 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about dark matter and how we can detect dark matter not just in galaxies but in clusters of galaxies on the larger scales of the universe. So what is dark matter? Well, we're still trying to figure that out, but let's look at what we see here. Normally we look in astronomy at what we call ordinary matter, that's what we've been studying essentially in this class. This gives off electromagnetic energy. So it may give off radio waves or x-rays or visible light, and that's things that we've studied like the galaxies, the stars, the planets, etc., all the other objects that we have studied in this class. And we've looked previously at dark matter in galaxies, and we find that 90% of the mass of many galaxies is in a dark matter halo. So a lot of that material is unseen. We saw this in rotation curves of galaxies, and it exists in all galaxies. As you may recall, what we expect is the rotation curve to look something like the lower line here. However, what we find as we look further and further out past the visible edge of the galaxy is that the velocities continue to increase, and by under gravity that would mean there has to be a lot more mass out there than what we can see. So let's look now into clusters. How can we determine the mass of galaxy clusters? Well, there are a couple of ways that we can do this. We can use the motions of the galaxies. What we mean is that we see the galaxies together. That means there must be enough mass to keep the galaxies from escaping from the cluster. The galaxies are there, and they've been there for a long time, so there must be enough mass there to bind that galaxy cluster together. And we can see that in looking at the distribution of material needed in the images here in order to keep these galaxy clusters confined together. And again, it is a lot more material than what we see in visible light. Now we could also see this through gravitational lensing. The gravitational field of a cluster bends the light of distant galaxies. We've looked at this previously. We will see multiple and distorted images of the galaxies. So you can see all of the different distorted regions here, which are all different lensed galaxies from far beyond the galaxy cluster that we see. Now the amount of bending depends on the mass of the cluster. So we can use that. How much bending is there to figure out how much material must be present to account for that bending? And there is far more than what we can see in the mass of the galaxies themselves. So while there is dark matter within galaxies, there is even far more within the clusters of galaxies that we see. And that's a lot of material of a type that we cannot understand. So when we look at the distribution of dark matter, we see that the dark matter clumps together, so it does behave under gravity just like ordinary matter. The difference is it does not give off any type of electromagnetic radiation. This clumping gives us the voids and the filaments that we see, and then the ordinary matter clumps along the dark matter. So the dark matter forms the gravitational structures of the universe, and then the ordinary matter clumps along with that, making it visible where that dark matter is. Remember, the dark matter otherwise is completely invisible to us as it does not give off any type of electromagnetic radiation. How much dark matter is there? Well, if you recall the mass to light ratio we talked about, for the sun it is defined to be 1. In many galaxies we see values of around 10, that's for like our Milky Way. For small clusters of galaxies it's 100, for large clusters it's 300. Note how it's increasing. These large ratios means there are substantial amounts of dark matter present, and that has to be there, and we can see how this increases as we go from just the material around us to material in these very big clusters of galaxies. The question is, and we try to figure out, what is this dark matter made up of? Well, there are two things that are looked at, and those are the machos and the wimps. Machos stands for Massive Compact Halo Objects. So those are things like ordinary material. Things like black holes, which would be hard to see, brown dwarfs, white dwarfs, these would be very hard to see out in the halo of the galaxy. They're not directly visible, but we could see their gravitational effects on other material. However, measurements show that there cannot be enough of these to account for the dark matter needed in the halo of our galaxy. There has to be far more material than can be accounted for by any type of ordinary matter. So the other one we look at is wimps. Wimps are weakly interacting massive particles. And these are exotic particles, things that do not emit electromagnetic radiation. One we've talked about previously is the neutrino when we talked about the sun, and how the sun produces its energy. The neutrino just passes right through everything and is not affected by other ordinary matter particles. So could there be some exotic particle like this? Like a neutrino, but that has far more mass in order to explain the amount of dark matter that is present. Now the other thing that you'll sometimes hear about when we look at dark matter is hot and cold dark matter. Hot and cold simply refer to the speeds. How fast are those particles removing? Remember, that's what we define temperature as. Temperature is how fast things are moving. So hot dark matter would smear out clumps and would keep clusters and filaments from growing. Cold dark matter would grow clumps of matter. This is most consistent with current observations. So what we think would be some kind of cold dark matter slowly moving particles and sometimes of exotic particle, example of one of the wimps that would most likely account for the dark matter. Simulations using cold dark matter match the overall structure of the universe. If we leave dark matter out of the simulations, we cannot form the structures that we see in the age of the universe. However, models such as this one starting very far back close to the Big Bang and allowing those to form the dark matter condenses out first and then the visible matter clumps along those dark matter filaments. So we can get structures that look then today like what we see the universe. At least the overall properties look the same. So we tend to think that dark matter is going to be some kind of exotic particle, some kind of exotic cold dark matter. Now one other thing to look at here is not only dark matter but we have dark energy as well. We know that dark matter makes up the vast majority of the matter in the universe. But dark energy is even more prevalent. We look at for every one gram of luminous ordinary matter, there are four grams of non-luminous ordinary matter. That's just hydrogen and helium that are not glowing. There are 27 grams of dark matter and 68 grams of dark energy. So for every one, and you use grams here, you can use whatever units you like, there is a lot more stuff that we cannot see. And what it tells us is that only about 4 to 5% of the universe is ordinary matter. The things that we have been studying over the course of these lectures account for only about 4 to 5% of the material in the universe. So we will come back and look at dark energy a little bit more in another lecture. So let's go ahead and finish up with our summary. And what we've looked at this time is that there are vast amounts of dark matter in both galaxies and clusters of galaxies. The dark matter is believed to be composed of cold, meaning slowly moving, exotic particles, what we call cold dark matter. And the luminous ordinary matter, the stuff we've been studying, is only a tiny fraction of the mass of the universe. So that concludes this 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.