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

thank you all for coming tonight you know it's a special night tonight for science who knows what I mean anniversary? Do you know what I'm talking about? someone what is it? Well, yes, but the light falls. is celebrating a particular event in the history of science that was the confirmation of Einstein's general theory of relativity 100 years ago today The observations of the eclipse were used to confirm the prediction that Einstein made in the call referenced in fact we have a show tonight on PBS so after this show if you want more you can run home turn on channel 13 from 10 to 11:30 you have a show that was filmed on this same set in February but tonight we focus on a related idea that has a deep connection to gravity theories our understanding of the force of gravity because we are going to discuss the possibility that many scientists have accepted over many decades that there is much more to the

universe

than meets the eye and the The reason we have come to this conclusion is by virtue of studying the gravitational poles that occur in the cosmos and coming to the conclusion that the things we see cannot give rise to the gravitational attraction we observe and a way of thinking about this is just Getting into the topic is imagining that you know you're flying to New York City at night.

Many of us, maybe all of us, have done this and when you fly at night of course the only thing you can really see are the lights on the buildings now we all know when we see an image like this that there is more structure to the image of reality than the lights themselves, how do we know? Because we all have an intuitive understanding of gravity and if there were no buildings, if there was no structure out there, these lights would fall to the ground and since they don't fall, we know that there is something out there that is holding them up and of course the light. of the day we can begin to see what that structure really is and of course we all know what the structure is, it is the architecture, the physical composition of the buildings themselves, but without being able to see that structure itself if we only have access to light , which is the only thing we have access to.

When we look at the

universe

we have to infer the existence of

dark

things that are not visible at the moment and that is how we come to the conclusion that when we talk not about an urban landscape but about the universe, there could be more out there. there than it seems, so this is an idea, it actually has a long history and I won't take you through the whole story, it's a fascinating story, but I'll just give you some highlights towards the end of the 19th century as began to have an increasing ability to observe the night sky, scientists began to try to really understand the movement of stars and one scientist in particular, Lord Kelvin, had the idea of ​​modeling the stars in the galaxy as if perhaps They were 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 a box of gas, apply the same ideas now to stars moving inside a galaxy and When he did that analysis he came to the conclusion that there could be things out there that we can't see, in fact, in his own words he described it like that;

However, it is likely that there may be up to a billion stars, but many of them may be extinct and

dark

and nine-tenths of them, although not all dark, may not be bright enough for us to see at their distances. real. Many of our stars, perhaps a large majority of them, may be dark bodies, so the possibility that perhaps most of the things that are out there could be dark, this is an idea that goes back to the late 19th century. 19th century and then another great mathematical scientist, Ray Poincaré, was inspired by Kelvin's analysis to do his own version of that analysis and came to a different conclusion, but he did introduce an important, yes, obscure phrase, he did it in French, of course, but of course it's dark

matter

, so this idea of ​​dark

matter

goes back to the late 19th century and early 20th century and as people started thinking about this idea over the following decades, it's this guy. from here do you know who he is?

Does anyone know, this is Ricky, yes, Fritz Zwicky, who was a Swiss-American astronomer, a wonderful character, but he started studying the motion of galaxies in the Coma Cluster, which is a few hundred light years away. and he again discovered that the motion of the galaxies was such that it could not be due solely to the ingredients which he could see by virtue of their light. He concluded that there had to be additional dark matter out there that would be responsible. because of the gravity that was pushing and pulling on these galaxies, but it's actually the work of this person here, Vera Rubin, who really clinched the case in the minds of many physicists and many astronomers about the existence of dark matter that is out there. because what she did was she studied the motion of stars in swirling galaxies and discovered that the galaxies spin too fast and that the stars should be ejected right now.

One way to think about this we can do a quick little demonstration of this, you know, if you have it, for example. a very simple pedestrian situation where you have one wheel on the right and you have water in the wheel and we all know we don't have to do this, but it's fun to see if you take this wheel and it turns slowly enough, very little. The water will fly out but of course you give it a real twist and the water droplets fly out. Thank you very much and the idea was that the galaxies were spinning as fast as I was spinning the wheel and I said the stars should be flying. as did the water, but Vera Rubin discovered that the stars are not flying and therefore there had to be something else that was attracting them towards something darker because we do not see it giving rise to the gravitational attraction that kept the stars. galaxies together and of course they weren't just images, there was a mathematical analysis behind this when you do the math which we won't go into any detail, but the expectation was that the further you got from the center of the galaxy, the slower the stars should be moving, but in fact their observations and analysis showed that that did not seem to be the case, the speed of the stars at the edge of the galaxy was too fast relative to what we thought it should be, they should be flying, but They weren't and hence this notion that there should be some dark matter out there and when you put the numbers in you find something quite remarkable if you calculate how much dark matter there must be out there to contain these galaxies. together we find that it is four five times more than the amount of matter (protons, neutrons, electrons) that make us up, so we are talking about the fact that most of the matter in the universe could be dark matter and, in fact, even We will talk about a different subject.

A kind but perhaps related dark entity called dark energy later in the show. The bottom line is what constitutes you and perhaps everyone else as a small portion of the massive energy budget of the entire universe and that is a remarkable pie chart. showing us that we know a lot about reality, but it may be that much of our focus has been on the small part of the full story now, before introducing the participants who will discuss this further with us. I want to mention a point that is actually related. to the centennial anniversary of the general theory of relativity, which is when Einstein was doing his calculations on relativity in November 1915, he focused his attention on a particular enigma, a particular problem that had to do with the motion of the planet Mercury, which had been It had been known for a long time that Mercury's orbit was not doing what the gravitational equations said it should, instead the orbit was moving a little each year and to explain this change in orbit some astronomers introduced the possibility that perhaps there is a dark planet. out there called Vulcan, a hidden planet, an undetected planet that was pulling on Mercury and that's what was causing the orbit to change.

