It's something that is pushing everything apart from everything else.

Is it like something between all the particles?

Kind of like like Sampson pushing on pillars that, Yeah, I think it's like a yeast in your bread that causes it to rise.

It's kind of pushing the structures in the universe apart from one another.

When you're lying in bed thinking about it, what's it look like to you?

Do you do sort of visualize in any way?

Or is it do you to see equations and numbers bouncing around your head?

Yeah, I don't I don't think we know enough about it to kind of say, Oh, it definitely looks like less Yeah, to me, it's, um it's it's equations and numbers on the one hand on observations we have of the universe.

On the other hand, well, we have some ideas of what it is.

So one of those is more is a thing called the comedian, So these are particles that changed their mass, depending on where they're living and changing the mass of the particle allows it to hide from some of our experiments.

So the problem with dark energy is that as well as pushing the expansion of the universe apart, it ought to come along with an extra force as well.

So they won't.

We ought to see kind of here between you and me, that be an extra kind of interact extra traction on we.

Don't you see that?

And so trying to explain why we don't see those forces has been one of the big problems of understanding dark energy shortly.

That means it doesn't exist, like I can accept chameleons and mass is changing and things fluctuating, but they're still would be something.

It would just be the quantity off.

It would be changing.

But you're saying we see nothing say comedians.

Help us explain why we see nothing.

Comedian becomes very heavy inside me and say the matter inside May any comedians that are produced there, they can't, they can't get out.

And so they can't transmit a signal to you because because they're saying Max, if they get trapped, these aren't protons.

They are electrons about new drones.

They're not faux tones than just some just just a thing.

Another.

Another new thing that that we think helps us solve some problems that we don't understand.

The chameleon's abundant.

Yeah, um, so dark energy makes up 70% of the universe.

So at the moment, say yet that they're everywhere.

So 70% of this stuff in this room are these particles which are changing mass and doing their thing.

And you're telling me we just can't see them?

We can't see them.

Yeah, so maybe maybe not 70% of the stuff in this room, because 70% of the stuff in the universe, But obviously matter is more densely clumps here than on average in the universe.

But but, yeah, there's still a lot of them around, and we don't see them because they're really good at camouflaging themselves.

This feels a bit like you're telling me there was a room full of elephants and the elephant can change its mass from one time to two tons to half a tonne to 500 grams toe, 90 tons.

But I would still be saying elephants, right?

So But the fact that they the fact that they changed their masses is what makes him hard, hard to see.

There are force carrying particles, so in the same way that photons carry electromagnetic interactions.

So they actually say the reason that we see election romantic interactions in our daily Nice, but we don't see weak interactions in our daily lives.

Even though the series look very similar is because the particles that transmit weak forces are very heavy, so they can only transmit forces over very short rangers.

Where's the photons?

Massless can transmit information over, you know, as long distance as we want.

Um, and so that's why the fact that the mass of the comedian changes makes it hard to see because it's it's only transmitting interactions that were very, very short distance scales that we don't experience.

But that doesn't sound like a problem off its changeability.

That sounds like a problem.

Just if it's massiveness, right?

Is the problem of these particles of massive or that they are changing.

So the problem is that if they're going Thio, help you explain why the expansion of the universe is accelerating.

They need to be really light because they need to be having effects over distance girls, the whole side of the universe today.

So that means that needs a light particle.

But then we'd expect to see that in our daily lives, and we don't.

So we need to explain how it's changed from one behavior or very, very, very long distance scales to two different behavior here in the room.

So is the suggestion that these chameleon particles become low mass to do their business and then go back to hiding his big mass?

Exactly.

Yeah.

So they quickly come out of their cage.

You do there.

Nor the work and the hard again.

Yeah.

Okay, that seems convenient, right?

Say, then we have Thio.

Uh, yeah, that would be too convenient.

We have to look at what you could do to try and see them because they don't hide everywhere.

What you're looking for are extra forces in the universe.

And you can do that in all sorts of ways you can.

You can have two masses in a laboratory experiment and see if you can measure for the forces between them.

So people like Cavendish did that for the first time to test test gravity.

People have been doing that for a long, long time.

You can measure our planet's orbit in the solar system that tells you about the forces between the sun and the planets, the moon in the earth.

And you can look at how stars come together to form Galaxies.

How Galaxies form clump together to form clusters.

If you can measure that precisely that those will tell you about extra forces in the universe.

Problem is, though, the rule.

The experiments from which the comedian Heinz I thought we've already found it because way.

See this expansion, increasing acceleration.

So it's not hiding if it's showing itself on the biggest stage possible on the Universal State, we see one consequence of dark energy.

But 11 observation isn't enough to build a whole theory on.

We want to understand all the ways in which dark energy behaves, and in particular we would.

We would want to understand how it interacts with the matter fields that were made off.

Um, and that's very difficult, understand, just from observing how the expansion the universe has evolved, what we need to try and find the comedian is to measure forces on things that are really tiny, because if we're looking on really tiny scales with really tiny particles.

Then there isn't space for the communion to hide more times than Kevin.

Yeah, So we're talking atomic distance gales.

We now have the ability to do this to measure forces on individual atoms on dhe.

So you and this relies on some things some principles of quantum mechanics.

