Archive: Dark Matter, the mysterious substance in space

Dark Matter is a proposed form of matter that does not absorb, reflect or emit light. It is known to make up 27% of our universe, while the matter that makes up stars and galaxies accounts for only 5%. In this archive episode, first released in March 2015, Melvyn Bragg and his guests discuss dark matter, the mysterious and invisible substance existing throughout the Universe. 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. Today astronomers and particle physicists are still fascinated by the search for dark matter and the question of what it is. One leading theory is that Dark Matter is made up of WIMPS (Weakly Interacting Massive Particles) that are big, heavy and move slowly, interacting with gravity but not other particles. Scientists have developed multiple theories about the make up of this substance, and they are able to test them through remarkable computer modelling that simulates the construction of the universe. With Carolin Crawford Public Astronomer at the Institute of Astronomy, University of Cambridge and Gresham Professor of Astronomy Carlos Frenk Ogden Professor of Fundamental Physics and Director of the Institute for Computational Cosmology at Durham University Anne Green Reader in Physics at the University of Nottingham Producer: Thomas Morris In Our Time is a BBC Studios production A world-class seminar every week. Spanning history, religion, culture, science and philosophy, In Our Time celebrates the pursuit of knowledge and the enduring power of ideas. In each episode, host Misha Glenny and expert guests explore the characters, events and discoveries that have shaped our world.

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• 2:20Speaker 04

  Hello, Misha Glenny 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 hypothesised that much of this missing matter was simply invisible to telescopes. It's now called dark matter.

• 3:04Speaker 04

  Dark 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.

• 3:30Speaker 05

  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 Fritz 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.

• 4:11Speaker 05

  With me to discuss dark matter are Caroline 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 Franch, Ogden Professor of Fundamental Physics and Director of the Institute for Computational Cosmology at the University of Durham. Caroline Crawford, what's the start of the story of the discovery of dark matter?

• 4:38Speaker 06

  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 realises is that he could use the motions of the galaxies to virtually weigh the whole mass of the cluster. Every galaxy, the way it moves, it orbits through the cluster, it's kind of reacting

• 5:22Speaker 06

  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 than you can, you would otherwise guess. And that mass, that gravity's anchoring everything to keep it as a 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 telescope when we look at a cluster of galaxies.

• 6:00Speaker 05

  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.

• 6:11Speaker 06

  Well, yes, at this, 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.

• 6:37Speaker 05

  And can you just identify for us why it's proving to be so difficult to identify this dark matter?

• 6:46Speaker 06

  Yes, and the key here is in the name. Originally, it's called dark because it is, 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, because light is the only way that we can

• 7:30Speaker 06

  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.

• 7:50Speaker 05

  I made large claims for this in, 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, isn't it?

• 8:00Speaker 06

  It is absolutely fundamental to everything in the universe. Dark matter is what anchors all structures together. Without 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.

• 8:28Speaker 05

  And Carlos Franch, can you explain what a galaxy rotation curve is and why it's important in this investigation?

• 8:37Speaker 04

  Yes, the galaxy rotation curves play a key role in, 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. A 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 around 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

• 9:22Speaker 04

  the sun. And then 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 the centre 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 kilometres per second, regardless of where they were. And that was immediately recognised 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,

• 10:04Speaker 04

  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. And 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.

• 10:29Speaker 05

  But it's sort of another add-on gravity, isn't it? Another form of gravity?

• 10:34Speaker 04

  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, it makes a contribution, but it does make, it makes the lion's share of the contribution to the gravity in the outer parts of the galaxy.

• 11:02Speaker 05

  You 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?

• 11:10Speaker 04

  Oh, yes. When I look at the simulations, it's pretty astonishing because the foresight of these two Princeton physicists, Jerry Alstiker and Jim Peebles, was really quite amazing. 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, these discs 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.

• 11:54Speaker 04

  So that was another important piece of evidence for the existence of dark matter in galaxies like the Milky Way.

• 12:01Speaker 05

  Did they correct their experiment so it didn't buckle next time?

• 12:03Speaker 04

  Yes, they very ingeniously, because these are 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 some unseen, they call it halo, a clump of dark matter, and then a beautiful, stable galaxy was possible. So yes, that was another very important advance, convinced theorists at least.

• 12:31Speaker 05

  Why did they call it halo?

• 12:33Speaker 04

  Well, I don't know whether they feel saintly, or I've never understood why it's called a halo. It 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, all astronomers have saintly tendencies, and it's expressed sometimes in our language.

