Hello and welcome to In Our Spare Time.
For the next seven weeks, I will be joined by 21 students from across Oxford University discussing their academic and intellectual passions.
Each week we will have a different theme, ranging from Marxism to mediaeval song Cicero to some time.
Perhaps you will be able to guess the long running radio floorshow on which this concept is based answers on a postcard piece.
For over three millennia, astronomers have been looking to the heavens.
Yet less than 100 years ago,
observations were made suggesting the existence of a previously unknown substance permeating the universe in vast abundance,
yet invisible to even the most powerful telescope. This substance has been aptly christened dark matter.
And though in nature it seems to be ubiquitous, all attempts to explicitly detect it have hitherto been unsuccessful.
So what is dark matter? Why do we think it exists?
And what has it got to do with a four ton tank of liquid argon two kilometres underneath Ontari?
My name is Alice Walker, and with me to discuss current research into dark matter are Peter Hadfield,
a 38 year old student at Lincoln College from day, a 38 year old student at Modelling College and Tulsa.
Bromwich, a second year student, also modelling college.
Thank you for joining me, Peter. Perhaps we could start by discussing the historical background of dark matter.
What were the first experiments that people did to suggest that there was other stuff out there than what we could see?
So the very first indication there might be some kind of substance out there in the universe that wasn't giving off light dates back to the 30s,
it was only as early as the sort of 1920s that people actually grew to understand that there was things outside of our galaxy.
The most famous evidence for the existence of dark matter came in the 60s and 70s, where people studied how these galaxies rotating.
I'm sure everyone listening kind of knows the kind of iconic image of spiral galaxies.
If you kind of look at the dynamics of how these stars are rotating, they appear to be rotating much faster than how much matter is.
You can understand how much matters in stars and it's actually much faster.
So people propose the existence of a new type of matter that we didn't know about.
So if I understand it correctly, you'd expect from the matter we see that the stars around the middle would rotate quite fast,
but then it would things would tear off as you got outside the galaxy and you said that's not what is observed.
If you do a lot of how fast start rotating relative to how far away from the centre you would expect for this to drop off very fast.
Whereas we see that one's even far on the outside of the galaxy are going extremely fast.
This is directly contrary to the notion of what you would expect if there was only if all the mass in the galaxy was inside the stars.
So you would be looking for. So what you're basically doing is you're it's it's like you're the centrifugal force
equal to the gravitational attraction of all the matter that you've got in the middle.
And what you're expecting is one over the square root of the distance from the from the centre of you, your galactic structure.
And you'd expect that to fall off according to that relationship and that it would be very, very clear.
I mean, is that sort of Newton's law of gravitation? Yeah. Yeah. So it's this is just a very technical term.
Yeah. Yeah, very. But what's so dramatic about when you actually plot this,
so you measure the velocity of different objects within the galaxy at various different places using their Doppler shift and other techniques.
And what you find is that it radically diverges from that pattern.
So you see a very, very flat distribution suggesting that even though when you look at the galaxy,
you see that most of the stuff that's giving off light that we observe is concentrated in the middle.
All of the stuff around the edges behaves as if as if there's like a continuous distribution of mass, almost like a halo around what you're seeing.
And that's what's referred to often as a dark matter halo.
So this kind of invisible mass that's causing things further out to be travelling much faster than they should be.
You've got objects behaving as if there's mass there that you can't see.
So that's where the name dark matter comes from, is because we it does not give off light in the way that other luminous matter does.
So you'd expect you look up into the sky where you see light that's coming from stars.
And so you assume that that is where the matter is concentrated. But this dark matter doesn't give off light in that way.
So we can't detect it in traditional methods using telescopes and such.
Yes, because this concept. Yes. Right.
OK. So so where we are, we have these rotating galaxy clusters, which we can't explain with the mass we can see.
But reading your notes, there are other ways since those early experiments that Peter's been telling us about,
we want if you want to come in on on the other things, it's about gravitational lensing.
