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particle physics.

And in this last video, we will go into some more depth

on the preceding discussion of dark energy and dark matter.

To do that, we have invited ourselves to a colleague from the

theoretical physics department of University of Geneva.

Let me present to you Ruth Durrer, professor of theoretical physics

who is a specialist on cosmology,

the physics subject which remains most

concerned with these two phenomena, thus far inaccessible

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I think there is little doubt on the existence of dark matter, is there?

Or am I mistaken, are there still people who believe that

dark matter does not exist?

>> Yes, there are such people.

Dark matter, as you have already heard in the preceding videos,

has been confirmed by only one interaction, which is gravity, and by

no other, so they ask the question: should our

theory of gravitation – to discover dark matter, one only needs

Newtonian gravitation – should it be modified at large distance scales.

There are people who have tried that, and who were able to explain, for example,

the fact that the rotational curve of galaxies does not descend

like one would expect, if gravitation obeys Kepler’s law.

But even if one can modify

gravitation such that one obtains flat rotational curves also,

there is still a lot more confirmation for the existence of dark matter.

>> Like gravitational lenses… >> Exactly.

>> …the motion of galaxies,

and their interactions, like the Bullet Cluster for example…

>> Exactly.

And if you want to explain all that by modifying the theory of gravitation,

you can for example do this with what is called TeVeS,

but it is a horribly complicated theory and one makes hypotheses which are much more

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complex and non-minimal, than to just add a particle.

So, I would say that 99% of

the scientists are convinced that dark matter

is what it takes and not a modified theory of gravity.

>> And in that case, there should be a particle which constitutes

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dark matter, and one should be able to see its interactions, either among themselves,

as we try with AMS, or with normal matter as out colleagues

try in Zurich, for example, in their underground experiments, or

by the artificial creation of this matter, at the LHC for example.

So, up to now, we have not succeeded to do so,

but it is never too late to try.

So, if such a particle existed,

what would be the implication for the Standard Model of

the existence of an additional particle, which we need to explain this, don’t we?

>> Of course,

such a particle is beyond the Standard Model of particle physics

but there are plenty of hypotheses on the existence of such a particle.

For example, in supersymmetry.

one expects that there be a stable supersymmetric particle.

Or one can add a heavy neutrino, to explain,

for example, the generation of quarks and leptons in the universe

and this could, at the same time, also play the role of dark matter.

A low mass neutrino does not work, because it has too large velocities

to be confined in galaxies, etc.

So, we need a particle with a minimal mass of at least

some kiloelectronvolts.

Standard candidates rather have some hundred GeV of mass.

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If it is comparable to weak interactions, in the coming years

one should discover it either at LHC, or with direct or indirect

interactions. But if the interaction is much weaker,

if we had, for example a gravitino, which interacts via the

gravitational force, we will not discover it by these experiments.

>> But we are still in good hope that during our own active lifetime

we will know of what is consists.

>> Yes, I do hope so.

It is one of these problems, which are with us since 70-80 years.

>> Since Zwicky, yes.

>> Yeah. >> So, on the contrary,

I have much less hope as far as dark energy is concerned, no?

So, can you share with us your vision of dark matter.

First of all, according to you, what is it?

>> I don’t know.

This is the honest answer, we must leave somethings to do

for young people, to explain to us.

I don’t know what dark energy is.

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So, I can tell you that current data are in good agreement with the

assumption, that dark energy is just a cosmological constant.

But I will talk a little about how

one has discovered the existence of this dark energy.

One has seen that the universe is currently in an accelerated expansion, and not

as one had thought, that due to gravitation the expansion would slow down more and more.

In fact, one has found that, since a while,

this slowing down has reversed and has become an acceleration.

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And this acceleration,

one can explain it, if there is a component practically with negative gravity.

So, we need, for example, something which has

a very strong pressure, a negative pressure.

Negative energy density is not possible

according to the Friedmann equations

which are a consequence of the Einstein equations…

>> Well, we have scratched the surface of these equations a little bit earlier in the course,

so, if you are no more familiar with this,

please watch again the previous video, which talks just about the Friedmann equation.

