PUBLIC LECTURE
Taking measure of our universe
The Shine Dome, Canberra, 19 August 2008
Professor Brian Schmidt
The Research School of Astronomy and Astrophysics
Australian National University
Professor Schmidt received degrees in physics and astronomy from the University of Arizona and went on to complete his master's and PhD in astronomy at Harvard University. He is currently working at the Research School of Astronomy and Astrophysics at Australian National University, and is leading Mt Stromlo's effort to build the SkyMapper telescope, a new facility that will provide a comprehensive digital map of the southern sky from ultraviolet through near infrared wavelengths. He was elected to the Fellowship of the Australian Academy of Science in 2008.
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It is a pleasure to be here today and to tell you a little bit about the world and the universe that we live in. As Kurt said, the universe really encompasses everything, but today I am going to talk about one of the biggest things and that is cosmology: the study of the universe itself and how it changes and evolves over time. We are going to try to ask some big questions that astronomers have been asking themselves for the last couple of millennia.
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The questions we are going to try to answer tonight are: how big is the universe and how do we know why it is that big; how old is the universe and why do we know it is that old; and what is the universe made of? We might even speculate about what the universe's future is. To get us all on the same page, I guess I need to start off with a little tour through the universe so that we get an understanding of where we sit.
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Astronomers use the speed of light to judge distance because it is the easiest thing for us to use to understand the vastness of space. Our Earth, for example, is about a 40th of a light second across, and that is because light is travelling at 300,000 kilometres per second. We all understand how big the Earth is, and a 40th of a second is not very big, especially when you have been watching the Olympics and seen what a 40th of a second looks like when someone touches out someone in the swimming. But, if you look at things on an astronomical scale, you can see that that a 40th of a second that we live in is really a minute part of the universe.
Let's go to the Earth–Moon system. It is 1½ light seconds between the Earth and the Moon; light takes 1½ seconds to reach us. So, when Neil Armstrong stood on the Moon and announced to the world that he had done so, we actually heard that 1½ seconds after he had done it, and that is because light is the rate at which information travels. So, although that was as early as we ever could have heard him do it, it really was 1½ seconds later than when it actually happened.
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The Sun is five light second across. I think we fail to realise how big the Sun is. It is much bigger than the distance from the Earth to the Moon.
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If we go out further, we know – at least I was always taught in school – that it is eight light minutes from the Earth to the Sun.
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And, moving out, we know that our solar system – which extends out to, I am going to say, Pluto; I was not terribly happy about getting rid of Pluto as a planet but, at least for a while, we are stuck with it – is 10 light hours out to Pluto and beyond. So that is the scale of our solar system.
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Moving on, the nearest star system, Alpha Centauri – and, in case you do not know, that is this little star here, which you can look at as the brighter of the pointers to the Southern Cross; go out and look at it tonight and realise that star is the nearest thing to the Earth; it is about 4.3 light years away – is roughly the same size as our Sun; it is just slightly larger than our Sun. If our Sun were a pea and Alpha Centauri were another pea, it would be located roughly at Goulburn and beyond, which is about 200 kilometres away. But it is actually further than Goulburn; it is more like Mittagong. If you think about it, we are two stars in one of the most complicated, fanciest places in the universe, a galaxy, which I will show you next, and you have a pea and a pea separated by a distance of 200 kilometres; everything else is empty space. That is what space is like; it is vast and empty. That is in the most complicated thing, which we call a galaxy.
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We think the Milky Way looks something like this – a fried egg on its side or a pizza – and we are located roughly 30,000 light years out from the centre.
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If we look at this galaxy from the top, we think it looks something like this. We can see other galaxies, and this is another nearby galaxy that we think the Milky Way resembles, but it is very hard to see our own galaxy because we are stuck in the middle of it. The galaxy is full of all sorts of things. It is full of about 10 billion stars like our Sun, but it is also full of dust, gas and other various things. But dust, gas and stars are the most common things in a galaxy. Then there are some other things that we are going to learn about tonight, such as something we call dark matter. So our galaxy, as I have said, is this concentration of lots and lots of material, yet our Sun and the nearest star are like peas lying 200 kilometres apart.
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Let's move out and things become even emptier. We have something that we call the local group, which is our little group of galaxies: the Milky Way, which is surrounded by two satellites, the Magellanic clouds, which you can also see – they are up especially right at the beginning of the night right now; and the two nearest galaxies, the Andromeda and Triangulum spirals, which are about two million light years in distance from us. Everything else in between us is empty space. There are almost no stars between here and there; there are a few, but most of them are in these little galaxies.
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Moving on further, in Australia we have had the ability to go out and make one of the most comprehensive maps of the cosmos. This was made by Matthew Colless, who is an Academy member here, with the Anglo–Australian Telescope and his colleagues from both Australia and Great Britain. Each dot on this map is a galaxy roughly like our own Milky Way. Each dot is 10 billion stars. You can see that the universe is not nice and smooth; it is lumpy and it looks like a sponge. That is because we believe that gravity causes these galaxies to form these types of structures. One of the ways that we understand the galaxies and the universe around us is by modelling how gravity makes this cosmic foam and it eventually forms galaxies like the Milky Way.
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Looking on further, if we take a picture with the Hubble space telescope, we can see the most distant galaxies possible for humankind at this point. We can look back about 12 billion years, and these little tiny dots here are roughly 12 billion light years in distance. They are a galaxy which is probably a little smaller than our Milky Way because they are forming still. That is because the universe, we believe, began roughly 13.6 or 13.7 billion years ago. So with the universe, when we look back, we are actually looking back into time. If you remember, I told you that information from Neil Armstrong took 1½ seconds to get here. Well, information here is taking 12 billion years to get to us. So we are actually looking back into the past. This is almost a time machine that astronomers have available to them. Unfortunately, we cannot look into the future; we can only look backwards.