This was the idea until Einstein came along and with a deeper understanding of the force of gravity and his general theory of relativity he was able to fully explain the data without any obscure elements, which is just to say that you have to have an open mind. As we will see in the discussion in the future, maybe there are dark things out there, but maybe our understanding of gravity needs to be deepened like this. The historical example shows us and perhaps a deeper understanding of the force of gravity could explain the anomalous observation, so this is a point we will return to, but the fundamental question we will discuss is whether it is a case very similar to this urban landscape.

We see the bright things, the stars, the lights on buildings, there are additional dark things out there and if there is what it is made of, those are the questions for tonight and we are lucky to have some of the world's experts to talk to. discuss these questions. our first participant is the director of the Kavli Institute for Particle Astro

physics

and Cosmology. It is playing a leading role in modeling and mapping 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, our next participant is a professor of

physics

and astronomy at Johns Hopkins University and a science writer and author of his work on the cosmic microwave background, galaxy formation, nature exploration, dark matter, please welcome Joe Silk.

All right, we're also joined tonight by an associate professor of physics at Princeton University whose research focuses on the nature of dark matter. She has tested dark matter theories using data from a wide range of experiments. Please welcome María Ángela Santi. Alright, our last guest tonight. He is a professor of theoretical physics at the University of Amsterdam. His research concerns string theory, quantum gravity, black holes and cosmology. He has received the Spinoza Prize, the highest prize available to Dutch scientists. Please welcome Eric Berlin day. Very good, thank you all for being here. for this discussion about the dark things that are in the universe and I just want to start by amplifying the point that I was making at the end so when we think about this puzzle, there is movement in the universe that it seems like we can.

I don't explain it using the things we can see using light. There seem to be two general ways to approach this problem. It could be that there is more out there than meets the eye. There could be dark things. You can actually display this on the screen. If you accept these two possibilities, but it could also be that our understanding of forces, and in particular the force of gravity, needs a deeper explanation, a more complete understanding, just like Einstein's general theory of relativity, for the movement of the planet Mercury, in part. of our discussion tonight will be these two possibilities and I'll just lay out the teams, so to speak, so that these three people here I think it's fair to say that they are more on the dark side of the explanatory possibilities and There, on the far left or On the far right, from your perspective, is Erik for Linda, one of the most thoughtful creative and insightful physicists I've ever met in my entire life, so I won't bias the conversation, but he has a very interesting take, but I think. it's fair to say a speculative idea is more on the right side of explanation, so quickly, where do you see your ideas fitting into this?

Did you feel like it developed to the point where there's a competitor for the dark stuff that's going to do it? be part of the focus or is it still a work in progress as busy understanding gravity in a more fundamental way. I mean a lot of colleagues that I do this with are trying to combine general relativity with quantum mechanics with the use of string theory and this has become a new framework in which we can explain where gravity comes from starting from a more microscopic picture and in fact I think we can explain these phenomena without the need for dark matter, but this is a theory that in some ways has started to develop, but we need to do more work to eventually have everything ready, so It's not like it can explain everything yet, but there is the first hint of a new theory and, in fact, no dark matter particles are needed to explain this additional gravity. that we've been looking at and just to give a snapshot so that people who don't follow the details of this in the research literature, is this a minority opinion equivalent to one page?

How would you frame the number of people who are thinking this way? versus the dark things are certainly in the minority and of course I would like to encourage more people to think about it this way and I think that once we start trying to understand gravity better and have developed that theory, this will automatically come out, but thisvery natural options that didn't make it yet, that was compelling, but it also did, the numerological aspect always made me feel a little uncomfortable, it's this point that Mariangela made, that if it's not that the parameter space opens up too much and That's an interesting situation we're in.

I mean, I think it's interesting that the interesting article where the original articles were seems incredible, yes, those models have already been discarded. Now we have a much broader parameter space that we have to analyze, but in the old days, when there were amazing things and that drove the focus on a particular category of so-called weak dark matter candidates, yeah, I didn't do that. I mean, thank you, so, so, so, weak means what massive particle that interacts weakly and the supersymmetric partners provide a class of candidates that fit that particular description and could be Dark Matter, yes, I mean several about the trace of the particles. that mariangela was already talking about being weak so if this is a viable possibility the key of course is to go out and find these particles and that's something we've been trying to do for a while so maryland can help us get through it .

The approaches that people have proposed to try to capture one of these particles or create one of them, sure, yes, with the weak ones in particular there is a three-pronged approach, so the first is to try to produce it in the laboratory and in that case, you look at a really powerful collider like the Large Hadron Collider, where you take two protons and smash them together. It's really high energy and you hope that in that high energy collision you can produce some new exotic heavy state that could be the particle of Dark Matter, so using the energy of the incoming particles and it is equal to MC squared you want to transmute that energy. in these exotic people, that's right and in a sense, if we can create this in the lab, that's the best scenario because then it's a controlled environment that we could go into and we can actually study what you know and come up with the properties of particle physics of that. particle well, how is it going?

