Good evening, ladies and gentlemen, and welcome to the 13th RNC electorates.
Tremendous to see so many people here. I don't think we're quite going to need the overflow, but it's getting a bit tight.
People who are late will come in the front. So particular.
Welcome to part school from Newbury I somewhere up there.
I think I saw you. Thank you for coming. It's nice to have you with us. So it was nice to have school groups here.
So it's my great pleasure and privilege tonight to introduce to you the 13th Quincy lecturer, Professor David Spergel.
He's the Charles Young professor at the astronomy department in Princeton,
but also holds a number of other appointments at the Institute of Advanced Study in Princeton.
Two at the Institute for the Physics and Mathematics of the Universe.
I think that's a wonderful name directed by a previous NC lecturer, Hitoshi Moriyama, and he's just taken on the role.
He's the founding director of the Centre for Computational Astrophysics at the Flatiron Institute in New York.
So we certainly wish, David, all the best with that new enterprise, which is very exciting.
He's no stranger to Oxford. He first visited here, I think, as a graduate student working with James Binney for a little bit in 1983.
His work on the cosmic microwave background has made him world famous.
He has made a major contribution to our understanding of the standard cosmological model,
an understanding about the age, the shape and composition of our universe.
He also has held a number of influential posts in the United States leadership posts.
So he is the chairman of the National Academy of Sciences Space Study Board, and he's the co-chairman leading a new mission called W First,
which may maybe or maybe not [INAUDIBLE] say something about he has a string of honours, far too many for me to mention all of them.
But most recently he was awarded the Dannie Heineman Prize of the American Astronomical Society.
He he was a member of the W Map team that won the group a prize for their work on the cosmic microwave background that that satellite did.
He won the Shaw Prize in 2010. He was made a member of the National Academy of Sciences.
In 2008, he was a MacArthur Fellow and also won the Yes, Helen B won a prize.
So he's a man of enormous distinction.
He has published a lot of papers, 280 papers and supervised 25 research students.
So it is my great pleasure to introduce David Spergel to give the 13th Tinsley Lecture a simple but strange universe.
David. Thanks.
It's a pleasure to be back in Oxford.
And in many ways, the theme of this lecture is the remarkable progress we've made since I was a graduate student here in 83.
In understanding our universe, understanding its structure and origin and properties.
And the bottom line of the talk is really the title.
Our universe is remarkably simple. And remarkably strange.
Much simpler than I thought it would have been back in 1983 when I was a student here.
But also much stranger. And what do I mean by that?
We can take a simple model, one that I'll talk about through word torque, which we call the Lambda CBR model.
That model has really five basic parameters the age of the universe, the density of atoms,
the density of matter, how lumpy the universe is, and how that lumpiness varies with scale.
So with those five numbers I could fit.
Hundreds of millions of pieces of data I can fit are measurements of the microwave background, which will be the theme of much of my talk.
I will sit observations of a large scale structure from things like the Sloan Digital Sky Survey, Supernova measurements.
Many of the observations,
the expansion of the universe from the Hubble telescope all the ways we have of measuring the universe are all fit by these same basic five numbers.
So as a scientist, that in some ways can't be simpler.
I know all the properties of millions of galaxies determined by five numbers.
It's a remarkably simple universe. It's also really strange.
Adam's. The stuff that makes up us in this model makes up only 5% of the universe.
There's six times as much stuff in the form of what we call it, five times as much stuff and what we call a dark matter.
We don't know what the dark matter is. It's stuff that clusters gravitationally on small scales and cluster.
It feels the effects of gravity. It could be some new subatomic particle.
That's our current best guess, but we truly don't know.
And even more strangely, there seems to be energy associated with empty space, something we call dark energy.
You know, our best guess on what that is is something we call vacuum energy.
Energy associated with empty space. But we don't know what makes it.
And something I'll touch on in my talk.
But really what I think is driving a lot of what we're planning to do in cosmology
in the next several years or ten years is understanding what the dark matter is,
understanding the nature of the dark energy, the origin of cosmic acceleration, and.
You know, understanding the universe,
the origin of the universe's early acceleration that gave rise to the structure we see understanding what we call inflation.
As an American, a couple of days after the election.
I can't resist talking about one idea for cosmic acceleration.