• 12:54Speaker 05

  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?

• 13:08Speaker 06

  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 Wiki's research showed us is that the galaxies we can see maybe only make up a few per cent of the total amount of stuff of matter 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-emitting gas.

• 13:51Speaker 06

  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 pressure 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.

• 14:32Speaker 05

  So that's proceeding by elimination, isn't it? You're finding out what it's not.

• 14:36Speaker 06

  Exactly. We're looking for the things we can see, and then still seeing that there's something else there as well.

• 14:42Speaker 05

  And 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 on that, trying to find something that you can't see?

• 14:54Speaker 06

  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.

• 15:05Speaker 05

  There's something called velocity dispersion, one of the many phrases I've come across with delight while preparing for this programme. Can you tell us what significance that has?

• 15:13Speaker 06

  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 expect it.

• 15:25Speaker 05

  And what does that mean?

• 15:27Speaker 06

  So this basically comes back to what Carolyn was already telling us about. It was the velocity dispersions that Wiki measured and found that they were far, far bigger than you would expect if all the material there was what you could see.

• 15:39Speaker 05

  Are these something we can learn from, these velocity dispersions?

• 15:43Speaker 06

  We've learnt a lot from them already, but I'm not sure that there's anything more we can learn.

• 15:48Speaker 05

  What did we learn?

• 15:49Speaker 06

  That the galaxy clusters weigh far more than the galaxies we can see.

• 15:53Speaker 05

  Can you just, is it possible to quickly explain how you're looking at this stuff, and you can work out how it weighs?

• 15:59Speaker 06

  So this is because it's the force of gravity that's moving the things we can see around, and 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.

• 16:17Speaker 05

  Are you using weigh in the sense of the word, as I would use weigh, you know, a pound of this or that or the other?

• 16:23Speaker 06

  More or less.

• 16:25Speaker 05

  I'm still baffled.

• 16:27Speaker 04

  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 just mean measuring how much gravitating mass the object contains. So Melvyn Bragg has a mass of, I don't know, 80 kilos, and that's your weight as well as your mass.

• 16:58Speaker 05

  Get it. Fine, thank you very much for that. Carolyn, can we talk about spiral galaxies and elliptical galaxies?

• 17:06Speaker 06

  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. And 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 in the stars of the galaxy.

• 17:50Speaker 06

  The great thing about elliptical galaxies is that they're much more massive than spiral 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, and 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, you know, whether they be spiral or whether they be elliptical.

• 18:31Speaker 05

  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?

• 18:42Speaker 04

  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, which would be the equivalent of one day in a human life, so 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...

• 19:29Speaker 04

  Which by then had cooled down to a mere 2.7 degrees above absolute zero because we'd 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. In around, at the turn of last century, one of the most important discoveries of physics ever was made, when a NASA satellite looked, 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.

• 20:14Speaker 04

  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. The 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 learned from that is that the universe had not only ordinary matter, like

• 20:58Speaker 04

  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 of 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, and from the vibrations in the box, we try to infer what's inside it. Well, this is very similar. These microwave background, that sort of vibrations, and by looking at the vibrations, we can infer what the universe contains.

• 21:38Speaker 04

  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.

• 21:49Speaker 05

  That's terrific, isn't it? I mean, I'm just in wonder at all this sort of stuff, it'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? He said it broke, 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?

• 22:20Speaker 06

  Right, 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.

• 22:46Speaker 05

  Why did it cool down?

• 22:48Speaker 06

  It's basically coming, as the universe expands from the Big Bang, the temperature drops.

• 22:54Speaker 05

  What's causing it to drop?

• 22:55Speaker 06

  It's basically conservation of energy. You've only got so much energy, and so as things expand, the energy has to go down, and hence the temperature has to come down.

• 23:03Speaker 05

  Okay, so we're on the track now, aren't we? We're trying to find out about this dark matter. What's the most convincing, in your view, observational evidence for the existence of dark matter?

• 23:14Speaker 06

  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 bent.

• 23:38Speaker 05

  If it hits a big mass, it has to go round it.

• 23:41Speaker 06

  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. And so the light from the galaxy, instead of travelling to us in a straight line, gets bent around the galaxy cluster.

• 24:26Speaker 06

  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 that Carlos was 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.

• 25:07Speaker 05

  Carolyn.

