Please join me in welcoming a commitment. Thank you. Thank you. It's not a very good lecture tonight. Sorry. Good. Yeah. I want to tell you about the exciting discovery of an object, set of objects called Little Red Dots, which we'll get on to. Spoiler alert, they're small and red and look like dots. And the reason it's not a great lecture is that I reckon we're two years away from knowing what these things are. So I don't have a punchline. In recompense, what I can do is tell you everything that we know about these things, more or less, and use that to illustrate the state of our knowledge of the universe. The story starts really with the launch of the JWST, a wonderful space telescope, a Canadian, European-American mission that was launched on Christmas Day five years ago now. This is it after its release from the Ariane rocket carried it's heading off to its station about a million miles from Earth, where it spread its solar panels looking something like this, and unveiled and unfolded the largest mirror of its type ever flown in space. And this is a telescope that's optimized for the infrared part of the spectrum. So just beyond the red light that our eyes can see in the infrared. And the main reason why we wanted a massive infrared space telescope and worked for about 30 years to make it happen was that we could want it to look at the distant universe. As the universe expands, light from distant objects is stretched into the redder end of the spectrum, what we call the redshift. And so if you want to see the distant universe, you want to look in the infrared. And JWST, after about six months of commissioning, started to send back beautiful images of all sorts of objects. This is a slice from a large survey called Iger that was looking at a particular set of galaxies. This is a pretty large patch of the sky, and almost every dot that you can see here is a galaxy, a system of a few hundred billion stars, not a star. The only stars are the things with spikes which form a foreground. But as images started to come down from JWST, as astronomers just started to pour over this data, as they looked at the objects that they'd spent, in some cases a career waiting to see with this telescope, they began to realize in the background of these images, in almost every image that JWST takes, there are little red dots, points of light that are particularly red in the infrared. So they have longer wavelength light than would be expected. And they sort of scatter the background. So there are a few in this image. You may have spotted some of them as I've been talking, but we can zoom in on them and we can see this collection of mysterious objects. And these have created so much excitement that this week, when the next year's worth of allocation of JWST time was announced to the world's astronomers, we spend our time competing with each other for access to these marvelous telescopes. People write proposals, spend many months honing their arguments, and to be honest, we got enough work to keep 11 or 12 JWSTs busy. But these little red dots, which were unheard of when the observatory was launched, about 10% of the proposals that were successful are going to be used to look at this mysterious set of objects. So this is the hottest topic in astrophysics right now. And it's also a reminder of why we build in astronomy, we build observatories, not experiments. We don't build tools that do one thing. We build systems that can take images or provide data on many different things so that we can be surprised. In the history of astronomy, whenever we've predicted what the big discoveries from a new telescope or a new instrument are going to be, we've tended to be wrong because it's the things that you don't expect that really excite you. And so that's why I was really excited to talk about little red dots tonight. But it's also good timing to be thinking about an unusual set of galaxies. So to certain scene, we should remind ourselves that we live in a galaxy called the Milky Way. When I was a kid writing addresses, you know, your full address is, you know, whatever road, whatever town, you know, in my case, Devon, England, Europe, the Earth, then you go the solar system, and then you go the Milky Way galaxy, and then maybe the universe, maybe the Virgo supercaster in there. There's part of that cosmic address system is that we live in a galaxy. Uh, this is our best image of the Milky Way. This is obviously an artist reconstruction from all the data that we have being inside it. And it's a system of a few hundred billion stars in a nice flat spiral shape. And I I've talked about this before, but I don't think I've said on this stage that it's still surprising to me that this idea that there are galaxies beyond our own, that we have things like the Mercury, that we belong to the system, is recent. Back in 1904, the great Agnes Clark, who was one of the first people to make a living out of being a writer on astronomy, uh wrote a Marvel's book called The History of Astronomy. And it's it it in each chapter, she takes a topic, say Mars, and she spends maybe a paragraph giving you the history. And then the rest of the chapter is everything we now know about Mars. She would have been a marvellous Gresham lecturer. Um, but in her chapter on the wider universe, she says that the idea of galaxies beyond the Milky Way, the idea there are other systems like this out there, is a long-forgotten and discredited hypothesis. And that was 1904. In 1920, there was what's called the Great Debate, astronomers meeting in Washington, D.C., for a public debate over whether the faint fuzzy things