space that we have to look at but right but in the old days when there amazing thing and that sort of drove the focus on a particular category of dark matter candidates called wimps yes I didn't want to say so thank you so so so so wimp stands for what weakly interacting massive particle and the supersymmetric partners provide a class of candidates that fit that particular description and could be the Dark Matter yeah I mean several over the print of the of the particles that mariangela was already talking about be wimps so if this is a viable possibility the key thing of course is to go out and find these particles and that is something that we've been trying to do for a while so maryland's it can take us through the the approaches that people have put forward to try to actually capture one of these particles or create one of them sure yeah so with with wimps in particular there's a three-pronged approach so the first being let's try to produce it in the lab and in that case you look at some really powerful Collider like the Large Hadron Collider where you take two protons and you collide them together really high-energy and you hope that in that high energy collision you might actually be able to produce some new heavy exotic state that might be the Dark Matter particle so using the energy of the incoming particles and equals MC squared you want to transmute that energy into this exotic folk that's right and in some sense if we can create this in the lab that's the best scenario because then it's in a controlled environment we could go in and we can really study the you know and get at the particle physics properties of that particle alright how's that going we haven't seen it yet [Laughter] but not for want of trying really really hard right okay and we're still still trying right it's not as though the game is over but so far I mean the LHC has been running for the last few years and will be continuing to run so far we haven't seen anything but you know people are you know there's gonna be more data and people are coming up with new and creative ways of analyzing it so we never know when that surprises so that's one approach trying to actually create it in the laboratory other approach the other approach is to to look it for it in the sky look I'm sorry to look forward in the sky yes so if you have to dark matter particles that interact they could sometimes very rarely but sometimes produce a little flash of light right and so you can look for this annihilation process by searching in parts of galaxies where you expect there there might be a lot of dark matter and see whether or not there's an excess of light over what you would have otherwise have expected to see and how's that going also the lots of data we haven't seen it yet again not for want of trying really really hard and Jody would that be your assessment the beautiful part of this argument is the very same effects that stop this particles being created in the early universe still give you a value of their interactions if ever they could find one another they would produce again a flash of gamma-rays actually yeah a cascade of quarks and whatever and so the trick is you look in the vast depths of galactic space the volumes are so large that you can look for the cumulation of these rare events and look for gamma rays you can see here the gamma-ray sky as measured by - what gamma rays are just silly okay they're photons which are thousands of times more energetic and penetrating the next age okay there are hundreds of MeV x-rays rkv so really really hard photons produced in nuclear explosions I mean if ever you're unfortunate enough to be near a nuclear explosion then you would be rated by gamma rays that would be one of the the more catastrophic things that could happen to you but fortunately the gamma rays don't get through the Earth's atmosphere that we look for them from satellites in space so and there they can penetrate can propagate freely and so in the center of our Milky Way galaxy which is where the dark matter mostly accumulates you would expect there to be a slight excess of gamma rays if the dark matter is indeed the WIMP type of dark matter and so many years ago even before the this firm this satellite was launched to look at the gamma rays there were hints there was an excess of something very far going on there based on radio observations then when the gamma-ray satellites are taking data and they model all the various contributions of gamma rays for example you get gamma rays when high-energy cosmic rays hit ordinary clouds of gas they've been as gamma rays but you know exactly where the clouds of gas are you know what the cosmic rays are so you can correct fall of this and when they did this they found it in the central degrees of our Milky Way there's this tiny excess of gamma rays not explainable by anything known at that time and so straight away the particle theorists the dark matter addicts said this could well be the signature been looking for of annihilation dark matter and Eiling with itself and giving you gamma rays and they said well if we take exactly the same interaction strength that's predicted from the fundamental theory and we take the standard model which you'll hear about in a little bit for the Dark Matter distribution in galaxies lo and behold to get the right flux and that looked very attractive it wasn't quite proof because there's always a bunch of sceptics around and the skeptics said well look we know there are certain types of stars they're rapidly spinning pulsars which the Fermi satellite also saw and they said weather could be a lot of those in the center but so faint that they're really hard to see but in humouredly they could give you the diffuse gamma rays and so the jury is sort of out on one side or the other it could be the Dark Matter very elusive it could be these missing bozos so there's a new test we can do which we're trying desperately to do now there are tiny galaxies going around the Milky Way dwarf galaxies which are full of dark matter very few stars and they almost certainly don't have these distracting millisecond also gamma ray sources right and so we're looking at those I'm staring at them with this gamma ray telescope to look for evidence of gamma rays there are you know we've looked at twenty of them one sees the gamma rays from one or two at marginal levels so it's impossible to stay statistically whether we found