I would say one of the most remarkable stories of my my own.
Experience, professional lifetime experiences has been the transformation of cosmology from a subject which was
described in once by John Peoples as an area that really wasn't fit for grown adults to work in,
to becoming, have become a remarkably precise and extremely important domain of theoretical astrophysics in some ways,
perhaps even sort of the crown jewel in the subject of cosmology.
That description I can remember actually as a graduate student in Berkeley.
Some of the early microwave background experiments showing grotesque distortions from simple black bodies and people taking it seriously and putting
considerable amounts of efforts into explaining what enormous source of energy could possibly have knock the universe out of kilter in this way.
In any event, we all know those measurements were in error.
The post-Soviet era paralleled with a whole new dawn on the subject.
The subject really begins after to establish that the universe, the microwave background is nearly perfect.
Blackbody at 2.7 degrees.
So one of the reasons why this subject is so interesting and has become so rich is that I think this is probably David Spergel, who puts it this way.
The universe has had its baby picture taken by looking at the time and the ratio of the time of recombination, the cosmic microwave background itself.
It is possible to, by studying its intricacies and its details and its statistical properties,
there's an incredible amount of information that can be extracted from that source of data,
but only to those still in the methods, by no means straightforward.
And that brings me to today's speaker, Joe Donnelly, who is one of the most important leaders in this field.
Joe was an undergraduate at Cambridge who graduated in 2001, came here to Oxford as a postgraduate postgraduate worked with Peter Ferrara,
took a degree in 2005 and after three years of postdoctoral work with the team at Princeton has been on the staff pretty much continuously since 2008.
Joe, as I say, is one of the great leaders in this field and she's been widely recognised for her work and for her
ability to extract useful information out of this swarm of photons that form the microwave background.
Her work has been widely recognised.
She won the Maxwell Prize and the Fowler Maxwell Medal and the Fowler Prize and as part of her work on W,
she was one of those recognised with the Cosmology Gruber Prize in 2012 as well.
And we're very fortunate to have her today and she is going to tell us about her work cosmology from the microwave background and thanks to you.
So thanks for that very kind introduction. Yeah. I want to tell you about the work that that I've been doing and we're doing here in
Oxford on the microwave background and some experiments that we're involved with.
You can't do good work without amazing students and post-docs,
and I'm lucky that we have many of them here and many of them in the audience and you know who you are.
So, okay, so it's this year is actually the 50th anniversary of the discovery of the microwave background, 1965.
This this faint light was picked up by China Penzias and Robert Wilson in New Jersey when they weren't even looking for it.
They saw it by chance. They saw something that they that they they saw in the empty sky this faint microwave signal that turned out to be light.
The earliest light we have of all. Now, here's a picture of it.
This is a map. This is a picture of the cosmic microwave background.
This is light that's been travelling since 380,000 years after the big bang.
In the hot, big bang model, the universe has been growing, expanding,
cooling down and transitioned at this particular epoch from being this hot, ionised plasma where we have this this fluid of photons,
electrons and dark matter particles around transition from being ionised to being perfectly neutral, where before this epoch,
photons would be bizarrely busily scattering the free electrons in this plasma without being able to travel freely through space.
But as the universe cooled down, it hit a particular temperature that at that point,
from that point onwards, photons could now travel freely through space because atoms were now neutral.
Hydrogen atoms are formed. Helium has been formed. Dark matter particles continue to be around that they don't affect the photons anyway, we think.
And photons could then travel freely through space to us and we get to catch capture some of those photons.
They're hitting us now, today. They've been travelling for the full age of the universe with very little interruption.
And this map here is made. This is the more exciting one I'll show you.
See, it was made by the Kobe satellite in the 1990s.
Of its average temperature. The average temperature of this light. And on average, you can see the orange ness of it.
This is a map of the full sky of the CMB unwrapped onto the flat, the flat oval.
It's all 2.7 degrees Kelvin. And the fact that it's all the same temperature and its distribution in terms of frequency is
a perfect Blackbody was the nail in the coffin of any model but the Big Bang 50 years ago,
and then even even more so when Kobe came out with its results.
So it it's existence tells us a lot,
but the details in it tell us even more and the details in it what we've been measuring over the last
20 years in greater detail after Kobe came in terms of satellite missions to the sea and became a map.
But then most recently, we've had the Planck satellite.
So the Planck satellite is a European mission, European led mission that launched back in 2009.
It's pictured here before launch that the Planck satellite with this it's about a person is about
this size and the mirror is in there and it's about almost two metres or about my arm span.
So it's a big it was a big beast sent up sent up from French go out to this point L2 a million miles from here
where it could quietly and calmly observe and observe the sky and pick up the microwave background signal.
And it did so with greater sensitivity and better angular resolution than any of our previous experiments.
So we've been involved in that here in Oxford.
