I. It is truly a great pleasure here to address everyone today. And Roger, although you say that we did not cross at Arizona. You are incorrect. You just did not know who I was, but I knew who you were. So that's the beauty of the way science works. So I do actually remember at least one colloquium that you were at that you gave that I remember when I was an undergraduate. Astronomy is a very small field, and so we tend to know everyone, even from a young age.
And that's one of the beauties of our subject, is you really do have an opportunity to be part of what is a very vibrant field, but it's one that, due to the scarcity of telescopes, is actually a relatively small field. And so for me, being able to come back to Oxford, I am here amongst friends because I know most of the astronomers here, at least the older ones, I'm beginning to lose track due to being a vice chancellor.
But today I want to talk about the state of the universe with some reflection into what happens every year in the United States, where you talk about the state of things. And in 2017, I'm just going to give you the answer to the state of the universe that I'm going to try to go through and try to figure out why we think the universe is this way. But in 2017, we know the universe is expanding. We've known that since 1929. We know the universe is very close to 13.8 billion years old.
We know that the universe is close to what we would say geometrically flat. And I'll explain what that means. It means that the universe behaves as your intuition said it should. But interesting, according to our theory of gravity, the universe could well be very unintuitive in terms of it could be bent or curved, as we would say. But quite remarkably, the universe is made up of a lot of things, roughly in equal proportion. There's something we call dark energy, which is what is related to that.
The discovery that me and a large team made around the acceleration of the universe. We'll talk about what that stuff is, dark matter, which has been known or hinted at for almost 90 years at this point. That is roughly a quarter of everything that makes up the universe. The stuff that we study here on earth. Atoms, baryons, as we like to say as astronomers, that's only 5% of the universe.
That's the stuff that we can actually tell exist here on Earth, except for the other things such as neutrinos, which of course we measure here on Earth as well. There are small but non-negligible part of the universe. And finally, photons. We take photons for granted. They're ubiquitous in the universe. They used to be the most important thing in the universe when it was young.
Now they make up about one part in 20,000 of the universe, but they remain a very important part of the universe's history. And so they're worthwhile thinking about. So what is unusual is that within physics, you might expect the universe to be dominated by one thing with everything else being orders of magnitude less important.
And yet we have at least three things roughly in equal balance, four things within factors of a thousand of each other, and five things within factors of 10,000 of each other. Why is that unusual? Well, if we were to take a snapshot at any other time of the universe, we would not have so many things, roughly in equal proportion. And so the evolution of the universe, you would expect that in the distant future, it turns out, dark energy will be everything that will be the most important thing.
So we'll see in the future. At the very early times of the universe, photons were everything. And in between we're at this special time where things roughly in balance, and that remains a bit of a mystery and makes you wonder if you're not missing part of the overall story. So let's go back to the beginnings of cosmology and understand how this story has emerged and why you should believe a story where 95% of the universe is stuff that we don't understand very well. Dark matter and dark energy.
And why you should believe that, because it's pretty easy to have the view that if you have to literally make up 95% of the universe, perhaps you don't understand the story as well as you're saying. All right. So this story begins. It actually begins before Edwin Hubble. But here is Edwin Hubble, using the largest telescope on earth back in the 1920s, the hooker 100 inch telescope. And astronomy has always been driven by technology. And it is important being smart in astronomy.
But if you have a bigger telescope than anyone else, you don't actually have to be very smart. And for most of us, we always like to have the best technology that makes up for a lot of everything else. Hubble had access to the best technology and he went out and wasn't particularly brilliant, but he was clever enough to do a measurement that no one else was able to do at the time, which was to measure distances to objects.
Now, measuring distances and objects in astronomy is challenging because you can't just lay a ruler down between you and the nearest star or galaxy. Rather, you have to rely on how bright or how big things appear. And of course, the further away something is, the fainter it appears, or typically the smaller it appears, although that turns out not to be always true in cosmology.
But we're not going to talk too much about that today. And so he used the fact that objects appear fainter the further away they are. So he's able to measure their distances. He was able to take that measurement and couple it with one made by Vesta Melvin Slipher, where you could measure what we now call the redshift of a galaxy, how much its light has been stretched, but what Slipher thought was essentially a Doppler shift.
So if you have a galaxy, for example, that's moving away from you, its light will be stretched red and you can measure essentially its velocity by the Doppler shift or as we would say now within general relativity, which we'll talk about in a second, you measure it. Redshift, how much its light has been stretched. So what he found is if you compare the redshift from the spectra. So that is this axis. So if light has been stretched just a little bit, that means the light is made redder.
You'd plot it here if it's been stretched a lot. You would put it up here. If it was stretched such that it was actually compressed coming towards you. It would be down here. You can note there are no objects down there. That was one of the big mysteries, is that everything was moving away from you.
And he made this very famous diagram in 1929 that the further an object is away fainter stars in a galaxy, then the higher the redshift, the more it appeared to be, the faster it appeared to be moving away from you and from this data. And it's always important to realise where conclusions come from. They come from messy little diagrams that look like this. He was able to say that the universe was expanding. And why did he say the universe was expanding? Well, you'll see.
I've just expanded the universe there for you. And if I expand the universe and I overlay before and after, you can see what happens as I expand the universe. I have nearby objects. They have moved just a little bit when I've expanded the universe. And so if you're nearby, so your stars are bright and the universe is expanding, I expect the universe to have stretched only a little bit. That means the photons will have a low redshift. If you are further away, well, then what do you see?
You see these objects are further away. They've moved a lot. So they will have a higher redshift because their photons have been stretched by the expansion of the universe. So it's a natural thing that you expect within a universe that's expanding to have this feature that the further the faster the motion or the more of the stretching. So if the universe is expanding, we can think and a thought experience what happens in the past.