We haven't seen it yet, but not for lack of trying really hard, right, okay, and we're still trying. It's not that the game is over, but so far I mean the LHC has been running for the past few years and we will continue to run so far, we haven't seen anything, but you know, people know there will be more data and people are coming up with new and creative ways to analyze them, so we never know when that will surprise. so that's one approach, try to create it in the lab, another approach, the other approach is to look for it in the sky, look, I'm sorry to look ahead, in the sky, yeah, so if you have dark matter particles that interact with them .

Sometimes, very rarely, but sometimes it produces a small flash of light, so you can look for this annihilation process by looking in parts of galaxies where you expect there to be a lot of dark matter and see whether or not there is an excess of light . about what you would otherwise have expected to see and how that's going, plus the huge amount of data we haven't seen again, not for lack of trying really hard, and Jody, would that be your assessment? The beautiful part of this argument is the The same effects that prevent these particles from being created in the early universe still give you a value for their interactions.

If they could ever meet, they would again produce a flash of gamma rays, actually, yes, a cascade of quarks and whatever, and that's the trick. If you look into the vast depths of galactic space, the volumes are so large that you can look for the accumulation of these rare events and look for gamma rays. You can see here the gamma ray sky measured by: which gamma rays are just stupid. Okay, they are photons that are thousands of times more energetic and penetrate the next era. Okay, there are hundreds of MeV rkv X-rays, so really, really hard photons produced in nuclear explosions.

I mean, if you're ever unlucky enough to be near a nuclear bomb. explosion, then you would be tested for gamma rays, that would be one of the most catastrophic things that could happen to you, but fortunately gamma rays don't go through the Earth's atmosphere, so we look for them from satellites in space, so it can penetrate there and propagate freely, so in the center of our Milky Way galaxy, which is where dark matter primarily accumulates, there would be expected to be a slight excess of gamma rays if the dark matter is truly the WIMP type of dark matter and so many years ago, even before this company launched this satellite to observe gamma rays, there were indications that something very far away was happening 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 gas clouds, they have been like gamma rays, but you know exactly where the gas clouds are, you know what rays are cosmic to be able to correct them. fall of this and when they did this, they found that in the central degrees of our Milky Way there is a small excess of gamma rays that cannot be explained by anything known at that time, so immediately the particle theorists, the addicts of dark matter, they said this could well be the signature they've been looking for of annihilating dark matter and Eiling with itself and giving them gamma rays and they said well if we take exactly the same interaction strength that is predicted from fundamental theory and we take the standard model that you'll hear about in a little bit for the distribution of Dark Matter in galaxies here's to get the flow right and that seemed very attractive, it wasn't quite proof because there's always a lot of skeptics around and the skeptics They said, well, look, we know there are certain types of stars.

They are rapidly spinning pulsars that the Fermi satellite also saw and they said that the weather could be many of those in the center but so faint that they are really difficult to see, but with humor they could give you the diffuse gamma rays and so on. The jury agree on one side or the other, it could be Dark Matter, very elusive, it could be these

missing

idiots, so there is a new test we can do that we are desperately trying to do now that there are little galaxies orbiting around the Milky Way. Dwarf galaxies that are full of dark matter have very few stars and almost certainly don't have these millisecond distracting gamma ray sources, so we're looking at them with this gamma ray telescope to look for.

There is evidence of gamma rays, you know, we've observed twenty of them. One sees one or two gamma rays at marginal levels, so it is impossible to remain statistically independent of whether we find them or not. All we can say is that we need more and better. telescopes, that satellite was built down the hall for me and when people were working on it, I think we really hoped that when it launched we would immediately see those bright signals from those dwarf galaxies, yeah, and that didn't happen, so you know . There were models that could have been whip versions of the weak that could have had booming signals from dwarf galaxies and we're definitely not seeing that, so we've ruled out a bunch of things with the same excess that we see from the center of the galaxy if that kind of dark matter when we dwarfs would see it marginally now we need bigger telescopes, that's what astronomers always say, so let's turn to telescopes and think about cosmology, so Risa, this is an area that you have spent a lot of time working on Dark Matter as a key ingredient in how the universe has evolved and how the structure has formed, so let's now move on to look at the deep sky for dark matter signatures, so tell us some of the approaches that have been taken used to discover the role of dark matter in forming the things we know about galaxies and other structures in the universe.

We already talked about how dark matter you know if there is dark matter if it is a particle, it is you. we know four or five times the amount of normal matter, yes, 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 predicts something about the fluctuations there. It should be very early 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 extra stuff and a little less stuff, so my understanding of gravity doesn't is just as complicated as Eric's so most of what I do is think about the same kind of gravity that you and I interact with the Earth with or you know the Earth revolves around the Sun so that kind of gravity we can put it into our computers and we can Let's take the fact that there were little places in the universe that were a little bit denser or a little less dense and we can figure out gravity and so we can figure out what kind of structure forms in the universe in that context and the idea is that when it's a little bit denser it attracts 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 also make predictions about where galaxies should form and in our current theory of galaxy formation, yes, one side of the screen was a prediction from those dark matter simulations and how the structure forms in a model where you know that 85% of the mass is dark matter, the other side is real observations where we are actually going out and mapping the distribution of galaxies and this is just a computer that you put there, you put it there and you see what It happens essentially whenever you get enough dark matter in one place, that's where you expect a galaxy to form, you basically get enough If you get enough dark matter, the gas can start to cool and form stars, and this on the right side, those Observations are from a telescope called the Sloan Digital Sky Survey, which actually mapped the 3D positions of approximately two million galaxies.