they saw in the sky really were distant galaxies or whether they were just part of the Milky Way. And it wasn't until uh late 1924 that the debate was settled. At that point, Edwin Hubble, in a story that I've told in a previous lecture, was able to measure the distance by looking at a particular class of star within these galaxies. He measured the distance to six nearby galaxies and showed that they were so far away that they had to be distant from the Milky Way. And he leaked the news to the New York Times. So this is the actual New York Times article. Uh, they spell his name wrong, which I think is brilliant. Uh, but you can see the headline, Dr. Hubble confirms the view that there are island universes similar to our own. I like this bit in the middle where he says the results are striking in their confirmation of the view that these spiral nebulae are distant stellar systems, found to be about 10 times as far away as the small magnetic cloud, and a distance of about a million light years. It's about a factor of two wrong, but not too bad. This means that light travelling at the rate of 186,000 miles a second has required a million years to reach us, and that we're observing them by light which left them in the Pliocene age upon the Earth. Um, so we've got the basic idea that there are other galaxies out there, and I love the fact that this is the scientific discovery of the first half of the 20th century, at least in astronomy, and it's on page eight of the New York Times. There it is in the top right, just above uh some fetching fur coats, and I was very taken with whatever the Hydra Bob is, uh, which is apparently the latest hair extension technology. Um, so this was big news, and then astronomers heard about this, astronomers who didn't read the New York Times heard about this, on the 1st of January 1925, uh, when Henry Russell gave a talk to the American Astronomical Society on the technical details of this discovery. And so at this point, you may be remembering that this lecture tonight is called the Universe's Hundredth Birthday, and you may be thinking that I'm a year out, right? We should be celebrating the anniversary of these discoveries in 24 and 25. But I chose the word universe deliberately because I think the vision of a universe that we'd recognize today, this vision of a uh cosmos filled with galaxies, which are spread out amongst the cosmos, where it's not just a small group of local systems, but in fact a near-infinite, perhaps actually infinite array of galaxies, is set out by Hubble in a later paper on the extragalactic nebulae from 1926, which also has fetching photographs of many of the galaxies taken with the telescopes at Mount Wilson in California. And this paper is astounding. He's still only measured the distance to six galaxies. But he talks in this paper of about 400 of these things. And he makes one crucial assumption. He says, okay, we can't measure the distance to all of these things, but let's just assume that they're all the same brightness. They're probably not, but that's probably a good guess, right? We don't know what these things are, but maybe they're all about the same brightness as the Milky Way, say, or as Andromeda, our next nearest galaxy. And if we make that assumption, we can guess their distances because we can see how bright they appear in the sky, and we can see how bright they would be at different distances. So if I take a galaxy, say, if I have two galaxies and I've assumed they're the same brightness, they have the same number of stars, the same types of stars giving out the same amount of light, and one is twice as far away as the other, it will appear four times fater, just from the laws of geometry and optics. And so on this foundation of observations of 400, he's able to build this picture of a universe that has a density of galaxies. He says uh somewhere in here, there's one galaxy per 10 to the 17 cubic parsecs, so parsecs about three light years. And so he can work out that the universe is mostly empty, which is an astounding fact, I think, but the cosmos is mostly empty and we have these galaxies spread throughout it. He goes on to link these observations to recent solutions to Einstein's theory of relativity and the equations that it contains, which describe what he calls a boundless yet finite universe filled with galaxies, and he looks forward at the end to a reasonable increase in the speed of photographic plates and the size of telescopes, so that it may soon become possible, he says, to observe an appreciable fraction of the entire Einstein universe. And this was his project, really, was to try and stretch out further through the universe and connect to what we'd see as a modern view of the cosmos. This is the Sloan Digital Sky Survey. It's data I worked on 20 years ago, but it's our best view of the local universe. So you really see this Hubble picture here of individual galaxies separated by vast distances, scattered through space with a particular density. And you can see a feature in this which Hubble didn't know about, which is that there are places in this universe where there are lots of galaxies and places where there are very few. This lumpiness is something that we've discovered more recently. So we now have this view of the universe a hundred, just a hundred years ago. And the next challenge was to try and understand what was happening to the universe. Now, to Hubble, that was a question of whether the universe was really expanding or not. Um other results that I've talked about before indicate that the galaxies were rushing away from us, that we're in this expansion