Astronomers, of course, are always asking to build bigger and bigger and more expensive and more expensive telescopes. That is what astronomers seem to spend most of their time asking, begging, pleading and whining about. The reason we want to do that is, the bigger the telescope we build, the further back in time we can look. But things get very hard. As I have already told you, we cannot look back too far, because we end up seeing this.
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This is the universe. It is the entire sky, not seen by an optical telescope but seen by a telescope which is sensitive to microwaves, just like those that you might have cooked your vegetables with tonight. You see here the entire sky, which represents the universe almost 14 billion years ago – 13.6 billion years ago – and we are seeing the universe when it was so young it was hot. It glowed like the Sun. It was about 4,000 degrees centigrade at this time. Each of these bumps you see – and we are going to talk a little bit more about them later – is a concentration of material, but not much. These concentrations of material are what eventually form galaxies like our Milky Way – but not just a single galaxy. Each of these little bumps probably contains roughly 10,000 galaxies' worth of material. But we are looking a long way into the past here, so things are looking quite small.
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Before that, we have what we call the Big Bang, the time when we think the universe was formed. One of the reasons we believe that the Big Bang is the way the universe was formed is: how else would we have understood this picture where we have the universe appearing to glow like the Sun and being 4,000 degrees? We will talk more about the Big Bang in just a second.
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Let's go back and look at the history of cosmology. To my mind, the modern era of cosmology really started with someone who is my own personal hero, but you probably have never heard of him.
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His name is Vesto Melvin Slipher. Vesto Melvin Slipher worked in Arizona. His principal job was to monitor the canals of Mars for Percival Lowell, who was fascinated with the possibilities of life on Mars. But Vesto did some other stuff on the side. One of the things he did was to take the light from galaxies – the Andromeda Galaxy, for example – and spread it out with a prism or a grating, which is something that reflects instead of transmits light and has the same effect of spreading the light into the colours of the rainbow. If you do that for a star like our Sun, you do not just see this beautiful rainbow; you see a bunch of little lines. If I could get you a really big prism, you would see this, even if you used it on sunlight. You would have to shine it through a little slit so that you could break up the Sun's image into a little piece and then you would see these lines. These are lines are the fingerprints of atoms.
Every atom – hydrogen, helium, iron, uranium, europium and technetium – has a fingerprint of colour where it emits and absorbs. For example, there is sodium. Sodium has two little lines right next to each other which have a distinct yellowy-orange colour. Astronomers love low-pressure sodium lights. Indeed, Mount Stromlo convinced Canberra to move to low-pressure and high-pressure sodium lights in 1994. The government did it because it saved them money on a power bill by about a factor of five. But we wanted it because they stick all of their light out in this colour, and that is because they are made of sodium. That is the fingerprint of sodium. Astronomers can use that to say, 'Ah, there's sodium in the Sun.' Helium is called helium because it was detected first in the Sun before we ever discovered it on the Earth. So it is named after the Sun and they eventually figured out what it was here on Earth, but it was first seen in the Sun.
When Vesto Melvin Slipher saw this light in the galaxy, it did not look like the Sun. It was actually stretched; all of the lines were moved a little bit to the red. Light can be stretched to the red if an object is moving away from you. If an object is moving towards you, it will be stretched to the blue. We call that the Doppler shift. He did this for not just one galaxy; he did it for about 50. Each of these exposures would have taken him several nights where he would take a photographic plate and stick it in the telescope. He would keep the telescope pointed all night, moving it by hand to keep it on to the galaxy and, at the end of the night, he would take it out and carefully put it away. The next night he would put the same photographic plate in, line it all up and do the same thing. He would do it four nights in a row and then he would develop the film and get something that looked like this. When he did this, he discovered that the light of almost every single galaxy was shifted to the red. Every object appeared to be moving away from us. This was quite a cosmic conundrum in 1916 because it seemed that we were in a special place, a place which everything was moving away from.
How did we solve this? We started measuring distances to these galaxies. Astronomers are really stuck because we cannot put a yardstick down in space to measure the distance easily to a galaxy. We measure the distance to the Moon, to the Sun and even to the planets by radar. We measure the same way we would measure distances accurately here on Earth. You bounce a signal there, it comes back and you time it. That tells you how far away the object is. But, when you get to the distance of a galaxy, unfortunately it would require the entire power of the galaxy to measure the distance to the next galaxy because they are so far away.
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However, imagine that I have what we call a standard light bulb, a 100 watt light bulb, and I have something that measures how bright it is by counting how many photons from that light bulb arrive per second. If I move that light bulb to half the distance and then count how bright it appears, it is going to be four times brighter. We call that the inverse-square law. It is something that Newton figured out, among other people: that objects get fainter by the square of their distance. Effectively, things get very faint very quickly as they get further away.
How do we use this?
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Edwin Hubble, who had access to the best facilities in the world in 1925 or so, used the 100 inch Hooker telescope in Mount Wilson in California and started looking for the brightest star he could see in all of Vesto Melvin Slipher's galaxies. He assumed that the brightest star he could see in any given galaxy was the same as the brightest star he could see in every other galaxy. Why did he assume that? Well, you know, first order: it's got to be true to some level, and it turns out that it is good to a factor of about five or 10. But for astronomers factors of five and 10, when you are worrying about factors of a million, are not so bad. He found that, the larger the redshift – that is the more the object was stretched – the fainter the stars were or the further the stars were. In 1929, he announced this and told everyone that the universe is expanding because of this.
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You may ask why he made that deduction. It is actually quite simple. But, before I show you how and why he made that deduction, let me just show you his data.
