Why does dark matter matter?

Thousands of people die every year waiting on transplant lists. And lungs are in particularly short supply. Now scientists might have found a way to increase the numbers of donor organs that are suitable for transplantation. In experiments using pigs, which have lungs very similar to ours, Matthew Bacchetta from Vanderbuilt University has found that if he takes injured lungs that would normally be unsuitable for transplant, and plumbs them in to a potential recipient’s circulatory system for a day or so - but keeps the lungs outside the body in a special organ chamber - nourished by the healing effects of the blood supply, they recover very rapidly to a state that means they can then be moved inside the recipient. Matthew spoke to Chris Smith...

Matthew - Lungs are extremely sensitive to injury from gastric aspiration, pulmonary contusion, meaning that the lung gets bruised, getting infected while the patient is on a ventilator so they could develop a bacterial infection like pneumonia, and those are the primary reasons that organs are deemed unacceptable for transplantation. The major thrust of what we've done here is replicate the injury that we see in humans. We used what we call a gastric aspiration; what that basically means is that the patient has taken gastric contents, which are very acidic and caustic, into their lungs and it causes inflammation like a severe pneumonia so that the organ cannot be used. And what our system has enabled is the organ to regenerate or repair itself over time.

Chris - How have you done it?

Matthew - We first failed a lot trying different types of ex vivo systems, meaning the organ was placed outside the body separated into a sort of machine perfusion system. And after being very frustrated and failing repeatedly, we eventually sort of had the 'eureka' moment where we said we can't replicate a whole system but what we can do is attach the organ to a natural host or recipient. In other words, the organ could be attached to somebody who needs potentially a lung transplant and that body provides the whole natural system required for wound healing. What we have essentially done is plumbed this organ into the potential recipient who provides all of the critical factors in their blood that enable the organ to heal.

Chris - So where did the lungs sit then, are they in the bath next to what will be the patient? When you actually come do this you’re gonna end up with some tubes coming out of the individual bringing the blood to and from these lungs which are going to be outside their body next to them?

Matthew - That's correct. That's exactly what we do. They are in a specialised container - it actually looks very similar to what we would do for a patient that was on dialysis, they would have blood coming out, it would go into the dialysis machine and then that blood is returned to the patient.

Chris - Does the lung breathe as well as? Are you pushing air in and out of the lungs you do this to keep it natural and what it would be expecting were it inside the body, to create as much a mimic for what the real body environment would be like?

Matthew - It does. We connect it to a mechanical ventilator and we can actually measure the performance of the organ in real time.

Chris - So you're looking at how much oxygen is getting pushed into the blood that your pushing through it by that set of lungs, so that gives you a marker for how well they're behaving and what the improvement is?

Matthew - That's correct, exactly. So we can monitor that over time and that gives us a benchmark to monitor the improvement process and to also let us know when we've reached a level that's normal.

Chris - Why is this better than just putting the lungs into the individual full stop? Because you're basically doing the same thing, you're sending them a blood supply, you're sending them an air supply, why is that better doing it outside body than just putting them in?

Matthew - Yeah, that's a great question. The major difference is that you have to subject the patient to a very invasive procedure. I have to remove their lungs, I have to put in new lungs, and we know that the lungs are damaged, that they're not really acceptable for a transplant. And then I have to support that patient with damaged lungs which causes a profound inflammatory process and so the body actually does not actually work as effectively in healing those lungs and the patient becomes unstable because they're now relying upon an injured organ to keep them alive.

Chris - And how did the recipient fare while they've got this extra set of lungs hitched up to them, and not just any old lungs, someone else's lungs, some other animal's lungs, and diseased lungs at that? Was there an obvious burden on the individual or did they cope well?

Matthew - They actually coped remarkably well. They were hemodynamically stable, meaning that their blood pressure, their heart rate, and all of the other physiologic measures that we use were normal and stable.