that started with a Big Bang. And so he, his big project was to try and count the galaxies at different brightnesses. And the idea being that if the universe is expanding, as you get to fainter galaxies, you should see more of them. And so, as long as you keep this assumption that all galaxies are the same brightness, he could literally just count the galaxies. And this is a story that's told in a recent book by Richard Ellis, a distinguished astronomer called When Galaxies Were Born. And by 1936, Hubble has literally counted 40,000 faint galaxies, recording their positions and their brightnesses on big photographic plates obtained with the Californian telescopes. The problem is that he realizes his telescopes aren't big enough. We already said that he was hoping to see a fraction, a large fraction of the universe. His telescopes aren't big enough to make this possible. And so, pretty quickly, there's a clamour for new instrumentation, led by this guy. This is Hale, who uh had fundraised for many of the big telescopes at the start of the 20th century. He wrote a marvelous article in Harper's magazine in 1928, in which he was trying to explain why we need bigger telescopes. And instead of giving details of the outcome, what science you might do with a big telescope, he talks about the problem as he sees it. And he says, look, starlight is falling on every square mile of the Earth's surface. And yet the best we do is gather up and concentrate the rays that strike an area a hundred inches in diameter. It's the diameter of the biggest telescope mirror at the time. You could just hear that he thinks this is a waste of starlight, and that the only solution is to build a bigger telescope. I had a simmer thought when James Webb, JWST, first flew, realizing that those infrared photons had been passing through the past the Earth for all of the Cosmos's history, all of the Earth's history. And for the first time, there's this golden mirror in the way, able to capture some of them. Hale's words and a lot of politicking and fundraising eventually led to the building of the 200-inch telescope at Palama, which is still operational today. It was the largest telescope in the world from about 1950 to about 1990, and plus or minus a Russian one that was built in the 80s, that we could discuss some other time. And it continued this Hubble project of trying to count faint galaxies. But it was quickly realized that we're missing something here. That Hubble's assumption that all galaxies must be the same brightness was not only overly simplistic, but it misses out a big feature of what's happening in the universe, which is that things change. The galaxy population evolves. The galaxies that existed perhaps a little while after the Big Bang won't be like the ones that we see today. Or at least we shouldn't assume that they are. And so astronomers started to look not just for evidence of the expanding universe by counting galaxies, but for evidence of evolution, of changes in the stellar population and the galaxy population. And this was the effort that would take extra-galactic astronomers most of the 20th century. Richard, in his book, does a good job of the history of this. We'll just stop off at one point. Let's take a halfway point, roughly 50 years between Hubble and now, so the late 1970s. Here, by this point, astronomers using telescopes like the Anglo-Australian telescope and down in Eastern Australia to take photographic plates like the one at the bottom here. Now, this is not a glamorous scientific survey. This is a set of blobs in a noisy image because they're pushing the telescope and the capabilities of the photographic technology to its limits. But particularly British astronomers, actually, at the time, had an advantage in that they had automatic plate scanning machines. So you get these giant photographic plates, you expose them for hours to record these faint blobs, you bring the plates back to either Edinburgh or Cambridge, and there you have a software-driven system that can scan the plate very carefully, recording the brightness of everything that shows up in that photographic image. Then you get something like the data at the top there, and each blob is a faint galaxy that you can measure the brightness of. And we can play this game again of just counting them and seeing what has changed. Of course, you need a theory to compare against, and that's where the woman on the right comes from. This is Beatrice Tinsley, a New Zealander Kiwi, who was the first uh female uh professor of astronomy at Yale, who was remarkable in applying physical principles and theories to predicting what would happen to the galaxy population. And it was a crucial piece of work that I think is underrated. She died young and I think was forgotten for a few years. But in the late 1970s, from plates like this, astronomers were able, like Bruce Peterson, were able to create their galaxy counts. And the the black dots here are the galaxies. And I want to show you real data. I know this isn't the most exciting graph that you've ever been shown, and that the material for this lecture promised beautiful images. But this is what we've got. But basically, the bright galaxies are on the left and the faint galaxies on the right, and then the higher up the dot is, the more of them they are. So you can see that there are more faint than there are bright galaxies instantly. But the crucial point here is these lines, they're they're predictions from Tinsley's models. And this top one that's labeled IB Evolution, this is the line you'd expect if what happens is that you form a galaxy in the early