them or not all we can say is we need bigger and better telescopes that satellite was built down the hall for me and when the people were working on it I think we really hoped that when it launched we would sort of immediately see those brilliant signals from those dwarf galaxies yep and that didn't happen so you know there were models that could have been whip versions of the wimp which could have had booming signals from dwarf galaxies and we're definitely not seeing that gotcha so we've ruled out a bunch of things the same excess that we see from the center of the galaxy if that sort of Dark Matter when the dwarfs we would be marginally seeing it now we need bigger telescopes that's the or what the astronomers always say so let's turn to telescopes and and thinking about cosmology so Risa this is an area that that you've spent a lot of time working on Dark Matter as a key ingredient in how the universe has evolved and how structure has formed so let's now turn to looking into the deep sky for the signatures of dark matter so so tell us some of the approaches that have been used to figure out the role of dark matter in the formation of the things that we know about the the galaxies and other structure in the universe we talked already about how dark matter you know if there is dark matter if it's a particle it's you know four or five times the amount of normal matter yep so we can take that model and we can say well what does that predict for what happens over the last 13 billion years in the universe yeah so that model first it predicts something about what fluctuations there should be very early on in the universe which we can measure from the Cosmic Microwave Background we say fluctuations I mean places where the universe is there's a little bit of extra stuff and a little bit less stuff so so we my understanding of gravity is not as complicated as Eric's so most of what I do is just think about the same kind of gravity that you and I interact with the earth or you know the Earth rotates around the Sun so that kind of gravity we can put that into our computers and we can take the fact that there were little places in the universe which were a little bit more dense or a little bit less than and we can turn out on gravity and so we can actually figure out what kind of structure forms in the universe in that context and the idea is where it's a little bit denser it pulls in more so yeah right and where there's a little bit less stuff you know you get empty regions when you do that you can actually make predictions also then of where the galaxies should form and in our current theory of galaxy formation yep one side of the screen yes was a prediction from those simulations of dark matter and how the structure forms in a model where you know 85% of the mass is dark matter the other side is is actual observations where we're actually going out and mapping the galaxies distribution and this is just a computer you put in there you put it in there you see what happens essentially whenever you get enough dark matter in one place that's where you expect a galaxy to form basically you get enough you get enough dark matter then the gas can start to cool and form stars and this on the right hand side those observations are from a telescope called the Sloan Digital Sky Survey which actually mapped the positions in 3d of about you know about two million galaxies actually later this year we're going to start kind of the next generation of that called the dark energy spectroscopic instrument we're going to get about 10 times as many galaxies and so we'll have a much better map of what that looks like but the point is if looking at this beautiful video the left hand side the right hand side they look they basically look the same right yeah and so this is actually what this is actually showing is that the other thing that dark matter does so it forms that structure and that structure just like we talked about gravitational lensing with with the perihelion a mercury it just how we talked about gravitational lensing with those micro lensing events it impacts the shape of galaxies a tiny bit so if a galaxy were round it gets distorted it looks tiny bit it looks distorted because it's light got distorted over the several billion years that it took to get to us because because there was stuff in the way and that stuff we assume is the dark matter and it has an impact that's right so so that stuff if it is dark matter actually changes the shapes of galaxies and so the thing you're seeing here is is actually a map of where all of the mass in the universe is inferred from the positions of these galaxies so that was a map made by the dark energy survey and actually this map here you're seeing this is the first time it's been shown in public this is one eighth of the sky it just tells what the color code so the color code basically tells you about where the mass is in the universe so and and and this is actually looking at the mass primarily about six billion years ago okay so these are the these are galaxies that span all the way from sort of 1 billion light-years away to about 7 billion light years away and so and there's I think about a hundred million galaxies that went into making this map over one-eighth of the sky and we're actually seeing where in the universe is there more stuff or less stuff and by measuring how this evolves over time we actually also learn something about the expansion history of the universe which tells us about dark energy so overall through these simulations and through the relationship to observations you are honing an ever more precise understanding of how much dark matter there is and its distribution throughout the universe so this I gather is just adding more and more weight to your argument that the dark stuff is it's real and it's actually out there so so from the work that you've done what would you say is the most I mean do you ever ask yourself whether the dark stuff is real or is that just a foregone conclusion and the approach that you you take in your work on a day to day basis I sort of assume it's there because it's a model that works extremely well so we can you know we can use that model and we can make a huge number of predictions so the things that I talked about already it allows us to say how much there is and where it is but it doesn't tell you what it is so our best guess is that it's a particle but there are other