In Our Time · · Transcript & summary by WhipScribe
Three working physicists explain, in plain language, how astronomers can be sure that something invisible exists and why they still disagree about when it will be found.
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“So often I say my career has been a failure because I set out to rule this out and I can't.” Carlos Frenk · at 36:00 — open in the transcript →
“It's all still tantalisingly circumstantial.” Carolyn Crawford · at 50:54 — open in the transcript →
Most of the matter in the universe gives off no light, reflects none and casts no shadow, yet it holds galaxies together. Melvyn Bragg puts the puzzle to three physicists: Carolyn Crawford of Cambridge, Anne Green of Nottingham and Carlos Frenk of Durham. The case they assemble rests on gravity alone. Galaxy clusters swarm too quickly to stay bound, stars at the edge of spiral galaxies orbit as fast as those near the centre, and hot X-ray gas in clusters and elliptical galaxies should have boiled away. Gravitational lensing then shows where the unseen mass sits, in broad halos around the visible galaxies. Crawford's point is that independent telescopes, wavebands and assumptions all return the same answer.
The second strand is what the stuff could be. Faint ordinary matter, such as planets, brown dwarfs and dead stars, would glow enough for modern detectors, and Frenk argues that the microwave background left by the Big Bang settles the matter by fixing the amount of ordinary matter to about 1 per cent. Neutrinos move too fast to build galaxies from the bottom up. That leaves heavy, aloof particles called WIMPs as the favoured candidate, with axions and warm dark matter still in play. Frenk's computer universes, seeded with cold dark matter, produce galaxies his colleagues cannot tell from real ones.
The last strand is the hunt. Underground detectors wait for a rare collision with an atomic nucleus, CERN might manufacture the particles outright, and gamma rays from the Milky Way's centre or extra positrons in space could betray particles annihilating each other. The panel splits on timing: Frenk expects success soon, while Crawford calls every signal so far circumstantial.
Ogden Professor of Fundamental Physics and Director of the Institute for Computational Cosmology at Durham University, who has built computer simulations of the universe's evolution since the 1980s.
On In Our Time, Carolyn Crawford, Anne Green and Carlos Frenk explained that dark matter is inferred from its gravity on things that do shine. Galaxies in clusters move too fast to stay together, stars in the outer Milky Way orbit as fast as inner ones, and hot gas in clusters should have escaped. Crawford stressed that different telescopes, wavebands and assumptions all point to the same missing mass.
Anne Green of the University of Nottingham called gravitational lensing some of the nicest evidence for dark matter. Einstein's general relativity says mass bends space, so a massive galaxy cluster bends the light of a galaxy far behind it into multiple distorted arcs. Mapping those arcs shows how mass is spread out, and it reveals large lumps of dark matter surrounding the visible galaxies, the halos that simulations predicted.
Carolyn Crawford said the first suspects were dim ordinary objects such as planets, gas clouds, brown dwarfs and black holes, known as baryonic matter. Any of them warmer than absolute zero would emit radiation, and in the quantities needed today's detectors would have seen the glow. Carlos Frenk added the clinching argument: the cosmic microwave background measures the amount of ordinary matter to an accuracy of about 1 per cent, and it falls far short of the total mass.
Anne Green described weakly interacting massive particles, or WIMPs, as the most popular and best motivated dark matter candidate. They barely interact with each other or with normal matter, which explains why none has been seen, and they would weigh from a few to a thousand times a proton. They would be produced just after the Big Bang in the right amount, and they appear in particle theories proposed to unify the fundamental forces.
Carolyn Crawford said neutrinos looked promising because they are real, plentiful and uncharged, but experiments put their mass too low, and being light they move close to the speed of light. Such fast hot dark matter would build huge structures first, while the real universe grew from small structures up. Carlos Frenk said his first 1980s simulations of a neutrino universe looked nothing like ours, his first big disappointment as a scientist.
Carlos Frenk of Durham described four steps. Start from the tiny quantum fluctuations left by inflation, assume what the dark matter consists of, instruct the computer to solve the equations of physics including Einstein's relativity, then let it run, often for months. With the right dark matter assumption the results look like the real universe, and he shows colleagues simulated and real galaxy images side by side.
The panel described three approaches. Detectors in underground mines wait for a rare head-on collision between a WIMP and an atomic nucleus, seen as a tiny temperature change or a single flash of light. CERN's Large Hadron Collider might create the particles, though Anne Green said that alone would not prove they make up the dark matter in galaxies. Space instruments look for gamma rays and antimatter from WIMPs annihilating each other.
Carlos Frenk told Melvyn Bragg he expects dark matter to be discovered within five years, comparing the competition to the 1950s race for the double helix and noting detector sensitivity has improved by factors of billions in 30 years. Carolyn Crawford said she could not be as optimistic, since experiments keep improving without finding the particles. Frenk said finding it would rank alongside Darwinian evolution.
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2:07continuous use of Shell V-Power Nitro Plus and gasoline direct injection engines. Actual effects and benefits may vary. See shell.us slash more dash protection for more information. Hello, Misha Glennie here. Welcome to this week's pick from the In Our Time back catalogue. In Our Time explores the wonders of human knowledge, but one thing I find most exciting is finding out what things we still don't know. One of the best examples of this is in the archive edition I've chosen for this week, on dark matter. In 1932, the Dutch astronomer Jan Oort noticed that the speed at which galaxies moved was at odds with the amount of material they appeared to contain. He hypothesized that much of this missing matter was simply invisible to telescopes. It's now called dark matter.