universe, it forms all its stars at once, and then you have a slow decay. So it's an extreme version of the possible history of galaxies. You see, that doesn't fit the data at all. A better fit is this line, this dashed line called SSF Evolution, which is sort of a steady star formation. So this is a set of galaxies that form stars throughout their lives, and you can see that that fits reasonably well. But the details of this graph, which we don't need to go into, and in particular the fact that there were those faint galaxies found in such great quantities, told astronomers that the galaxy population was changing, that we have to deal with a universe where things in the past are not necessarily the same as things they are now. This will be important when we get back to the little red dots. To bring this bit of the story to a close, the real revolution came in the 90s with the advent of bigger telescopes again. These are the Keck telescopes in Hawaii on the surface of the Sacred Mountain of on the summit of the Sacred Mountain of Mauna Kea. These used uh hexagonal mirrors rather than a big solid piece of glass. They used a jigsaw of hexagonal pieces to be able to make bigger mirrors. And they saw deeper and further than other telescopes have been able to do. The red thing on the left is the record holder for most distant galaxies from 2013, discovered by the Kex and found 700 million years after the Big Bang. And Hubble, of course, and other space telescopes, as I've described in previous lectures, look deep into tiny patches of the sky and give us iconic images like the Hubble deep field. Another image here, like the JWST one that I started with, where almost everything that you can see is a galaxy, a system of a few hundred billion stars rather than a star itself. And so by this point, we can piece together the whole history of galaxy evolution and formation. And we discover that these galaxies come in a multitude of different forms, from spirals like this one. This is M51 interacting with a smaller galaxy to the Sombrero galaxy in this iconic Hubble image. And even that these shapes that we see in the galaxies, well, they're sculpted by encounters between the galaxies that Hubble's island universes, his isolated star systems in an otherwise blank and empty cosmos, do over the fullness of time, over the billions of years that have existed since the Big Bang, combine and collide with each other. This is the antennae in another beautiful Hubble image with a close-up on the right of the star formation and the disruption that's caused by the collision between galaxies. And whereas maybe 20 years ago we might have been describing encounters like this as rare, we've come to realize that this is a fundamental part of the life of many galaxies. I talked about the Milky Way's history when I talked about Gaia, but you can see on the left here, in a simulation of about 10 billion years of cosmic history, with the expansion of the universe removed, you can see that if you follow any one of these dots, which is roughly a galaxy-sized halo, you can see how it encounters and collides and interacts with both other galaxies but also the dense gas in these filaments of this lumpy universe. And so those interactions cause quite dramatic changes to the galaxy. On the right here is a very recent image just from last year, actually, I think it's from earlier this year, of a spiral galaxy from JWST, where in red you're seeing star formation that's been induced by a collision between this galaxy and one which is a little smaller than it. Thing that's just here. So the the yellow galaxies are a nearby cluster, the pink thing is a distant galaxy, um, which has been magnified by uh its light's passage through the gravitational field of this nearby cluster. And the press release for this object for this image says the pink galaxy has distinct spiral arms, but clearly resembles a jellyfish with tangled tentacles because its light has been warped and bent by gravity. Um I'm not a jellyfish expert myself, uh, and I think clearly is pushing it. But you sort of see what they mean. This is a sample of what's happening uh a few billion years ago. So we know that galaxies exist, we know the universe is filled with them, we know they evolve, and we know that they're shaped partly through interactions. But there's another story playing out here, which is that there's another process that's absolutely key to understanding what's shaping the galaxy population. And astronomers sort of stumbled upon this one back in the 1960s when they were looking at this object. This is a Hubble Space Telescope photo of it from more recent times. Um there's an object called 3C273, and that 3C in the name indicates that it was in the third Cambridge catalogue of radio sources. So this is an object that was discovered when astronomers were first using radio telescopes to map the sky. Having found several of these things which appeared to be point sources, so they looked like stars, and in fact they got the name quasi-stellar objects, sort of seeming stars, or we would call them quasars today. The question was, well, what are they? And if you've only got the radio data and all you can tell is that there are radio waves coming from this point in the sky, it's very hard to make progress. So there was a competition to try and connect these things to what we might see with optical telescopes, with telescopes that use the kind of light that we're sensitive to. But this is a difficult process. Radio telescopes, because of the long wavelengths, don't give you a very precise precision of the position of an