possibilities and you know there could be other possibilities that give you the same distribution of galaxies the same distribution of mass and I think one of the interesting things is that there are clues so the things we talked about so far are sort of looking at the universe on very large scales yes but different kinds of particles make different predictions for what dark matter should look like on small scales so this wimp we talked about yep it's one version of a kind of particle that's called cold dark matter and that basically means it doesn't travel very fast it's cold it's sort of heavy it doesn't move as quick it doesn't move very quickly and this other this warm one kind of moves more quickly and the consequence of that is that it most work quickly so it actually erases the small stuff and so you would actually expect in those kind of models to have fewer tiny galaxies than you would in the models on the right and it makes lots of interesting predictions for the numbers and and how they do when you compare to observation where does the observations drive you toward left hand side right hand side definitely so far to the left hand side so there are versions of so the most extreme version of warm dark matter which hot dark matter is for example Mary Angela mentioned nutrients so that would be hot dark matter we've art we have incredibly light particles that's the key thing and they move really fast and so all of those little clumps would be erased and so already so Joe was talking about the dwarf galaxies actually in the last five years or so we've discovered a whole bunch more tiny galaxies that are orbiting the Milky Way there's more than 50 now and we haven't seen gamma rays from these galaxies but we do see that even though they're super tiny the tiniest ones have like 300 stars but they see and maybe you look at how the stars are moving it implies that they have like a hundred million times the mass of the Sun in dark matter and so we've found a lot of those and that actually kind of tells us that we don't live in a universe which is mostly warm dark matter it could be a tiny bit warm that's like almost cold right there are still a lot of sort of missing structure if you look at this model it could predict many more cliffs sub halos that have not been observed yet I mean so there's still something to be confirmed these pictures here are just the dark matter not all of these are things that we expect to light up so in our in the context if the picture on the left is correct then there would be a lot of dark matter we call them Dark Matter halos these clumps of dark matter we know that in in this model that above like 10 to the 8th like a hundred million times the mass of the Sun those kinds of halos should form galaxies but below that they're not in this model that we wouldn't be able to see the galaxies now there may be other ways to see them that we're also thinking about so let me ask you two questions along those lines so there's a tendency in trying to address the question of what is the dark matter to think that there is a thing a particular variety that will explain all of the dark matter could it be that the dark matter is a sort of smorgasbord a few black holes a couple of wimps you know some other things mixed in it could it be a melange in that way or is it going to be sort of one thing I think it's totally possible it's a Milosh I mean I think most of the time we talk about dark matter is being really simple because that's an easier thing we want to say is the dark matter a wimp is the dark matter in accion is the dark matter black holes but you know we don't know and if the dark stuff is an outcome - you just have just one more question before I forget it you know could you imagine that the dark stuff lives in a kind of dark sector which has its own I don't know standard model of particle physics and as all these other thing but it has no interactions with our world besides gravity and therefore extraordinarily difficult to directly detect because it simply passes through us without any impact beyond the gravitational force which is incredibly weak is that a possibility that you that you pursue as well as that I think it's definitely a possibility I think you know we know that our owns we know the standard model which we've actually already mapped out is very complex yeah so in that sense it's a little bit naive to think that this whole the whole rest of the universe is only one thing it's just a simpler possibility to think about so there could be like dark worlds dark people dark everything in some dark well most of the candidates we have right now for dark matter would not create dark worlds or dark people I think that the Dark Sector models are very interesting and with the null results that have been coming out of these experiments so far I've been gaining a lot of attention over the last few years and one of the things that's really exciting I think about these types of Dark Sector models is the types of predictions that they make are very different from the predictions that you get for wimps so we have a whole slew of experiments this massive experimental program that's been really targeting wimps from over the last few decades we haven't seen it and now with all these new ideas about these dark sector models people are starting to think oh gosh like we need to start branching out and build different kinds of experiments so that we can really capture the full range of these possibilities so where do things stand on on heading off into that wild new territory that will differ from the focus of attention for 30 years is is that an active area now it's yeah it's very active and but it's also not like we're going into the big Wild West we could do it very systematically so you can say we have the standard model we have a dark sector and then there's certain rules for how that you can communicate with each other it's a finite set of rules so that that allows us to list all of the possibilities and then we can think about experiments that would be targeting all of those possibilities one very natural consequence is that the Dark Matter particle is lighter than you'd expect for a wimp so just give us a sense of scale when we talk about wimps the typical size relative to the proton is about 100 times as much right so when you say light you mean so now we're going down to like 10 to the minus 3 10 to minus 6 times the