 Hello, Professor Attish. Can you hear me? I cannot hear you, but I can somehow I cannot hear you. Sorry. Hi, welcome. All right. Okay. Welcome, Adnan. Thank you. Thank you for coming. Maybe. I'm sorry about the delay, technical delay, but it's okay. Yeah. We got accustomed to it. We got accustomed to it. Okay. Please stop. Okay. Let's get started. Okay. Hello, everybody. So on behalf of the ICTP and its scientific community, it's my great pleasure today to welcome you to this first lecture of the Salam Distinguished Lecture Series of 2021. So starting from today, 27th of January, we will be hosting three days of exciting events focusing on the theme of cosmology, featuring the annual Salam Distinguished Lectures and then ending on Friday with the celebration of 2019 Dirac Medal winners. And they are also all cosmologies. So this is really a week of cosmology. And as you know, cosmology has really exploded in the past many years. And today's speaker as well as the Dirac Medalists are among the leaders who have sort of made this major scientific development possible. So it's actually a great pleasure to have Professor David Spargel with us. Normally, this is a very festive occasion and we mingle together and after the talk, there are a lot of discussions. But unfortunately, we have to make do with a Zoom seminar. So it's going to be a virtual format. And it will also be live streamed on the ICTP's YouTube channel. So this year's Salam Distinguished Lecture is Professor David Spargel, who is the director of the Center for Computational Astrophysics at the Flatiron Institute in New York. And his lectures will be on topics ranging from the study of the universe's initial conditions, all the way to the use of machine learning techniques for modeling the universe. This is an annual series as some of you know by renowned scientists with the view that not only to showcase the important research developments, but also to provide a kind of a vision for the future. And I personally am looking forward to the lectures from David to get a view of all the exciting things that are happening in cosmology. So let me just say a few words about KFAS and then I will come back to the introduction to the speaker today. So ICTP and the Kuwait Foundation for Advancement of Sciences, we renewed our agreement in 2018 to provide support for scientists from Kuwait and the Arab world to participate in ICTP activities. In fact, this was extended until 2024, which means that KFAS will continue to support ICTP postdoctoral fellowships, various scientists coming here, and activities such as this Salam Distinguished Lecture series and many other conferences. And in fact, this year, the three diploma students at ICTP Belawi from Algeria, Husuna from Tunisia, and Hussein from Egypt are sponsored by KFAS. And I'm delighted to now introduce you the Director General of KFAS, Dr. Adnan Shiba Beldin. He's a very good friend of ICTP for many years and a staunch supporter of ICTP's activities. He's the Director General and himself, a physicist, a nuclear scientist with PhD from University of Berkeley. And he has served in leadership position in many international organizations including OPEC, IAEA, and UNESCO. He's also the recipient of prestigious awards like the Chevalier de la Légion d'Honneur from the President of France and the gold medal of the presidency of the Italian Republic just to name a few. So I would request Adnan to say a few words. He's joining us from Kuwait. Thank you very much, Director Adish Davulkar. I call you Adish as a good friend. I want to start by also welcoming Professor David Speudel and thanking him for accepting to give his lectures at the KFAS sponsor distinguished Salam lecture series. Good evening to all participants at this lecture tonight from all over the world. Normally they will be from Trieste and ICTP where I would love to be but hopefully next year I will be. And as I start my very few remarks I would just say how delighted and honored really Adish I am to be with you. And it's encouraging to see that despite all of the challenges that we are facing due to these unprecedented times as we all know. We're still able to capitalize on modern technology advances to meet and to have an in-depth discussion of scientific pertinent topics such as the topic of tonight about the cosmos. And it's important in those times to celebrate science, especially in the last in the light of the past year. When science has taken the front seat, not only to mitigate the impact of pandemic but also at the forefront of potentially eradicating and any complicated disease like this. And therefore we are here to discuss the cosmos, but we are also to celebrate in essence the importance of scientific research, basic scientific research in driving knowledge to further advance our societies. And the global community as well as to understand how the universe works. I would like to take this opportunity to commend you, Adish, and your colleagues for continuing this annual series that gives us perspectives. And I look forward to listening to the insights and perspective of Professor David Spergel. If you allow me, I'm not going to repeat what you said about the partnership between CAFAS, Long-Standing Partnership. You have said it eloquently, but I just say that this partnership and so far as I am concerned started in 1976 before most of you were born. I would say that with confidence. And I remember those days in the summer of August 1975 or 1976 when I visited Trieste and met the distinguished Professor Salam. And we struck a friendship of scientists, young like me and senior like Salam. And he wouldn't hesitate to call me from Kuwait Airport saying, Adnan, I am at the airport with you send somebody to pick me up. I'd like to come and give a talk. And you know his talks in Kuwait, in Kuwait Institute for Scientific Research at that time or CAFAS or the university captured audiences, not only the scientific community but captured policymakers. He gave a lecture in 1990, after 1990, after the liberation of Kuwait at the Tuas, the Third World Academy of Science International Conference that was organized in Kuwait. And he was seated because he is a Nobel laureate and such a distinguished scientist, he was seated next to the late Emir of Kuwait, Sheikh Jabir, who started CAFAS. And the late Emir started CAFAS with partnership with the private sector because CAFAS is funded, you did not mention this and I could mention it, CAFAS is funded exclusively by the private sector. And we are very, very pleased that the private sector listened to Professor Salam when he came to Kuwait about the importance of basic science and the importance of setting funding with contribution from the private sector. So I am really very happy to share with you this thing. Now, obviously things don't go