So now, let's turn our attention to large-scale structure in the universe. And first, let's review some of the basic observations about large-scale structure, and how we did get to measure it. This is of course a numerical simulation, not the real universe. But it kind of shows what universe might be look like, visible galaxies embedded in this cosmic web of dark matter filaments. So the densities in the early universe serve as seeds for all of us. I will talk about how that happens. And so, they fall upon themselves using their own gravity, and they create smaller pieces. And that's how galaxies and clusters and everything form. So, on scale larger than galaxies, we talk about large-scale structure. And that could be just general field or groups and clusters and so on, but distinct from galaxies. And when you look at the sky, you don't see just kind of uniform web of matter. You see galaxies as very distinctive little island universes. And the reason for that is important. They actually form with an additional process atop of what drives formation of large-scale structures, which is pure gravity. So the way we map this out is we need to measure distances to galaxies that form large-scale structure. And we do this by measuring redshifts, the velocities of galaxies. You probably know that universe is expanding, and the recession velocity for galaxies is proportional to its distance. We'll go through that in more detail later. But the point is that if you can measure the distance that can be translated into the radial distance to the galaxy. And with the two coordinates in the sky, you kinda where it is. So in order to map three-dimensional structure in the universe, you need to measure distances. And large surveys that measure up to a couple million redshifts by now have been done, and we have a fairly good understanding of the morphology of large-scale structure. Now, cluster of galaxies is something that was very obvious and visible very early on. But amazingly, took long time until by 1980s to realize that actually galaxies are not uniformly randomly peppered through the universe with occasional blob like a cluster. But they actually form coherent structures of filaments and sheets and whatnot. So this is the 6,000 brightest galaxies in the sky. And the number was chosen because with the naked eye on a good sight, perfect vision, you can see 6,000 stars in the sky, well, in two hemispheres. And so this is plotted in the equatorial coordinates, which you remember is just outward projection of Earth's equatorial coordinate system. And so there is a dark band in the sky. Any guesses what it could be? It's called a zone of avoidance. Yes. >> Doesn't the Milky Way block it out? >> That's correct. It is dust of the Milky Way absorbing light from the galaxies outside. And as soon as people realize that there is such a thing as interstellar absorption became obvious why this is the case. They're of course galaxies behind it just as well. You will also notice that there are some interesting structures and features, and they are labeled here. But here they're shown in more detail. Now this is a different coordinate system. This is in a coordinate system galactic coordinates, so galaxy plane is at the equator. And this is galaxies from two mass, that's a two micron all sky survey near infrared, so it's easier to see through the dust. And some of the important structures are labelled. You can look at this later, but you can see that there is clearly non-uniform distribution. Now starting from smaller scales, there is the local group about which we spoke already, and it's our immediate neighborhood but 2 Mpc in size. Milky Way and Andromeda are two big galaxies and a whole lot of smaller stuff around them. And our local group is one of many, many other groups. In fact, most galaxies in the universe are in groups. And we belong to what we call the local supercluster, which is a bigger structure about 60 Mpc. And it's flat, well, it's six to one flattening ratio. And it's centered in the vertical cluster or galaxies about 15, 17 megaparsecs from us. And it includes some other core clusters and lot of other groups which are labeled here, and kind of indicated in this classic 3D plot. So, superclusters are the biggest structure we know about. And so far, a number of them have been mapped around us. But, in some sense, their boundaries are somewhat arbitrary. Once you get on the scale of hundreds of megaparsecs, you're really talking about describing the overall density field of the large-scale structure. So one of the first large lecture series was done for a Center for Astrophysics at Harvard Smithsonian. And they did it again. And since they couldn't observe enough galaxies to cover the whole sky, they used a new strategy. That instead of doing a full sphere, they just did cuts. They would do thin slices in Right Ascensions, sky turns, but very narrow in Declination. And that gives you a fair sample of what universe might be. You're losing one dimension, but it can extrapolate. It's all the same. And then notice immediately that there are all these interesting structures. They're centered in a rich cluster of coma. There is feature pointing at us, and those have been called fingers of God pointing at you. We now understand where they come from, but we also see there are all these filaments and voids. And because this is essentially 2D cut through a 3D structure, chances are that what you're looking really is more like a sponge like structure. A thin slice through sponge might look like this. So what we're measuring really is not the distance, it's redshift. And that is combination of the actual cosmological expansion velocity, which we could translate directly into distance. But also galaxies have their own velocities that have been caused by gravitational infall. And so in the real space, you say if you have a dense cluster of galaxies. Galaxies in it will have velocity dispersion kinetic energy and random motions just to balance against the gravitational potential of this massive cluster. You're not observing sideway motion that's much to see. But you can see the radial component. So that radial component adds to the overall cosmological expansion velocity. And so instead of seeing a round blob in the redshift space, you see it highly elongated, elongated by the dispersion of galaxies along the line of site. And in a thinner and less dense structure, what's happening is the galaxy is still falling in. So galaxies on the far side of it are coming towards you. Galaxies on the near side pulling a little extra away from you. That kind of piles them up at the distance of this filament, and so the filaments look thinner than they really are. So both effects play some role, and I can model them exactly. So the field has been really transformed by very large redshift surveys. There were two of them, the first one is called a 2 degree Field redshift survey. It's named after the instrument, that Anglo-Australian telescope in Australia. And they measured quarter million galaxies, not over the entire sky, plus quasars and whatnot. And that was surpassed by the mother of all surveys, the Sloan Digital Sky Survey, which surveyed about third of the sky and measured redshifts close to million sources by now. So this is what slice cut through 2dF survey looks like. Now, this is going much deeper than previous, older CFA redshift survey. You can actually see labelled on the cone how far it goes, both in terms of redshift and billions of light-years. So this is obviously plot for public consumption because astronomers would never use light-years, they use megaparsecs. And again, you see the same structure goes on and on. No voids and filaments and dense concentrations and so on. But you don't see structures that are bigger than about 100 megaparsecs. And that's about it. Beyond that is pretty much uniform areas. The same kind of thing was observed Sloan Digital Sky Survey going a little deeper, and again, this is projecting their redshift volume onto two slices in the sky. So there is some smearing. And again, see the same kind of pattern of voids that are typically of the order of tens of megaparsecs in size. And measure filaments connecting and ending in clusters of galaxies. Now to go deeper, there isn't enough telescope time in the world. So instead of that, people do what, say, census studies do. We don't need to ask every American about something. You can only ask representative sample, a few thousand say. And so the equivalent of that is doing a deep, deep redshift survey in a narrow patch of the sky. Those are called pencil beam surveys, or sort of a drill thin cone all the way out through the large redshifts. And the idea there was to probe galaxy evolution, but turns out also to be constraining structural evolution. And now this has been done, again, on industrial scale, couple of hundred thousand faint galaxy redshifts have been seen. And lo and behold, we see the same kind of structure all the way out. So these are two little slices through one of the surveys of Penn of Keck telescope. And, they're exaggerated in thickness because it's a really narrow beam compared to the depth. But you can see that it's intersecting same kind of structures, sheets, filaments, voids, and so on. And if you plot the density redshift distribution, which is the distance distribution, see these spikes. Every time you cross one of those filaments, you see excess number of galaxies. So it's just like nearby, and this is interesting in and of itself.