3:02Dark matter is known to make up nearly a third of our universe, while the matter that makes up stars and galaxies and all things around us accounts for only 5%. Melvin and his guests discuss the fascinating theories proposed about dark matter, what's it made of, how it behaves, and what this elusive substance actually is. I hope you enjoy listening. Hello. Something in our universe is missing, or rather almost everything, most of the matter in existence. Scientists first noticed this in the 1930s, observing that galaxies were moving much faster than expected, and at such speed should have dispersed or evaporated. They theorised that there must be something as yet unknown keeping the galaxies in place. The Swiss astronomer
3:48Fritz Zwicky in the 1930s called this missing matter, at first, and later as we know it now, dark matter. At least one of our guests today claims that once we do know what dark matter is, we will have solved one of the greatest mysteries in science, linking the Big Bang with the creation of galaxies, planets, Earth, and everything on it, including us. With me to discuss Dark Matter are Carolyn Crawford, Public Astronomer at the Institute of Astronomy, University of Cambridge, and Gresham Professor of Astronomy. Anne Green, Reader in Physics at the University of Nottingham. And Carlos Frenk, Ogden Professor of Fundamental Physics and Director of the Institute for Computational Cosmology at the University of Durham.
4:30Caroline Crawford, what's the start of the story of the discovery of dark matter? The primary evidence for dark matter is astronomical observations. And as you said in your introduction, the story starts back in the 1930s with the astronomer Fritz Zwicky, who was identifying, classifying, studying clusters of galaxies. And a cluster of galaxies is where you have a whole swarm of galaxies. You've got thousands, hundreds of thousands, all contained within a fairly small volume, a few millions of light years across, a few tens of millions of light years across. And they're all bound together under their sort of mutual gravity. So you have these galaxies that are swarming through, and what Zwicky realised is that he could use the motions of the galaxies to virtually weigh the whole mass
5:14of the cluster. Every galaxy, the way it moves, it orbits through the cluster, it's kind of reacting to the gravitational pull of the rest of the galaxies in the cluster. And as you described, he discovered that they're moving too fast. They're moving at speeds of the order of 1,000 kilometres per second. The whole system should have just dispersed out into space. Unless you've got more mass there, more gravity there, then you would otherwise, I guess, and that mass, that gravity is anchoring everything to keep it as one bound entity. And at that point, he identified this idea of a missing mass. There's extra mass there, there's extra gravity within the system, but it's missing from our normal view through the
5:55telescope when we look at a cluster of galaxies. So having theorised that, the search then began in the 1930s to discover what that was, identify it and line it up with all the other forces. Well, yes, at this stage you're still realising that there is this extra component of a cluster of a gravitationally bound entity that's missing. And Zwicky started with the idea of calling it a missing mass and within a few years the term changed to be one of dark matter, which is what we recognise today. And it comes, again, originally from the observational evidence, from clusters and also from other gravitationally bound systems. And can you just identify for us why it's proving to be so difficult to identify this dark matter?
6:44Yes, and the key here is in the name. Originally it's called dark because it's not luminous. It's very, very faint. That was the original description of it. We can now extend that to say you have some matter. So it has mass. It's got gravity. But the problem is it doesn't interact with light in any way. Not only does it not give off light, it doesn't emit, it doesn't radiate light, but it doesn't reflect light. It doesn't block light to create shadows. It doesn't absorb light. And so you have this matter out there, and it's in huge amounts, sort of five times the amount of the so-called ordinary matter, that we can't see because of the light it produces. And that's a huge problem for astronomers
7:26because light is the only way that we can really test what's out beyond our solar system. And that's why it's so elusive. It's not giving off light. We can't see it with our telescopes. All we can see is the gravitational effect this extra mass has, how it pulls on objects that are luminous. So it's a kind of one step removed. We're inferring it's there from its influence rather than seeing it directly. I made large claims for this. I think it was in the trailer earlier on about without dark matter there'd be nothing at all, no planets, no galaxies. It is as important as that, discovering what it is, is it? It is absolutely fundamental to everything in the universe. Dark matter is what anchors all structures together.
8:05Without dark matter, we couldn't have created galaxies and clusters of galaxies. We wouldn't have the current universe we see if you didn't have dark matter that initiated that process right at the beginning of the universe. So this is the link that you described. You need dark matter right at the early start of the universe in order that these structures start to form and you get anything resembling what we see around us today. Carlos Frank, can you explain what a galaxy rotation curve is and why it's important in this investigation? Yes, the galaxy rotation curves play a key role in the story because they provided evidence that was in many ways neater than Zwicky's evidence. So let me tell you what a rotation curve is.
