11. The Cosmic Microwave Background - Sean Carroll - Dark Matter, Dark Energy: The Dark Side of the Universe

This lecture concludes the focus on the early universe, the second of the two larger cycles so far. Alternating between observation and theory, we used our development of the standard model along with general relativity from the first cycle, to characterize the early universe and its relations to dark matter. The stages of how we got to know, and why we believe in our current theories were accomplished, as outlined in the first lecture. Further speculations on dark matter will round out this second cycle, to be followed by the third on dark energy.

The previous lecture concentrated on matter in the early universe. Yet Sean restated that the early universe was radiation dominated. The Friedmann equation predicts recombination after 380,000 years. This is an atomic physics term referring to the electron returning to the atom after ionization. In our case the electron is not actually returning, but combining with a nucleus for the first time. Like so many other cases, recombination is actually a misnomer.

Before recombination the photons interacted with the free electrons and ionized nuclei so much that the scatter made the universe opaque. At recombination the temperature dropped enough to allow electrons to combine with nuclei to form atoms. The radiation does not interact as readily with atoms, meaning no scattering took place and the universe became transparent. But this radiation is the cosmic microwave background (CMB) seen today at a temperature of 3K. This snapshot of the 380,000 year old universe was as hot as our sun's surface. The scale factor of 1/1000 implies a temperature of 3000K. From our perspective, expansion over time has stretched the CMB's peak wavelength to the microwave portion of the spectrum.

The whole story of the first theories and observations of the CMB is well told by Sean. But the modern counterpart gets a little fuzzy, literally and lecture wise. Compared to the detailed particle physics descriptions of just seconds after the big bang, our picture 380,000 years later of the CMB seems to almost be the opposite. We have no theory of what the CMB should look like. The image of the CMB is quite good, showing the expected variations in temperature. But what those variations represent can be hard to describe. We can only predict their appearance over several angular scales. Sean goes through them a bit fast and without graphical support, so I will do my best to help out here.

The CMB snapshot being only 380,000 years old implies that a large blob of ordinary matter, say 1,000,000 light years wide, would not have had nearly enough time to gravitationally collapse. A blob as large as the universe is old, 380,000 light years wide, would still not yet have the time to evolve. A blob this size would appear just greater than one degree wide on a CMB map showing the whole sky.

A much smaller blob, far less than one degree wide, would indeed have time enough to collapse. But then enough time also left over to rebound, to become less dense and to cool off! This sets up an interesting effect much like that of an oscillating sound wave. Overall, the blobs on this much smaller scale would produce a smooth CMB map.

A medium blob just less then one degree wide would be just right, in that it would collapse but then not have time to rebound. A blob on this scale would show up best as a temperature variation in the CMB. If this sounds crude and primitive, especially compared to the nucleosynthesis from last lecture, I agree. It reminds me of the collapsing solar nebulae story, mostly elegant words to hide our ignorance. It doesn't help any that the course guide is more confusing than the lecture either. I'm finding that to be true in general for this course.

I think the three examples of blobs are also trying to say that a detector has to be set to observe for only certain sizes or temperature variations in order to actually see the fluctuations. But the real fascinating theory is how dark matter behaves with respect to the blob scenarios.

The dark matter initially collapses along with the blobs of ordinary matter. But the decrease in volume that serves to increase interactions and heat up the ordinary matter, does not happen to the non-interacting dark matter. So while ordinary matter rebounds, the dark matter continues to collapse.

At various stages the two are in and out of phase with each other, actually producing more temperature variations in the CMB. Supposedly we can see this effect to the degree that it implies a factor of five times more matter is needed than ordinary matter. This agreement with the dark matter implications from galaxy cluster dynamics, galaxy rotation curves, and primordial nucleosynthesis from last lecture, is of course very reassuring. Though Sean states it would actually be somewhat nice if the CMB result did not agree with the others, showing there was something new we didn't understand!

The grand conclusions are twofold. The critical density we so far had been setting equal to one as part of our own measure, is really equal to one according to the CMB. This implies there really is more than just ordinary and dark matter in the universe; what we now call dark energy.

The CMB also allow tests of our more advanced theories on the early universe such as inflation and polarization. The big bang is the poor mans particle accelerator, or so they say.

In the last lecture, we began our paleontological exercises, starting with looking at fossils from the Big Bang, leftover particles from the earliest moments of the universe's history, both using them to check that our theories of it are on the right track, yet also to put constraints on the stuff that makes up the universe, and to learn about our universe today.