Later this year we're going to start the next generation of what's called a dark energy spectroscopic instrument. We'll get about 10 times as many galaxies, so we'll have a much better map of what it looks like, but The point is if you look at this beautiful video, they look on the left side and on the right side, they basically look the same, yeah, and this is actually what this shows is that the other thing that dark matter does is form that structure. and that structure, just as we talk about gravitational lensing with perihelion and a mercury, just as we talk about gravitational lensing with those microlensing events, impacts the shape of galaxies a little, so if a galaxy were round, it would be distorted and it looks small. it kinda looks distorted because its light was distorted over the several billion years it took to get to us because there were things in the way and those things we assume are dark matter and it has an impact, that's right, so those things if it's dark matter it actually changes the shape of galaxies and what you're seeing here is actually a map of where all the mass in the universe is inferred from the positions of these galaxies, so that was a map made by dark matter. energy study and actually this map that you're seeing here is the first time it's been shown in public this is one eighth of the sky it just says what the color code is so the color code basically tells you where the mass in the universe so and and and this is actually looking at mass primarily about six billion years ago, okay, so these are galaxies that extend from sort of a billion light years away to about 7 billion light years away. away and so on and there are I think of a hundred million galaxies that were used to make this map in one eighth of the sky and we are actually looking at where in the universe there are more or less things and by measuring how this evolves over time, We actually learn something too. about the expansion history of the universe that tells us about dark energy, so in general, through these simulations and through the relationship with observations, an increasingly precise understanding of how much dark matter there is is being refined and its distribution throughout the universe, so I gather that this is simply adding more and more weight to your argument that the dark things are real and are actually out there.

For the work you've done, what would you say is the best? I mean, do you ever wonder if the dark things are real? Real or is it just a foregone conclusion and the approach that you take in your work day to day I guess it is there because it is a model that works very well, so we can know that we can use that model and we can make a lot of predictions, so the things I already talked about allow us to tell 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 they 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 that we talked about so far are a kind of observation of the universe on very large scales. big, yes, but different types of particles make different predictions about what dark matter should look like on small scales, so this funky thing we're talking about, yes, it's a version of a type of particle that's called cold dark matter and that's basically it.

It means that it doesn't It doesn't travel very fast, it's cold, it's a little heavy, it doesn't move as fast, it doesn't move very fast and this other one, this warm one, moves faster and the consequence of that is that it works faster, so which actually erases the small stuff, so you would actually expect that in those types of models there would be fewer small galaxies than in the models on the right and it makes a lot of interesting predictions about the numbers and how they work compared to the observation where Do the observations take you towards the left side, the right side, definitely, so far towards the left side, so there are versions of the more extreme version of warm dark matter, which is hot dark matter, for example, Mary Angela mentioned nutrients, so it would be hot?

Dark matter, we have art, we have incredibly light particles, that's the key and they move very fast, so all those little clumps would be erased and Joe was already talking about dwarf galaxies in the last five years. We've discovered a lot of smaller galaxies that are orbiting the Milky Way; Now there are more than 50 and we have not seen gamma rays from these galaxies, but we do see that, although they are super small, the smallest ones have about 300 stars. but they see and maybe you watch how the stars move, which implies that they have like a hundred million times the mass of the Sun in the dark matter, so we have found a lot of them and that actually tells us that we don't live in A universe that is mainly warm dark matter could be a little bit warm, that is, almost cold, many types of structures are still

missing

.

If you look at this model, it could predict many more cliff subhalos that haven't been observed yet, I mean there's still something to confirm. These images here are just the dark matter, they're not all things that we expect to light up, so in In our context, if the image on the left is correct, then there would be a lot of dark matter, we call them Dark Matter halos. dark matter clusters we know that in this model, above 10 to the power of 8, about a hundred million times the mass of the Sun, those types of haloes should form galaxies, but below that they are not in this model and we would not .

Being able to see galaxies now there may be other ways of seeing them that we're also thinking about, so let me ask you two questions along those lines, so there's a tendency to think about when trying to address the question of what is dark matter? . that there is something, a particular variety, that will explain all dark matter, could it be that dark matter is some kind of smorgasbord, some black holes, a couple of weak ones, you know, some other things mixed in, it could be a mixture in that way or it's going to be something like that, I think it's totally possible, it's a Milosh.

I mean, I think most of the time we talk about dark matter it's very simple because that's the easier thing we want to say is dark matter. a weakling is dark matter in action is dark matter, black holes, but you know we don't know and if dark matter is a result, you only have one more question before I forget it, you know, could you imagine that the dark matter? things live in some sort of dark sector that has its own, I don't know, standard model of particle physics and like all these other things, but it has no interactions with our world other than gravity and is therefore extraordinarily difficult to detect directly because it just passes through us without any impact beyond the gravitational force, which is incredibly weak, is a possibility that you pursue and I think it is definitely a possibility.

I think you know we know our we know the standard model that we actually have. already mapped out it's very complex, yeah, so in that sense it's a little naive to think that all of this, the rest of the universe, is just one thing, it's just a simpler possibility to think about, so there could be dark worlds, people dark, all dark. in some dark well most of the candidates we now have for dark matter would not create dark worlds or dark people. I think 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 I think is really exciting about these types of Dark Sector models is that the types of predictions that they make are very different from the predictions that you get for the weak, so we have a lot of experiments, this massive experimental program that has really been targeting the weak for 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 God, as if it were necessary. start to diversify and build different types of experiments so that we can really capture the full range of these possibilities, so where are things as we move into that new wild territory that will differ from the spotlight for 30 years?