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This is distance measured by how bright a star was [pointing to graph], and this is velocity measured by how much the spectrum was shifted to the red. It is kind of a mess, but the general trend is that, the further away you are, the faster you are moving. I will show you this data because it looks terrible. Hubble's a famous guy so, when I show you my own data later on, I figure that you're going to give me a break because I reckon my stuff looks better than his.
Why did he say the universe is expanding? I have a toy model of the universe here.
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Thanks to the wonders of Microsoft and PowerPoint – as long as they do not crash on me, which is always a risk – I can blow the universe up; I can expand the universe by a few per cent. Imagine that I overlay those little toy universes here. Let's look at what happens. When I expand the universe, things that are nearby move a little bit, and things that are a long way away move a lot. Imagine that I was measuring a velocity as this thing expanded. In an amount of time something moves a little bit and in an amount of time something moves a lot – high velocity, low velocity. The further away an object is, the faster it appears to move away from you, in an expanding universe. This is why Hubble said that the universe is expanding.
It turns out that Hubble also said it because Albert Einstein had developed an equation set called general relativity which, it turns out, predicted that the universe should be expanding or contracting. We will talk more about that – why it ended up not predicting it. The other thing that is neat about this is that I do not just have to overlay the universe here; I can do it from the perspective of someone sitting here [pointing to the screen], and they will see exactly the same thing. In an expanding universe, everyone sees the same thing. Everyone sees that, the further away something is, the faster it moves away from you. So it gets rid of that problem of us having to be in a special place in the universe.
So the universe is expanding. Let's run it in reverse and see what that means [runs movie in reverse].
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It means that everything is going to get closer and closer, on top of itself, so that at some point the galaxies next to us are where we are now; we share the same space. As you collapse something – if you think of having a big balloon and allowing it to shrink, shrink and shrink, it keeps shrinking rather than just riding up into the elastic – everything ends up in the same spot. That is the time which we call the Big Bang. If you do the math, you will find that eventually you should reach an infinite density. That is the time we talk about as the Big Bang. So a universe which is expanding naturally has a beginning, but it does not necessarily have an end, as you will see.
I am going to show you a graph.
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Imagine that I have two galaxies separated at a time, right now, by some distance. As the universe is expanding, I can run it back in time so that, at some time in the past, those galaxies have no separation. That is the time of the Big Bang. This rate is telling you that the line's slope – the angle of the line – is how fast the universe is expanding now. So, if I measure how fast the universe is expanding, it tells me how old the universe is. To measure the time of the universe, where the galaxies are on top of each other is the time of the Big Bang.
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We call the rate of expansion the Hubble Constant, in honour of Edwin Hubble. Astronomers have funny units. We typically talk about the Hubble Constant having a value of between 50 and 100. The units we use are kilometres per second per megaparsec; they do not mean much to you. But we can change how astronomers measure the Hubble Constant in to something that might make sense to you. When we measure the rate at which the universe is expanding, we are effectively measuring the age of the universe in billions of years – it is 13.6 or 13.7 billion years old.
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How are we going to measure the expansion rate of the universe? It turns out that we need to know how much the universe has stretched. Vesto Melvin Slipher figured that out. We do it in exactly the same way; we just do it much more efficiently with modern telescopes. We measure how much a galaxy's light has stretched, and that allows us to measure how much the universe has stretched. We can measure how much the universe has stretched to within roughly 10 centimetres per second. That is a lot slower than I can walk. It is amazing to think that I can look halfway across the universe, billions and billions of light years, and I can measure how fast a galaxy is moving to a precision better than I can walk. That is how good we are at measuring stretching.
Measuring distances is much harder. How far away are the galaxies? It's hard because you cannot go out and just buy these light bulbs and rulers at the hardware store. I have already told you that what Hubble used was good to a factor of 10. Well, that is not very good. If I want to know the age of the universe, I've got to have things at least to a factor of two. That tells me the age of the universe to a factor of two.
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The way we do it now was invented by Henrietta Swan Levitt, who was an astronomer or an astronomer's assistant at Harvard. She was put in charge of looking at this galaxy, the small Magellanic cloud. Her job was to look at the variable stars in this conglomeration of stars with a telescope. If you look at this little thing – it might look like a little smudge – it is actually a galaxy of about 100 million stars like our Sun. So it is a little galaxy. It is not 10 billion; it is 100 million. If you look at this, some stars pulse; they get brighter or fainter over time. We call those variable stars. She found that the longer it took a star to pulse – the longer the period – the brighter the object.
This group of stars are almost at the same distance, so we knew that these stars were all at about the same distance. Henrietta's work showed that we had a real cosmic light bulb that we could use, the stars, which we called Cepheids.
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The idea is that these things are like giant ringing bells. The bigger the bell the lower the pitch. If you go to the Carillion, the things that make the lower notes are the bigger pieces of metal because they resonate; they are larger. Low sounds are larger waves. When big stars pulsate, sound is moving through them and they ring just like bells; the bigger the star, the lower the pitch, the longer the period.
You can imagine that you get something like this. You have a big star pulsing really slowly and a smaller star pulsing very quickly – and big stars are brighter than little stars, which is not surprising. We can figure out how far away the little star is because it is pulsing very quickly; and we can figure out how bright the big star is, how many watts it is, by the fact that it is pulsing slowly. That is only true if we know how far away the objects are; we have to know the distance to at least one of these things well so that, from our physical law, we can figure out how many watts there are.
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How do we do that? If we know exactly how far away an object is, we can measure how many watts a Cepheid light bulb is. We had a lucky break, called Supernova 1987A. It occurred in the large Magellanic cloud and was an explosion of a star. The star – and I am not going to talk too much about this type of supernova today – went from being something like 100,000 times brighter than our Sun to being a billion times brighter than our Sun. It did that in a few seconds and it lasted for several months.