It's an idea called the big riff. So the idea that the universe is not just accelerating as it is now,
but this acceleration will go faster and faster with time and eventually run away and tear up the universe completely.
Tear apart every atom. This possibility is consistent with the data.
And in fact, if you look at some of the data, we have even favours it slightly over our standard model.
So I was interviewed by New Scientist five days ago, and I don't know if they'll go with this quote, but I think they may.
They asked me what I thought the big rep. I said I viewed it as something like the Trump presidency.
Terrifying to contemplate, but I didn't think very likely.
So in order to get into the story,
I want to begin by introducing first special relativity and then general relativity and then use that to construct our basic model.
One of the key things to think about when we're thinking about astronomical observations is light travels at a finite speed.
It takes light about a trillionth of a second to get from the front row to me.
So people in the front row see me as I was a 10th of a second ago in the second row, two trillions and so on.
I don't change that much on that time scale. So you don't see something very different from the back as we do with the front.
So you see, as you look out in space, you look back in time.
When we look at a nearby store, we see it as it was five or ten years ago.
The further out you go in space, the further back you look in time.
We see the Andromeda Galaxy as it was a million years ago.
And if we if there's life on the Andromeda Galaxy that has a telescope looking at us,
it sees the Earth as it was a million years ago because it takes light time travel.
So the further out we go in space, the further back we go in time.
We now have observations with the Hubble telescope that look out for what are 13 billion years, see galaxies as they were 13 billion years ago.
And as we look out at the microwave background, the leftover heat from the Big Bang,
one of the themes of this lecture, we'll see the universe as it was 13.7 billion years ago.
So for us, the key idea of special relativity is light travels at a finite speed.
All right, everyone mastered special relativity. Good.
All right. Now we're going to move on to general relativity. General relativity.
As one of my early mentors, Johnny Wheeler taught consists of two ideas that matter tell space how to curve.
And the curvature of space tells matter and light how to move.
So the picture to have is shown here. This is the sun curving space around it and light from a distance start beamed or even a nearby
star lines up behind the sun being deflected as light from the star moves around from the sun.
So as you can see, if you are an observer on earth,
instead of seeing the star or having the star blocked by the sun, you'll see the star at a different position.
And this was the shift in position was one of the great predictions of general relativity
and one that was tested by Eddington and others in the Great Eclipse expedition.
And. I recently learned with the 100th anniversary of Einstein's theory, a more complete story of what almost happened.
So Einstein's first version of the theory was not complete.
And in his first version, his first calculation of the defection of starlight.
His answer he got was off the right answer by a factor to.
He only later completed the theory in late 1915 and came up with the full answer.
There was an earlier eclipse expedition hat which would have tested his earlier wrong answer.
It got cloudy. So what could have happened is Einstein made one prediction.
The observations were off by a factor of two. And when he came up with the new theory that matched that, we wouldn't have taken it so seriously.
But fortunately it was cloudy. His results were published with the correct number.
And in 1919, here in the pages, this is the New York Times.
Lt's all askew with the heavens. Men of science in those days, mostly men.
Now we hope more men and women of science, more or less a dog lover.
The results of the and Einstein's theory triumphed.
And I like the stars. Not where they seemed or were calculated to be, but nobody need worry.
And in fact, it's true that they weren't where they seemed.
But the important thing is where they were, where Einstein calculated them to be, that this effect is happening.
And based on this and if I have time at the end of my talk, I'll say a bit about the recent local results.
If all reinforces the idea that general relativity is the valid theory to describe the universe on its larger scales.
And general relativity makes this very surprising prediction that the universe is expanding.
It says that space and time are not absolute.
But we really should only talk about the properties of space and time in relative terms,
in terms of the distance between you and me, the distances between objects.
And if I can tell you about distances, I can tell you about geometry and structure of space.
I flew last night from New York to London.
We flew. Over the pole. If I continue on in the next few days, I will not do this trip.
You can fly New York to London, London to Tokyo, Tokyo to New York just by telling you those distances and travel times.
You can figure out that the earth is not flat, but curved.
And that's basically what we do with general relativity in cosmology.
By measuring distances to objects and travel time, we infer the structure of space.