A more recent observation that people have been able to come out to kind of confirm these ideas as the idea of gravitational lensing,
Einstein's theory of general relativity is the kind of upgrade to Newtonian gravity that explains how things behave.
I'm going to get really strong. And that actually describes that when there's a gravitational force, light is curved around,
that this was the first reason that evidence that showed that gravity was right.
People observed libraries being bent around the sun during the eclipse.
We can do the same with dark matter if you look at the galaxy and see how strong the light has been around it.
That implies that there's much more mass there than you can see in the actual stars, which again,
agrees with the amount of mass that we would infer exists there from the rotation of the galaxies.
So you have one object that's to be closer to another side of the galaxy that's behaving as the lens is kind of close to us.
And then you've maybe got another galaxy or something else far away in the distance.
And illusion, perhaps like a lens can focus light around it in the strength of that light is dependent on the
distribution of mass for that distribution of mass doesn't agree with where the stars are,
but it does agree with the distribution of mass that one would infer from the
rotation that we were discussing earlier now where the interest of fairness,
we ought to say, is although the main theory,
it's not the only theory that's been posited to explain these gravitational anomalies for want of a better term,
a modified Newtonian dynamics is what you said.
A friend wants to tell us a bit about that and why it's not the main theory before it tries to tell us.
And the idea is modifying Newtonian dynamics is that rather than introducing a new particle,
we could change Newton's law of gravitation or change the laws of dynamics to accommodate these new observations.
And it actually works quite well for observations and galaxies.
But there are problems with modifying Newtonian dynamics.
For example, it was recently observed we had this great observation of something called the bullet cluster,
and this is two galaxy clusters that are colliding. So this is a really spectacular event and we're really lucky to have been able to observe it.
How many light years across? We're talking about for these clusters of millions and millions of things.
I think it's about three billion light years away.
And then in the order of millions of light across, yes, I knew that big objects, they're the largest gravitationally bound structures in the universe.
They're huge. And these two are colliding.
And we can use these gravitational lensing techniques to work out where all of the mass in the collision is.
So we can sort of map out the mass and we can also see whether or not there is with telescopes.
And what you see is that the visible matter is getting this like horrible jumble in the middle where they collided.
If you imagine two cars colliding, you've got like a car crash and it would all be horrible place.
And that's what we see with the visible matter. But when we Mapai the dark matter is dark matter is most of the mass of the clusters,
we find that it's basically just passed straight through each other night.
If you if you had two waves of water, just go straight through each other.
So you have this horrible crash in the middle and dark matter on either side.
And this is exactly what we'd expect from dark matter, because dark matter doesn't really interact with normal matter or wave itself very much.
So it doesn't really do collision if it just passes through. And it's very hard to see how you get that pattern with modified Newtonian dynamics.
It's currently can't be explained by no, I haven't seen anything.
Have you seen an attempt to do it? I, I haven't not with the example of the cluster.
I mean, the bullet cluster is used as one of the sort of smoking gun of the existence
of dark matter as like a particle as opposed to a modified gravity theory.
Yeah. So we have these particles, but there are also lots of particles in the universe that they could be.
So do one of you. Maybe people want to take us through some of the candidates that people came up with, for what dark matter could be.
You know, how some of the dust. So as a kind of the seventies when people who kind of originally measuring the rotation curves,
my understanding was that there wasn't enough understanding of the astrophysics,
of how stars and galaxies behaved, that it could have still been like another form of matter.
Look, we know that actually only a very small amount of normal matter is in stars.
Most of it is kind of gas around the galaxy.
So are kind of people's understanding of gas wasn't sophisticated enough at the time to definitively say that we needed a new type of particle?
And subsequently, since people have kind of understood more where the gases we've understood like how many?
Brown's wolves, which are like a kind of like I didn't start,
maybe they could have been loads of them that we didn't know about, but now we think we can rule that out.
