That's something like 13 billion years old.

And it's doing something unusual when you look at distant Galaxies.

By that, I don't mean Galaxies nearby, our own Milky Way, like the Andromeda Galaxy's.

I'm talking about Galaxies that are tens of millions of light years away from us, and you look at how, then moving with respect to one another that generally moving away.

Now that's that's no unexpected, because we know the universe is expanding.

So if the universe is expanding, then it means the space between Galaxies is is is stretching.

But what's going on that's unusual is that it's not just moving apart.

It's moving apart, moving apart faster and faster, so the universe is actually accelerating.

So then you say, OK, well, fair enough.

The universe is accelerating.

What's weird about that?

What's funny about it is that if you just think of what's in the universe, we've got radiation and we've got matter, makeup us, and then we got doubt matter.

We would it actually expect that the gravitational pull from those objects would slow down that movement so that the universe is expanding because of the hot, big bang of the energy from the hot Big Bang.

But then gravity begins to pull things back, and we should expect the Galaxies to be slowing down.

But they're not the moving away faster on DSO.

It implies that one of the things that implies is that there's some form of energy density being pumped in that's causing it to expand its overcoming.

The natural pull of gravity is like an anti gravity.

Andi, this has been coin.

Does dark energy is that it's a bad name, really, But it's but it's on it.

Not only is doing this is dominating all the other energy contributions in the universe today.

In fact, if if you if if you add up all the types of energy that you could have, then you've basically got the radiation in the universe.

We have a matter content in the universe, and you've got this dark energy and you've got the possibility of the curvature of the universe acting like an energy source.

When you add all of these up to come to unity to in terms of these funny units, the dark energy is about 70%.

So 0.7 of this contributions from this weird stuff that we don't know, we don't understand the doubt matter, making up about 27% as well, so together than making up 97 98% of the of the energy density of the universe that we just don't understand what it is.

But today we're gonna talk about the dark energy.

So one of the things that I've been working on for the last few years is trying to understand the source of dark energy.

Now it could be that the source has always been there.

Andi.

It's been there throughout the history of the universe, and it's just that it's come to dominate everything today or in the recent past, by recent past.

By the way, I mean, within the last seven billion years or so, we're talking Cosmo, leave you here.

It could be that it's been constant.

That would be a term cut.

That would be a quantity that is known as a cosmological constant, and we can come back to that.

Another possibility is that it's something that has been evolving with time on that it had it was sub dominant early on, but perhaps it mimicked the matter and radiation and you notice it pretended it was like matter and radiation.

It followed the the density of matter and radiation dropping, dropping, dropping.

But for some reason, about seven billion years ago, it came to dominate everything and that those air colds generally coming to the name of quintessence theories where it's evolved with time.

Then there's possibilities that it's not due to some energy source it all that what we're seeing here isn't either.

Cosmological constant isn't some new form of energy, but what we're seeing is the first evidence that Einstein's theory of general relativity needs modify on.

We're seeing that it needs modifying on these large scales the size of the universe, the observable universe on these air called modified theories of gravity on DDE.

So these three possibilities are all being actively pursued, a za way of understanding the observation that the universe is accelerating today and I've been involved in a kind of all three of them trying to come up with models which try and mimic these.

So then it doesn't have to be dark energy because maybe everything doesn't need.

That's right.

If so, of course, when we say Einstein's got it wrong, it's a bit like he's got it wrong.

Perhaps on these largest scales, he's clearly brilliantly correct on solar system.

Everything within the solar system is beautifully explained by Einstein's theory of gravity.

In fact, to be honest, if I say it quietly, pretty much everything on even cosmological scale seems to be brilliantly explained by Einstein's theories of gravity.

So any modification would have to be kind of quite small.

But it could be that we just need these small modifications, in which case we don't need these new exotic forms of energy, energy density.

Thio explain it.

As you say it is due to some change in the gravity of the curvature of the universe that that that's leading to the observations That universe appears to be accelerating.

Why do you say Doc?

And he's no good.

What would you call up?

Well, I've just been thinking about that because I don't think dark energy is such a great name.

That's there's lots of things that a dark then It's not particularly that that's special.

Lots of things have got energy on this.

It's not really that that's special.

But the thing that makes dark energy the important quantities it is is that the pressure it has is negative.

It's got negative pressure, and that means it actually causes things toe push apart.

That's the way that you can imagine it.

So some of the discovered you can think of it.

It's been having a bit like attention on associated with it, and it's smooth.

That's the other thing about it.

It's It's a very smooth components through.

It's pretty uniform throughout the whole universe, so something like a smooth tension might be.

It might be a better thing toe have rather than rather just moved pension.

I guess the problem with that name is it doesn't it doesn't allude to the fact that it's completely backward.

Don't know.

It makes it sound like like we might know what we're talking about.

All that we know is that when you write down the equations, Einstein's equations on Dhe allow for something that can have a negative pressure.

It will do the job for you.

That's the clear.

So a cosmological constant will do that for you as well.

These concocted models that you have these contestants models, they will do it for you.

And then, of course you can.

You can also use your modify gravity, and the modifications will do it for you.

There is one other thing that will that can effectively do it for you, which is is less in favor but hasn't been completely ruled out.

And that is maybe the universe isn't a smooth on his uniform.

As we thought, maybe we knew of the re structure in the universe, right?

We know, Look at you and I were quite different.

Andi, Even though you're down the gym and I'm down the gym, we're still quite different.

And you So there are structures out there.

