Sean Carroll outlines his plan to cover nothing less than the history, current beliefs, and future prospects of cosmology in only 24 lectures. This larger structure is to be repeated in smaller cycles throughout the course, helping one to retain the material long term. The three part cycle includes a review of how we got to where we are, why we currently believe our theories, and then speculations on what our theories imply. The first cycle reviews how astronomical observations culminate into the theories of Einstein and particle physics. The following cycle combines these latter two with more recent observations into an introduction for dark matter and dark energy. The last cycle concentrates on the leading theories of what the latter two are composed of, and how we might develop a test for them.
The previous TTC lectures on these subjects were given by the very animated Alex Filippenko. In fact, the webcasts of Alex's astronomy courses at Berkeley show him to be even more entertaining in the role of professor! Sean Carroll seems more reserved and serious, maybe due to the more quantitative approach. If Sean can somehow present the material on a whole new level than Alex, I can see the value of this course. If not, then it just seems like duplicate material. The course guide is enhanced with extra material in the appendix, in addition to the usual timeline, glossary, bio, and bibliography.
Welcome to this course on the dark side of the universe. Not the dark side of "the force," since we're not going to be talking about good and evil very much, but the part of the universe that we don't see directly, and nevertheless modern physicists and cosmologists have been able to piece together.
The news we'll get across in the next 24 lectures is that we have basically figured out what the universe is made of. It's an impressive accomplishment, and is only something that happens once in the history of the human race, when you figure out what the universe is made of, and it happened! It happened at the very end of the 20th century, at least we're pretty sure it happened. We have a picture that holds together very well, and we'll be getting the evidence for why we believe in that picture, and what the other possibilities are. These other possibilities are even more dramatic than the idea that we got it right, so it's a no-lose proposition!
Before going into any of the details, we'll give away the punchline away right at the start, and explain what it is that we think we have figured out. It's basically encapsulated in this pie-chart. It's nice to know that all the secrets of the universe on the largest scales can be fit into one tiny little pie-chart that reveals a lot.
This pie-chart is telling us the ingredients of which the universe is made, and the amount of slice that you have as to how much of that stuff you have in the universe. That little yellow slice in the pie-chart, 5% of the universe, is what we call ordinary matter. By this, physicists mean every single particle that we've ever detected directly, in any experiment ever done anywhere on earth, ever.
So Sean is made of ordinary matter, we are made of ordinary matter, this table, every star, every planet, every bit of gas and dust that we've seen, is ordinary matter, the stuff that we see directly. All of this is only 5% of the universe.
Yet 25% of the universe is the red part of the pie-chart, what we call dark matter. It's matter, it's stuff, it's some kind of particles that move around, but it's dark, we don't see it directly.
The rest of the universe, 70%, is something even more exotic than dark matter, called dark energy. The idea of dark energy is something that is smoothly spread out throughout the universe. So there's this same amount of dark energy here, as over there, as somewhere in the desolate cold of inter-galactic space. Yet 70% of the universe is this smoothly distributed kind of energy we call dark energy. We know something about it, so it's not that we know nothing. Yet it's a very interesting picture.
So there's a lot to mention about this inventory we're claiming to having. One the one hand, when we call something dark matter and dark energy, it makes it sound like we don't know very much. What is this stuff? Yet that's not precisely true. We'll make it clear over the course of these lectures that we do know some things about the nature of dark matter and dark energy. There's things we don't know and also things we know.
The important part is that the dark matter and dark energy are different than ordinary matter. It's not that we've just missed something, like some stars we haven't seen yet, or some gas and dust spread smoothly throughout the universe, and this is really the dark matter. The truth is that all of the particles, all the ordinary stuff, all that's made of atoms and molecules is counted in that ordinary matter.
The dark matter and dark energy are made of something we haven't detected directly. We have evidence for believing that, and we'll find out what that is. All this, of course, unless we are missing something, unless we're getting something wrong! So it is possible that the inferences that we use to go from observations of the universe to the existence of dark matter and dark energy, have gone wrong somewhere along the way. It's not that we've made a mistake with the observations.
