Welcome everyone.
As we all rearrange ourselves.
Today we're going to talk, in this talk, about the state of the Universe.
Now, you've had the benefit of several talks of cosmology.
I'm going to try to tie it all together.
So, in 2015, what is the state of the Universe?
Well, the Universe is expanding.
The Universe is 13.8 billion years old.
The Universe is close to being geometrically flat.
And the Universe is composed of a mixture of things, including dark energy, dark matter, atoms, neutrinos, photons,
and a few other things of lesser amounts.
Now those are words, and I want to explain why we know
and think that we understand the state of the Universe so well.
So let's first start with the expanding Universe.
Now Saul Perlmutter talked about this, but in case people missed that,
I figure it's good to review just a little bit.
So the expanding Universe goes back a long time.
At least to 1929, when Edwin Hubble went out and looked at galaxies, we didn't really know
what galaxies were, except for collections of stars, until about this time.
And he looked at how bright the stars appeared in those galaxies.
And he noticed that the galaxies, some had brighter stars, some had fainter stars.
And he attributed that brightness to their distance, of course,
the further away an object is, the fainter it's going to appear.
Another person, Vesto Slipher, most of you will not have heard of, but he's a great astronomer,
and his family gave me a scholarship, it turns out, so that was one of the reasons he's great, for me.
Is that Vesto realized that he could go through and see an effect, the Doppler Shift.
He thought it was the Doppler Shift, we now call it Red Shift.
That galaxies' light was stretched.
And some more, some less, but almost all of them had this effect.
And if you plotted the amount of stretching of light,
which is equivalent to their velocity, or apparent velocity, away from us,
you get this plot that Hubble made where you have a trend.
And what I love about this plot, is this is the plot that makes us realize the Universe is expanding.
And that's not beautiful-looking data.
It's data that a biologist might be proud of.
(laughter)
But physicists, maybe not so much.
Yet it tells you the further away an object is, the faster its apparent motion.
So, from that, we learned that the Universe was expanding.
Why? Well let's think what it means to be the further, the faster the motion.
I have a toy Universe here, which I'm going to expand, alright?
So if I take the Universe, and I make it a bit bigger, in between before and after,
and then I overlay those two things, what do you see?
You see that for nearby objects, their motion has been small, they would have a low velocity.
Distant objects, well, they will have moved a lot in that same amount of time.
They'll have an apparent high velocity.
And it doesn't matter where you are in the Universe, everyone sees the same thing.
In a Universe that's expanding, the further away something is, the faster the apparent motion.
Now it turns out, this was all predicted, sort of, by theory.
Theory from none other than Albert Einstein.
Now before we get to that, let's think what it means to be expanding.
If I'm expanding, well, you're getting bigger and bigger apart.
But I can run the experiment in reverse.
What do I get?
I get things getting closer and closer and closer, until everything in the Universe is on top of everything else.
That's the thing we nominally call the Big Bang.
So if you measure how fast the Universe is expanding, that's giving you an idea of how old the Universe is.
And this was such an interesting thing to do that I decided to do it for my PhD thesis.
And from a graph you can think of it this way.
Right now, you look at galaxies, and they have some separation.
I run the Universe back in reverse, based on how fast it's expanding.
And I can simply read off, when the two objects are on top of each other, to the age of the Universe.
And so, here I am, at the end of my PhD thesis, three years, eleven months and four days,
but I wasn't counting.
(laughter)
Showing my PhD supervisor Bob Kirschner the answer I got.
So the answer I got was that the Universe was roughly 14 billion years old.
Now I'd like to say that I get credit for measuring the age of the Universe.
The answer is, I didn't convince that many people with my thesis.
But I did get the correct answer, which is important.
Actually it's not that important, but that's OK.
OK, so the theory that ties this all together comes from Einstein.
Einstein had this revelation in 1907,
which was that acceleration due to moving faster and acceleration to gravity,
in his mind, must always be essentially the same, they always were equivalent.
Kind of a small thought, that took him eight years to sort out.
He had to learn a lot of mathematics, which I can still challenging this day, but essentially the idea is,
imagine you're in a box, or in a room.
