 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to discuss the expanding universe, and we'll look at how we discovered this and what it actually means. So, let's go ahead and get started. And first of all, how do we discover the expansion of the universe? Well, this actually goes back into the early 1900s, and one of the people studying this was Vesto Sliffer. And what he was doing was to study actually looking for planetary systems. So, he was collecting spectra of the spiral nebulae. Now, he was working at Lowell Observatory, which is known, which is the area where Pluto was discovered in 1930. And he was looking for planetary systems, that something that the Lowell Observatory was concentrating on, looking for planets. And he was obtaining the spectra of these, and what he found is that most of them exhibited large red shifts. So, what he'd see is this is the pattern you should see for the spectral lines, and the patterns were all shifted. Everything was shifted to the red. And, as you may recall from the Doppler effect, this means that the galaxies are moving away from us. So, he found that all of these galaxies were moving away from us. And, George's Lamatra in 1927 published a paper suggesting an expanding universe, which was something, interestingly enough, consistent with general relativity, although Einstein had added in a cosmological constant to make it so that the universe did not necessarily expand, but it was something that general relativity could have predicted. And he used Sliffer's observations as his evidence. So, this was some of the very earliest evidence of the expansion of the universe. Now, also sometimes known for talking about the expansion of the universe was Edwin Hubble. And Hubble, in 1931, published a paper relating velocity of recession, so how fast a galaxy was receding, to the distance of the galaxy. And, what he published, his original data from 1929 is shown here, and that's only this very small portion for very nearby galaxies, only going out to a few million light-years. In 1931, he had added more data, and now going out to a hundred light-years, and was able to see a very distinct relationship between the velocity here on the y-axis and the distance on the x-axis. Now, this was very important because it becomes another way to determine the distance. The velocity is very easy to measure, and once you measure the velocity, you can run across and down and find the distance using this graph. So, the better we can determine this line, this becomes a new method of determining distances in astronomy and can work out to the very edge of the universe. And he gave us what is now known as Hubble's law, and Hubble's law simply says that the velocity is equal to a constant times the distance, meaning that if you wanted to find the distance, you just take that distance is equal to the velocity that you measure divided by Hubble's constant. And that was, again, would be a way to determine distances to the galaxies. So, first thing we have to do is understand a little bit about what Hubble's constant means. And what it is, Hubble's constant is actually the slope of the line that we saw in the previous slide. If we can figure that out, we can then use this to determine the distance to any galaxy that we can get a spectrum of, and that is beyond our local region of space. Our current estimate of Hubble's constant is about 22 kilometers per second per million light-years. Very strange units, not something that we're used to. What does this mean? This means that a galaxy at a million light-years away would be receding at a speed of 22 kilometers per second. A galaxy at 10 million light-years away, 10 times further away, would be 10 times this. 10 times 22 kilometers per second, or 220 kilometers per second for its recessional velocity. This is, again, just the current estimate, and it is still subject to revision, but over the last few decades we have been able to narrow down this value more and more with more detailed measurements. How about some variations? How constant is Hubble's constant? Does Hubble's constant vary? And one of the questions can be, is Hubble's constant really a constant? And what we have to remember is that when we look out in space, we look back in time. So what we mean now is that if we are using this to determine distances, which is what we want to do, it means that we have to make the assumption that Hubble's constant that we're using was exactly the same 10 billion years ago as it is today. If Hubble's constant has changed over that time, then this equation will not work. So what kind of things might cause this change? Well, gravity is one example. We remember that galaxies are pulling on each other, which should slow down the expansion rate of the universe. If gravity were the only force involved. So that means that galaxies should have been moving faster in the past and slower now. So that means that Hubble's constant would have changed just because of gravity. Now there's a lot more going into this, but just to keep that in mind that Hubble's constant may not really be a constant. So what do we mean by the expanding universe? Let's take a look at this here. And what we mean by a uniformly expanding universe, first of all, to start out, what this means is that what it means for expansion is we come back to what we call the Copernican principle. And we give Copernicus credit for this because he was the one who moved the Earth away from the center of the universe. And what the Copernican principle means is that we are not at any special point in the universe. And when we look at this, it can be a concern when we look at all these galaxies receding from us. Does that mean we are at the center? And that's actually not true. Every observer would see exactly the same expansion. So it does not matter