object on the sky. And so if you take an optical image of the possible area where this quasar exists in the radio, you find that there are many hundreds or even thousands of potential sources. But in 1963, there was a brilliant coincidence, which is that the moon happened to pass in front of this object on one particular night. And so that meant that by observing it was possible to record precisely when the radio signal blinked out because the moon was in the way. And so that gave you a very precise precision, and an astronomer called Martin Schmidt was therefore able to go and identify the optical counterpart. And he said the only explanation for the spectrum of this object involves a considerable redshift. And so he ends up realizing that this is an object that's shining brightly, but is actually two and a half billion light years away. And yet it's detectable with relatively small telescopes. And so then the question is: well, what could be powering such an energetic source, particularly as observations with Jodrell Bank and with other radio telescopes showed that this thing was very compact. It may be not much bigger than the solar system. And so you need something that's luminous enough that we can see it from two and a half billion light years away across a considerable fraction of the universe, and yet small enough that it explains those radio observations. Well, we now know the answer to this. We know that the answer is that these things are powered by black holes at their centers. Black holes are the collapsed remnants of massive stars. We have one at the center of our galaxy, and we know that partly because we can measure the motion of stars. This is real data taken over about 10 years. We can measure the motion of stars around an invisible object at the center of our black hole, center of our galaxy, marked by the star here. And in recent times, radio astronomers have used a collaboration of telescopes around the world called the Event Horizon Telescope to take images of the luckiest light in the universe. This is actually in M87, a nearby galaxy, but the donut here is really light from a hot disk of material that surrounds these black holes. We can make quite impressive made-up visualizations of these things as well, if you prefer this to an orange donut. I think it's got a certain grandeur that the raw data misses. But the point is this: you have a black hole in the center. So an object that's so dense that light can't escape. And yet it glows brightly. And it glows brightly because as material falls down towards the black hole, it forms this disk of material. We call it an accretion disk. And that accretion disk is heated to extreme temperatures, partly from the gravitational effect of the black hole, but also just from friction amongst material in this disk. And in fact, if you want to turn matter into light, the most efficient way we know of doing that in the universe isn't to form a star. If you form the sun and wait 10 billion years for it to live out its natural life, you end up converting less than 1% of the Sun's mass into light. If you throw that same amount of material onto a decent sized accretion disk, about 40% of it in some circumstances can be liberated as light. So black holes are really amongst the, or the phenomena that surrounds them are amongst the brightest phenomena in the universe. And it was realized that this was exactly the sort of explanation that could account for what was seen in these quasars. A discovery made in the late 1990s, or the theory of which was worked out in the late 1960s, after previous papers by Aman Sandage and a Russian cosmologist called Yakov Zaldovich, but by Donald Lyndon Bell in Cambridge, who was the person who worked out the theory that you could get enough power out of an accretion disk to explain these quasars. And that was pretty much Donald. But also, it's a slight modicum of revenge. He was in the audience in the first ever talk I gave as a PhD student. And I was very nervous and I didn't really understand the science that I was talking about. And I sort of knew the words that I wanted to say in the right order, and I could tell he was in the front row. And about two minutes into a 10-minute talk, I clearly said something interesting because he was sort of fidgeting and getting excited and kept half asking a question. And so I my brain became 90% occupied with what Donald was going to ask me. I got to the end of the talk, and before any the chair could even say, has anyone got any questions? He jumped up and he started talking. And I heard this, I just had this buzzing in my head. I didn't really follow anything he was saying. It was quite a technical argument about actually counting galaxies. Um and I was just standing here thinking, I hope there's a question at the end of this that I know the answer to. And there was, because he stopped speaking and said, Would you care to comment? And I said, No. And sat down. Um, it all ended fine. Over dinner, he he took me aside and spent an hour giving me the longer version of his question, and it turns out he was right and I was wrong. But but um but but I've never forgotten that encounter. And there's something of that in his paper on quasars, that clarity of taking one idea. He knew that these things were compact, and he knew that there was this theoretical idea that you could get energy out of an accretion disk around a black hole. And this paper just takes those two things, puts them together, and then just works out the consequences that explain these objects. And so if you look at astronomy textbooks, they often tell you that there