Phantom Dark Energy (full interview with Ed Copeland) - Sixty Symbols

0:01 what's going on in the the universe of the universe.

0:05 Well, Brady, there's been a couple of uh recent results

0:08 that have been published um which I think are pretty interesting.

0:12 They concern what the universe is made of and what it's doing.

0:16 So, I thought we could talk about them.

0:18 I think uh one is uh sort of saying everything as it should be, stay calm.

0:25 The other is like, oh wow, something big's maybe happening here.

0:30 start getting excited if you're a theorist.

0:32 And so I thought we'd try and place them in some

0:35 sort of perspective and just describe them a little bit.

0:39 Let's remind ourselves of what we think the universe is made of.

0:42 Okay, the universe is here.

0:45 We're in it, but it's very big and it's made of lots of different things.

0:50 Uh you and I and the particles that we're made

0:53 of, we kind of club them together and we call them barons.

0:57 They make if if the if you add all the masses

1:00 or all the energies of of everything that makes up the universe

1:03 together to make it to one just we normalize it so

1:06 it's all to one then um then the baronss are about 5%

1:11 of that you and I make up no more than about

1:13 5% of the total energy content of the universe is this like

1:19 uh atoms or protons or absolutely atoms protons electrons light and you

1:26 know you everything you can see around you it's a negligible

1:30 amount but it's an important amount because we wouldn't be here

1:32 without it but it's about 5% about 25% or so is thought

1:39 to be in what we call cold dark matter we've never

1:42 seen it but we an individual cold dark matter particle we don't

1:46 know what it is we've looked for it we're still looking

1:49 for it and we've got lots of ideas on trying to find

1:51 it whether it be direct detections underground in or through the Large

1:57 Hydron Collider looking for effects

1:59 or indirectly from things emitting from galaxies,

2:02 but we've not been able to pick out an individual particle.

2:04 But the the way light gets bent as it passes through galaxies

2:09 and clusters of galaxies seems to indicate that there it has to be there.

2:14 It's about 25% of the total energy content of the of the universe.

2:19 So when you're talking about the universe,

2:22 I think of mass like how you know kilograms

2:24 and of of yeah you know of us but even that you measure

2:30 in energy yeah people tend to measure things in terms

2:32 of energy we can convert it to mass it becomes a ginormous

2:36 number in my head I'm thinking E= MC^² so C is

2:40 the speed of light which is a constant so E energy

2:44 is the same as mass that is what we tend

2:47 to think about when we we say things in terms of energy.

2:51 We we're really meaning mass as you as you say.

2:53 Okay.

2:54 So, we've got to 30% with Yeah, we've got to 30%.

2:56 So, we've got to the galaxies and the structures

2:58 of galaxies and but the universe is uh expanding.

3:03 Well, that's okay.

3:04 We thought the universe would be expanding because um it began with a some

3:08 sort of big bang things be some initial conditions caused it to expand.

3:12 But with a universe that's just made up

3:14 of the matter in the it should actually eventually slow down.

3:17 It should be decelerated.

3:18 So, it's expanding, but it's like someone's putting their foot

3:21 on the pedal of a car and it's slowing it down, decelerating it.

3:24 Why is that?

3:24 Because everything attracts back to itself.

3:26 Everything attracts back.

3:27 Yeah.

3:28 Gravity sucks, right?

3:30 Gravity sucks.

3:31 So, so everything gets further away because of the initial propulsion,

3:34 but then eventually it gets it slows

3:36 down because the gravity sort of saying, "Whoa,

3:39 come on, come on, slow down." But in the early

3:43 90s and then in particular the about 1998 um astronomers looking at first of all

3:50 the clusters of galaxies but then looking at supernovi exploding

3:54 in in in distant star in distant galaxies realize

3:58 that this can't be right that the universe has another

4:01 component in it and that component which we've dubbed dark

4:06 energy makes up about 70% of this 60 yeah 70%

4:12 of this right 25% in barons 25% or so

4:16 in in cold out matter and this stuff is like

4:19 70% of of of the energy content of the universe

4:23 and it's not only present it's driving an the universe

4:27 to accelerate so the ex the distant galaxies rather

4:30 than slowing down as I as you might think they

4:33 should accelerating apart someone's putting their foot on the accelerator

4:38 not on the brake A couple of quick questions.

4:43 First, you said' 90s and 98.