When we talk about the universe expanding. It's not that it's expanding into something, but rather the distances between objects are growing.
The distances between us and a distant galaxy keeps growing.
With time, the whole universe is stretching. So you should have this picture of the universe getting ever bigger.
As expansion goes forward.
The answer I like to give to the question of what the universe is expanding into is to say the universe is expanding into the future.
That I think of the universe as a sphere. I suppressed one dimension and it's a balloon that's being blown ever bigger.
And as just born bigger, the distance between dots on that balloon get larger and larger.
Now let's run that picture backwards. Our universe today is two dimensional version is on a surface of a big balloon.
So we go back in time. The universe gets denser and denser.
Eventually we reach the initial singularity, the star of the Big Bang, where the balloon collapses to a point.
Now you notice there's no particular place of the sphere.
That's the centre of the explosion.
The Big Bang happens everywhere and the universe is expanding from that point in time, 13.8 billion years ago to the universe we see today.
I think this is conceptually the hardest thing for us to think through, because it's most different from our ordinary notion of space and time.
One of the surprises of the last 20 years in this was we had evidence of this before the supernova observations in the late middle 1990s,
but was really strengthened in the definitive observations came from those.
We see that the acceleration of the expansion of the universe was behaving in a very strange way.
What I was taught we expected to happen was gravity should slow things down.
If I toss didn't want to move, my room came up in the air.
Gravity makes it come down. It slows it. Even if I was incredibly strong and able to toss the so fast it would go into orbit.
Gravity still has the same effect of slowing its departure, and we thought that's what gravity would do to the expansion of the universe.
The universe would be expanding, but gravity would slow its expansion.
So the expansion rate of the universe, the size at which the rate at which that balloon is blowing up, would decelerate.
It would slow. And the strange discovery is that our universe is accelerating, that its expansion rates growing with time.
All right. Now let's go to our basic picture. That we have of the universe's history.
We talked about the universe expanding with time, being denser in the past, in addition to being denser in the past.
It was much hotter in the past. The universe is filled with what we call the microwave background radiation.
This is the leftover heat from the big bang. For those of you older members of the audience who remember watching television when it didn't
come through a cable but came from an antenna and you switched your television between stations,
about 2% of the static that you saw was the microwave background.
So those of you old enough to remember some static on your television, you've seen the microwave background with your eyes.
And that's left overheats everywhere. Today, it's a temperature of about three degrees above absolute zero.
But as you go back in time, it gets hotter and hotter. When the universe was half its present size.
So the distances between galaxies down by a factor to the temperature was six degrees, a thousandth of present size, 3000 degrees.
And the further back we go in time, the hotter and hotter the universe gets.
Much of what we'll talk about today involves observations of what the universe looked like 13.7 billion years ago when
it was roughly a thousand its present size and the temperature of the microwave background was about 3000 degrees.
The reason we're observing that time is before that time the universe was ionised, and after that time, electrons and protons combined become neutral.
So we'll turn to the microwave background. The microwave background was first detected by Penzias and Wilson in the sixties, in the 1990s.
A star of the nineties, the Kobe experiment. First, not only so, the microwave background was remarkably uniform and thermal on its largest scales,
but was able to pick up tiny fluctuations at a part one part in 100,000 in the microwave background temperature.
And they won the Nobel Prize for this first discovery.
And this gives us a picture of what I think of as the universe baby picture what the universe looked like when it was only 380,000 years old.
Much of my own work in the past 20 years has involved working with data first from the Wilkinson microwave anisotropy probe,
a mission that NASA launched to make a more precise version of the microwave background.
This is an experiment that we named after our colleague Dave Wilkinson, who helped start the experiment, passed away with cancer.
Soon after, we got the data. This experiment was launched deep out into space.
You want to get away from the Earth's radiation, get away into a benign as environment as possible,
out to four times the distance of the moon, to what's orbit around what's called the second Lagrange point.
This is a point that is so far from the earth that instead of orbiting the Earth,
the satellite follows the Earth as it moves around the sun, always staying with the Earth, the moon in the sun behind it.
This is a really optimal place to put a telescope if you want to look out in space so that this is where the Planck satellite is.
This is where the James Webb Space Telescope is going to go.
It's a nice place to live if you a telescope.
