 Greetings and welcome to the introduction to astronomy. In this lecture we are going to talk about dark matter, and that is some of the unseen matter that makes up a lot of the material in the universe. In fact what we see in terms of stars and galaxies and nebulae and planets and everything that we normally study in astronomy is found to only be a very tiny portion of the amount of matter in the universe. Dark matter is just that, it is completely dark, and we cannot see anything of it except for its gravitational effect, so it gives off no specific types of electromagnetic radiation that we can see. So how do we know that it exists if we can't see it? Well, what we can do is make some various kinds of measurements to try to better understand dark matter. So normally we look at what we call ordinary matter. That is the stuff that you and I are made up of as are all the galaxies and stars and planets. This gives off electromagnetic energy, and if you remember that includes things like visible light, and it also includes the rest of the electromagnetic spectrum whether it be x-rays and gamma rays or radio waves. But dark matter is different, and that turns out to be about 90% of the mass of many galaxies is located in a dark matter halo. How can we see that that exists? Well we look at galaxies like this and we measure the rotation of their stars and the gas that we see in them. So as we move out we watch and we see what the rotation, what the velocity of those stars is, and we expect that it should increase as you start from the interior and work out or it should increase, but then we expect when you get out here towards the edge of the galaxy where you're not seeing anything else beyond it, it should expect to decline, and this is what we see within our solar system. The objects further away from the sun, once they are outside all of the mass in the solar system, which is essentially all of the planets, then the velocities decrease as you get further away. What we observe from stars and galaxies is that it continues to increase out to the edge of the visible galaxy, and then even looking at hydrogen clouds from 21 centimeter lines, it still continues to increase. There is no sign that it is decreasing down to the level that we would expect, and what that means is that there must be a lot more matter in the galaxy in order to explain this rotation curve. The only way we can explain that is through a lot more matter within the galaxy. And we see this in most galaxies, in most or all galaxies. So what can we learn about dark matter in galaxies, and how can we use this to determine clusters? How about in clusters of galaxies? That was just in one galaxy. We can use the motions of the galaxy. There must be enough mass within the galaxy cluster to keep the galaxies from escaping. Otherwise, things like clusters and superclusters would not remain. They would disappear over time. So we would not be able to see galaxy clusters. Galaxies would just be spread out all over the universe. So we can use their motions and doing that we can then estimate what the dark matter distribution has to be like in order to keep the clusters together, to keep them the galaxies from spreading apart. Because if we go just by the galaxies that we see themselves and add up their mass, there is nowhere near enough mass. Those galaxies would eventually and very quickly on astronomical time frames spread out over the universe. Now another way we can see this is through gravitational lensing. And what we see is that gravitational lens is a bending of light due to the predictions of Einstein's relativity. And the cluster gravitational field will bend the light from more distant galaxies. So distant galaxies behind this will give us multiple and distorted images. So we see some here and here and all these images have been distorted by the light not only of one big galaxy here but of all the other galaxies around it plus any dark matter that is present. So the dark matter, while it does not contribute to the light of the galaxy cluster at all, does contribute to its mass and will cause the bending to be more significant. We can use Einstein's model of gravity to be able to explain, figure out how much mass there must be to explain the distortions that we see. And when we do that we find out that there is many times the amount of matter that we need that must be present that we simply cannot see. So what about this dark matter and how much of it is there? Well we can take a look at that and see that if we want to go back and recall we had looked at the thing called a mass to light ratio and if you recall the sun had a mass to light ratio of one. It just meant that it had a mass of one solar mass and a luminosity of one solar luminosity. Galaxies are showing a mass to light ratio of ten, meaning that they have ten times the amount of mass for each luminosity unit. So there is a significant amount of dark matter. As we get further and further up into small clusters and then into large clusters the mass to light ratio increases. And what that tells us as we see the increase here in the table is that there must be substantial amounts of dark matter present in these clusters, otherwise we would not get these kinds of large number. 