are normal galaxies and there are quasars. There are these two types of objects. You're either a galaxy with an actively growing black hole that outshines the rest of you, or you're a nice quiescent galaxy like the Milky Way. But we actually know that's not true now, that galaxies can switch on and off between these states. And we know that partly because of an object that volunteers in our Galaxy Zoo project, where we ask people to sort through images of galaxies. It's a project that's still ongoing online if you want to participate. We put new galaxies up just the other week. Um, but participants in that project in particular, one Dutch participant called Honey van Akel spotted this galaxy and asked a sensible question, which is what's the blue blob next to the galaxy? And um she called it, uh, she's Dutch, and she called it the Vorvet, which I understand is Dutch for thingy, roughly, but we adopted it as a technical term. And we end up looking at this thing with the Hubble Space Telescope. And what's happened here is that this is hot gas, it's about 50,000 degrees, and it's being excited by light and a jet that comes from material falling onto the black hole in the center of this neighboring galaxy. Except that when we went looking for the accretion disk, we use an X-ray telescope to try and see the hot gas around the black hole that must surely be there. We don't see any. And what we worked out was that this is a system that in the last 50,000 years has switched from being a quasar with a bright accretion disk to a normal galaxy. If you'd been around 50,000 years ago, this thing would have been visible in binoculars, you would have been able to see material falling onto its black hole. And now we don't see any sign of that. So galaxies can switch to and from. There's a whole nother lecture in the fact that galaxies can change state, that their black holes can be growing or quiescent. And that idea is going to be important. Let's come back to the little red dots. With that sketch in mind, so we've got galaxies as isolated systems that interact via merging, but also have these black holes that grow and influence the rest of the galaxy. We can start with my favorite kind of astronomy. We found a thing. What do we know about the thing? Well, we know that these things are little because they're dots. So, okay, that's two-thirds of the name accounted for. Um, but they're also red. So the immediate suspicion is that these things are red because they're distant. I told you, reminded you, that the expanding universe stretches light as it passes through it. And so red might signify very distant. And the first year or so of these things, once they started showing up in the background of images, people argued about whether they were really distant or not. And it turns out they are. We know that. We've measured their spectra, we've looked at the light, we can measure the redshift directly by recognizing things like the features of hydrogen in a spectrum, seeing how far they're redshifted. And some of these, not all of them, but some of them are from a really early time, maybe 500 to about 700 million years after the Big Bang. So about 13 billion years ago. There are some local ones which have been studied in more detail, but most of them are early. They seem to be a feature of the early universe that we don't have today. There aren't any neighboring the Milky Way. It's not that we've missed them locally. We needed to look at the distant universe to see them. So then you've got, even when you take into account the distance, they're still pretty red. And so this is immediately puzzling because your first guess is okay, this is the early universe. We expect galaxies to be forming back then. And we know from the kind of work that I shared with Tinsley, we think that most of those galaxies will have started with a burst of star formation. So it's not that unexpected to find a population of brightly star-forming galaxies in the early universe. That would make sense. But there are three problems with that simple explanation. One is that when you form stars, you tend to form all kinds of stars. You form tiny red stars, red dwarfs, you form sun-like stars, the yellow uh stars in the middle, and you form the big blue and white giants. But most of the light comes from those massive stars. They're bigger, they're brighter, they dominate. It's only after the population has had time to evolve, and the big stars have gone bang in the supernova that you're left with the red giants. So if you have a galaxy that's just formed stars, these should be little blue dots, not little red dots. There's also, they're also too bright. To get to the brightness of these things at the distance that we see them, we would have had to have about 10 billion to a hundred billion stars, sun's worth of stars. 