 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss the formation of structure in the universe. So how did the structures that we see in the universe form? Now that is not talking about the small scale structures, things like planets and stars and galaxies that have been looked at in the past. We are talking about the overall large scale structure as to how clusters and superclusters of galaxies are distributed in the universe, and how that formed. So let's go ahead and get started here. And what we see is how did galaxies form? So first of all, looking a little bit at galaxies here, and what we saw, you know, how did the galaxies form? There are two different ways we can think about this. We can call it top-down or bottom-up. So a top-down method says that all gas formed the stars very early on in the history of the elliptical galaxy, and we see an example of one of those here. And then it was just slow evolution afterwards, minor changes. Or we could have bottom-up formation, which means that the smaller galaxies formed first and giant ellipticals formed through mergers. And when we look at the distant, and we look at very old galaxies, we see that they tended to be a lot smaller. So it could be either of these two models, or it could be a combination of the two. So we could put the two together, and there could be some combination of these that actually explains how galaxies might have formed. Now, let's look at spiral galaxies, and we can look at the same kind of thing. We can look at bottom-up formation, and they formed over a longer time period. And then they had a rapid collapse. So when we look at these, we look at a rapid collapse very early on. Here, steps one through three, where we had a hydrogen cloud collapsing under gravity and forming a basic spiral galaxy. But then there were mergers. So you could have a small galaxy orbiting around and over multiple orbits, it eventually becomes engulfed in this. And then this will change the size of, for example, the bulge of the galaxy, and will change the galaxy. So as an example, that can possibly explain the different classifications of spiral galaxies. As the spiral galaxies are classified by how large their bulge is, and that could tell us something about the number of mergers, perhaps, that number, maybe the number and type of mergers that they have gone through. We can also get evolution between types of galaxies. At one point, it was thought maybe that elliptical galaxies could turn into spirals. Not really possible, because where does the gas come from? So a spiral galaxy has cold gas and dust, and elliptical galaxy doesn't. So if we're going to do this, where did that gas come from? Now, could you turn spirals into an elliptical galaxy? Well, not all by themselves. How do you un-collapse a galaxy? Well, a galaxy sitting here by itself is not going to be able to un-collapse from a disk back into something more spherical. However, a massive collision could do this and could take two spirals and form them into an elliptical galaxy, which is what we believe could happen to the Milky Way and the Andromeda Galaxy in a few billion years. So let's look a little bit at the largest scale structures, which is what we really want to look at here. And what we see is that the large-scale structure that we see formed very quickly. It did not take a lot of time for it to form. It just took the first couple of billion years. And one of the problems with this is that this is not possible if we use only ordinary matter, only the matter that we are made up of. So that's the typical matter that we've looked at. There is no time. We would not have any such structures as we see here in this model. We would not be able to see in 14 billion years there would not have been enough time for it to form. So if we run models and we run them with just ordinary matter, this would take many times the age of the universe to get down to the stage where we see the structures that we see today. Now we can do this, however, if we use cold dark matter. So when we use cold dark matter, we can actually take material that was relatively uniformly spread out shortly after the Big Bang, watch it condense over time, and then eventually end up with the structures that we see today. That can be done in the 14 billion years that we have as the age of the universe. So we know things had to happen. So this is one of those things where we make an observation. We know that the universe was able to form these structures in 14 billion years. So if ordinary matter was not sufficient to do that, we need some other kind of model that will help update that and be able to demonstrate that, yes, it can happen. So how do we think this structure formed? Well, the initial universe was very smooth. Not quite perfectly smooth, but almost very uniformly spread out, and we see that in the background radiation today. However, early on, even in that time, when the universe was pretty smooth in ordinary matter, in dark matter was able to form clumps early on. So before the ordinary matter was able to form clumps, the dark matter was, these clumps had more gravity, and then gathered more ordinary matter and dark matter into them. Eventually, these elongated clumps would form the filamentary structures that we see in the universe today. So this is the way of doing it, is that the dark matter condensed first. So first the dark matter, and then first the dark matter was able to condense, and then the ordinary matter kind of gravitated into the dark matter. So then we got the ordinary... So then we got the ordinary matter condensing, condensing together. So the dark matter condensed first. The ordinary matter, the stuff that we've been studying, galaxies and stars, condensed second. So within these clumps and filaments that we see, structures began to form from the bottom up. So first large star clusters in small galaxies form, then black holes would form through many collisions, giving us the AGM that we've looked at, and the galaxies would then form into clusters and superclusters. But it is the dark matter that formed the basis of this, and the basis of the structure that we see, and this process is still ongoing. So ordinary matter is still being drawn into these filaments, condensing things, putting more material in a smaller portion of the universe over time. So when we look at clusters, how did clusters form then? Well, first we formed the small clouds, these would merge into galaxies, and then the galaxies would come together. So things formed in a scale. This is where we would form the galaxies, galaxies would then form here, and then those galaxies would then become clusters of galaxies. So things built up over time. And again, it comes down to that dark matter is what started all of this. If it were not for dark matter, we would not have the structures that we see in the universe today. There simply is not enough time for the small amount of ordinary matter, everything that we see in stars and galaxies, to have formed those large-scale structures, the filaments and the voids that we have looked at. So let's finish up here, as we do with our summary, and what we've looked at is that galaxies form through a combination of a top-down and a bottom-up process. And dark matter is required. We have to have dark matter to explain the structures of the universe we see today. There is no other way that they could have formed in such a short time of just 14 billion years. They would not have had time. And we can also reproduce these. We can reproduce the general features that we see using the cold dark matter. So if we put cold dark matter into the simulations, we are then able to reproduce in the proper amount of time the structures that we actually see in the universe today. So that concludes this lecture on the formation of the large-scale structure of 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.