Mapping the Heavens to Understand Dark Matter and Black Holes

so welcome to the Fermilab for tonight's lecture before I introduce tonight's speaker I do want to remind you to please turn off and silence all of your electronic devices photography and recording is not allowed this will be on YouTube afterwards I think so there's no reason for you to record at anything I also wanted to let you know about some of our upcoming events this is actually a pretty busy month here at Fermilab for our for arts and lecture series in addition to tonight's lecture next Saturday we're hosting a one-woman performance entitled the other mozart which is based on the true story of Mozart's sister who is also a musical prodigy equal and talent to her much more famous brother as we encourage you to come out to that that is next Saturday on May 19 we'll have another entry in our lecture series and this one is by Jorge Cham of PhD comics and Daniel white s'en who will explain why the vast portion of our universe is still a mystery to scientists this also marks a very special year for Fermilab 2017 is the year that we turned 50 years old so we have lots of special events for that you can check our website for that but the one that I wanted to especially bring to your attention is on September 23rd when we plan to host our largest ever open house so now it's my great pleasure to introduce you to tonight's speaker dr. Priya Natarajan dr. Natarajan is a theoretical astrophysics and the Department of astronomy and physics at Yale University she is interested in cosmology gravitational lensing and black hole physics she has degrees from MIT in Cambridge in 2009 she was awarded the Guggenheim Fellowship and has been named a fellow of the Royal Astronomical Society the American Physical Society and the Explorers Club and I learned at dinner that she's also a regular contributor to the New York Review of Books her research involves mapping the detailed distribution of dark matter in the universe exploiting the light the bending of light and route to us from distant galaxies in particular she's focused on making dark matter maps of clusters of galaxies the largest known repositories of dark matter in the universe her first book which is aimed at the public isn't I mapping the heavens the radical scientific ideas that reveal the cosmos and was published in 2016 and received an honorable mention in the cosmology and astronomy section so please without further ado welcome to dr. Natarajan first of all thank you so much for coming it's a real pleasure and a privilege to have the opportunity to share with you some of the ideas the motivations for writing this book and also talk about the research work that I do so I wrote this book because I was really really perturbed at the level of scientific denialism in society today and it is my personal opinion that the one of the reasons one of the key reasons that there is rampant denialism is because the process of science is not really well understood by the curious public and so my personal take has been to try and reveal and demystify the process of science and in particular focus on two attributes of science which is what makes science exciting for a scientist for extracting scientist but that might be quite confusing for someone who is not involved in the scientific process and these two concepts are the fact that science is inherently provisional so our knowledge is of science of any phenomenon is of best to date and it is circumscribed by the quality of data the quality of evidence that we have if we have better data we have more evidence in support of our current understanding then we refine our understanding and we get a more sophisticated and nuanced picture of a natural phenomenon and we are continually honing our understanding and therefore as scientists we have to be nimble our minds have to be ready to change and I think this this part of the process which is the beauty of science because it keeps us on our toes it keeps us forever open minded I think that is an aspect that would be really helpful for the curious public to know so part of the motivation thank you part the motivation for writing this book was to reveal how scientific ideas in particular radical ones in cosmology the path to acceptance of these radical ideas and here I focus really on the kind of push backs and the challenges to new ideas but scientists themselves within the community have provided and this is just to humanize science to reveal that scientists and science is a human endeavor and it is laced with subjectivity and just as it is fired by our passions so often accounts of science and scientific discoveries write out the passions of individual scientists right because we're supposed to be extracting objectively these truths from nature and white lab coats or something and that is really not an appropriate description of how science is done ever and more so than ever today the way science works today and so I wanted in this book to really talk about how science works reveal this through some key ideas transformative ideas in cosmology in the last hundred years the reason for picking the last hundred years is actually quite simple right in 1914 we believed that we were unknown in the universe there were actually no other galaxies we did not know of the existence of other galaxies right and we've come a really really long way we've understood today we understand that there are billions and billions of other galaxies out there and not only do we know about other galaxies and the overall global structure of the cosmos we have a very very detailed understanding of the history of the formation of structure in our universe how the galaxies came to be to start with and we also know closer to home we also now know that there are many many planets much like our own around other nearby stars to us and therefore in an odd kind of way we are significant and yet insignificant in some in the sense that we may not be unique we may yet be unique for the particular form of intelligent life that our planet harbours but this behooves us to take much more cosmic responsibility to guard the earth and take care of our resources conserve them and try to live a sustainable life so I think what I want to show you today is give you a cosmic view and the idea is that having a cosmic view really helps us as human beings have always done to find our place in this grand scheme of things so I focus on the journey of ideas from proposal to acceptance and as a young girl growing up in Delhi so my first foray and taste of research came from maps I was obsessed with maps as a child I was obsessed with atlases I was obsessed with terrestrial maps celestial map and and I think I therefore you know anything I write had to do in my own work as you will see a little bit later also has to do with cartography of an odd kind mapping dark matter so in the book I use maps as very literal devices as well as metaphorical devices so maps in some sense codify our state of knowledge what we mapping is sort of knowing so if we look back in history when we look at the conceptions of cosmos looking at the evolution of maps we get a really good idea of how our views got refined transformed and so on so I used I used some old maps to tell some interesting stories about the evolution of our cosmic view and as I said I wanted to humanize the scientific process so I really talked about the resistance from within the scientific community for even though we are trained to be open-minded at the end of the day we are human and so I focus on the controversies that arise intellectual constant controversies but also the personal sides to the controversy a personal clash of personal ambition and and you know motivation of wanting Fame and so on amongst our interests in particular cosmologists when new ideas are proposed so I do a bunch of case studies and I look at the some of the grand ideas that have been proposed in the last hundred years so the idea of the expansion of the universe that was discovered by the astronomer Edwin Hubble the discovery of dark matter dark energy black holes exoplanet Oh in the book I have each chapter I devotes to each of these radical ideas and then I drill into the conception of this idea the press first presentation of this idea and the arc of acceptance and I think the reason why I said you know it's a bit of a rash a lot of my colleagues are writing books and you know everyone feels now that they have so much to say that they want to write a book and you know for me in particular what was really exciting about being an active scientist working in some of the really exciting problems in the frontiers of theoretical astrophysics today sort of trying to understand the nature of dark matter trying to understand the growth information of black holes is that this is a very special time and I feel particularly grateful to be able to be alive now and the reason for that is that we are at a point where there's a grand convergence there's a grand convergence in the our theoretical the sophistication of our theoretical understanding of the cosmos the kinds of instruments that we have invented in order to get evidence from the cosmos to validate and invalidate theories and the computing power that now allows us to analyze the kind of data that we can get so they are all extremely well aligned even twenty years ago we did not have the kinds of detectors we now have that span the entire range of wavelengths so we didn't have the kind of copious data even twenty years ago so the last ten years have really the Golden Age of cosmology and for me in particular it's it's really pointed because now is a time where as a young scientist in cosmology you can propose a radical new idea of your own and within your lifetime it's there to be tested either validated or invalidated so this is really a very very special time so anyway let me move on to to talk a little bit about sort of the history of our