So, if the famous lambda is the only thing which exists,

this would be a property of our universe, which was created together with our universe, wouldn’t it?

So, by chance, well, it has the value it has,

and we must live with it, no?

>> It is more complicated than that.

The problem is that this lambda, this cosmological constant,

must have changed, for example,

if the interactions of the universe passed through a phase transition.

>> Yes.

Like inflation, to just name one.

>> For example, during inflation.

And during that transition, the cosmological constant has changed

its value, which was 10 to the power 120

greater that the value we measure today.

So, this value requires an enormous fine tuning.

>> I.e. of the initial conditions to get there.

>> And most of all this is not what we call

a « natural » fine tuning

in the sense that in field theory

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this constant is not protected from corrections.

If one modifies the theory a litle bit, this constant changes enormously, etc.

So, from the point of view of a quantum field theory,

such a constant is very, very unnatural, unexpected, bizarre.

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>> So, all of this requires effectively

that his field is a quantum field, no?

Is it really necessary that this be so,

or could it not simply be a classical field,

which does not have a quantum representation, which does not exist

in a theory at small scales, does not exist or is not necessary?

>> Well, the cosmological constant itself is not a field, but it is

an effective constant, which comes from the quantisation of all other fields.

And that the electron is well represented by a quantum field,

or the quarks, I believe that you have discussed that in your course.

So you cannot say that at very small scales, this is not the case.

I would even say that usually all fields, even effective ones,

even non-fundamental ones, can be described by quantum fields,

that they exhibit a quantum nature if one talks about very small excitations.

>> Like phonons in a solid, for example.

>> Exactly, like phonons in crystals.

But here, we have two explanations, which are

a little bit more satisfactory, but more complicated and also not very attractive.

One is, one adds a scalar field,

which is not constant but nearly constant.

And with that, if the field is sufficiently constant,

one can explain dark energy.

But the difference to a cosmological constant becomes more and more

diffuse there.

Another one is that one changes the theory of gravitation at large scales.

You see, one has tested Einstein’s gravitation for distances beyond

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a tenth of a millimetre,

a hundredth of a millimetre, up to the size of galaxies,

and maybe at the distance scale of the universe one must add modifications.

This can be done, there are modifications, for example concerning the graviton,

which can have a very small mass.

This mass must also be fine tuned.

But at least the quantum corrections, if one develops a quantum

theory of gravitation, are small, they are logarithmic corrections,

which do not blow up everything, when one

describes the facts by a quantum theory, like the cosmological constant does.

>> I believe that it is, however, correct to say that this form of energy is not

easily amenable to human experimentation, is it?

This is a phenomenon, which really only exists at a cosmological scale,

and which stays confined to this domain.

>> If it is a cosmological constant,

it is accessible only by gravitational forces.

All other forces are not sensitive to a constant

difference in energy.

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But, if it is not a cosmological constant,

but if it is a modification, lets say, take this example,

a modification of gravity, one should be able to also see it

on other scales, with very high precision experiments, etc.

Or, if, for example, the graviton had a mass,

this would mean that the graviton has five helicities and not only two.

Ans maybe with certain experiments, still gravitational ones,

one would be able to excite these other helicities of the graviton.

So, there would be this other possibility of see that.

>> Experimental possibility.

>> Yes.

But I would say also, however,

that a cosmological experiment is a human experiment.

So, this is not to scale, but the scale

tested at LHC is as far from the human scale as the cosmological scale.

>> There, I admit, that is true.

>> You agree, Martin.

>> OK.

So, thank you very much for this interview.

I think that it is very important also

to indicate like an open window towards the future of our profession.

Because the course does not stop here.

It invites you to stay interested in the forthcoming

experimental as well as theoretical progress in the field.

So, I invite you to have eyes and ears open,

for what will happen, and for what will probably again revolutionise our field

in the coming decades.

Thank you for your attention.

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