So that's why in the universe back in reverse and see what happens. Well, if you go back in time, everything in the universe is closer and closer and closer. And there is a time when everything in the universe is on top of everything else. The Big Bang, the time of the Big Bang is essentially a natural occurrence in an expanding universe. And you will think of the time what happened before the idea of the Big Bang.
We actually had the idea of stuff being magically created out of the aether as the universe expanded. That was the only way to kind of avoid something like the Big Bang. So the natural occurrence, if you think back in time, is everything should just be closer and closer, more and more dense back in time. So if you think. Oh, I didn't know I had. Sorry about the sound here. The Big Bang. It's always worthwhile. People always saying, so what's the Big Bang?
Well, the answer is, I have no idea what the Big Bang is it at the time, about 13.8 billion years ago, where the universe was created into its current form, where it was expanding. Why was the universe expanding? Well, that's the way it was born. What happened before the Big Bang? I don't know. I don't even know what the Big Bang itself was. I know what happened after the Big Bang. I know that right after the Big Bang, the universe was much, much more dense than it is right now.
Orders upon orders of magnitude more dense than it is right now. I know that the universe was expanding. It was hot. It was billions upon billions of degrees, very, very dense. And it has been in a trajectory expanding ever since. And as it's expanded, it's cooled. And we see. So the last thing the first thing we can see is the universe is the cosmic microwave background. So that is an image taken with the plonk satellite.
That is the entire sky, the entire sphere of the sky mapped on to that type of projection. And you can see that the universe is full of very faint bumps, which we call essentially fluctuating temperature fluctuations. And those bumps are typically have a typical scale, as you'll see, of about one degree. The universe at this time is about 2700 degrees Celsius in well, actually, it's about 3000 degrees Celsius at temperature.
And the universe at that time was at the temperature where hydrogen was able to grab back to its electron. So before that, it was ionised. And the universe, when it was ionised, the electrons are out there. Electrons scatter light. So as a photons trying to make its way through the universe, it can't. It has to bounce from electron to electron. 380,000 years after the Big Bang, the universe expanded and cooled to the point where hydrogen was able to what would say recombine gravity's electron.
That means the universe suddenly went from being opaque, full of fog to being clear. Kind of like Oxford today. And so you get then this essentially back 13.8 billion years ago, 380,000 years after the big bang, a fog bank, which we can see right now when we look back to that time before that time, we can't see any further because light can't penetrate through that fog. But it does give us this amazing observation we'll talk about later on, which we can use as a giant experiment of the universe.
So. After that point, we can ask ourselves, how do we think about this in a mathematical sense? So you can always graph things. So imagine you have two galaxies right now separated by a certain distance, and I can run the universe in reverse when I run the expansion. The universe in reverse. You get this time when the two galaxies we see moving apart, now we're effectively on top of each other. That time then, is the time of the Big Bang when they're on top of each other.
So the slope of this line and the steepness of that line, that is the expansion rate of the universe today. So you can infer the age of the universe by essentially just measuring how fast the universe is expanding it today. Running it back in time. And that gives you the age of the universe. So that's actually what I did for my PhD thesis, which I won't talk too much about now, but has been a series of experiments.
And when you do that, you get that the age of the universe is roughly 14 billion years old. So the expansion of the universe tells us roughly the age of the universe. Now that way of looking at things is as actually is. Here is my thesis. Going in and doing this, showing me at a much younger age and me showing my PhD supervisor, Bob Kirshner, that I thought the age of the universe was roughly 14 billion years old.
Or in the units of a Hubble constant, measured in the units of kilometres per second, per megaparsec. That's what an astronomer does. That tells you that if a galaxy is a megaparsec away, megaparsec away being about 3 million light years, it will be travelling roughly 70 kilometres per second in terms of its redshift. That's the equivalent of a Doppler shift. So. Now that's sort of where the observations took us in the expansion of the universe back in 1915.
So 102 years ago, right now, Einstein released his his equations of general relativity. And they started in 1907 when he described having this amazing thought. And he said that he had this thought when he saw someone fall off the roof of a building. And instead of and this is why Einstein was a different person than you and I instead of calling an ambulance. He thought and he said, I don't think that people that person's feeling gravity as he falls.
He's in free fall. And I think this acceleration exactly cancels gravity. He feels they exactly cancel out. And I think that in every situation in the universe, that cancellation is going to occur. So that's Einstein's big thought. Now, it took him eight and a half years to figure out what that meant. If you have that thought that gravity and acceleration are equivalent.
And the speed of light is constant and all frames you end up having to create a very complicated theory general relativity, which includes the idea that, uh, that, uh, space can be curved. So just going back to this, the idea is that there is no experiment that allows you to figure out if you're in a rocket ship accelerating at 9.8 metres per second squared or sitting on earth in a box being accelerated by 9.8 metres per second squared by gravity,
they are absolutely equivalent. No way to tell the difference between the two. And so this is the experiment that made Einstein famous amongst the general public equals m.c squared. Special relativity made him famous amongst physicists. But this is the thing that got him on the front pages of the newspaper spectrum, especially when Arthur Eddington went through and helped organise an experiment to look in an eclipse to see if his theory which predicted that space would be curved.
And therefore, if you looked at stars at an eclipse, for example, here is the real data from 1919. This is the data, this one that's too cloudy. This is the data where the Astronomer Royal didn't manage to get his telescope focussed properly. But you can still get the positions of the stars relatively accurately. And under the equations of Einstein's general relativity. Space is curved, distorts the light path, and stars will be displaced in an eclipse from where they would be otherwise.