Why could we do this? If we look at this object today with the Hubble space telescope, this is the thing that exploded [pointing to the slide] and there are these little rings of gas. Stars, before they die, throw off gas, which lights up like a giant neon sign. We can do geometry with this to figure out the distance very accurately. I will show you basically how it goes.
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Here is the ring [pointing to the slide]. When the supernova exploded, the ring was not lit up, because the supernova is the thing that put the energy in to light this ring up. When the supernova exploded, there was no ring. The supernova exploded and then the light from the supernova travelled to the Earth on a nice straight line. But the ring is not directly in the line between Earth and the supernova, so the light had to travel down a little bit to reach the ring. Eventually, it reached the ring and it slowly lit up the ring over time, because the light had to travel to the various parts of the ring first to light up the ring and then the light from the ring travelled to the Earth.
The geometry is such that, if you figure out how long it took the light to travel from the supernova to the edge of the ring, it allows you to measure the distance; because we know how far this is in light days, months or years and, from the Hubble space telescope, we know how big things appear on the sky.
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It turns out that it took 240 days for the sides of the ring to light up. That is, this part of the ring lit up 240 days after the supernova exploded. That means it took light 240 days to travel there and, once it left there, it is the same distance from that part of the ring as here to the Earth. So it is 240 light days from here to here [pointing to centre of supernova and then the ring].
The other thing that we have measured with the Hubble space telescope is that this ring is 1/2200ths of a degree across, so it is pretty tiny. But that is why we have the Hubble space telescope; to make really fine measurements. Using simple trigonometry, which I am not going to bore you with – but, if you feel up to it, you can try it at home – we can deduce from this that this object is 168,000 light years in distance.
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That allows us then to apply our physical law to light bulbs to calibrate how many watts a Cepheid variable star is. So what you end up with are all these Cepheid variable stars. The longer ones are brighter. How many watts are they? A bright Cepheid is about 1031 watts, so that is a '1 with 31 zeros behind it watt' light bulb. So they are bright but, of course, they need to be because they are a long way away.
What do we do next? This was done by another Academy member, former Director of Mt Stromlo, Jeremy Mould, and his collaborators. They used the Hubble space telescope, but not to look at the large Magellanic cloud, because the large Magellanic cloud is orbiting the Milky Way. It is not expanding at all with the universe; it is actually orbiting, just like the Moon is orbiting the Earth. They went to a much more distant object called M100, a big galaxy, and they looked and found about 100 of these things – fainter, brighter, fainter.
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They tracked how long it took for these things [on slide] to pulse; they measured how bright they appeared and they knew how many watts they were. That allowed them to measure the distance to this galaxy [pointing to galaxy on slide].
So they measured the distance to this galaxy and they measured how much it was stretched – about 1,100 kilometres per second.
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That allowed them to measure the Hubble Constant: the rate at which the universe is expanding to this galaxy. You had to do it to many galaxies, so that is what they did. They found that the Hubble Constant was about 70 kilometres per second per megaparsec. That means that the universe is about 14 billion years old.
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That tells us about how old the universe is, but we have to worry about some other stuff.
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Gravity is pulling on the universe as it expands and that slows the universal expansion rate down over time.
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So the graph I showed you was kind of wrong because, if the universe is full of stuff that has gravity – and we are here, so it has at least a little bit – instead of the straight line, the universe is actually going to slow down over time. So you will have a different trajectory.
It will be expanding faster here [pointing to slide] and slow down over time. That means the universe will not be as old as we expected. If it has enough stuff, you could even imagine the universe being roughly two thirds of the 14 billion or less than 10 billion years old. That would be a problem because, from measuring how old the oldest stars are by using the physics of how stars like our Sun live, we think that the universe must be older than 12 billion years old – and that is just from looking at these stars.
We can look into the past, but what happens in the past influences what is going to happen in the future. Right now, we have the universe. If the universe is not slowing down in the past, it is not going to slow down in the future. It just keeps on getting bigger and bigger. It is an infinite universe. It goes on for ever; it is without end. On the other hand, if the universe is slowing down, you could imagine it eventually stopping and starting to collapse. In this case, you have the Big Bang over here and, in this case, we get the Big Bang backwards, which we like to call the gnaB giB.
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The slowing down or speeding up of the universe affects how old we think the universe is. It tells us, potentially, of the ultimate fate of the universe, and it turns out that it also tells us the shape and the weight of the universe.
Space is a funny place. Einstein described space with his equations of general relativity, and it can be curved. It is curved in a fourth dimension that you and I cannot understand easily. I have met people who claim that they can understand a fourth dimension. They are not typically people that I like to eat dinner with; they are a little unbalanced, but that's okay. But we can collapse this down into three dimensions and try to get at least a basic idea. The way that I understand this is through mathematics, but we can visualise it in two dimensions.
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So a universe that is heavy and that is going to end in the gnaB giB is closed. It wraps onto itself, just like the Earth wraps onto itself. If I go here and I head out in that direction on the planet Earth, I will eventually end up where I started. In a universe like this, if I head out this way, I will eventually end up where I started. A lot of time will have gone by, but the universe wraps on itself. You might say, 'Well, how does that happen?' It happens in a fourth dimension. You can't easily visualise it, but you can imagine doing an experiment where you send a graduate student out that way and you say, 'Let me know when you get back and we'll see if it's the way the universe is.' I have not really found anyone who has volunteered to do it. It would probably take hundreds of billions of years, from what we know about our universe.
Then we have a Light universe. It turns out that a universe that has very little in it – that is, one that is infinite – bends away from itself into infinity. Note that this universe is finite: it wraps onto itself, so it does not go to infinity. This one wraps away from itself; it goes on for ever and it has very little stuff in it.