And it's a it's a significantly large collaboration with many people in the UK, many people in Europe and internationally too.
It's a it's a large a large body. I mean, we showed you some results from Planck,
but at the same time we've also been heavily involved in this experiment called the Atacama Cosmology Telescope,
which is, we think, the highest experiment, high telescope on earth.
Someone may have just put one slightly higher. It's on Cerro Toco in northern Chile right by the Bolivian border and it's
pictured here hidden inside a ground screen that shields it from the mountain.
And inside there we have a six metre telescope designed, custom built to look at the microwave sky to measure the CMB light.
And inside it is a very sensitive camera that's pictured here.
And I'll show you in a bit more detail later on. We had this large group in Oxford to work on it.
Their name, Zahir, and he he'd lead a lot of the analysis for this project.
So the sort of be the observation, doing analysis and doing observation to the C and B means you either are in space
for the satellite or you do your observations as best you can from the ground.
And the best locations we found so far from the ground are northern Chile, where it's incredibly dry.
So up here, we've got you quantify it in terms of perceptible water vapour,
how much water vapour you'd have if you compressed all the water vapour above you into a into a small into water?
And this is point five millimetres. It's really dry, but some of it snows, which is annoying.
And so to there and the South Pole, the optimal places for observing this microwave light, you want it to just be as dry as possible.
Okay. So using Planck, we've now been able to zoom in and make that orange orange map more interesting.
What I'm showing you here is, is the measurements by Planck of the microwave background, the fluctuations about the mean.
So I've subtracted the mean and showing you just show you the deviations about.
That's where the colour scale here is hundreds or up to 100 micro kelvin plus or minus.
So the, the little, the little features you see are typically one part in a 100,000th of a degree.
So the very small. And so it's taken these sensitive instruments to map them out,
although we have seen them previously with this kind of progression of ground based, balloon based and then satellite missions.
So what we're seeing here is the fluctuations in the light where you got the blue is ever so slightly colder than average and the red is hotter.
And these little features in the temperature of the light show us reveal the features in the universe.
At 400,000 years, we're seeing a picture of part of space, as it was at 400,000 years.
And the temperature of the light is tracing out the underlying features in the universe, the density of the underlying space.
So back at that time, the universe was almost completely featureless. We didn't have the large cosmic structures.
Things hadn't collapsed and formed large gravitationally bound objects.
We had an almost completely smooth universe with these tiny density fluctuations about the average, about the average density.
And we're tracing them here with the temperature of the CMB light because on average,
while on the large scales where you have a slight, where you had a slightly denser region in the universe at 400,000 years,
that slightly denser region actually turns out to be a cold spot in the CMB because that denser region will have more light there,
but it has to climb out of a deeper potential well to get out to us.
So a slightly dense region comes out cold in the CMB and vice versa.
So we're basically capturing this picture of the density fluctuations at that early time.
And the reason we've been able to then use that to extract so much science or so many,
so much the precision cosmology that Steve is talking about is because everything was very it was linear.
These fluctuations are tiny. And we think we understand the physics of what was going on before then.
And we can evolve the physical behaviour of the universe from time, zero up to time and be formed.
And that's not the case later on in the universe. We could do an awful lot later in the universe, but things definitely get messier.
Things definitely get more non-linear and harder to model.
And so in a way, it's sort of easy to look at the CMB.
And so, again, these are these are then the seeds of cosmic structure, these tiny fluctuations we're seeing here.
If you have a density at slight over density at this time, then over the next millions of years that would gravitationally collapse to eventually,
after about a million years or so, form the first stars and the first gravitationally significant objects.
So we'd be able to track back the very the very formation of cosmic structure using this.
Now what let me let me then get out some dirty laundry. So when we show you these these nice maps of the CMB.
First of all, we've done a lot to them to get there. There's a huge amount of work that happens to make a map of the CMB.
And one of the most obvious things that happens is that you actually just measure
microwave light and you have to work out what is the CMB and what's everything else.
Because we're sitting here in the Milky Way and we're looking out through it and we're also looking out through the rest of the universe.
And we're picking up this light that's been coming in from from the most distant place possible.
So I just wanted to show you what we actually measure, for example,
with the Planck satellite at a few of the different wavelengths that it gets to see.
Now Planck has nine wavelengths and I'm showing you maps of the sky and three of them centred at around 1 to 200 gigahertz,
which is about two millimetres in wavelength. And this is the sweet spot for measuring the CMB.
It's the wavelength where it peaks and other things don't.
Okay, so if you go to slightly lower wavelength, sorry, sorry, slightly lower frequencies,
then you start getting swamped by emission from synchrotron radiation.
So electrons spiralling the magnetic field of our galaxy emitting synchrotron, and that increases that low at lower frequencies.
If you come up you so you'll see this, this red emission is is light from the galaxy and much of that would be synchrotron emission.