And that displacement is not where you might naively predict it, making a few assumptions under Newton's laws of gravity if the displacements are a factor of two. And indeed, these measurements at the time were shown to reproduce Einstein's theory, not Newton's. The best job you might do with Newton's laws.
And so that is what made Einstein famous, because this is one of these unique times in astronomy or in science period, where someone has a thought based on aesthetics of how the universe should be. And it has turned out to be the way the universe really is. Normally you do things because there's a problem. There's an observation that doesn't make sense.
Now it turns out that this did explain some observational problems with respect to how Mercury's orbit evolves, but that was realised after he was doing his work and it made predictions and every prediction that general relativity has made has thus far come true very spectacularly with the discovery of gravitational waves that we've seen with the mergers of black holes in the last couple of years and neutron stars more recently.
So it's a remarkable theory and it's got its legs to do cosmology very early on. Now, one of the things that Newton could never reconcile with his theory of gravity is imagine I have a universe that goes on and on and on. What happens? How does gravity work? And it turns out under Newton's laws, there is a single solution to that type of universe, which is the universe has nothing in it. That is the only self-consistent solution for the physicists and the audience.
Use Gauss's law. And that's the that's how you get the solution. So that's not a very good solution. And Newton, this really drove him nuts. He could not solve that thing. And if you've ever got a chance when visiting your favourite place on Earth here in Oxford, Cambridge, you ever get to see some of Newton's little notebooks? He did all those arithmetic to the width of the page. Why? I don't know. But 45, 50 digits kind of random.
It was. Yes, he did. It carried as much as all of his calculations, as many digits as to fit across the page. Think what Newton could have done if he would have just kept significant figures. Unbelievable. Newton was not a he would have been an interesting person to work with. Anyway, so these are sort of the founders of cosmology, because it turns out you could with Newton, with general relativity, solve the problem. Imagine a universe that goes on. You could ask those questions sensibly.
And so Einstein turned out to be the second person to do it. The sitter was the first person. And his first solution, because Einstein's equations are very complicated. And he said, let's just think about a universe that is empty. So that made, it turns out, the equations of general relativity, relatively simple. But even in that very simple model, you got a universe that was essentially dynamic. Einstein was not fond of an empty university, so that was ridiculous thing to talk about.
But he struggled in 1917 getting a universe that was anything other dynamic, and other than that, being dynamic in motion, they didn't really understand the idea of expansion of the universe at the time, and so they didn't really know what it meant. But in because of that issue of wanting to make the universe static, Einstein introduced this term very famous term, known as the cosmological constant or what we think of space having energy itself.
Friedman was the first person to really go through and do the solutions for cosmology as we think of them today. Did that in 1923 and he had to make an assumption to make the equations of general relativity solvable. And that assumption was pretty simple, which is that the universe is homogeneous. Also isotropic means it doesn't have a preferred direction. Homogeneous means that more or less any part of the universe is like any other part of the universe.
And, you know, we sort of know that's wrong because here on Earth, it's different than, you know, the middle of space between stars. But it turns out if those lumps and bumps of the universe are averaged over, once you get to a few percent of the size of the universe, then you should expect the Friedman's version of the universe to be correct. And he allowed us to actually create what we call a standard model. Now, I'm going to show you just as out of interest for that.
We do have equations that solve this, but the equations of general relativity are very complicated. They're actually the equivalent of ten nonlinear partial differential equations that you have to solve simultaneously. That's very hard to do. Almost impossible, one would say. But if you have this thing that Friedman did, you can break it down into a single, ordinary differential equation, which for some of you who don't do physics and math, that still looks like gobbledegook.
But the good news is that's something that after your first year at Oxford doing a physics degree or a math degree, you can probably solve. So it means it becomes impossible to solve to something you can solve. And this equation basically says that, yes, the universe is in motion and it has something we call the scale factor. So that's how big a ruler is in this universe.
And it changes size over time. And you can measure it turns out the how big that ruler is with the redshift you measure how much light is stretched as it travels through the universe, and you can measure, it turns out to one part and a million. How much that ruler is changing over time.
So it's something we can measure very accurately. It's the same thing effectively that allows us to measure the motion of planets around stars, but it also allows us to measure that very important part of the universe. The other thing that you need to worry about in this don't worrying about speed of light and gravity is the density of the universe. And it turns out this factor K, which is the geometry of the universe.
So the geometry can have three values, which we'll talk about here in a second. But the universe is quite simple. So in that framework, we have lots of things we need to worry about. We need to have what we call the Hubble parameter or the Hubble constant. That's the rate the universe is changing right now, and it has a value of roughly 14 billion years. There is this notion of what we call critical density.
So it turns out if the universe has higher than that density, which you can calculate just from constants and the Hubble constant, then the universe has geometric, has a geometry where it bends onto itself. I'll show you a little diagram of that in a second. If it's less than that, it has a geometry where it bends away from itself as the shape of a hyperbola. And that density is a very small number. It's roughly ten to the -27 kilograms per metre cubed.
Now, given that the Earth has a density of 5500 kilograms per metre cubed. You can realise that if we're anywhere close to the critical density, this place, this dividing line, the earth is not a typical place of the universe and it certainly is not. It turns out, as we'll see, that indeed we think we have within probably one part intended for right now that density in the universe quite remarkably.
The actual density that we have we do relative to that critical density, and we use a term called Omega. So if I have, for example, 5% of the critical density in atoms, then I would say omega in atoms is 0.05. And that's just a useful shorthand for us to do. And it turns out density can be in anything. In these equations it can be atoms could be something like dark matter can be something like dark energy. They're all equivalent or photons, they all have energy.