Human beings are very talented at using a cognitive skill called “transitive inference” - using information about things you know to draw conclusions about things you don’t know. For example, if you know that A is bigger than B, and B is bigger than C, you can tell that A is bigger than C without having to look at them side by side. We know humans can do this, but it’s something of an open question which other animals can do it as well. Professor Elizabeth Tibbetts from the University of Michigan has been investigating whether one of mankind’s most maligned foes, the wasp, is capable of using this advanced cognitive technique. She spoke to Ben McAllister...

Elizabeth - Long ago people thought that transitive inference was based on logical reasoning and we thought only humans were capable of transitive inference and, not too surprisingly, before long we found that humans were not the only ones. It turns out that a huge range of vertebrates can do transitive inference so primates and birds and even fish.

Ben - Wow. So it lives in that bucket of things that we used to think were kind of unique to the human experience, but we are rapidly learning is becoming a much much smaller bucket?

Elizabeth - It's a very small bucket I think. There had been one study on transitive inference in a non-vertebrate and that was done in honeybees, and they found that bees couldn't do transitive inference. And so I think that wasps are way smarter than bees so I wanted to test whether wasps could do it.

Ben - And for anyone out there who isn't super fond of wasps, you heard it here first, wasps are not only scarier than bees they are indeed infinitely more cunning as well, so add that to your consideration. What did you do with this study specifically in order to figure out if wasps could use transitive inference?

Elizabeth - What we did is we trained them to a bunch of colours. So, for example, we would train them that blue was better than green, and then we would train them the green is better than purple, and then we would train them that purple was better than yellow. So they had all this information and now we asked them to make an inference, so asked them what you like better green or yellow?

Ben - Right. And they've never seen green or yellow together before?

Elizabeth - Exactly. They've never seen green or yellow together. Some of the time green has been good, some of the time green has been bad so there's nothing that should be inherently different about the stimuli.

Ben - How do you go about training a wasp that green is better than say any other colour?

Elizabeth - We train them in this tiny little maize. It has to be tiny because wasps are tiny. Some of the bottom is electrified and then some of the bottom isn't. So when we're training them that blue is better than green, blue is a safe area in the maze and green gives them a little electric shock.

Ben - How exactly do you figure out what's a little electric shock for a wasp?

Elizabeth - I would say it's trial and error. But I promise no wasps were harmed in this experiment. We want them to learn so we don't want them to be freaked out or really worried or anything, right. So we just give them enough shock so that they act a little uncomfortable so they start moving around more quickly and try to get away from it.

Ben - And so they just spend a bit of time in this maze until they eventually land on the part of it that doesn't shock them and that part corresponds to the colour that you want to train them is good?

Elizabeth - Exactly. They move around the maze and they eventually go to the part that's safe and they're like oh my gosh, it's safe and there's the colour green - green is great.

Ben - Okay. What did you do after you trained them?

Elizabeth - After we trained them we had to test them, so we put them in the middle of a box and then we tested which colour they prefer to go to.

Ben - And you had no electric stimuli or was that still present?

Elizabeth - There were colours on either end and there was no electricity to cue them, but the idea is that they've learned that green is good, then they would go to the green side. So we tested them on the colours we had originally trained them on just to confirm that they had learned what we trained them on, and then we also tested them on those new transitive pairs.

Ben - Okay. And what did you find?

Elizabeth - We found that wasps do have transitive inference. So they took all those trained pairs and they seem to kind of organise them in their mind linearly, and then use transitive inference to choose between stimuli that had never been next to each other before.

Ben - That's fascinating because, as you mentioned before, previously someone else had found that bees are incapable of doing this. Surely a bee and a wasp have pretty similar sized brains, right?

Elizabeth - Yeah. Bees and wasps both have similar sized brains and their brains are really tiny, about the size of a grain of rice. I think the difference between bees and wasps isn't really that wasps are just geniuses and bees are dumb, it's more about what the social life of wasps and bees are like. All the workers on a bee colony are about the same, they spend their time foraging, but on a wasp colony there's all sorts of interesting dominance relationships. They have linear dominance hierarchy where the dominant wasp does most of the reproduction and the subordinate wasps do most of the work, and so figuring out how dominant other wasps are in wasp land is incredibly important. For example, if you've beaten Jane in a fight before and you see Jane beat Susan then you can infer hey, I'm probably going to be able to beat Susan. So that kind of thing is really important for wasps and not important for bees.

