Capability. Once again, everybody. And a very warm welcome to this set of Saturday Morning Theory lectures.
This is actually the third set of of of these lectures.
And I'm very glad to see that their popularity is not waning.
It's great to see so many people here here this morning.
Now, this this lectures is about cosmology, which, of course, these days is the, you know, one of the forefront fronts of of physics research.
But it wasn't always like that. And 100 years ago, the the the forefront of physics research was somewhere else.
It wasn't actually in Oxford is the first thing one should be fairly honest about.
And so Oxford University famously had the opportunity to hire a Helmholtz to one of its senior senior chairs and instead hired a guy called Clifton.
Ring any bells? Who sort of blocked the entire system for about 50 years as he sat in his chair?
But it all changed in the early years of of the 20th century.
And I think that the thing that changed the psychology was was a guy called Henry Moseley,
who was the person who established the relationship between atomic number and the central charge of the nucleus by doing X-ray spectra experiments.
So many of you may vaguely remember seeing Moseley's graph somewhere up in the in the Clarendon Laboratory.
Now, Moseley was killed in the First World War in 1915, so his hundredth anniversary is is coming up.
And this is really by way of a trailer. So so in order to to honour Moseley's legacy.
And, you know, so I think make the point that, you know, we're here now doing all these things,
you know, partly because of a mindset which was going around. Then we'll be having, you know, a number of events to to celebrate Moseley's centenary.
Of course, we're not celebrating his death, but we're set up, which was very sad, but we're celebrating the achievements, his scientific achievements.
So there will be a number of sort of Moseley specific events over over the next couple of years.
And the physics department in his college, which was Trinity College and in fact the University Museum of Science,
we will be working closely together to make sure that we, you know, do it in the best possible way that we can.
Now, those of you who were at the alumni weekend in the autumn will remember that we've also launched the Henry Moseley Society,
which is a sort of basic society for people who want to support the physics department and its and its work in the future.
And if you weren't here that alumni weekend to want to learn more about it, you can do some leaflets.
There'll be some leaflets outside. And there are also details about what well, how it works on on the website.
So so back to back to today.
So we're doing doing cosmology. And we're going to start with the spear who it looks like somebody in a balloon
on the left hand side and some very high tech picture on the right hand side.
So severe. So that John says today's theme is the interface between particle physics and cosmology.
I'm the head of the Particle Theory Group.
And as you would see in the talks that follow, we have diversified immensely from what you might consider to be the code of particle activity.
We impinge on our neighbouring sub departments, on astrophysics and on particle physics.
We are garnets, we go wherever the action is and we try and apply the same underlying core principles that you have heard
about in the last couple of lectures through the problems that we now encounter at this interface.
So the last two sessions are of course going to be a hard act to follow, and I'm very happy to see that so many of you have come, nothing else.
It keeps the room warm. We have switched of the heating to safe, but actually I've just turned the air conditioning on, so it should be all right.
And the other departure from the previous format is that instead of having all the talk given by senior staff today,
I've asked one of my graduate students released on offer who is from Heidelberg and is just finishing here to talk about dark matter.
And he will be followed by Dr. David Marsh, who's a postdoc working in a group, is from Cornell University of the Swedish Belgian.
And I hope that you will then get a feel about what we actually work on.
So these won't just be grand overviews of the subject. We'll try and give you a flavour of the kind of problems we're engaged with.
So that means necessarily that it will not be as accessible at every moment as when, for example, the first talks.
But I hope you will understand that our purpose is really to try and convey some of the excitement at the coalface, as it were.
Okay. So on with the show of James Binney has asked me to give an overview to the all the three dogs because they
will be focussed on specific topics and I think that's necessary and I'm already 8 minutes into my time,
so I'll go even quicker than I had done when we rehearsed it yesterday.
But let's see. So the motivation for what we are doing is starts about 100 years ago, 1912.
That gentleman on the left is an Austrian physicist, Victor Hess, who got into a balloon those days.
There were nice suits to do experiments and went up and discovered cosmic rays, particles that are impinging on us from all directions.
And space was origin is still a mystery and which extend up to very high energies.
That's how particle physics started. And I would say that was the birth of what has come to be called astroparticle physics,
which is this intersection, as I indicated there, and the particles.