And then due to energy, mass equivalence and general relativity or special relativity, you can convert them back and forth by equals m.c squared so I can make a photon the equivalent of an atom. All right. So graphically, what does this look like for those of you who aren't cosmologists? You can tune back in to my less technical slides. So this is a universe that's empty. Imagine you have a universe that has no gravity in it. It has no nothing in it.
If it's expanding, it doesn't change. It just keeps on getting bigger and bigger at the same rate it costs forever. That kind of makes sense. On the other hand, if the universe has a fair bit of stuff in it, that is. Let's say that the attractive matter in the universe is between zero and the critical density. So it's heavy, but not too heavy. Then the universe slows down over time means that the universe isn't going to be quite as old as we might otherwise think it might be.
For measuring the Hubble constant. And it means that it will slow down over time. If the universe is heavy that it has an attractive matter greater than that critical density, then the universe expands, slows down, and then goes in reverse. So all these universes begin with the big bang, but only the heavy ones end with a going ab. Get the big bang of reverse. All right. Now, I talked about geometry. So geometry is related to how heavy the universe is.
The difference is that when you measure the geometry of the universe, you get to weigh everything in the universe, not just attractive gravitational matter like I just showed you. So if the universe has the critical density, it's flat triangles add up to 180 degrees. However, if the universe is heavy, then it curves onto itself. And triangles have greater than 180 degrees in them, just like triangles do on the earth.
Of course, we don't think of triangles right here because this part of the earth is flat, so I only see a little part of it. But when I see the whole earth and you do a triangle on a globe, it has more than 180 degrees. You can do that experiment at home if you'd like to try it. If the universe is light, it turns out the universe naturally has the shape of a saddle or hyperbolic geometry, and triangles add up to less than 180 degrees.
And so if you're going to try to move around in this universe, the geometry becomes a little more complicated. And these two compared to this one, although you can actually calculate that again with what we would call a metric to to sort that out. All right. Now, imagine you could go and measure the past of the universe. How would we do that? Well, light only travels 3000 kilometres per second. And imagine I look at something billions of light years away.
Light takes billions of years then to reach me. And when I see an object billions of light years away, I'm actually looking back on the universe as past. So we can go through and we can, for example, measure the Hubble constant. That's how fast the universe is expanding. And we can measure it now, and we can measure it long time ago, and we can see what the universe has done over time. If the expansion rate hasn't changed, then I know the universe, for example, is coasting.
On the other hand, if the universe is slowing down faster than, for example, this critical line where the universe has critical density in things like atoms and it turns out dark matter, attractive gravitational matter, then I know that gravity wins and the universe is heavy. It's finite because it's going to curve onto itself. And that means that has a finite volume, but it also means it's finite in time. It's going to end in the future in the big bang and reverse.
If so, that's a universe that's expanding faster in the past and slows down at a rate faster than that yellow line. The other side of that line, well, gravity loses. That means the universe is infinite. It's infinite in space. It has the shape of a hyperbola or a saddle. It goes on forever, but it also keeps expanding forever. So it's infinite. And time and space. All right. How would you do this? Well, in 1994, I just finished my Ph.D. and we had the idea of using type one supernovae.
So what's a type one? A supernova? Well, imagine two stars, one, you know, not dissimilar to the sun. So as stars chew through their nuclear power, our nuclear energy from their hydrogen, they tend to puff up. And as they puff up, if it's next to another star, it can donate its material, a fair amount of it to the other star.
It eventually will run out of its energy, its hydrogen and its core, and probably even work through its helium and will blow off its outer envelopes and create what we call a white dwarf star. So that's the core of our sun. When it dies, 5 billion years will be a white dwarf star. The other star, which is now the heavy star, will eventually pop up and it can start doting, maybe donating its material to the white dwarf.
And when it reaches this magic value of about 1.38, three times the mass of the sun, it becomes unstable. This is called the Chandrasekhar Mass, and it will cause that star to detonate as a giant thermonuclear bomb. Not one the size of a suitcase, but one the size of the sun. And so all of that nuclear fuel creates an amazing explosion that produces about 6/10 of a solar mass of iron. So the iron in this room will be created largely in these explosions.
Now, the way I've just showed you is one possible way of making one of these explosions. It turns out that it may well be that you don't actually create the explosion directly. You have to make a white dwarf. So the first star will evolve, become a white dwarf. The second star will also evolve and become a white dwarf. And then they will rotate a revolve around each other over time.
And as they do that, give off gravitational waves and the same process that has been discovered that causes their orbits to get a little closer and a little closer. And they might well merge in some cases and create an explosion. That is probably my best guess. The most common way to make these explosions. Although I will say it remains a bit murky and a bit of a mystery exactly how we make this.
But the beautiful thing is, no matter how you make them out, you have a big ball of stuff that comes together whose physics is quite well understood and understood. And you get a giant thermonuclear detonation made up with a bunch of iron in its core, expanding hot. And it gives you essentially a light bulb that produces a lot of watts. How many watts? Oh, ten to the 43 watts. So that's a lot of watts. So 5 billion suns worth of light.
Now, a group that I was working with in my PhD thesis down in Chile. Known as the control search went through and systematically charted how bright these objects were in the universe. And they discovered in 1994 that there was a essentially a trend which the brighter the object was, the more iron it produced and the slower its light curve was. That is it rose and fell more slowly if it was bright compared to being faint.
But if you were to calibrate that effect, you could measure distances with these objects. By essentially how bright they were when they were a long ways away to a factor of about 6% accuracy. So 6% for measuring distances in 1994 was about two and a half times better than any other method. And it's still essentially the most accurate method we have for measuring distances to this day.