8:49A galaxy like our own Milky Way is essentially a disk of stars that are rotating round and round the centre of the galaxy. Now, a rotation curve is just a curve that describes how fast stars are moving round the centre at different distances from the centre. Now, according to Newton's theory of gravity, we would expect the stars closest to the centre to be going around faster than the stars further out, just like in the solar system, where all the mass is in the sun, and then Mercury goes around really, really, really fast, much faster than the Earth, which goes Mercury goes around really, really, really fast, much faster than the Earth, which goes around much faster than Jupiter, say. So astronomers were expecting that when they were able to measure the speeds of stars around
9:33the center of the galaxy, they would see exactly that. To their horror, in the 1970s, they found that actually the stars were moving more or less at the same speed, a few hundred kilometers per second, regardless of where they were. And that was immediately recognized as a very serious problem because essentially the stars in the outer parts of the galaxy are just going too fast. And if all the material that produces the gravity was in the stars that we can see, those stars, far-flung stars, should have already been ejected from the galaxy. The galaxy, they should have just been tossed out of the galaxy, but they were there. So it followed that there must have been something we cannot see that is producing the gravity that keeps the galaxy in place,
10:18and that then provided very, very clear-cut, neat evidence, although not accepted by everyone, for the existence of dark matter in galaxies like our own Milky Way. But it's sort of another add-on gravity, isn't it? Another form of gravity? It's another form of gravity. It adds to the gravity that we can see, but it actually overwhelms the gravity of the stars that we can see. So if all there was in the Milky Way was the gravity produced by the stars that we see, then the stars in the outer parts of the Milky Way should be rotating at a much lower speed than they are. So, yes, the dark matter makes a contribution, but it does make the lion's share of the contribution to the gravity in the outer parts of the galaxy.
11:00You mentioned the Milky Way. In the 1970s, there were computer simulations about the Milky Way and discovering dark matter there. Could you tell us about that, Carlos? So they did the first simulations in a computer of a disk of galaxies like our own. And again, to their horror, they found that if all they did was put stars in there, this disk would buckle up. It would crumple up into a kind of horrible looking bar. And they came up with the idea that in order to make galaxies stable, one required this unseen component of dark matter. So this was what we call theoretical evidence, which of course is not as compelling as seeing the real thing, but in this game, it's as good as it gets. So that was another
11:53important piece of evidence for the existence of dark matter in galaxies like the Milky Way. Did they correct their experiments so it didn't buckle next time? Yes, they very ingeniously because it is very primitive. Computers in the 1970s were laughable by today's standards. I mean, the simulations were nothing compared to what we can do today, but they did manage ingeniously to assume there was someone seen, they call it Halo, in the clump of dark matter, and then a beautiful, stable galaxy was possible. So yes, that was another very important advance, that convinced theorists at least. Why did they call it Halo? Well, I don't know whether they feel saintly or... I've never understood why it's called a halo.
12:37It is a clump. I guess clump is not such an elegant word as a halo, but I think the idea is that most of it is in the outer parts, outside the galaxy. But I always think that, you know, us astronomers have saintly tendencies and it's expressed sometimes in our language. Yeah. Anne Green, can we just stay with this galaxy notion, which Carolyn raised at the start? Why is a galaxy of such interest to people like yourselves who are investigating this problem? So galaxy clusters are interesting because they actually tell us quite a lot about the properties of dark matter. So as Carolyn's already told us, what Zwicky's research showed us is that the galaxies we can see maybe only make up a few percent of the total amount of stuff of
13:21matter that's in the galaxy. Now the optical light that we can see with our eyes is only actually a small part of the electromagnetic spectrum. And when you look at galaxy clusters with detectors that are sensitive to different wavelengths, you see some very different things. And so, for instance, in the 1970s, astronomers started looking at galaxy clusters using X-ray telescopes. And what they found is as well as the galaxies we can see, the galaxy clusters contain a large amount of hot X-ray telescopes and what they found is as well as the galaxies we can see the galaxy clusters contain a large amount of hot x-ray emitting gas and there's a balance going on gravity's trying to pull this gas in pressure's trying to stop it collapsing and by looking at this balance between
13:57pressure and gravity you can again weigh the x-ray emitting gas and you can also compare that to the mass of the cluster as a whole and so what they found was there's actually a lot more hot gas in the cluster than there are galaxies about a factor of 10 roughly but still that's not all of the missing mass there's still five or six times as much stuff in the galaxy cluster on the whole as there is this hot gas so that just added some more information about what the dark matter had to be. It wasn't this gas, it was something else. So that's proceeding by elimination, isn't it? You're finding out what it's not. Exactly. We're looking for the things we can see and then still seeing that there's something else there as well.
14:41And so how did the whole business start about looking at something that you can't see? Or trying to find. I'd be more sensible than that. Trying to find something that you can't see. So I guess you have to find other ways of trying to see it in inverted commas. Look for effects it can have which aren't necessarily just seeing light coming from it. There's something called velocity dispersion one of the many phrases I've come across with delight while preparing for this program can you tell us what significance that has? So actually that's something really quite simple dispersion just means spread so when we say that the velocity dispersions are very large we just mean that things are moving faster than we expected. And what does that mean?
15:25So this basically comes back to what Carolyn was already telling us about. It was the velocity dispersions that Zwicky measured and found that they were far, far bigger than you would expect if all the material there was what you could see. Are these something we can learn from, these velocity dispersions? We've learned a lot from them already, but I'm not sure that there's anything more we can learn. What did we learn? That the galaxy clusters weigh far more than the galaxies we can see. It baffles me. Can you just... Is it possible to quickly explain how you're looking at this stuff and you can work out how it weighs, what it weighs? So this is because it's the force of gravity that's moving the things we can see around.