And slowly, kind of like all these kind of conventional solutions, it's kind of been ruled that people are like,
well, neutrinos or a type of particle that doesn't really interact with anything. Maybe there's just a lot more to tarantulas than we thought.
But again, like that's been ruled out for various reasons. And slowly, we've kind of like ruled out everything, conventional leaving.
But it has to be some new type of particle that we think we don't know about.
But hopefully we'll find these things, those machoism wimps, as is so much as.
Well, it's a it's an acronym cimetidine much I think is a massive compact object, et cetera.
Yes.
So those would actually, I think, be made out of normal matter to be something like how many black hole or if in fact the only thing that have stars.
Yeah, yeah. Yeah. So I think you can still have models where these work, but it's a bit of a stretch these days.
So we say rules out maybe. Could you develop this a bit.
So how do we move these things out?
So what's really interesting about dark matter is that it's kind of it's the way a lot of particle physicists are now working on it,
the evidence and limitations that we place on it. A lot of those come from astrophysical constraints.
So in terms of the dark matter density required in the universe for these very complex models that we have of how the universe evolved.
So it's called the lander called Dark Matter Model. It places very strict constraints on the mass and the the velocity of these particles.
So when we say like machos ruled out, that tends to be from astrophysical measurements.
We don't see enough of them to account for the density required to fulfil the conditions of this kind of evolution of the galaxy
that very much agrees with what we observe in the universe today and how we would explain the evolution of the universe.
So it's interesting that you sort of you're looking for something very specific on a very small scale in particle physics realm.
But the constraints that you place and how you rule out potential particles or windows of other things is based on astrophysical theories.
And that's kind of a lot of historical fruitful interaction between particle physics and astronomy.
Like the chemical element, helium was first discovered on the sun through emission of specific frequencies of light.
That helium gave off before we discovered constraining how many neutrino species the world was kind of constrained
through cosmological observations before particle physics could kind of understand how that was working.
So hopefully the same will happen. Yeah. So we've ruled out brown dwarfs out neutrinos or neutrinos moved out in the same cosmological
approaches or I think they're not they don't exist in sufficient quantities and also they're too hot.
The issue is that one of the other reasons, yet another reason that we need dark matter is for structure formation.
So you look in the sky, you see all kinds of structures that stars and galaxies and galaxy clusters.
And if you trace the evolution of the universe from the Big Bang to today, just looking at normal matter,
there just wasn't time for that amount of structure to form because all the normal matter within what we call a thermal soup.
So it was all just really hot and interacting with each other so it wouldn't have gravitationally clusters.
So you need dark matter to decouple from that family soup early because it doesn't really interact
much and then cluster it forms kind of blobs which are gravitational gravitational well,
which the normal matter can then falling to and form galaxies and stuff.
Neutrinos can't do that because they're too light, so they just move really fast for ages.
So you're saying dark matter is responsible for the universe being in the sort of galaxies with have formed without dark matter?
Yeah, we've heard of neutrinos, ruled out ground rules and we're down to constrain possibility.
So there are these wimps that we began to talk about.
And also some of the friends will come in on actual accidents from.
Yeah, so an accident or an interesting kind of dark matter.
So an accident is kind of like a particle that is motivated by various problems in particle physics, and it's also motivated by string theory,
sorry, string theory, which you might know is kind of our only working theory of particle physics at very, very high energies.
Totally unconfirmed, of course, and at motile is.
As little vibrations, the strings, which is not as silly as it sounds,
I know string theory predicts loads and loads of accidents, axioms now accidents are very, very light particle.
And you might remember I just said that light particles can't be dark matter. So Acción Dark Matter works a bit differently.
So we kind of have to take a step back and ask what is the particle?
And today, particle physicist believe that particles are ripples on quantum Fayose,
which are like fundamental failures that span the whole of space and time, like ripples on water.
And that's a particle and a normal particle. It's like a little bit like if you have like you mess up a bathtub when you see,
like a little bit on the surface with Axium dark matter, we're doing something different to the field.