4:45 I thought it was like back in the days of Edwin

4:47 Hubble that it was shown that the universe was expanding.

4:50 Sorry.

4:50 Yes.

4:51 Edwin Hubble showed the universe was expanding.

4:53 Quite right.

4:53 In fact, a guy called Sliper even earlier may have been

4:57 sort of the first person to infer there was something going on.

5:00 But uh that's different from saying the universe is accelerating.

5:04 Oh, so the universe is he indeed showed it was

5:07 expanding and growing but we thought it was slowing down.

5:11 And in fact when I began my PhD

5:13 in the mid 1980s everyone said the universe is slowing down.

5:17 It's late 1980s early 1990s and in particular

5:21 1998 they realized it's going whoosh.

5:23 It's accelerating.

5:24 So this 70% this is the dark energy.

5:28 This is the dark energy and one of the big

5:30 goals in cosmology now is to try and understand its origin.

5:34 What is it that's doing it?

5:36 And it it has to have a particular

5:38 property in order to allow the universe to accelerate.

5:41 And maybe it's worth because all of these papers refer to these properties.

5:45 So matter that you and I are made of and radiation they

5:50 we have both a a dens an energy density energy and a pressure.

5:56 So particles hit things.

5:58 Radiation pressure is something that people at school,

6:01 children at school get to see.

6:04 The photons can get emitted and and little

6:07 uh aluminium foil or gold foil can get

6:10 pushed up by the pressure of the radiation

6:12 that's been so there's a positive pressure pushing out.

6:17 You and I are made of particles that are not moving very quickly.

6:20 So then we are non-relativistic.

6:23 So our pressure is almost zero.

6:25 We we had you know the pressure of the particles

6:28 in this room is not making much impact

6:31 on on the on the walls of the room for example

6:35 anything that's got a positive pressure leads to a decelerating universe.

6:39 It slows it down.

6:40 That seems counterintuitive because you're talking about pushing

6:43 an impact and that but you're saying that stops.

6:45 Yeah.

6:45 Because the energy that's that's transferred to it from the ex

6:48 from the expanding universe causes the universe to slow down.

6:53 Einstein showed us a very important result that is the curvature

6:57 or the expansion of the universe is linked to the matter in the universe.

7:01 They're not independent of one another.

7:02 And so if something's happening to the matter,

7:04 a compensating thing is happening to the expansion of the universe.

7:08 And so this causes the universe to slow down.

7:12 But dark energy is a type of energy that has a negative

7:16 pressure which is sort of hard to get your head around.

7:20 But the the effect is it feeds back into the expansion

7:24 of the universe and it causes the universe to expand.

7:27 It's sometimes known as thought of as anti-gravity.

7:29 I don't like to call cause it that.

7:31 But if you think of gravity sucking then

7:35 dark energy will try and expand it back out, accelerate it back out again.

7:40 So dark energy has a negative pressure.

7:43 Does it have a negative energy?

7:45 No, it has a positive energy.

7:46 So it's not like anti- energy.

7:48 No, it's not like anti-energy or anti-mass.

7:50 No, it's its own thing.

7:52 Okay.

7:53 And the type of properties that type of substances

7:56 that can have this property are the most

7:59 famous is the cosmological constant which we've talked about

8:02 in the past that Einstein introduced into his equations.

8:05 Actually to try and stop the universe expanding.

8:08 But by to to make it stop, he had to make the universe closed.

8:12 So our universe is spatially flat.

8:14 It's just keeping going.

8:15 He made the universe closed.

8:17 And by doing that, gravity wants to pull it

8:19 in, but the cosmological constant wants to push it back out again.

8:23 And and Einstein said, well,

8:25 there's a there's a a st there's an equilibrium point

8:27 where they balance and that leaves you with a static universe,

8:30 which is what he wanted.

8:32 Turns out that's an not a good solution.

8:34 The universe is spatially flat.

8:36 So the cosmological constant is there and it's causing

8:39 or something like it is there and is causing this acceleration.

8:43 So we have this negative pressure.

8:46 We have other things that could act like

8:47 a cosmological constant or act with a negative pressure.

8:51 You're talking about the cosmological constant like

8:53 it's a thing like it's a like it's

8:55 a kind of particle or a kind of I always thought cuz it was a constant it

9:00 was just a number like it was a correction but you're the cosm but when you

9:04 say cosmological constant you're talking about some kind

9:07 of yeah we're trying to understand what it is.