mass of the proton right so considerably lighter so five eight orders of magnitude yeah exactly and and then that has significant effects for how you look for it because imagine that you build an experiment where you want to look for a Dark Matter particle coming in and knocking into your target inside your experiment if the Dark Matter particle is heavy it'll come in and knock that target and you'll be able to see it very easily if the Dark Matter particle is light it'll come in and it won't knock it and then we never see the Dark Matter particle but we can see the kick the atom inside them what sort of atoms are we imagining kicking in this yeah so this well a lot of the experiments are using xenon there's some experiments using argon where are these experiments we put these experiments deep underground so usually like underneath mountains or down the in mines because the signals are so so faint you're looking for these atoms just giving you a slight little jiggle after these collisions so you need to make sure that you are sort of shielded from any other kinds of backgrounds that can mock up that type of signal and it's in this scenario there have been signals at least reported in the literature I don't think any of them are really believed right is that the general consensus yeah that's right the current state of affairs right now is that the there's very strong limits from these experiments that exclude whatever signals were claimed by some other experiments so the consensus right now is that those other experiments haven't been reproduced although there's continuing efforts to try to and do the the to those who have claimed positive signals are they still supporting their previous result are they also agreeing that they're inconsistent with other experiments and probably are not correct no they're they're continuing to run and continuing to stand by their original results and it really will take repeating that experiment by a separate group using essentially exactly the same type of setup to really be able to confirm it one way or the other now if the park is much lighter you're about to tell us before interrupt you sir so if it's much lighter than these experiments would differ and some yeah so if it's much lighter and it comes in and it hits that nucleus the nucleus is in and a jiggle very much and so it becomes really hard to see it so but building an experiment where you're looking for those electron recoils is different than an experiment is looking for the nuclear recoils and so there's a lot of brainstorming right now as to how to do that and some initial efforts to get that underway gotcha so is it a fair summary to say that there's a lot of indirect evidence for the existence of dark matter the searches to actually find it directly are at best inconclusive but perhaps some would even say it's beginning to close the window on certain of the favored candidates over the past so three decades so it's a precarious situation if it's something more exotic like primordial black holes that that would be an interesting approach but not the one of the particle physics nature and moreover as we also noted supersymmetry forgetting about dark matter we've looked for that at the Large Hadron Collider we haven't seen that either so that window is sort of closing so that sort of as a natural segue at least to thinking about alternative approaches that might really stand outside the the box of thinking that people have been within for a long time so Sarek why don't we turn now to some of the things that you've been thinking about which is I understand it begins with trying to get a fuller grasp on the underpinnings of gravity itself and it's sort of a nice again day to be talking about it general relativity confirmed 100 years ago you know today and that was at the time the deepest understanding of the force of gravity you're trying to go forward from that so just give us a sense of where you've been going in these ideas yes so Einstein game but his theory are more than 100 years ago and has been confirmed and of course it has also been very good in predicting things like gravitational waves black holes and they have been seen now so a lot of predictions have come out and and have been confirmed but this has to do with gravity that's very strong it's a particular where we see the dark matter happening is something that where gravity is very weak and this is where I think modifications might happen and our understand just give us a sense of that so when you say strong you don't mean that gravity is intrinsically strong you mean there's more stuff right my stuff or so acceleration is strong in this where where a lot of matter is put together if we look at larger scales then then the accelerations are much smaller these are more diffuse and more diffuse but by thinking about black holes in particular we have tried learned more about where gravity kind of comes and this is sort of a development that that started in the 70s with the work of Stephen Hawking and I with my colleagues have been thinking about these black holes also by combining it with what we know about quantum mechanics and then we discovered that there is actually deep relationship between gravity and well entropy and thermodynamics and it's from that those considerations that that we start seeing that there's a deeper understanding of what gravity comes so can we pursue that just for a few minutes just to give people a sense of that so so can you tell us what like so there's a little puzzle that was raised I guess a long time ago by John Wheeler yes so what was that so he was indeed the first person to really ask questions about black holes first of all that was he he coined the name yeah black holes 112 Street and Broadway actually yeah I know I'm not joking it was at the Goddard Institute for Space Studies it was during a talk but no black holes is where the matter is so densely packed that light cannot even escape and and there's some some imaginary sphere around it that if you go behind and beyond that and you cannot escape anymore we call that the horizon and so John Wheeler asked the questions about well the laws of thermodynamics also whether they would apply in that situation and so he had a thought experiment where where is the a cup of tea and so this is a cup of tea and in the tea there are molecules going around and there's a temperature and if you look at the motion and