Physics, of course, has reached its pinnacle now in the Large Hadron Collider at CERN,
about which I hope there'll be a future dedicated session someday.
And cosmology is represented here by this map of the sky, as taken by the Planck satellite and the microwave radiation,
which was the first evidence that we live in a universe that at one time was hot and dense and therefore
subject to the same laws of fundamental physics that we study today in accelerators such as this.
So you see this prophetic statement by Viktor Hess on the occasion of his receiving the Nobel Prize.
And the other one, which I also like and which has motivated me considerably, is Yakov,
which pioneering Soviet cosmologist and father of the Soviet hydrogen bombs.
He never got out much. He recognised as soon as the microwave background radiation was discovered that we could in fact study
the universe as a whole and try and infer things of interest to fundamental physics from there.
Okay, so let me take you on a whirlwind journey through this interface, which has been very active for, I would say, the past 30 years.
That's how long I've been working in it anyway, and try to give you a flavour and context for the talks that are to follow.
So very quickly, you saw what Mr. Hess, Mr. Hess carried two counters of with him.
These are ionisation chambers. They showed that in addition, instead of falling off as you went up, actually went increased,
which is not what you expect if it does due to radioactivity in the ground.
And just as we have two experiments at CERN today, Atlas and CMS had two chambers.
Okay. It's always good to have good information. And it was a amazing discovery.
And what it really triggered, that's what intrigues us theorists was further work.
You heard about the Dirac equation from James Binney in the first session,
the Relativistic Wave equation that in fact gave you a solution that suggested that for every particle that we know,
there is another one of opposite charge and the C mass and Dirac himself couldn't
figure out whether this was really true in nature until a man called Carl Anderson
at Caltech observed that a a particle going through a plate and therefore slowing
down was actually curving the wrong way from what you would expect for an electron.
And certainly we had doubled the number of particles in the universe in the same
way that today people talk about perhaps invoking supersymmetric partners.
So you been there before a mathematical equation, predict something that turns out to be what nature has chosen to be.
This is miraculous when we see it and we witnessed it again last year with the discovery of the Higgs.
So this is what gives us the confidence that we are doing the right thing. We are on the right track.
And of course, the parallel developments in experimental physics were equally crucial in order to make these discoveries possible.
I know that several of you are experimentalists or have worked in the area.
Then of course you know the story you call up predicted the presence of existence of mesons, heavier particles mediating the strong force.
Initially, another particle which looked like the pion was found.
It turned out to be just a heavy cousin of the electron.
And the question that Rabi asked allegedly in a Chinese restaurant who ordered that still holds today?
We don't know the answer. That's the notorious flavour problem while the three generations of particles that look identical.
And then this series went on to bore the word when a series of discoveries, one of the brothers, the U.K. physicist, played a notable role in them,
not least, Don Perkins was the professor of particle physics here for many years, and all that has now resulted in a magnificent edifice.
But let's go back to what theorists were thinking around the same time,
and I thought I would show you something that you may not be so familiar with because this Russian physicist did not live very long.
As you see, he was executed by Stalin,
but he recognised that the three dimensional couplings that had been talked about already that mentioned four constants of nature.
Absolutely. The action quantum of the action that Planck had given us,
the speed of light that we already knew from Maxwell as emerging out of the unified electromagnetism that he had constructed.
And of course, Newton's constant that these corresponded to various aspects of the magnificent theoretical edifice that he urged us all to pursue,
to unify quantum theory and special relativity and gravitation into what today we would call the theory of everything.
And so this has come to be called Brownstein's Cube. You see, if at the origin we have Galilean or classical mechanics,
Newtonian Newton rules so the in the speed of light but infinitely sea goes to zero.
Then you added are the. I'm sorry. Otherwise, you would have to be living here at the Theory of Special Relativity.
If you marry that with gravitation, you have general relativity.
If you marry that with quantum mechanics, you have quantum field theory.
Perhaps the greatest intellectual construction of the past century.
But we still have this other vertex here to get to, which is where you all these three constants play a role.
And the scale of that or the energy scale of that is parameterised by the combination of this constants,
which is that anomalously high energy, the Planck scale.
And I'm setting this up so that David Marshall talk about superstring theory and its possible observable consequences.