Active area now, yes, it is very active, but it is not like we are going to the great 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 are certain rules about how. that can communicate with each other is a finite set of rules that allows us to list all the possibilities and then we can think of experiments that would target all of those possibilities. A very natural consequence is that the Dark Matter particle is lighter than you would expect for a weak, so give us an idea of ​​the scale when we talk about weak, the typical size relative to the proton is about 100 times larger, so When you say light you mean that now we are going down. like 10 to the power of minus 3 10 to minus 6 times the mass of the proton, considerably lighter, so five eight orders of magnitude, yeah, exactly, and then that has significant effects on how you look for it because imagine you build an experiment where you want to look for it. a particle of Dark Matter that enters and hits your target within your experiment.

If the Dark Matter particle is heavy it will come in and hit that target and you will be able to see it very easily if the Dark Matter particle is light it will come in and not knock it down and then we will never see the Dark Matter particle but we can see the kick of the atom inside them. What kind of atoms do we imagine kicking? Yes, so, well, a lot. Of the experiments use xenon, there are some experiments that use argon, where are these experiments? We put them deep underground, usually under mountains or in mines, because the signals are very weak, you're looking for these atoms, they only give you a slight movement after these collisions, so you have to Make sure you are protected from any other type of fund that can simulate that type of signal and it is in this scenario that at least signals have been reported in the literature.

Don't know. I think what you really think is that whichever of them is right is that the general consensus, yes, that's right, the current state of things right now is that there are very strong limits on these experiments that exclude any signal affirmed by other experiments, for The consensus at this time is that these other experiments have not been reproduced, although there are ongoing efforts to try to do so. For those who have claimed positive signs, do you still support your previous result? Do you also agree that they are inconsistent with other experiments and probably not correct?

No, they continue to run and maintain their original results and it will really be necessary to repeat that experiment with a separate group using essentially the exact same type of setup in order to confirm it one way or another. Now, if the park is much lighter, you're about to tell us before we cut you off, sir, then if it's much lighter than what these experiments would differ and some do, then if it's much lighter and you come in and hit the core in which the core is located. It moves a lot, so it's very hard to see, but building an experiment that looks for those electron recoils is different from an experiment that looks for nuclear recoils, so there's a lot of brainstorming going on right now about how to do it. to do that and some initial efforts to get it going, I've got it, so it's a fair summary to say that there is a lot of indirect evidence for the existence of dark matter.

Searches to find her directly are inconclusive at best, but perhaps some would even say. is starting to close the window on some of the candidates favored over the last three decades, so it's a precarious situation if it's something more exotic like primordial black holes, that would be an interesting approach, but not nature's of particle physics and, furthermore, As we also notice supersymmetry, forgetting about dark matter, we have looked for it at the Large Hadron Collider and we have not seen it either, so that window is closing, so it is a Kind of a natural transition at least to think about alternative approaches that might really be outside the box of thinking that people have been in it for a long time, so Sarek, why don't we move on now to some of the things in which What have you been thinking?

I understand that it starts with trying to get a more complete understanding of the fundamentals of gravity itself and it's a good day to talk about it again. General relativity was confirmed 100 years ago, as you know today, and that was at the time the deepest understanding of the force of gravity that you are trying to achieve. From that, give us an idea of ​​where you've gone with these ideas. Yes, Einstein's game, but his theory dates back over 100 years and has been confirmed and of course it has also been very good at predicting things like gravitational. waves from black holes and have been seen now, so many predictions have emerged and been confirmed, but this has to do with gravity which is very strong, it is a particular where we see dark matter occurring, it is something where the Gravity is very weak. and this is where I think modifications could occur and our understanding just gives us a sense of that, so when you say strong you don't mean that gravity is intrinsically strong, you mean there's more to it, right?

Or so, the acceleration is strong in this place. where a lot of matter comes together, if we look at larger scales then the accelerations are much smaller, they are more diffuse and more diffuse, but in thinking about black holes in particular, we have tried to learn more about where gravity comes from and this en A kind of development that started in the '70s with the work of Stephen Hawking and I and my colleagues have been thinking about these black holes also combining them with what we know about quantum mechanics and then we discovered that there is actually a deep relationship between gravity and entropy and thermodynamics and it's from those considerations that we start to see that there is a deeper understanding of what comes with gravity, so can we continue with that just for a few minutes just to give people any idea about that?

Can you tell us? I suppose it was a long time ago that John Wheeler posed a little riddle. Yeah, so what was that? In fact, he was the first person to ask questions about black holes. He was the one who coined the name in the first place. Yes, black. holes on 112th Street and Broadway actually yes, I know I'm not joking, it was at the Goddard Institute for Spatial Studies, it was during a talk, but there are no black holes where matter is so densely packed that light can't even escape and there is some imaginary sphere around it that if you go behind and beyond that and you can no longer escape, we call it horizon and that's why John Wheeler asked the questions about the laws of thermodynamics and also whether they would apply in that situation and why That had a thought experiment.

Where's a cup of tea? So this is a cup of tea and in the tea there are molecules going around and there is a temperature and if you look at the motion and all the random motion of those particles there, they represent a certain amount of entropy, yeah, and you can think about the entropy as if it tells you what are all the possibilities that tea can be in and one thing you can do is apply the laws of thermodynamics and entropy always has to increase, that's why we ask This also applies when we take a black hole nearby and we throw the cup of tea into the black hole because what happens is that whatever was in the cup disappears from our sight and the entropy that is there we no longer see.