Are you being accelerated by 9.8 meters per second by Earth's gravity?
Or are you actually in a rocket ship, being accelerated by 9.8 meters per second.
There's no way to tell the difference under his assumption.
And that assumption has never been shown to be wrong.
So astronomers are the ones who actually vindicated Einstein's theory of gravity.
How? Well, by looking at eclipses, and this is the real from 1919.
It's terrible, it's out of focus, but it was good enough to show
that stars next to the Sun were displaced
due to the gravity of the Sun and the way that the Sun curved space.
Because Einstein, to solve this seemingly small problem, had to say that space was curved by mass.
And that was a huge leap forward.
Many people were, of course, sceptical.
Others weren't.
But it's been vindicated, and that basic idea has never been shown to be false.
We know it doesn't work with quantum mechanics,
so there is at least something that's probably not right somewhere.
Alright, so Einstein actually became a household name due to, not special relativity, but to general relativity.
He was all over the front pages of the newspapers.
And so it was really an idea that came out of pure thought.
And that is very, very rare within science.
Normally you have an idea, there's a problem, you need to solve it.
There was no problem at this point that Einstein was thinking about, he just said, this must be the way world was.
So, in the coming years, de Sitter and Einstein first tried to figure out what this meant for cosmology.
Newton could not solve cosmology, that is, how the Universe behaves, with his equations of gravity.
The only way they made sense is if the Universe had nothing in it.
So de Sitter and Einstein tried, and then Alexander Friedmann in St. Petersburg came along and said,
let's assume the Universe is the same everywhere.
Homogeneous.
And isotropic, it doesn't rotate, or something.
And he came up with a series of solutions.
Einstein knew about those.
But, at the same time, Georges Lemaître, a Belgian astrophysicist, came up with them independently.
And then said, the mathematics means that the Universe is actually expanding.
So, unfortunately for Lemaître, he met up with Einstein in 1927, showed him his work, presented it to him.
And Einstein said, your mathematics is correct, it's already been done by Alexander Friedmann,
but your understanding of physics is abominable.
Lemaître was forgotten, for a while.
Hubble came along, and said, the Universe is expanding.
He didn't tell Einstein, he just put it on the front page of the New York Times.
And so we remember Hubble, we don't remember Lemaître.
But you should, because Lemaître did it, as part of his PhD thesis.
So what does Friedmann's equations say about the Universe?
Well, they say that the Universe is empty,
it just keeps on getting bigger and bigger and bigger and nothing really happens.
It just, it's on this straight line.
Not dissimilar to a ball, if you threw it out in space.
On the other hand, if you have a light Universe, gravity is going to slow down the Universe a little bit.
But the Universe will be able to expand and expand and expand forever.
On the other hand, if you fill it full of stuff that Einstein had already thought about,
dark energy, energy that's part of the fabric of space, this stuff makes gravity push rather than pull,
and you might get a Universe that would exponentially expand, very very quickly.
Finally, there was the, kind of most intriguing model, the heavy Universe.
The Universe that has so much gravity, that it expands, stops expanding due to the effects of gravity, goes in reverse.
So all the universes seem to start with a Big Bang,
but only the one that is heavy ends with a gnaB giB, that's the Big Bang backwards.
Alright, so another feature of curved space is that a heavy Universe literally curves around itself in four dimensions.
Given enough time, you can imagine heading out in this direction, and eventually coming back to where you stand.
What's the Universe bending into?
Well, it's bending into this time thing.
On the other hand, if the Universe is light, it bends the other way, the shape of a saddle.
A Universe that has the shape of a saddle, triangles, for example, and this is an experiment I've always wanted to do,
you send a graduate student off this way, another one off that way, and in a billion years,
travelling at about 99% of the speed of light,
we get together, and we measure the angles of a triangle.
And we ask, what are they?
In the light Universe they add up to less than 180 degrees.
In the heavy Universe, they add up to more than 180 degrees, just like they do if you did this experiment on Earth.
Get a globe, if you don't know what I mean.
Finally, there's the "just right" Universe.
That Universe precariously balanced between the finite and the infinite,
where triangles add up to 180 degrees.
So there is a geometry to space that is inherent with general relativity.