And the goal was to look out this time at which the universe was first ionised.
Our first show, we actually observe in terms of what we see in the sky and then talk about what this means.
So we observe the sky at different frequencies. This is observations, and we're showing the whole sky around us as an ellipse.
You can see this is just like taking a globe and projecting it onto the sky.
So you're seeing at the top of the picture is the North Galactic Pole.
The bottom of the pictures, the South Galactic Pole and those bright red region is our galaxy.
And the striking thing you see in this picture is the bright red stuff.
That's our galaxy. That's not stuff coming from the universe.
Lots of interesting things about our own galaxy that one can learn from this data, but that's not the focus of this talk.
We're going to look at the stuff either above and below this bright red region and that stuff coming from outside our galaxy,
from the microwave background. As we go up in frequency, the galactic emission from relativistic electrons, synchrotron emission orbs gets dimmer.
It's easier to see the cosmological signal that's been travelling to us for 13.7
billion years and eventually we can combine this and get a very clean map of the sky.
And until the Planck's measurements in 2013, which I'll talk about next, this represented our best map of the sky.
One of the features I like to show in this,
and I think this is important for people looking at data to be careful about patterns is the letters H which appear in the map.
And I actually first saw this in the map back in.
23 after we released the data and I went to a talk at a conference where we showed our results and there was a talk by Stephen Hawking.
And I was sitting next to my colleague Lyman Page, who designed a lot of the experiment.
And Lyman pointed out, I said, Hey, students put in some of the data as a joke.
And we looked. Our data. And it was there.
Now. This can have a number of meanings.
This is in the Michael Scott I used to say, because this is back at 23,
that these were actually Saddam Hussein's initials and bad on the wife was God.
And then I sometimes argued when the Planck data would come out and we could see things with higher precision,
we'd see the two missing letters about the cosmological message that was left when this universe was made.
But I think more likely, this tells us something actually about evolution.
We evolved to find patterns and data.
And we find patterns and data, whether they're there or not.
We're selected for finding patterns. And the analogy I think of is you see some rustling in the grass and it looks a little bit like a tiger.
Do you jump or not? There were some humans who are very calm.
They see it and they say, you know, nine times out of ten, that's not a tiger.
They wouldn't run. Our ancestors there did jumping once they ran every time.
And the one time in tennis was a tiger. They didn't get. That's why they're our ancestors.
So we kind of selected for for erring on the side of seeing patterns when they're not there.
That's, I think, important to keep in mind when we look at any data.
That's why we do statistics, because we have to be careful for that.
All right. So how do we do statistics on this data?
What we like to do is measure the lumpiness as a function of scale, measure the temperature on different scales.
Basically, what we do to go back to this picture is we throw down circles and we measure how lumpy the universe is.
What's the average fluctuation as a function of the area of that circle?
And. Measured as a function of scale, angular scale.
For the physicists in the audience, we're taking spherical harmonics and looking at a power spectrum.
And this is a power spectrum. For the non physicists, it's how what are the temperature fluctuations versus angle?
And what you see here and this is the data of as of about five years ago will show some more recent things going forward is.
Our data. And our best foot model. And you see that the model does a beautiful job of fitting the data.
It captures the statistical properties of what we see.
Not only can we compare that to our observations of the microwave sky, we could take that model fit it to.
The microwave data ran a simulation of large scale structure.
Look how clustering forms and compare the pattern of galaxies that are predicted to what's seen in surveys like the Sloan Digital Sky Survey.
See what the model predicts for the universe today. And actually to skip forward.
This is one way of looking at the data, again, the lumpiness versus scale.
This is a plot from Rene Lozzi. This is from her Ph.D. thesis work at Oxford, this plot.
And this shows the fluctuations versus mass scale.
The read points in this plot come from the microwave background observations I've talked about.
They measure how lumpy the universe was. What, 13.7 billion years ago?
The blue points are coming from the Sloan Digital Sky Survey.
They're measuring how lumpy the universe is today, the the blue or the cyan points right here in the middle.
Read points that beautifully.
Our ideas about our model does a beautiful job about protecting the data over a factor of 10 to 13 in scale and ten to the eight in time.
That's remarkable for a model with five numbers.