16:03And the gravity is sensitive to everything. It doesn't matter whether we can see it or not. The pull of gravity is just there. So it's looking at the gravitational effects of the stuff we can't see on the stuff we can see. Are you using way in the sense of the word as I would use way? You know, a pound of this or that or the other? More or less. I'm still baffled. It's just mass. So when you say you weigh 10 kilos or 100 or whatever, it just means you have a certain amount of mass. And when you put it on a scale, it registers as weight. But weight and mass, as we learned from Newton and Galileo, are one and the same thing. So when we talk about weighing something, you mean measuring how much gravitating mass
16:48the object contains. So Melvin Bragg has a mass of, I don't know, 80 kilos, and that's your weight as well as your mass. Get it. Fine. Thank you very much for that. Carolyn, can we talk about spiral galaxies and elliptical galaxies? Carlos has already explained how a flat spiral galaxy is rotating. We can use that rotational motions of the stars to work out there's this extra dark matter in a spiral galaxy. You can do something similar with an elliptical galaxy. Now, most galaxies in the universe are these kind of ball-shaped elliptical galaxies. They don't have that neat pattern of rotation, but still you've got a whole... Again, it's a swarm of stars that are just responding to the gravity of the galaxy.
17:32And Anne's mentioned velocity dispersion of galaxies in a cluster. You could look at the motions of the stars within the elliptical galaxy and, again, you find they're responding to much more gravity than there is if you just count up all the... what you assume if you look at all the light and the stars of the the galaxy the great thing about elliptical galaxies is that they're much more massive in spiro galaxies there's much more dark matter there and you don't just have the individual stars and how they move through the galaxies but again like the clusters you have a big halo of x-ray emitting gas. This X-ray gas is at temperatures of millions of degrees. It's a plasma of fast-moving charged particles.
18:10And these should have just, again, dispersed. They've got so much energy, they should just scatter into space, unless you've got more gravity there to anchor them to this galaxy. So it's just another line of evidence that this dark matter is endemic to all galaxies in the universe, whether they be spiral or whether they be elliptical. I see. Carlos, can we talk about the cosmic microwave background and what corroboration that gives us for dark matter and where it takes us? Yes. So the cosmic microwave background radiation is nothing less than the heat left over from the Big Bang. It's quite remarkable. The Big Bang was very hot, and it had lots of radiation. And we know that when the universe was about a mere 350,000 years old,
19:01which would be the equivalent of one day in a human life, so it's still very young compared to its present age, this radiation was just emitted. And as the universe expands, the radiation cools, and this radiation was discovered amazingly in 1964 by two very famous engineers, actually, Penzias and Wilson. And this radiation, which by then had cooled down to a mere 2.7 degrees above absolute zero, because it had been travelling for so long, appears in the form of microwaves. Now, this discovery in the 1960s really nailed down the Big Bang theory, because here we had evidence that the universe had once been very hot and had been expanding and moreover this radiation brings us news about this baby universe. At the turn of
19:54last century one of the most important discoveries of physics ever was made when a NASA satellite mapped the temperature of this microwave background radiation, as we call it, the heat from the Big Bang, and found that the temperature was actually not uniform, but patchy. It had little spots of, little, little, I'll tell you what I mean by that in a minute, but patches of hot and cold radiation. Now, when I say little, these are very, very tiny irregularities. So it was a huge feat of engineering to be able to detect these tiny differences in the temperature of this radiation from one place to another. Now, it turns out that the spotty universe, the spots, tell us about the contents of the universe.
20:40The early universe was much simpler than the universe today. And we can read off this pattern of hot and cold spots, what the universe must have had in order to produce such a pattern. And what we learn from that is that the universe had not only ordinary matter, like the matter of atoms of which we and the sun and everything else that we can see is made of, but there had to be something else, something, some form of elementary particle, different from ordinary atoms. And that is the dark matter. Now, one way to think about it, I like to think about it like, so when, so if you're given a present in a box that's wrapped, and you don't know what's in it, what do we all do? We shake it, right? And from the vibrations in the the box we try to infer what's inside it
21:26but this is very similar these microwave background are sort of vibrations and by looking at the vibrations we can infer what the universe contains so this was really a very convincing evidence that the dark matter not only is there but must be made up of some exotic kind of matter, elementary particles of some kind. That's terrific, isn't it? I mean, I'm just in wonder at all this sort of stuff. That's why we do the programme. Anne Green, can I just take that on from Carlos? 350,000 years in to the existence of... It broke away. Why did it break away then? What caused the breakaway? You said, didn't you? I'm giving this question to Anne, yeah. Stuff starting being emitted. What happened then to make it be emitted?
22:18Right, so up until then, the universe was a very hot, dense place and everything was broken down into nuclei, which are positively charged and electrons which are separate. And if an atom tried to form, a very energetic photon, a particle of light, would come along and kick the electron out of the atom again. So at that point we've got this thick, gloopy mess of particles that are scattering off of each other all the time. However, at that point, then the universe has cooled down enough, the energy has dropped so that atoms can form. Why did it cool down it's basically coming as the universe expands from the big bang the temperature drops what's causing it to drop it's basically conservation of energy you've only
22:55got so much energy and so as things expand the energy has to go down and hence the temperature has to come down okay um so we're on the track now, aren't we, of trying to find out about this dark matter. What's the most convincing, in your view, observational evidence for the existence of dark matter? Well, there are lots of things. We've heard already about galaxy rotation curves and galaxy clusters, but some additional really nice evidence comes from gravitational lensing. So gravitational lensing is a consequence of Einstein's theories of gravity. And so one of the things his general relativity tells us is that mass bends space. And therefore when light travels through space, its path gets
23:35bent. If it hits a big mass, it has to go round it. It's more, you can consider space like a rubber sheet. And so when you put a heavy object in it, it gets bent down. And so the light, as it travels through, travels along the rubber sheet and gets bent towards the heavy object and round it. And so by looking at how the path of light is distorted, you can map out how space is bent and therefore how the matter is distributed. And so what's particularly useful is what's called strong gravitational lensing, when you get something really heavy bending space a lot. And so in particular here, sometimes we're very lucky that we've got a big galaxy cluster, and then a long way behind it is a galaxy.