We know that there are that galaxy is a kind of clustered together on in filaments on those filaments kind of surround void, so voice where there is lower density than on the average.

But the thought is that as you get to bigger and bigger scales in the universe, that that the distribution of those voice becomes a bit more uniforms Andi there that their effect drops off, giving you this net homogeneity.

Things are very smooth, but maybe that's not the case.

you know, maybe actually, the universe is is homage in homage ing.

Yes, it's not homogeneous, nice and smooth and live scales.

And if it's in homogeneous, then and we happen to be living in a void in a region that's gotta lower density than on average than we would actually perceive the universe to be expanding at a different rate from outside on, it could be that that's what we're seeing here, that the effect of this acceleration is, and that's what an acceleration is.

You're seeing a different rate here, compared to here on the effects of the explosion is just that.

But because there are so many, that's probably not the case on.

The reason why it's not the case is is because of the the the fact that we can measure the different things in the universe and from it in first, something about the distribution of the matter so we can measure the microwave background radiation.

We can measure the distribution of the hot and cold spots in the microwave background.

We can measure the distribution of Galaxies and when you do in particular the microwave background on look at the distribution of the hot and cold spots.

It becomes clear that in order for this in homogeneous model type of models to work, then you need to be in a very special place in this void.

In fact, you need to be very close to the center of the void, and then you begin to ask, Well, why is it I would be very close to the center of a void could be anywhere in this void.

So there's some high degree of fine tuning going on in order for you to explain the apparent acceleration of the universe from the idea of these in homogeneous distribution of matter.

So it's kind of being it's under under threat a CZ a model, but it hasn't been ruled out yet, but it's certainly very fine tuned if if under the theory of I know there are always possible was cause if it is the cliche, dark energy is this, like some kind of particle or field that can be measured is that is that thing is the thing with mass or no mass has a core energy that can be measured in jewels.

Maura's this like it's dark and you just feel like I shall I get my cup of get my cup to show you the how much energy there is.

Okay, if you just hang on, I'll go get it.

What a shame has taken it.

I think I'm just taking it to university.

And she had a dark energy cut, but that is it for this cup here.

It's about this size this cup has in it about, I think, is a yack to Graham of Oh, 10 to the minus 24 jewels of energy of dark energy.

Because dark energy is smooth ride, it's throughout the whole universe.

So it is.

It has got an energy has got on that.

It's got an energy density and so you can work out how much energy is in a cup.

And I think it's about 10 to the minus 24 jewels, which is no, not a huge amount, but it's it's it's there.

So what makes it spent?

You said, What is it?

Well, we don't know what it is, and that's why we try lots of things that can and mimic its effect.

The the key ingredient isn't particularly that it's got an energy.

That's lots of things have got energy matter has got energy.

Radiation has got an energy associated with it.

That doesn't make it.

That doesn't make the end of the universe accelerate.

It actually slows down the universe.

But what The things that have energy also often have pressure.

And now we usually think of pressures of a positive quantity.

You think of particles banging against the surface of a container, creating a pressure pushing out.

And that's that's a positive quantity of radiation has in the in the early universe.

Relativistic red particles have a radiation pressure, which is 1/3 of the energy density had dark matter particles and the particles that you and I am made up off there called dust particles written.

That's the general name given to them.

They've got basically no pressure because they're hardly moving.

They're moving non relativistic Lee.

So you think of putting some doubt matter or some barriers that were made of in a in A in a tin on, they'll hardly hit the side of the tent because they're moving set so they don't create a pressure.

So that's got zero pressure or very close to zero.

What mix Dark Energy special is that the pressure, it has actually is negative on DSO.

It's you need.

You need something that can that can produce, have a positive energy density but have this negative pressure that's acting in the opposite direction to what you might imagine.

And there isn't any standard stuff.

Does that the stuff that you and I are made of, the particles that you know it just does not do that.

You need something more a bit more exotic on the took that the two favorite where's of doing it are basically to use what the particle cosmologist loves to use, which is a scale of field on a scale of field has both.

A It's a it's an object that's got a value everywhere on Dhe.

Its energy is made up basically of a kinetic energy.

So the kinetic energy of motion of that object on the potential energy that sort of telling you at any given point in space and time.

What's its potential associated with it now, if it turns out that potential dominates over the kinetic, so the kinetic energy of the field is Sasa letting around a small compared to its potential energy, then it will have a negative pressure, and it can cause a kind of a repulsive effect in terms of causing acting against gravity on dhe.

And so that's what a quintessence scenario will do for you.

You work in a regime where the potential dominance over the kinetic energy and it will give me this negative pressure.

It's what a cosmological constant does, in fact, in a cosmological constant.

It's a unique case where the energy density is exactly minus the pressure.

So they balanced that they're equal in magnitude, but opposite in sign, and that hasn't has the same effect.

The data suggests that the universe is perfectly consistent with the cosmological constant so that the acceleration of the universe has bean driven by something that looks like a cosmological constant.

In actual fact.

Just recently, there's a set of death have been coming out.

If you look at the plank on dhe microwave background data, if you look at people at the data coming from, people have looked at some supernova.

That, by the way, is the way in which we infer what the universe is doing.

You you will come back to that if you look at the that they're debtor they're actually finding that the pressure is actually less than the pressure is less than the energy dense, the minus.

Okay, What they're finding is that when I divide the pressure by the energy density for a cosmological constant, that number would be minus one.

If I do the same for ordinary matter, that number is positive.

So the pressure is 1/3 of the energy tested for radiation.

The pressure is zero for dust.