Ben - I would also say probably important for humans depending on who you ask.

Elizabeth - Yeah, definitely important for humans.

Ben - That's an important thing to know. Do you think there's scope for extending this kind of reasoning to dealing with other animals or other kinds of animal cognition?

Elizabeth - I bet that many other insects are capable of transitive inference. I think we just haven't tested them yet. I think one of the messages is that animals can be really good at what's important to them. We think of humans as being like the best at everything, but lots of animals are amazing at really specific things they need to do to be successful.

Ben - Intelligence doesn't necessarily correlate just the size of the brain but also to the tasks that need to be undertaken?

Elizabeth - Exactly. You don't need a big brain to do complicated things. Even a tiny little brain can do complicated things if the animal needs to be able to do it.

We’re going to be delving into the mysterious stuff that makes up a massive amount of the Universe. But we can’t see it, and we’ve haven’t the foggiest idea what it is. So how are we trying to actually find out, and how do we even know it’s there at all? Ben McAllister has been finding out...

Ben - I'd like to tell you a story. It's a story about galaxies, black holes, stars, planets, people, and everything else in the universe. We now know that all of the big stuff in the universe - people, planets, stars - is made up of a handful of different kinds of particles. These tiny little things like atoms which are made up of protons, neutrons and electrons. We've come to know quite a lot about those little things; how they make up bigger things over the last few hundred years. Collectively, astronomers call all that stuff baryonic matter, and that's kind of all there is, right?

I'm here to tell you that that's not the case. The baryonic matter - people, planets, stars - is only a very small fraction of the whole story. In today's program we're going to hear what we know about the rest of it. We're going to hear about dark matter.

To give you a taste, dark matter is this mysterious stuff that occupies the universe. It's enormous, there's five times as much of it as there is regular matter, it's everywhere. Right now, as you listen it's passing right through your body; we can't see touch or feel it. But before we get to what it is, we have to go back a bit. Have you ever looked at the stars and wondered if there's more out there? If you have, you really aren't alone, humans have been doing it for as long as there gave been humans.

Professor Lord Martin Rees, Astronomer Royal...

Martin - It was actually some of which emerged in the 1930s through the work of Fritz Zwicky, who was a Swiss-American astronomer; and he was studying the distribution of galaxies. Each galaxy is of course as big as our Milky Way, so he was looking at the universe on very large scales. He realised that the galaxies weren't distributed randomly, but they were clusters, and these clusters obviously seem to be held together by gravity. But when he measured the speeds of these galaxies he found it was surprising that they weren't flying apart, because the energy corresponding to those speeds would overwhelm the gravitational force holding the cluster together if that gravity was just due to the galaxies. He inferred that there must be some extra material that bound the cluster together, and this was the first really serious evidence that there was some dark stuff in the universe over and above the gas and stars that are visible.

Ben - For decades we've been observing things like this. Strange movements of large bodies in space that can't be explained if we only consider the matter that we can see. It all comes down to gravity. Gravity is the main force that governs the way things move around in space. It's a force that exists between any two things that have mass and it pulls them together. Gravity gets stronger the more mass there is but importantly, it gets weaker the further apart the two things are.

In space, when we look at the stuff we can see like stars, for example, we can estimate how much mass there is in the system; and then, by using the laws of gravity - what we call Newtonian gravity - we can model the way we expect the mass to move. When things don't move the way we expect, say they move much faster for example, it implies that something's missing from our picture. There's some extra force making things move around faster which points to their being some extra mass to provide that extra force...