Of course if you make one observation, it's always possible that you've made a mistake. Yet at this point it's where we've made observation after observation and keep getting the same result. So there's something going on in the universe, and not that we've made a mistake in taking our data. Yet we could be making a mistake in interpreting the data somehow. The way we get from observations to the idea that there's dark matter and dark energy is to use the force of gravity. It's always possible that there's something wrong with our understanding of the force of gravity and we'll be talking about that possibility.
So one of the themes throughout these lectures is going to be what we know is true, what we believe pretty strongly is true, versus what we speculate might be true. We know that the universe is about 14 billion years old, if you count from the Big Bang to today. It's not worth talking about the possibility that the universe is only 1 billion years old, since we know better than that. We believe that 95% of the universe is a dark sector, 25% of it is dark matter, 70% is dark energy. Yet we don't know that for absolutely sure, because there are interesting alternatives. Yet that is the theory that fits the data by far the best.
Now we start speculating about what is that stuff? What is the dark matter? What is the dark energy? Where do they come from? Why do they have the amounts that they do? Then we're into a regime where we're making hypotheses, and we'll be interested in figuring out which one of those is true, but we don't quite know yet.
So over the next several lectures we're going to be talking about how we got there, why we believe that 95% of the universe is in this dark sector, and where we go next? What is going to be the kind of ideas we're approaching and the experimental techniques we're going to be using to get there?
Yet first let's step back, put ourselves in context, and talk about what we're doing over the course of these lectures. Talking about dark matter and dark energy is participating in a kind of discussion that has been going on since ancient times. We're asking, "What is the world made of at a fundamental level?" The idea is that everything you see, whether us or this table or a star or galaxy, can be reduced to a certain set of small components. All the different kinds of things you see, the difference between us and this table, is that the small components that make them and us, are arranged in different ways.
It didn't have to be that way. It could be that different things were just different. The stuff that makes up this table is just a completely different kind of stuff that makes up us or the stars. Yet way back to the ancient Greeks, people have been wondering about the other possibility. Maybe there's only a small number of fundamental elementary constituents, and they get rearranged in different ways to make very different looking things.
Now if you were an ancient Greek, what you would say the elementary constituents are, are the elements, earth, air, fire, and water. You would explain different substances that you saw, by combining earth, air, fire, and water, in different proportions and in different ways. Now that idea didn't fare very well! It didn't really become a scientific theory. Yet the important insight was made that is a small number of ingredients, and all we do when we describe the world is take those ingredients and put them together in different combinations.
That is still the kind of idea we're pursuing in science today. It's been very successful, so we're going to be talking about what those modern elementary constituents are. These days, it's not earth, air, fire, and water, but elementary particles. We have a set of particles, a set of individual tiny dots, that arrange together in different ways that pull on each other, using different kinds of forces, and make the stuff of which we are made. Now it was Democritus back in ancient Greece who first proposed that the universe is made of individual particles, and he called them atoms. So an atom to Democritus was the tiniest little thing that you could make. If you took an object, you could divide it up into pieces and pieces, yet there's a point at which you can't divide it anymore. There's a smallest possible piece, and Democritus called that an atom.
So chemists in the 19th century took the elements of which we're made, carbon, iron, hydrogen, helium, and so forth, and realized that there was in fact a smallest particle of which chemical elements were made. So they called these smallest particles atoms. Now these days, we know that the atoms we call atoms, are not the truly elementary particles. They're not indivisible, so you can actually divide atoms up.
Yet it remains true that if you want to keep something being a chemical element, then it must be made of atoms. So we have an interesting way of characterizing all the atoms we see, in the Periodic Table of the Elements. We have different kinds of atoms, arranged in different patterns which give us different kinds of elements, and the chemists of the 19th century figured out how this was made. You get, these days, over 100 different chemical elements.
That could have been the end of the story. That could be the universe in which we live. The chemical elements were the different elementary particles, and what we called atoms in the 19th century were in fact what Democritus was calling atoms.
Yet that turns out not to be true. You take an atom, and these days we know we can smash them, take them apart. What do we see? These days what we call an atom, we realize is a nucleus made of protons and neutrons, where protons are heavy positively charged particles, neutrons are heavy neutral particles, joined together into an atomic nucleus. Around the atomic nucleus are spinning much smaller particles called electrons, which are negatively charged.