So, when I finished my PhD, I went off to Australia,
and on the way to Australia I was able to work with several of my colleagues,
and specifically Nick Suntzeff in Chile, to devise an experiment to measure the Universe's past.
And that was to look back in time and literally see how the expansion rate of the Universe was changing over time.
We can do this, because when we look a long ways away, it takes billions of years for light to reach us.
And in 1994 there was new technology, and there were new ideas of how to use supernovae
to go through and see the trajectory of the Universe.
So this is what Saul Perlmutter talked about in his talk.
And essentially we wanted to see how the expansion rate of the Universe changed over time.
And we could see whether or not it was going to, for example, exist forever, or collapse into something analogous
to the Big Bang in reverse.
So the supernovae were the new ideas done by a group in Chile.
The technology was developed for six years by Saul Perlmutter, and we sort of married these two
together in what was quite a competitive experiment.
Saul and I always had a lot of fun with it.
I'm not sure that all of our team members did.
But competition was great.
Because it meant we always knew the other team was going to push us, in terms of how efficient we acted,
but also point out any mistake we might make.
And so, after several years, we came up, this was our data,
and we noticed immediately that the data was in the wrong part.
The distant data was in a place of the diagram where the Universe was expanding slower in the past,
and seemed to be accelerating.
Einstein told us we need something funny for that, stuff we call dark energy.
This is the work that produced the two papers of the two teams in 1998.
And, of course, Saul and I are here, but we represent about 60 people who did this work,
our team and Saul's team are both represented in Stockholm.
A great party for us all.
Alright, so what's pushing on the Universe?
Well, Einstein told us it's this stuff called dark energy.
Well, what is dark energy?
He called it the cosmological constant.
It really is energy that is part of space itself.
And it turns out, it does, through his equations, cause essentially gravity to push rather than pull.
Now, if you go through and did a detailed analysis, as we did in 1998,
you could see that you needed a mix of stuff,
you needed about 30% of the Universe to be gravity that pulls, and 70% to be gravity that pushes.
Alright, so that was the conclusions there.
Now, let's think, again, if we take the Universe, and we compress it, what does that do?
Well, if you do that experiment here on Earth, for example you'd take a little piston and I'm going to compress it,
or this gentleman is going to compress it, things get hot.
So there's a little piece of cotton wool in there, and he just took a glass piston and pushed it,
and it heated up, to the point where you could set the cotton wool on fire.
The Universe is very similar.
It has a certain temperature now, when the Universe was more compact it was hotter.
And if you go back in time, you reach a point, to when the Universe was roughly 3,000 degrees.
So, 3,000 degrees is when hydrogen becomes ionized, so, before that,
the electrons were essentially not attached to their protons,
and, to a photon, that means they look like a giant target.
Likes to scatter off electrons.
So the Universe was opaque.
Right now it's not, we can see through it.
And so that led to the discovery, which hopefully Bob Wilson, you would've heard him describe,
and this is one of the most remarkable discoveries of the twentieth century, where you look out in all directions
and the Universe is glowing.
And it's glowing because it was opaque and hot, and things that are opaque and hot glow.
Not a lot like your stove or the Sun.
And so, 13.8 billion years ago, what is what the Universe looked like.
And in this image, we see many interesting things.
The one thing that we can imagine is imagine that we're bumps and wiggles in the Universe.
They're going to create sound waves, and those sound waves are going to move out, at the speed of sound.
And they're going to get out to, essentially 380,000 years' time, that's the speed of sound,
and they get dragged around a little bit by the Universe, but let's not worry about that.
And so those sound waves have a scale, which we can calculate using physics, to about one part in 10^4.
It's a ruler of incredible precision.
Especially by cosmologists, who normally are used to dealing with factors of two.
And so, if you have a ruler, you can look and say, ah, I can figure out how far away that object is.
And how does it work?
Well, it turns out there are many things that cause that ruler to have a size,
but the most important one of interest is the geometry of the Universe.
If the Universe is curved, the ruler gets distorted.
It's not dissimilar to looking in a rear-view mirror.
The entire Universe acts as a lens.
If it's flat, it doesn't get distorted.