Finally, we have the Just Right universe. If you have just the right amount of stuff in a universe, it is neither heavy nor light; it is just right and it is what we call flat. It bends neither away from itself nor onto itself. It is also Just Right because theorists, for about the last 20 years, have been saying, 'For us to understand the Big Bang, we think the universe must be flat right now.' I have always been sceptical, but I am an observer; I like to go out and measure the universe and see what is going on. The theorists, when I grew up, said, 'You don't need to measure it; we already know the answer. First principles: don't worry about it'.
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When I came to Australia, I decided that I was going to try to measure the universe's past. How would I do this? I could measure how fast the universe is expanding now, using the Hubble space telescope, and then I could compare that to what it was doing in the past. As I am looking a long way away, I am looking into the past. So I could imagine looking and measuring the Hubble Constant now – I did that for my thesis – and, if I look back five billion years with the universe still expanding at the same rate, I would know that the universe was not slowing down and, therefore, that the universe was going to be infinite and would last for ever. It would also tell me that the universe is pretty close to empty, which would be kind of scary, given that we are here. On the other hand, you could imagine the universe slowing down over time. If I measured how fast the universe was expanding back in time – and it was much faster in the past – and if the universe was on that side of the line [pointing to slide], I would know that gravity would win and the universe would be finite and end in the gnaB giB. On that side of the line, gravity loses the war, the universe keeps expanding for ever and the universe is infinite. It seemed like a good experiment to me. We also had the idea that the universe might be speeding up, which was heresy back in 1995, but at least I was smart enough to put it on the diagram at the time. As you will see, that is what the universe appears to be doing. It did not meet my expectation.
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How are we going to do this? We use something called Type la Supernovae, a type of exploding star, which I will now describe. The Sun is going to last for about 10 billion years and, in about five billion years from now, it is going to become what we call a red giant star and collapse to become a tiny star called a white dwarf about the size of the Earth. If our Sun was not a single star but a binary, the same thing would happen and the bigger of the two stars would become a red giant first. Stars are big and they do not get destroyed but, when the second star then starts dumping material onto that white dwarf, that white dwarf gets heavier and heavier. When a white dwarf becomes 1.4 times the mass of the Sun, it explodes; it is a giant thermonuclear bomb that is roughly four billion times brighter than our Sun. So that is 1036 watts or four billion Suns.
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These things take about 20 days to reach their maximum brightness and then they slowly fade into oblivion over time. What a great standard candle – really bright – and we have reason to believe that they are very similar. When we measure them, they are all the same to about 15 per cent, which is about as good as we can do in astronomy. The next thing that we have to do is go and find them.
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Here is an image of a sky that we took with a large four-metre telescope; it was one of the largest telescopes in the world in 1995. Almost everything you see here is a galaxy. That's a star, that's a star, that's a star, that's a star and that's a star in our own galaxy. Everything else is a distant galaxy. What we have to do is find the exploding star. I will help you; it is right there [pointing to arrow labelled SN]. It is a little star next to a galaxy. It is probably not the best way to tell a supernova, because there are lots of stars next to galaxies. We actually take two images. Here is our first image taken on 28 April and we had taken an image 24 days earlier, and you can see that nothing has gone to something. This object was roughly five billion light years in distance, as we measured it, which means that it exploded before the Earth was formed. So we are looking that far back into the past.
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We did that many, many times. In 1998, we presented our data to the community. I will say that it scared the living daylights out of me, because I was expecting the points to tell us whether or not the universe was going to end in the gnaB giB. I expected the points to be down here somewhere. You can see that, with these nearby points, some of them lie above, some of them lie below but, on average, they sort of mix in. You cannot tell the difference between the different models. However, these points clearly – I should say, the top to the bottom – tell you the range of uncertainty of a given object. None of these objects, even with their uncertainty, say that the universe is going to end in a gnaB GiB. It is very depressing; that is the way I wanted the universe to end.
I figured that the universe, therefore, would be slowing down but not very quickly – and, if it is slowing down but not very quickly, the universe would be infinite. My theorist friends would be wrong, which would be an achievement in itself. But, alas, the universe was doing something unexpected. All the points, on average, lie up in this acceleration part of the diagram. The universe is seemingly speeding up.
This was done by two teams. One was partially based here, in Australia; the other one also was partially based here but was led out of Berkeley in the United States. So it was a big team. I will just show you the large team.
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These are the 20 people who worked on this for three years to make the observation receiving a prize late last year. The interesting thing is that we agreed on this measurement. The two teams independently came to the same crazy answer at the same time.
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To make the universe accelerate, you have to push on it rather than pull on it. Gravity pulls. What is pushing on the universe? What could do that? The thing that was coined by Mike Turner, at a meeting here in this Academy in 1999 is something that we call 'dark energy'.
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What is dark energy? Dark energy is stuff that theorists dream up. Essentially, it is material that is tied to space itself. It is energy that is part of space, and it turns out that it pushes rather than pulls on the universe.
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Einstein was actually the first person to dream this up. He called it the Cosmological Constant. Our measurement suggests that 25 per cent of the universe is made up of normal stuff that pulls but 75 per cent of the universe pushes, and that is what we call the Cosmological Constant. The Cosmological Constant was originally proposed by Einstein to counteract gravity and make the universe static – not move. When he dreamt up his equations in 1915 and then applied them to cosmology in 1916, he saw immediately that the universe should be expanding or contracting. He looked around and said, 'Geez, the universe isn't doing that. What am I going to do?' Then he said, 'Oh, I can actually add an energy to space, which is completely allowed; it's a little ugly, but I can do it. It doesn't change anything else, and that would be to counteract gravity.' Of course, he later called it his greatest blunder – and in modern terms, as I said, we call this the 'what is in space when you think that nothing is there'. It is the energy of space.