If you have the same number of electrons spinning around the nucleus as you have protons inside the nucleus, the total amount of charge in the atom will be zero. A typical atom has zero net electrical charge. So we can take that atom, which is an electron or set of electrons spinning around a nucleus, and we can divide it. We can say, move the electron over there, and we have a nucleus left.
Then we can divide that. We can take the nucleus and we can divide it into protons and neutrons. So it is natural to ask if we can take protons and neutrons, and divide them up into something even smaller? This is something not figured out until the 1960s and 1970s, but we now know that the answer is yes. Protons and neutrons are made of smaller elementary particles, which we call quarks.
There's something called the up quark with a charge of +2/3, and something called the down quark which has a charge of -1/3. As we go later in lectures, we'll learn of all sorts of different elementary particles and we'll realize that sometimes that names for them are very clever, sometimes they're very boring. So you have the proton with two up quarks in it, and one down quark. You can do the math, which comes out to a +1 charge. The neutron on the other hand, has two down quarks and one up quark, so it is neutral.
So that is another layer of structure. We see the direction in which we're going. We go from atoms which make chemical elements, we divide them into electrons and nuclei (plural of nucleus), we take the nucleus and divide it into protons and neutrons, which we take and divide into quarks.
It's very natural to ask at this point if there is yet another layer. If we took an electron or a quark, could we divide those into smaller pieces? The answer is we don't know, but most of us believe that we cannot. We actually think that electrons and quarks are part of the bottom layer of which ordinary matter is constructed. We'll go into the evidence for that a little bit later.
Yet the key point here is ordinary matter coming out of up quarks, down quarks, and electrons, so we can make everything we've ever seen! That's enough for all of chemistry, all of economics, psychology, literature, and so forth! It's all up quarks, down quarks, and electrons!
If we look out into the sky, we see stars, galaxies, gas, and dust. Again, it's just quarks and electrons arranged in different combinations. We know that the quarks are there, because we've done experiments at particle accelerators. We take heavy particles with high energies and zoom them towards each other at very high velocities. They smash into each other and they make more particles. That's how we discover new and new layers, but we think that the tiniest particles have now been discovered.
The next question is what are the heavier particles that we haven't yet discovered? We have a good reason for understanding why we haven't discovered heavy particles, namely that a particle accelerator takes more and more energy to make heavier and heavier particles! Einstein taught us that energy is mass times the speed of light squared, E=mc². So if you want to make a more massive particle, you need more energy in your particle accelerator, which costs money. So there's a financial limit right now on how many heavy particles we can discover and we're trying to push that more and more beyond.
The question is what other ways are there to discover new particles? We have a very nice picture of almost everything we've ever seen. We have the quarks and electrons, the ability to put together atoms, into molecules, into proteins, into desks and us, and stars, gas, and dust!
Yet is it possible that there's other stuff out there that we haven't seen in the laboratory? How would we know? The answer is to look into the sky. The question that we're thinking of here, is that when you look at the sky, you see things, stars, galaxies, stars being formed, etc. Yet is that what there mostly is in the universe? Are the things that you directly see, the same things that the universe is really made up of?
The other possibility is that the things we really see are more like a decoration. So we are more like the olive in the martini of the universe. It's the part you see, but maybe not the important part for the whole concoction. So how would you know if there was stuff in the universe, if you can't see it? By construction we're saying that there's stuff there that we don't see, how are we ever supposed to figure out that it's there? Of course we're asking the question in a fairly leading way, and should be clear that it's very easy to become convinced that something exists, even though you don't see it.
For example, there is air here in this room. We're convinced that it's here, even though we don't see it. We can see right through the air. Well how do we know? How do we know for sure that there is air in the room? There's many different ways we could demonstrate that. One way is just to wave our hands in the air and fell something reacting against them. The air exerts a force on our hands, even though we don't see it. We have other senses besides sight, that can feel the air in the room, even though it's invisible and the air is dark in some sense, and doesn't emit or absorb light. It's easy enough to tell that it's there.
So how do we do the analogous thing to waving our hand, except that we do it in the sky? The answer is we can't just put our hand into the sky and wave it around, but we can feel the force of gravity, or more precisely, we can detect the influence of gravity on other celestial objects. So if there's some stuff out there in the universe, some stuff that exists, and there's as much of it or even more of it than there is ordinary matter, that stuff will create a gravitational field. We can then detect its gravitational field, which provides us the secret to detecting the dark side of the universe.