We're making it oscillate all as one. Like, if you drop something really heavy in a bathtub and it will flush from one side to the other.
So it's the feel of moving in a different way.
And when you work, work it all out, that behaves like it was a heavier particle like particle, dark matter.
OK, your power of metaphor is wonderful, seeing the bottom in my mind's eye.
OK, so we have actions. On the one hand, we have wimps.
What do we stand for. So wimp stands for weakly interacting massive particle basically.
So what's really tantalising about sort of having a particle dark matter is that we have lots of theories that predict particles that could,
if they existed, fulfil the criteria necessary.
So as well as axioms, we have heard of supersymmetry and the supersymmetric particles.
So which is Bahadoor? So it's basically an extension to the standard model where you have a whole
nother set of of particles to the existing ones that we know about the heavier.
And these are predicted by supersymmetric theory to explain various problems that we have with the standard model.
So a standard model is it's our basic kind of like the periodic table for particle physics.
It's the particles we know of and how they relate to each other as embodied in a series of equations that
we can use to predict how they will interact and explain how they work on a fundamental sort of level.
So these are the particles that go together to form a nucleus of atoms.
Yes. As well as other particles, such as we've mentioned previously,
neutrinos and electrons going around the outside of the atoms, as well as more kind of exotic things.
But things that have been measured and found and that we understand quite well from experiments such as the Large Hadron Collider,
where we study them.
So the hope is that this new dark matter particle, whatever it ends up being, we could add to the standard model or be slightly separate.
Yeah, no. So the these I mean, there are lots of suggestions the standard model is incomplete as it stands at the moment.
So which is why there's so much active research into looking for explaining contradictions that we see.
One of the things that's dominated particle physics studies for quite a number of years now is the supersymmetric theory,
this idea that there's this whole other family of heavier particles and we are now at the
stage where we have colliders that can reach the energies to detect these particles.
And the hope is that we'll start seeing some of them soon. Now that we're at those energies at the LHC, they none of them have been detected yet.
But if they are, some of the lightest amongst them are very good dark matter candidates.
And they all come under the class of these wimps weakly interacting massive particles that cigarettes very nicely into dark matter detection.
We have the substance that makes up 80 percent or more of all matter.
But how can we try to detect it,
given that we can't see it and it doesn't interact with itself or with with other matter to talk mainly about the astrophysical experiments?
Peter. Yeah. So there's lots of ways you can indirectly constrain properties of dark matter,
for example, wasn't discussed other galaxies to form in these dark matter halo.
So by learning how these galaxies, what we can kind of tell how the dark matter of behaving,
which kind of like it can lead us closer and closer to understanding how it behaves,
although we've been talking about that might have been something that weakly interacts if it interacts just a tiny,
tiny bit, if it's dense enough, if there's enough dark matter, you can still see it interacting with itself.
So some people believe that if you look to a part of the universe where there's the densest bit of dark matter,
you might be able to see photons being given off by particles finally being dense enough that they're colliding with each other.
So there's maybe some hints that this has been detected through photons being given from these really dark, dense regions of dark matter.
That we think could only be produced by these particles collide and give them a function that's still kind of in a kind of speculative.
What is this part, the line spectra that take the pulse of exactly what we're talking about?
Yeah. So this is as you said, when we look at galaxy clusters, we see an excess of photons.
So particles of light over what we would expect, one really small energy.
So all the extra photons have very similar energies. And that's quite exciting programme that had different energies.
You're thing maybe we've just modelled how the cluster works a bit wrong,
but when they all have the same energy as very few ways you can produce that one,
which is what we call an atomic line, which is when the electrons in atoms change from one energy level to another,
they produce a photon and that produces a light. So all the programmes have the same energy.
When we say light, we're talking about spikes on a graph. Yeah, it's not quite a line.
It's like a very narrow bump, a very narrow. But it might as well be.
Yeah, it might as well be a light is the point. All the photons are basically the same energy.