So you can look at those sound waves and you can say, how big are they?
Well, you do that experiment, and this was done, starting in 1998, just the same time as our experiment,
and you can say, OK, if the sound waves are small, it's a Universe that's light.
A heavy Universe, they're big, and somewhere in between, they are somewhere in between.
So, when you do that experiment, it turns out the Universe is just right.
It has got that perfect amount of material so that space is neither curved on to itself or away from itself.
Fantastically accurate experiment.
This is why we say the Universe is geometrically flat.
Alright, now, this, there's so many things we can do with the cosmic microwave background,
it's sort of our best laboratory.
One of the things, you know, is that these bumps and wiggles are going to become the galaxies of today.
So in Australia, back in the early 2000s, some of my colleagues went out and mapped out 221,000 galaxies.
People in the United states did a million galaxies a few years later.
But you can go through and measure, essentially, gravity in action, because gravity's going to take those bumps,
and it's going to make a pattern.
We use computers to see what that pattern looks like.
And it depends on, it turns out, what the Universe is made of.
And how much gravity it has.
So you can look up here, at the real Universe, where it says "Observed", and here are four different universes,
made up of different things.
And your eye is pretty good at doing the Fourier transforms we really do to compare those data sets, and,
if you're looking at that,
all figure out which one you think is right.
If you said that one, you're right.
That is what the data looks like.
And that is a mixture of five parts dark matter, which I haven't talked about yet, one part atoms.
What's special about dark matter?
Dark matter doesn't interact, except for by gravity.
It goes right through itself, it goes right through the Earth, it goes right through atoms,
it essentially is invisible to itself and everything else in the Universe.
It only has gravity.
That makes it insanely easy to calculate, because you literally just have to put Newton's laws in, and it works.
You don't have to worry about pressure or anything.
So it's very convenient that the Universe made the Universe out of this,
because if it didn't, it would be really hard to calculate.
So, dark matter.
Well, we don't quite know what dark matter is.
It has gravity, but we think it's probably some undiscovered particle.
And we can sort of get a sense to say this stuff really exists.
Here are two clusters of galaxies which are made up of atoms and dark matter.
When you clash atoms, what do they do?
They light up, make x-rays, and they condense in the centre.
If you look at where the atoms are in this picture, well, they're in the centre.
If you map out the mass, by using how space gets distorted through what we call gravitational lensing,
you can see where the mass is.
And the dark matter seems to have gone right through itself,
as these galaxies have collided, and the atoms are stuck in the centre.
Just like we see.
Every way we look at it, we see dark matter in this five to one ratio.
Not just here, but, again, in the cosmic microwave background.
Why? Because those sound waves are dependent.
The wave action in the early Universe is dependent on what the Universe is made out of.
If you plop something into a pond, and it's made out of honey as opposed to water, the waves look different.
And so, the Big Bang essentially, as I'll show you, threw gravel into the pond, those waves fluctuated out,
and we get a snapshot as the Universe became transparent.
And that's what the Universe looks like.
So what does the wave action look like?
Well, we measure it in the form of, essentially, how many waves there are at different scales.
So you get this very complex curve.
The curve is the theory, the dots are the data.
You don't get many experiments better than this.
And, if you change anything by even small amounts, the theory no longer fits the data.
And it says the Universe is 25% dark matter, same answer we get in the other place.
It says it's 5% atoms, or baryons, as we like to say.
It tells us approximately, it tells it very accurately, but it's roughly,
there are 10^9 photons for every atomic nucleus.
And that there are 3 low mass neutrinos.
Heavy mass neutrinos, 4 neutrinos, all screw things up.
Alright, that's what we get by doing that wave action.
So, when you put this all together, you get this quite remarkable but unexpected story.
Flatness, that means all of the matter in the Universe adds up to 100% of enough to be flat.
Turns out about 30% of it, when you look at how much gravity there is, is stuff that's attractive.
So we need mystery matter, 70%, the same stuff that the supernovae experiments
that Saul and I have been talking about seem to require, to push the Universe apart.
So we apparently have our Universe, which we are only 5% of.
Because we have all these constituent parts, and we know how they work together,
we can very accurately trace the Universe back to the time of the Big Bang.