understanding of the cosmos so ever since the Neanderthals were able to look up at the night sky they probably wondered at the cosmic drama that was unfolding every day and night so one of the first depictions of the cosmos that we have is this Nebra sky disk this was excavated in the saxony-anhalt region of germany and this is a copper plate it has a depiction of the Sun the crescent moon and the play D star cluster we have no idea what this was for what they used it for and why they did it we have no idea but we know that this is the earliest sort of depiction of the night sky and then if we move on in time in 7th century BC we've discovered they'd be of uh north of tablets from one of my favorite ancient civilizations the Mesopotamians who were complete inveterate charters of the night sky so cuneiform their script is now being disappered so we know exactly what these tablets contain so this particular one is the very famous Venus tablet and it marks it charged the positions of the planet Venus this is just awesome right so these guys 7th century BC already knew the difference between a star and a planet right so this really kind of awesome so let's jump on to one of my other favorite maps from the The Late 1300 so even the Mesopotamians who were charting the night sky were not looking for an explanation the way we understand an explanation today they were not looking for causation they were not trying really to understand what was going on in the night sky what caused it they were merely trying to do well I say merely but you know it's a pretty grand idea to have had in retrospect you know one can say merely they were trying to make connections between the celestial and the terrestrial right so that's what they were trying they were trying to make connections between floods floods and rain and thunder and the night sky right so they were trying to make these correspondences so the first shift that you see in attempts to try and understand causation comes from this kind of map where you see a depiction of the earth and the celestial sphere and you see two angels here mechanically turning the crank to account for night and day and for the season so the sort of the earliest sort of depiction where we see and attempt to give some kind of explanation for what was going on in the night sky and you all oh you all know of course that at this point the cosmic view that ruled our understanding was the Ptolemaic view the Aristotelian Ptolemaic view and remember at this time the cosmos was just our solar system so it we were confined in now in terms of our understanding to just the solar system and so here of course the view was that the earth was at the center of the solar system and the planets revolved and the Sun revolved around the earth all right and of course in 1543 something fundamental happened Copernicus completely shifted came up with the most radical idea at that time shifted the pivot from the earth to the Sun and proposed that the Sun was at the center of the solar system and that the earth orbited the Sun this was extremely radical in fact he sat on this idea for a long time and it was published soon after his death it turns out that as with all radical ideas and we will see today I'll talk about two radical ideas and the arc of their acceptance I'll talk about the idea of dark matter and the idea of black holes but this fundamental shift in our cosmic wheel and all these radical ideas it's very hard for the full radical idea to be accepted so often see this phenomenon of sort of a halfway house idea so you can kind of shift your thinking just a little bit so here is Richie oles depiction of the solar system so he and Tycho Brahe he had this halfway house model where basically the earth still was at the center of the solar system but the Sun had the other planets moving around it and the Sun was still moving around the earth right it was halfway but it was too hard to go all the way so it sort of halfway and so here you see a very interesting depiction so what you see is first of all the one-eyed Argos with a telescope at this point Galileo has already repurposed the spyglass he's pointed it up at the night sky and started making observations right and so here you have urania the muse of astronomy who is now weighing these two ideas intellectually so one of course is the Copernican model and the Braja model so those are the two models and down here you see Ptolemaic cosmos that have been discarded right it's been pushed out it's no longer fashionable and so I think you know this is sort of what I mean when I say I'm using maps metaphorically I'm using these diagrammatic representations that actually show you so before you had scientific papers and peer reviews that adjudicated evidence and arguments you had these kinds of depictions that slowly persuaded and allowed us to change our conceptions of cosmos so here is a depiction of bra haze model nice animation of it where you see basically the earth is still at the center of the solar system and the Sun is moving around with a few planets that are moving around the Sun and of course the transformed view that we finally had was the Copernican view that was accepted which was a complete displacement moving the pivot from the earth to the Sun for the solar system so this this was dense and you can imagine right that in 1543 Copernicus we could not even have imagined that by mm we would have launched say two satellites Voyager 1 and 2 that would actually leave the confines of the solar system I'm just mentioning that here because to caution us that you know the path of future science can never be predicted right who could have imagined definitely cover would not have imagined that we would have sent off Voyager 1 and 2 that would actually lead the solar system anyway here we are this is where we are today and this is our current understanding of the cosmos and what we now know is that our universe started from a very hot dense state and we mark that time as the origin of time we start measuring time T equals 0 from this this instant it's referred to as the Big Bang right after the Big Bang when the universe was in a very hot dense state it was very compact as well and there was a very short but rapid exponential expansion of the universe's effort that's referred to as inflation and following inflation you had this sort of primordial soup from which basically matter condensed and then you form structure you form the first stars you formed the first galaxies but what we know about the sequence of formation of structure in our universe is that dark matter is what structures the universe it's in the driving seat and we know that we have a lot of independent many many independent lines of incontrovertible evidence that show us that this kind of picture this picture with the total age of the universe being about 13.7 billion years that's from the Big Bang to today we know the timeline quite well and incidentally we also have direct data from very very early from the infancy of the universal universe is about four hundred thousand years old this is the cosmic microwave background radiation this is the relic radiation the relic photons the light right off of the Big Bang that then have been streaming towards us that is detected today that is no longer so energetic because as be as the light has been coming to us from the Big Bang from sorry from about 400,000 years after the Big Bang it has encountered all the structure that has formed since all the galaxies and the stars and we detected today the universe in the meanwhile has expanded and not only has it expanded we now know that the expansion was an accelerated expansion quite recently and we now detect these photons in the microwave so we see this via this is a map of the cosmic microwave background radiation which has this hot and cold spots and these middle spots this sort of pock marks that you see are actually rich with information because these are the relics that are left behind with the encounters with all the galaxies and the first stars that formed since the radiation was coming toward us so this is a picture that we have of a current picture of the universe in addition to understanding the dynamical way in which the universe has formed we also know in quite exquisite detail also from the Cosmic Microwave Background and many many other independent lines of evidence the contents of the universe we know that much of the universe most of the matter in the universe is constituted by this peculiar thing peculiar particle possibly called dark matter and that the bulk of the energy density of the universe once again is dominated by this entity called dark energy and this dark energy is what we believe propels the currently measured accelerating expansion of the universe just as you need gas to run the gas pedal to accelerate your car we think that dark energy is the thing I mean it really is quite wait we don't know what it is we think that it is it corresponds to some sort of energy of space itself that is causing the accelerating expansion but that is the primary constituent of the universe and surprisingly and sobering Li the stuff that we are made of ordinary atoms everything that in the periodic table is a paltry 4% of the universe other we we know that and as I said the bulk of the matter that we infer in the universe we believe is dark matter and this dark matter is not only ubiquitous not only the most dominant component but it's also quite exotic first of all it's not the stuff on the periodic table secondly we believe that it is some kind of particle that form very early in the universe because it structures the universe we know it structures the universe it forms the scaffolding in which galaxies form the first stars form and the other thing that we know about dark matter is that at the moment the evidence as I'll show you suggests that it's a very weird particle it's a particle that moves very sluggish ly very sluggishly compared to the