I was in my process of moving to Australia. I was down in Chile here with Nixon staff who had helped do that, worked out in Chile, and we discussed the possibilities using this new technique and the new emerging technology to go through and actually do an experiment to look back in time with these exploding stars and measuring distances, measuring redshifts and measuring, therefore, the expansion of the universe back in time.
And the challenge was technical in that you had to these these exploding stars were very rare. They only occurred every several hundred years. So you literally needed to look at tens of thousands of galaxies in a night to have the chance of seeing one. Now, 1994 was the year that large digital devices we call CCDs emerged for astronomers to use. And so we would take roughly on the SETI oh four metre telescope, a thousand images like this in a night, each one being roughly four megapixels.
But we were using Pentium 200 computers, if you remember those. Some of you have no idea what I'm talking about really, really slow compared to your iPhone. We had one gigabyte hard drives at the time, which we're very excited about, except for we took 50 gigabytes worth of data and we had to find the needle in the haystack. And I wrote software that essentially took data like this and was able to discover the new objects that appeared these exploding stars like this one.
And just to show you how we know that was a supernova is we would take one image, we would take one a few weeks later and we'd look for nothing to become something. So this object turned out to be roughly 5 billion light years in distance. So it exploded before the earth was formed. That is the power of cosmology being able to look back in time. So now I think you're going to unfortunately get a audio thing. Good. So I take you to Cuba just to give you a sense of what it was like.
Here we are on the SA Tololo four metre telescope. There's an extensive. Now we get six nights a year. We have the largest allocation of telescope time in the world. Six nights a year. You got to have to a picture before and a picture after. So everything has to work perfectly. And as the data comes in, we're looking at it to make sure everything's fine.
And meanwhile, I have written the software that goes through and aligns the data and matches it and then subtracts it and looks for the little needles and Aztecs. It doesn't work particularly well, but it's better than doing it by hand, which would have taken the age of the universe to look through. So we have a team going through a much younger me trying to get through this data as fast as we can because 18 hours later in Hawaii, we have telescope time on the cat telescopes.
These are the largest telescopes in the world, the only ones we can really use to get redshifts of. In 1995. Let's see Adam RIESS and Alex Filippenko there. And it's this this amazing cycle of having to pore through all that data and then follow up to say, here's the redshift, and this really is an exploding star. I like to say it went smoothly.
It was completely crazy. I ended up not sleeping, getting heart palpitations, trying to work 22 hours a night for these three days in a row because everything broke every time I did it. But anyway, it did work in the end. And here's what the results look like when we came in at the end of 1997. So 20 years ago today, I had no idea that the universe was accelerating. 22 weeks from now. I had my I had my first idea.
So I'm almost at the 20th anniversary when I realise the universe might be accelerating, but this is what the data looks like. Each supernova and these objects were existing done by the chill. The Choi Linos provides a measurement of how fast the universe is expanding compared to the average. And you can see these nearby objects. You can't really tell what's going on.
They don't really lie in this part of the diagram or this part of the diagram, but these distant ones, none of them lie down here. Not a single one is consistent. Each one provides a measurement. And you know, of all these objects, not a single one light lies down there. So the universe is not slowing down enough to be sure that we're completely convinced from this data that the universe is not going to end up going in reverse. The universe is not finite, but interesting.
When you do the statistical analysis, you realise that the universe with about 99.9% confidence is not in the yellow part of the diagram. Rather, it's above the line up in this part of the universe where the universe was expanding slower in the past and it actually sped up over time. Now I realise that's not a brilliantly convincing diagram.
In the same way, Hubble's diagram wasn't brilliantly convincing either, but at the same time a team led by Saul Perlmutter was getting exactly the same answer. And in 1998 the two teams had essentially the same experiments. And when you combine them together, your 99.995% sure. And while it's not quite good enough for a particle physicist, it was enough for us to say at least there was evidence as opposed to a demonstration of that the universe was speeding up over time.
And it's for that work that our team and Saul's team were awarded the Nobel Prize in physics. And I do say it was a true team effort. Nobel Prizes are about the science, I think, more than individuals, although individuals tend to get a great deal of notoriety and at all. But these are two great teams that contributed to that. So what on earth would cause the universe to accelerate? Well, Einstein came up with it in his idea in 1917.
The idea of what he called a cosmological constant energy is part of space. And in those equations, if you have energy filled filling all of space evenly, it causes gravity to push rather than pole. We always think of gravity as being attractive. Well, it doesn't have to be in general relativity. It can be repulsive under the right conditions of how material is distributed. So the cosmological constant has those conditions. And so we're not positive. It's a cosmological constant.
So we generically call it dark energy. So when I first saw that data back in 1997, people always asked me, Did you have a eureka moment? And the answer is no, I did not have a eureka moment because I just thought it was wrong. And so your eureka moment never happened because you were continually trying to look for the mistake that caused you to get this answer. And at some point you kind of shrug your shoulders and say, I don't seem to be able to make it go away.
I guess we're going to have to publish this and that probably my thought wasn't, Oh, I'm going to win a Nobel Prize. It was, Oh, I guess I'm probably going to have to leave the field of astronomy because no one is going to believe me. So that is sort of the way it happened. So the detail analysis of that data was that the universe is a mix of normal gravitating matter. So stuff that behaves like you're used to 30% and 70% stuff which is pushing the universe apart.
That's what you needed to have to make sense of our data. All right. Now astronomers are sceptical folk, as they should be. And so people, I think, had a healthy view of saying, great, that you guys are making these wild claims. We want to measure them other ways. It makes sense. And so there are a variety of ways to go out and and measure things.