all the the random motion of those particles in there they represent a certain amount of entropy yep and you can think about entropy as sort of telling you what are all the possibilities that the tea can be in and one thing you can do is apply the laws of thermodynamics and an entropy always has to increase and so we asked us is also apply when we take a black hole in neighborhood and throw the cup of tea into the black hole because what happens is that whatever it was in the cup disappears from our view and the entropy that is in there we don't see anymore even if the cup breaks we would not know it because it would go into the black hole and everything disappears then somehow this law of thermodynamics that the entropy has increased has still to apply so we ask the question where does this entropy of the black hole where where is it sitting and so Stephen Hawking and also jacob bekenstein basically answered that question and they realized that when you throw something into a black hole that the horizon gets slightly bigger so the black I'll eat some stuff and just gets a little bigger yes and and they indeed we then came with the proposal that the amount of entropy that we souped associated with it black hole is actually given by the area of that horizon and that was a beautiful idea and they wrote down some beautiful equations for it so this is the you need a picture of the black hole and you can see the horizon there and it is so if you throw in one particle in the first black hole the second black hole is slightly bigger and it will have a bit more information and this is this one little bit that has been added so here you see the other black hole which is slightly bigger and we're gonna compare indeed what is the amount of information in there well squares on it and and that actually represents one unit of information and and this can be described actually using indeed is these laws of thermodynamics namely the mass of the black hole represents a certain amount of energy and the entropy is then the area and indeed the area will increase is the energy increases and that's also the same equation that we know from thermodynamics and now the idea comes namely these thermodynamic laws we can really explain by thinking about the microscopic motion of molecules so we understand precisely what entropy is we know what temperature means namely as a statistical measure of the energy per particle and so we can arrive those laws but now we want to derive actually the same laws for gravity actually they have the same form and so the gravitational loss of black holes and actually the einstein's equations look like the equations that well the thermodynamic equations in that sort of a remarkable statement right I mean thermodynamics was developed in the in the 1800s and its ways to understand things like you know steam engines the things of that sort and you're saying that there is a deep relationship between those laws that have nothing to do with gravity right they just have to do with things that we see in the world around us and you're saying there's a deep connection between those laws and the laws of gravity that's correct and this is something we have been discovering say over the last three four decades and a particular last 10 years there's a big development trying to indeed understand these gravitational equations that Einstein wrote down from this deeper underlying description it's a you entropy some principle you could have a conversation with Albert Einstein and say al the equations that you wrote down on November 25th of 1915 I can give you a deeper explanation for where they come from yes and I think that is the same way that he actually of course explained in a deeper way what Newton's equations were saying and so every theory in in physics that we've known about eventually will be surpassed and they made it sort of well taken over by a noodle theory doesn't mean that the old theory is wrong simply it's explained at a deeper deeper level but there can be dense circumstances where the new theory works differently and this is where I think indeed when we are dealing with horizons and we can see these temperatures appearing so one of the predictions of Hawking was indeed that black holes don't only have an entropy but also a temperature and that they emit for instance radiation and that's also a remarkable statement because normally when we talk about black holes we don't imagine anything coming out of them yes so this is indeed the discovery of Hawking that black holes aren't really black I mean black wouldn't mean that nothing can come out but he discovered that because of this quantum mechanical properties of the horizon they carry a temperature and that means that they even really radiate and can possibly even evaporate and so this is all about black holes and of course you've been talking about other things mean in the universe about even also the dark energy and what has to do with dark matter so my idea actually indeed is that we in order to understand this dark matter phenomena it's not sufficient to only focus on that we also have to understand this dark energy component that you talked about I mean the fact that we don't understand 95% of the energy in the universe why would we focus only on one component and I think that the dark energy is first the thing that we also have to understand more and better in a microscopic way so when we spend a split second on that since we mentioned it but we didn't really say what it is so like 1998 this wonderful discovery that the distant galaxies are rushing away ever more quickly accelerated expansion of the universe completely unexpected yes as we sort of see here everybody thought that over time the distant galaxies would be rushing away ever more slowly since gravity tends to pull things together but it's going faster and faster and the explanation that came out in the late 90s was there's an energy stuff using space it's dark it doesn't give off light and it's giving rise to a kind of a repulsive gravitational push that's making everything rush away yes so that's it's dark energy and and and then so your view is that there's a connection between dark energy and dark matter yes I mean the fact that there's dark energy in the universe has a very important consequence and we indeed things keep moving away from us and even accelerating that also means that if we look further things are moving