Once you have a model like this that fits the data, you want to ask what happens when the data improves?
We now that we've based one way to think about this is you basically can fix the parameters with just the W map data.
Measured things with data like the Sloan survey. Future surveys,
which we'll talk about later like the large lr sas t get higher precision
measurement for Planck and ask us the data improves as the models still work.
The Planck satellite was a European space agency led mission that mapped the sky and high
resolution and higher sensitivity of the W map so the next generation and studying this.
And. It reached with higher precision, remarkably similar conclusions.
And this shows first just a comparison of the two datasets.
These are the two maps of the sky, and reassuringly they see the same thing.
We can compare this more quantitatively by measuring the lumpiness versus scale, the power spectrum of the different experiments.
And you can see here first in black that cross what you get from cross correlating the W map and
Planck data and in read the order correlations of the Planck data and they agree incredibly well.
And you can see the Planck data is higher precision so that the red lines is
certainly consistent with the black data at the scales at which the noise is low.
In both experiments, they agree almost identically. Point by point.
And at the smaller scales here. Well, they they agree overall but Planck has high precision.
We can also check our data by comparing the Planck measurements with ground based measurements.
So from the ground we can zoom in on small patches of high resolution.
And this shows two sets of maps, one made by the Planck experiment,
one made by an experiment that we're part of at Princeton called the Atacama Cosmology Telescope.
And they have consistent results on these scales.
Here's another way of seeing the state of play.
This is that same model that we picked to the W map data I showed the W map data sort of cuts off around here.
We could take ground based data that's shown in the red and the green.
And. Compare it to the model. It fits beautifully.
The Planck data fits beautifully. We have a very consistent story where the different experiments basically see the same
thing with the more recent experiments achieving higher precision and resolution.
But we can look at this basic pattern of those sound waves and a bunch of different ways and test things in a bunch of different ways we could.
You've seen the pattern in temperature, the two points below.
So what we get when we measure the temperature and polarisation data and the polarisation and polarisation data.
So here we fit our best model to the Planck data. It's done by the Planck team.
We now see some showing here and you can see that red curve.
Which was not fit to the temperature of polarisation data. The red curve is completely independent of the data.
It's a fit from other data and the blue points.
The data. It's just remarkable how successful the model is.
So the two lower parts are based on large scale structure observations.
These same sound waves that will show a different representation of the data shortly that generates structure in the microwave sky.
The sound waves in the early universe also produced a structure in the distribution of galaxies.
And the plot above comes from work from the Sloan Digital Sky Survey.
Some recent measurements and.
This is structural in the galaxies. We see the same sound wave that we see in the early universe.
We can see in the microwave sky. It all, it'll galaxy distribution.
It's all consistent. And it's a wonderful confirmation of this basic model that goes back to work by Sonja Films, although dovish in the 1970s.
And here's a quote from some named Enzo Darvish saying that a detailed investigation of the spectrum
of fluctuations may in principle lead to an understanding of the initial density perturbations.
Since the distinctive periodic dependence of the spectral density of perturbations on wavelength is peculiar to ET about it perturbations.
We see just what they predicted. I actually like to show the data in this way.
This is taking the temperature data taken.
On the left on this side, cold spots on this side of the map, hot spots.
So I go to the map. You saw I take every cold spot.
I plot the data around a cold spot. I stack up.
Hundreds of thousands, 100,000 cold spots in the map.
I stack up 100,000 hotspots. You could see around every cold spot is a hot red.
Around every hot spot is a cauldron. The bottom plot is.
The data itself. The top part was the theory.
There's other plots for the pattern and polarisation. What we're seeing here are sound waves in the early universe.
What's going on physically is we start out with the regions that have more dark matter, more electrons, more protons, more pressure.
If you have higher pressure? Well, if you create a region of high pressure, it produces a sound with.
That sound way propagates up. So that's the Soundwave propagating out in the electrons and protons and photons.
They're all coupled. The dark matter is not coupled.
It doesn't interact with things. So the dark matter stays in one place.
The sound wave moves out.
Where you have excess dark matter, there's more gravity that produces a cold spot where you have excess variance of photons that produces a hotspot.
The model predicts there should be dark hot spots, hot rings around every dark.