Yeah. But we know all the time because we can measure them on.
So we can subtract those off. And once you've done that, you're left with this line at three and a half electron balance.
And this could be dark matter particles, decaying entity photons.
And then the energy of the photons would just be set by the mass of the dark matter particle.
So that would then reduce this very narrow line.
But this is so speculative because astrophysics is just so complicated that maybe there's just something else that we hadn't really thought of yet.
I think some people have suggested that maybe there's some potassium.
So, yeah. So to subtract off the atomic lines is quite a hard procedure.
And one thing you've got to do is work out all the abundant sets of all the different elements in the cluster.
And that is the suggestion that we did it wrong for potassium and the paper.
Yeah, the paper was called Dark Matter Going Bananas. It was a cunning pun because bananas have lots of fruit.
So do you really have to take into account, like, all like hundreds.
So do you think there not a hundred because the very heavy elements are found in clusters.
You can only really make them artificially on earth, but you do have to take into account a lot of them.
It's a really big job. There's really only been made possible by computers.
OK, so that's the astrophysical observations you made to tie up something that you spoke
earlier that Einstein thought we could try to find dark matter in particle accelerators?
Yes, definitely. And there's lots of active research working on exactly that at the moment at the LHC.
So what you're doing there is in a similar way to the astrophysics where you're looking for an excess of photons, where you're not expecting it.
There they as I said, as we've mentioned, that don't match particles, you can't detect them.
So when we do these massive collisions in like linear colliders like the LHC,
how you would identify the dark matter particles have been produced is that you see kind of a lack of energy.
So, for example, if you imagine two things colliding and one of them, you see bounce back the other way,
but there's nothing in the other direction you that that violates conservation of momentum.
So you've got some missing momentum, some missing energy from that collision.
And that's how you infer that's something else that you're not detecting is being produced.
So you look for these very specific events where, for example, monologist,
where you get like a huge jet of hadrons in one direction and nothing in the other direction.
So forgive my ignorance. What's a natural? So hydrogen is what you generate loads of in the LHC?
Yes. Yes. So Hadron is is something that's made of quarks.
So it's the stuff the ordinary matter is composed of. So because we're smashing together hadrons in the LHC, generally, when you smash them together,
you're going to produce a whole bunch of hadrons alongside anything else that you're interested in.
So you just call it a hydrogen jet because it's a mass of hadrons that gets chucked in one direction.
And they're very useful because you get this very strong signal in one direction and then nothing in the other direction.
So one of the difficulties is that, of course, neutrinos, as we've mentioned before,
are also not detected and they're inferred in the same way from the sort of missing momentum.
But what they're doing in collider is looking for potential decays where you
do see this sort of this mono jet or some sort of similar interaction with.
Momentum more often than can be explained by neutrinos and therefore you infer that
there's some kind of other non detectable particle being created in that machine,
but I wouldn't necessarily be what we observe in the bullet cluster.
Exactly, exactly. Which is why, although these experiments are very interesting in terms of looking for possible regions,
for dark matter, in terms of isolating even more precisely what mass to expect and things like that,
the only way that we can actually say that we have detected dark matter is through actually detecting the dark
matter that's all around us and that involves a very different type of detector and different type of experiment.
You are involved in the design of some next generation of accelerators.
It's a football programme. Terminix is one of the things that you're looking towards.
Detectors are designed in a very different way to the LHC.
So I actually work on I mean, we talked about the fact that the LHC is Hadron Collider, so I actually work on future Lepton colliders.
So leptons are more fundamental. So that's things like electrons.
And what we're doing in these future machines, hopefully, is it means that the collision is much, much cleaner.
So we have a much, much more detailed picture of the energy that we're putting in and the energy that we're getting out.
So the problem with hadrons is they're made of quarks.
So it's a bit like smashing two balls together, but inside each ball is lots of other little balls.
So you're never sure from which little ball collision has actually generated your event.
So that's kind of an uncertainty on the energy that you're putting in, whereas with leptons, they're fundamental.