You know that we are following the same program that was outlined for us nearly 100 years ago.
But today we have, of course, the standard model which most of you know about.
This deserves a talk by itself, which I'm not going to give you.
But you have heard and you have read that the last missing piece of the standard model of unified
electroweak interactions and the strong interactions had one missing piece the Higgs boson.
That, through the mechanism of spontaneous breaking of a gate symmetry gives masses to
the W and Z bosons which mediate the weak nuclear force that has was missing,
it says, yet to be confirmed,
and now it has been spectacularly found through processes that create the Higgs boson through in the Large Hadron Collider.
And you observe here that it's decayed to four leptons through this Feynman diagram.
And then we observe its decay due to photons through this other Feynman diagram.
And the two giant experiments with 4000 people each have managed to carry out this
amazing endeavour of showing that a mathematical trick that Peter Higgs discovered,
which enabled him to satisfy two loopholes that had to be plugged, namely the existence of massless particles that were not seen,
and that this need to give masses to the Higgs boson so we can fractions which to explain why they're short range,
that this could be achieved in an extremely elegant and economic way,
which however, until two years ago,
even provenance theorists I think I'm thinking for example of there are two hooft did not quite believe and yet that exists.
Richard has once again chosen to obey the dictates of the mathematics that we can construct.
I find it quite awe inspiring that this is actually the case, and we now know that this famous Mexican hat potential,
which has a centre of symmetry but the universe live somewhere on that,
didn't go vacuole, which are degenerate,
that we are sitting somewhere out on the ring and we know the potential, we know the height of that potential.
We knew the vacuum expectation value of the Higgs. We know everything about it now, and it looks just as predicted in the standard model.
The couplings are proportional to mass. This is amazing.
But this is, of course, just the beginning. This has been talked about for the last 40 years.
That's how long it took to find that particle. What is next?
So here I have to be slightly more technical, but please bear with me because you can see how this feeds into the talks that will follow.
We have a Lagrangian for the standard model now, which are written in very schematic form.
We have been introduced to the gradients earlier and you will recognise that there are terms here which look like credit terms.
There are things that look like the potential, the size of fermions, the find, the Higgs field.
This is the coupling that gives masses to the fermions, to the so-called couplings.
This is the for example, it could be the electromagnetic field. It could be the electroweak field.
These are the reasons that mediate forces, and we have very normalised opportunity.
In other words, one in which you can calculate quantities to very high level of position by a trick of cancelling infinities off against each other,
which nonetheless gives numbers.
But this somewhat dodgy approach which agree with experiment to now eight or nine decimal places you got to seeing is believing.
Okay, but this is not all. What we now recognise is that this is just the tip of the iceberg.
You can write an infinite number of other terms which you can add to the Lagrangian of the standard model which are not normalised.
But. They reflect physics, new physics, which violates the symmetries of the standard model.
The standard model, for example, has the barrier number of particles conserved.
It's a global so-called global quantum number, but we know that it cannot be exactly conserved because it is not a gauge quantum number.
There are no longer influences government barbarians bought from gravity, of course.
And that means that I can write down a term that makes protons leaky.
And we know that protons must decay at some level because otherwise they could not have been born.
That's what a philosopher once told me.
And I think that there's a point there, and the proof don't have to be born because we are here to prove that they have.
They didn't exist earlier and now we are here. And similarly, neutrinos carry ID number four, lepton number, quantum number, which is also violated.
And that allows a neutrino to be its own antiparticle because it's a neutral particle gain in so-called majorana models.
And here is a term that allows for that. We believe we might have seen this not definitively.
We have seen neutrino oscillations.
That means that the mass the race is now on to find out if they really had their own antibody to live their masses of the majorana nature.
So these are just technical terms and throwing them out.
I have no time to explain them in detail, but I hope you are getting a idea of the vast landscape of phenomena into which all these things intersect.
And we understand why these quantities are so slow or so small,
because the purpose of the purity of the cut off in the denominator, this is the energy scale at which new physics will emerge.
We don't know what that scale is, but what we do know is that there is a tension within the theory that tells us that
that new speed must be within reach of colliders such as the Large Hadron Collider.
How do you know that? Because in order to keep these guys small, m has to be large.
But there are two terms in the theory which are of dimension less than four.