That's dark spot. That's exactly what we said. And all those ripples we saw are just different math ways of representing this pattern we see.
In the sky. So we're seeing just what the model predicts.
And I've mentioned already we see this in the pattern of galaxy distributions in the Sloan survey.
And here's another representation of it. We actually make a lot of use of these ripples.
So now that we've got these sound waves where we can use the first the map and now the Planck data to measure the size of these rings.
That's now a cosmic ruler that we call baryon acoustic oscillations.
And I can use that ruler to measure the distance to different objects.
So I take galaxies in a survey like the Sloan survey.
And I can measure the distance to them as a function of redshift.
And this plot shows the scale of this ring in excess galaxy count that we see as a function of redshift.
And it compares it to our theoretical model, this five parameter model that I talk about.
And you can see the model does a beautiful job of fitting the data. So we really have this consistent cosmology.
And this was true back in 2003 when we had the W map data initially came out,
but over the last 13 years, the data has improved enormously and the pieces all still fit.
So. Are we done? I don't think so.
I would argue that in a sense we've only just begun because we have this model.
And this model requires a lot of new physics that requires dark matter.
We don't know what it is, requires dark energy. We don't know what it is.
Another side piece of open physics is we don't know why there's more math bearing on why there are more protons and antiprotons.
We don't know why the universe doesn't have equal amounts of ordinary matter and anti-matter.
The capacity of Genesis. Nor do we really understand what generates the fluctuations.
We have this idea called inflation that I'll say it a little bit about. But we're really not sure where that comes from.
We'd like to answer these questions. And for me, as a cosmologist, I think what will be driving the field over the next 20 years or so,
looking ahead, is answering these questions What is the dark matter?
What is the dark energy? What is the physics of inflation? And that will drive both theory and experiment.
So a little bit about inflation.
In our standard cosmology, in this expanding universe model, what we would expect is that the size of the universe grows with time.
And this is a plot the size of the universe versus time.
The key idea in the inflationary universe model is that very early in the universe's
history it underwent a period of very rapid expansion that we call inflation,
and this period of accelerated expansion would produce both fluctuations in density and fluctuations that we call gravitational waves.
And what's. You know, and so inflation makes these key predictions.
We want to test these predictions. And again, the microwave background data is very useful.
It lets us show, as we saw already, that the fluctuations are full of these 80 biotic fluctuations, their sound waves.
We can also look at the statistics of the fluctuations, and this shows the distribution of the number of hot and cold spots versus scale.
And they make nice Gaussians. They're nice bell curves that really perfect all curves.
The number equal number of hot and cold spots and their statistical properties are exactly fit by Gaussian.
So that's just a beautiful confirmation of the predictions that the fluctuations are Gaussian.
They're close to scale, invariant. The amplitude is the same on all scales and the fluctuations that we call super horizon.
I could look at two points in the sky that are widely separate.
In fact, so widely separated that in our.
Standard picture where we didn't have this early inflating stage.
Those two points in the sky have never had the chance to communicate.
So if they've never had a chance to communicate, how do they know to have correlated fluctuations?
Well, either they were imprinted someone, you know, they are put in sort of my hand.
At the Big Bang or we're there somehow from a pre big bang phase or they were generated during this period of exponential expansion.
So that, you know, a very important prediction.
So if we look at what are the key predictions of the model, the key predictions that the universe's geometry is flat,
that the super horizon fluctuations are 80, about if the Gaussian are close to scale invariant.
I missed one on my list of. Inflation confirms our observations.
Confirm those first five predictions. What we are left with is a sixth prediction that there should be a background of gravitational waves.
And. I realised when I walked into this room that I think I was in this room.
I guess this was just about look at the date three years ago when this newspaper came out.
And I don't want to talk about Putin's invasion of Crimea, but rather the claim that space time ripples were seen.
There was a group called the Bicep Collaboration.
Led by John Kovac from Harvard that claimed that they had seen gravitational waves in the early universe.
And we were at an acting workshop in Oxford.
We were watching all this, the press conference remotely.
The Internet crashed. We were able to finally download the paper.
We put the paper up and we were very confused for reasons you'll see.
So what was going on here? These were observations of patterns of polarised light.
And what light is an electromagnetic wave?
Electromagnetic waves come with different polarisations.