smoothly in science and things don't go smoothly in life and we can see vivid account of that in the pandemic that is touching us. But it's also the celebration of the advocacy of Salam and his foresight that created momentum in developing countries to set up and establish basic science as well as applied science. He had a dream of having similar ICTPs or satellite ICTPs in the Middle East. But we have many more than just simply satellites of ICTP now. We have top-notch universities in some part of the Middle East. And I think the vision of leaders like Abdul Salam and he, I am happy also for CAFAS that has been able over the years to support scientists from the Middle East and from developing countries so that we can continue the legacy. So I am really in concluding my remark, but I want to say that across all fields of knowledge and their application, basic science is behind it, behind the productive as well as the service sectors of the modern economy from energy, environment, transportation, health and food. Without basic science, what we consider as applied or technology transfer and deployment would not really be possible. Since it's very essential that we reflect the lessons learned from Dr. Salam, the late Dr. Salam, and I pledge that we will work with ICTP to ensure that ICTP will continue to flourish. And we, you can consider us as your partners and your endeavor to increase the partnerships throughout the world, but in particular in our part of the world. Thank you very much and I wish Dr. Spirgill and everybody a very successful lecture series and I'm very anxious to listen to the lecture tonight. Thank you, Adish. Thank you very much. So on behalf of ICTP, I would, and we are really grateful to the support of CAFAS and I hope that I think you're very right that the importance of fundamental science perhaps needs to be underlined even more into this world. And we hope that we continue this partnership and perhaps find new and we have actually many exciting plans in which CAFAS can play a role. So now let me say very quickly a few words about today's speaker, David Spirgill. As I mentioned, he is the director of the Center for Computational Astrophysics at Flatiron in New York and his professor emeritus at Princeton University. And he has been one of the leaders in microwave background observations, in particular the results of the WMAP satellite which has played a very significant role in establishing the standard model of cosmology. He has also made important contributions to the study of dark matter, formation of galaxies, and search for exoplanets. And currently he is the one of the leaders of the Simons Observatory, which will bring this microwave observations to a new level of precision. So it will open new vistas and of the W first space telescope. He is the recipient of in a very distinguished career of many prizes, including the 2018 breakthrough prize in fundamental physics, the Heinemann Prize, the Shaw Prize. He's a membership of the National Academy of Sciences and so on. It's a long list, but I would also say that he has, he's been a supervisor of an impressive number of students and a role model for the new generation. One note I can say that when I was a graduate student in the physics department, he was a kind of a young star assistant professor in the astrophysics department at Princeton. And is it so happened that my first paper was not in string theory, which was my thesis, but in cosmology with his first student who was John Quashnak. So I think it's kind of from, I would like to just add that my personal observation that I think he's one of the very few people who are equally at home in pen and pencil theory and numerical calculations and observations and talking to experiments. I think this is a very unique ability that he has to sort of be able to talk cross talk to different tribes and sort of drive the these collaborations and that is reflected in the achievements throughout his career. So without further ado, I request David to proceed with his lecture. So thank you for that very nice introduction. I want to begin with actually regrets I mean I regret that I'm not in Triesta. Mostly for myself I was looking forward to spending the week there and meeting the students and meeting the spending time with the faculty and it's a delightful place to be. It's a pleasure and an honor to be giving these lectures together with the time and the awarding of the direct prize to leaders in cosmology people whose work shape the field and have shaped my own research. And I hope this will be an enjoyable week, but like many things in the 20 in 2020 and unfortunately 2021 it seems to be an extension of 2020. We do the best we can in the environment we live under my own personal good news is soon after the talk on Friday. I'm scheduled for my first shot. So that's good at that is the moment. Besides as an American on January 20 with 2021 truly begins and look forward to when all of us have the are on the other side of 2020. But now let me turn to cosmology and it's been a wonderful time to be a cosmologist. Many of the ideas that have shaped the field and certainly the observations that have shaped the field have all happened since I was a graduate student and are we have cosmology has gone from a field of intense exciting speculation to one where we have a well established now standard model that fits the host of data. And the goal of this lecture today is to talk primarily about the observations that shape that. Tomorrow I'm going to talk more about the theory talk, both discussed the standard theory but also stress how we can use both the current and upcoming observations to learn more about fundamental physics. And in the final lecture, I want to turn to the application of machine learning as a new tool that's revolutionizing many fields of physics and many fields of study of data, and talk about how we may be able to use in the coming years to get an even deeper understanding of the universe. Let me begin in a sense with the answer what is a merge from cosmology over this time and I think what is a merge is that we have a, in many ways a very simple model. A strange model that fits our astronomical data. And what are the elements in that simple model. Well, the laws of physics are valid throughout space and time what we learned the laboratory at my cosmologically. We live in a universe whose total energy is at the zero the geometry of the universe is flat as is predicted by the inflationary model in which the universe went through a period of early acceleration and one of the predictions of this and what an area where the prize and direct prize winners played a central role is this period of acceleration produces a newly scale invariant spectrum of fluctuations Gaussian random day fluctuations I'll talk a bit about