Then as you increase in frequency, you can see the galaxy, this signal from the galaxy, the red stuff is getting brighter.
And what you're seeing there is more emission thermal emission from dust grains,
micron sized dust grains in the in the galactic plane that are being heated up by starlight and are emitting thermally.
And that also sends a signal that we can we can see now, happily,
when we're looking at the temperature of this light, you can see the CMB fluctuations pop up.
Just you can see them here and here. The galaxy obviously isn't it isn't swamping the signal in the temperature.
This map of the temperature of the of the CMB.
But certainly what we have to do is we combine this information to extract out what's the blackbody signal.
And luckily other things that come from the galaxy aren't blackbody.
And we use that fact. We use that information to tease out that the CMB signal.
Okay. So. What? What we've been used to seeing in CMB observations is measurements of the temperature of the light.
But what we're now getting from Planck and from our new ground based instruments are new views of the universe through the CMB.
And one of those views is through the polarisation of the light.
And this isn't very nice. This isn't a very nice image. I'll show you nice one in minutes of you can measure the intensity of the photons of the CMB.
But you can also measure that polarisation. You can measure that linear polarisation in the Q Stokes vector, or they use stokes factor.
So you're measuring. So we just defined the key stokes factor is polarisation in this direction and you in that direction.
And so any measurement you can, if you have a polarisation sensitive detector,
you can measure the polarisation of the light and these two different in these different directions, not just its intensity.
And that's what we've now done with the Planck satellite. And those fluctuations are shown here in this not very nice greyscale.
And the range of fluctuations is very much smaller than the temperature.
This is now a few micro kelvin fluctuations.
Now, I'll show you a little zoom in. That's maybe a little pressure taken from from our telescope.
Acts of a of a a zoom in on like ten degrees at the skies about this speck on the sky showing the
fluctuations in the CMB with this Q type stokes vector and a U type stokes vector the different patterns.
Okay, now why does this arise? So I said before that the CMB temperature kind of traces the density of the universe back at 4000 years.
Now the polarisation roughly traces the velocity with a motion of the the photons in the baryons.
At that time, you can only get you only get CMB polarised, you only get a polarised photon arising.
If you have two things happening you need,
if you have a free electron and you have Thomson's scattering of it and if the radiation pattern incident on that photon has a quadrupole pattern.
So if you imagine a photon scattering out towards you and you wouldn't know whether it's going to be polarised or not if it's got hotter light,
for example, coming in from the two sides and colder light from top and bottom.
When that scatters out, it's going to come out with a net polarisation along this direction.
Whereas if it had, for example, well isotropic or even dipole pattern,
if you had the same amount of radiation coming in from side to side and top and bottom, when it scatters out, it will be polarised.
So you need this particular quadrupole pattern where you have this, you know, sort of hot, hot pattern side to side and cold, top and bottom.
And you get that from the motion of, um, of this tightly coupled photon baryon fluid that,
that it, that existed before the CMB became completely neutral.
So when we, when we map out the anisotropy in the polarisation of the light, we're basically measuring the velocity of the of the fluid at that time.
And so we're getting so you imagine that, you know,
you've got a map of kind of the the density of space for 2000 years and a map of the the velocity of space to lots of stuff in space at that time.
And I'll show you the science we can extract from the moment. Okay.
So these are little maps that we made from from Acts just last year.
And what we do, we extract the information we try and extract from them is we quantify it in terms of the type of polarisation we see.
And you can decompose it into these two types of polarisation and B modes,
where if you take a map of polarisation, if you pull out the pure divergence part of the field.
So at any point, any polarisation which is pure radial or tangential or at any point in space,
that's the emo type of polarisation that looks has this kind of pattern around any point.
Or you can pull out a curl type of polarisation that is a pure B mode.
And this is useful because both because otherwise your polarisation is coordinate dependent and depends how you define your map.
Whereas these things are coordinate and dependent. But more importantly,
it's the physics that we can extract from the different kinds of polarisations the motion of
the photon baryon fluid act recombination when the c b formed should only produce this kind.
It shouldn't produce this kind. This kind comes from a more exciting thing, which is gravitational waves.
We'll come back to that. Okay.
So with Planck, we get the temperature and the polarisation and we get a third thing to now that we didn't used to have,
which is we get some measure the lensing of the CMB.
So you measure its intensity, you measure its polarisation.
But we can also now measure how much it's got bent by the gravitational effect of all the stuff, but it won't pass on the way to us.
It's just simple, you know, but matter there.
It bends light. Einstein tells us that.
And it didn't used to be possible to measure this because CMB photons typically only get bent or distorted by about two or.
Minutes. So if your experiment hasn't got that resolution, you can't see it.
But Planck has got just about enough resolution and set in the ACT, and Chile has definitely enough resolution and can measure it.
And so what we have here, what this map is now, is a picture of the lensing potential where think of it.