Martin - If you had found, for instance, that Jupiter was going round the Sun as fast as the Earth was, you'd have had to infer that there was a lot of mysterious mass outside the Earth's orbit, but inside Jupiter's orbit. So Jupiter was feeling not just the mass of the Sun, but something extra which the Earth wasn't feeling. Something like that, on a far bigger scale of course, happened when people studied the outer parts of galaxies. They found that the material was going round faster, the outlying stars and the gas at large distances was going faster, and this implied that the stars in a galaxy were not the dominant kind of mass, and that a whole, this galaxy like ours was embedded in what came to be called a halo of some material which was not emitting any light but was exerting a strong gravity, and was dominating the gravitational pull in the outer parts of the galaxy.

Ben - We've arrived at the point in the story where, thanks to observations of bodies moving around in space, we're pretty sure we're surrounded by massive amount of dark matter. Again, it's moving through your body right now, and it massively outweighs the regular matter that we understand, we just don't know what it is. We've since figured out a little more about it, but not that much more. It's a new frontier, a new region to explore. We do have some theories to explain the phenomena we see, some of which don't actually include any dark matter at all...

Martin - And there is, of course, the idea that we are wrong about gravity. And of course, all the arguments where you infer a mass from the motions of planets and stars and galaxies: that is assuming, in a sense, Newtonian gravity. So some people are proposing other ways where we wouldn't need to have dark matter at all, and we would simply have a different theory of gravity. But I think most people are against that, because first of all there's no particular reason why we should be surprised by dark matter. There's lots of scope for dark matter particles. And secondly we'd be jettisoning a lot of good data if we abandon the idea that we understood gravity. I would still bet that it's most likely that dark matter is in some kind of particles.

Ben - A number of experiments around the world propose to try and detect these particles as they pass through the earth and we'll hear more about some of those later. But why should we care about this? We can't see, touch or feel it, it's just this mysterious stuff that floats on by...

Martin - Well, we know everyone has gazed up throughout human history at the stars and wondered about them. One of the great achievements of cosmology is to understand the structure of the universe - why there are stars, why there are galaxies, why they are clustered, and the details of that. This is only giving us a consistent story if we have the presence of dark matter which is, on average in the universe, five times as dense as the gas and stars that we see. And I think this success is one of the great achievements of modern science. I would say it's up there with the standard model of particle physics in the genome. When the history of science is written I think the fact that we understand cosmic evolution, and why galaxies exist, is really a very great achievement.

Ben - If that doesn't do it for you, consider this. Think about everything humans have accomplished with an understanding of just one sixth of the matter in the universe. Computers. Modern medicine and spaceflight. All of art literature. Imagine what we could do if we could unlock the rest.

We talk about the dark matter being five times as abundant as regular matter, but that can be a little hard to comprehend. Ben McAllister has prepared a little breather from all this hard science, to help us wrap our minds around it...

Ben - That sound you’re hearing right now, focus on it. Let the loudness of that sound represent the amount of regular matter in the Universe - the stuff that makes people, planets, stars etc. Now, do you hear that low sound slowly fading up? That sound represents the amount of dark matter. Not quite there yet, hang on. There we are. Hear how it swallows up the regular matter? We’re sitting in a sea of this stuff all the time. Let’s just listen for a minute.

Chris - Quite relaxing to listen to! We’re pretty sure dark matter is made of unknown particles that don’t interact with each other, and now we’re going to learn about two of the best candidates for those particles are, and how we’re trying to detect them.

This week we’re delving into dark matter and so far we’ve heard about particles called WIMPs that might be what dark matter is made up of. But there are other possibilities too. One is a particle called an axion, and now scientists are developing experiments to try to detect them. Gray Rybka is from the Axion Dark Matter Experiment (ADMX) which is in Seattle, Washington, and claims to be the most sensitive axion dark matter detector on the planet, and he spoke to Chris Smith.

Chris - So Gray, first of all, what's actually wrong with a WIMP?

Gray - What's wrong with WIMPs? Well, I'd rather talk about what's right with axions. It's a particle that was predicted by nuclear physics to solve a very subtle problem, and yet when we look at how many of these axions should be produced in the early universe you get almost exactly the right amount of dark matter. I find that's too good a coincidence, you've got to look for it.

