 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to look at formation of the structure in the universe. So we're going to look a little bit about the largest scale structures in the universe. So let's go back and start looking at galaxy formation. How do galaxies form? Do they form top down or bottom up? What does that mean? Top down means that all the gas formed stars early on and nothing much happened. That it was just a very slow evolution. So essentially a great burst of star formation billions of years ago. And then the stars just go through their lives. All the gas is used up so no further stars would form. A bottom up formation means that smaller galaxies formed first and then giant ellipticals formed through mergers of these smaller galaxies. So remember we know that mergers are important. So is it this or is there some combination of these two models that put together to decide how ellipticals form? How about spiral galaxies? Well spiral galaxies we look at bottom up formation because it's over a longer period of time. There is a rapid collapse. Things collapse down quickly forming the basic spiral galaxy. And then mergers would then add to the galaxy. So galactic cannibalization as they cannibalize smaller galaxies would allow that. So we have the very rapid collapse from a hydrogen cloud which collapses under gravity. And then the ancient stars dominate. And then the spiral arms can be built by companion galaxies can be increased by the gravitational interactions which will then cause more star formation and highlight those spiral arms. Can you get an evolution between spiral and elliptical galaxies? Well this is one thing we looked at when we talked about the Hubble classification and could ellipticals turn into spirals? This is not possible. There is no way an elliptical is going to gain gas and dust to be able to start forming stars. However you can go the other way. In fact a massive collision of two spiral galaxies could form an elliptical galaxy and this is perhaps what will happen with our own Milky Way and the Andromeda galaxy when they collide in several billion years that we could actually form an elliptical galaxy as now part of our local group. Now this is the smaller scale for just looking at galaxies. How about looking at this on the larger scales? How do we form the large scale structure? Well we know that this large scale structure formed fast. It just took the first few billion years and this cannot be done. It is absolutely not possible using only ordinary matter. Just look at the material we see in stars and galaxies and gas clouds and from radio emission. All of that material and we let the simulations run under gravity. They will form structure but it will take a much longer time than possible. There has not yet been enough time for such structures such as the filaments and voids that we see today to have formed. Well we need something else. We need some other type of material and that's what we've looked at is the cold dark matter. If we have significant amounts far more than we see in the visible matter these can reproduce the structures that we see in the universe today. How do we go about forming those structures? So let's take a look. We form structures. Initial universe was almost completely smooth. If it was completely smooth we wouldn't have anything but since it was not quite perfect dark matter was able to clump. So dark matter because it does not interact with light with energy with other matter except through gravity it was able to form clumps. While ordinary matter was still being affected by the radiation, the electromagnetic radiation the dark matter does not interact through electromagnetic radiation. So its gravity was able to start it clumping. First this gives us a jump start on forming the structures. So the dark matter started forming clumps and these clumps had more gravity and gathered the ordinary matter. And these clumps then evolved into the filamentary structures that we see today in the universe. So it was because of the dark matter that was able to collapse before that gives us the structures that we see and allows us to actually be here seeing the clusters that have actually formed. As these clumps form the filaments structures begin to form from the bottom up. So first you form large star clusters in small galaxies. This is what we see when we look at the very early universe. Remember we can look back and see what galaxies looked like shortly after the formation of the universe. They are not the large spiral and elliptical galaxies that we see today. They are large star clusters, small galaxies. Then black holes would form at the center and they grew through many collisions and that gave us the active galaxies that we looked at. Those galaxies then grouped into clusters and then into super clusters and finally into filaments and voids that we see. This is an ongoing process. This still is going on today. This continues. So let's look a little bit here. One more section on this and let's look at this. So how do we form a cluster? Well first the small clouds formed and these merged to form the galaxies. Galaxies then came together into clusters and then finally super clusters eventually giving us the filaments and the voids that we see today. So small clouds here formed together to form various galaxies. So we form a couple different galaxies whether it be spiral elliptical depending on how those clouds combined. And then those galaxies could then merge together to form larger galaxies and then eventually the galaxies would group into clusters and finally into filaments giving us the structures that we see today in our universe. So let's go ahead and finish up this section with our summary and what we've looked at is galaxies form through a combination of top down and bottom up processes. Dark matter is needed to explain how the structures that we see in the universe today. These are things we actually see could have formed in such a short time. And the large scale structures that we see can be reproduced using simulations that include very significant amounts of cold dark matter. So that concludes this lecture on formation of structure in the universe. 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.