Back to 380,000 years, and then it's a very small extrapolation.
And it tells us that the Universe is 3.8 billion years old.
Sorry, it's 13.8 billion years old.
The best number is 13.82, people get out in a fight if you say it's 13.78 now.
So we've really got it aged very accurately.
Now let's go before the CMB.
You make the Universe smaller, it gets hotter and hotter.
Eventually you get to the point where the photons are so hot that they can create things.
For example, they might create an electron-positron pair.
Now an electron-positron pair get together, they like to get back together and produce two more photons.
One of the interesting issues of all of the particle physics people are looking at, is they have
this self-contained model, that seems to all work,
but we know from astronomy there are a bunch of unanswered questions from this point.
Why do atoms exist?
Because, in a hot universe, all the equations are such,
that every time photons get together, they make and destroy matter evenly.
Yet we have this ratio that there's 10^9, a billion photons, per every atom.
There is an asymmetry in the equations we don't understand.
Because, as they stand now, we aren't here.
The entire Universe is just photons.
So something's going on we don't understand.
We of course haven't discovered dark matter.
We think it's a particle, but we haven't seen it.
What is this dark energy stuff?
We really don't know there.
We have learned almost nothing about dark energy since our discovery in 1998.
It's a sad state of affairs.
Why do neutrinos have mass?
There are reasons from astronomy, and from experiments here on Earth,
that we're pretty sure they have a little bit of mass,
but the equations don't tell us that either.
And finally, how does gravity work with other forces?
That's sort of the huge question in physics, the big one, of unification.
Now, if we run the Universe back to a little more moderate temperature, 800,000 degrees,
the whole thing acts as a nuclear reactor.
And so you can very clearly predict what the nuclear reactor's going to do.
You go through and do the equations, same ones that we use for doing nuclear bombs and other things,
and you sort of get a universe that produces a set of elements:
hydrogen 75%, helium 25%, and trace amounts of other things.
Exactly what we see in the Universe.
Everything else, the carbon, all the stuff that makes biologists excited,
well that's all happened lately in the stars that we look at.
So we know the Universe was hot, it cooled.
Here, those little sound waves are only fluctuations, in one part in a hundred thousand.
They grew by gravity, and something happened.
About probably 100 million years later, first stars formed, and suddenly, 13 billion years ago,
we had this really exciting Universe
that looks not that dissimilar to today.
We're at the point of being able to go and ask questions.
So one of the things I've been working in, is taking and looking at every single star in the Milky Way,
and asking, can I find stars that only are made up of hydrogen and helium?
Those would be the ones made right after the Big Bang.
So, we're looking at billions upon billions of objects, and last year we found the closest thing yet.
It's an object that has no iron in it whatsoever.
Exploding stars make iron, they also make other things.
So this object has literally no iron.
One part in 100 million.
Less than the Sun.
And so this is a spectrum where you can identify the iron, there really is nothing there.
It does have a little bit of calcium, and other things.
So we think this star was created out of the ashes of one of the original stars of the Universe.
We only have one, we're going to be, over the next five years, hopefully finding a whole bunch of these.
And piecing together how the Universe was formed from the fossil relic.
But we can also, with the next generations of telescopes, look literally back to this epic,
and hopefully see the objects being created.
In real time.
With the James Webb space telescope, and the extremely large telescopes.
So, we can tell, though, that the Universe right now is 2% made up of heavier atoms.
So stars have taken that initial mixture and created 2% of the Universe and transformed it through nuclear reactions,
into the things that make, quite frankly, life here on Earth quite interesting.
Except for those of you who like to play with helium.
OK, another big question, what is the Big Bang?
This picture is really funny.
It's very lumpy here, but we have incredibly put up the contrast.
The temperature is the same on both sides.
How can we understand that?
When, it turns out, if you go through and make a diagram, or you sort of look at how light looks at,
you see, that if I look that direction,
and I look the other direction, and I go back to a distance of 380,000 years after the Big Bang,
you can go through and then make those light cones and you can say that piece of light and that piece of light,
never can see each other, and yet they're the same temperature at one part in 10^4, almost.