So if I measure the temperature of the CMB in some direction in the sky,
then I can think of the lensed CMB temperature as the unknown signal coming from some direction plus a lensing angle.
So every photon gets the photon that started off in some direction appears to come to me from a slightly deflected, deflected direction.
So every point in the sky has a deflection angle.
How much it got bent by. And I can write the angle as the gradient of some lensing potential field.
And that's what's shown here, where, again, our blue is, is slightly less lensing potential and red is slightly more.
And what that corresponds to is an integral of all of the matter between here and our scattering.
If you have more matter between me and where this can be formed, then it's going to bend the light more.
And if you have less matter, it's going to bend the light less.
So this really is it's a weighted it's a weighted sum because how much the light gets bent depends on where the stuff is.
But it is a weighted sum of all of the matter in the universe. From here it's a lot of scattering.
And that's powerful because that includes all of the invisible stuff, that includes all the dark matter.
And so we can use it to look to to understand dark matter in the universe with with Axe in Chile.
We've also managed to zoom in. I haven't got the plot of it here.
We can take part of that sky and zoom in and get an even better, better measurements of that lensing at high resolution in a bit to the sky.
Okay so so as so we use, I think in the last few years we've gone from having measurements of the temperature of the CMB to now having temperature,
polarisation and lensing. And this is really telling us a lot.
So it's helping us answer these questions that we have, that questions we have about the geometry,
the contents of the universe, and the initial conditions.
How did it start? What happened to start the expansion of the universe and what are the properties of the dark sector in particular,
including cold, dark matter, dark energy and also hot dark matter presumed to be in the form of neutrinos.
And we can learn all of this from from from the CMB.
So what do we do? Sydney, these maps we have right?
We have these maps that we very well.
But then we extract statistics from them and at the main statistics that we pull out from all of these three maps,
temperature, polarisation, lensing is their power spectrum.
That two point function pretty much that all the information,
the power spectrum contains all the information in those maps because they're more or less Gaussian fluctuations,
which means if you compute the power spectrum, you capture all of information that's in them.
So essentially what you're doing is taking you know, you're doing a for a transform.
But because we're doing it on the sphere, we take an angular power spectrum.
So we decompose the maps into spherical harmonics and look at there and look at that, that power.
And so I'm going to be showing you a few of these these these plots, these statistics.
So what I'm showing you here is this is this is this statistic, the angular power spectrum as a function of angular scale.
So large scale over here, 90 degrees in the sky, down to degree scale down to some degree down here.
And you can also think of that as multiple moments ranging up to a few thousand.
So large scales through small scales. And what we see is this incredibly rich statistic, right?
It's the power spectrum of the CMB. Sky is by no means featureless.
Right. It's not flat. It's got all these it's got has got all these peaks and troughs.
And what this is capturing is the physics of what's been happening up from time, from time, zero, up to when this could be formed.
And the physics we think is is pretty straightforward.
We think we had an initial input of primordial fluctuations, and we think those were perhaps put in by mechanism.
We call inflation an exponential expansion of space where quantum fluctuations on very small scales could have been imprinted.
Expand it to macroscopic scales. I'll come back to inflation in a bit.
Some mechanism put in primordial fluctuations and they then evolved over the next 400,000 years.
If these fluctuations were on such a large scale that they're actually beyond the horizon of the universe,
then they won't evolve and they won't have evolved at all between time, zero and time recombination.
And actually, this is what we're seeing on these very large scales here that we know that that's the way we plot.
It says that when you see a flat line here, these are fluctuations that have not evolved by the time that CMB formed,
but smaller with smaller wavelengths that are inside the horizon of the universe.
Those fluctuations evolve according to what's in the universe.
So we've got this mixture. We think of a mixture of dark matter photons and baryons and also neutrinos.
The photons in the baryons, a tightly coupled together, the photons, a scattering of the electrons, and that tightly coupled photon baryon fluid.
In that fluid, we set up sound waves because you have this competition between the gravity of the baryons and the pressure of the photons,
and that sets out sound waves in this in this plasma.
At the same time, you have dark matter particles that are just gravitationally collapsing because they're not interacting with the with the baryons.
Okay. And if you then so you let these sound waves evolve and you capture them after 400,000 years and
different length scales will have reached different points in their evolution of their sound wave,
their oscillations. So for example, we find this peak in the CMB at one degree,
and that corresponds to the wavelength of an oscillation that just had time to reach
its maximal compression of the acoustic wave at the time that the CMB formed.
And then a smaller wavelength that might be at the next peak just had time to have one compression in a rare fraction before the CMB formed,
and another one had, you know, compression, RAF action compression when the CMB formed.
And so you have this what we see here is this is this is this rich acoustic peak structure with harmonics of these, uh, of these sound waves.