Even if the cup breaks we wouldn't know because it would enter the black hole and everything would disappear, so somehow this law of thermodynamics that theentropy has increased has yet to be applied, so we wonder where this black hole entropy comes from where is it sitting? So Stephen Hawking and also Jacob Bekenstein basically answered that question and realized that when you throw something into a black hole, the horizon gets a little bit bigger, so the black hole will eat some things and it will just get a little bit bigger. , Yeah. And in fact, then we came up with the proposal that the amount of entropy that we increase associated with the black hole is actually given by the area of ​​that horizon and that was a beautiful idea and they wrote some beautiful equations for it, so this is You need an image of the black hole and you can see the horizon there and it's so if you throw a particle into the first black hole, the second black hole is a little bit bigger and it will have a little more information and this is this.

It's been added a little bit, so here you see the other black hole that's a little bit bigger and we're going to compare, in fact, what is the amount of information there, squaring it up and that actually represents one unit of information and this. It can actually be described using these laws of thermodynamics, that is, the mass of the black hole represents a certain amount of energy and the entropy is then the area and in fact the area will increase if the energy increases and that is also the same equation we know. of thermodynamics and now the idea arises: we can really explain these thermodynamic laws by thinking about the microscopic movement of molecules so that we understand precisely what entropy is, we know what temperature means, that is, as a statistical measure of energy per particle and so we can get there. those laws, but now we want to actually derive the same laws for gravity, they actually have the same form, so the gravitational loss of black holes and indeed Einstein's equations look like the equations that complement the thermodynamic equations in that kind of remarkable statement, right?

I mean, thermodynamics was developed in the 19th century and their ways of understanding things like steam engines and things like that and you're saying there's a deep relationship between those laws that have nothing to do with gravity, right? ? It has to do with things that we see in the world around us and you are saying that there is a deep connection between those laws and the laws of gravity, that is correct and this is something that we have been discovering, say, for the last three four decades and in a particular period. In the last 10 years there has been a lot of development trying to really understand these gravitational equations that Einstein wrote down from this deeper underlying description.

It is an entropy of some principle. You could have a conversation with Albert Einstein and say all the equations you wrote on November 25,. 1915 I can give you a deeper explanation. Because where they come from, yes, and I think it's the same way that of course explained in a deeper way what Newton's equations and therefore all the theories said. in physics that we have known will eventually be surpassed and they did. Just because a noodle theory is well assumed doesn't mean the old theory is wrong, it's just explained at a deeper level, but there can be dense circumstances where the new theory works differently and that's where I think when we're dealing . with horizons and we can see these temperatures appear, so one of Hawking's predictions was, in fact, that black holes not only have entropy but also temperature and that they emit, for example, radiation and that is also a notable statement because normally when we talk about black holes we don't imagine anything coming out of them, yes, so this is in fact Hawking's discovery that black holes are not really black.

I mean, black wouldn't mean nothing can come out, but he figured that out because of this quantum. The mechanical properties of the horizon have a temperature and that means they actually radiate and possibly even evaporate, so this is all about black holes and of course you've been talking about other bad things in the universe, including the darkness. energy and what it has to do with dark matter, so my idea actually is that to understand this phenomenon of dark matter it is not enough to focus only on that, we also have to understand this component of dark energy that you talked about, I I refer to the fact that we do not understand 95% of the energy in the universe why we would focus only on one component and I think that dark energy is the first thing that we also have to understand more and better in a microscopic way, so when we spend a fraction of a second since we mentioned it but we didn't really say what it's like in 1998 this wonderful discovery that distant galaxies are moving away more and more rapidly accelerated expansion of the universe completely unexpected yes, as we see here everyone I thought that over time distant galaxies would become They would move away more and more slowly since gravity tends to bring things together, but it goes faster and faster and the explanation that emerged in the late 90s was that there is an energetic matter that uses space, it is dark, it does not. they emit light and it is giving rise to a kind of repulsive gravitational push that makes everything move away yes, so it is dark energy and then your point of view is that there is a connection between dark energy and dark matter yes, I mean the The fact that there is dark energy in the universe has a very important consequence and in fact 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 over which things move away with the same speed. speed of light and then you get the beautiful conclusion and I that acts like a horizon, that is, anything that moves faster with the speed of light we can no longer see, it's like with a black hole we can't see what what's inside, but now our own universe. will have a horizon that we will not be able to look beyond and that is actually happening in the universe that only has this dark energy, so the expansion is actually a rate is actually a constant, so what Hubble discovered, this expansion rate is can express in a clinic, yes. constant that basically tells you how big the universe is because it will tell you where the horizon is located and that horizon has many other properties like black hole horizons and this is where I make the connection between dark energy and what we're talking about so Let's listen to it, so the connection you establish is the following, namely, that the entropy that we talk 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 is actually energy In the universe there is also a temperature associated with the death horizon, so it actually also satisfies all the laws of thermodynamics, but then you can ask where is this entropy and this temperature associated with the dark energy that we have added to our universe.