But I should say that our data, uh, you know, made a very, very funny prediction, which is that if the universe is full of dark energy, it has this funny phase that it's going through right now where the universe is. Expanding. The more it expands then, the lower the density of stuff like us is in the universe. Why? Because we're here and the box around us gets bigger, so our density drops. The cosmological constant is part of space itself.
So it's density stays the same. So as the universe expands, we become less and less important. The universe is made up of stuff which can essentially push harder and harder on itself. And so you get an exponential runaway where the universe exponentially expands. And that's what the future will be in the past. Well, it turns out this new phase of the universe where dark energy has a density equal to our density. So that's the time when the universe can start to accelerate.
That only happens 6 billion years ago. According to our measurements. So the universe has only recently started to speed up. Before that, the universe would have been sufficiently dense that the atoms and other material, which we call dark matter. We'll talk about in just a second. Well, that would have been slow had the density to slow the universe down. So we would have expected that the universe would have been slowing down. And then it's kind of taken off over the last 6 billion years.
So that's what the model says should happen. All right. So you can go out and start testing aspects of this model. And one way to do it is just to measure how much gravity there is in the universe. Turns out gravity makes very specific signatures within the distribution of galaxies. And you can go through and you can measure that within a computer by essentially taking the initial lumps and bumps of the universe and just allowing gravity to evolve over time.
And you get a universe full of galaxies that have a very funny foam like structure and the nature of exactly how much gravity there is in the universe and the nature of how that gravity is occurring is that atoms, is it dark matter? Which is dark matter is just something that essentially goes right through itself. So it only interacts by gravity. Well, you get different models of the universe, and so you can go through and observe the universe.
And this is something that we did as a joint experiment between Australia and the UK back in the early 2000. So that's the real universe and these are different models of the universe and you can look up there and say which of the mock universes looks like the real universe?
And if you said that would be right, that is statistically the one that looks most like our universe, and that turns out to be a universe that has 30% of the critical density in gravitational gravitationally attractive material. And it turns out most of that has to be a form of dark matter. Stuff that only interacts by gravity, doesn't have pressure, and it goes right through itself. All right. So that turns out, is about six times more gravity than the atoms we can account for in the universe.
And so we do have, again, this notion of dark matter material that has gravity but doesn't seem to interact by any other means with atoms or even itself. And so wherever we look in the universe, sphere and galaxies, we see dark matter, or even in galaxies, the scale of galaxy clusters where galaxies come together.
And this very famous image of the bullet cluster where dark matter in blue has seemingly gone right through as the clusters of collide, where the atoms in pink have cropped up like you expect atoms to do, and interacted and formed a bit of a train wreck in the centre so we can dream up of some particle like a neutrino but not a neutrino.
Because neutrinos are of course going right through the earth right now because they weakly interact, which means they effectively can go through light years of lead without interacting with more than a 5050 chance. So it could well be that type of object. But at this point we have not been able to discover it in any way, shape or form here on Earth. Not for lack of trying. So dark matter seems to be with us now. You can go through and say, okay, dark matter, I don't know about this stuff.
Here's an experiment with the cosmic microwave background, noting that these little bumps and wiggles are actually our waves sort of left over from the universe right after its formation. And it's essentially sound waves splashing around the universe. And the physics of sound waves is complicated but very, very well understood due to just the basic physics that we understand here on Earth.
And if you have material and, for example, you throw a rock in a pond, the waves depend on what the ponds made out of. It's made out of molasses. It's different than if it's made out of water, for example. And the wave action allows you to see what the universe is made out of. So the Big Bang and the period of what we call inflation is like throwing a bunch of rocks into the pond.
The waves splash around for 380,000 years, and then you get a map of what those waves look like in the cosmic microwave background. And so it turns out that we can measure how big the waves are. And so this is big waves, small waves. And depending on variance atoms, the amount of atoms you have in the universe, you get a different pattern. The amount of dark matter plus atoms you have, you get a different pattern.
It turns out that when you actually measure the sound waves, that's the pattern of the little red dots and the line run in there is the model that effectively is the model that is indicated of 30% dark matter. Ah, 30% gravitational matter, 25% of so it's essentially 25% dark matter, 5% atoms, 70% dark energy. That's what the model looks like. It goes right through the dots. You can't make that stuff up in advance, right?
You could have every person on the planet scroll out for their entire lives lines and not a single one would ever fit the data as well as that as as as well as the data is fit by that model. So it's a remarkable fit and it tells you something's going right. And that says that the ratio of dark matter to atoms is six and a half to one very accurately measured. Not going to go away. So this is why we think the universe really has dark matter in it.
One of the things you can do is measure the geometry of the universe very accurately, and that's because the sound waves, if the universe is curved, get magnified or magnified based on the shape of the universe because light paths are curved and that's like magnification. You can think of it looking at the different sides of a spoon. So if you look at how big those sound waves look, this is graphically what it looks like.
But let's just look at it here. You can see that the sound waves get magnified in this universe, magnified in this universe. But it turns out they look exactly like this. So they have the scale exactly of a universe which is flat. So we know to very high accuracy that the total matter density of the universe is almost exactly the critical density. So you put this all together and you say everything in the universe adds up to the critical density.
30% of it from that map of galaxies appears gravitationally attractive. That means you have 70% mystery matter. The same matter we need to have. Explain the dark energy that supernovae come up with. So wherever we look at it, we have a universe that seems to be 70% gravity that pushes 25 or 30% gravity that pulls 95% of the universe is in stuff that we cannot observe here on Earth. It's crazy, but it fits literally everything measurement we can make.