Big Bang nucleosynthesis gave us a very nice way of measuring the total amount of ordinary matter in the universe, total amount of atoms, of stuff made mostly of protons, neutrons, and electrons. The answer is there's not enough stuff there to account for all the matter we perceive in the universe, so there must be dark matter that is the something else.

Today we're going to look at an even more basic relic of the Big Bang, which is photons. In particular the CMB, the background radiation left over from when the universe was about 400,000 years old. We may have already heard of the CMB in the context of providing evidence for the Big Bang theory. When the CMB was first discovered in 1965, as we'll talk about, it was still plausible to believe that the Big Bang wasn't right. There was an alternative called the steady state cosmology, in which the universe had been expanding forever, with more matter continually being created in empty space. There was not some hot, dense, original state, out of which everything came. Yet the Big Bang model made a very specific prediction, that there should be a leftover, very cold radiation, from that initial hot state. That's what this leftover radiation is today, the CMB.

Also, the CMB is important, because it provides static for our tv sets. If you have a non-cable tv, about 10% of the static that you see on your screen, is actually from photons coming from the cosmos in the form of the CMB. We only figured that out, after we'd discovered it by other methods.

Today we'll not really be so interested in the existence of the CMB, not really in its use for testing the Big Bang theory, but to look at the tiny variations in the temperature of the CMB from place to place. The very early universe is smooth, and the earlier you go, the smoother it is. So we talked about Big Bang nucleosynthesis and treated the universe as exactly the same everywhere. The CMB is the first time when the slight ripples in the universe, the slight perturbations in density, make an appearance. The effect is that things are a little bit different from place to place.

So the CMB provides us with a snapshot of what the perturbations in the early universe looked like, 400,000 years after the Big Bang. So by both looking at what they looked like then, and comparing to what they look like today, we can learn a tremendous amount about the constituents of the universe. Ultimately the CMB will provide us independent evidence, both for dark matter and for dark energy. Dark matter we'll talk about today, while dark energy we'll get to in a later lecture.

So the game we play by now should be familiar. We start with the universe now, and wind the movie backwards. We ask what happens when you take a volume of the universe today, and squeeze it like a piston, make everything more dense and squeezed into a smaller place, increasing along the way, the temperature. As the scale factor goes down, the temperature goes up, in exactly the inverse proportion.

So if the CMB we observe today is at about 3 degrees Kelvin, as a very cold set of radiation particles, when the universe was 1/1000th its current size it was at about 3000 degrees Kelvin. The reason why that's an important temperature is that it's when the photons were sufficiently hot, that when they were banging into atoms, they had enough energy to strip the electrons away from them.

In other words, at moments before the universe was 1/1000th its current size, there was too much hot radiation to allow atoms to exist. You had free electrons running around, unattached to atoms, and you had free protons and other atomic nuclei. They wanted to get together but they kept running into hot photons which prevented them from doing so.

So all this was happening, like we said, at about 400,000 years after the Big Bang. More specifically, 380,000 years after the Big Bang is the slightly more precise moment when it happened. We know all that from the Freidmann equation, the equation in General Relativity that relates the expansion rate of the universe, to the stuff inside.

So what the Friedmann equation is telling us, is that at earlier times than that, the universe was hot enough that everything was ionized, individual atoms were just a nucleus and the electrons were moving freely. That means that the universe was opaque. Remember that photons like to interact with charged particles. They can interact with atoms, as the photons cannot go through a table which is opaque, yet if atoms arrange themselves in the right way, photons will just go right through them. The air in a room has this property, where photons of visible wavelengths, travel right through it unimpeded.

However, when you're ionized, when you have free electrons going around, no photons can go through you unimpeded. Those photons will keep running into these free electrons, which will make them bounce and turn in a different direction. If you were alive during that era of the universe's history, which you couldn't be since it was far too hot, somewhat like living on the surface of the sun, yet if you put your hand up in front of your face, you couldn't see it. The photon couldn't make it from your hand to your eyeball, without bumping into a whole bunch of things. It was as if the universe was immersed in a thick, soupy fog, so you couldn't see anything.

There were a lot of photons. There's a rule of physics that says when something is hot, it gives off a lot of radiation, called black-body radiation. It doesn't matter what color the thing is, or how its excited the different atoms or anything like that, but it's just that any lump of stuff at some fixed temperature will be giving off radiation all by itself. This is how you can see people in the dark with infrared goggles, as their body temperature is enough that they're giving off thermal radiation, blackbody radiation. It's the same reason why a heating element will be glowing red, since it's just giving off blackbody radiation.