24:18And so the light from the galaxy, instead of travelling to us in a straight line, gets bent around the galaxy cluster. And so instead of just seeing the galaxy, straight line gets bent around the galaxy cluster. And so instead of just seeing the galaxy the cluster acts like a lens. It creates images. And so you get multiple images of the galaxy distorted into arcs. And by looking at the positions and the features of those arcs you can map out how much space has been bent by and therefore how the matter is distributed. And what you see is you see peaks where we know the galaxies are in the matter distribution, but surrounding the galaxies is a big additional lump of dark matter. And this is the dark matter halos
24:55that Carlos has been telling us about already. So it's not just telling us that the dark matter is there, but it's telling us where the dark matter is, spread out, extended around the galaxies. Carolyn? I think the thing that is important as well is that you have all these different ways of detecting dark matter whether it's from a spiral galaxy, elliptical galaxy whether it's the motions of galaxies in the cluster whether it's through the fantastic gravitational lensing Anne was just describing and all of these involve different detectors different telescopes, they're taken in different wave bands, they're making different telescopes. They're taken in different wave bands. They're making different assumptions about the physics required in the interpretation.
25:32And yet it all comes back to the same basic answer. There's overwhelming evidence that we need more mass there than we see from the light that's available. So when did they start theorising? We've talked about Zwicky mentioning. What did they begin to propose further on from that? Well, if you've got some mass that's incredibly dark, the obvious place to start is that it is some ordinary matter that's just not luminous, and you might hear want to start thinking about things like a planet, or a gas cloud, or perhaps what happens when a star reaches the end of its life and it turns into a black hole, or some other compact object, or even things called brown dwarfs, which are things that didn't quite get massive enough to turn into a star and shine properly.
26:11All of these are made up of ordinary matter. We call this baryonic matter because it's made of atoms and atoms are made of neutrons, protons and electrons, which are known as baryons. So baryonic matter means ordinary matter. So your first idea that is naturally explored is that you've just got vast quantities of rocky planets or lumps of rock or failed stars or black holes. But there are problems with this interpretation where you cannot get the observations to match. Many problems with the interpretations, but the most basic thing is that if you have ordinary matter that's not at absolute zero temperature, it is going to give off some kind of radiation and if it's a planet it could give off infrared radiation a gas cloud maybe would absorb light
26:55and you have the problem now that with today's detectors if there was enough of this ordinary matter in the quantities we need to account for the dark matter, we would have detected the glow from it. So the people, quite rightly now, have largely dismissed the idea that this dark matter is ordinary matter that's just very faint. And then the problem is, of course, you have to go to a much more exotic kind of explanation, and that's when we get to non-baryonic matter. And that's when Carlos comes in with your computer modelling. Yes, so before I tell you about the computer modelling, which I will do in a second with great pleasure, I think to me really what clinches the fact that the dark matter cannot be ordinary matter
27:36is this microwave background radiation that we were talking about before because that unambiguously tells us how much ordinary baryonic matter there is in the universe. No questions asked. It's a really precise measurement with an accuracy of 1%. And it tells us how much total mass there is. And the two years don't add up. The bulk of the mass has to be something different from baryonic dark matter. So to me, that is really the argument that clinches it. In addition to the ones that Carolyn gave. So let me tell you about the computer simulations, which is another. Which is what you do. Yes, what I do. I make my living from that. You sort of make a hell of a reputation on that as well, but never mind. We're certainly living in this studio.
28:18But I work on this day to day, but coming here allows me to step back and realize how amazing it is what we actually do with these computers because what we do essentially is to recreate the entire evolution of our universe. And it sounds grandiose but it is. Now the way we do this is as follows. We now know that when the universe was very young and I really mean very very very young. Much younger in your terms. I'm very suspicious of you a lot with figures. Well, it's a decimal point and then imagine 34 zeros and one, that fraction of a second. Okay, I'll give you young. We need young. Very young. Now, we now know, and we have evidence for this from the microwave background as it happens, that the universe began
29:04with a big, not just a big bang, but a big period when we call this inflation, when it expanded very quickly for a short period of time. And the main thing about inflation, from our point of view, is that this process seeded the universe with tiny little irregularities, what we call of quantum origin, we call these quantum fluctuations. Now these small irregularities are the we call of quantum origin, we call these quantum fluctuations. Now these small irregularities are the initial conditions for everything that evolved in the universe thereafter, for galaxies and for everything else. So the way we do the simulations is quite simple. We have these initial conditions, starting state, which we represent mathematically and feed that
29:42into a big computer. Secondly, we make an assumption about what the dark matter consists of. Thirdly, we instruct the computer on how to solve the equations of physics, Einstein's relativity, and so on. And fourthly, we let it compute, often for months in a row. Big computers can do this. And what comes out at the end, and this really is quite astonishing, are universes, when you make the right assumptions about the dark matter, that look just like the universe in which we live. I know the latest generation of simulations is really quite astonishing, and I like to challenge often my battle-hardened astronomy colleagues by showing them images of galaxies that came out of a computer from this process, from these quantum fluctuations to the present, alongside images of real galaxies.