And then you get this damping effect known as silk damping, where the CMB,
where the structure is actually a bit, a little bit damped or very much damped because of,
because during the time of recombination, while the CMB is transitioning from being neutral of ionised to being neutral,
photons have time to diffuse and wash out this structure.
So we get this damped effect.
So we think we have this incredibly, in a way, this simple model that fits this great curve, fits this data really remarkably well.
What we call Lambda CDM, and it's a flat, geometrically flat universe.
It has these baryons photons called up matter three neutrino species and also a cosmological constant that we don't understand,
of course, yet, but we think it's there.
And if you put so if you put it and two parameters that describe the initial fluctuations just to an amplitude and a scale dependence.
And if you put if you describe the fluctuations by just two numbers, and then you put in just those those components of the universe,
and you evolve that physics through each recombination, you see exactly this.
Right. And so this this grey curve is the theoretical prediction from our numerical codes that, you know,
Einstein, Boltzmann solvers that that numerically integrate these equations from time 0 to 4000 years.
And, you know, it fits. It's kind of amazing.
It didn't we didn't have to have a model that fitted this data, this incredibly rich data with a lot of features.
And, you know, there's a universe that goes right through it and we've kind of got used to that, right?
We're like, Yeah, yeah, Lambda CDM It works, but it didn't have to.
And the fact that it continues to work kind of continues to surprise us or it should do anyway, right?
Because we've seen it. We kind of had this Lambda CDM model in mind for a while and with the map data
we measured these first three peaks that said Peak wasn't mentioned very well.
And then we've been kind of revealing unwrapping this kind of fourth, fifth, sixth, seventh, eighth,
ninth acoustic peak down here with the Planck data shown in blue and then data from our telescope act.
And then the South Pole Telescope, another ground based experiment in the South Pole.
So in the green an orange remarkably consistent and extending this down here.
So these are many decades of scale. And the fact that we can continue to explain it is pretty remarkable.
And this grey curve is fixed, that data. But actually, if we take in the curve, it just fit to the first.
But the data would have from map then the stuff that fits. Now Planck we got for free, you know, it just, it just works.
And so this has allowed us to measure the primordial fluctuations rather well and measure
the properties of the universe that the amounts of stuff in the universe rather well.
These numbers don't and are going to be a measure to 2% level.
But, you know, ultimately, one doesn't just want to measure numbers to tip at a precision.
You want to actually find out something new and answer questions that you don't know the answer to.
And one of the we've had a few things that we didn't know the answer to.
And we thought and by adding in the polarisation map, we've, we've been able to learn or constrain more things.
So what I'm showing you here now is underneath that temperature curve,
I'm showing you the angular power spectrum of the mode of the polarisation pattern measured from Planck.
And this is now you're seeing now these acoustic peaks again down here.
Note the fact that it's, you know, 1 to 2 orders of magnitude smaller.
So these measurements that small and hard to make from, from Planck and from our initial data from our act experiment in Chile.
Now so the and so this the grey curve that beautifully matches that data,
this new data from Planck, the polarisation data, this curve was not fit for that data.
The curve just goes bang through the data points. And this is the prediction.
This is the prediction from Lambda CDM. This is this you take the theoretical curve that fits this data.
You say, I'm going to take this Lambda CDM model. Tell me what I should predict for the polarisation of the light.
And it's this curve down here and it's really doing it's really doing pretty well.
And the fact that we now have this additional probe and say and it's just this as an aside,
it's just kind of neat, although I don't have a ruler long enough, we try and do it.
I told you that the polarisation roughly measures the velocity of this this fluid.
And if you do draw the line, you see that the peak of that matches up with the trough of that and the peak of that with the trough of that.
And so they are offset by that that the derivative of the other.
So you get the peaks in the polarisation where you get the troughs in the sorry,
where you get the minima in the temperature, which is what you'd expect. And we find that correlated to I just haven't plotted it on here.
So this actually the fact that this works so well is again, the, the conclusion we come to is Lambda CDM works.
But again, we didn't need to come to that conclusion and before Planck,
before new experiments and in Chile in the South Pole there was this kind of whole array of possible cosmologies that we thought might be possible.
And the data now really limit those. So one of the things that we thought might be possible was extra relativistic species in the universe.
We normally assume there are three neutrinos and nothing else, no extra particles, no additional neutrino species,
no sterile neutrinos, no other particles that that could be around that could have decoupled early on relativistic ones.
And before before we had these measurements, there was actually scope for having maybe four or five neutrino species, additional relativistic species.
Now, there's very little scope for that. The constraints now from this data that the neutrinos must be the number of relativistic
species that we assume to be neutrinos must be 3.1 plus a minus point three.
So still a little bit of wiggle room in there. But before this data, it could have been like ten.
That would have been fine. It also greatly limits the scope for fluctuations that could be described by any more than just two numbers,
just a power law description of the primordial fluctuations.