• 25:08Speaker 06

  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 wavebands, they're making different assumptions about the physics required in the interpretation, and 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.

• 25:39Speaker 05

  So when did they, when did they start theorising? We've talked about Zwick, you mentioned him. What did they begin to propose further on from that?

• 25:48Speaker 06

  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 here 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. All 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

• 26:30Speaker 06

  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 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, and 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. And so the people, quite rightly now, have largely dismissed the idea that this dark

• 27:13Speaker 06

  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.

• 27:25Speaker 05

  And that's when Carlos comes in with your computer modelling act.

• 27:28Speaker 04

  Yes, so before I tell you about the computer modelling, which I will do in a second with great pleasure, the, I think, to me, the, really what clinches the fact that the dark matter cannot be ordinary matter is 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.

• 28:09Speaker 05

  Which is what you do.

• 28:12Speaker 04

  Yes, what I do, I make my living from that, and.

• 28:15Speaker 05

  Well, actually, you're sort of making a hell of a reputation on that as well, but never mind, we're certainly a living in this studio.

• 28:20Speaker 04

  But, you know, from, I work on these day to day, and so, but coming here allows me to step back and realise 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.

• 28:46Speaker 05

  Much younger in your terms, I'm very suspicious of you a lot with figures.

• 28:49Speaker 04

  Well, it's a decimal point, and then imagine 34 zeros and one, that fraction of a second.

• 28:56Speaker 05

  Okay, I'll give you young, we needn't go.

• 28:58Speaker 04

  Very young, fine. Now, we now know, and we have evidence for this, from the microwave background, as it happens, that the universe began with 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 the 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

• 29:43Speaker 04

  and feed that into a big computer. Secondly, we make an assumption about what the dark matter consists of.

• 29:52Speaker 04

  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 for 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 now, 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. And I challenge them

• 30:34Speaker 04

  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.

• 30:51Speaker 05

  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?

• 30:59Speaker 06

  Right. Well, particle physicists are very creative. We're very good at inventing particles. Sometimes they turn out to exist, 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 huge range of different masses and different properties. 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-

• 31:25Speaker 05

  Isn't that rather unfortunate? Never mind.

• 31:27Speaker 06

  So I think that was actually the astronomers we've got to blame for the WIMPs, but anyway.

• 31:30Speaker 04

  It was all on purpose.

• 31:31Speaker 05

  Was it?

• 31:32Speaker 04

  Yes, because the alternative that Caroline was talking about before, where these ordinary matter, Jupiters, fake stars, they were called massive astrophysical compact halo objects, or Machos. So it was Machos versus WIMPs.

• 31:46Speaker 05

  I see.

• 31:47Speaker 04

  That's where it comes in.

• 31:48Speaker 05

  I'll just leave it at that. And I interrupted you.

• 31:49Speaker 06

  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 weakly 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-triple. 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 it turns out that they

• 32:31Speaker 06

  turn up in particle physics models that have been proposed for other reasons, and specifically to unify the four fundamental forces we know about into a single force. So that's why WIMPs are probably the best particle dark matter candidate.

• 32:45Speaker 05

  Caroline, are there any alternative views about the particles making up dark matter?

• 32:50Speaker 06

  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, 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 them, 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

• 33:35Speaker 06

  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 of clusters. That's not what we see, and it's not what the models predict. So you're producing, they're very sort of, 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'd

• 34:19Speaker 06

  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 lightweight 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 the dark matter.

• 34:50Speaker 05

  Carlos, Frenk, you've been computer modeling 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?

• 35:02Speaker 04

  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 of 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. It 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

• 35:47Speaker 04

  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. And now, thirty five years later, I'm still trying to do it. So often I say my career's 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 of our universe. Now, however, the case is not closed yet, because-

• 36:25Speaker 05

  You've still got a chance.

• 36:26Speaker 04

  We still get a chance for 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. They are not distributed at random. They appear, they form along filaments. Yes. And that cosmic web was actually predicted by the computer simulations in the 80s and 90s, and has now been detected in 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 the tricky business

• 37:11Speaker 04

  problem you've been putting off. Claude is 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. 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 your picturing, attach your design system, and it comes back in your own design language, ready 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,

• 37:54Speaker 04

  or strategizing your next business move, Claude extends your thinking to tackle the problems at matter. For problems worth solving, get started with Claude at claude.ai/solve. That's claude.ai/solve. And check out Claude Pro, which includes access to all of the features mentioned in today's episode. Claude.ai/solve.