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We are not at the end of the story; let's see what else happened. I told you earlier about cosmic microwave background – the glowing of the universe when it was 3,000 or 4,000 degrees centigrade back 13.7 or 13.6 billion years ago. You can see all these bumps and wiggles. They are concentrations of material, back, that were forming into the first galaxies. We can use physics to understand how big those things are. This is going to be the hardest thing I talk about in this lecture.
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Gravity causes things to collapse: when you cause things to collapse, they heat up. For example, this is how the diesel engine in my car works. The piston pushes down on the diesel, the diesel heats up to a point where it explodes and that causes the piston to go up. In the universe, gravity causes little bits of the universe to collapse and that heats them up. But, when you heat something up, it provides pressure. Things get hotter and they also get higher pressure, and the pressure pushes back against that gravity.
Pressure operates at the speed of sound. You can measure the speed of sound in the earlier universe to exquisite accuracy. It is roughly the speed of light divided by the square root of three: 57 per cent of the speed of light. You know that the universe is only 100,000-or-so years old at this time, so the biggest bubble you could possibly see would be the age of the universe times the speed of sound.
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Physics tells us how big the biggest blobs are that we can see in the early universe. How does that allow us to measure anything interesting? Measurement tells us how big those blobs appear now and, remember, the further away an object is, the smaller it appears. Well, in a curved universe that is not necessarily true. So how big these blobs appear can tell us the geometry of space.
Let me just try to give you an idea of how this works. Imagine I am looking out at a universe. I have a curved universe in blue and a flat universe in red. The Earth is up here, looking at a blob. That blob in the flat universe looks the same size as that blob in a curved universe because the light is bending on its way here. They are the same size. We know how big those blobs are. By seeing how big those blobs appear, we can measure whether or not space is curved this way, is flat or curves away from itself.
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That was the hard part of the lecture; hopefully we will not have anything harder than that. This is the more heuristic way: in a universe which is open, the blobs appear very small. Remembering that it is the age of the universe times the speed of sound across, basically the biggest blobs are 57 per cent of 100,000-or-so light years across. If the universe is flat, the blobs will be this big; if the universe is closed, they will be that big. Measuring the blobs – this is a science diagram – tells us the size. It turns out that they are the biggest bumps in the universe that you can measure. If you ask whether the universe is finite or infinite, it appears to be on the knife edge. The knife edge is flatness. I was so depressed when I saw this because it meant that my theorist friends were correct. They always told me that I did not need to measure anything. Of course, I did not measure this; people with the WMAP [Wilkinson Microwave Anisotropy Probe] satellite and other balloon experiments measured this. But, as close as we can tell, the universe appears to be flat.
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What else can we do? Here, in Australia, we did the most extensive survey of the cosmos, the 2dF RedShift Survey. Another survey came afterwards in the US called the Sloan Digital Sky Survey, which allows us to measure the force of gravity in the nearby universe. This was done up at Coonabarabran – which you should visit if you are ever up near the Warrumbungles; it is a beautiful spot – and they were able to weigh the universe.
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How much gravitating material is there in the universe? They did the weigh – the measurement – and found out that the universe had 28 per cent of the amount of gravitating material necessary for the universe to be flat. So what is going on here? Perhaps you are going to ask, 'Twenty eight per cent gravitating matter?' The universe, as measured by its geometry, is saying that it has 100 per cent of the material; and the supernovae are saying that they have 25 per cent gravitating matter, close enough to 28 per cent, and 75 per cent dark energy, which will affect the geometry of space in the same way that gravitating matter will. So there may be consistency, it would seem.
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What do you think might be pulling on the universe? Atoms pull on the universe; that is the obvious thing. But everyone in this audience has heard probably of something called dark matter. Why do we think that the universe is full of dark matter?
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Here is the nearby Triangulum Spiral, which is one of the nearest spirals to us. This is observed with the Hubble space telescope in optical light. If you look at this with radio telescopes, it looks like this. Here we are looking at hydrogen. Blue light means that stuff is coming towards you and red light means that stuff is going away from you – forgetting about the expanse of the universe; we have subtracted that off. So we can see that this whole thing is rotating, as shown in this diagram. The centre of the galaxy is at zero but, as you go out, the whole thing is moving at about 120 to 130 kilometres per second rotation.
Rotation of a galaxy is balancing, by centripetal force, the gravity of the material. The faster the galaxy is rotating, the more mass there is. If that material is further and further out, you have to have more and more material to keep something rotating at the same speed further and further out. So, by looking at a galaxy and measuring how fast it is spinning, you can figure out how much gravity there is. For example, if you do this for the Earth rotating around the Sun, you can measure the mass of the Sun very accurately. That is how we do it. What happens? You go through and you measure gravity and this object and, if you go out to even 40,000 light years, which is way out here, you find that the mass seems to be getting bigger and bigger the further and further you go out, as measured by gravity.
Galaxies are made up of stars, gas and dust, all of which we can, more or less, measure with telescopes. If you add up how much stars, gas and dust there is, in the middle of the galaxy it is everything. But then suddenly things start to depart.
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This departure, we believe, is dark matter. That is, about only one in six things that gravitate in the universe are atoms. This is one of the reasons, but not the only reason, that we believe that there is dark matter; it is the only one I am going to talk about. In and around the galaxy, there is roughly six times more dark matter than matter, gas and stars.
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If I do some cosmic subtraction the cosmic microwave background tells us that, of all the material, we have 100 per cent, which is necessary, compared to being flat. I told you that the 2dF RedShift Survey and the SDSS experiments together say that 27 per cent of the universe is gravitating. If I subtract those things, 73 per cent of the universe is mystery matter – the same mystery matter that appears from the supernovae to be causing the universe to be accelerating.