It's a bit like doing a double slit experiment where you put the particle in a superposition of states, one of which travels through the left and slipped, one of which travels through the right hand slipped.

And if you don't watch, which slipped the particle gay street, you an interference pattern.

So it's like you had a wave instead of a particle.

So what we do is is we take an atom on DDE.

What we what we'd like to do is put it into a seafood position of two states, one of which feels a stronger force.

What a rich field, a weaker force.

So if you wanted to measure gravity system, the gravitational strength, the gravitational attraction of the Earth, what you do is he'd have one atom moving horizontally on another one.

That was that was doing some parabolic trajectory.

Okay, say so.

They take different paths under gravity.

This is the same thing.

This is the same atom.

Instead of going through both the left and the right slit of the double slit experiment, it takes both paths as long as we're not watching it.

And then it's only at the end.

When the T path three converge.

Then we do measurements.

And what state we observe the atom in tells us about the strength of the force that was acting on it while it was in this superposition of states.

So you can you can tell what experienced on both pervs.

Yeah, it can tell you both stories.

Yeah, exactly.

Tells you about the differences between the tea packs.

So the comedian is an extra force.

Say we could put some some lump of matter in our vacuum chamber and then be an attraction between our atoms and found this on extra comedian attraction between between the atoms and an R source mass.

But that why this is so good for looking for comedians is that because atoms are small, they the comedian can't hide from these atoms.

It has to talk to them, And that's different from any other experiment that we've done looking for these extra forces before.

And that's just that's what makes it super sensitive.

Tiu the existence of comedian fields.

Why does the chameleon have to talk to an atom but doesn't have to talkto Jupiter or bowl of lead or son right s.

So we talked about the fact that the changing mass of the particle tells you about the distance scales over which it can transmit forces.

Unless the comedians mass can become smaller than the size of the atom, it it can't hide from the atom.

Right?

Where is where is Jupiter is so big?

Easy for the comedian kind of interaction scale to become smaller than GPS, Aaron.

And then it can't talk.

How do you do this experiment?

Because any chamber you build is gonna be made of staff is gonna be here on earth, and it's gonna be electromagnetic forces going on, presuming there's gonna be so such a mess of force that this actually is experiencing anyway.

How are you gonna I select the chameleon's force, Right.

So, actually, um, yeah, that's that's obviously the tricky bet.

Say you have your afternoons in particular, it's hard to shield the gravitational effects of the so What we do is we arrange for the comedian forced to be acting horizontally while the gravitational forces acting vertically on.

And then you can you can set up your regiment in which is, uh, in a way so that your only sensitive to horizontal forces and in that way you can you can decouple the two effects Electromagnetic forces have to exist within our experiment because it's how we control the the atoms, how we move them around and say they're they're a bit trickier to deal with.

But if you're if you're careful and then they're sort of sub dominant thing that you have to you have to control.

Presumably you don't go to the cupboard in the lab and open up a jar of chameleon particles.

How do you You just bank on chameleons being in there?

Yeah, there, there, every last sight there, they're gonna be inside your vacuum chamber are planning to build this experiment at Imperial.

It's an experiment you can do with with current technology.

All you need is a vacuum chamber and a laser.

The laser is what helped you move the atoms around on.

Then he just needed a way of cooling the atoms down so that there isn't a lot of certain formation kind of happening that's destroying the the effects that you're looking for.

These are all things we can d and this is an experiment that lives on a table.

You have one atom, you shine a late so and this atom is living in its ground state.

Okay, so really, it's not excited s normally rubidium ladies.

Andi, you shine the fight on on the atom on if if you've chosen your laser beam carefully, your atom absorbs that fate on and it gets excited.

But now you have to conserve momentum.

You in your atom was standing still, but the photo on that came in had some momentum.

Now there's no fight on any.

Well, so you're Adam had to pick up the momentum of the incoming Vital.

So now you've got a way of moving atoms around.

But if you don't make a measurement at that point, you don't know if the atoms absorb effect on or not.

So now it's It's in the super position of states, one of which is standing still, one of which is making it all you do that three times.

Basically, you give it a kick in one direction, give it a kick back on.

Then there's a final kick from the fight on the brings our two paths back together.

So one of which was just standing still, one of which was moving the atom or they're part of the atom.

The part of the state of the atom that that made that's been moving around has been going placer to us.

A Mass, which is a source of of our comedians, is, is the source of this extra comedian force.

And so the two paths have experienced kind of different different comedian effects.

So when you bring them back together, ones that have been traveling are in their excited state.

The one that's been staying still is in the ground state, so you could make a measurement of your atom, and the probability that you see it in its excited state is proportional to the strength of the comedian force that has been feeding as it's been moving around.

So you're measuring how many atoms come out in the excited state, and that tells you about what extra forces have been have been acting on the atoms.

This is quite exciting.

This is like, this is this is this is really showing science working.

The point is, Hook reports this.

Okay.

How did he make it up?

Did he have an aberration in his telescope so he couldn't see it.

Did he have a spot in his eye?

Okay, until somebody else has seen him has reproduced it.

You can't be sure of that.

But you see the principle.

It's by publishing.

Here you are stimulating debate, stimulating more observation, and the science becomes that bit more secure.