So the early universe, we know had to be filled with radiation, because it was at a high temperature. It has a blackbody spectrum which we can actually test by observing the CMB today. So the moment of the universe becoming about 3000 degrees Kelvin, is when recombination happens, when electrons can finally get together with protons and other atomic nuclei, to form atoms.

Recombination is a word from atomic physics that is used when you take an electron off an atom, and it then goes back. So you've removed it, and now it is recombining. Sometimes people say you shouldn't use this in cosmology, since it's not recombining. It was the first time these things ever combined! That's being a little too tricky in fact, since in the real world, what happens, even in cosmology, is that a typical electron and proton will combine and recombine many times at that temperature of 3000 K, before they finally settle down.

Yet when they do settle down to form atoms out of the ions you used to have, the universe has now become transparent. You had a lot of radiation running around, you had blackbody radiation suffusing the universe, and now that radiation stretches freely throughout empty space. It can go through the gas of hydrogen and of course can go through the dark matter and neutrinos, they won't bother you!

So that photon now has a path that instead of being really short, can stretch for billions of light years. All that radiation was being created at every point in the universe. So we, living here right now, can look around and see that radiation coming at us, if nothing ever happened to it from the moment of recombination, 400,000 years after the Big Bang, until today when we bump into it. So the universe becomes transparent at about 3000 degrees Kelvin, which is actually pretty hot, similar to the surface of the sun. The sun is not emitting microwaves, but visible light, which is why we see it.

Yet of course, what happens is that the universe expands by a factor of 1000 in between recombination and today. So the light that was given off at recombination, was actually similar to what we see from the sun, yet gets redshifted, stretched in wavelength by the expansion of the universe. Today it has reached to the microwave regime. The wavelength of this stuff from the early universe is the same as what you use in a microwave oven to heat things up. It really is that kind of microwave.

So this entire story of the early universe emitting blackbody radiation, it being opaque when the universe was ionized, and then suddenly becoming transparent when recombination happens, was worked out in the late 1940s and early 50s by George Gamow and his students, Ralph Alpher and Robert Herman. We heard about Alpher and Gamow in a previous lecture. They also worked out Big Bang nucleosynthesis for the first time, and the prediction that there should be something called the CMB, was actually made by these physicists as a spinoff of their work on primordial nucleosynthesis.

So they are making these predictions by about 1950. They didn't have very good astronomical data, but were still ale to predict that the CMB should exist and should have a temperature of something like 5-10 degrees Kelvin, as their guess. It turns out to be more like 3 degrees K, but that's extremely good, considering the quality of data they had at the time.

Again however, they were ahead of their time. They made a prediction by about 1950, and no one had the technology to even go look for this stuff. It was very unclear to Gamow, Alpher, and Herman, when they were writing their papers, whether anyone would ever be able to even detect this CMB radiation they were predicting.

As it turns out, in 1965, Arno Penzias and Robert Wilson, working at Bell Labs, succeeded in detecting the cosmic microwave background. This was back in the day when Bell Labs, owned by Bell Telephone, would do basic research just because it was interesting! They were not trying to build anything better. Well, of course they were all trying to build better things, but they were investing in research because they just wanted to learn about the universe!

So radio-astronomy was a hot, new topic at the time, and Bell Labs was heavily invested in radio-astronomy. Penzias and Wilson were young radio-astronomers who had built new receivers and were trying to calibrate them into looking at different objects on the sky. They built a large, horn-shaped antenna, located in New Jersey, in Holmdale, and tried to bring it online for the first time.

Penzias and Wilson didn't know that there was any such thing predicted as the CMB. They were interested in looking at radio-astronomy sources, stars and nebulae which were inside our galaxy. Yet when they were trying to calibrate their new telescope to make sure it was working, they kept getting noise, some background buzzing that they couldn't get rid of. The reason they were puzzled was no matter which direction they pointed their telescope, they got the same kind of noise. To them this meant there was a problem with the telescope, not something they were seeing in the sky. If it doesn't matter where you look, it's probably because your instrument is faulty somehow.

Yet they went through every single check they could think of, trying to remove this noise from their instrument, and even climbed into the horn to scrape off the pigeon droppings from the inside of the instrument, because they were a dialectic material, giving off microwaves. Eventually they realized they couldn't get rid of the signal, and realized they were actually receiving microwaves from the sky.