away faster and eventually there will be a distance where things are moving away with the speed of light and then you get the beautiful conclusion and me that acts like a horizon namely anything that moves faster way with the speed of light we cannot see anymore it's like indeed with a black hole we cannot look what is inside but now our own universe will have some horizon that we cannot look beyond and that is happening actually in the universe that has only this dark energy in it then the expansion is actually rate is actually a constant then what Hubble discovered this expansion rate can be expressed in a clinic yeah a constant that tells you basically how large the universe is because it will tell you where the horizon is sitting and that horizon has very other properties as black hole horizons and this is where I make the connection between dark energy and the thing we talk so let's let's hear it so the connection you draw is is a following namely that that the entropy that we talked about that black holes have we can also associate an entropy to the horizon that our universe will have and that horizon only appears because there's actually dark energy in the universe there's also a temperature associated to death horizon so it actually also satisfies all the laws of thermodynamics but then you can ask well where is this entropy and this temperature associated to it's actually associated to the dark energy that we've added to our universe so I say that the dark energy is the thing we really have to understand because that will carry also an entropy and a temperature which is the one that we can calculate using the same equations that that Hawking and bekenstein found for black holes but then applied to our universe dark energy was essentially invented by la Mettrie and then you know and discovered half a century later it's a constant in our science equations which Einstein himself put in but the Metra realised that it was due to these tiny quantum effects and he even called it dark energy so a constant of nature that could be the dark energy and that accounts for everything it's a tiny constant we don't know where it came from it dominates the universe now gives us the acceleration but you know there are other constants in nature too though I don't think one is trying to explain at the moment the unified theory maybe someday we will what what's wrong with that just saying dark energy is this constant dark matter some some other problem so this constant describes more than 70% of the energy of our universe and then we just put one constant there well the 5% we're talking about that is ordinary matter well that's where all interesting stuff happens that's our current theory and that's precisely what I'm okay so for black holes we would have said something similar a black hole only has a mass and well maybe it can rotate we would not be able to explain its entropy by thinking about a black hole in that way in order to and explain that entropy that's something we learned from string theory we have to add many more degrees of freedom and we have to indeed explain these what's happening on on the horizon of this black hole we have to do something similar for our universe so adding describing dark energy simply as a constant is making an approximation for instance that we can describe everything that's here in the room by just adding the temperature and that's not describing really what's going on in the room with all the motion of the molecules that's in there so there's a lot of things we're missing if we describe dark energy with only one we also heard a lot of very detailed evidence of these beautiful simulations that allow us to have a video on left hand and right hand side which is virtually indistinguishable at least to the naked eye between simulation and observation we see these beautiful explanations of the rotations of galaxies and so forth how far can you go in in this novel approach to explaining these kinds of detailed features of the world without invoking say dark matter so what is needed to describe the observations is that there's an additional gravity that keeps these galaxies together I mean it's the additional pool that we want to explain so if there's an other explanation by understanding gravity better that there is an additional pool we can reproduce many of these results so I see Dark Matter almost as a sort placeholder in a way that we can put it extra matter there but we describe in effect that it actually is due to the gravity itself so when it comes to a simulations would work in a very similar way if you would have an understanding of gravity that adds this additional force can you talk about the the swirling yeah I mean the beer Rubin you know observing you how would you explain that okay so there indeed I explained that I want to understand the dark NSEL swirl also and so dark energy for me as an entropy in it but also when the matter is there actually it has an effect in interaction with this entropy that's in the dark dark energy and we can probably show this a min so indeed this is what what you talked about before namely this is the expectation for this rotation curves where that where on the vertical axis we have the velocity into the right we have the distance and then you see the velocity going down what's actually being observed is that it's indeed flattened and there's an important hint in in how where that happens turns out that it happens at a moment when the acceleration drops below a certain critical value and if we express that value numerically we find the connection with the expansion rate of the universe this Hubble constant to me that's an important hint that there may be a connection between what's happening here and what is causing this expansion namely the dark energy and so that indeed I can explain why there would be an additional force due to the dark energy here's the the the galaxy that's rotating and we won't understand why there's an additional pool yeah but this has to do with the presence of the dark energy that's in there so if we take the dark energy and add it to here you actually will see that in this neighborhood of the galaxy itself the dark energy be has been expelled and it tries to sort of reach or push back again and it's sort of the interaction between the dark energy and the matter that's there well that will give the additional force that is responsible for keeping the