30:32And I challenge them to tell me which is which. And more often than not, they fail. So we can create realistic universes in the computer that are beautiful, except we know everything about them, so long, so long as we make the correct hypothesis for the nature of the dark matter. So Durham challenges the world and not for the first time. Anne Green, what's the current mainstream view of the particles that make up dark matter? Right, well, particle physicists are very creative. We're very good at inventing particles. Sometimes they turn out to exist, for for instance the neutrino and antimatter and sometimes they don't you just have to go looking so there's a wide zoo of possibilities for the dark matter with a huge range of different masses and different
31:13properties but probably the most popular and arguably the best motivated are things called weakly interacting massive particles or WIMPs for short and they do just isn't that rather unfortunate never mind so I think that was actually the astronomers we've got to blame for the WIMPs for anyway. It wasn't on purpose. Was it? It is because the alternative that Caroline was talking about before were these ordinary matter Jupiters, failed stars. They were called massive astrophysical compact halo objects or MACHOs. So it was MACHOs versus WIMPs. I i see that's where it comes i'll just leave it at that and i interrupted you but anyway for the wimps at least it's a good acronym they do exactly what their name says they're weakly interacting they interact only
31:54weakly with each other and the normal stuff so that would explain why we haven't seen them to date and they're very heavy they weigh maybe a few times a proton, up to a thousand times what a proton does. And they're a good dark matter candidate for two reasons. Firstly, they'd be automatically produced a tiny fraction of a second after the Big Bang in the right amount to be the dark matter. That's somewhat non-tribal. You could have far too many of them or nowhere near enough. But the WIMPs, they have just the right density to be the dark matter. And then the other reason we think these particles might exist is that turns out that they turn up in particle physics models that have been proposed for other reasons and specifically to unify the
32:35four fundamental forces we know about into a single force so that's why WIMPs are probably the best particle dark matter candidate. Carolyn, are there any alternative views about the particles making up dark matter? Well, again, before you move to WIMPs, a lot of which, as Anne says, are quite hypothetical particles that emerge out of theories, one candidate that was proposed was the idea of a neutrino, and at least this is a particle, we know it exists, it fills the universe, it's everywhere. It doesn't have a charge we know it exists it fills the universe it's everywhere it doesn't have a charge so it doesn't interact with radiation and you could think that if each of these tiny particles had a certain amount of mass and there are so many of them trillions of
33:15them that you could account for the dark matter the problem with this is first of all that the upper limits to the mass from experiments are too small now for a neutrino to account for the dark matter. But more fundamentally, if the neutrino is light, it's moving very, very fast. It's moving at close to the speed of light. And this has implications for the size of structures that it starts forming. Carlos has described these computer simulations where you have the dark matter starts clumping together in the early universe. If that dark matter was moving very fast it's very difficult to trap it in small condensations the kind of structures you grow are enormous on the scales of galaxies on the scales
33:55of clusters that's not what we see and it's not what the models predict so you're producing they're very sort of um it's producing top-down, you know, you start with large structures going to small models, where what we see in the universe is the rather inelegantly termed bottom-up version, where small structures start first and grow to larger. So the point about neutrinos is they travel too fast. They predict too large structures that don't fit the computer models or the observations, and they don't have the right amount of mass. So even though neutrinos look good to begin with, I think they're largely discounted now. And then there are other examples of, again, exotic particles. There's the axion, which is an example of one of these
34:38lightweight hypothetical particles that emerges from a theory, but it's very difficult to track and, you know, again, is out of the mainstream ideas of what could cause a dark matter. Carlos, Frank, you've been computer modelling on many things, but including cold dark matter. We have cold dark matter, hot dark matter, and warm dark matter. So you've been concentrating on cold dark matter. Why is that and what have you found? Well, the reason I've been concentrating on cold dark matter is because I started with hot dark matter in the 1980s. The hot dark matter were the neutrinos that Caroline just talked about. And my great, big, first disappointment as a scientist was running the first simulations
35:18of a universe where we had assumed that the dark matter was made of neutrinos, which are also known as hot dark matter, for the reasons Caroline explained, they move very fast. And it was so disappointing when we saw the universe come out of a computer that didn't look anything at all like the universe in which we live. So that was a big disappointment. But this is the 1980s, I was young then, and cocky, and I thought, right, we now ruled out hot dark matter, let's go for the next target, which was cold-dark matter. Let's rule that one out. That's the way you make science. You rule things out in order to eventually be left with the correct assumption. So I set out in the 1980s to rule cold-dark matter out.
35:57And now, 35 years later, I'm still trying to do it. So often I say my career has been a failure because I set out to rule this out and I can't. Now, so cold-dark matter is a very different kind of particle and it is exactly the sort of particles that we need to put into our computer simulations to produce these faithful representations of our universe. Now, however, the case is not closed yet because You've still got a chance. We still get a chance for something in between. And this is fascinating from the point of view of particle physics, because when we look... So, cold dark matter essentially explains everything we see on large scales. It explains something we call the cosmic web, which is the way in which galaxies are distributed in the universe.
36:41They are not distributed at random. They appear, they form along filaments. Yes, and that cosmic wave was actually predicted by the computer simulations in the 80s and 90s and has now been detected in surveys of galaxies. However, the dark matter could still be warm and it's very difficult to tell cold from warm, but we're trying. Sometimes we all need an extra helping hand, especially with a tricky business problem you've been putting off. Claude is the AI for just that. It's the collaborator that understands your entire workflow and thinks with you, not for you. And Claude isn't just for quick questions. Hand it a whole task, like turning a folder of scattered notes into a report, and it does the work and hands back something finished that you can open and edit.