First, purely by accident they were informed by a friend that there was a preexisting prediction for a CMB. Second, there was another group down the street at Princeton, New Jersey, who knew about this prediction and were trying to detect it. Bob Dickie, a physicist at Princeton, along with his graduate students Jim Peebles and David Wilkinson, knew about the CMB. Dickie had predicted it independently after Gamow, and they were engaged in an attempt to build a receiver to detect it. They were building something on the roof of the physics building at Princeton University, not knowing there was a much better telescope which was by accident, down the street discovering the same thing.

So at the end of 1965, Penzias and Wilson published a paper, while Peebles, Roll, Dickie, and Wilkinson published a theoretical paper explaining what Penzias and Wilson had seen. These two papers appeared side by side in the Astrophysical Journal. Penzias and Wilson's paper was entitled "A Measure of Excess Antenna Temperature." They didn't say out loud that what they had detected was in fact, the relic radiation from the Big Bang. They admitted in the paper that this was a very promising interpretation of it, but just like Hubble, years before, they were good observers, telling you what they saw in the universe, leaving it to us to interpret where it came from.

So Penzias and Wilson won the Nobel Prize for their work by 1978, yet nobody won any Nobel Prize for predicting the CMB! Alpher and Herman, who were still alive at the time, while Gamow has passed away, thought that they would win the Nobel Prize, since they had predicted this wonderful thing that was then observed. Yet they had predicted it so early, that everyone had forgot about it! Even though they kept trying to tell people, they never really got the recognition that most people now think they deserve, for doing both Big Bang nucleosynthesis and the CMB.

You do, by the way, feel bad for the poor photon, from the CMB. We're talking about a photon, which in the very early universe, kept scattering off of electrons. Then the universe became transparent and that photon traveled across tens of billions of light years, without bumping into anything, only to land in New Jersey, to be detected by some physicists. Yet it was for a good cause, because the CMB today is very useful to us, for doing astrophysics.

So the important thing when they were discovering the CMB, was that it was there, with a temperature to be measured. The CMB at that time, to the best observation anyone could make, was perfectly smooth. You look in every direction of the sky and you get the same temperature. Yet everyone knew that couldn't be strictly true. In our current universe, it's not perfectly smooth, yet more or less smooth in sufficiently large regions. So if you go back in time, it must have been pretty smooth, yet not perfectly so.

That meant that there must be fluctuations in the temperature of the CMB, which grew into the larger perturbations we see today. You could even figure out how large the fluctuations in temperature should be. They should be one part in 100,000. So if you see the CMB being at a certain temperature at one point, you go over to a point next to it, then it should be different by 1/100,000.

So that is where the action is today, in measuring the perturbations in temperature of the CMB, and using them to characterize the behavior of structure formation in the universe, and the ingredients that go into that structure as it forms. These tiny variations in temperature of the CMB were finally detected in 1992 by COBE (Cosmic Background Explorer), a NASA satellite. It was the second most complicated satellite NASA had ever built, after the Hubble Space Telescope.

So COBE just sat there, taking data. At the time when launched, there was a previously existing experimental result, which brought into question the fact that the CMB was really a blackbody. Some people had put up a telescope on a rocket, which measured a deviation in the blackbody spectrum. People were worried about that, so the very first thing COBE did was to verify that the spectrum of the CMB was indeed a blackbody. There was a talk at a AAS meeting that got a standing ovation, which rarely happens! The fact that COBE was able to verify the CMB as really a blackbody, was big news.

A year later, COBE went beyond that and found the tiny fluctuations in temperature from place to place in the CMB. A few more years later, by 2006, the boss of COBE as a whole, John Mather, and George Smoot, who was the boss of the specific instrument which found the temperature fluctuations, were awarded the Nobel Prize for doing that.

Since 1992, since COBE found that there really were fluctuations in the CMB, we have gotten much better at measuring these fluctuations. We've developed a large retinue of different ways to take pictures of the CMB. Here is an image of one such CMB experiment. This is the Boomerang experiment, which was launched on a balloon in Antarctica. The idea of a balloon is that you get above most of the atmosphere, so you have a clear view of the sky. The idea of Antarctica is that there are winds that go around the continent, so this is a very unusual kind of physics experiment, where you build it and then launch it in a balloon. The wind carries it off to the east, and you see it go that way. Then you wait two weeks and see your experiment coming back to you from the west! It has circumnavigated the continent once, so over the course of two weeks, you can collect that much data. Then you push a button, it falls down, and you collect your data in the experiment.