speed of light in the universe right it moves very very sluggishly it does not feel it interacts very weakly with itself and with anything else in the universe the only force that it really feels and exerts is gravity because it has matter and therefore because it has it has gravity it aggregates and it clumps and it clumps in a very peculiar way so unlike our common understanding of gases and other atoms where basically if you have two particles ordinary atoms that approach each other they will actually collide and they have pressure because they collide they collide with each other they collide with the contents of a container if you think of gas and so they have pressure so the other thing we believe about the properties of dark matter as inferred from data that I'm going to just show you is that it is collisionless what that means is a to dark matter particles that approach each other will simply graze by and then gravity will just bring them back and eventually they will kind of coalesce and clump and cluster however they do not have pressure so this is great this sounds pretty peculiar already so what is the evidence and you might say okay this is great but PA you know we've heard about all these invisible entities and medieval times right there was my ass which was supposed to be the fluid that caused diseases before we understood pathogens then there was phlogiston before we knew oxygen and then of course there was ether right and all of these invisible entities went away and you might say that's great so if dark matter going to go the same way a dark energy and dark matter going to go the same way and well I am hoping to persuade you that no dark matter is not going to go the same way at least not from the evidence that we have at the moment and the reason for that is that that two completely independent lines of evidence very compelling evidence that show you that you need a large amount of dark matter with the kind of peculiar properties that I just outlined and these two lines of evidence are because dark matter is matter and it exerts gravity it there's an it plays an important role in affecting the motions of stars and galaxies in its vicinity right so there's an impact on the dynamics on motions and it also the presence of dark matter is also revealed because of the impact that the presence of Dark Matter makes on the deflection of light on the propagation of light over cosmic distances in the universe the reason I demarcate these two different kinds of evidence two independent lines is that this the impact of the gravitational effect of matter and the impact on motions right with something that is predicted by Newton by Newton's theory of gravitation so from the 1600s we had this brilliant insight about how gravity works that's what Newton told us Newton told us that if you have two objects that have mass M 1 and 2 then gravity is the force of attraction between these two masses and it falls off of the square of the distance between them right however we know that Newton's conception of gravity was completely upended by Einstein by his general theory of relativity because Einstein explains to us why gravity behaves the way it does not just what it is how to calculate the force but why right and took this this prediction of light bending is made by Einstein's theory of general relativity so whether you want to whether you have a cosmic view that is still stuck at Newton or you're willing to change your mind and be open well we don't have a choice right we all carry GPS is our phones tell us where we are and without Einstein's theory of general relativity none of that would work right so we know that Einstein's theory of general relativity works and it actually predicts light deflection so I'll show you these two very very different cosmic world views that both give you the conclusion lead you to the conclusion that there is huge amounts of dark matter in the universe and how its distributed and in particular there's a class of astronomical objects where you can actually do both of these two both methods you can get a census of the dark matter using both of these methods so we'll talk about that in a minute but the discovery of dark matter was actually empirical okay this was not an esoteric mathematical idea it came out of observations and so the first mention of dark matter was by Fritz Zwicky in a paper in 1933 where he looked at this astronomical object called a cluster of galaxies he looked at a nearby cluster of galaxies which was called coma that which is called a coma cluster and a cluster of galaxies is essentially an agglomeration of about a thousand galaxies what you see here is yellow fuzzy dots are all galaxies that are at the same distance from us they are held together by gravity okay and they're stable and so what he did was that he measured the speeds of these galaxies and he found that they were whizzing around at much higher speed which would suggest that they should all be flying off unless there was more matter than we can see here that contributed to the gravity the gravity contributed just by the visible stars that are seen in these galaxies is insufficient by a factor of ten to account for the motions these galaxies that are held together stably would be flying off unless there was a lot of unseen matter here that was contributing to the gravitational pull that held the cluster together so he proposed the idea of Dunkel material and I mean back you know is a Newtonian description so it's the impact from the motions right so that argument that I just gave you in 1937 removed by this time in 1916 already Einstein had presented his theory of general relativity and people had worked out all the consequences and one prediction was the deflection of light so in 1937 he applied the argument of if there was a lot of unseen matter here then applying Einstein's theory of general relativity you would predict that there would be a lot of light deflection that should be seen now at Ricky's time in 1937 the deflections were large but we did not have the instrumentation to actually detect it right so unfortunately Ricky's proposal sort of fell by the wayside there was another reason why it fell by the wayside which is you know Ricky was an extremely creative guy but he had a few Hicks but a lot of misses in terms of ideas his crazy radical ideas right and so many proposed this idea people thought well you know this is a bit too crazy and so this one fell by the wayside sadly so one other phenomenon that I talked about in the book is sometimes radical ideas have to be discovered rediscovered really discovered before they actually make it to the realm of acceptance and that is definitely the case with dark matter so in the 1970s Vera Rubin and Kent Ford were looking at a slightly different problem they were measuring the speed of stars from the center of a galaxy outward so they were looking at the speeds at which stars were moving around in a galaxies typically spiral galaxies what they were originally studying and what they've measured so they were first of all they were unaware of Ricky's work okay so they were looking at galaxies they were not looking at clusters and what they found what is expected if the only gravity that matters in terms of accounting for the motions of the stars was the matter that you see in the stars then the speeds of the stars you expect to do something like this red curve so you kind of run out of stuff as you go to the outskirts to the edge of the galaxy and then the speeds of stars and gas that's sort of sitting outside falls however what they measured was this white curve and I'm very peculiar and to try to remind you why that's peculiar let me let me try to sort of motivate so let's look at a solar let's look closer to home let's look at the solar system if you look at the speeds of planets around the Sun so remember in the solar system the Sun is the most massive body it gravitationally dominates our solar system and therefore the force of gravity falls off as you go further out and the speeds of the planet actually fall as you go further out so here I reveal my blind spot Pluto for me is still a planet I am unable to give it up because you know it destroys my tiny childhood impression of the solar system right so there you go and so what you find here so you know this curve of this plot of the speeds versus distance is called a rotation curve and when you plot this for the solar system you see that it falls off like this and this tells you immediately that the dominant gravitational body in the solar system is sitting somewhere here in the center and as you go further out these things are wimpier in terms of their mass and so that's what this tells you right so let's go back and look at what Vera Rubin and her collaborators measured right so we now have a top view of a galaxy so what she measured once again what the speeds of stars from this Center outward and this is a kind of curve that you got so what this tells you is that the gravitationally dominant matter is dispersed everywhere in the galaxy it's distributed in a much more extended fashion and it kind of follows the light but not quite because it extends well beyond where the light is dimming so this was evidence for the existence of an invisible and of matter that is smeared that is kind of piled up in the center of a galaxy and then yet is smeared all the way to the outside okay so this is to just show you the contrast between the solar system and what is seen and this you know and this is not one-hit wonder if this is routinely seen in almost all galaxies even today so this suggests that you have a lot of invisible dark matter this was the idea that was proposed and you have a lot of invisible dark matter that is distributed piled up in the center where what you're really seeing is the tip of the iceberg in terms of the total matter distribution in a galaxy and in fact there is a dark matter that is extending well beyond where you