I say that dark energy is what we really have to understand because that will also entail an entropy and a temperature, which is what we can calculate using the same equations that Hawking and Bekenstein found for black holes but then applied to our dark energy. universe was essentially invented by La Mettrie and then, you know, and discovered half a century later, it's a constant in our scientific equations that Einstein himself introduced, but Metra realized it was due to these little quantum effects and he even called it dark energy so a constant of nature that could be dark energy and that explains everything it's a small constant we don't know where it came from dominates the universe now it gives us the acceleration but you know there are also other constants in nature I don't think one is trying To explain the unified theory right now, maybe one day we will know what is wrong with saying that dark energy is this constant dark matter, some other problem, so this constant describes more than 70% of the energy of our universe and then we just put a constant in there, well the 5% we're talking about is ordinary matter, well that's where all the interesting things happen, that's our current theory and that's precisely what I'm okay with, so for those black holes we would have said. something similar, a black hole only has mass and well maybe it can spin, we couldn't explain its entropy if we think of a black hole that way to explain that entropy, that's something we learned from string theory, we have to add many more degrees of freedom and we actually have to explain what is happening 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 example, that we can describe everything here in the room by simply adding the temperature and that doesn't really describe what is happening in the room with all the movement of the molecules that are there, so we are missing a lot of things if we describe dark energy with just We also heard a lot of very detailed evidence from these beautiful simulations that allow us to have a video in the left and right side that is practically indistinguishable at least with the naked eye between simulation and observation, we see these beautiful explanations of the rotations. of galaxies and so on, how far can you go in this novel approach to explain these kinds of detailed features of the world without invoking, say, dark matter?

So what is needed to describe the observations is that there is additional gravity holding these galaxies together. I mean it's the additional group that we want to explain, so if there is another explanation by understanding gravity better, that there is an additional group, we can reproduce many of these results, so I see Dark Matter almost as a placeholder in a way that we can put. There's an additional issue there, but we describe that it's actually due to gravity itself, so when it comes to simulations they would work very similarly if you had an understanding of gravity that adds this extra force.

Can you talk about gravity? the swirl, yeah, I mean the Rubin beer, you know, looking at you, how would you explain that? Okay, so, in fact, I explained that I also want to understand the NSEL dark eddy and therefore dark energy for me as an entropy in it, but also when matter is In fact, it has an effect in interaction with this entropy that is in dark energy and we can probably show this in a moment, so in fact this is what you talked about before, that is, this is the expectation for these rotation curves in the vertical.

On the axis we have the speed to the right, we have the distance and then you see that the speed goes down, what you actually see is that it is actually flattened and there is an important clue as to how that happens: it happens at a time when the the acceleration falls 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 for me is an important indication that there may be a connection between what is happening here and what is causing this expansion. i.e. dark energy and I can actually explain why there would be an additional force due to dark energy.

Here is the galaxy that is spinning and we will not understand why there is an additional pool, yes, but this has to do with presence. of the dark energy that's there, so if we take the dark energy and add it here, you'll see that in this neighborhood of the galaxy itself the dark energy has been pushed out and it's trying to reach out or push back again and it's kind of an interaction between the dark energy and the matter that is there that will give the additional force that is responsible for holding the galaxies together and it's more than just images what you're saying it's mathematical analysis so there's one I'll actually give.

Tomorrow there will be a presentation here in the workshop where we will show these equations and, in fact, you will be able to calculate numerically what is the additional force that one expects and you will find that it indeed has the correct value to predict, in reality, even this flattening of the rotation curves , This is not like this. In a particular situation, of course, dark matter is responsible for many other things, yes, not only the rotation curves of galaxies, it is now used to explain the formation of structures and, as we saw well, it is important to explain what we see in the cosmic microwave background and for that, in fact, I need to develop the theory further so that we can also explain those things, but they haven't done it yet, well, that's the idea, so yes, but no It's like it's been completely solved, but what I realize is that what is needed and what makes these models work is that there is an additional component that gives you a gravitational potential extract that keeps the things together, but in all these calculations it is never essentiallet a particle be the only thing it really uses.

The additional gravitational field, so the particle nature of what is now called dark matter, is never 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 mathematics can even be It's very similar, so what do you think? They know they've been looking for these particles for a long time, they haven't found them and here's an approach that may not need them for anything convincing or yes, the first demonstrations of the dark began. The matter came with Rubén, yes, and we saw a beautiful explanation about the use of dark energy, but also even before that it was Vicki, yes, with the clusters, and it turns out that if I believe that you apply your same theory to the clusters, you do all attempts so far, you need Dark.

Matter too, otherwise one cannot fit into this new type of theory of a Dark Matter field to explain the movements of galaxies, new classes, the ghost part and the fly, otherwise, that is true, Do you agree with what Joe says? So, there are attempts at something like that. In all the main attempts, you are talking about different theories and then you come out of the one I wrote, where it would have too low a factor. I think in my case it may actually fit into the outside of these groups, the problem really appears more. In this, in the center, where there is generally a very strong concentration of what would then be dark matter and here I think there may be our explanations that are necessary, but this is certainly a region where we also have supermassive black holes.

They have formed and many other things are happening in the central part of these groups, where I think the explanation of what is observed there may even have to do with understanding those phenomena in a more fundamental way, so I agree. agreement that a group still provides A challenge certainly was the core part, the external parts. I think I'm pretty sure they can be explained the same way I explained the rotation curves of galaxies, so I agree with Erik that we haven't proven that. It's a particle and until we actually see evidence that we know something that comes from that particle, then we won't really be convinced, but I think what's so compelling about the idea of ​​dark matter is that there are these particles that are predicted.

As we discussed for totally independent reasons, we didn't talk much about the action, it's another particle that you know could be Dark Matter, it would behave for structure formation very similar to the weakling and what I find so compelling is that you take that . A simple idea, you know, take the simplest version where it's a particle and then you just use basic gravity as we understand it and it makes a wide variety of predictions, it predicts the Cosmic Microwave Background, it predicts essentially the last 13 billion years of our universe. the way from small galaxies to the scale of the entire universe and it does it in a sense quite naturally and you need a new particle, but then you can make many correct predictions and you can test them and we are testing them at Very high precision now, yes, That's why the physics community for so many decades has had that as an explanation of the paradigm, but I guess the part that still feels deeply unsatisfying is that we have yet to get it right, it's very unsatisfying.