• 38:16Speaker 03

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by Miguel.

• 38:35Speaker 01

  Man, I'm a black and Mexican kid from San Pedro, and I grew up in Inglewood as well. I never liked to be like boxed in. Both of my parents colored outside of the line. I mean, even them being together at the time in LA was just not typical. And so I think that has always been a part of my DNA. There's a defiant something in me that feels like what we get to do with with what we're given that's greater than us is to maybe connect people to ideas that they wouldn't naturally, you know, connect to. That's really me. I'm just kind of like defiant with love, though.

• 39:11Speaker 03

  Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.

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• 40:52Speaker 05

  Anne Green, how close a scrutiny will Carlos's experiments stand up to Bär?

• 41:03Speaker 06

  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 certainly not too warm.

• 41:13Speaker 05

  Right. Carlos, 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, right, okay? Never mind. What about direct detection?

• 41:30Speaker 06

  It's almost impossible, just by its definition, to directly detect...

• 41:34Speaker 05

  As you said earlier on in the program.

• 41:36Speaker 06

  Yeah. These wimps, they move through matter. They will flood through the earth all the time. You know, even my hand here, you'll have several hundreds of thousands passing through my hand every second. You know, 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 perhaps for evidence from a particle collision that some of the energy and momentum's been carried off by an invisible particle, which reveals the wimp. Or you're looking for those moments when, in passing through all the ordinary matter, there's... Even though the wimp is tiny and the nucleus of an ordinary atom is tiny, there's a head-on smash.

• 42:20Speaker 06

  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.

• 42:31Speaker 05

  So you must be very excited, Carlos, that turning on CERN today again.

• 42:35Speaker 04

  Oh, yes, absolutely.

• 42:36Speaker 05

  Because that's indirect evidence, isn't it?

• 42:38Speaker 04

  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. But 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, perhaps I'm an optimist, that in the next six months to a year, then newspaper headlines will say the dark matter has been made in Geneva of all places.

• 43:15Speaker 05

  Well, as Ricky started his first... No, he was in America.

• 43:18Speaker 04

  He was in America, but he was Swiss. He came from there, yes.

• 43:20Speaker 05

  Anne Green.

• 43:21Speaker 06

  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 were 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.

• 43:52Speaker 05

  Are there... There are your computations. Are there other groups of people doing similar or analogous computations, computer computations you're doing? Carolyn, do you want to talk about other work that's going on?

• 44:05Speaker 06

  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. So it's an experiment. So 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 matter. You hope that they all come to the same answer.

• 44:32Speaker 05

  Sorry, Carlos, you want to come in?

• 44:34Speaker 04

  Well, I was going to say, just to supplement these detection experiments that we just heard from Anne and from Carolyn, that there is another way that one could potentially detect dark matter. And I thought I'd done it, actually, but it tend not to be the case. And that is this. Like everywhere in nature, there are always exceptions. We said the dark matter is dark. Well, it is dark most of the time and in most places. Occasionally, it can shine. Would you like me to tell you about that?

• 45:07Speaker 05

  I'm agog.

• 45:09Speaker 04

  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.

• 45:28Speaker 05

  That's always baffled me while we're here now, never mind.

• 45:30Speaker 04

  Well, it's because the universe, for reasons we're beginning to understand, is mostly made of dark matter, sorry, of matter, not antimatter. But the microwave background radiation we were talking about before came from an annihilation in the early universe. But let me get back to our story here. So the dark matter is almost... It's 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

• 46:13Speaker 04

  we call gamma rays, which is radiation even more energetic than X-rays. 2005, NASA launched a satellite, the Fermi satellite, to look for gamma rays, for gamma radiation. And there are now claims that the center of the Milky Way is glowing in gamma rays and that that is a signal of dark matter in the center of the Milky Way.

• 46:37Speaker 05

  And what are the forms of detection, Alba?

• 46:40Speaker 06

  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.

• 47:16Speaker 05

  And right 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?

• 47:24Speaker 06

  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. It's... You know, we're all poised for exciting news, but we don't know when it's going to come.

• 47:52Speaker 05

  And when we do know, Carlos Frenk, what's it going to do to the nature of our understanding of the universe?

• 47:57Speaker 04

  Well, I 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'd 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.