9:09 It's a thing though in my head.

9:11 It's a thing, right?

9:12 And um so one of the things that so constant is quite a misnomer.

9:17 A constant energy density.

9:19 Maybe you'll So something some material of some

9:23 form that has a constant energy density.

9:26 That is what the cosmological constant is.

9:28 Okay.

9:29 Do scientists have any idea?

9:30 I mean you know lots of particles and you know

9:32 lots of energies and fields and either real or imagined.

9:37 What's imagined that could have a positive energy like all

9:40 the other energies we know but somehow has this negative pressure?

9:44 Has someone come up with that?

9:45 So the the the obvious one is this cosmological constant.

9:48 But that but no but you don't know what that is either.

9:50 I don't know what it is.

9:51 No.

9:51 So that's I'm talking about what what's something that could actually do it?

9:55 What's a physical scale?

9:56 A scalar field like the Higs field.

9:58 We well but we won't we only know one scalar field in nature.

10:03 But the Higs field or another scalar field,

10:06 a scalar field is a is a a field of nature that's just got a value everywhere.

10:11 It doesn't have a direction.

10:12 Hence scalar.

10:13 A vector field like an electric field or a magnetic

10:17 field has both a value and a direction.

10:19 A scalar field is like um a temperature, right?

10:23 A temperature in this room.

10:24 It can have different values in different parts of the room,

10:27 but it's a scalar quantity.

10:28 You don't say it's 23° C in that direction.

10:33 So a scalar field is a a quantity that can have a negative pressure if

10:39 it satisfies certain conditions on its how fast

10:42 it's moving compared to its energy, its potential.

10:45 What do you mean how fast it's moving?

10:46 Because you said it hasn't got direction.

10:48 A scalar field can change in time.

10:50 Sorry by you, you're too smart.

10:52 It can change in time.

10:54 So like a temperature, right?

10:55 You you don't Yeah.

10:57 Okay.

10:57 That's a that's a good example.

10:59 If I um cool if I put ice into uh into water, I'll cool the water down.

11:05 So there's a a rate of change of that temperature.

11:07 That's the that's what I'm talking about.

11:09 It's with respect to time, not not with respect to distance.

11:12 It's not a velocity.

11:13 Okay, that's a good point.

11:15 Okay, so there's some kind of scalar field, right?

11:18 Could do it right now.

11:20 The big question is what is the dark energy?

11:23 The best fit models for our universe to date have

11:28 been a model where you say it's a cosmological constant.

11:31 So my my universe consists of basically four things.

11:36 It consists of the cosmological constant.

11:39 It consists of cold out matter and it consists of the baronss.

11:43 And by barons I'm clubbing everything in together.

11:46 It could have a curvature but we but the data suggests the curvature is zero.

11:51 So we've was three things uh cosmological constant cold dark matter barons

11:58 and curvature and I'm saying the curvature is looks like it's zero.

12:02 Okay.

12:03 So it could be that and and the data has been saying that is the best fit.

12:09 So what what I what do I mean by that?

12:11 Because we now have a background cosmology,

12:15 we can solve Einstein's equations and we can ask what happens

12:19 in this this universe when photons of light pass through it.

12:23 What happens to those photons when the if I look

12:26 far enough back in time to the cosmic microwave background which

12:30 it was just to remind you the cosmic microwave background

12:34 was emitted way back about 340,000 years after the big bang.

12:38 Remember the universe is 13.7 billion years old.

12:42 It's when the photons decoupled from the particles and just started propagating.

12:47 That's the cosmic microwave background.

12:49 And there are slight deviations that you experience

12:52 in this cosmic microwave background related to the early universe.

12:55 And these satellites, these amazing cosmic microwave background

12:58 satellites have measured these fluctuations.

13:01 The the way the fluctuations change as a function

13:04 of scale on the universe either small scales

13:07 or bigger scales or all the universe that you

13:10 can see is a dependent depends on your background cosmology.

13:14 In other words, you can constrain the values

13:17 of the parameters called matter cosmological constant

13:21 barons by looking at these peaks and troughs

13:24 in the in the cosmic microwave background.

13:26 And the data that fits beautifully is lambda CDM.

13:30 And in fact, the the first paper I sort of want to touch on is the ACT paper,

13:36 the Atakama cosmology telescope,

13:38 which has been has just released six years worth of data

13:43 from 2017 to 2022 when it's I think it stopped operating.