Yes, but you know there are many possibilities, right? I mean, I really hope we're okay, you know, figuring out which one is right, but there are a lot of possibilities and we're on the cusp experimentally of being able to test those possibilities, so I think that's what makes this field so exciting, Would you say in the next five years what we expect, Nick? and if we were having this conversation a decade from now and assuming funding levels hold up, you know it's not clear, you know, but you know, a decade from now, how would this conversation be different?

I don't think we know, I mean, we might find it, you know, in the next five years. I guess my question is whether it's possible to go back ten years and we're still not sure or would it be that we've ruled out this enormous amount of possibilities and it's starting to feel like we're grasping at straws on the approach of particles to dark matter as well? So my point is that the pot, the range of possibilities is pretty big, so you know, if we still haven't found a weakling in ten years, it probably doesn't mean it's definitely not a woman, but it means it's probably a weakling that's hard to find, that's cool. song title and definitely, if we have this conversation in 10 years, I guarantee we will know a lot more about what dark matter is not, Joe, so just speculate in ten years, where do you think it will be said?

This is what will happen in 10 years. years and I know speculation is what it is let me even give you 50 years actually yes so we are going to build bigger telescopes okay we are going to feel bigger accelerators and the reason is China is getting involved yeah , they want to build the largest in the world, therefore the West will do something similar and these will be accelerators ten times more powerful than that. It is CERN that will take us to the limits of what the Dark Matter particle could be. It will have new types of gamma ray telescopes and eventually we'll build telescopes on the Moon, which is a stable platform with no atmosphere, bombarded for billions of years by dark matter particles and other things, it's also a great place to start re

searching

dark matter, as well I think that many experiments are very, very expensive, but science you know, at some point it is done and will be done, so I will discover something even if it is not what we were looking for.

Is there only Mary? Is there a point in your career in the future where you would say if you cross that threshold and we haven't found evidence of dark matter of the direct kind that we're talking about, that you would say you know it's time to change perspective, do something else or Take a completely different approach, well, I guess. We should always keep an open mind and investigate completely different approaches simultaneously, but I don't think you know, even if in my lifetime we never find the dark matter particle, I mean, that seems like a totally viable possibility to me.

I mean, because when we just ask in the most model-independent way, like let's leave out Susie, let's leave out blimps, let's leave out stocks and just ask what is the range of masses in which this Dark Matter particle It may be the most model-independent constraints that we can obtain, we only need the mass of Dark Matter to be such that we can form galaxies and the mass range is orders of magnitude, going from 10 to minus 22 electron volts until reaching black holes. , TRUE? weak, which is what we've been spending all of our time on, really most of our time really focusing on, there's a small portion in that space, so in the next five or ten years, yeah, I think you know, given the amount of effort we are making.

We've put that sliver in, we'll probably know one way or another whether that hypothesis was viable or not, even that's how science works. You start with the hypothesis, you see, you choose the place where you think it will be and then do it and then you know it says yes or no depending on what the evidence gives you, but as we get further away from that, the possibilities are so enormous and the types of experimental signals that I would look for in that range are so diverse that you know, it takes you hundreds of years to figure out we're almost lucky, yeah, so final question from CEREC, you know, it's a bit of a funny situation, so you and I are theorists. of strings, yes, how famous is a theory that has no experimental evidence to support it at all, yes, so the question we usually get asked is when do we give up on these ideas and yet, wonderfully, you are moving forward in a direction that tries to make contact with observation, but in a very unexpected way because for a long time the community has been obsessed with dark matter as a solution to the various problems that we have talked about here today, so how do you reconcile that ?

I mean, is there a point where you think we'll be gone? So long without confirming supersymmetry or finding extra dimensions or any of the other strange qualities that it's too speculative to push this kind of direction and you could just say no and we could end the evening no, no, no, I don't think The ideas we've developed there are important and will teach you a little more about what gravitation is, but I think as string theorists in the string theory community we also have to ask questions that can connect to the observations and I think cosmology is also one of those areas, I mean, right now we're working with a cosmological paradigm that uses dark matter and so on and then relies on general relativity, but I hope that we can develop string theory to the point where we have a Really this picture more microscopic in terms of a theory that combines it with quantum mechanics and then we can also understand the universe with dark energy in it and I am convinced that once we have this theoretical framework we will also be able to address questions. that have to do with observations up to that point, I think we should look for dark matter and do all these experiments because sometimes you have to look and try all the possibilities before you are convinced that there may be another way to look at this, so I think you should work on both sides why we look at observations, actually better observations, even of the associated phenomena, that is, the expansion history of the universe, what is happening in these galaxies and clusters, getting better precision data that will help, I want say, I hope even that there will be time when we can develop the theory, our string theory, guided, yes, by observations and we return to all our time when they go hand in hand and we all get along, no, but, at first glance .

I've certainly spent the last 10 years a lot of my time reading about what people have been looking at in cosmology and so on, yes, because it can also give me ideas about what direction to look for and I think that our universe is not the kind of universe that the string theories are studying now, I mean, they are more interested in a very idealized model without dark energy and that is completely super symmetrical and so on, but that doesn't seem to describe our world, but if you wanted to understand what it looks like in our world I think we also have to consider the data very diplomatically, so thank you, it's been a fascinating conversation and I hope we resolve this in 50 years, but who knows, join me in saying thank you. the group here thanks