• 48:37Speaker 05

  Well, thank you very much, Carolin Crawford, Anne Green, and Carlos Frenk. We have a new publisher for In Our Time, Simon Tilletson. His predecessor, Thomas Morris, is moving on, and alas, out of the BBC. For five years, Thomas was a wonderful man to work with and a fine producer of this program, widely appreciated. He'll be much missed. Next week, we'll be discussing the great 11th-century Persian intellectual Al Qasri. Thanks for listening.

• 49:06Speaker 05

  With a few minutes of bonus material from Melvin and his guests. No, Tom was great.

• 49:11Speaker 04

  I remember Tom, yes.

• 49:12Speaker 05

  Yeah.

• 49:12Speaker 04

  He was great, yes.

• 49:13Speaker 05

  Yeah, he was, he was, he was. Well, thank you very much. Did we get most of the... Usually when, 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?

• 49:23Speaker 04

  I thought we covered everything. Do you like it?

• 49:25Speaker 05

  I loved it.

• 49:26Speaker 06

  The only thing I'm stuck in at the end, it's also important from, we've focused on the astronomy, but you know, 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's going to probably...

• 49:41Speaker 04

  It's new physics beyond the standard model. It'll give us some clue about unifying gravity with the other forces.

• 49:46Speaker 06

  Yeah, so it'd be a complete door to a complete new world of physics. You don't sound convinced.

• 49:52Speaker 04

  No, no, I'm not convinced that we will know about quantum gravity or about things like that, but... It'll tell us something, it'll point us in the right direction rather than all going in the... I, I think the, so, 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, but 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.

• 50:18Speaker 05

  Yeah.

• 50:18Speaker 04

  That people knew something like that had to be there and there are groups everywhere trying to get to it. Here we have the same situation. There are groups all over the world competing. It's a, it's a, it's a real race because...

• 50:29Speaker 05

  Where are you? Are you, are you, would you, should I put money on you then, Carlos?

• 50:33Speaker 04

  Yeah, sure. The, I, I think the, I would say, I really, I mean, looking at the sensitivity of these experiments, now how they've improved over 30 years. I mean, they improved in sensitivity by factors of billions. And I think there 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.

• 50:54Speaker 06

  But I mean, we're getting to the point...

• 50:55Speaker 05

  Well, if they don't, I mean, sorry, Carlos.

• 50:56Speaker 06

  Well, so I was going to say, with all of these things, they've all given tantalizing evidence which could be dark matter. But I mean, it's like these, these facts I talked about where they're looking for the signal of the, you know, the collision. They're, they're running and the first results, they discovered nothing. The gamma rays that you described from the centre of the galaxy, there 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.

• 51:31Speaker 05

  Should we have heard that in the programme? Have we misread, misled our listeners?

• 51:35Speaker 04

  No.

• 51:36Speaker 06

  Well, I don't, and maybe I'm not as optimistic as Carlos. I think that as, as we go, we keep pushing the experiments further and further and we're still not finding these wretched particles.

• 51:46Speaker 04

  No, look, you didn't expect to find them. But listen, Carlos, you know full well, nature doesn't give up its secrets cheaply. It makes you work for them.

• 51:55Speaker 06

  That's true.

• 51:56Speaker 04

  But if you work hard enough, it reveals them to you. So I think we're just working hard enough now. And you know, it's a very, very difficult experiment. It took 30 years to discover these temperature irregularities in the microwave background. We knew they had to be there and the, 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.

• 52:17Speaker 06

  Yeah.

• 52:17Speaker 04

  And this is difficult, but I, I, they have to be there, they have to be. The universe doesn't work without it.

• 52:22Speaker 05

  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. The tomb of comfort. So you're really, it's a good track record there. You should be okay. Anyway, here's Simon, our new man. Thank you for listening to this archive episode of In Our Time. Every Monday, I revisit one of more than a thousand episodes in our archive. And on Thursdays, you'll still find brand new episodes of In Our Time.

• 52:52Speaker 06

  Listen first on BBC Sounds or wherever you get your BBC podcasts.

• 53:00Speaker 06

  Sometimes we all need an extra helping hand, especially with a tricky business problem you've been putting off. Claude is an 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. 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, ready to edit right on the canvas. It works with your files and the tools you already use, like Gmail and Slack.

• 53:43Speaker 06

  And whether you're debugging code at midnight, building a financial model or strategising 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/solve. That's claude.ai/solve and check out Claude Pro, which includes access to all of the features mentioned in today's episode. Claude.ai/solve.