13:49 Now they've analyzed all the data from the cosmic

13:53 microwave background fluctuations that it can see and pretty much

13:57 everything is consistent with this model and the impact is

14:00 I mean I think it's a beautiful piece of work.

14:02 It's very technical which I way beyond what I

14:05 understand in terms of how they have done some

14:07 of the analysis but their conclusions they they they

14:11 try to let things change from the cosmological constants.

14:15 So let's call it the lambda CDM paradigm.

14:17 That's the one that we've been discussing.

14:19 Lambda CDM.

14:20 What does that mean?

14:20 Is that the four things?

14:21 Yeah.

14:22 Well, yeah, cuz I'm taking Yes, I'm taking the the curvature to be zero.

14:26 But lambda is cosmological constant.

14:29 CDM is called that matter, but the baronss are sort of included in there.

14:32 Right.

14:33 So, you're saying lambda CDM this understanding

14:36 of how these four things work in the universe.

14:40 People like you are constantly twiddling the dials to say, "Oh,

14:43 could there be more of this or less of this?" But you

14:46 always have to come back to this touchstone of the cosmic micro background.

14:50 Very good.

14:51 And if it doesn't fit that, then it's out the door.

14:53 There's this famous quote, right, from Fineman of um you can have the you

14:58 can have the most beautiful mathematical theory in the world,

15:00 purely elegant, totally elegant.

15:03 If it doesn't fit the data, it's not right.

15:07 So you you have to come back to the data there.

15:11 Why do people like like me not just say something like well

15:15 it's just lambda Einstein intro it's just lambda get you know go

15:19 and do something decent go and more interesting why are you worried

15:22 about it and and the reason we're worried about it from a theoretical

15:26 standpoint is that there are big issues with the value that lambda

15:30 must have in order to fit the data there are other processes

15:34 that occur in nature lambda lambda being the cosmological constant there are

15:38 other processes that occur in nature which contribute to what lambda should be.

15:44 And when you calculate them,

15:46 they're generally way higher than the value of lambda that we see.

15:50 In other words, the value that lambda would

15:52 have when we take into account our theoretical

15:55 models of what we think lambda should be is quite different to what we see.

16:00 And so some of us think there must be something else going on.

16:04 that there must be a reason why lambda is playing no

16:07 role and that the dark energy is nothing to do with lambda.

16:12 It's it's it's being cared for by something else.

16:16 And what's actually driving it is maybe one of these scalar fields.

16:20 And that's where we're looking for what we call

16:22 dynamical dark energy because these scalar fields evolve in time.

16:26 We just discussed it.

16:27 The temperature changing over time is like the scalar field changing.

16:31 And so it rolls down.

16:33 its own potential and its contribution to the dark energy changes over time.

16:39 So if you had an experiment which could

16:41 change see evidence of of this changing over time,

16:45 it would mean the dark energy isn't a cosmological

16:48 constant because the cosmological constant would be constant throughout time.

16:51 Its contribution would be constant throughout time.

16:53 If you had this dynamical dark energy, it would change over time.

16:57 And up until about a year ago,

17:00 all the data was suggesting there's no evidence for a dynamical dark energy.

17:05 Nothing out there.

17:06 Just coming back to ACT for two seconds.

17:08 Yeah.

17:08 Well, we can go back for more.

17:10 It's such a good piece of work.

17:11 But the ACT, what ACT did was they used this telescope.

17:14 Are they more accurately measuring the cosmic microwave background

17:18 which in turn gives you a more uh constraining Yeah.

17:23 the data is constraining your model more and more and more.

17:27 And it's worth pointing out um there was another breath there's been

17:31 a number of breathtaking cosmic wave

17:33 background experiments and we've discussed them.

17:35 One is I mean the original was Kobe which

17:38 first saw these fluctuations back in the early 90s.

17:41 Then W map which is a a Princeton le collaboration

17:46 saw what we call the acoustic oscillations these peaks and troughs

17:50 in the in the temperature of the microwave background and then

17:54 there was plank and plank we did a lot on where

17:57 they really found clear evidence for these peaks as of and what

18:01 am I drawing here I'm draw I'm drawing the the amplitude

18:04 of the fluctuations as a function of of of length

18:08 scale and large lengths are here on the left and small

18:11 lengths are here on the right and plank sees big fluctuations

18:15 on large lengths and then it gradually comes down and then

18:18 eventually their sensitivity drops at a I can talk about

18:23 wave numbers but there's no no need I mean now what

18:26 ACT can do so plank's doing large lengths to some

18:29 intermediate scale here act does this intermediate scale to even smaller

18:34 scales so ACT sort of matches on to plank and then

18:38 goes further and so it It's matching onto plank is beautiful.