And I'll come back to that.
There's no you can't there's not room for putting in additional features in the early universe that could come from cosmic defect or
magnetic fields or even from some models that produced that would have dark matter annihilation that could produce an extra signal.
There were just this whole this whole set of things that could have been possible that we're not seeing.
And so maybe that's.
Or maybe that saying, okay, the universe really does look too simple,
even though now we have these still these kind of big questions that we don't know the answer to.
The Fed the Fed's statistic is the lending of the CMB.
I just want to give you an image of what lending does to the CMB so that top panel shows the lens and lensed CMB.
Okay, so what you see, we should say, is the effect is not ignore the fact that the bottom line is moving.
It shouldn't be of the effect is not strong.
Okay. But it's coherence. You're seeing the large you're seeing this the things coherently shift.
And that's because you're having this background light currently distorted by large cosmic structures and the structure,
the kind of degree scale on the sky. When you work out the lensing and process and then compute,
it's again got a map of the lensing potential and you compute it and get a power spectrum again for a transform.
How much power is down? Different angular scales? This is again a power spectrum we show.
And this past spectrum we're looking at the CMB.
So power structure was a function of angular scale where this is like a degree and this is below that.
And now here again, okay, so the black curve, I think here is the nice old lambda CDM cosmological model that was fixed.
The Planck temperature data not fit to this data.
And then here is the map here, the measurements from Planck,
including either temperature or temperature optimisation to extract the lensing shown here and even the biggest slight dip here.
We don't think it's significant or maybe it's not significant. It certainly fits this model rather well.
And once again, you know, to belabour the point, it didn't have to fit.
There are different universes that could have fit. Both the temperature and the polarisation data are not fit here.
Lambda CDM comes out, comes out, winning again.
And the kind of universe is that it could have been a universe that's a bit geometrically curved,
that's maybe a little bit closer, a little bit open, and it's not completely flat.
I'm measuring the amplitude of this. This spectrum limits that to a couple of percent.
What the curvature could be actually adding in the positions of galaxies constrains like even further.
But this is, this is, this is, uh, this is, you know, new and powerful measurements.
And it also constrains the neutrino mass from the C and B alone to be less than 0.7.
So this is this is interesting. I think this is this is saying this is the sum of the neutrino masses.
This is saying 95% confidence. How much could it possibly be in terms of what we see cosmological?
And we're saying that that if it's more than 0.7, even the mass of the neutrinos, then it won't fit this data.
And the reason. So this is I think this is this is interesting,
but it's also the path to what we think we know we're claiming is a future detection of neutrino mass cosmological.
Well, they will have to do that in the context of of a discussion with particle physicists and hopefully agreement with particle physics experiments.
Okay. So I want to just have a note and I would kind of talk a bit about why why the lensing helps us with neutrinos.
So much so. After we measured the cleansing of the CMB, we're measuring basically an integral of the dark matter out from here to the scattering.
And what I can think of is if I increase the neutrino mass some of the neutrino masses,
then I should think roughly of switching cold, dark matter into matter.
Dark matter that started off hot and then became cold.
So cold, dark matter we think has been cold all the way through the universe.
It's perhaps a wimp decoupled very early on and it's been known relativistic for the whole duration of the universe.
Neutrinos, in contrast, are much lighter, which means that they started off as relativistic particles.
They started off like radiation. And then as the universe cooled down, depending on their mass, they suddenly became not much of a stick.
So if you think of a neutrino particle as basically being a hot, dark matter particle and then a cold, dark matter particle.
So now today it's all cold. All in all, all of the dark matter, including the neutrinos, are cold.
But to begin with, they weren't. And that means that we have we have a way of detecting their effects because logically,
because what they do is cold, dark matter clumps together and collapses gravitationally.
But hot, dark matter free streams. And it doesn't clump.
So during the epoch of the universe, when the neutrinos were behaving relativistic,
only when they were hot, they basically washed out the formation of cosmic, large cosmic structures.
But during the epoch, when they were cold, they were fully contributing to the regular growth of cosmic structure.
And so this has a very visible effect on the clustering of matter in the universe.
I'm showing it, first of all, here, and I'm still using a power spectrum, but now I'm not showing you the lensing.
I'm showing you the power spectrum of just matter.
The two point function of what dark matter should look like,
how the clustering of dark matter looks as a function again of scale from large scales to small scales.
And if I have basically a zero no neutrino mass,
then the kind of lambda CDM zero neutrino mass curve has this particular shape that basically looks like that blue curve here.
Now, if I increase the neutrino mass, then what I do is I dump, I reduce the amount of clustering on these small scales.
And you can look at the most extreme thing where it's one IB the sun and you lose structure on these small scales,
but you don't lose any structure on these large scales.
And these small scales correspond to scales that began to evolve earlier in the universe and earlier was when they were still hot.