Just quickly, you can also measure the photon density by essentially how many photons are in that cosmic microwave background. It's a very small number. You can measure the mass of the neutrino very accurately because if the neutrino has mass, it smears out the galaxies in the universe because it pulls gravitationally on the universe when it's young.
And so you can actually, right now, with our measurements that we have, tell that the mass of a neutrino must be less than this very small number of 0.25. We know that from particle physics it should be greater than 0.05. So that seems to be the masses that are Trina is live between those numbers. So we have a universe which seems to be able to fit almost everything we do.
It goes through and it relies on this idea of inflation, which I haven't talked about today, where the universe is sort of born with splash marks and whatever inflation is, we don't really know. We have models, but I would say they really are toy models at this point. The universe has got four photons. Neutrinos, baryon, dark matter, dark energy. It's homogeneous. General relativity is important. And we have this essentially, you know, reproduces every measurement we can make.
The only place that we seem to have a problem is with the Hubble constant, it turns out. So Adam RIESS, who I won the Nobel Prize with, has been working very hard on this, and he is able to measure how fast the universe is expanding.
Starting essentially with stars we call cepheids and measuring their distance geometrically, going to objects that contain type one a supernovae and calibrating exactly how many watts are in a type one, a supernova, and then going out into the distant universe and measuring essentially how fast the universe is expanding. And that work gives the Hubble constant in these units, astronomers units of 73.
When you measure this curve through those lines with the cosmic microwave background, you have part of that. That measurement is the value of the Hubble constant, and that gives you a measurement of 67.8. Those are there's some tension there. They do not completely line up. I have been suspicious. I've been having a student going through and Reanalysing Adam Reese's work thus far. I would say Adam gets an A for his work. I'm sure Adam would appreciate that.
But we have we didn't know what the answer was. We completely hit it, did all the work, and we've reproduced, not this current one, but his previous one, and seems to more or less be consistent. So I don't know what's going to happen there. This is the one [INAUDIBLE] in the armour that I know of is comparing how we measure the Hubble constants from these two methods.
So the only problem, of course, is we need to make up a lot of things here, inflation, which we haven't talked too much about, dark matter and dark energy. So there are actually some questions left to answer. Cosmology is not done, and I'm going to finish on these questions. Okay. It's great. We talk about dark matter. What is dark matter? We don't know what dark matter is. It might be a particle.
We know there's a lot of gravity out there and we know we don't see any sign of any particles interacting. The only way we're going to tell some things, they are through interactions. We don't see that. So we need to figure out what dark matter is we had hoped. Maybe the LHC at CERN would show something supersymmetry. Something that might create one of these particles. Nothing. Nada.
We have big vats of xenon sitting underground, waiting for a dark matter particle to come and ping the xenon and put out a photon. Nothing yet. And indeed, those are getting quite substantial experiments. So we cannot seem to identify what dark matter is. What is dark energy? Well, dark energy is spread absolutely smoothly across the universe. The only way we know how to detect its presence is the effect on the overall expansion of the universe.
That is the only way anyone has been able to figure out how to measure dark energy, noting that in the entirety of Earth there would be a microgram of it equivalent in the entirety of the Earth then, because there's just not a lot of density of dark energy at any one place, although over the whole of the universe it adds up. There's also basic questions why? Why do neutrinos have mass at all? The standard theory of particle physics says neutrinos should have no mass.
So there's a mystery there. We know they seem to have mass. We also have the issue of the universe actually having matter in it. All the equations say that every piece of matter in the universe would have been born with matter and anti-matter, or just born really, really hot. And as it cooled down, it should have formed equal amounts of matter and anti-matter. And yet what we see is that for every photon or every billion photons, there's one atom in the universe.
And so that seems we would expect there to be zero atoms per any number of photons under our current equation. So there's some asymmetry in the equations. That is, the universe cooled, allowed matter and anti-matter not to completely annihilate. And then finally we have the question is, what is this thing we call inflation, which seeded the universe at a very earlier time and made the universe full of bumps and wiggles? And we think sort of set the initial conditions at the time of the big bang.
These are the big questions, not of astronomy. They're also the big questions of particle physics. And one of the interesting things you will note is that when I was a graduate student, particle physicists used to make fun of astronomers as stamp collectors as what they often called us. But you note that these are the big problems of particle physics, and they're all astronomical.
So we are now much more in the books of particle physics than we were when I was a graduate student, where we were simply stamp collectors. So lots to do. We are not anywhere near done. I have no idea how we're going to solve any of these problems, but that is for people, some of the people in this audience to do. So it's never been, as my Prime Minister would say in Australia, never a better time to be an astronomer than right now. Thank you very much. Tremendous.
Brian, thank you very much indeed. I'm sure lots of questions. So please first. So. Yeah, it's really interesting. This tension between the distance, father. The plan. Is there any chance this some kind of systematic with. That's when I see, you know, the. So the question is the you know, what might be leading to that [INAUDIBLE] in the armour where the Hubble constant is measured by Planck? The cosmic microwave background disagrees with the supernovae. So I was I know supernovae.
And while I love supernovae, I also know their faults. So one of the reasons I went through and I have a graduate student working to reproduce the work is because I was suspicious that there might be systematics. But I will be honest, we haven't worked through the entirety of the thing yet thus far. I would say I don't see any. Anything that would contribute enough to create the issue. The problem is we have thousands of objects of supernovae right now.
And so you're comparing samples and samples, and it's kind of hard to imagine why this sample would be completely different than a sample out here that we've we can match as best we can. And to get a big uncertainty when you match samples, you know, the central limit theorem does tend to apply. And it's just very difficult to make to create a systematic error that would be of the size that we see.