Alright, so we need a trick.
And the trick is something called inflation.
And inflation allows a time in the Universe where the Universe exponentially expanded and stopped for some reason.
And so that makes that diagram look a little funny.
And I seem to have lost one of my slides unfortunately.
It also takes tiny fluctuations caused by quantum mechanics, and blows them up to the size of the Universe.
Seems crazy? Well, if you don't have this, the Universe, instead of looking like what we see it,
it would look like this, there would be no structure in how gravity could grow things.
It also turns out, is a way of taking a very lumpy universe and as it accelerates, it flattens it out.
It matches all these observations we've made of the Universe.
But it does mean that the Universe had this funny time,
where we think it essentially expanded by, you know, umpteen orders of magnitude,
in a tiny fraction of a second, and then essentially made the Universe as we see it today.
So that brings the question, what is the Big Bang?
Well, I don't know what the Big Bang is.
To me, the Big Bang is whatever caused the Universe to inflate.
Now there's a time, probably, before that,
but essentially that magnification has washed out almost anything we can know.
And it may well be that the Big Bang is the inflation, or maybe there's something before, we don't know yet.
So I'm afraid I don't know what the Big Bang is, I only know what happened after the Big Bang.
And what's our future?
Well, I'm sad to say, that our Sun's nuclear reactor's getting stronger and stronger.
In about the next 500 million years it's going to get very hot on Earth.
Not the little degree or something we're seeing right now, fluctuation, we're talking 50 degrees hotter.
So, in 500 million years, we're going to have to find another home, or we're going to cease to exist.
Eventually, 5 billion years from now, the Sun really does run out of nuclear fuel,
and we are definitely doomed at that point.
So, as the Sun expands and becomes hotter, life's going to be uncomfortable.
But I have hope that we will be able to move to another planet, maybe not quite like Interstellar,
but we will continue on and probe the Universe.
And what are we going to see?
Well, it turns out, we have lots of places to go.
There's 100 billion stars in our own galaxy.
And if you go through and zoom in from our galaxy, and look at a tiny piece of sky,
we see literally, in this tiny little piece, postage stamp, you know, 20 thousand galaxies.
And so that is a very rich Universe.
But it's finite, quite remarkable.
Because, if we look over the entire sky, you see essentially everything.
And so, if you want to go through and ponder how insignificant we are, when you just do a quick little
back-of-the-envelope calculation here.
So, how many stars in the Universe?
Well there's about 260 billion galaxies, if I add up all those pictures.
There's about 100 billion stars in each one of those.
It turns out about one in five of them, we think, has an Earth-like planet, from latest measurements.
So there's a lot of stars in the Universe.
More stars than there are grains of sand on planet Earth.
So we are insignificant.
But think that we, 17,300 years ago, in a cave not too far away from here, our forefathers made this picture.
We have the Pleiades stars, the same ones we see today, almost every human's been able to see them,
they have Taurus, the Bull, the same place where it is on the sky today.
The same description we use today.
We call these the Seven Sisters, the Aboriginals of Australia called them the Seven Sisters,
they'd been isolated from Europe for tens of thousands of years.
Stars are our history.
And what is remarkable is that we have been able to go through, as insignificant humans,
and piece together the story I have shown you today.
But time is of the essence, both because I've run out,
but also because the Universe is exponentially expanding, we're at the beginning of it.
And so, the more space expands, the less significant we are.
The more space expands, the more dark energy can push it apart.
Eventually, the expansion of space is going to happen faster than light's ability to traverse space.
So galaxies we see today will be lost in the future.
So when you read my grant proposals, please fund now, because we have limited amounts of time.
(laughter)
So, to be clear, we're not expanding.
I just want to make sure you understand that.
We're in a very dense part of the Universe.
And gravity has collapsed our part of the Universe, we are not expanding.
However, when we look out, as cosmologists, to the rest of the world, the Universe will be accelerated out of sight.
Now we don't understand what dark energy is, but unless dark energy suddenly fades away,
for some reason we cannot foresee, the Universe will, at an ever increasing rate, expand, fade away,
leaving me and my fellow colleagues unemployed.
Thank you very much (applause)