So when you smash them together, you know very accurately how much energy you're putting in,
which means you can much more accurately reconstruct your missing momentum from whatever happens afterwards.
So the hope is that in these future machines, it'll be similar sorts of experiments to what's been done.
GHC looking for these Monegasques with missing momentum that would be done with much higher resolution.
So yeah, and also very light hearted high statistics, which is what's required for all of these experiments,
because it's hard, because we're looking for like it's not even a needle in a haystack.
Yeah. The Milky Way don't exactly. The Milky Way.
So we start to talk about direct detection on Earth, which is quite a lot of research is going to.
But I think before we do that, I'd really like to hear what some of you are doing at the moment.
So we've heard a bit about Two-Thirds Collider design. Peter, what are you thinking about at the moment?
So I'm thinking about the kind of interaction between how galaxies evolve and how the dark matter distribution evolves, as we discussed earlier.
We think that dark matter halos, these blobs of dark matter,
we believe exists are crucial for the formation of consciousness and said we are now being able to understand, like the structure of the universe.
We kind of think of the universe that on large scales it's a kind of continuous thing.
But if you kind of see Minta segments, this is kind of rich structure and you get the galaxies and clusters of galaxies,
clusters of clusters of galaxies and so on.
So I kind of use kind of what we believe is the kind of theoretical kind of structure of dark matter distribution.
The universe. People are often surprised by this kind of like, well, isn't dark matter this kind of mysterious thing?
Kind of in my work, the dark matter is the thing that we understand very well,
because it doesn't matter in theory if if it is to be believed, it behaves in a very simple way.
It doesn't interact with anything. So the structure of dark matter, we think is behaving this very simple way.
What's much more complicated is how all the rest of the universe,
the stars and galaxies and everything, kind of formed kind of on top of that structure.
So all these kind of large groups around the world,
data from telescopes in Chile and Hawaii to kind of probe what galaxies are doing kind of from right now.
And as I'm sure some listeners know further away, is further back in time.
So you can kind of plot galaxies from now back to 10 billion years ago and kind of look at how
they're statistically arranged and kind of what structures in that can then relate that kind
of observed structure of galaxies to the kind of theoretical distribution of dark matter and
that kind of information about how these galaxies are forming in relation to that matter.
So, for example, like the kind of core problem and this is if you can't imagine a halo of dark matter
or the kind of DASSIN that is kind of believed to have fallen into the centre, kind of formed the galaxy.
But if you kind of let go progressively more massive kind of dark matter holds, the galaxies do get bigger, but like not much.
If you kind of go to a dark matter 10 times as big, the galaxy might only get two times as big.
And kind of understanding why you get this diminishing returns, why kind of really hard to build these more massive galaxies,
even though you've got all the gas in these halos, is a kind of ongoing problem of research from what's going on in the actual world at the moment.
So I'm working on two sort of.
Kind of. One is the dark matter is that we've already spoken about, but actions can also behave as what's called dark radiation.
So this is just very fast particles that we can't see.
So neutrinos, if you mentioned earlier, would be dark radiation.
But we know what they are. So we just call them neutrinos. And I'm looking at detecting actions using the fact that in a magnetic field,
actions have a very small chance to convert to a photon, which is converting to photons.
What axioms want to do so? Well, I wouldn't say I want to be a bit anthropomorphic, but that's just.
Yeah, that if you just work out like the equations of motion.
So in the same way that if you set over a pendulum swing backwards and forwards,
actions go backwards and forwards between axioms and photons, except they spend typically it starts off as an accident.
It won't spend all that much time as a photon. It's slightly more complicated than that because it's a quantum thing.
But it's so we have the small chance to convert into a photon.
And it's like a very small chance, like a case in point in ten thousand to one in a billion, depending on the field spin,
which is why they can still be dark matter, although, of course, you'd be able to see them.
And happily that are magnetic fields and galaxy clusters.