18:43 It matches really well and then goes further and it fits beautifully

18:46 these lambda CDM models and so they are really constraining these models.

18:52 They then do what all of us theorists like to do

18:55 is we we ch we allow the models to fluctuate.

18:58 We say imagine that the obvious one

19:00 is let's imagine the dark energy isn't constant.

19:04 And so they do they let it vary.

19:06 they have some representation of it varying and they constrain

19:10 th that representation to my surprise they have this they do

19:15 it and they they actually say they have to combine

19:19 their results with with another set of results from DESI which

19:22 we're also going to talk about and when they do

19:25 they say there's moderate evidence that the dynamical fit is slightly

19:31 better than the cosm than the lambda CDM fit which

19:35 I found intriguing But overall they said there's no real evidence.

19:40 It's a such a small change that that you

19:42 can't say this is evidence of a better fit.

19:45 But they allow lots of different things.

19:47 And the one that I should mention to you is

19:50 um they allow for the dark matter to be axons.

19:57 So we think of dark matter conventionally we've thought of dark matter

20:00 as being wimps weakly interacting massive

20:02 particles that come linked often to super

20:06 symmetry very massive objects and we've not seen them we've not detected

20:10 them and so it's caused a a shift in the community towards

20:14 much lighter dark matter particles called axons there's a lot of work

20:19 going on in axons and to my amazement if if unless I'm

20:21 misunderstanding it they're saying in terms of the contribution to dark matter

20:26 axion like particles can only make up about 5% of the dark matter.

20:32 That's a big that could be a really big result

20:34 because people are busy looking to see if axons can be

20:37 all the dark matter and if there can only be

20:39 5% then the dark matter has got to be something else.

20:42 Okay.

20:43 Okay.

20:44 I've moved away from the dark energy briefly.

20:46 Okay.

20:46 So, so ACT have given us a better idea about the cosmic microwave background

20:52 and in turn have let us look more closely at how well our models are working.

20:57 Very good.

20:58 And they're saying the models are looking pretty good.

21:00 They're saying there's no reason to go beyond Lambda CDM.

21:03 That's what their bottom line seems to be to me.

21:05 Lambda CDM is fitting really well,

21:08 which is what you explained right at the start though.

21:10 Yeah.

21:10 The Okay, what's going on?

21:13 Talk to me about let's get dynamic.

21:15 So, let's get dynamic.

21:16 So, a year ago, there's another fantastic experiment.

21:20 Let me just I I'm rubbish on experiments, but let me just give you some numbers.

21:25 There's this amazing result.

21:27 It's called the dark energy uh spectroscopic instrument, DESI.

21:32 The hint is on the in the title, right?

21:33 They're looking for dark energy as well.

21:35 They have over the course of three years they have taken spectroscopic details

21:40 in other words light coming measured the light

21:42 coming and the wavelengths of light coming

21:44 from 30 million galaxies in three years which is just think about how many

21:51 a day they're managing to do and the reason they can do it is

21:55 I if I understand right the plate which collects the light has been designed

22:00 in such a way it basically has 5,000 points in it which can they

22:06 can move around to look at at any one time 5,000 different galaxies and take

22:12 the light from them and that's one observation and then let's do another one.

22:16 So every time they're doing five I mean it's just staggering.

22:21 So they're looking at these galaxies which are

22:24 much closer to us than the CMBB is.

22:27 And from there they're also trying to infer you know by looking at what we

22:31 call baron acoustic oscillations and the baron

22:34 acoustic oscillations tell us about the background cosmology.

22:38 They're looking at these baron acoustic oscillations

22:40 linked to these galaxies and from it they're trying to infer the cosmological

22:45 parameters the lambdas the cold matters.

22:47 a year ago they they said they've got tentative

22:51 evidence that the best fit to their models is not

22:55 a lambda CDM and in fact when they fit

22:58 to lambda CDM they find real constraints conflicts if you like

23:04 with um the CMBB results and they said the way

23:08 they can reconcile it is by saying the dark

23:12 energy isn't a constant it's evolving so it's changing over

23:16 time and they they then constrain the way it changes.

