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particle physics.

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

Â on the preceding discussion of dark energy and dark matter.

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

Â theoretical physics department of University of Geneva.

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

Â who is a specialist on cosmology,

Â the physics subject which remains most

Â concerned with these two phenomena, thus far inaccessible

Â 1:19

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

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

Â dark matter does not exist?

Â >> Yes, there are such people.

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

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

Â no other, so they ask the question: should our

Â theory of gravitation â€“ to discover dark matter, one only needs

Â Newtonian gravitation â€“ should it be modified at large distance scales.

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

Â the fact that the rotational curve of galaxies does not descend

Â like one would expect, if gravitation obeys Keplerâ€™s law.

Â But even if one can modify

Â gravitation such that one obtains flat rotational curves also,

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

Â >> Like gravitational lensesâ€¦ >> Exactly.

Â >> â€¦the motion of galaxies,

Â and their interactions, like the Bullet Cluster for exampleâ€¦

Â >> Exactly.

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

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

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

Â 2:53

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

Â So, I would say that 99% of

Â the scientists are convinced that dark matter

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

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

Â 3:19

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

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

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

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

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

Â but it is never too late to try.

Â So, if such a particle existed,

Â what would be the implication for the Standard Model of

Â the existence of an additional particle, which we need to explain this, donâ€™t we?

Â >> Of course,

Â such a particle is beyond the Standard Model of particle physics

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

Â For example, in supersymmetry.

Â one expects that there be a stable supersymmetric particle.

Â Or one can add a heavy neutrino, to explain,

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

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

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

Â to be confined in galaxies, etc.

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

Â some kiloelectronvolts.

Â Standard candidates rather have some hundred GeV of mass.

Â 5:14

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

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

Â interactions. But if the interaction is much weaker,

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

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

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

Â we will know of what is consists.

Â >> Yes, I do hope so.

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

Â >> Since Zwicky, yes.

Â >> Yeah. >> So, on the contrary,

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

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

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

Â >> I donâ€™t know.

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

Â for young people, to explain to us.

Â I donâ€™t know what dark energy is.

Â 6:25

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

Â assumption, that dark energy is just a cosmological constant.

Â But I will talk a little about how

Â one has discovered the existence of this dark energy.

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

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

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

Â this slowing down has reversed and has become an acceleration.

Â 7:02

And this acceleration,

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

Â So, we need, for example, something which has

Â a very strong pressure, a negative pressure.

Â Negative energy density is not possible

Â according to the Friedmann equations

Â which are a consequence of the Einstein equationsâ€¦

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

Â so, if you are no more familiar with this,

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

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

Â this would be a property of our universe, which was created together with our universe, wouldnâ€™t it?

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

Â and we must live with it, no?

Â >> It is more complicated than that.

Â The problem is that this lambda, this cosmological constant,

Â must have changed, for example,

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

Â >> Yes.

Â Like inflation, to just name one.

Â >> For example, during inflation.

Â And during that transition, the cosmological constant has changed

Â its value, which was 10 to the power 120

Â greater that the value we measure today.

Â So, this value requires an enormous fine tuning.

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

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

Â a Â«Â naturalÂ Â» fine tuning

Â in the sense that in field theory

Â 9:00

this constant is not protected from corrections.

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

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

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

Â 9:20

>> So, all of this requires effectively

Â that his field is a quantum field, no?

Â Is it really necessary that this be so,

Â or could it not simply be a classical field,

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

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

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

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

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

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

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

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

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

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

Â >> Like phonons in a solid, for example.

Â >> Exactly, like phonons in crystals.

Â But here, we have two explanations, which are

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

Â One is, one adds a scalar field,

Â which is not constant but nearly constant.

Â And with that, if the field is sufficiently constant,

Â one can explain dark energy.

Â But the difference to a cosmological constant becomes more and more

Â diffuse there.

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

Â You see, one has tested Einsteinâ€™s gravitation for distances beyond

Â 11:15

a tenth of a millimetre,

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

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

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

Â which can have a very small mass.

Â This mass must also be fine tuned.

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

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

Â which do not blow up everything, when one

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

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

Â easily amenable to human experimentation, is it?

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

Â and which stays confined to this domain.

Â >> If it is a cosmological constant,

Â it is accessible only by gravitational forces.

Â All other forces are not sensitive to a constant

Â difference in energy.

Â 12:26

But, if it is not a cosmological constant,

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

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

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

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

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

Â Ans maybe with certain experiments, still gravitational ones,

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

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

Â >> Experimental possibility.

Â >> Yes.

Â But I would say also, however,

Â that a cosmological experiment is a human experiment.

Â So, this is not to scale, but the scale

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

Â >> There, I admit, that is true.

Â >> You agree, Martin.

Â >> OK.

Â So, thank you very much for this interview.

Â I think that it is very important also

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

Â Because the course does not stop here.

Â It invites you to stay interested in the forthcoming

Â experimental as well as theoretical progress in the field.

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

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

Â in the coming decades.

Â Thank you for your attention.

Â [MUSIC]

Â