10 billion to a hundred billion solar masses. That's the Milky Way today, give or take a factor of two or three. And yet we're saying that has to happen within the first billion years of the universe's history. That's very difficult to explain, and no one's predicted that. And then the third problem is that these things are little. There's a joke circulating that these aren't little red dots, they're just big red dots a long way away. But even when you take that, it's very far that Ted, but even when you take that into account, they're still little. And so to cram that many stars into such a small system, you're basically like the center of the densest cluster of stars that we know across a whole galaxy. And such systems would be unstable, and also we don't see anything like that today. So maybe they're not lit by stars. Well, let's go for our other explanation. Maybe if they're not on the galaxy side, they're on the quasar side. Maybe these are lit by growing black holes at their center. Well, the colours don't quite work. You take a normal growing black hole with an accretion disk. You don't match the colours that we see. And there's another problem as well. Um I did want to show some of the technical data. I don't want to go into the details particularly, but this is data for one little red dot that's now been intensively studied. It's a relatively nearby one. It's been given the name the cliff, um, which is, I suppose, as good as any other name. And the purple points are from the cliff. And what you can see is so this is um redder light is over here, and bluer light is over here. And the higher a point is, the brighter this object is at that particular um wavelength. And what you could see is that as you go from sort of the far infrared, very red light, towards the optical, it gets brighter and brighter and brighter, and then there's this dip that drops down. Now, this is a familiar thing. It's called the barma break, and it exists in galaxy spectra where you have excited hydrogen and lots of hot stars. So this is usually a sign that there are young hot stars exciting the gas that they're forming within in a galaxy. We see it in the most star-forming galaxies in the local universe. And so people started to say, okay, maybe this is a sign that this is star formation, but we still have our three problems. There's we've got to form 10 billion stars in this compact dense system, and then we still don't explain the colour. So the Balma break is really tricky. We see it in many systems. These red dots are from a big JWST survey, which the title of the survey is Red Unknowns, Bright Infrared Extragalactic Survey, which some of you may have worked out makes the acronym Ruby's, uh, which is quite nice. I think they came up with the acronym first. Um, but these rubies, these red dots, and all have this barma break and yet don't have properties that are consistent with being primarily made of stars. And so people start to say, okay, can we do something weird with a black hole at the center? Let's have a black hole, let's put lots of material onto an accretion disk so that it's growing, and then let's hide it under a blanket. In this case, a blanket of dust, of small particles that would perhaps obscure the accretion disk itself. And then what we're seeing is this blanket of dust. And there are theoretical papers like this one that came out last year that show that you can get this dip. If you look at, say, the blue or the yellow or the black line, that if you have a thick enough blanket, you can get something that looks like this barmer break that has this dip. And so this seems like a promising model. What we've got is a hidden black hole that's growing rapidly and then this blanket of dust. Maybe there's something in the early universe that means that galaxies form in this way that we don't see around us today. Maybe dust is less common now. Or maybe these black holes are growing rapidly and so are embedded in their galaxies differently. And there's there's evidence that there are black holes there in this spectrum of this red dot, and this is one nearby-ish little red dot. Um, this is the spectrum again, but the crucial point in this one is this bottom left thing here, where we've zoomed in on one particular bit of spectrum. We've split the light out into its rainbow. And what you can see is there's a line here which is just hydrogen emitting hot hydrogen in the disk around a black hole. But the fact that it's broad, that there's a width to that line tells you it's moving rapidly. And so this is evidence that there is a disk of material there and that it's moving rapidly around the black hole. It's exactly what we see locally when we have growing AGM. From that width, because you can tell how fast the gas is moving, we can measure the mass of the black hole. This one has a black hole at its center. So the Milky Way has a black hole at its center that weighs a few million times the mass of the sun. This one weighs in at a hundred million times the mass of the sun. And yet it's observed about one and a half billion years after the Big Bang. So this black hole has to have grown very quickly. We don't know how to do that. We know how to form black holes. Black holes form when you have a massive star and it explodes, and the core collapses to form a black hole. So from a star that's maybe 20 times the mass of the sun, you might get a black hole that's one or two times the mass of the sun. Something like that. Which can then grow steadily over time as material falls into it. And there's a reasonably well-understood limit as to how fast black holes can grow in normal circumstances. They're fastidious eaters. There's a maximum speed with which they control material. If you pour more material on, the disk gets hotter, it heats up, and it expands, stopping material falling onto the black hole. So if you take even what would be a very big star, say a hundred solar mass star, form a black hole in the early universe, and then feed it at the maximum rate it'll take, all the way up until the time that we observe this one, you're about a factor of 10 off the mass that's predicted here. And so you can't start with a normal black hole. You need to invent some way of growing an exotically large black hole to start with. So something we don't know about is happening in the early universe. So maybe at the center of the first galaxies, and remember these red