So they are super normal move. And this term shows that the Higgs particle,
which is the fundamental particle asking the particle the first to be first fundamental scheme is to have been found.
It propagates through the vacuum and sees everything in the universe, all particles that go through them through their mass.
So, for example, it covers the top quark and gets a mass correction to its own, which diverges as the square of the cut-off.
And this is not tolerable. The fusion would not make sense.
The measurements are not make sense. If we can attain that quantity and you can see that the only way to do that
is to have the aim of ordering TV so that the Higgs mass is within control.
And then, of course, we have the opportunity to look for processes that might be new Windows beyond the standard model.
Without actually having to construct large machines, we can get access to high energy scales through these real processes.
And one of the ways to cure that problem is to invoke another symmetry between bosons and fermions,
which contribute to the opposite sign to that divergent laws so that they cancel out.
And that's called supersymmetry, which I'm sure you have heard about.
But the race for finding these particles is very urgent, and we are definitely expecting the next run of the Large Hadron Collider in two years time.
We'll either find them or show that we have been barking up the wrong tree and we
are really the weirdest people in the universe that when you are proved wrong,
we actually quite enjoyed. It's nice to be wrong. It's nice to be reminded that there are yet more wonderful ideas to discover, right?
As long as somebody funds us to do these experiments.
Now, the link to cosmology that I'm trying to highlight here is that this new physics provides candidates for new,
stable particles in nature other than the ones that we are made of,
the ones that we have lived with all these years, the ones that astronomers have been concerned with for most of recorded history.
We now understand that dark particles, dark matter, dark energy makes up most of the universe,
and their identity must be sought for a new physics beyond the standard model.
And that, of course, is what physics will talk about following me.
I also want to point out the biggest embarrassment we have in theoretical physics that
there is one more term which is allowed by all of the symmetries that we know of,
which is the vacuum energy of the theory. And as you can see from here, the scale is at least of order 1000 GB to the fourth power.
That's a trifling matter of 60 orders of magnitude larger than the maximum that is tolerable from cosmology.
So I think it's a safe understatement to make that we don't really understand how gravity governs to the standard model,
and nonetheless, we have to pursue the goal of constructing quantum gravity where presumably the clue lies.
We still do not have a clue. And that, again, is something which one might see.
Weinberg calls it The Bone in our Throat, which I think is a rather apposite description.
But it is also the beacon for the next generation of theories to attack.
So I think we'll leave it there. So let me very quickly, in the remaining few minutes and through the general construction of cosmology, then again,
we have some assumptions, except that these have not been as well tested as the standard model of particle physics.
We assume the simplest possible kind of spacetime, which is called the Friedman Robertson Walker metric,
after the people who first came up with it, which basically looks like the metric of special relativity.
You can see the square minus square. You are familiar with that, except that this is not time.
This is something called conformal time, because the metric is just a sheet of graph paper which allows you to measure things.
But this graph paper, somebody's stretching it. And the scale of the graph paper we call the scale factor of DX.
And that means that this innocuous little substitution means that all your information about how spacetime and booleans behave is now
going to be examined because this gives rise to a dynamical situation where the scale of spacetime itself is changing with time.
And that means I rather put it that way than, say, the universe is expanding, because that begs the question of what is it expanding into?
And the answer is nothing. The metric is all that there is. Mathematically, we have to deal with the metric.
We should not ask questions like, What is the universe expanding into?
But we can certainly draw a picture of what I'm talking about, which shows us that the centre of the universe,
where we of course belong and the Big Bang is this dashed line surrounding us.
The radial direction is what's called redshift, which is, as you know,
the movement of spectral lines towards that end of the spectrum, indicating that the scale of the universe is smaller and smaller.
As you went out, it was infinitely small that thanks to this mapping, it is a finite surface here and a photon from the Big Bang,
good sort of random walk there until it got to the point where the universe becomes transparent because it's expanding and becoming more dilute.
And at that point, it can come straight to us and that.
Typical structure on that last scattering surface, you can see,
is such that only points which are about a degree apart, can be causal contact at this point.
At this point cannot have been in close contact. And yet when you look at the sky, it looks totally smooth.
And that raises fundamental questions about how that happened in the early universe around here, which we call inflation.