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40:08The ocean connects us. Find your connection at montereybayaquarium.org slash connects. Anne Green, how close a scrutiny will Carlos' experiments stand up to bear? Well, they tell us an awful lot, and in particular the most important thing is that the dark matter has to be cold or maybe a little bit warm, but sort of certainly not too warm. Right. So are we saying that dark matter... What about direct detection? Is that possible? I mean, there's different ways of getting there. And Carlos is ridiculously modest. I mean, he's made an awful lot of progress from the notes I've read. Never mind. What about direct detection? It's almost impossible, just by its definition, to directly detect... As you said earlier on in the programme.
41:03Yeah. These WIMPs, they move through matter. They will flood through the earth all the time. Even my hand here, you'll have several hundreds of thousands passing through my hand every second. It depends on which WIMP you want them to be. They will pass through your detector. So how you need to... The direct detection is really looking for evidence that a WIMP has been there. It's almost like looking for a ghost. You're looking looking for evidence that a wimp has been there it's almost like looking for a ghost you're looking perhaps for evidence from a particle collision that some of the energy and momentum has been carried off by an invisible particle which really is um which reveals the wimp or you're looking for those moments when in passing through all the ordinary
41:40matter there's even though the wimp is tiny and the nucleus of an ordinary atom is tiny, there's a head-on smash. And that head-on smash between the WIMP and the nucleus of an atom imparts some energy, which is then released perhaps as a microscopic temperature change or just like a single photon flash of light. So you must be very excited, Carlos, that turning on CERN today again. Oh yes, absolutely. Because that's indirect evidence, isn't it? Well, actually CERN today again. Oh yes, absolutely. Because that's indirect evidence, isn't it? Well, actually CERN could actually make dark matter. Make it in the laboratory. I was pretty confident that in the first run of CERN they would find it, but they have not in the first run.
42:18But many people believe that they will see evidence, either direct or indirect, by discovering something called supersymmetry, which is the theory that predicts the WIMPs. One of the theories that most naturally predicts the WIMPs. So I'm hopeful and perhaps I'm an optimist that in the next six months to a year, the newspaper headlines will say the dark matter has been made in Geneva, of all places. Well, was Ricky started his first... No, he was in America. He was in America, but he was Swiss. He came from there, yes. Anne Green. So, as Carlos says, it would be fantastically exciting if the LHC managed to make WIMPs, but on its own, that wouldn't solve the dark matter problem because it wouldn't actually tell us that those particles
42:58were the dark matter in galaxies and galaxy clusters and across the universe. So, that would be fantastic, but it would only be the first step. We'd still want to do the sorts of things that Carolyn's been talking about, as in the direct detection experiments in the lab, trying to look for the dark matter particles themselves interacting with nuclei in the lab. Are there other groups of people doing similar or analogous computations, computer computations, as you're doing? Carolyn, do you want to talk about other work that's going on? Yeah, well, with any result, you always want to compare it to other simulations. And so there are several groups who are doing amazing computer simulations. And so it's an experiment.
43:43So in the same way that you have many different computer simulations competing and you hope agreeing with the answer it's the same way as you will have many different kinds of experiments and different ways of looking for the direct detection of the dark dark matter you hope that they all come to the same same answer sorry carl do you want to come in well i was going to say, just to supplement these detection experiments that we just heard from Anne and from Caroline, that there is another way that one could potentially detect dark matter. And I thought I'd done it, actually, but it turned out not to be the case. And that is this. Like everywhere in nature, there are always exceptions. We said that dark matter is dark. Well it is dark
44:26most of the time and in most places. Occasionally it can shine. Would you like me to tell you about that? I'm a gog. What happens is this. If the dark matter is a wimp the chances are that these are very strange particles because they're their own antiparticles. Now, you know that when matter and antimatter come together, they blow up, they annihilate, producing a puff of radiation. That always baffled me while we were here. No, never mind. Well, it's because the universe, for reasons we're beginning to understand, is mostly made of matter, not antimatter. But the microwave background radiation we were talking about before came from annihilation in the early universe. But let me get back to our story here.
45:12So the dark matter is very likely its own antimatter, but it's so diffuse that particles of matter and antiparticles of dark matter never collide, except in very extreme situations, like, for example, in the center of the Milky Way. There, the densities are so huge that the particles of dark matter actually collide with one another. And because they're on antimatter, they produce radiation, very energetic radiation, what we call gamma rays, which is radiation even more energetic than x-rays. In 2005 NASA launched a satellite, the Fermi satellite, to look for gamma rays, for gamma radiation. And there are now claims that the centre of the Milky Way is glowing in gamma rays and that that is a signal of
46:01dark matter in the centre of the Milky Way. And what other forms of detection are there? Well, when the WIMPs come together and annihilate, as Carlos has just described, as well as producing high-energy gamma rays, they can also produce antimatter, things like positrons and antiprotons. There are also experiments, for instance, something called AMSO2 on the International Space Station that are looking for the antimatter. And in that case, we possibly have seen an excess in positrons. But what's not clear there is whether it's due to dark matter annihilating. It turns out there are also possible astrophysical explanations like pulsars and supernovae. So in that case, it's not so clear that that's a sign of dark matter.