see the light so this cartoon shows you our current conception of galaxies so let's now go to the second idea for the evidence for dark matter the idea that comes from the deflection of light so in Newton theory of gravitation at the time people believed that light was composed of particles of these particles called corpus coals so the idea was that if you had a very massive star massive object then the particles of light would get deflected in their path if they strayed nearby because of the gravitational attraction by the more massive body so the Newton and Solar computed what that deflection angle would be it turns out that Einstein's Li conceptualization of gravity that actually explains the light bending that is just off by a factor of two compared to this flawed idea of light particles and so it's kind of in it's like an interesting coincidence that you're just off by a factor of two pi is a very important factor of two because concealed in that factor of two is a completely conceptualization of gravity which is what Einstein gave us and so I Stein proposed this theory and I'll just mention to you what the basics of this theory are in a minute and interestingly this is one of the cases in physics where you have a theory that was that he came up with one of the reasons we are all such great fan the Feinstein is he came up with the theory ex nihilo there was no anomalous observation he was trying to explain this came out of a profound insight bit of our linkage between physics and geometry that that and mathematics that Einstein was able to put together and of course one of the interesting things is the dispute actually predicted light bending and it was measured in actually 1919 that's a typo there to confirm his result so what was what was predicted by Einstein we'll talk about why this actually this phenomenon happens in a minute but what he so during a solar eclipse what he predicted is when you have a solar eclipse the Sun the moon and the earth line up as we are going to have one that's going to be visible from the United States in August this year if you look at the positions of stars during this alignment the stars appear to be at a location which is different displaced from their actual position and so it's this shift that Einstein's theory predicts and that was measured so the actual positions of two stars are this these two positions and in fact you say you end up seeing this guy here and this guy here so they're actually displaced from their original position and why are they displaced they are displaced because of as I said this radical reconsolidation of gravity where Einstein postulated that one can think of the universe as a four-dimensional entity this entity called space-time and the in which the presence of matter creates potholes or divots in the space-time so the end our universe can be imagined to be this four dimensional structure and we are all anything that has mattered that's embedded we are all embedded in this universe and anything that has mass will cause a little potholes in space-time and the depth of that pothole the severity of that portal depends on how much mass there is how massive the object is that is causing the pothole as well as how compact that object is how the matter is distributed whether it's very tightly packed or if it's fluffy so here is a depiction of a vine Stein's prediction which is you have a cartoon here shown of the fabric of space/time through gridded here and you have a cluster of galaxies so we're back to galaxy clusters all of Fritz Zwicky and what you see is a divot that is made here due to the large amount of dark matter that is associated with this cluster and so when you have light from a distant galaxy that is coming towards us light rays are coming towards us remember there's nothing above space-time there's nothing below space-time so any light ray that comes to us from this galaxy will actually have will track and follow every bump and pothole in space-time and it will carry the imprint of everything that it has encountered every pothole that it has encountered much like my car the axle of my car is getting slowly damaged by the potholes in my local highway i-95 I'm sure the imprints are there already right so similarly you have these light rays that get deflected from the path because they have encountered these potholes in space-time so this is and so the question is okay so what do we actually see what do I mean that light light rays get distorted or they get deflected so what's the consequence the observable consequence is that when you see distant galaxies and you have a very very massive object that causes a deep pothole in space-time and to have that kind of matter that causes a deep divot you require dark matter so in this case you have a distant galaxy that looks like a fuzzy blue dot and you have an intervening galaxy with a huge dark matter halo rather like the ones that Vera Rubin and her collaborators found then what happens with the light deflections from the source is that the shape of this galaxy that we ultimately see it's completely bent out of shape it's full it out into an arc okay and this is a kind of dramatic effect that you see in gravity so this is a real Hubble Space Telescope image and this is sort of the work that we my research group and many other groups around the world do which is to try to use this image because we know what the undistorted shapes of galaxies are by and large because the objects these massive lenses that deflect light are very rare so when you look at a patch of sky you know what the distribution of undistorted shapes is and so when you see something like this you can work out how much matter you need in here to produce this kind of dramatic effect and so the interesting thing is that just another schematic to remind you so you know when you see these distant galaxies their shapes are distorted so in addition to distorted shapes something something else more dramatic happens occasionally when things really line up you can think of light as light rays or sort of tubes occasionally when the pothole is so deep these tubes get split and what I mean by they get split then you end up seeing multiple copies of and distant galaxies you produce multiple copies fake copies when in reality there's only one galaxy the images are real and that the reason they are real is because you see multiple copies and I'll just show you in a minute so the kinds of images the kinds of distortion that are produced are completely dictated by where this distant object is where the lens there's a huge amount of dark matter somewhere associated with the galaxy or with the cluster of galaxies what shape that distribution of dark matter is and the relative distance to us so it's very much like those optics experiments you might have done in high school where you have a lens and you have some light source or a candle you have a screen and you move things around and occasionally you'll see an inverted image you'll see a magnified image you'll see a D magnified image depending on the relative orientation and the distance between the screen the source and your lens so it's very similar to that and in fact we can infer from the kinds of distortions that we can see the way in which matter in the lens should be distributed so this is another mark that just shows you a distant galaxy now if the dark matter was distributed like a like in an egg shape then you would see particular configurations if it was like a sphere you would see particular configurations and if it was just a lot of little blobs of dark matter you would see particular configurations and these are all Hubble Space Telescope images so these are all seen so just to press the point a little bit to sort of give you a feel of the range of things that you could see it's a little animation where basically I model the background distant galaxies all as little blue circles and then I have blocked a lens a clustered lens like one of the ones that Ricky found I flopped that in front has a large amount of dark matter remember it has 90% dark matter the galaxies that you see are only 10% of the matter there and the distortion so nothing is moving in the universe it's just to show you the range of distortions that I'm showing it as an animation so you end up seeing these highly distorted arcs and arc let's notice that in this frame there is not a single galaxy that looks like the original little blue circles everything is stretched out and that's because the light has been deflected through the pothole that is generated by this big cluster and I'm showing the effective aportfolio it's almost like you're looking down and you are seeing this is a deep part of the bottles so they're like these two potholes and this is the kind of distortion that is formed and to remind you remember every individual galaxies also from their Rubens work we know has this extended Dark Matter halo so when we look around galaxies we see a systematic distortion of distant sources these blue blobs lie behind the lens they are more distant but their shapes get distorted so this is the kind of systemic effect that we can see and remember the multiple images so this is a Hubble Space Telescope image of a cluster of galaxies so this is a cluster of galaxies causes a very deep divot a deep pothole in space-time enormous amounts of light bending in fact such dramatic light bending that you end up seeing five copies of the image of a distant galaxy in fact the trick is that there is a - one here that I have not circled intentionally I wanted you to look for it and find it so these are basically you can tell okay how can you tell there are four four copies five copies of the same distant galaxies remember every cosmic object has a fingerprint it has a unique fingerprint its spectrum so you can go out we've gone out and measured the spectrum the energy that is coming from this image the light that is coming from this image you collect that up and