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We live in what I would describe as a very messy universe. I am sorry that these numbers are changing; you will have to add everything together but in different ways. In the final analysis, we believe that the universe is 72 per cent dark energy – and we think that might be similar to what Einstein dreamed up, but not necessarily so.
Twenty four per cent of the universe is dark matter. What do we think dark matter is? Again we do not really know. We suspect that it is a particle that is yet to be discovered. Some of us are hoping that it may be found in the new Large Hadron Collider that is coming on line as we speak. But it is quite likely that it will not be discovered there, because the particle probably is too heavy and they will not be able to create it in that experiment – but we could get lucky. However, the interesting part is that this would be a type of material never yet discovered and it would have the ability to travel right through the Earth; that is, it does not like to interact with atoms. They say, 'That's crazy.' But we already have a particle that does that: neutrinos. The average neutrino goes right through the Earth. Indeed, the average neutrino will go through a light year of lead before it gets stopped. They do not interact with much. What we are thinking is that the universe has a lot of something that has not been discovered that can travel right through things like the Earth, and that is why it is so hard to detect. Then four per cent of the universe is atomic matter, which is the stuff you and I are made up of and what we look at and see in the universe.
So it is kind of a depressing situation in that we are simply the tip of the iceberg. However, I really should say that, as crazy as it seems, we now have thousands people making measurements, thinking about it and trying to find a flaw in this craziness. As yet no-one has found anything, in my opinion even close to a flaw in this theory. They will be famous if they do because the community, I guess, has gradually and begrudgingly moved towards accepting this view of the universe. Me too; it seems crazy to me, but it seems like the universe that we live in.
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So it turns out that dark energy is in a battle with dark matter for domination of the universe. This is the World (Wide) Wrestling Federation view of the universe that we live in. What is going on? Dark energy is tied to space itself. The more the universe expands, the more dark energy is created. Dark matter is particles – and that is gravity, like you and me. As the universe expands, extra amounts of it do not get created. So its density, the amount in any given part of space, decreases, but the density of dark energy remains fixed. So, as the universe gets bigger and bigger, the dark energy gets stronger and stronger and stronger. So it would seem, from what we have measured, that 75 per cent of the universe is already dark energy and that dark energy has won the battle of the universe. I have no idea whether dark energy is the good guys or the bad guys in Harry Potter, but I guess I would like to think of it as being the good guys – because dark matter really sounds a little more evil.
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But let's see if we can now speculate about the final act of the universe. Where is the universe going?
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It would seem that, if you just took everything we see at first principles, we are going to end up with something that we call the Big Chill. Dark energy gets created exactly as space gets created. Dark energy has won the battle of the universe and will continue to accelerate the cosmos forever, to infinity. The creation of space will start happening everywhere eventually even more quickly than light can travel. So, as the universe expands, light emitted from this object will get pulled along and left behind. So, if the universe expands faster and faster over time, the light trying to travel to us from distant objects will just fail and get dragged behind by the universe's expansion. You might say, 'No, nothing can go faster than the speed of light.' The universe is expanding; it is creating space. Nothing is actually moving when I say that the universe is expanding. It is actually the creation of space rather than the motion of material. So, yes, that can happen more quickly than light can travel.
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What we will see eventually in this is actually a very bleak ending. We will look out and see nothing. Cosmology, as I do it, is done – because there is nothing to see. Everything in the universe has been expanded beyond our ability to see. Gravity is dominating and has already won here, because we are standing on the Earth. The universe is not expanding on Earth. The universe is not expanding in our galaxy, and that is why our galaxy exists. It turns out that the nearest few galaxies are actually falling into our part of the universe. The universe is not expanding there; gravity has already won the battle. So we are going to end up living in a super galaxy that will be created with the nearest galaxies – that is where gravity has won – and the rest of the universe will be invisible to us.
A more exciting ending and certainly a much more speculative one is that it is possible, according to some people, that dark energy would actually be created a little faster than space is being created. So, under Einstein's version, if I double the amount of space, I double the amount of dark energy. But theorists can dream up some form of dark energy – they just write a little equation; they do not think too much about it – where, when you double the amount of space, you end up with slightly more than twice the amount of dark energy. In this case, the universe expands more and more quickly over time but in such a way that it can eat in where gravity has already won. Every piece of this universe will eventually be pushed away from every other piece of the universe, even at the subatomic level.
If you run through the equations, you find that this is a very exciting universe because the end happens very quickly. Stars in galaxies start blinking out. Then, in the last 15 minutes, you go from being able to see all the stars in our own galaxy to seeing nothing; all the stars in the universe just fade away, as they get pushed away from you. It is very exciting. It leaves nothing or at least what I would call a very good definition of nothing.
The good news at least is that, as near as we can tell, this is not what the universe is doing, but we cannot rule it out. However, it is not going to happen tomorrow; that I promise you.
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Finally there is what I call the Big Rip-off, which is what the theorists always tell me: 'Oh, we're going to make dark energy do anything we want in the future.' So in this case it is absolutely impossible to predict the universe's future. It is kind of frustrating, but a theorist has the right to make dark energy do anything it wants to. Until we can actually find dark energy or figure out what it really is from a fundamental theory, we are sort of stuck in this limbo land of not really knowing what is going to happen in the future.
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Where does this leave us? It sort of leaves us with how the universe is going to end. It is probably going to end not with a gnaB giB but with a whimper – but just possibly it will end in a scream during the big rip.
Discussion
Professor Kurt Lambeck: Thank you very much, Brian. It is quite remarkable, actually, how one can make a tome out of a subject where four per cent of the material we understand and the other 96 per cent we do not. There is certainly a lot of scope for the theoreticians here. Are you happy to answer some questions just for a few minutes?