46:43On a range from 1 to 10, Carolyn, how far are you along the path to getting to where you hope you'll get to about understanding dark matter? Well, it's one of these things that it could change in the next few months if the LHC is successful. These experiments are running all the time, either on the space station or in mines underground, looking for evidence of WIMPs either from annihilating or colliding with atomic nuclei. Anything could change. It's really exciting. We could know the result in a month. We might have to wait 20 years. We're all poised for exciting news, but we don't know when it's going to come. When we do know, Carlos Frank, what's it going to do to the nature of our understanding of the universe?
47:24I think it'll completely change our perspective of who we are and why we're here, because the discovery of the dark matter would corroborate this amazing cosmic story that we've been unveiling over the last 30 years, in which the dark matter is the main protagonist, because it is the dark matter that is responsible for the formation of galaxies. Without dark matter, there'll be no galaxies, no stars, no planets, no people. So to me, our origins are intimately linked to the nature of the dark matter. So to me, the discovery of the dark matter would be an advance to human knowledge on the same level as the discovery of Darwinian evolution. Well, thank you very much, Caroline Crawford, Anne Green and Carlos Frenk.
48:08We have a new publisher for In Our Time, Simon Tillotson. His predecessor, Thomas Morris, is moving on, and alas, out of the BBC. For five years, Tom was a wonderful man to work with, and a fine producer of this programme. Widely appreciated. He'll be much missed. Next week, we'll be discussing the great 11th century Persian intellectual Al-Khazali. Thanks for listening. And the In Our Time podcast gets some extra time now, with a few minutes of bonus material from Melvin and his guests. No, Tom was great. I remember Tom, yes. He was great. Yeah, he was. Well, thank you very much. Did we get most things? Usually when we have this little PS, I'm told about the things we didn't talk about. So what didn't we talk about that was important?
48:50I thought we covered everything. Did you like it? I loved it. The only thing I'm stuck in at the end is it's also important from, we've focused on the astronomy, but these are new exotic particles that we don't know exist. So the particle physicists will also be really happy should we find dark matter. Because when we find it it it's going to it's new physics beyond the standard model it will give us some clue about unifying gravity with the other forces so it will be a complete door to a completely new world of physics you don't sound convinced no no I'm not convinced that we will know about quantum gravity or about things like that but It'll tell us something which will point us in the right direction
49:28rather than all going in the... I think the... So, you know, I've said this for a long time. The discovery of the dark matter is within the next five years to be expected. I've been saying this for 15, but now I really mean it. Because, I mean, there really is a race. It's a little bit like, I'm sure it must have been like this in the 1950s with the double helix, that people knew something like that had to be there, and there were groups everywhere trying to get to it. Here we have the same situation. There are groups all over the world competing. It's a real race because... Where are you? Should I put money on you then, Carlos? Yeah, sure. I think the... I would say, I really... I mean, looking at the sensitivity of these experiments,
50:06how they've improved over 30 years, I mean, they've improved in sensitivity by factors of billions. And I think that now the experiments in the regime where you do expect these particles to show up, and if they don't, then that would really be very, very bad news for physics. But, I mean, we're getting to the point... Well, if they don't, they... Sorry, Carla. Well, sorry, I was going to say, with all of these things, they've all given tantalising evidence which could be dark matter. But I mean, it's like these facts I talked about where they're looking for the signal of the collision. They're running and the first results, they discovered nothing. The gamma rays that you described from the centre of the galaxy,
50:42there could be other sources for that light in the centre of the galaxy. Similarly, the positrons that Anne mentioned on the cosmic ray detectors, again, they could be from dark matter annihilation. It's all still tantalisingly circumstantial. Should we have had that in the programme? Have we misled our listeners? No. Maybe I'm not as optimistic as Carlos. I think that as we go, we keep pushing the experiments further and further and we're still not finding these wretched particles. No, look, you didn't expect to find them. But listen, Carol, you know full well, nature doesn't give up its secrets cheaply. It makes you work for them. That's true. But if you work hard enough, it reveals them to you. So I think
51:26we're just working hard enough now. And again, these are very, very difficult experiments. It took 30 years to discover these temperature irregularities in the microwave background. We knew they had to be there, but the experiments had to be built. So it's only 30 years. The first dark matter experiments were in 1982, so we're only 32 years, 33 years since. And this is difficult, but they have to be there. The universe doesn't work without it. I must say, I love the fact that you're working in Durham, where people used to come. It was one of the great shrines in the early Middle Ages where people came for miracles. It's a good track record, though. You should be okay. Anyway, here's Simon, our new man.
52:08Thank you for listening to this archive episode of In Our Time. Every Monday, I revisit one of more than 1,000 episodes in our archive. And on Thursdays, you'll still find brand new episodes of In Our Time. Listen first on BBC Sounds or wherever you get your for you. And Claude isn't just for quick questions. Hand it a whole task, like turning a folder of scattered notes into a report, and it does the work and hands back something finished that you can open and edit. Claude can now draft a doc or deck right in front of you, and Claude Design now lives right inside Claude. Describe the website or prototype you're picturing, attach your design system, and it comes back in your own design language,
53:05ready to edit right on the canvas. It works with your files and the tools you already use, like Gmail and Slack. And whether you're debugging code at midnight, building a financial model, or strategizing your next business move, Claude extends your thinking to tackle the problems that matter. For problems worth solving, get started with Claude at claude.ai slash solve. That's claude.ai slash solve. And check out Claude Pro, which includes access to all of the features mentioned in today's episode. claude.ai slash solve.