you see they're identical so they correspond to the same objects of five copies of the same object so this is a very very spectacular image of a cluster lens this cluster is called a bell 2218 so what you are seeing here by now you're all experts you can pick out the lens images you can see all these stretched out images already so these are distant galaxies that lie beyond the cluster that have been bent out of shape right so what we can do now is look at the shapes of these distortions you may remember it looks really like the animation that I was showing you right but the stretching you can see these arcs and arc lifts and what you can do is look at the extent of light bending and back out how much matter you really need intervening along the line of sight to produce all this dramatic lensing right and you might say okay this is all great but you know what you care okay that dark matter you don't know what dark matter is and so why you care if you can map it and you can figure out how much dark matter there is in the lens so there's a very interesting reason why this is of consequence so what I'm going to show you now is a simulation of the formation of a region so this is all dark matter this is just mocked up to look like light this is all dark matter particles that don't stick with each other that don't collide there collisionless they only aggregate via gravity and what you're seeing here is the process by which structure forms a cluster forms in the universe so this is the dark middleweight dark matter settles in the universe gravitationally and this is a zoom in of this region that over time is going to form a cluster like the one we just saw in the Hubble Space Telescope in so what you see here in pink right these are all clumps of dark matter so if dark matter is really the sluggish particle cold dark matter that we think it is and the leading candidates for that kind of dark matter are these particles the weakly interacting massive particles wimps I apologize for the corny acronyms that astronomers come up with all the time this one is the candidate particles are called neutrally knows we haven't found them right so that's the minor fly in the ointment and major embarrassment so we we know exquisite exquisite amounts in exquisite amounts of detail how the dark matter is distributed and so on but we don't know what it's made of right and that's why we are resorting to doing things like this indirect probe so if you look at so this is a simulation from some of my colleagues about the same patch of the universe again this is all dark matter just mocked up to look like light and if dark matter was cold dark matter and collisionless then you should see a lot of clumping a lot of structure a lot of fuss well if it was a different kind of particle warm dark matter this is a particle that is not as sluggish you don't see as much clumping it's more smoothly distributed so the idea here is that if we can exquisitely map dark matter that is calling the lensing that we see with the optics of the Hubble Space Telescope can we discriminate between these two kinds of dark matter so this is something that I had started this is a problem that I had started any other reason why you have to do all these indirect stuff is we haven't found the particle yet we've been looking for it for about 20 years no luck yet but you know I'm optimistic however in the meanwhile right you want to see what you can do so I have been trying to our current theory suggests that dark matter is this cold dark matter so soon after my PhD I started working on the stuff so this is sort of you know the gravitational lensing mapping and I thought I'd become really famous because you know I'll find a crisis in this cold dark matter model that's instant fame you find a crisis in your and it turns out you know I've been working really hard and you'll see having found a crisis the model works incredibly well dude I mean to the level to the precision that we can test it at the moment so once again this is to show you the cold dark matter patch and the warm dark matter and so if you can map the clumps of dark matter if you can figure out how dark matter is heaped you have some chance of discriminating between these two so let me show you the results of trying to do this exercise mapping the dark matter for that cluster so just to quickly show you we are looking down this is sort of a top view you can imagine we are looking down almost at sort of the pothole that is generated by this cluster and this region where you have that looks like a bunny rabbit any distant galaxies whose light traverses through the bunny rabbit will be multiplied imaged so light rays will be split so what do we do we can transform that to a dark matter map that looks like this so this is the Dark Matter distribution in that cluster you see these two heaps and a lot of other clumps so in a way right what we're really trying to do in this exercise right it's like being able we are trying to map sand dunes right how sand dunes are piling how they change and so on and so forth it's like mapping the sand dunes on a beach and trying to figure out what a grain of sand is made of it's really the best analogy that I give it I can give for what we are trying to do now right because we haven't found the particle yet so this is the latest and the greatest that this is the best that we can do this is one of the most recent deepest looks of the night sky from the Hubble Space Telescope as part of this project called the frontier fields project and so this is a cluster of galaxies a very massive cluster of galaxies that is we know what caused a huge divot in space-time there's a lot of like bending you see about a thousand background galaxies that are bent out of shape in this image so when you reconstruct the amount of matter that you need to produce a deflection that you see you derive this blue fuzz so this is all the unseen s'matter that is required to be there to produce what is seen in the Hubble Space Telescope image so what you can do once again is you can generate a map and this is a map that we just put out last month and we had a press release this is one of the most detailed maps of Dark Matter ever produced and so this is how it is heaped and as I mentioned earlier I kind of you know gave you the punchline it agrees incredibly well with the predictions of the cold dark matter model at the moment so we are we're back to square one in terms of corroborating that model although we haven't found the particle yet and as I mentioned several times it really not found the particle so there was one controversial claim for the detection of dark matter by an Italian group an italian experiment called gamma and and now finally replications right so the power of science is being harnessed amid you know this replication was not funded earlier only very recently these replications have been funded same detectors same same methodology independent groups doing the analysis collecting data so in about three years time we will know whether that claim holds water or not right and we've just as I said you know there many different experiments nothing yet but you know I as I mentioned I am optimistic because you know the recent announcement of the detection of gravitational waves from two colliding black holes they've been looking for it for 45 years and it was just detected right so let's now move on to a different radical idea and this one is fundamentally different from dark matter because it was not empirically discovered so the idea of a black hole was originally proposed it was a mathematical idea it was a mathematical solution to one of I'm Stein's equations field equations so it was and this was basically the black hole solution is the shape of space the distortion that is caused around a very very compact mass so it's the shape of space the shape of the pothole right around very very compact mass so that was a mathematical solution and so no one believed that black holes would be really I signed himself didn't believe that black holes would be real he actually never thought that his field equations would have any exact solutions these are the complicated equations maybe you'll have an approximate solution solution to them right but what happened is that within months of Einstein's lectures and he's talking about presenting his theory of general relativity calls what child found this exact solution but they were believed to be a mathematical curiosity no one thought that they could be real right you know once again if we look at Newton and we look at this classical idea remember I talked about how light was believed to be particles then actually John Mitchell who use these ideas of Newton's argued that you could have something that could be a Dark Star you could have a star that is so massive that it attracts all the light particles that stick to it and so there's no light coming out right that's not quite what a black hole is as we'll see but it's kind of you know the Newtonian analogues to what a black hole might be so the Astrophysical use of the term is attributed to Princeton physicist John Wheeler in 1964 and then you know black holes became real because you found these cosmic objects that had the properties that were ascribed mathematically to the solution of black holes and we have much to my surprise but I was researching the book the term black hole actually originates from India it originates and it actually is the was the site it's a pretty sad story it's the site of a prison it's a prison where the local in the Wahb had actually imprisoned soldiers from the East India Company the colonizers and they had been left overnight many of them had been sucked into this tiny tiny room and very few survived actually by all accounts nobody survived so a black hole was basically a point of no return and this predates Einstein's theory or the mathematical solution or whatever as we will see this is a very very apt description for