today and return more to tomorrow or observations have support and confirm this theory. And these fluctuations evolve. In ways in which receipts and I have been that is. We believe that one of the first winners of the direct prize El Dovich developed in the 1970s. And that basic pictures then confirmed by our observations. And it's a model in which by basic numbers, the age of the universe, the density of atoms, the density of matter. How lumpy the universe is and how that lumpiness varies with scale the latter two to be more quantitative is the amplitude of the power spectrum and scale dependence. Those five numbers describe all of our astronomical data. And we'll talk a little bit later about some of the potential tensions in the model, but it's been overall remarkably successful. It's been, you know, incredible to be to see the data improve and improve and improve and the model continuing to fit. So we talked today about this improvement is we went from the early data is from Kobe to WBAP now to plank and moving forward to data from the atomic cosmology telescope and towards the Simon observatory and towards the future. The data continues to improve the model continues to work. And now the strange. The strange part. And we've heard this often enough, so that were some of us have become accustomed to it but some point to remember how strange this is. Atoms the stuff we know and understand make up only about 5% of the universe. Most of the matter in the universe is in the form of dark matter. We don't understand who's properties we don't understand. We have some constraints on it, potential candidates vary in mass from 10 to the minus 22 easy axioms to 10 to the six solar mass black holes. I think that's a pretty big range of ignorance that we don't know what makes up most of the matter in the universe. We don't have an idea what the particle mass is to an enormous load of magnitude. Right, so I think for the students out there. That's a pretty fundamental problem to solve. As a cosmologist, one of the most important things I can tell particle physicists is what we know is it's not ordinary that it requires physics beyond the standard model. Perhaps the most important piece of physics beyond the standard models we know that the universe is not even dominated by matter that most of the universe today is dominated by dark energy. It seems to be a very close to that we're not certain of this, a vacuum energy and energy associated with empty space. And while the fundamental scale for this is the Planck scale, the energy we observe is down at about 10 to the minus 40 feet scale. So again, there's these enormous numbers we don't understand. So we have a month. So this is sort of the conclusion of the whole talk in the sense is we have this simple strange model that works. Well, how do we get there. I want to, before going on just to congratulate again, the prize winners, it's a terrific group of people I've admired through my career. And it's a pleasure and an honor to have my lecture linked to these awards and looking forward to their presentations and talks on Friday. I'm going to begin with the basics and just for some of you, you're familiar with this but this is a broad audience so let me start with special relativity. The key idea in cosmology is when we look out in space, we look back in time. When we look out to the nearest stars. We see them. Those are a few light years of four light years away as they were four years ago we see the Andromeda galaxy as it was a million years ago. When we look out to the microwave background we see the universe as it was about 13.8 billion years ago. The data I'll be showing, I'm sorry, like, is primarily observations of the universe's baby picture, its physical conditions, only about 380,000 years after the big day. The second ingredient beyond special relativity is general relativity. Matter tells space how to curve. Johnny Wheeler, my academic great grandfather taught me the curvature space health matter. And it's now been 100 years since the great confirmation of general relativity, the first measurements of during the solar eclipse of the curvature of space. You know, the notion that I love this New York Times headline stars not where they were are calculated to be but nobody need worry. It's like to say, this is an example of fake news by the New York Times, because of course the important point is the stars were exactly where Einstein calculated them to be. And this was the beginning of general relativity theory that has now been remarkably beautifully tested with the results from the Lego experiment. When we apply general relativity on the cosmological scale, we get a picture, which is our expanding universe model. It's often called the big bang model and I think that's really the wrong name for it. I think it's better to think of it as an expanding universe model. That's the part we don't understand the initial singularity is where we will really become ignorant where our knowledge of physics breaks. And the picture one ought to have and when I think of is, we, I suppressed one dimension I think of us living on the surface of a sphere, and I think of the universe expanding out with the radius being time. What everyone often gets when you talk about cosmology is what is the universe expanding into, and I like to think of the universe as expanding into the future, that it is growing in the time direction. And one of the remarkable things is it is expanding as an accelerating, and this red rate of growth is growing with time. Of course we can run that picture backwards as we run the picture backwards, as we go back in time, the universe gets denser and denser as it shrinks harder and harder. Eventually we approach towards the initial singularity, and you'll notice that is not a singularity in at a position in space, the big bang didn't happen somewhere. It happened at a moment in time, and it happened in a moment in time everywhere. And I think that's very clear when you think about that picture. What does that give us for a picture. So now as we look out as cosmologists as we look out in space we look back in time. We're looking back to this early universe. As we go further and back it gets harder and harder. We're looking back and this is picture shows we now running time forward we were seeing when we look at the microwave background the observational signature of the end of inflation. We're seeing a microwave background whose photons were produced about a month after the big bang that interacted with radiation and matter over the subsequent 300,000 years. And most of what we see in the microwave background represents light that last interacted with electrons over 13 million years ago, and have been traveling to us since. And the fact that lets us look back so much and is this fossil light, let's