So galaxy clusters have these magnetic fields which stretch over billions of light years.
So these are really ideal conditions to observe axioms.
So I'm looking at what kind of signal would expect from Axium to vary from conversions and galaxies and galaxy clusters.
And then also mapping this to dark matter is for the three and a half billion signal, which we talked about earlier.
Yes. And so if you make a prediction of where you should see this.
Yeah. The signal for reactions to the functions of some of these experiments and see
this without being used to make any kind of smoking gun corrections system.
So we need improvements in astrophysics. Excitingly, that is an excessive X-ray photons and galaxy clusters to a straight line.
And it's also known or in excess and low energy X-rays and galaxy clusters that could be explained by accident.
But it's still debated because astrophysics is so complicated, it can be quite hard to work out whether you have an excess or not.
And normally when you're putting particle physics against astrophysics, astrophysics, which occasionally.
So it's just about looking at the anomalies that are there and thinking about how we could explain them
in other ways and also making predictions so we can rule out high levels like temperature conversion,
for example, by just the fact that we don't see loads of photons kind of ruling out a parameter space.
It's almost as important as trying to discover things,
because if we can just sort of rule out one of the particles that don't exist, then we'll have less particles that might be right.
We have about 10 minutes left and we're moving to one of the most active areas of current research,
which is trying to detect wimps, which is the other candidates at the moment for the atoms.
What it might be say that there are loads of the experiments around the world, but no one will talk about now is called deep three 600.
Yes, that's right. Yes. Which is under two kilometres.
But that is kind of a little bit about that. Yeah.
So before I shifted into designing accelerators, I did my master's project working on research and development for the deep three six hundred.
So this basically this is the direct detection technique.
So what you're trying to detect is dark matter particles in the universe that are flowing through
the earth and through us all the time from our own galactic dark matter halo and how you do this.
It's basically like a sort of a target experiment. So you go to somewhere, first of all, where there's not a lot else going on.
And a good place for this is underground, which is why these experiments tend to be they tend to be very, very low elevations.
So in particular, this one is at the Sudbury Neutrino Observatory in in Canada.
And it is an active nickel mine.
And it's two kilometres underground where they've created this laboratory for doing these type of neutrinos and dark matter experiments.
Because if you're trying to detect something that's sort of flying through the atmosphere, the problem we've got is that it's very messy.
So there's like a lot of other stuff going on. So cosmic rays hitting light as they hit our atmosphere, generate these massive showers of particles.
So the problem is that most of the time, even if you did have a dark matter particle going through your detector,
you wouldn't see it because there's too much else going on. What if we just go back a step?
So we've been saying for sure that dark matter doesn't interact with matter.
But you'll tell me that it does. Just a very small of it's very, very, very rarely.
So you also don't know everything, don't you?
The theory is that if it's a particle and if you throw enough of them antimatter and you watch for long enough,
at some point you will get a head on collision between dark matter and ordinary matter.
And this is actually how neutrinos have been detected in lots of Nobel prises
recently for experiments that have made very detailed measurements of neutrinos,
made fantastic discoveries using exactly this method of generating basically a massive
target mass and then watching it under very clean conditions for these very rare times
when these otherwise undetectable particles literally collide head on with something in
your detector and generate either sort of photons of light or some kind of other sort of.
It's usually a scintillating target so that when it receives the kick from this direct collision,
it produces photons of light, which can then be detected. The huge challenge of this is that, as you said, this this happens.
I mean, it's predicted to happen incredibly rarely, if at all.
So you need massive, massive experiments and they need to be in very, very clean environments.
So by going deep underground, well,
that means is you've got two kilometres of rock to basically stop all the other mass that you've got going on from cosmic rays,
whereas the dark matter doesn't care. It just go straight through most of the time.
But I mean, the sad thing is, in a director of the kind of tunt scale,
you're still expecting only a few events of these to happen per year, of course.
So you have to watch very carefully. You have to make sure that, you know,
everything else that could potentially be getting in there and creating signals that could look like dark matter.