The electric field could point this way or the electric field could point this way.
Scattered light tends to be polarised when.
No other pocket. It's sunny. Out I go. The beach.
I bring with me and one of my pockets.
Sunglasses. I don't. But you know what they look like.
Some of the reason I do that is polarised light, scatters off the water and tends to scatter one polarisation.
So if I have sunglasses that block a polarisation, I can see more easily on a sun.
The microwave background scatters off of electrons and it produces a polarisation pattern.
And the pattern we see in polarisation could be divided into two types what we call an old pattern, the one on the top.
Those are symmetric. Under narrow reflection. You can take this picture reflected in the mirror.
The emos look the same. You look at the bottom part of the picture, the beam holds, the beam loads.
You shine them, put them in the mirror. They look different.
Variations in density produce only emotes.
Gravitational waves produced both emotes and emotes.
And this is why cosmologists were so eager to check them out.
I'm still over eager to protect emotes because.
They would be the signal gravitational waves. So why would we be excited to see gravitational waves?
Well, it's a glimpse into the universe's first few seconds.
It's testing this idea of inflation. It would open up a new world of cosmology.
So when this map came out showing this playing female pattern and here's the data that they published.
That we were all excited about when I was sitting here in Oxford. This red curve is what we expected without gravitational waves.
The points are their data and the curve is a pattern is a fit through the data that says that there's a gravitational wave signal that's pretty good.
Looks like they Antarctic gravitational waves. The people involved were already claiming the Nobel Prize.
There was a lot of excitement. There's like over a thousand papers people got named most influential.
My experience in this was a bit like the small child.
In the story of the Emperor's New Clothes. Having worked in the microwave background for many years,
I knew that dust was polarised and felt they had not shown that there was any yet, that their signal was not due to dust.
So I went around and started to worry about this.
And since the leader of the experiment was one of my senior thesis students long ago,
I wrote to him a seven page note and said, Here's the concerns I have with the data.
Have you address them? For the students and the audience.
If your advisor from 20 years ago writes you a 27 page note about this thing that you're pushing for the Nobel Prize,
you should read it and answer it. At least say a polite note.
Thank you for your note. That's that's a good social suggestion for the high school.
There are some students here. When your teacher brought you here, writes you a note in 20 years that says, aren't you worried about this?
Answer. So we went.
And checked and said, all right, let's take the data and try to see what the dust model and these were our fits with the dust model.
And you can see our colours in the blue and their curves and the red all go through the data.
And then we went and looked at the plant data they used for analysis and they did something.
That I would call slightly aggressive with the data. The Planck team had not yet released their data.
They had talked about the data in a conference and there was a conference proceeding where they had a PDF from the talk on the web.
They downloaded the PDF and they scanned it.
They use the scanned PDF for their analysis.
The problem was they didn't read the slide.
The slide says not C.I. B subtracted.
It's a technical thing, but it's a nice little bit of algebra. The C D is the cosmic infrared background, so it's the stuff from galaxies.
The polarisation they measured in the map that they used for the correction was the polarisation of the galaxy,
plus the polarisation of those of all these distant galaxies.
Our galaxy. Distant galaxies. Divided by the intensity of our galaxy and the intensity of distant galaxies.
The PCI term is zero. If you these two terms are about the same.
If you ignore one, you get the wrong answer. They assumed one was zero.
So you go back you correct for there the term for what you were told on the slide was there and it all goes away.
And that was our conclusion.
And now that the Planck data is published, the Planck team has done its own analysis and concluded that all that excitement was for nought,
that it all just turned out to be polarised dust. And we can now at least turn this around and say.
Now that we know that's mostly dust, we can constrain the amount of gravitational waves.
And based on that, we could actually focus in on the type of inflation that we think is going on.
Where do we go from here? The next steps is to make better measurements of the microwave sky.
And rather than do it at a single frequency, we want to do this at multiple frequencies so that we can handle dust.
The project that we in Princeton are part of, and Amelia Calabrese, who's here in the audience,
is this is part of our team is something called Advanced Act, where we're going to use five frequencies so we can get a handle on the foregrounds.
We're actually now looking at this is how well we hope to do with measuring the gravitational wave signal.
We also want to measure the effects of lensing on space distortions in space between here.