23:20 They they use a particular way of parameterizing it

23:23 because we don't know what the dark energy is.

23:24 So we don't know how it's really changing.

23:26 So what you try and do is you you try and parameterize it.

23:29 You try and think of possible ways it

23:31 can change and and constrain those possible ways.

23:34 So they do that and they said back a year or so ago

23:39 it was about a we call it a three sigma two to three sigma.

23:43 Well, if if in in the world of particle physics,

23:46 three sigma is okay, that's interesting, but I'm not going to get too excited.

23:49 Two sigma is could go away any day.

23:53 Five sigma in particle physics is that's whoa detection.

23:56 Higs Higs detection was a five sigma of it.

24:00 So they have gone on and they've been carrying on doing this analysis.

24:05 They've now produced their second set of data

24:08 and they're now saying the evidence has gone up

24:12 depending on which set of data they use has gone up to a order of four sigma.

24:17 Okay, so it's getting more significant.

24:19 Now why am I I mean I I personally would love it if there's dark energy,

24:25 dynamical dark energy.

24:27 What's the claim here?

24:28 is the claim that at different points in time

24:31 there's different amounts of dark energy in the universe.

24:35 Yeah.

24:35 Yeah.

24:36 Yeah.

24:36 And we all traditionally think energy can't

24:38 be created or destroyed or go anywhere.

24:40 And yet at different times there seems to be more floating around.

24:43 So the sum of the energies will always be one.

24:45 Do you remember the the the cold out matter

24:47 and the baronss and the dark and they'll always sum to one.

24:51 So that that's consistent with it overall not getting changed.

24:55 But it's how it sums to one that's changing.

25:00 So when it's a cosmological constant,

25:03 it's its contribution to this to this sum evolves in a certain way.

25:10 So it it it's it begins negligibly and then it rises up.

25:14 But dynamical dark energy will just have a different evolution as it works

25:19 up to become it has to look a little bit like a cosmological

25:22 constant today because that fits so well with CNB but earlier on it

25:26 didn't need to and it will have a different evolution and what

25:29 Desi are claiming is that they could can begin to see evidence

25:33 of this evolution being different to what the cosmological constant is now there

25:39 if that's true it's amazing I mean because it means that there must

25:44 be something else out there in the universe that we just don't know.

25:47 We don't know what the cosmological constant is as as a as a quantity,

25:51 but we know it could be there.

25:52 Einstein said it could be there.

25:54 It's in his equations, this thing.

25:56 But this dynamical dark energy can't be

25:58 a cosmological constant because it's evolving in time.

26:02 And the and the cosmological constant,

26:03 the energy density of a cosmological constant is constant in time.

26:07 And that means it has to be a physically different thing.

26:09 It has to be a physically different thing, completely different.

26:11 And the obvious thing for it to try and be as a scalar of field,

26:15 but there are other possibilities that it could be

26:18 that it's a representation of a maybe we haven't quite understood

26:22 how matter couples to gravity and that that could could

26:26 lead to some evolution that looks like this dynamical dark energy.

26:30 And so so there's this potential strand, right?

26:36 There's the ACT result which is saying pretty much everything's

26:38 consistent with lambda and there's this dark energy survey result

26:43 saying um desi result sorry saying actually things are beginning

26:47 to look like there's a cosmological that there's dynamical dark energy.

26:51 The thing that concerns me a bit about the last result, the Desi result,

26:55 is that when Desi just look at the data

26:58 from their own galaxies themselves and and don't add anything else,

27:03 everything seems to be consistent with the cosmological constant.

27:07 When they add in they they they add in some co

27:11 cosmic microwave background data as well to pin some things down,

27:15 things get a bit more strained, but there's still sort of consistency.

27:19 The thing that really begins to drive the the move

27:23 towards dynamical dark energy is when they use supernovi.

27:27 And they use three different sets of supernovi.

27:30 So these are one's called the pantheon set,

27:33 one's called the union set, and one's called the dezier 5 set.

27:36 Three separate sets.

27:38 And these supernovi are made up of each one's got a,600 supernovi in there.

27:45 There's some overlap between them because

27:47 they've they've looked at the same objects,

27:50 but these are giving quite different represent, you know,

27:54 um degrees of um evidence for the dynamical dark energy.

27:59 They're not giving the same.