Professor Brian Schmidt: Certainly. People who want to ask elaborate, hard questions should talk to me afterwards; but good generic questions that most people in the audience want to know the answer to I am happy to answer now.
Question: Thank you much. This is an old question that you would have heard many times but it is something that I cannot understand the answer to: what is the universe expanding into? I understand the concept of someone blowing up a balloon and gathering that you are a spot on the balloon. Just a second question, in case I do not get another chance, is where did the Big Bang come from? How did that happen from nothing?
Answer: You are not going to like either answer I give to you. To answer what the universe is expanding into, it is really expanding into that fourth dimension, which is related to time. So I always like to say that the universe is expanding into the future, but that does not help you to visualise it, because it is a fourth dimension. Theorists come up with 11 dimensions, which we can expand. They go in infinite directions beyond where we can comprehend. So it is expanding into this fourth dimension that you cannot really have any intuition of. It is sort of like saying what is the Earth bending into? It is bending into the third dimension.
Of course, we can understand the third dimension, but the fourth dimension is much harder to understand. You just have to measure the effect that it has. It turns out that you can go through and take the various measurements that we make and say, 'Is there a fourth dimension?' The answer is yes. If you look at why light bends around the Sun, it is because light has to travel in a fourth dimension further, which is no way to really understand what it is really doing in three dimensions. Probably, you are not terribly happy with the answer to the first question.
Why did the Big Bang happen? We do not know. The reason we do not know is that physics is sort of set at the time of the Big Bang and we have no way of extrapolating what we know now to then. However, theorists are happy to think about this and to provide explanations. One idea is that it is some weird quantum fluctuation which formed a hunk of dark energy and caused the whole thing to run away, effectively. So we are seeing that the Big Bang sort of looks exactly like what we are seeing now – the universe running away. If you asked where the Big Bang occurred – I am not sure whether that is what you asked, but I will give the answer anyway – the Big Bang occurred everywhere in the universe at the same time. So the Big Bang was here, it was there, it was there and it was on the other side of the universe, but it all happened 13.7 billion years ago at the same time. But, fundamentally, there is no answer to why the Big Bang happened. We just have not got there yet. Our tools of physics really are not obviously going to get us there in the long term, but we will see.
Question: How do you reconcile the Big Bang with a flat universe? Can you explain that to us?
Answer: In trying to understand the cosmic microwave background, although there are all those bumps and wiggles in it, if I look at that background there and there at the same time and measure the temperature, I find that it is the same temperature in one part in 100,000. So it is incredibly smooth. From physics, the only way we know of making something that smooth is for those two pieces of the universe to have been contact with each other so that they could equilibrate to the same temperature.
If you run the universe back in time, the only way we can get it to do that is if the universe, right at the time of the Big Bang, was expanding not normally but in the way we say it is doing now. It was accelerating and we call that inflation. It turns out that Inflation, when the universe accelerates, tends to make the universe flat. It tends to flatten out the universe by making it really, really big. Think of it this way: if you take a big balloon that has a nice curved surface, expand that balloon by a million and then look at that surface, that surface no longer looks curved to you; it looks flat. That is why, to understand why the universe is so smooth, they have to have this universe puff up when it is really, really young, and that causes the universe to be flat. That is why they say that.
Question: The central idea about energy is that it is conserved. At what point in this analysis is it not conserved any more?
Answer: Conservation of energy is a good principle here on Earth, and we are not getting rid of that. If you are going to conserve energy in the universe, you have to make sure that it is consistent within Einstein's equations. Einstein's equations are a little different from what you are used to. Under Einstein, energy conservation is not kinetic energy/potential energy in the normal way. There is something that we would call conservation under Einstein's laws; it is not energy in the way you would normally think of it, but it is the same basic concept. Fundamentally, it comes down to energy, as you think about it, not being conserved in the universe. It is not guaranteed to be, under Einstein's equations.
Question: If the universe is expanding and eventually going on that curve where it collapses in on itself, could that be the cause of another Big Bang?
Answer: That would be the idea and that is why I like it. The answer is: maybe, but you do not know. It sounds really good that you would have the collapse and then a Big Bang would happen again. But, again, when you get that infinite density, our physics breaks down and we think all the laws of physics would be reborn differently from the way they are now. It sounds good and it sort of feels right but, when you really look at it, you cannot actually easily make predictions. Some people do, and there are some methods that people are using where this happens, but I would still call them extremely speculative and there is no way to test them at this point.
Question: (inaudible) but does all the stuff that you have been talking about mesh with superstring theory?
Answer: The question is about superstring theory and how this meshes with it. It has caused the string theorists a lot of headaches. String theory sort of puts together everything about gravity and quantum mechanics. The way they do this is by working in 10 and sometimes 11 dimensions, and the equations become horrific. There are only about 15 people in the world who can really work in this space, and I am certainly not one of them. However, one of the things that superstring theory predicted was that the universe could not have a cosmological constant. We have measured a cosmological constant and so you would think that string theory would be dead and buried. But they have now figured out that the chances of our having a universe with a cosmological constant is about one in 10 to the 500th, so it is not likely.
They then say that the only universe that we could possibly exist in is this universe that has the cosmological constant; therefore, we know that we are in that 10 to the 500th chance. So then they base all their theories on that extremely poor probability that we happen to be in there. Perhaps it sounds crazy to you and, yes, it sounds crazy to me too. But the people who are doing this are a lot smarter than me and I have trouble arguing with them; I just shake my head.
Kurt Lambeck: I think that is a good note on which to thank Brian and to enter into the cold outside. Thank you very much again, Brian.
Brian Schmidt: I will stay around for people who want to ask more questions.