300 is an incredibly large mass to light ratio and that's what we see for some of these really big galaxy clusters. So not only are there a lot of galaxies there, but for each galaxy there can be ten or a hundred or a couple of hundred galaxies worth of matter that is completely invisible and that's what we mean by dark matter. It's not just a few extra dim stars or a few black holes. It means that for each galaxy that you see in an image you have to imagine that there can be even a hundred galaxies worth of matter scattered around and is in some form that we cannot see. And what could it be? What could this dark matter be composed of? Well, there are a couple different possibilities that we talk about and those are the machos and the wimps. The machos are massive compact halo objects and that is essentially ordinary matter. Things like black holes, brown dwarfs, white dwarfs, these are all things that are very hard to see and would be incredibly faint. So we wouldn't see them directly but we would see their gravitational effects. We would see the effect of their gravity. Measurements have been done and show that there are simply not enough of these. So there are not enough of them to account for the amount of dark matter needed in the halo of our galaxy, meaning that if they don't work for our galaxy they're probably not going to work for other galaxies as well. The wimps on the other hand are weakly interacting massive particles. These are exotic particles that do not emit electromagnetic radiation. So these are unusual subatomic particles, not things like protons, neutrons and electrons that make up ordinary matter, but other more exotic types. And those are what we think may make up the dark matter now because it does not seem like any type of ordinary matter can possibly work. Now there are two, we can look at dark matter in two models and those are hot and cold dark matter and if we remember hot and cold temperatures refers to the speed of the particles. Hot dark matter would be things that are moving very quickly. Cold dark matter would be things that are moving slowly. Now hot dark matter would not do a whole lot because it would smear out the clumps and would inhibit the growth of clusters of galaxies. On the other hand, cold dark matter would not be moving very quickly and it would grow clumps and give us something consistent with our current observations. The model here running time forward as we go across and then down is a cold dark matter simulation and it starts off with the material relatively uniformly spread out and over time it condenses down and gives us a structure relatively similar to what we see today. So these simulations can give us things that match the structure of our universe. Again, not precisely, it's not going to match up exactly where the galaxies are, but the overall pattern is what we are looking for. Now the other thing that we have and that we want to mention is what we call dark energy. Dark energy is another mysterious substance quite different than dark matter. Now dark matter as we said makes up the majority of the matter of the universe far more than the than any of the ordinary matter that we see. So this chart kind of breaks it down. What is the universe made up of? Well, heavy elements that make up the earth are about three one hundredths of a percent. Things like stars are about half of a percent of the mass and energy of the universe. Hydrogen and helium get us up to about four percent. So about four to four and a half percent of the material that we see is in ordinary, what we would call ordinary material, everything we study in an astronomy class. That would be the nebulae, that would be stars, that would be galaxies, that would all be included in these three here. Everything else is this dark matter which we have been looking at making up 25% vastly outnumbering by five times the amount of ordinary matter. And even more so is this dark energy. And dark energy we will talk about in a coming chapter. But dark energy is another mysterious substance that seems to permeate the universe and actually makes up most of the mass energy of the universe. Now as a comparison what we can look at here is if we imagine one gram of luminous ordinary matter, that's the ordinary stuff that we see every day. For every one gram of that there would be four grams of non-luminous ordinary matter. So that would be things that we can't see. Hydrogen gas that would not be glowing, things that would not be glowing. Most of the material in the universe is invisible to us. But for those five grams of what we call ordinary matter, there would be 27 grams of dark matter and 68 grams of dark energy. So in reality only about 4 to 5% of the universe is the ordinary matter. That is the stuff that makes up everything with us here on Earth, that makes up all of the stars and the nebulae and the galaxies and everything that we've studied in astronomy to this point. That's only about 4 to 5% of the universe. And the rest of it is in this mysterious dark matter which we've looked at in this lesson and in dark energy which we will come and look at coming up. So let's finish up this lesson on dark energy with our summary. What we find and have talked about is that we know that there are vast amounts of dark matter in galaxies and in clusters of galaxies. The dark matter is likely composed of cold, meaning slowly moving exotic particles, what we call cold dark matter. And as we looked at at the end, luminous ordinary matter, again, stars, galaxies, planets makes up only a tiny fraction of the mass of the universe. So that concludes our lecture on dark matter. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.