us study the properties, both of the early universe what happened in the first moments, and to learn a lot about its subsequent evolution. And then this picture just reminds you what we're looking back in time. We're looking back to a period when the universe was on us. And that reminds us that as the universe expands the density of matter drops as does the density of radiation. Right before the ionized universe makes a transition to becoming neutral, the universe underwent a transition from being radiation dominated to being matter dominated. So it needs to become dark energy dominated. And we look at the microwave background, we're looking at physics back in this time we're undergoing this transition. And we'll talk more about the properties and fluctuations and how they evolve. In the second lecture, but for now I wanted us to think about the fact that we're observing the universe at this primarily at this moment, a period of time where it made the transition from being ionized to being neutral. So I'll begin by talking about some of the older results and then show how things are built on this from observations with Wilkinson microwave and esotropy code. And this shows a picture of the satellite, and my late colleague Dave Wilkinson, one of the pioneers in the field from his earliest gains he was involved in the first discovery of the microwave background. He was part of a team working with Bob Dickney as Hedzius and Wilson discovered the microwave background, just down the road at the laboratories there was a parallel experiment that Wilkinson was part of David then played an important role in many of the early experiments in the field, then was one of the founders of the Kobe mission, and then played an important role in starting what was then the microwave and esotropy code which after Dave passed, we named after him as W map. So W map measure the sky and this map shows the sky around us, projected onto a sphere. And the thing you notice first is the big bright red thing. That's our galaxy, our galaxy's fascinating but it's not the subject of this talk, but it does show one of the challenges we face as cosmologists observing the microwave background. We need to separate the contribution from the galaxy from the contribution from the microwave sky. This is at times caused us trouble some of you may remember the claims from the bicep experiment of observations of gravitational waves, which turned out to be dust from the galaxy. And that's something we need to be careful about and one of the things that we've done with W map, and plank that's even greater extent is one observe the sky at many many frequencies, and by doing that you can see the sky at different frequencies, the galaxy looks different for the microwave background, and we can use that to clean out the sky and get a global baby picture, and this is what the microwave style looks like. And it is as far as we can tell, well characterized by Gaussian random field something I'll talk more about that tomorrow. You know, if you can stare at something long enough, you do find patterns this is one of my favorite patterns there's the letters as eight that appear in the microwave sky. I think this mostly tells us that our eyes are very good about forming finding patterns, and we need to be careful about how we apply statistics. As an aside, I would be very wary of claims coming from even distinguished mathematicians like Roger Penrose of unusual patterns, because you can't just find things by I need to do statistics. I don't think this is a big message left on the universe as a way of praising Stephen Hawking's contributions. It was a great physicist but I don't think his initials are rich on the one on the microwave sky across the scale of the universe. What are we seeing when we look at the microwave sky. We're primarily seeing the signature of sound waves. We start out with regions that are dense, those dense regions we have regions of excess pressure, I'll create one right now here's a pressure over density generates a sound wave that propagates out. And you can see that sound wave propagating out. And what's left behind in this picture is the dark matter. So we actually get to see the combination of the dark matter in the battery on to the microwave sky. So if we look at the observation we're actually seeing a combination of many different ways down to random field, or responding that way we have an overlap of waves. We can actually go back to the underlying physics by taking on that microwave background, or the hot and cold spots and stacking them on the hot and cold sport. This is a technique that to come out soon one of my former students really led development where you take the hot and cold spot stack the temperature fluctuations around the stack of polarization fluctuations around this is an analysis done with the client data here. And you can see in the figure above the pattern of cold spot defy with a hot ring around it on the scale of a degree that hot ring that scale of degree is set by how far a sound wave can travel. That's actually a very nice ruler, because we know exactly how far that is nature is holding up a ruler to us. This is actually how we measured the geometry of space. This is a picture has this ruler held up to us and we can see how big that is. It also since we have a ruler there that also lets us measure the age of the universe. Because we know how big that it is in physical length. We measure the angle, and it's just geometry that then gives us the distance that the photons travel. And this is what gives us the age of the universe with an accuracy of a percent. And looking at the amplitude of that sound wave and the damping, the soap damping signature from the diffusion, we can measure the barrier density from that. And the beautiful agreement between the upper and lower panel so the beautiful agreement between theory and observation will show that in terms of power spectrum pot later but you can really see it in these hot and cold spots. Here's the Planck satellite that we drew that data from. And as I mentioned, while WMAP looked at five frequencies, Planck looked at nine. All the way up to these higher frequencies where the signal is dominated by dust. And this combination of multiple frequency data is what's essential for drawing inferences about the microwave sky. As a kind of buyer's warning, if anyone tells you anything about gravitational waves, again, based on a single frequency, don't trust them. You really need multi frequency data to make important inferences. And this is a higher resolution version of the baby picture I showed earlier. Here's the Planck baby picture. And you'll notice that Planck have higher sensitivity. So things are higher resolution. But we've got as we'll discuss beautiful agreement between the data. This is now