There were other similar experiments and telescopes observing the CMB in Chile, California, and at the south pole where we've gotten very good at measuring the CMB in different ways. Perhaps the best image we have of the CMB comes from MAP (Microwave Antisotropy Probe), renamed WMAP (Wilkinson Microwave Antisotropy Probe) after Dave Wilkinson, one of Bob Dickie's grad students in the early days, who became a pioneer and sort of the grandfather figure of CMB research. He was an important person on the WMAP satellite, yet passed away just before the first results were announced.

So we see an image of WMAP itself, a satellite which can observe the whole sky. That's the real difference between a satellite imager of the CMB, versus a balloon or ground-based imager. If you're sitting on the ground, or even in a balloon that is not very high above the ground, you can't see what is going on beneath you, behind the earth. A satellite can get a 360 degree view of everything that is happening in the CMB. What WMAP did was exactly that, and it returned this image, an iconic picture in cosmology, a map of the tiny temperature fluctuations in the CMB. The blue regions are a little bit cooler than average, the yellow and red are a little bit hotter than average. This is the snapshot of what the universe looked like about 400,000 years after the Big Bang.

So what do we learn from this? Where did this picture come from, and what is it telling us? The point is that we don't have any way, of predicting precisely where a cold spot will be, and a hot spot will be, on the CMB, anymore than we have any way of predicting where a galaxy will be in the sky. What we have are statistical theories, that predict we should get so many perturbations like this, so many like that.

So what you need to do is observe all of the CMB, and characterize the statistics of the different patches of hot spots and cold spots. That is something you can compare to a set of predictions. So how do you make that set of predictions? You're going to consider the life of a blob of stuff. So we imagine we have a little blob of stuff at very early times in the universe's history. A little blob that has a tiny bit more density than the surrounding plasma. So its wants to contract under the force of gravity, to get smaller and more dense.

Yet it may or may not succeed in doing that. What happens to this blob of stuff will depend on how big it is. Take a blob of stuff that will be, for example, lets say, at recombination, one million light years across. Well the age of the universe at recombination is only 400,000 years. So a blob of stuff that is a million light years across, doesn't have time to collapse.

So when we look at the CMB, it turns out that every angular size we're looking at, that is greater than one degree, corresponds to a physical size that is greater than 400,000 light years. So on scales greater than one degree across, when we look at the CMB, we're looking at things that did not have time to evolve. They are not telling us anything about the evolution of stuff in the universe, because they didn't have enough time to evolve themselves.

So let's look at stuff that was smaller than that. Let's look at blobs of stuff that did have time to evolve. What happens to them? So you have an over-dense region of the early universe with slightly more matter in it. So when it begins to evolve, it will contract. When stuff contracts, just like the universe or a piston or anything else, it heats up. So what initially happens is, the region of stuff which is slightly over-dense, contracts, heats up, and that forms a hot-spot you can observe on the CMB.

Yet you need to ask what will happen next? It's exactly like a sound wave, here in the room or anywhere else. There is a region of increased pressure, yet that pressure pushes things away, so things bounce back and become less dense than they were before. So you have a blob of stuff that contracts under its own gravity, it heats up and becomes hot, yet there's also more pressure that pushes against it and that same blob expands and becomes under-dense and cold. Once it becomes under-dense, it is surrounded by regions that are denser than it is, and the same process happens. The blob bounces back and forth. It's truly an acoustic wave, moving through the primordial plasma, just like a sound wave moves through this room. A moving region of high pressure, low pressure, back and forth.

So that would be interesting all by itself, yet there's more physics going on than just this oscillation. There are two important pieces of physics, one being that it doesn't oscillate forever. It is damped, just like when we say something into this room, you hear a slight echo, but you don't hear it forever. The sound waves bump into stuff and die away. These oscillations in the early universe will eventually damp out and stop because things are mixing, moving from dense regions to under-dense regions.

So if you look at very small regions, very small blobs of stuff, it is true that they oscillate, yet they damp out and stop. So for very small parts of the CMB, you don't see that much antisotropy. You don't see that much variation from direction to direction, or very much perturbation in the temperature of the CMB. On small scales, things have had time to smooth out.