galaxies together and it's more than just pictures you're saying that mathematical analysis so there is a I will actually give a presentation here tomorrow at the workshop where we'll show these equations and indeed you can work out numerically what is the additional force one expects and you find indeed it has the correct value to predict actually even this flattening of rotation curves this isn't in a particular situation of course dark matter is responsible for many other things yes not just rotation curves of galaxies it's used now to explain structure formation and as we saw well it's important for explaining what we see in the cosmic microwave background and for that indeed we need to develop even the theory further so that we can also explain those things but they've not done that yet is that well that's this idea so I have but it's not like it has been worked out completely but the thing that I realize is that that what is needed and this is sort of what makes these models work is there is an extra component that gives you an extract gravitational potential what keeps things together but in all of those calculations is never essential that it's a particle the only thing that really uses the additional gravitational field so the particle nature of what now is called dark matter has never is not essential for those things so if I can explain the same gravitational effect without invoking dark matter I can still reproduce many of those calculations and the math may even be very much the same so what do you guys think you know you've been searching for these particles for a long time haven't found them and here's an approach that may not need them convincing at all or yeah you began the first demonstrations of dark matter came with Ruben yes and we saw a beautiful explanation with using dark energy but also even before that was Vicki yeah with clusters and it turns out that if I think if you apply your same theory to clusters you do all the attempts so far need Dark Matter as well otherwise one cannot fit this this new type of theory of a Dark Matter field to explain the motions of galaxies new classes the ghost and fly part otherwise that is that true do you agree with what joe says so there are attempts to sort of main all of the attempts you're talking about a different theories and then exit the one I wrote down indeed where it would have a factor that is too low I think in my case actually can fit the outside of these clusters the problem really appears more in this in in the center where there is usually a very strong concentration of what then would be dark matter and here I do believe that there may be our explanations that are necessary but this is certainly a region where we're also supermassive black holes have been formed and many other things going on in the center part of these clusters where I do think that the the explanation of what being observed there may have to do with even understanding those phenomena in a more fundamental way so so I agree a cluster still provides a challenge certainly did the central part of it the outer parts I think I'm pretty confident that they can be explained in the same way as I explained the rotation curves for galaxies so I mean I agree with Erik that we haven't proven that it's a particle and until we actually see evidence you know of something that comes from that particle then we won't really be convinced but I think the thing that is so compelling about the idea of dark matter is that there are these particles that are predicted as we discuss for totally independent reasons we didn't talk much about the acción it's another particle that you know could be the Dark Matter would behave for structure formation very similarly to the wimp and the thing that's so compelling to me is that you take that simple idea of you know take the simplest version where it's one particle and then you just use basic gravity as we understand it and it makes a huge range of predictions it predicts the Cosmic Microwave Background it predicts essentially the last 13 billion years of our universe all the way from tiny galaxies to the scale of the whole universe and it does that in some sense quite naturally and you do need a new particle but but then you can make lots of predictions right and you can test them and we're testing them at very high precision now yeah which is why the community of physicists for so many decades has had that as the paradigm explanation but I guess the part that feels deeply unsatisfying still is that we've yet to actually get our hands on it right it's very unsatisfying yeah but you know there's lots of possibilities right I mean I really hope we all right you know figure out which one of them is right but there's lots of possibilities and we're just at the cusp experimentally of being able to test those possibilities so I actually think that's what makes this field so exciting would you say in the next what are we looking next five years Nick and if we had this conversation a decade from now and assuming that funding levels stay you know it's unclear you know but but you know a decade for now how would this conversation differ I think we don't know I mean we could find it you know in the next five years I guess my question is is it conceivably to come back ten years and we're still mmm not sure or would it be we've ruled out like dunt dunt dunt these huge number of possibilities and it starts to feel as though we're clutching at straws in the particulate approach to dark matter so my my view is that the pot the range of possibilities is pretty big so you know if we still haven't found a wimp in ten years it probably doesn't mean it's definitely not a women but it means it's probably a wimp that's hard to find [Laughter] that's a great song title and definitely if we have this conversation in 10 years we I guarantee you we will know a lot more about what dark matter isn't Joe so so just speculate ten years from now where do you think will say here's what's gonna happen in 10 years and I know speculation here's what it is let me even give you 50 years actually yeah and so we're gonna build bigger telescopes okay we're gonna feel bigger accelerators and the reason is that China is getting involved yeah they want to build the world's biggest therefore the West will do something similar to and these will be accelerators ten times more powerful than that it's CERN that will take us to the