representing the data in terms of a power spectrum. So we're looking at the amplitude of the signal as a function of scale. The black points are data from WMAP. The black points, the circles are data from Planck. And then I'm also showing on this plot data taken from two of the current leading ground based experiments. And this is the older data, but we'll see some newer data soon from these experiments, looking at data from the Atacama cosmology telescope in Chile and the South Pole telescope, not surprisingly located at the South Pole. And the first takeaway from this is the remarkable agreement between all these different experiments. These are using different technologies at different locations, independent teams. And we're measuring signal whose amplitude, you notice the units on the graph, are mill, are micro Kelvin. We are measuring temperature fluctuations at the levels of sensitivities of parts in million. And we're measuring them across the whole sky. And the independent experiments are seeing the same signal. That's a great experimental from my, it's really good. And the other triumph is the curve that goes through this. And this curve, which is describing the behavior of those, you know, what the ingredients in that curve were basically two fold ingredients that we talked about. We start with Gaussian random field fluctuations that are nearly scale and vary the physics that Stara Binsky and Mokhanov taught us about, you know, in the 70s. In the 70s. And the theory that Rashid and Sonia and Zoldovic and peoples and others developed in how those sound waves respond to the fluctuations. We combine those two, and that produces the theory. And that theory is a beautiful fit to the state. In fact, I think this spot and some of the others will show later tremendous crime. One of the things that we as physicists and then, you know, like many great accomplishments and as many parents failures usually only have one success as many. This has been a tremendous thing to be part of. So we'll look at a number of other ongoing experiments just give you a sample of what's going on with right now in this space. This shows the experiments as a function of sky area and as a function of resolution. So we can see a W map and plank covering the whole sky. W map with a resolution at of about 12, you know, 12 arc minutes, Planck's resolution more than a factor to better its higher frequencies. On the ground experiments like act and SPT it's even even higher resolution, but not covering as much sky. And on the ground experiments like bicep cat, which are looking at gravitational waves things we'll talk about tomorrow more on not having the same resolution as these others, but having a tremendous sensitivity the other dimension in this spot. And we'll, we'll look at some of the results from these experiments over the next couple days. Turning to results in the outer karma cosmology telescope when I'm part of this is an experiment based in Chile, and really a grown to be a real international collaboration with collaborators from Chile and South Africa, the United States, Canada, Britain, France, many parts of Europe, Japan, Taiwan, and reflecting I think they're really international nature and how we besides today. We're in the desert here at, you know, a height of about 17,000 feet over around over 5000 meters. This is one of the coldest and one of the driest places in the world so a tremendous place to operate. And here's the experiment and most importantly the team of people who have been working to make this happen. Right now the team is led by my colleague Suzanne Stads, it's my arrows not showing up. She's over in the right corner. What have we seen, and this just stresses this point I've tried to make already about the remarkable agreement between the different experiments. Here's what plant sees. Here's what axes. You can see it's higher resolution that higher sensitivity now we're able to see some of these point sources, but the overall picture agrees remarkably well and observation. And one of the higher level takeaways is on the observational side it says that how beautifully the different experiments agree, and we're measuring not only temperature but also polarization. And we're looking at the pattern of polarization that we see on sky. What we do when we analyze our polarization is we split the polarization pattern into those that are symmetric under mirror reflection those we call emotes. And notice if I took that he likes signature on the right and reflected in the mirror, it looks the same, and those are anti symmetric under mirror reflection those are be modes. Any tensor field like this can be decomposed into in the modes densely fluctuations produce only emotes. We need either gravity gravitational waves which will produce be modes and that's one of the things we'll talk about. Or as the photons propagate from the surface of last scatter to us emotes can get distorted to meet the modes and those gravitational lensing induced emotes are something that will see soon in the data. And polarization maps, and here are running running a little late saw skipped on it delving too deeply into this, but we now have high resolution temperature, temperature polarization and polarization. And this attempts to summarize the current state of the field, where we're showing here the current data showing temperature temperature polarization polarization emote and be mode signals on the bottom temperature polarization. And again, the, the two big takeaways I want you to take from this picture is the remarkable agreement between these different experts, you can see five independent experiments here. We're no longer showing double the W map data but as you've seen that agrees with plank nicely on scales where W map and plank has relevant sensitivity. As we push to these smaller scales were now complemented by the ground based results. And you can see the remarkable agreement between the experiments between experiment and and theory. And because of this we can use this kind of data to make pretty accurate inferences about the basic properties of the universe. And overall this consistent cosmology, what that fits that might play background data is also broadly consistent with the supernova data, large scale structure data gravitational lensing data, as other ways of getting at ages and big value processes. In the last moments of my talk, what I'd like to discuss is other ways in which we use the microwave background. We've talked about it primarily is coming from the surface of last scatter, but the microwave background propagates from the surface of last scatter to us. And as it propagates it interacts with matter and electrons along the lens site. It interacts with electrons. It gets its spectrum can get distorted by gas pressure. So our measurement that we call