And I'll talk about that in a second. But meanwhile, by combining this metric with this description of the dynamics,
according to Einstein, which takes this metric, constructs objects from it,
called the curvature tensor and relates them to the energy density of matter,
which is the quantity here, coupled through Newton's constant that this simplifies it.
This is very, very complicated, but it simplifies into a simple equation that an undergraduate can solve.
And we do. And that can be written in terms of the energy density of matter, the curvature of spatial sections,
and this cosmological constant which allows it, which allows for general coordinated invariance.
We have to add it because we have to reflect the symmetries of the theory and that then describes itself into this simple equation here,
which tells us how the universe evolves.
And then when astronomers go out and measured the universe, we find that it is in fact in conformity with this.
But it is very, very hard. It's not what we expected.
Atoms, the stuff here made off with shine and emit light that are mainly in clusters of galaxies.
You'll hear about that from David Moss. They are only 5% of the universe.
We can take a good census from Big Bang Nucleosynthesis the formation of the elements and the famous 3 minutes up to the Big Bang.
And that raises yet another mystery.
There are no empty variants, although all the laws of physics that we know are pretty much symmetric between variants and antibody ons,
and yet there are just no antiquarians. Okay, big mystery.
Then there is the mystery of dark matter, which is about six times more.
Felix Karloff would talk about this and you have plenty of evidence for this from gradation curves of galaxies,
from gravitational lensing by clusters, from the formation of structure in the universe.
[INAUDIBLE] talk about it in detail.
And then we have evidence that most of the universe is in the form of some mysterious fluid with negative pressure,
which is causing accelerated expansion that we see here in this Hubble diagram of supernovae in distant galaxies,
and from observations of the fluctuations in the cosmic microwave background and the large scale distribution of galaxies
that this we do not yet have direct evidence for that is inferred from that equation that you just saw in the last slide,
which I have now written as the sum rule.
So we measured this and we measured this, and from that we inferred that this guy is one minus point three -0, i.e. about 2.7.
Okay?
But all this raises issues about this new physics that must lie beyond the standard model in order to give us possible candidates for dark matter.
And this, of course, touches on the cosmological constant problem.
So I'm personally not yet convinced that it actually exists.
We need to do further experiments, make more detail observations to find dynamical evidence for dark energy.
But we certainly are able to do better when you go to the early universe, because the early universe is very simple.
It's just a ball of radiation.
The radiation comes to dominate over matter because as you go back to the universe, the energies of photons and other monstrous particles increase.
Just as the rich shift, the universe expands the also redshift upwards when the universe contracts and therefore it takes over.
And we are therefore able to construct a permanent history of the universe and
put the energy scales that we know in physics on superimposed on top of that.
So here is the transition between three quarks and gluons and nuclei, which is nucleosynthesis.
Here is the transition between a plasma and a neutral atoms.
That is the last scattering surface of the microwave background. And when you go back,
these experiments at the at CERN have allowed us to extrapolate back to an
unimaginably small time shortly after the big bang into the minus 10 seconds.
Although our direct evidence stops here at about a minute after the big bang or maybe a second.
But we do know this physics beyond that, however, is just extrapolation and speculation, which is fun.
And that's exactly what we do, because we can confront ideas about new physics here with their consequences for the universe,
and we can confront that with the data and ask if it measures up.
Okay. So in addition to the Large Hadron Collider, we do need known accelerator experiments as well in order to throw light on this.
And one of the most important of those is the picture that you have seen earlier,
the picture of the microwave background, which is has been called the Rosetta Stone of cosmology.
And indeed it is because it is those fluctuations that you see in the microwave radiation that go through gravity in.
A sea of dark matter that dominates the universe to give us the large scale structure that we see today.
And we'll hear more about it later.
And the amazing thing is that our current picture of how those fluctuations arose is that there are quantum fluctuations
as big as the universe of a massless field that came to dominate the expansion of the universe at very early times.
Now, this sounds like a fantastic story, and obviously it needs to be demonstrated that this is actually true.
But mathematically it holds together, even though, again, it impinges on this cosmological constant problem, which is as yet understood.
But we do know this is in cartoon form, that that phenomenon can blow up a small universe into a large universe and explain
that horizon problem I alluded to earlier as to why different parts of the sky,
which could not have been causal contact, apparently were clearly in close contact.