dots are comments, and this must happen a lot. But maybe in the center of those first galaxies, there's a collapse of material straight down without bothering with a star. Maybe the density is high enough and conditions weird enough that you form a black hole straight away. That the first things to form aren't normal galaxies and stars, but these black holes. And maybe then you have time to grow one. That might be the right answer. But we have new results just from the end of last year from this telescope, from Elma, which is a radio telescope looking at short wavelength radio waves from a site very high up in the Chilean Atacama. It's an amazing telescope, it's an amazing place. It's high enough in altitude that if they have to do maintenance work on any of these dishes, they go and get them. And they're about 12 meters across, but they go and get them and they bring them down closer to sea level so it's safer to work on them rather than sending people up to the summit. And Elmer, which has looked at lots of these things along with JWST, has told us how much dust there is in these galaxies. And the blanket that we need to hide these AGN is looking very threadbare indeed. It's very hard to see how you can have enough dust to have these massively growing black holes and still be able to reconcile them with the properties of what we see with the little red dots. So these things exist. They have properties that we've now measured with pretty much every telescope that we can get our hands on. And they fit neither of these two models. They don't seem to be early galaxies that are forming lots of stars, and they don't seem to be straightforward black holes growing. There is, I should say, in the large pile of papers, Uh that on these things. There's some consensus that some of them are both of those things, but there are still some weird ones as well. Enough of them that we need a different explanation. And one of them I might have to tell you might be rubbish, but it's fun. And I think has a good chance of being an explanation for this phenomenon. We're going to invent what's known in the literature as a quasi-star. And this was first worked out in 2006 by a theorist called Mitchell Bagelman, who's now in Colorado, and working with Martin Reese, who used to be astronomer all, and a couple of other colleagues. And his recipe for a quasi-star is as follows. This is an unconvincing artist's impression, by the way, not a picture. You take a massive star, a really massive star, bigger than anything we otherwise suspected existed, say a thousand times the mass of the sun. Now, massive stars run through the fuel at the center very quickly. When the fuel runs out, they collapse, they form a supernova, and you end up with a black hole as the remnant. For such a big star, though, the theory is that maybe all of that can happen. But because you have a thousand solar masses of star, in the right conditions, a supernova, the explosion that normally rips the star to pieces, the shockwaves can be absorbed by the star. And so what you end up with is a black hole within a still functioning star. So I like to think of these as zombie stars. They've died, but from the outside you've still got a star. Now material's going to fall onto the black hole. That's going to form an accretion disk about black hole, and that's going to shine brightly, and that will take the place of the reactions at the center of these stars. And the back of the envelope, it's a bit better than that. A couple of chalkboards worth of calculations show that this would be stable for a few million years. So you can grow the black hole from about a thousand to about ten thousand solar masses. And in the meantime, you'd have this one source that would be growing very brightly, would appear as a little red dot. It's an incredibly neat explanation. It's an excuse to grow a really big black hole while shielding it using the rest of the star. Now, as I said, this might may be rubbish, and I want to give you an idea of the state of this theory by showing you the most technical diagram from the most recent theory paper on this. Okay? And I won't go into the details, but the GIST gives you some idea of the level of simulation that we've got. You've got a star, then you form a black hole in it, and then the black hole grows, and then you get rid of the rest of the star, and you're left with an early quasar. Now there's a page of equations behind each of these things, and the time scales make some sort of sense. This takes about two million years, this takes two to five, then you've got about 20 to 50 million year period where you're growing your black hole, and then after 100 million years you get the quasar. But I want you to understand this isn't a supercomputer simulation, right? And nor when you look at this, I think, should you jump from there's a red dot there to there are now these things called quasi-stars, stars that are zombies with black holes at their center. But there might be. And more to the point, a hundred years ago, when Hubble looked and understood that the cosmos was scattered with these things called galaxies, he didn't know about the diversity of galaxies that was about to be revealed by telescopes. He didn't know that galactic mergers exist, he didn't know that black holes existed. But he did know that as we peer more deeply into the universe, as we collect more of that wasted starlight that so offended hail, we learn new things. And it's possible that these little red dots are just simply something that no one had thought about even a few years ago. I think that's fun. Thank you very much.