the thermals in the also govich effect measures the integrated gas pressure kinematics and so on which that gives us the integrated gas momentum. And as we measure gravitational lensing, we can measure the integrated matter distribution. So not only can we get from our microwave background and a picture of what the universe looked like in the past. We can also get these maps of the integrated distribution of matter pressure and momentum and we'll look at those maps shortly. And this to me is what is now the cutting edge observationally revealed one of the cutting edges is characterizing and measuring this with precision. Let's talk quickly about measuring the integrated mass as light propagates from the surface of last scattered to us its gravitational lens. Just as we talked about the beginning of the talk with gravitational lensing occurring as light travels from a star, the same deflection that Einstein predicted and was served observed in 1919. We're observing that same effect as the microwave background photons travel and are distorted is that cast in galaxies and large scale structure, and we can infer the integrated distribution of mass that way. And this shows a patch of the sky observed by act, where we've inferred the distribution of mass from the gravitational lensing of the microwave background. And the cold regions of blue regions here are low density regions. The orange to white regions are high density regions. And we're looking at density fluctuations in this spot on the scale of tens of megabytes looking at fluctuations in density of on the scales of about 10 to the 15 solar masses so really looking at the large scale structure of the universe and mass. And this picture is going to flip back and forth between observations of a matter distribution inferred from gravitational lensing of the act data and observations of the infrared light. So this is infrared light from galaxies from dusty galaxies. This is plants cry frequency measurements, when we take the plank data and use it not to study the microwave background but to study the best galaxies and trace the large scale distribution of stars. And you can see on the largest scales of the universe the integrated distribution of starlight and the integrated distribution of matter line up. So this gives us a way of characterizing that I'm showing pictures but obviously we can do a lot more quantitatively to understand the relationship between matter and light and and learn about galaxy formation and that's a whole frontier that's of active work. We can also measure the integrated pressure the, you know, direct 1985 prize slash 2019 prize event. And this effect distorts the spectrum. And let's let's measure the line of sight pressure. And this shows how this depends on frequency, and you can see how a cluster of galaxies at low frequency scatter looks like a cold spot, a cluster of galaxies the same type of cluster that is in the eye and his colleagues are observing the x-ray. We observe as here as a shadow that it casts against the microwave sky as the hot gas scatters it up and energy and these are observations from playing showing a cluster of a number of different frequencies. And these are our recent results that came out in September 2020 from the act experiment that shows a sample now of 4,000 clusters that we have detected as shadows against the microwave sky covering 13,000 degrees and this shows the sample of clusters from acting also the South Pole telescope, the PSE survey, and you can see we're now seeing over with act about a third of the sky, all of the clusters in the universe, whose masses exceed about three times 10 to the 14 solar masses. The reason why we're looking at act about this sample is because it's measuring this effect through looking at a shadow, it's luminosity does not depend on distance. We can detect a cluster regardless of its register, regardless of its distance from us. So this is pretty much a mass selected sample of clusters, a very powerful tool for studying both cosmology and galaxy formation and evolution. We can characterize what's going around a cluster by detecting kinematics in the answer of its effect. These are some results that we just put out with a series of papers from the acting showing the distribution of electrons around clusters and comparing it to the distribution of dark matter. And what we're finding is the electron distribution and the dark matter distribution are different. When we combine all this together, we're now really able to look around the galaxy, not only see the starlight and this is observations from Hubble showing the starlighter distribution around the typical galaxy. But statistically at least in stacking them, we can look at the average distribution of dust distribution of gas pressure distribution of gas density, we can trace what's going on on large scales. We can come with the electrons and protons around galaxies and see how feedback affects galaxy formation. This is something that's going to be important. I think for two related fields, for people interested in understanding how galaxies affect their environment and how they form. This is characterizing how feedback is working on the largest scales. People interested in cosmology, interested in using the distribution of matter to infer its basic properties. What has been the biggest uncertainty in these estimates is understanding the relationship between dark matter, which is the dominant form of matter which we can simulate easily. And the galaxies we actually see and how physics like star formation feedback and jets from massive black holes and you know they affect their environment, how that changes our observations is constrained tremendously by these observations. So that combination is a new way in which we're using the microwave background data as a tool to learn both about cosmology and galaxy formation. So today what I want to do is really talk about things at the level of the maps and give you a sense of how the maps are revealing the universe, how the microwave background and temperature and polarization shows the universe's baby picture revealing the universe that's both simple and strange, but also how we can use how the maps are affected by the intervening universe, and how these observations let us infer the projection distribution of mass momentum and pressure. And I think the, we've learned a lot from the data we've taken from Kobe and W map and plank in the ground based experiments today. I want to leave you with a sense and we'll talk more about this in the upcoming lectures that this is a field that will continue to evolve in advance we have new observations at higher precision that at a minimum will improve our ability to characterize the numbers we measure already. And we hope this will be one of the things for tomorrow will