Here in the distant universe. The gravitational lensing. That's sort of a guaranteed part of our signal.
Typically eager to be able to combine this with upcoming experiments like Elsa's teeth to improve, to look at this piece.
But we're hoping to get them a more complete picture and maybe make a definitive detection of gravitational waves if they're there,
because we have lots of frequencies. We recently announced that we're going to go beyond that experiment.
We've got $45 million from a combination of the Simons Foundation and the
universities that are partners to build a new telescope set of telescopes in Chile.
We just had a big meeting for the brought together.
The two teams involved are now combined into one team to measure the microwave background score with increased precision.
I want to advertise this because we're hoping that there'll be a British participation team where there's a group of
British scientists here and at other universities that we're hoping will join in this international collaboration.
This is in the same site as the ORMEAU Radio Telescope,
and it's basically looking at the same region of the sky as another very important upcoming experiment.
The closest to the Live Synoptic Survey Telescope.
This is a telescope that's going to survey much of the sky.
About two thirds of the sky get very deep images.
It'll do many things. But in cosmology, one of the things we're most interested in is it will trace the large scale distribution
of matter that we can use to test some of these theories on talking about and our test,
our ideas about dark energy and dark matter. And my final.
5 minutes. I want to say a little bit more about gravitational waves because.
I just spent 15 minutes telling you, 10 minutes telling you I don't believe gravitational wave detections.
Well, I didn't believe the claimed detection of gravitational waves in the early universe.
But there's another gravitational wave detection that took place in the last year that I think is remarkably compelling.
On. It tells us mostly about black holes.
But I think as a result, this also ties into this general theme of this lecture,
which is that general relativity seems to be valid on a remarkable range of scales,
and it reinforces the underlying principles that we use for a cosmological analysis.
So it logo is designed to observe is gravitational waves produced from the collision of black holes or black holes and neutron stars?
It's a truly remarkable experiment in that the gravitational waves they're looking for has the effect of shifting to mirrors.
In these experiments in Louisiana and Washington by less than the radius of a neutron.
So you're measuring distances to a changes, some distances to a part and ten to the 21.
And it was a tremendous technical triumph to build this.
It's also, I think, putting on my policy hat, a tremendous triumph for the government agencies involved actually to stick with it.
So they built this experiment. It had sensitivity of a part and ten to the 20.
They spent about $100 million on this. It worked as planned.
They saw nothing. The scientists said, let's spend more money to improve our sensitivity.
This is opening up a new window. The National Science Foundation to its.
Stepped up. Did it made the investment.
Improved sensitivity by another order of magnitude.
And now we're starting to see black hole collisions. And these sound waves are tiny displacements in the mirrors.
And what you see on one side are observations made in Washington, the other in the made in the detector, Louisiana.
And you can see the two experiments saw the same ripples, the same displaced, same sound waves.
At the same time. And the curse underlying them is our theoretical curves based on solutions of general relativity that.
You say that we're seeing the collision of two black holes. And the theory fits the data beautifully.
So this is important. They've detected the first collision of two black holes.
They've tested General to be one of its most important predictions, dramatic predictions, the existence of gravitational waves,
something that actually Einstein predicted, then thought wasn't there, then came back and deciphered.
Yes, it really is. They hard to work through, opened a new area of the universe.
I think it represents the fact that instead of just seeing the universe,
which is what we've done up to now through photons for the first time, we're hearing the universe.
And what we're hearing is it's playing Einsteins to. So let me conclude.
The. We are lucky to live in exciting times.
Cosmologists. We have discovered that we live in a universe that is both remarkably simple,
remarkably strange, simple model with only a handful of parameters fit all of our data.
But it's a model that gives us big open questions.
Dark. What's dark matter? What's dark energy? Was the universe accelerating?
What's exciting looking ahead to the next 20 years is that we can and not only can
we envision we are undertaking a series of experiments and satellites that will.
Test a lot of these ideas, the data will continue to improve.
Particularly, you know, things like the large synoptic survey telescope that will map galaxy positions.
These upcoming microwave background experiments, I think will give us new insights into the nature of dark energy and dark matter,
and perhaps we can even understand the origin of the universe. So let me stop there.
Thank you.