what a black hole actually is so to understand what a black hole is there are many different ways in which you can think about a black hole I like this particular one aside from the shape the distortion that it's going to cause in space if you look at the escape speed what is the speed that you need to escape the grip of gravity so for example if you look at the earth you need to launch rockets with boosters from Cape Canaveral to have them escape Earth's gravity right and you need to launch them with about 300 times the speed of sound that's how fast you have to launch a rocket in order to for it to escape Earth's gravity now a black hole is an entity from which such a launch speed has to be the speed of light so what that means is that not even light can actually escape from a black hole and we will see why that is in just a second so coming back to the connection with Einstein's conception of space-time so this is what if you had no matter in the universe space-time would be flat and if you had this is the divot that we saw that's generated by the Sun and this is for example the divot that would be part hole that would be generated by a neutron star which is much more compact than the Sun okay it has more mass than the Sun but it's actually very compact very dense to notice it has a deeper divot a deeper part holes in space-time black hole on the other hand causes a puncture in space-time and in fact there is this interesting boundary of visibility and invisibility this region called the event horizon this is a point beyond which even light cannot escape a black hole so that's sort of the place of no return I think this is just to reiterate what really Einstein's theory of general relativity did was to reekin sexualize the relationship between matter and space and he showed that you know mass as we've seen with the application to gravitational lensing that mass creates this curvature in space-time these potholes in space-time and in a way the way one can think about it is that space as the curvature in space is produced by mass of by the presence of mass and this curvature in effect and in turn a dictate how matter moves around in the universe so this up the black hole solution is a very interesting solution for unsigned field equations as I said that the sort of wholly boundary the event horizon and if light crosses into the event horizon it actually cannot make its way out so nothing nothing material or even light that causes the event horizon can make it back out they are interestingly there are light orbits that skim outside when light could get captured and be around for perpetuity this like Dante's Limbo right so you could actually be here in purgatory so you could be sort of captured and you could be of course if you were kick slightly in if you moved in and you went in you would be captured or if you were kicked out light could completely escape so you could graze a black hole at a certain distance from the event horizon and just be deflected and move away and then there's a there's a certain distance out where you would call the photon capture radius where you would just be orbiting forever and then there's the event horizon and once you cross it you're done so one of the other peculiar things remember I keep saying four-dimensional space-time so one of the peculiar things about black holes is that the nature of time is altered around a black hole around this puncture so for and outside if you have an in falling object the in falling object time really slows down as you fall into a black hole okay so the question is that's great but our black holes real so it turns out we now know that black holes inhabit the center of every galaxy pretty much in the universe including our own and so this is this we started off from a real Hubble Space Telescope image we are zooming in now we are moving into an artist's conception because we cannot resolve these tiny tiny distances in the center of a galaxy so this is an Arctic artists depiction of the presence of a central black hole in the centre of that bright galaxy that we just saw and this is the feeding gas disc this is the gas that is feeding onto the black hole where the gravitational effect of this black hole will call the gas to be rapidly move in will funnel it rapidly and pull it in and in the process the gas will get heated and we'll start emitting in the x-ray to start emitting light so what we end up seeing is never the black hole directly we cannot image it but what we end up seeing are the dying gas of gas and materials as they are falling into the black hole and we also know that black holes generate these Jets of matter and energy that we see this is of course an artist's impression and we know that black holes in the universe real black holes in the universe as I mentioned they inhabit the center so be the supermassive black holes these are black holes that have masses that are about a million times the mass of the Sun they're sitting in the center of the of galaxies and they happen they are in two states so they are either feasting and so if a black hole is feasting which means that gas is falling in it's being fed by that gas disc it glows the gas glows and so it reveals the presence of the black hole and this object is called a quasar and we see these objects out they are very very bright beacons and you can see them out to vast distances in the universe then you have these fasting black holes the ones that are not binging like the ones in the center of our own galaxy the Milky Way so there's not a whole lot of gas around in the Milky Way and it's just sitting there it's doing nothing all it does is occasionally capture a star or some gas that graces nearby okay so black holes we see in the universe appear to be in one of these two states and as I mentioned quasars are very bright growing black holes and they outshine a galaxy so here we are seeing a quasar and now we've removed the quasar and you're seeing the galaxy that is hosting the quasar in the center you basically did not see the storage of the galaxy because this quasar outshines the entire galaxy that's how bright it is so a lot of my own work has involved trying to understand how you form the first black hole see we know that most likely the first stars that form in the universe will exhaust all their fuel and they'll bleep leave behind little black holes if the mass of the original star was eight to ten times above the mass of our Sun so those guys will end up as little black holes but the question is for the origin of these supermassive black holes a billion times the mass of the Sun million to billion times mass of the Sun there's not enough time in the universe to start from a fiddly little seed and grow it rapidly to explain the quasars that we see so many collaborators of mine have and I have been working on these models to make very massive black hole seeds from the get-go in the very very early universe so now the most compelling evidence for the existence of a black hole comes from our neighborhood and that is from the motions of stars in that movie not working for some reason we try one more time okay it's not working so what you could see if this movie for some reason is hanging you would see these are the these are stars right around the black hole the location of the black hole in the center of our Milky Way is here and this black hole is four million times the mass of the Sun and what you would have seen in this movie our data tracking the motions of these stars they're moving quite rapidly and you would see these beautiful neat ellipses so it would look rather like the ellipses of describing the motions of the planets in the solar system where the black hole would be at one focus of those ellipses like so and this is measured this is real data and this is data from Andrea guesses group at UCLA so as I said some of the open problems in understanding black holes or how you form them how you grow them how you feel them and what is the impact that a black hole has on its surroundings because we are going to look for indirect signatures we know that we can get indirect signatures from the material that's falling in and feeling a black hole and growing it so these are some simulations that a student former graduate student of mine did where what you're seeing here is gas that's falling in and a small black hole that is going to be assembling at this location that sort of marked by the red pixels so it's getting denser and denser material falls in this is sort of a side view of the gas disc so it's sort of a a bloated disc that form the black hole is here and gas will trickle on to the black hole so this is how we believe now that black holes grow so we also know that in the universe you have galaxies that have a halo of dark matter that is not shown here so these are two spiral galaxies that have extended Dark Matter halos they have a lot of gas and stars and as I mentioned every galaxies we believe has a black hole in its center these two guys have black holes in the center and we know that in our universe structure assembles hierarchically you form small galaxies first and you grow big galaxies by colliding them and so the collision of these two galaxies will lead to the formation of a more massive galaxy and will cause a collision of the two black hole that will also merge and what you see I'll show you the movie one more time because now you can focus on the fact that these black holes you see the flickering they are actually feeding the flickering corresponds to feeding that feeding episode that I was showing you from from an accretion disk that's what is modeled in the simulation so you can see that as these two galaxies approach each other the black holes are being fed and they are growing and then eventually they completely crash into each other and they merge these two galaxies become one the Stars get spilled out everywhere the Dark Matter combines and hangs around in a big halo and the question