That's because we are assuming that the expansion did just go smoothly.
But what if it had gone up exponentially at one time?
Now, inflation is something those of you who work in the city are familiar with in a different context.
We actually like inflation because it solves problems. It creates something called DC space, which is very interesting.
Mathematically, it turns the curvature to zero, as is shown in this cartoon,
and the microwave background shows that the picture is in fact that the spatial
sections of the universe are flat rather than being positively or negatively curved,
because then this picture would look different, and we know that the universe looks like that.
So these are all magnificent achievements in delineating the nature of the universe,
which have come to a real head last year with the release of this blog maps,
which again you would have seen in the newspapers,
people wonder what what the significance of that is that you understand when you expand it in spherical harmonics and you plot the power spectrum.
And this power spectrum, let me just point out one feature,
which is that the present universe and everything it contains would be contained within about one degree of that map.
Okay. And that is why most of the fluctuations you are seeing in that map are on one degree scales.
This multiple corresponds to one degree. Okay, these are all other universes.
Of course, we don't see them because they are outside of Horizon now and we are of course inside of the universe.
So it's kind of bit weird to try and think about it.
And remember of course, that when we think of this micro photons coming to us, they're not actually coming to us.
We get there as well. We everything we had made of everything that we know, it was all did in the same place.
But it was going away from each other so fast that it has taken 10 billion years for that photon that started out right there to catch up with me.
Okay. It's a fantastic thought and I try not to think about it.
It's sort of it's sort of rather mind blowing to you over use a commonly used word.
But we can actually test the theory of inflation by studying this pattern in some detail.
And that's what our colleagues, for example, in astrophysics are engaged on, as are we,
because those fluctuations, as I said, go into the large scale structure of galaxies.
And I only have time to flash to this, which Felix will talk about later, which shows that the full spectrum of Galaxy clustering,
measured from large surveys of galaxies using fancy techniques,
is in very good agreement with the theory that I have presented to you somewhat sketchily.
And I just have one word to say. This is to provide a lead to the talk by David Marsh that this inflatable field is meant to be a scalar field,
which has its own potential, just like the Mexican hat potential you saw for the Higgs, which it evokes down.
And then then it falls into the minimum, it oscillates there and it transfers this energy to ordinary particles of the standard model.
It's just a problem. Undergraduate put salt book upward oscillators like two simple harmonic oscillators.
And in that process you create the hot big bang that we know and love.
And then these starts the history of the universe.
And during this process, I have no time to explain how the fluctuations that are created of quantum fluctuations get
stretched out to astronomical length scales and are then responsible for creating the galaxies,
the clusters of galaxies, the silver clusters that we see today.
So let me wrap up. We have this timeline that I already presented to you starting from today,
15 billion years after the big bang, going back to the formation of galaxies, the universe turns opaque.
At this point, we cannot see beyond that form because it's ionise and the electrons and protons are tightly coupled.
But then there is the synthesis of the elements at 3 minutes, which created helium made stars possible,
made life possible, and then earlier, still, we don't have a direct window, but we have physics.
We have calculations on the lattice or computers which allow us to understand this transition,
and we can also understand the electric phase transition now thanks to the discovery of the Higgs.
But beyond that is speculation. And we don't know this physics.
That's what you are engaged in.
And as it says here, although we can account for a vast body of observations and I justifiably can be proud of what we have achieved,
we should also be ashamed that we have no idea about this initial condition problems that we have thereby brought up.
Why is the universe asymmetric between matter and anti-matter?
What is the dark matter? How do you solve the cosmological constant problem and explain dark energy?
This is what we are struggling with right now. And the journey, of course, is the reward.
But I think since we are in the Danish drama lecture theatre and I came to Oxford 32 years ago to work with Denis,
who in fact turned me on to this subject. That was the best professional advice I've ever had in my life.
He said something which I'd rather like. None of us can understand why there is a universe at all, why anything should exist.
That is the ultimate question. But why do we cannot answer this question?
We cannot just make progress of the next simpler one of what the universe as a whole is like.
So what I'm trying to say is that for us, the journey is the reward, and I hope we will make that clear in the talks to follow.
Thank you.