Chris - If I could see an axion what would it look like? How would I recognise one if it came and slapped me round the face?

Gray - An axion is very light and very weakly interacting with anything. If you imagine something like neutrino, it's a thousand times lighter, much less strongly interacting. It's behaviour is much more like a radio wave than even a particle.

Chris - Given that dark matter's all about gravity, if these things are incredibly light does that mean there must be lots and lots of them to subserve enough of an effect then?

Gray - That is absolutely correct. There is a huge, huge number of them.

Chris - And if they're a bit like a sort of radio wave, how are you trying to detect them because we heard from Katherine how she's trying to find WIMPs using their incidental occasional collisions between the WIMP and things like sodium iodide, how do you try and find an axion then?

Gray - Well, the axion should interact very weakly with electromagnetism and that means that if you've got a big strong magnet you can convert axion dark matter into microwaves, and after that you just have to detect the microwaves.

Chris - But I thought the whole point of this was that we thought dark matter doesn't really interact with anything?

Gray - Well, it interacts very, very weakly. You need an extremely strong magnet and to see the signal, which is in the yoctowatts, “yocto” being the bottom of the SI units scale, you need both an extremely sensitive detector and you need very very low noise levels. The thing has to be cooled to a 100 millikelvin just to keep the blackbody radiation from making it impossible to see the signal.

Chris - Talk us through actually how your detector works then, so where is it built how does it work, and what does it do when it registers a hit? How will you know when you found in axion or two coming through?

Gray - It's at the University of Washington and the outside is an 8 Tesla superconducting magnet with maybe a 50 cm bore, about a metre deep of active area. Inside that we have a big tin can, well it's a copper can, and we can tune the frequency that that can resonates at, we can tune what microwave photons it picks up, and as we tune that  sweeping across frequencies, if we hit the frequency that corresponds to the axion mass it will start ringing up. And so we'll see extra power coming just out of the dark matter, and we pick that up with what's essentially a very very fancy AM radio.

Chris - Does that mean then that when they're in this intense magnetic field being resonated at the right frequency, it's actually the particles or the waves themselves the axions that are moving backwards and forwards in generating that signal that you are detecting?

Gray - Yeah, that's absolutely right. You can view it as the axions are turning into microwave photons or you can view it as just an interaction between the axion waves and radio waves.

Chris - Actually how close are you to being able to detect what you think is a real axion now because, obviously, there are sensitivities with all these experiments? And I mentioned at the beginning that you are laying claim to having the most sensitive way of doing this, so how sensitive is sensitive?

Gray - Well, this is the exciting part. After decades of work we finally have an experiment that is sensitive to the theoretically predicted axion interactions, which means now we're operating just any day as we slowly tune this radio like experiment we could make a discovery.

Chris - Will there be a repetition of this experiment because are other people also building similar detectors because obviously it's important to do replication, isn't it?

Gray - There are a number of people who are working on detectors, mostly they're working on ways of making them more sensitive. There's kind of an interesting thing about axions which is that once you know the exact frequency to look at it’s much easier to build an experiment. So once we've made a discovery, I think that very quickly, all over the world there will be people who will be able to build experiments, repeat the signal, see that yes we see this all over the world and start doing some axion astronomy with it.

Chris - And how does this tally with what Colin was saying about his modelling? If you actually feed in axions into models like Colin's do they actually fit the bill?

Gray - Yes. That's kind of the interesting thing is there are people working very hard to say ‘Is there a difference between axions and WIMPs and what we would see astrophysically?’ So far the answer is no but there's always hoping that more subtle models will be able to fish out the difference.

Chris - And is there any grounds for considering that they might actually be more than one particle, because you're in the axion camp, we heard from Katherine in the WIMP camp, is it possible that more than one thing is happening at once?

Gray - It's certainly possible. Nature can be very very rich. Just Occam’s Razor that we hope that it's just one thing. If it is many things that's great, it'll keep us physicists in business for quite a while.