is what happens to the two black holes how long do they take to merge into each other and the reason that's interesting is because when two black holes merge there's something extraordinary that happens which is that the at the point when they finally merge they generate a tremor an earthquake in space-time itself and you see the gravitational waves that are generated and these were detected for the first time from the collision of two black holes that are about 30 times the mass of the Sun by the LIGO collaboration last year but what I'm showing you here are the collisions of supermassive black holes a million to a billion times the mass of the Sun and these these collisions should be very frequent in the universe because we know that all the galaxies that we see has to have formed by these crashing by these writing in these wreckages right so so this is to show you this is a calculation that I did with a collaborator very long time ago where we proposed how to sort of understand how a black hole would spiral in a secondary would spiral in with a primary black hole in the center of that merged kind of train wreck of a galaxy that we just saw so this is a top view where you see the accretion disk is this red stuff that you see this is the location of one of the black holes say one that was at the center of the gap of the two galaxies we saw emerging and this is the shape of the the gas disc that is piled up so gas is filed up the black hole is here the central black hole is here gas is filed up around it and now we're going to plop the second hole on - you're getting a top view we're going to plop the second hole onto this and you can see the way it spirals in slowly into the gas and eventually makes its way to the center so what is very interesting about it is that gravitational waves are very very hard and challenging to detect so what we would like is we would like to have some other signal some other kind of light signal not a tremor in space-time but an actual light signal that we can detect that will tell us two black holes are going to merge there very soon and so what we see here rumored and the light signals are going to come from the gas and so what we see here are these waves that are generated in the gas that would glow so you would see light emanating electromagnetic radiation that will signal a place from where you could get gravitational waves because the two black holes will finally merge so as I mentioned this is the LIGO event and you might also remember these beautiful animations they showed us and this is the signal this is the chirp the actual dying gasps of these two black holes as they merge that was detected so as I said the black holes that I'm interested in are supermassive black holes that inhabit the Centers of galaxies and thanks to a real breakthrough in computational general relativity about ten years ago we can actually predict what those signals should look like and we know so this is the LIGO collaboration this is their sensitivity and and the frequency at which to about a hundred or so between between ten to 100 Hertz is roughly the region where you would see these gravitational waves for the supermassive black holes merging but the candidates that I am interested in you will need a LIGO kind of detector in space you cannot measure this much lower frequency and such a detector is planned by the European Space Agency it's called Lisa and I'm really hoping that within my lifetime this window is opened and that we actually see the gravitational waves from the merger of supermassive black holes so before I wrap up you might ask this is great so you've spent your entire life trying to work on and prove that there are these invisible entities and they are real and so why this obsession with in with the unseen being visible so it turns out that you know the way we make progress in our understanding I started out by telling you that we have to be open-minded you know to new ideas and changes and so the way in which often we have progress is when we have a theoretical prediction and then we make a measurement and confront them and they disagree right so there's a gap there's a gap between our expectation as predicted by the theory and what we observe and the gap can be really really important so people like me are looking for gaps so why are we looking for gaps because history teaches us something very very interesting so when the orbit of Uranus was measured with greater accuracy it deviated from Newton's prediction so we're talking in the 17 to 1800's it deviated from Newton's predictions right so there was a brilliant French mathematician Urbain levare who came along and said aha you know this caused a lot of commotion right because Newton's theory Newton theory of gravitation law of gravitation was universal and it was sacred I mean how could there be a deviation from that prediction right it sent everyone into a tizzy so he came up and he said well actually what really you know what's happening here the anomaly that we see has to do with the presence of another planet that lies beyond Uranus that is perturbing that needs to be taken into account to describe the orbit of Uranus and so he correctly predicted the location and the presence the presence and the location of Neptune Neptune was found and with the inclusion of tune the orbit of Uranus as observed was fully explained and Newton's laws remained intact okay now similarly a deviation was seen in the precession of the orbit the change in the shape of the orbit of mercury and that deviated from Newton's predictions as well and obey Allah very a you know clever man that he was said well same solution there's another planet between the Sun and mercury and he called it Valken right and they were all these expeditions to look for welker and some people found it some people didn't find it there is no Vulcan right and what was really needed and you all now know what was really needed right this anomaly pointed signaled that there was something deeply missing in our understanding of gravity Allah Newton and we needed to have this three formulation reconceptualization by Einstein to explain the precession of the orbit of mercury so in one case the gap led to a refinement of the current theory in another case it actually signaled the way to a brand new theory a radical new idea so what people like me are trying to do is to try and find gaps and to see whether we are in the Uranus situation or in the mercury situation and we know already that Einstein's theory of general relativity which has been tested on the scale of the solar system it works exquisitely and we know on cosmic scales because of like deflection that's measured that it works really well despite all of that we know that the theory is incomplete the reason it's incomplete is because we don't have a quantum theory of gravity it's not married to the physics of the smallest we don't have a microscopic description of gravity right so we know that we don't we have an incomplete theory so the question is can we find a gap that will actually signal the way forward to a radical a new idea and so people like me are waiting for that thank you very much for your attention so we have time for a few questions there are mics on either hand please raise your hand if you want to ask for a question and wait until the mic comes to you I am doctor in early in your presentation you mentioned that these ancient people looked at the just the solar system and I'm curious because of the obvious lack of light pollution at that time in ancient history how did they account for the millions of stars that they must have been able to see because of no distractions that we suffer from today with our industrial universe right I think that while so they documented the many stars but they sort of had a conception of a fixed orb of stars that surrounded our solar system so there was a cosmic view to the extent that you didn't have any details they didn't have an understanding of the motions they seemed like they were fixed stars so there was this idea of fixity of the universe and that was you know classically it was very appealing in fact you know even to Einstein it was very appealing Einstein really did you know like the idea of fixity so when one of the solutions to his field equations suggested that the universe was expanding and Hubble measured it he was a holdout he didn't want it because he liked that ancient idea of fixity of course you know he knew there were other galaxies other nebulae and so on which corresponding to other galaxies but I think the ancients saw they had a conception of cosmos if you look across cultures if you look at you know Egypt you look at India China the conception was that you had a fixed orb of stars around the solar system a great lecture thank you my question is what would be the consequences if that matter didn't exist well the convict wanted you know rather than consequences what would you need to dispense with the notion of dark matter so let me sort of turn your question around and so you know there has been there have been proposals one proposal in particular in which the suggestion is that a modification for Newtonian dynamics Newtonian laws that there's no reason that Newton's laws should hold in a spiral galaxy far away from us and so the suggestion is that there's some modulation in the force of gravity when you look at cosmic objects right so this theory they have been refinements in the theory they've attempted to explain away dark matter by the saying that you're changing the nature of gravity you're not changing you don't need matter you just change the strength of gravity it turns out that that's that idea and there's a version of that idea if you want to look it up it's called Teves te te ves model it works quite well to dispense with the idea of dark matter using this argument in galaxies but it cannot account for light bending which we see so at the moment we really don't have an alternative