So scales in the universe that began to evolve when the neutrinos were relativistic,
their structure gets suppressed compared to the larger scales in the universe that have been evolving.
While the neutrinos were not much of a stick. So you get this, you get this behaviour that the more and the more massive it is,
the more this contrast between, you know, neutrinos that behave like radiation and then matter.
And the more that you get, the more you have this, this, this spectrum damped compared to that compared to the large scales.
And this is what is leading to the current limits on the genomes from from the Planck lensing power spectrum,
which is basically an integral of this dark matter over, over the whole of the history is now a 95% confidence, less than .23 EV.
Okay. And that's I mean, that's a pretty strong constraint.
That's the kind of level that, you know, is hoped to achieve with the captured experiments.
So if we can make a at least a statement about what we think is the case from cosmology, then this should have interesting,
consequent implications for what one would expect to see from particle physics experiments.
And actually, in the next decade, we're hoping to make a detection of non-zero in a treated mass through its cosmological effect,
through its effect on the lensing of the CMB.
We know from oscillation experiments that the minimum mass of neutrinos should be 0.06 IB or 60 maybe electron volts.
And we're looking at its effect on this.
On the CMB lensing power spectrum, we should be able to get to a few sigma detection of that in the next decade.
And this plot is showing the fractional a fractional effect of changing the neutrinos on the CMB lensing power spectrum.
So this is this is a 1% shift, 5% shift, 10% shift.
And this is the curves for, you know, for some of the neutrino masses. It's 50 or 100, 150 electron volts.
So they're small shifts, but they are they got the shape dependent.
They look different. And we do think and this is work that we're doing at the moment,
is trying to establish how similar the effects can be to other cosmological features like the curvature of the universe, like dark energy.
But it appears that we can get to a.
Detection in a decade. But that detection is obviously it's indirect detection.
Right. We're saying with we're saying that we think we're seeing the effects of these these relativistic particles and we think they've got this mass.
So putting that in the context of then direct detection particle experiments will be exciting.
But obviously we'd like to be able to make the statement from the CMB and then see if if we see the same thing in particle physics.
Okay. Let me say something about inflation. Inflation is our best current model or scenario for the universe.
This exponential expansion of the universe driven by the potential of some scalar field,
it's, it's the most favourite model out there, but we don't yet know if it's what happened.
It seems to work so well because it predicts this exponential expansion.
Should it predict the flatness of the universe,
it should also produce the right kind of fluctuations that we see quantum fluctuations imprinted during the
inflationary expansion should will naturally produce this almost scale independent spectrum of fluctuations.
They should be Gaussian and they should be adiabatic, which means that fluctuations in all of the fluids in the universe should of traced each other.
So where you have had an over dense region of dark matter, you'd also have an evidence region of photons and baryons.
At the same time it's called adiabatic. And inflation predicts all these things.
And we're seeing all of those things from Planck and from Planck and from from ACT as well.
We're in particular seeing this very particular prediction of some inflationary models,
which are that the fluctuations are not not precisely scale invariant.
So scale invariant means that there are the injection of fluctuations at the beginning.
You have the same amount on all scales or on all scales.
But many inflationary models predict that you should get slightly smaller fluctuations on small scales because inflation has to end.
And the dynamics of that, the inflationary process, many of the models predict smaller, less power and small scales.
And so we've detected this now where we measure it by using the spectral index of fluctuations,
where scale independent is one and not scale independent is a bit less than one or a bit more than one.
And what I'm showing you here is just the angle of power spectrum, of the temperature fluctuations from Planck,
with the data in black, with the best fit curve, with a special index of one in blue.
And it doesn't fit. It doesn't fit like Six Sigma.
You need slightly less power at small scales shown by this red curve here to fit the data.
And this is new from Planck.
We had kind of hints at Three Sigma before, but this is Six Sigma and we think it's there and it still doesn't mean it's inflation,
but it, it's all looking a lot like inflation.
And so if it's not inflation, it needs to look a lot, a lot like this and make all these predictions as the last thing we're looking for.
My last thing would be if we found it's the beginning of a lot of things, a gravitational waves.
So inflation produces scalar fluctuations that are the things that cause density over densities and gravitational collapse of objects.
But it should also imprint gravitational waves in space and they should propagate.
So these are tensor fluctuations to the metric that just propagate as gravitational waves and do not couple don't produce density fluctuations.
And, you know, we want to find them because inflation happens then they should produce.
They should print it.
So sorry, this this little cartoon is designed to show what gravitational gravitational wave travelling towards you would do to space.
Right? A gravitational wave is going to distort space shrinking and stretching it as it goes.
It can have these two different oscillation patterns. Gravitational waves, if there were any gravitational waves travelling through the universe.
When the CMB formed at recombination, they would polarise the CMB light.