So imagine for example if the moon were invisible and transparent, if we couldn't see the moon whatsoever, and we could look into the sky with any sort of telescope we wanted and still could never detect that it was there. Could we nevertheless be sure that there was such a thing as the moon? Could we weigh it and figure out where it was?
The answer is yes, since back here on earth, we could observe the tides that occur on our oceans. Tides are due to the influence of the moon, on the oceans here on earth. In particular, the moon has a gravitational field, which pushes around the water in the oceans and bays on earth, so we can therefor detect just by looking at the waves and tides in the oceans, that there must be something out there exerting a gravitational field. You can even figure out where it was and even weigh it. You can figure out how much gravity had to be exerted.
Fortunately we can see the moon, so we don't have to do that. Yet you can do the same kind of process for other things in the universe. You can look out there in the universe, see the gravitational fields that are being created and then attribute them to some kind of stuff, even if you can't see the stuff directly.
So you see the possible loophole in the argument at the same time. In order to do this inference, you need to understand how gravity works. So even though we think that we do understand how gravity works, we'll take seriously the possibility that we're being tricked, that something else is going on, so that we don't understand gravity. We'll try to ask whether it's possible that there isn't any dark matter or dark energy, and instead that we just don't understand gravity?
The ultimate answer will be probably not, that we think we do understand gravity, that we think there is dark matter and there is dark energy, but we need to keep every single possibility open until absolutely sure.
The other thing we're trying to do, of course, is to go beyond looking at dark matter and dark energy through their gravitational fields, and detect them directly. So another possibility we'll be discussing is building experiments her on earth that will produce dark matter particles, or that will wait until dark matter particles come into the detector and bounce off of it, and therefor give us direct evidence that dark matter really exists. That may or may not be possible, and depends on properties of the dark matter. Yet we'll try to do that, and you never know what may happen.
So let's just outline the entire rest of the course. We won't keep any surprises until the end, so we'll give everything away. The picture we're painting in some sense, makes a lot of sense. Yet in other senses, it's surprising on some levels. It's better to hear the same story over and over again so it can really sink in, and we can understand what is going on. The first part of the course, lectures 2-6, will be about just looking at the universe. Actually going out there in the universe, looking with a telescope at what's in the sky, seeing how galaxies are distributed, where they came from, and so forth. That's part of how we begin to understand what the universe is made of.
Yet just looking doesn't get us very far, so at the same time in the same lectures 2-6, we'll also be talking about the underlying framework that we use to understand the dynamics of the universe. That framework was given to us by Albert Einstein in the form of his General Theory of Relativity, which we'll talk about as being a theory of space and time, and how it grows.
So through the first couple of lectures, we'll be going back and forth through looking at the universe, seeing what is out there, and then coming back here and thinking about it. When we think about what could be going on, then we get a better idea of what should be going on. When you then go back to look, you then say, "Aha, I now understand something about the universe that I didn't understand before."
What we'll see from looking at the universe and putting it in the context of Einstein's General Relativity is that first, the universe is a pretty simple place. It looks more or less the same all over the place, except that it's getting bigger. The universe is expanding, it used to be smaller and in the future will be bigger, which is an interesting fact we need to think about.
Yet the second thing is that the dynamics of the particles we observe in the universe can't be explained only by those particles. In other words, what we'll do is look at stuff in the universe from various perspectives, radio telescopes, ordinary telescopes, individual galaxies, clusters of galaxies, we'll take the universe as a whole and then put it back together.
We'll then realize that it doesn't quite make sense all by itself! The only way to make sense of the dynamics of stuff in the universe in the context of Einstein's General Theory of Relativity, is to imagine there is stuff there that we don't see. This is of course the stuff that we call dark matter. We'll see that if we imagine there is stuff in the universe that we don't see, then suddenly the dynamics of galaxies and clusters of galaxies begins to make sense to us.
So that is telling us that there is something out there, something in the universe that we haven't yet directly detected here on earth. We have a very good view of the kinds of particles that exist in atoms, and they don't seem to be the same kinds of particles that exist out there, that we are calling dark matter. So in lectures 7 and 8, we'll take a step backwards, back to earth, and try to talk about the actual particles we know and love, those particles in what we call the Standard Model of Particle Physics.