be able to reveal new systems. So let me stop there and thank you for your attention. Okay, thank you very much David for a wonderful talk. We have now time for a few questions. Let's see. I suggest that you write them in the Q&A or in the chat some of you have already written. Okay, one question is about primordial black holes. What are the sources of dark matter? What are the possible this is from unique Sen Gupta. What, what can be the possible sources for its formation and can dark matter halos result in generation of gravitational waves. Well, first let me start with the simplest case dark matter halos if they're smooth. The dark matter halos collide will not be a source of significant gravitational waves, even though they're massive their velocities are well. So the one way by the way to think about gravitational waves is that the amplitude of gravitational waves depends on the third derivative the moment of inertia squared. And that's because V squared V over C to the sixth hour. So it's the velocities and cosmology are typically about 1000 the speed of light. So the motions of galaxies themselves do not generate significant gravitational waves. The gravitational waves you see are going to be generated. Well, one of two ways primarily either in the early universe, when you have the velocities close to the speed of light during periods of phase transition or or during inflation itself when they're sourced, or this is what we seem to date is when you have compact objects that are moving around each other emerging very quickly. So the gravitational waves we have seen are from the collisions of stellar mass black holes that are a product of stellar evolution or the collisions of black of neutron stars. Okay, the next question is from Jorge Cervantes Cota. What would be the most sensible measurement to understand if dark matter is cold or warm. I think looking at the distribution of dark matter in dwarf galaxies. So a galaxy like our own is a massive galaxy is a massive water 10 to 12 solar masses. It has many satellites galaxies falling into it, whose masses are sometimes 10 to the five times smaller. In other galaxies, their structure is much more sensitive to the nature of dark matter. If the dark matter is warm, so that it is moving more rapidly. It will not be trapped so easily in gravitation potential wells because it's warm. And that will give you a dark matter profile that is more less centrally concentrated more smooth down. If the dark matter is sufficiently hot, it will not be able to cluster in these dwarf galaxy halos. So the existence of the dwarf galaxy halos, which I think we've now been able to infer with some more precision with observations from Gaia put places constraints on the temperature of the dark matter. So we know that the dark matter not hot. If it's warm, it's nearly lukewarm. Warm enough to cluster but perhaps not warm, you know, make warm enough to cluster in dwarf galaxies is an important construct. Okay, the next question is from Matteo Marcelli, one of our colleagues at ICTP from quantitative life sciences. So, can you please comment on dark matter interaction with visible matter? How is it possible to get such spectacular agreement between theory and experiment in a system in which only 5% we see only 5% and not so little about the 95%. I understand this is a really naive question, but the purpose is to answer these questions from people from slightly different fields. It's a very good question. So this comes to what do we know about dark matter? We know that dark matter doesn't do, we know, we can characterize things by their interactions. We know dark matter interacts gravitationally with ordinary matter. And we can characterize that entirely, but the gravitational interaction is entirely by its math. So once you tell me the density of dark matter, that's all I need to know for its interactions. And the other thing we know is its interactions electromagnetically through the weak interaction, the strong interaction, and the other interaction you can measure with ordinary matter is extremely weak. And we can tell that in two ways. If the dark matter would collide with add with protons or electrons, and have a significant cross section for interaction, it would change the way the microwave background would look. And also we would have detected in our underground experiments. So if the dark matter is some new subatomic particle, it would show up in experiments. And the fact that we haven't seen it yet tells us that dark matter is very weakly interactive. People talk about whence, weakly interacting mass of particles. But really what we know about it is its interactions are small. Therefore, it's pretty easy to characterize. You just have to put it into your model as something with gravitational interactions. Now, of course, there's a whole field of study and people write theses on what if my dark matter proton or dark matter electron interaction is just big enough to change the observations. But I see it and we analyze our data and we look for those effects. And so far we've not seen evidence for any dark matter interactions with ordinary matter at the level that would show up as an issue. But we continue to look for it. There is another question from a condensed matter colleague. So regards to David, the angular momentum spectrum you showed seems to fall off like L to the minus two. Why is that? The power spectrum, yeah. The power spectrum. It actually falls off and starts to get steeper and steeper. It actually approaches more like L to the minus four. And will this work? All right. The experiment of my blackboard behind me, which sometimes works. So I've talked about the microwave background as if it was coming. Yeah, please write in the lower half of the blackboard so that I've done this before. Okay, okay. Not there, not there below below but right to be lower. I'll leave it low. Okay, good. Um, so we talk about it as coming from this spin surface of last scatter. It's not actually that big. So if we look at the trend as a function of red ship, so looking out in the space back in time. Equivalently this is time going forward here. Where the microwave background comes from, sort of the photosphere the where the photons of last interactive is not a delta function in red ship. But has a range in red ship of a delta Z over Z of about a 10. So that what you're seeing is smoothed out. You're seeing a pattern basically the scouts and random field, but it's not just at this red ship this red ship in this region, but integrated through. And that smears things out on physical scales that are smaller than that distance that thickness of the surface of the scale. Another effect that's smoothing this out is what's called silk damping that effect arises when we've got the sound waves moving out. The sound waves