28:00 Some are giving two sigma results.

28:02 Some are giving four sigma results.

28:05 And then within the each supernovi data set there's these are made

28:12 of different um groups that have looked at small red shift versus large shift.

28:17 I'm getting a bit technical here and I don't mean to.

28:19 So all I'm saying is there is some questioning still about

28:24 the supernova element of it and whether it's been rightly interpreted.

28:29 Not there's no question about the desi

28:31 data which is just this breathtaking data.

28:34 It's the interpretation of how you're coupling the supernova in with it.

28:40 And it's important we get this right because from a cosmological standpoint,

28:44 from a theoretical standpoint, it's a massive difference whether it's

28:47 a cosmological constant or whether it's dynamical.

28:53 And there's one final thing about the dynamical

28:55 bit which is causing concern for theorists.

28:58 And that is it's driving us into a regime of parameter

29:03 space of theory space where we call it phantom dark energy.

29:07 Phantom dark energy.

29:09 I I have a little link to that because um it's the guy who

29:12 came up with the idea of phantom dark energy is a friend of mine,

29:15 Rob Caldwell at Dartmouth and he was doing this work

29:18 when we were both sharing an office at the Isaac

29:20 Newton Institute and I remember him telling me about

29:22 it and I was thinking that's bonkers because it's unstable.

29:28 Phantom dark energy will is such that it just

29:31 keeps driving the acceleration faster and faster and faster.

29:34 And in fact, the universe ends up going through what we

29:36 call a a big rip and it kind of tears apart.

29:41 But the data seems to be suggesting we're going down there into this regime.

29:44 It's going phantom.

29:45 Now, there are lots of ways of coming back up so you don't end up shooting down.

29:50 But at the moment, the fact is driving us down

29:52 into this region is a is a worry for theory people.

29:55 They're saying this we don't think this should be happening.

29:58 There's quantum mechanical reasons why we don't want to be in phantom regime.

30:02 We don't think it's consistent.

30:04 So there are ways around it but they're not obviously natural should we say.

30:10 So there's lots of questions going on just to and just

30:12 to show the impact of DESI the first paper that was

30:16 released just on a year ago now under a year ago

30:20 has got like 700 citations in a year and you know

30:24 in our field that's huge I mean the CMBB papers get

30:27 this as well but um yeah it's having an impact there's dynamical

30:32 dark energy which changes over time in this worrying but interesting way

30:37 or the traditional cosmological constant which is consistent but then you said

30:43 you said to me that the cosmological constant changed over time

30:46 as well but like that was in a more like I feels

30:49 like hang on they're both changing over time just clarify

30:51 that for me yeah thank you okay so this is the difference between

30:56 what we call the density which is the energy per unit

30:59 volume for a cosmological constant that's a constant and the thing that's

31:05 actually we tend to plot isn't the density It's the density

31:09 divided by what we call the critical density which is another density.

31:12 It's the density of matter that's required to give me a spatially flat universe.

31:17 And what you find for in in that case the one that I did that went up like that.

31:22 It's that parameter.

31:24 It is in in fact it's the sum of those parameters.

31:26 It's the sum of this density parameter.

31:29 It's called omega big omega row which

31:33 is the density divided by the critical density.

31:35 The sum of the omegas is one.

31:38 And it's that that's actually always conserved.

31:41 So the the actual density itself for um

31:45 for dark energy just remains a constant throughout.

31:48 Well done.

31:49 As I was saying, I was thinking what do I do here?

31:51 Okay.

31:52 I wasn't sure.

31:53 Let me ask you this.

31:55 If if it turns out that this Desi finding is heading

31:57 in the right direction and dark energy is dynamical and changes over time,

32:03 what will the implications be for someone like me, not a not a theorist,

32:09 but someone who is a fan of science and is

32:11 really interested and hopes to have some basic understanding.

32:14 How will my understanding of the universe

32:16 have to change that kind of interested layman?

32:20 It's going to mean that we've definitely got something new.

32:24 a new particle of some form or a new

32:26 interaction with gravity that we haven't fully explored.

32:31 And so it would mean that um the consensus

32:35 which has been there now for over a 20

32:39 years that we understand this as a cosmological constant

32:42 plus cold matter plus barons isn't the whole story.

32:46 And I I'd have hoped for and I'm sure for someone like yourself,

32:49 you'd find it interesting to know there's stuff

32:52 out there that we don't yet know and it's

32:54 worth going out to try and explore and find

32:56 out because that's the nature of human beings.