And roughly, you should imagine I told you before that to get a polarised photon you need a quadrupole pattern.
So you'd like more radiation this way than this way.
And actually, if a gravitational wave is passing through, then it distorts space in such a way that you get that exact pattern instant on a photon.
So if it's passing through, then the photon, that CMB photon that comes out, comes out polarised.
So it's an indirect, again, effect of the gravitational wave.
Now gravitational waves produce both patterns of polarisation, both emerge and b mode polarisation.
And so this is the space that we're looking, looking for and looking at once again, because it's our favourite statistic.
This is the angular power spectrum of the CMB polarisation as a function of angular scale up here.
This red curve actually is the curve that I showed you before with the Planck and the pole data on.
This is a thing we've actually now already measured, although we've got some work to do still we've seen it already.
What we're now looking for is this smaller blue signal, the blue curve down here,
that's the prediction of a gravitational wave signal, but whose amplitude is unknown?
I'm showing you two examples here of two different sizes. It could be it's got to now be less than about here.
We would have seen it, but it could be anywhere down here and it could be down through the floor.
It could depend on the size of it depends on the energy scale of inflation.
But its shape is is better known in green is actually a lens and b mode signal that
gets generated because the polarised E mode signal gets lot gravitationally lensed.
It's interesting in its own right, but it's not what we want to look for. We're looking for this this primordial signal from gravitational waves.
The thing we're searching for is this very small B mode signal.
I'm running short time. Let me just say a couple of things about the challenges there.
This is what many of us in the CMB community and are doing is is building experiments to search for these signals.
But this is the tiny signal. You can see it's down and it's below.
It's very much sub micro kelvin, we're talking dense and then a Kelvin range.
It's the kind of technology we have to use and this is what's in it's an act.
And surely thousands of detectors coupled together in array arrays in a telescope where you have thousands of very sensitive detectors.
There's just a little picture of all of our detectors for act where you have this these this array of 500 feet horns collecting C and B light,
splitting it into POLARISATIONS That then get instant on a transition to a sensible ometer.
So these are big emitters that change their resistance when they receive photons
and you couple thousands of those together to get as much sensitivity as possible.
And we've gone from having tens of detectors to hundreds to now thousands.
And you need that many if you want to get sensitive detection of the polarisation of the CMB and where this
is going is going from thousands to tens of thousands is where we need to go in the next in the next decade.
So this technology has had remarkable improvements in the last few years, which means we can now field thousands of detectors in the sky.
The other challenge is to see through the galaxy. The galaxy emits light.
I told you that already. It's also polarised that light and it's polarised because the magnetic field in the galaxy is not isotropic.
It aligns with the plane of the galaxy and that aligns dust grains and dust grains line up along the galaxy through radiative,
radiative talks and they are firmly along their long axis and that comes out polarised and we never knew how polarised it was going to be.
So we measured it with Planck just last year and it turns out it's quite polarised.
We weren't sure how polarised it was and it's about it seems to be up to about 10% or more polarised and that's actually quite polarised.
We optimistically thought it might be like 1% or 2%, but we didn't know and it turns out some of it's a few percent from it's 10%.
And that means it's really going to continue to be one of the main challenges to getting through to see and be a big bang signal.
What that also means,
this fact that the galaxy is quite bright means that it's going to that that the the AP might signal was in fact detected last year by the.
Bicep2 experiments operate in the South Pole, but that B mode signal turns out to have been almost certainly just a beam signal from the galaxy,
not a beam mode signal from the big bang from the CMB itself.
And if we zoom in on parts of the spectrum that it measured, then its measured data points well here in black.
This is the B mode power as a function of scale. But when you subtract off the signal from the galaxy, you get this blue curve.
So these blue data points, and that's completely consistent with this theoretical curve just from a lensing spectrum,
not from any gravitation, not from any gravitational waves.
So right now, we haven't seen any gravitational wave signals.
And in fact, they've pushed down the upper limits on the possible signal,
such that actually one of the simplest models of inflation is now basically disfavoured.
Quadratic potential for the for the inflation field that now is is kind of is is out.
What is disfavoured. So the path to getting to gravitational waves,
this is what we're going to be doing in the next decade is using ground based experiments with more and more detectors,
covering more of the sky with more wavelengths.
So we'll be mapping half the sky in a number of wavelengths with these many thousands of sensitive detectors.
And that can get us down to two to many factors lower than where we are now.
And we hope that we might get to see it. And we'll skip to actually.
Okay. So so all our CMB data really demands this simple model, this very simple lambda CDM cosmological model,
and it holds up to this fifties many new views of the universe that we have from these experiments.
The neutrino sector is fascinating and holds questions that cosmology can help to answer in the coming decades.
And in fact, if inflation is not the thing that began the universe,
then whatever it was has got to look a lot like it's searching for gravitational waves.
That search is still firmly on. So watch this space.