And you have to go to such precautions to make sure that your equipment.
Some people urged, yes, everything has to be completely radio pure because, I mean,
if your detector has anything that's naturally radioactive decay that's going to generate particles within your detector,
that could look like a dark matter signal and not be. So it is.
It is it is worse than a needle in a haystack because you have to cut out so much.
But as I said, what's what's hopeful about these experiments is that they have worked for neutrinos in the past in detecting in a very similar way.
So it's a known technology that's now being sort of expanded and evolved for dark matter.
So we're running out of time, but perhaps just before we leave that topic and move on to a few closing remarks,
you could tell me where the allegation comes in. So basically, I've seen a wonderful picture.
It's basically a big bowl of all.
So the noble gases are fantastic for dark matter detection because they scintillate and they're transparent to their own citizens.
So they give off photons when you hit them. Yeah.
So when you give them a bit of extra energy, they they produce these photons, which you can then detect.
So the deep three 600 experiment uses liquid argon.
There are other experiments such as the Lux experiment, which currently holds the world record for dark matter detection, which is liquid xenon.
I thought we hadn't detected it. No, no. So it's when I say world record for detection,
what we mean is so basically we've got these constraints on what we're trying to measure for this dark matter based on these astrophysical things.
It's got to be a certain mass. It's got to interact a certain at a certain rate, but that's still quite a large window.
So what these experiments are doing is they're trying to get more and more
sensitive to look at more and more of that kind of parameter space and then say,
OK, we've looked at all of these masses at this, you know, cross section and we haven't seen anything.
So at the moment, it's basically a null result. So the Lux experiment has ruled out the lowest cross section at a certain mass.
It's not seen as a world record for dark matter.
Non-taxable. Yeah, yeah.
So a couple of years ago when the results came out, the headlines were very amusing of big news, dark matter, not the.
But I mean, that's how it works. You keep proving these spaces until you gradually narrow it down and you hopefully discover something.
Yeah, maybe it's a funny few. Collins The floor is open next five years.
What are your predictions? What the new experiments being built, what you think might happen?
Well, the exciting part that's coming up in astronomy from our perspective, are probably things like the Euclid satellite,
which will probe galaxies going back to the beginning of the universe, like I was talking about earlier.
But the survey I work on is looked at a patch of sky the size of the moon in the sky.
So a small patch of sky will extend that to the whole sky.
Just a huge amount of data that we have to kind of probe the dark matter distribution,
much more accurate ways to understand how kind of structural it behaves over time.
And somehow that interacts with galaxies. The sky will do the same in radio, and this will give us lots of information not only about the galaxies,
but how the structures and to read things like Light Rasenberg or special events and
looking at galaxies in different frequencies gives you different information about them,
used to look at different parts of the universe and so on. And there's a lot of surveys that look at, for example,
the Sloan Digital Sky survey or start a new project on detecting something called baryonic acoustic oscillations,
which are basically detecting kind of sound waves of dark matter, which is an exciting project that's detected so far.
But we'll be able to that's been detected in the universe today,
but we'll be able to kind of put that back to kind of like the last six billion years, how these waves have kind of grown.
So that's what we're looking at over the next five to 10 years.
From two to say one exciting thing about the three and a half billion figure, I was told is that the satellite going up,
that's got a much better energy resolution and it will be able to tell by looking at
the line whether it's dark matter or astrophysics because they're different shapes.
So anyway, I'm afraid that would be the talk of the century. So that's very exciting.
And a final sentence from telephone. So if the LHC finds supersymmetric particles in the current run in the next couple of years,
that would be incredibly exciting because they would be fantastic dark matter candidates.
And in the meantime, there are something up to 50 direct dark matter detection experiments worldwide looking to directly detect it.
So any one of those could find a signal at any time. So it's very exciting.
So we should just keep our eyes peeled as you improve.
You're a fascinating 45 minutes next week scissoring.