32:58 That's what we want to do.

32:59 But we already don't know.

33:00 We already don't know what the cosmological constant would be.

33:03 So it's just a so it's just it turns out the thing

33:06 that we didn't know about it turns out that's not there at all.

33:09 There's something else we don't know about instead.

33:11 It's a good point.

33:11 And in fact, not only it's it's actually worse than that at some level.

33:15 the cosmological constant has a has this problem and and the problem

33:19 is the the the the what we observe it to be

33:23 if it's lambda CDM is much smaller than what we would naively

33:27 expect it to be and a number of us and you can

33:30 Tony for example and and and I've written it with po

33:32 with Paul Saffin and Tony and Christos um have have tried to come

33:37 up with arguments as to how we might explain away the cosmological

33:40 constant problem the dynamical dark energy doesn't explain away that problem,

33:45 this issue over the differences.

33:47 And what in my head I'm I'm hoping

33:50 is that there's going there's perhaps a symmetry

33:52 of nature that we don't yet haven't yet figured

33:54 out which forces the cosmological constant to be zero.

33:58 It's just not there because symmetry doesn't allow it.

34:02 And then but the d the dark energy is there.

34:05 We we measure it.

34:06 And so the idea be is then

34:08 that the dark energy is is represented by something else.

34:11 But but I I take on board your you're you're moving

34:14 one unknown to another unknown in terms of the cosmological constant.

34:20 You know what a number of us try and do is try and we

34:22 try to understand that there must be

34:23 some fundamental origin maybe in a cosmological constant.

34:26 And so you go back for example for those of us

34:29 who have worked on it and it's a string theory

34:31 where you you look for terms that are present that could

34:34 act like this constant contribution and they're there in string theory.

34:38 They can do it.

34:39 What we can't yet figure out is how to determine their magnitude,

34:43 but they could be there.

34:44 I think the brain problem I've had for a long

34:46 time that I'm struggling to overcome is I've

34:50 always thought as the cosmological constant as a number

34:54 like pi or or just like an an abstraction.

34:57 But when you say cosmological constant,

35:00 although there are numbers attached to it,

35:01 you are talking about a big bunch of stuff.

35:04 Yeah.

35:04 It's something.

35:04 Yeah.

35:05 Something.

35:06 Yeah.

35:06 It's a it's a physical thing.

35:07 It's a thing.

35:08 It's the thing.

35:08 Yeah.

35:09 And uh it's here.

35:11 Hi.

35:11 Yeah.

35:12 Why don't you show yourself?

35:13 Yeah.

35:13 It's not like pi.

35:14 Pi isn't here, but pi is a constant.

35:16 No.

35:16 No.

35:16 It's here.

35:17 Yeah.

35:18 Okay.

35:19 Is the fact that dark energy could be dynamical and have

35:23 changed over the history of the universe an inconvenience for you?

35:27 Because when you're coming up with models about how the univer No, I love it.

35:30 Yeah, I love it.

35:31 But you've always But you but it makes your job easier if it is consistent.

35:36 Yeah.

35:37 I mean it's true you can solve things easier with a cosmological constant.

35:42 Um the the equations you have to solve become more

35:45 readily uh you can do integrations better and but the prospect

35:51 of using observations to test a fundamental quantity like

35:57 a new scale of field just I think is fantastic.

36:01 I I I have to say I'm not yet convinced it's there,

36:05 but um it it would be nice if it was.

36:07 And um yeah, as I I mentioned briefly, that intriguing little line in the ACT

36:13 paper which basically says everything's cosmological constant.

36:16 Don't worry about it does have this one

36:18 little line that says but maybe it's not.

36:20 It says it okay moderately better fit.

36:22 And I was thinking what what's going on?

36:25 And so I'm going to try and talk to some of the people.

36:28 Um, I've got a good friend Joe Duckley who's involved in this.

36:30 So, I'm planning on an email to Joe saying what's happening here is now placed

36:36 and it was built for another large accelerator

36:40 called LEP which is the large electron positron accelerator.

36:44 So, that large because it's so big 27 km.

36:48 So, which spokesperson do you give it to?

36:50 Do you give it to the spokesperson that was

36:52 in charge you know when the discovery was made?

36:55 You give it to the spokes people who sort

36:57 of came up with the idea of the experiment.