which galaxy you are on. The galaxy here sees the other three galaxies in this image receding away from it. But if you were on this other galaxy, instead, you would see the same thing. You would see all the other galaxies receding from you. So what it means is the galaxy, the universe, has no center. So there is no center to the universe. Everyone will see exactly the same expansion. Let's look at a couple of examples to try to understand this a little bit better. We can look at an example of the expansion here. Let's look at a one-dimensional example, first of all. So a one-dimensional example we can think of as a ruler. And that's just because it's very simple. We look at an ant on a ruler. And if we look at the 2 centimeter mark, so our ant right here, we'll see all the other ants moving away and the more distant ants moving away faster. So this one, the ant here at the 4 mark, there's our ant at 2 and there's the ant at 4. This one expands at 2 centimeters per minute. So now it's gotten larger. So while this ruler still looks the same, now the centimeters are actually twice as large. So he has moved away at a distance of 2 centimeters per minute. The ant here at the 7 centimeter mark has now expanded at 5 centimeters per minute and the ant at the 12 centimeter mark has gone at 10 centimeters per minute. So, but it does not matter who you look at. If you pick out this ant, this ant still sees everyone else expanding away from him. So it does not matter where you happen to be located. Each of the ants is going to see the exact same thing, is going to see every other ant moving away from them and is going to see the most distant ants moving away faster. Now that's a one dimensional example. Just looking at one, let's go ahead and look at another example which would be a three dimensional example. And this could be what we call raisin bread. So baking some bread with raisins in it. And if you imagine as the bread rises, the raisins will then spread apart. And what we mean is that an observer on any of these raisins would see every other raisin moving further away and the more distant ones moving away faster. So this one here, we look here at the central raisin that we pick out and that's now over here as it expands. This one that as it expanded, this one moved from three centimeters to six centimeters. So it moved three centimeters away. This one that was way over here at 20 centimeters away is now 40 centimeters away. So while the universe expanded, things changed by greater distances. What was close to you is only a little bit further away. What was further away is even further away from you. So the expansion is the same regardless. And once again, it does not matter which raisin you pick. We picked one here, we could have picked another just as easily. But the view would be exactly the same and that's what happens to us. Within our galaxy, we see every other galaxy moving away from us. But if we imagine an astronomer in a distant galaxy looking at all of the other galaxies, they will see exactly the same thing. They will see our galaxy moving away from them and all of the other galaxies. So no matter where the observer is, they're going to see exactly the same thing. So what does this mean for the expansion? If we know this, what does it mean? What is really expanding is the space. The galaxies are not getting bigger. Galaxies do not get any larger. The size of the galaxy stays exactly the same as seen in the image here. The size of the galaxy is still the same. Galaxies are gravitationally bound together, so they do not take part of the expansion. What really expands is the space between the galaxy clusters. It's not the galaxies, not the clusters themselves, nothing that is gravitationally bound together. For example, within our solar system, the planets are not getting further apart by expansion. They are all bound to the sun by gravity. But when we look at the very largest clusters of galaxies and superclusters of galaxies, those are getting further and further apart. Now, one of the problems here is that in reality, there is no edge or center to the universe. So we do not have an edge, we do not have a center. In our examples, that doesn't really work out. If we look at our examples, we did have an edge to the raisin bread. And you could imagine that someone at the very edge, at the edge of the loaf, would see things a little bit differently because you'd be looking out into emptiness or into something else. Now, in terms of our universe, that's not how it works. So our universe would not have an edge or a center, but would actually involve an additional dimension. So you can imagine that the universe, we have a three-dimensional universe expanding into a fourth dimension. And that means that we're never going to see any kind of edge or any kind of center to it. Not in any dimension that we as three-dimensional creatures can actually observe. So let's finish up here. Space itself is expanding. It is the space that is expanding and is carrying these galaxies and galaxy clusters apart as it expands. So let's finish up here as we do with our summary. And what we've looked at in this section, first of all, the expansion of the universe is something that we've seen for the last hundred years. So we're able to get the very earliest detections of that. Edwin Hubble gave us a formalized version of his Hubble's law that related the velocity of an object to its distance. So giving us another way to be able to determine distances and one that worked out to the edge of the universe. And then to emphasize the expansion is an expansion of space, not a direct motion of galaxies. It is only the space between the galaxy clusters that is actually expanding. So that concludes this lecture on the expanding 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.