limits of what the Dark Matter particle could be will have new types of gamma-ray telescopes and then eventually we'll be building telescopes on the moon which is a stable platform no atmosphere bombarded for billions of years by dark matter particles and other things too a great place to start doing research to for dark matter so I think many many experiments very very expensive but science you know at some point does get done and will get done so I'm will discover something even if it's not what we were looking for is there just Mary owns that is there a point in your career going forward where you would say if we cross that threshold and we haven't found the evidence for dark matter of the direct sort that we're talking about that you would say you know it's time to change perspective either do something else or or take on a completely different approach well I think we should always be keeping an open mind and investigating completely different approaches simultaneously but I don't think that you know even if in the course of my lifetime we never find the dark matter particle that I mean that seems like a totally viable possibility to me I mean because when when we just simply ask in the most model independent fashion like let's set aside Susie let's set aside blimps let's set aside accion's and just ask what is the range of masses that this Dark Matter particle can be the most model independent constraints that we can get are just we need the Dark Matter mass to be such that we can form galaxies and the the mass range is orders of magnitude it goes from 10 to the minus 22 electron volts all the way up to you know black holes right and the wimps which is what we've been spending all of our time really most of our time really focusing on there's a tiny tiny little sliver in that space so in the next five or ten years yeah I think you know given the amount of effort we've put in that sliver we'll probably know one way or the other whether or not that hypothesis was viable even that's the way science works right you start off with the hypothesis you see you pick the spot where you think it's gonna be and then you go for it and then you know you say yes or no depending on what the evidence gives you but as we move away from that the possibilities are so huge and the kinds of experimental signals that you would look for in that range are so diverse that you know it takes you know very you know hundreds of years to just sort it out almost we're lucky yep so CEREC final question you know it's kind of a funny situation to be in so you you and I both string theorists yes which famous is a theory that has no experimental evidence supporting it whatsoever yeah and so the question that we're generally asked is when do we give up on these ideas and and yet beautifully wonderfully you're pushing in a direction that's trying to make contact with observation but in a very unexpected way because for so long the community has fixated on dark matter as a solution to the various problems that we've spoken about here today so how do you reconcile that I mean is there a point where you think that we will have gone so long without confirming supersymmetry or finding extra dimensions or any of the other strange qualities that it's just too speculative to push on this kind of a direction and you could just say no and we could finish the evening no no no I don't think that the ideas we have developed there are important and they will teach you some more about what gravitation is but I do think that as a string theorist in the string theory community we have to also ask questions that can be connected to observations and I do think that also cosmology is one of those areas I mean at the moment we are working with a cosmological paradigm that uses dark matter and so on and then it's based on general relativity but I hope that we can develop the string theory to the point where we have a really this more microscopic picture in terms of a theory that combines it with quantum mechanics and and then we can also understand for insisted the universe with the dark energy in it and I'm convinced that once we have this theoretical framework we can also address questions that have to do with observations until that time I think we should be looking for dark matter and be having all these experiments because sometimes you have to look and test all the possibilities before you get convinced there may be another way of looking at this so I think you should work on both sides why we look at observations actually better observations even of the phenomena that are associated to is mean the expansion history of the universe what's going on in these galaxies and clusters getting better well precision data that will help I mean I hope even that there will be time that we can develop the theory our string theory by being guided yeah by observations and we go back to our whole time when when they go hand in hand and we all just get along no but but at the eye I have certainly spent the last 10 years a lot of my time reading about what people have been looking at in cosmology and so on yeah because it can give me also ideas of what direction to look for and I do think our universe is not the kind of universe that the string theories are now studying I mean they're mostly interested in a very idealized model with no dark energy and which is fully super symmetric and so on but that doesn't seem to be describing our world but if you wanted to understand what it looks like in our world I think we also have to take the data into account very diplomatic so so thank you it's been a fascinating conversation and hopefully we will resolve this before 50 years from now but but who knows so please join me in thanking the group here thank you [Applause] [Music]

Channel: World Science Festival

Views: 586,971

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Keywords: Searching for the Universe’s Missing Matter, Brian Greene, what is dark matter?, how to detect dark matter, Mariangela Lisanti, Joseph Silk, Erik Verlinde, and Risa Wechsler superfluid, shaking up the dark universe, dark energy, dark matter, galaxies, particles, natures dark side, farthest reaches of space, antimatter, dark fluid, Standard Model, Supersymmetry, Large Hadron Collider, world science festival, best science talks, 2019, The Joe Rogan Experience

Id: 1VajnuxMJmU

Length: 82min 24sec (4944 seconds)

Published: Fri Oct 11 2019