are waves in the electron photon fluid. They're regions that are hotter and colder regions with more photons are hotter. As we all know, photons diffuse from hot regions to cold regions that damp fluctuations on smooth scales. Below a scale that corresponds physically to about five arc minutes. The fluctuations are damped. I actually think a better way of thinking about the physics of fluctuations is rather than thinking about them in the power spectrum of those acoustic fluctuations let's have a present data. I'll go back to the picture I showed earlier of the sound waves moving out and the pattern of hotspots, surrounded by cold rings, and cold rings to surrounded by hotspots. You can think about what we see in that power spectrum as the foyer transform of that cold spot defined divided by the hot ring. In that picture. The damping at high L is just the thickness of that hot ring. The fact that that hot ring is not sharp. And the reason it's not sharp is photon diffusion that smooths out the ring. So that's the physical model I have to explain why things drop off the way to do. Okay, thank you, David. And this is one of these many moments where I wish I was in front of you on a black and footsteps work through this more in more detail. Yes, I think yes, this is this personal interactions are really what is missing. Okay, I have another question from a colleague from CISA Francesco Benini. What do observation tell us about the properties of dark energy. And I guess what he has in mind is beyond it being the vacuum energy. We often characterize this by the equation of state of the dark energy. So we, as the universe expands. We can look how the density varies with expansion factor the density of matter as you learn. The density of matter is the universe expands for matter goes at eight to the minus three. I double the size of the universe. The volume goes by factor eight the matter density goes down by factor eight matter. So that so matter goes as eight to the minus three. Radiation goes as eight minus four is the universe expands it gets colder. We characterize how the dark energy behaves by this factor w where a goes is to the minus three times one plus w where w is what we call the equation of state of the dark energy. W would equal minus one for a vacuum energy. So observations roughly tell us w is what minus one plus or minus about point oh five, depending on what combination of observations might be constraints, as weak as point one right now, as good as maybe point oh five. The next generation of experiments and there's a number of observations doing this is going to improve this by by an order of magnitude. And of course, the big question is, will this continue to be close to minus one, or will we see deviation from this, telling us that we're seeing evidence not for a vacuum energy but a dynamical field. And, but at the moment everything is consistent with a vacuum energy basically, we can also characterize the properties the dark energy another property is not just how the density of the dark energy is the universe expands. But how was how it responds to gravitational fluctuations. That's effectively the sound speed of the dark energy. Again, we don't see any evidence for deviations from vacuum energy and its behavior in terms of how it affects the observations from microwave background and large scale structure. But this is very much something that we're trying to improve our observations of a teach mentioned early on that I've been involved with actually mostly involved lately with making the political case to the Senate and House committees to restore the budget canceled by Trump in the previous administration successfully to keep the W first mission alive. We succeeded. And one of its many goals is to better characterize the properties of dark energy. Yeah, so maybe I can ask, so what are the prospects in W first to major time event to see hints of the time dependence or Well, what we've optimized W first for is to carry realize the dark energy properties of higher register. So, the European satellite Euclid will be launching first. It is observing gravitational lens and primarily at optical wavelengths. It will be telling us a great deal about what's going on with dark energy at red ship one. W first, we're observing as the eye in the name says infrared, the name by the way is evolved. We've now named it the Nancy Grace Roman telescope Nancy Grace Roman was the first kind of astrophysics of NASA. She's was a very important pioneer in the field she was the one who, you know, decided to build Kobe right she's the, you know, well, John Mather was the scientific lead, you know, Nancy Grace Roman was the one who actually decided to spend the money to build Kobe and selected it and select and, you know, led the agency as it built hubbles and was a real pioneer in space astrophysics. And so it's now the Nancy Grace Roman space telescope. So the Roman telescope is optimized in the infrared. So it will tell us about the universe at register to and more and that combination will tell us whether we see the dark energy of all the time. Okay. Okay, I think we can probably take a last question I think we have another session with the students later on. What can we learn about neutrinos from the CMV. Ah, that's a fine question and that's about 15 minutes of my talk tomorrow. Okay, very good. Okay. So, are there, let's see, let me just see in question and answer. Okay, I think I have asked most of the questions. So, okay, at this point. Actually, Joanna, this interaction with students starts at in 15 minutes, right. I would be for the students who are listening I'm actually happy. I can be there I'll be there five minutes early. So I'll, I'm going to get a glass of water. Okay, for an hour, and then we'll join with the students shortly at that number. Okay, very good. So let's thank David once again. Okay, my, okay, I will clap on behalf of everybody. Thank you very much David wonderful talk. Thank you for listening and my regrets again that I'm not with you. Yes, and we look forward to your second talk tomorrow and I will join you shortly with the students. The idea is of course to leave the students completely alone with you so that they can ask whatever comes to their mind so I will just make the introductions and disappear. Thank you. Thank you for joining us and thank you everybody for being here. And also let me thank you again for your support, your support of ICTP as an outsider. It's been a wonderful institution that's had a very important effect on physics. Yes, and KFAS has been really our very strong supporter and we really appreciate it greatly. Thank you. And you're muted this. I said thank you very much and I enjoyed it although I'm not familiar with the topic that much but thank you David, excellent talk. Okay, bye. Bye bye.