On large scales, things have not had time to evolve from medium scales that are interesting. That's when things can collapse, heat up, then expand and cool down. Yet the other new piece of physics is of course, dark matter. The story we we're just telling of stuff collapsing and heating up, is one of ordinary matter. It's the story of atoms, of protons and electrons. When we say heat up, we mean stuff bumps into other stuff. It exchanges energy, and that's what causes it to heat up.

Yet dark matter is collisionless. Dark matter just collapses. Dark matter doesn't heat up. So in the first stages of this process of an over-dense region, it's a nice, very convenient feature of the universe, that if a region is over-dense in ordinary matter, it is also over-dense in dark matter. They go hand in hand.

So when things begin to collapse, both the dark matter and the ordinary matter collapse together in the same place. Yet the ordinary matter heats up, becomes high pressure, and then uncollapses, becoming less dense. Yet the dark matter just keeps collapsing, while the ordinary matter is oscillating back and forth. What you get is a series where the ordinary matter is in-phase with the dark matter, then out of phase with the dark matter, then in-phase, etc. Back and forth, the dark matter is either helping the ordinary matter collapse, or hindering it from expanding.

In other words, when we look at the CMB, at the pattern of splotches of hot and cold spots on the sky, certain sized splotches are being helped by the dark matter, and certain sized splotches are being hindered by the dark matter. You will get more variations in temperature at certain scales, than you will at other scales, if there is such a thing as dark matter.

So this was a theoretical prediction that's made very cleanly and crisply ahead of time. People knew what to look for, so when WMAP, other satellites, and experiments, were looking at pictures of the CMB, they were able to then do statistics on what kind of patches existed. How big the different splotches were, how much variation you had in temperature from place to place.

The answer is that you need about five times as much dark matter, as there is ordinary matter, in order to explain the pattern and temperature variations in the CMB. That number, five times as much dark matter as ordinary matter, should be familiar to us. That's the number that comes out of taking the total amount of dark matter that we infer from dynamics, and comparing it to the total amount of ordinary matter we infer from Big Bang nucleosynthesis.

In other words, completely independently, from either individual galaxies, galaxy clusters, or Big Bang nucleosynthesis, the CMB is telling us that we need non-baryonic dark matter. The patterns of hot and cold spots in the CMB, would by themselves be enough to imply that we need dark matter in the universe. Yet it is very good for the standard cosmological model, that this implication from the CMB, matches implications we had from other ways.

Of course it would be even better if it didn't match! That would mean that we're learning something new, that we don't understand what is going on, that it's a clue to something else, that we haven't yet figured out. Yet it's nice to have a theory that works. What the CMB is telling us, is that the theory of dark matter, plus ordinary matter, works. That a universe in which 5% of the critical density is in ordinary matter, 25% in dark matter, is one that fits not only galaxies, clusters, and Big Bang nucleosynthesis, but also the CMB.

So that is a lot of information we're able to extract from these patterns of hot spots and cold spots on the CMB. Yet we're not done yet! The CMB turns out to be a treasure trove of information, and we're going to get a lot out of it. It is certainly evidence for dark matter, and we'll show in a later lecture that it is also evidence for dark energy. The CMB implies very strongly that the total density of the universe, is equal to the critical density.

So far we've been using the critical density, the density of stuff that you would need to make the universe spatially flat. That's just a convenient way to measure how much stuff there could be in the universe. It might be that the actual density is only 30%. The CMB is telling us that this is not right, that the actual density of stuff in the universe is one, in units of the critical density. The actual density we have is critical, and the universe is spatially flat. So in other words, not only is there dark matter, more than ordinary matter, but there is also something else. There is also something besides the ordinary matter we see, and the dark matter we don't see, and that is going to be dark energy.

That's one thing we get from the CMB, and another one is testing our theories of the early universe. In a later lecture, we'll talk about inflation, which is our best candidate theory for where the initial fluctuations and density came from. Inflation is very much a speculation at this point in our history, we don't know whether it's right or wrong. Yet it makes some predictions like how things should appear in the CMB, that have turned out to be true. Yet we're still trying to do better. We want to know not only the temperature of the CMB in all directions, but how inflation makes predictions for the polarization that we see in the CMB.

So we are by no means finished observing the CMB. We are building new satellites, like the ESA (European Space Agency) satellite called PLANCK that will be launched in a few years, to measure the CMB to unprecedented precision. We're hoping that as we study this relic radiation from the Big Bang, with greater and greater instruments, with higher and higher accuracy, we will continue to learn more surprising things about the universe.