The biggest uncertainty, I think, is still measuring quite crazily the nearest by seven stars and getting that giant geometrical measurement correct, which is essential. So that's the part where I think it's most. Perhaps most problematic. But I think you also need to realise that the CMB measurement of the Hubble constant is not as clean as people think.
Right? If you measure the cosmic microwave background on the larger scales of fluctuations, you get a significantly, as in three sigma different answer than if you measure on the high scales. So the difference is sort of in the high 66.8 to 70.5 or something, depending on who you talk to. Get David Spergel going on this and [INAUDIBLE] have a field day, so it may be multiple things.
Not quite right and everything's fine. We don't know yet, but I would say we now also need to understand the CMB side. It's not absolutely rock solid on what should be there. Paris. Oh, Iris, good to see you again. You too. Those experiments, all kinds of questions. Oh, these are always dangerous. So let's take you out, cosmologist, and put you back 6 billion years. To the point in the curve where it's where the second derivative. Yeah it stocks. Or let's put you a billion years before that.
Yep. In this the same universe. Are you in a position, Observationally, to determine that in a billion years time, the universe is going to start accelerating? So if you go slow so let's say we go back to 7 billion years ago, the answer is with the current level of technology, we would be scratching our heads because we would say it's just not quite right. But I don't know if we would be prepared to say, oh, yeah, omega, omega lambda.
So the the fraction of the universe and in dark energy is 0.3, which is what you'd be seeing. Okay. So I think so the answer is we might be able to will be seeing the discrepancy with the current well quality of data we have, but we would be confused. Now imagine even worse, that the Earth was born in the first billion years of the universe.
Completely possible. There was already the already almost certainly already planets like Earth formed at that point, and that life formed instead of four and a half billion years. Over a couple billion years. I don't think we'd say that's impossible either. So we're looking at two or 3 billion years after the Big Bang. Then the matter density would be, you know, omega matter would be 0.95 omega lambda .05, and we would be an absolutely no ability to be able to actually say it's there.
So that's you know, it's problematic. We live at the right time to be able to measure this. And of course, whenever you live at the right time to do something, it always makes you suspicious that maybe you don't understand what you're measuring. So but it it does, you know, the data is pretty impressive how well it matches on to the basic model is to your question it. The UN. The lack of protection of documents that was suggested has led some to suggest that at 1:00 there's about.
Thought about it. Yeah. So the problem of a modified theory of gravity is that. You know, general relativity. Has worked very well, predicts things. So people people's ability to come up with a self-consistent, modified theory of gravity that you can actually sensibly test has been challenging. There's been a few. The ones that you can really test in detail, as far as I can tell, have all been ruled out. So it it's something we need to be thinking of.
But you do need to have something that's a bit of a a strawman to test because you can create little factors and say it doesn't work in these regimes. But I would say you can't do some of the the detailed tests. For example. One of the curves I showed you is the cosmic microwave background. That curve requires. Dark matter as described, which is a clumping particle like thing that does not have any pressure terms, that only uses acts by gravity.
So, you know, what I want to see is a modified gravity thing that gets you the cosmic microwave background in some sensible way. And I think that's incredibly hard. You could, of course, create the initial conditions that match it perfectly, but that doesn't count. That's just faking it. So I'm I remain deeply sceptical, but not to the point where I tell people they shouldn't do it. I would say I want to have something I can sensibly test. I don't think we're there yet.
Maybe the last question for a particle physicist, but there's one. Go ahead. So if it just continues to. So for me. Yeah. Well, okay. So I just happened to have slides. So it turns out that if you think about distant galaxies, um, they are moving away from us at an increasing and accelerating rate. And the way the equations work out is that if the universe is accelerating, there are parts of the universe which are effectively the distance between you and that object.
That is, the, the, the distance of the scale of the universe is increasing at a rate which is faster than light can move through it. So the photons from those parts of the universe will literally get stretched into oblivion before they ever reach you. So those objects effectively are beyond the horizon, at least in the future. So right now, objects which are at a redshift of about three, uh, so those are objects which are back 10 billion years in the past.
Right now, the light that they emit today will never, ever reach us. So in the future, all of the objects that we see will be further and further away such that, say, 150 billion years from now, effectively every galaxy that we see today will be so far away, it's it'll have such a high redshift be so far away, we will have no chance of detecting it.
There will be a sphere around us where the galaxies will be gravitationally bound to the Milky Way and will merge and create a super galaxy, a giant elliptical of some description that will happen. So we will have a sky full of stars, but not one that we can easily do cosmology on. If you think of that Galaxy Stars last 400 trillion years or something, the smallest stars. So eventually those stars will all die and start becoming white dwarfs or black holes.
During that time, the individual stars will have evaporated one by one. So eventually you lose through evaporation processes most of the stars, but there will always be probably a few of them. Black holes evaporate. And then the big question is, is the proton stable? So if the proton is stable, then we'll probably always have these little remnants.
But if, as people suspect, it's not stable on very, very long time scales, even the remnants of neutron stars and white dwarfs will also disintegrate over time. And so every if that's true, every they'll break into, you know, elementary particles. And so every elementary particle in the fullness of time will be separated by greater than the horizon from every other elementary particle. So you will truly end up with a very unexciting universe over time.
In that case, it's a wonderful place to that. So, Brian, you have given us a fantastic lecture describing a weird and wonderful universe, but you also gave us that insight into discovery where, you know, the common feature is, Oh, eureka, I now understand this. But actually people who've made discoveries, it's generally, Oh yeah, what am I doing wrong? That's the first sense that.