We said that electrons plus up and down quarks are enough to make all the stuff we are familiar with. Well how do we know that? So in lecture 7 we'll talk about what different kinds of particles there can be. There are boson particles which basically pile up on top of each other and make forces, and there are fermion particles that take up space, and those make up matter. It's good that they take up space, because otherwise everything would just collapse in together. So these different kinds of particles are both necessary in making an interesting universe, and thankfully they actually exist.
Then in lecture 8 we'll go into the specifics, given that we can have bosons and fermions. What are the particles we actually have in the universe? What is the complete set that particle physicists have detected? It's actually quite a nice picture that has been constructed. We've put together basically a mini-periodic table of the elementary particles of nature.
In that table there are still mysteries. There are still things we don't understand about why these particles, and why not those particles? Yet it's a very impressive accomplishment that's very consistent with a tremendous amount of data. It's kind of depressing actually to be a particle physicist. You don't like it when your theory is consistent with all the data. You like it when there are puzzles that you can try to explain.
So one project in particle physics is to push our knowledge into some region where the standard model of particle physics isn't good enough. Cosmologists have already done that. They've found dark matter and one of the things we'll be talking about is why we think that dark matter is not part of the standard model of particle physics.
Then we have some knowledge of particles, so we can move back in lectures 9-11 and apply if to the universe. In particular, we'll apply it to the early universe, the very famously labeled first three minutes of the universe are a time when things were crunched together, at much higher temperatures and densities, and all of those particles we discovered in the standard model of particle physics, were playing important roles in the very early universe.
So we'll take the universe we have today, and we're going to run the movie backwards to when everything was packed closer together at high temperatures and densities, and use particle physics to predict what things would have been like, and then wind the movie forward again. We'll start making predictions for what our current universe should be like, based on our predictions for what the past was like, and given what we think particle physics was like.
The remarkable thing is that it's going to work! We're going to get the right answer by making predictions based on what was going on in the first few seconds of the universe's history, which will turn out to be correct. That means we understand something important about what was going on just a few seconds after the Big Bang.
In particular, there are two phenomena we'll focus on during these lectures. One is Big Bang nucleosynthesis and the other is the CMB (cosmic microwave background). Big Bang nucleosynthesis refers to the time in the universe's history when it was a nuclear reactor that was taking individual protons and neutrons and fusing them together to make heavier elements. The specific detailed properties of what was going on when the universe was only a few minutes old, come into making predictions for how much of each kind of element we should have. That's the kind of prediction we can make, and it turns out to be right.
The CMB is also describing a relic from the earliest times. It is the light that was emitted by the universe at the earliest times. When the universe was young enough, it was opaque. You couldn't see your hand in front of your face, imagining that you were there at the very early universe, but when it was about 400,000 years after the big bang, it became transparent.
So suddenly the light that was emitted from the very earliest times of the universe, could stream across the universe into our telescopes and be detected today. It's been stretched into the microwave regime, just like your microwave oven at home, and we observe that today as the cosmic microwave background. It turns out you can learn a lot about the universe by looking at what the CMB looks like today, and we'll talk about that as well.
Then finally in lecture 12, we get to talk about dark matter in some detail. We'll talk about what it could be. We have already seen, just by looking at the universe and applying General Relativity, that we need to imagine that dark matter exists. That is the only way we can make sense of what the universe is doing. So we're going to say, "OK, the dark matter does exist. What is it?"
Well there are different proposals within particle physics for what the dark matter could be. There are proposals that it could be ordinary stuff, it could be stars that are just crunched together, it could be black holes. We'll talk about those, but we'll argue that this is not going to be the right answer. There isn't enough ordinary stuff in the universe to be the dark matter. Probably it is some new kind of particle.
So then we'll ask what the new kind of particle could be? The most popular candidate for this, comes from something called supersymmetry. This is a new symmetry of nature that relates bosons to fermions, and predicts a whole bunch of new particles. One of them could very naturally be the dark matter, so particle physicists right now are very carefully looking for where that is in the universe, and in their particle physics experiments. So we'll emphasize a lot, the possibility of supersymmetry for the most popular candidate for what the dark matter particle could be, yet there are other candidates as well, and we'll talk about them too.