a persuasive alternative to dark matter remember any alternative that you come up with has to explain everything that you see and make future falsifiable predictions so there is really nothing a no viable alternative at the moment and and other kinds of alternative you would look for as I mentioned or some kind of new way to think about gravity another V conceptualization of gravity that is sort of what would be needed but such a theory would have to explain the light bending as well and at the moment we don't have such as so knowing that everything in the universe is expanding will I eventually rip apart the black holes - no that's a great question so it turns out right when we think about the expansion of the universe right what we really want to think about is that the spaces between things are growing right so for example the Milky Way itself is intact as a galaxy so the Milky Way is not getting ripped apart why because you form a galaxy you say that you formed a galaxy only when the gravity of the matter has overtaken the expansion of the universe so it is held in balance this is why the distance between you know the center of the Milky Way and acetal is not changing it's not expanding so this is a great question because I think the way to think about it is when we say the universe is expanding is to just really think that this fabric of space-time is being stretched so what's really expanding is the distances between objects so if you think that way you can see immediately that the black holes will not be ripped apart so what's really happening is that you're really changing the distance between the black holes so and the consequence of this is that in the far future so not only is our universe expanding as I told you now we know as of 1998 this is quite brand-new discovery that the expansion is accelerating right so if we look into the far future the distances between us and our nearby galaxies is going to be going to become a very lonely neighborhood things are going to be stretching out and moving really far out much more rapidly um why does D'Arnot Eric not concentrate oh hi um yeah sorry like why does dark matter not concentrate in the center of galaxies instead forms a halo no it does concentrate as I said it does pile in the Centers of galaxies but it is not as tightly piled as normal matter which has pressure which collides and condenses so it's kind of fluffy but it is filed in the center and it is extended all the way out yeah well I kind of have two questions okay one is how close is their ass black how what I mean it's in that how close is the nearest black hole in the center of the Milky Way but how close is that how close of that in oh in in lightyears okay how close is a black hole at the center of our galaxy I don't know what units to be yeah about forty five thousand light-years between forty fifty thousand light years or so you know I was trying to come up with a useful ruler that would be helpful but forty thousand light-years and my second question is like how big is the actual core of it sorry my second question is how Bay's the actual core of it so your big is that how big is the black hole so the mass of the black hole in the center of our galaxy is four million times the mass of the Sun and you know the we don't think about the black hole as having structure right so what we know is what the masses and the size of the event horizon is proportional to the mass of a black hole so we can't really think about you know the core of the black hole itself but which is actually the singularity the pinch in space-time but the one at the center of our Milky Way is four million times but there are galaxies nearby that actually harbour black holes that are ten billion times the mass of the Sun okay I think we have time for two more questions so we're off RN is this safe all the way to there's somebody in the back step you have woman oh oh there is okay yeah please okay okay so I believe you actually touched on this in your lecture that last year I believe it was at the Large Hadron Collider the two sensors I guess Alice and Atlas maybe had independently detected some kind of signal at about the 750 giga volts or gigahertz rain and I haven't followed up on that it went away it was not statistically significant okay it went away all right that's why I want you and also I mean if we're I think this was going I mean does it have any consequences for dark matter it actually didn't have consequences for dark matter I mean so the interesting thing is that LHC per se or you know CERN per se we're not going to detect we're unlikely to detect dark matter direct directly in one of these accelerators so what the discovery of the Higgs actually did in terms of consequences for dark matter is that it ratified the standard model so it was that one missing piece in that entire puzzle of standard model and the candidate for dark matter that we are talking about they're all compatible with the standard model so that's the extent to which you know CERN and Alexei are and the Higgs were relevant to the Dark Matter problem one more over here yeah we're okay so so if dark matter and black holes both results in like changing the dynamics movement of stars and in changing the direction of light how do we distinguish between like lensing caused by dark matter and lensing and like dynamic caused by black holes like that and there's there other parts of it okay that's a great question perhaps I did not clarify so any matter will cause deflection of light and will cause gravitational lensing the question is how big is the mass how much mass is there because that is consequential for whether the deflection is detectable or not so the clusters of galaxies that I've been talking about I just realized that I didn't mention what their masses were the mass of a cluster the typical cluster that I showed you here was a few times 10 to the 15 times the mass of the Sun so that's the kind of pileup you need to see those deflections that you can pick out by eye in a Hubble Space Telescope image so the black hole in the center of a galaxy will contribute to the light bending but it's a very tiny it's a very small mass compared to the mass of the total galaxy so as I said for the black hole of the center of our galaxy is four million times the the Sun the mass of our entire galaxy including the Dark Matter halo is six orders of magnitude larger so but 10 to the 12 times the mass of the Sun so the deflection will be produced by anything that has matter be a dark matter ordinary atoms black holes whatever but it's the amount of mass and how the mass is distributed had that has consequences for the detectability of the deflection and the strength of the deflection and so with the long term history of black holes widen so with like well laughs into black holes like eventually so you're saying that it wouldn't be reared apart by dark energy but would Hawking radiation eventually well yeah but I mean evaporation of black holes we know at that time scale is longer than the ages or age of the universe so it's not a process that is of quantum so there's one other interesting thing that I realized that might be worth mentioning here the term of that photon ring that I mentioned that sort of the light skirting the outskirts so that is gravitational lensing by the black hole ah that's the bending of light by the black hole what I said you know Dante's purgatory it's going to be there forever it turns out that that ring the inner photon ring so there is an a very very exciting experiment that is currently underway called the event horizon telescope which is using the entire earth using radio telescopes on the entire earth to mimic the entire earth being equivalent to one radio dish like giant radio dish and looking at the center of the Milky Way looking at the black hole to see if we can map this shadow okay so that's called the event horizon telescope watch out folks I mean you know by the end of this year they've already taken data we should have some very interesting results so once again that would be the light bending that you would see by a black hole and we're hoping to actually detect it and the only one that we can really detect it for is the one the black hole that is closest to us which is the Milky Way the one in the center of the Milky I think there was one more question out there right just last question okay how dare Hannah program at the very top yeah stand up and shout please because we don't have a microphone yeah and so there's a great question the question is what is the interaction between dark matter and a black hole so I presume what you want to know is what happens when dark matter falls into a black hole can dark matter fall absolutely so one of the peculiar things about black holes is that once something crosses the event horizon anything you have no idea what it was so when we see a black hole when I say that the black hole in the centre of the Milky Way is four million times the mass of the Sun we have no idea how much of that mass came from stars and gas and what fraction of it could be dark matter or as I often joke you know I have these unmatched socks in my laundry so they could have gone into the black you have no idea right what went into the black hole but what we know from our current understanding of structure formation is the trickle down rate of dark matter is piddly compared to that of gas because as I mentioned gas actually has pressure it collapses it condenses and is really heaped in the inner regions of galaxies dark matter is heaps but it's fluffier so it's a very tiny trickle if at all that goes into a black hole okay that's all the time we have for questions here but we have a reception that is upstairs and dr. Natarajan has agreed to answer some more questions you

Channel: Fermilab

Views: 41,919

Rating: 4.6183987 out of 5

Keywords: Fermilab, Physics

Id: ba_6klZk4Nw

Length: 89min 27sec (5367 seconds)

Published: Wed May 10 2017