So then in lectures 14 and 15, we'll start talking about dark energy. This is not dark matter, but something different. It's very interesting that this 95% of the universe, which is the dark sector, is split into two very different kinds of things. So what we mean when we say that, is that in order to fit the data, we need to imagine that there are two different things out there in the dark sector.
One is the dark matter, which is a bunch of particles that settle down, pulling on each other, under their mutual gravitational field, and the other is the dark energy which is smooth. It does not clump together under gravity. It also does not go away as the universe expands. Ordinary matter, as the universe gets bigger and bigger, becomes less and less dense. Yet dark energy somehow persists.
So we'll talk about why we think there is dark energy. It turns out that the universe in recent times, has begun to accelerate. Not only is it expanding, but it's expanding faster and faster. This is not at all what we would have expected on the basis of a universe with just ordinary matter and radiation, even dark matter. The only way we can explain the acceleration, is to imagine there's something that doesn't go away, something called dark energy.
Then as a final consistency check, we're going to measure all of the stuff in the whole universe all at once. Just to make sure that we're not missing anything, we're going to use the entire universe as a scale, to weigh how much stuff there is, and find out if we imagine a universe which is 5% ordinary matter, 25% dark matter, 75% dark energy, all adds up to everything we see. That is the best evidence that we have, as we'll discuss, for the idea that we haven't missed anything, The idea that once we include the dark matter and dark energy, we have a complete inventory of what is in the universe.
So then of course we have to ask, what is this stuff? So in lectures 16-18, we'll discuss different possibilities for what the dark energy could be. It's something that's smooth, more or less the same everywhere, something that doesn't go away, so that gives us some clues for the different candidates of what it could be. None of these candidates, frankly, are very good. We're very much in the hypothesizing stage of doing science when it comes to dark energy. We'll at least talk about some of what these are.
Then by lecture 19 we'll take a step back and say, "OK, we've inferred the existence of dark matter and dark energy by measuring their gravitational fields. What is it that we don't understand gravity? As we said before, the idea that there is dark matter and dark energy, comes down to the fact that we need to invoke them to explain the motions of particles. Yet if gravity is different than we think, if Einstein was not right when he invented General Relativity, maybe we could do away with dark matter or dark energy. So we'll talk about that possibility.
In lecture 20, we'll pause a bit to look at inflation, which is a hypothetical idea in the very early universe. Yet it's closely connected to the ideas of dark matter and dark energy. Number one because the mechanism of inflation is almost exactly the same as that of dark energy, making the universe accelerate. Also number two, because inflation makes some predictions that turn out to be coming right, only if you believe in dark matter and dark energy. So we'll put those pieces of the puzzle together.
Now lectures 21 and 22 are like desert. We get a little treat for doing all this good work, looking at data. We're going to start imagining beyond thing we've actually observed. We'll talk about parts of the universe we have no access to, in what some call the multiverse, extra dimensions of space, and string theory, a hypothetical theory that weds together particle physics and gravity.
String theory should have something to say about dark matter and dark energy, but our current state of knowledge of the theory just isn't good enough to day anything for sure. So what we'll talk about with string theory is what might be true. If string theory happens in a certain way, what could the implications be for dark matter and dark energy?
Yet then to bring us back to earth, in lecture 23 we'll talk about how we're going to test these ideas. All through the course we'll be mentioning experiments and observations that have been done, and some that will be done in the future. Yet there's a whole bunch, a whole retinue of techniques that are going to be used to probe the universe on the smallest scales and the largest scales.
So we'll get an outline of what kinds of techniques are going to be used, not just in the next year or two, but the next ten years or 50 years, to learn more about dark matter and dark energy, as well as particle physics, string theory, and inflation.
So then finally in lecture 24, we'll talk about where we have gotten, and where we're going to go from here. We've learned a lot, a universe for which we claim to have a complete inventory. Yet in some sense, it doesn't quite match our expectations. This universe in which we are only 5%, puts us into a tiny little corner of the universe. We could have designed a much simpler and more elegant set of ingredients for the universe, but they didn't turn out that way.
So why is it like that, rather than some other way? How do we put these ideas into the bigger context? What can we learn, and why are we even thinking about these ideas?
So next time in lecture 2, we'll be very mundane and go outside on a clear night, talking about the universe and what we see.
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