 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the expanding universe and how we discovered that and what kind of measurements we can make. So how did we discover the expansion of the universe? Well, this goes back to the early 1900s with a gentleman named Vesto Sliefer of the Lowell Observatory. What he was doing was looking for planetary systems around the stars, so he was looking at the spiral nebulae thinking perhaps that they were planetary systems in the process of formation. Remember that until the mid 1920s we did not know that the spiral nebulae were actually distant galaxies and found unusually that they all exhibited large redshifts. And then following that in 1927, Georges Lemaitre published a paper suggesting an expanding universe. Now this is consistent with general relativity although general relativity was originally set up for a static universe which is the way it was set what was believed at the time, you could easily explain an expanding universe under general relativity. And he used his evidence Sliefer's observations. And this then leads us to Edwin Hubble who in 1931 published a paper relating the velocity of recession to the distance of a galaxy. And his original data here shown in red were first relatively nearby galaxies going only out to about 6 million light years. And then later data here going out in 1931 going out to nearly 100 million light years. And they found that there's a very direct correlation between the velocity of recession and the distance of the galaxy. And that is what is given by Hubble's law which says that V equals H times D where V is the velocity of recession, D the distance and H is Hubble's constant. Hubble's constant would therefore be the slope of this line. And once we could determine that accurately that allows us to find distances when we measure velocity. So we measure the velocity and we use Hubble's law to then calculate the distance. Now how does that work? Well if we can determine Hubble's constant, which as I said is the slope of the line, then we can use this to determine the distance to any galaxy. So it doesn't have to be any specific type of galaxy, we don't have to wait for a supernova. As long as we can get a spectrum of the galaxy and it is beyond our local region of space. There are some very nearby galaxies that move that have other motions, random motions that overwhelm the Hubble velocity. So we have to go out beyond that but out beyond a couple million light years we can start to see this trend. Now we can currently estimate that Hubble's constant is 22 kilometers per second per million light years. What does that mean? Well that means that a galaxy one million light years away would be receding at a speed of 22 kilometers per second. So it's 22 kilometers per second for every million light years the galaxy is away from us. A galaxy 10 million light years away we'd multiply this by 10, so 10 times 22 or 220 kilometers per second at which it was receiving. The farther away it is the faster it is receding. Now then the question comes up is Hubble's constant truly a constant? And we have to remember that looking out in space is looking back in time. So if we use this to determine distances that means we're assuming that Hubble's constant is exactly the same 10 billion years ago as it is today. And is that correct? Well what we might expect is what is shown in the little animation here we consider gravity. Well every galaxy pulls on each other and this would slow down the universal expansion rate and that's what we see here in this animation. The initial explosion goes and everything slows down and goes slower and slower. Now it doesn't mean it will stop it just may continue to expand at a slower rate. This is true if gravity is the only force involved. What it means then is that galaxies should have been moving faster in the past and should be moving slower now. That's if gravity is just the case and we'll find out what is the case later on. So a uniformly expanding universe. What does this mean for the expansion? Well it really means we go back to what was called the Copernican principle and if you recall Copernicus was the first modern astronomer to suggest that the Sun was the center of the universe rather than Earth moving Earth away from its special point in the center of the solar system. So we find that we are not at any special point in the universe and that means that there are no actually no special points within the universe as things expand. It doesn't mean that we are at the center of the universe. We see all galaxies receding from us so you might think well we're at the center of the universe but that's not true. Every observer will see the exact same expansion no matter where they are in the universe. So we are not at the center of the solar system or the center of the galaxy which we later thought we might be close to and we are not at the center of the universe. So everyone will see this exact same thing. How about some examples of this? So what can we look at this? Well let's look at it in a one dimensional example first. We have an ant on a ruler. Well the ant at the 2 centimeter mark will see all the other ants moving away and the more distant ants are moving away faster. So the distance here has increased less than the distance between the two more distant ones. So as this expands they are receding at different rates. So the furthest ant is receiving in this case at 10 centimeters per minute. The intermediate ant here at 5 centimeters per minute and the closest ant at 2 centimeters per minute. That's very similar to what we have in the universe. However again they're not at anything special. At the 7 centimeter mark if you measured this ant he would see exactly the same thing. So an observer any of these ants would see exactly the same expansion regardless of where they are placed on the ruler. Now we can look at a three dimensional example of this as well and that would be a piece of raisin bread here. So if you start off with the uncooked one here the raisins are certain distances apart. So we pick one raisin here and we have a raisin at 3 centimeters, one at 7, one at 16, and one at 20. And if we look at this then this one has gone to 6 centimeters, this one to 14, this one to 32, and this one to 40 centimeters. So they've all increased and the ones that are further away are increasing at a faster rate. They have increased 20 centimeters, this one has gone from 20 centimeters to 40 centimeters, went from only 3 centimeters to 6 centimeters. And again it does not matter which one you see. An observer on any raisin would see every other raisin moving farther away. And the more distant ones moving faster. Now these aren't the perfect examples because in reality in our examples well there is no edge or center to the universe. So those are two things that don't apply when we talk about the universe. So whereas you could imagine an edge to the ruler or an edge to the piece of raisin bread this is not the case in reality. What it is is that it is the space between the galaxies that is expanding. Not the galaxies or even the clusters of galaxies themselves. Under the expansion galaxies do not get larger. Planets are not getting any farther apart. They remain the same. They are bound by gravity which overwhelms any Hubble expansion on the local level. It is space itself that is expanding in between the large groups of galaxies and the galaxies are carried apart as it expands. So it is not anything that we see when we study galaxies they are not getting any bigger. Planets do not get any bigger. Galaxy clusters are not getting any bigger. They are just expanding and they are being pulled along as space expands outward. So let's go ahead and finish up with our summary and what we've looked at is the expansion of the universe we first looked at this over a hundred years ago now. Then Edwin Hubble formalized this into Hubble's law which relates the velocity of an object to its distance and we looked at the expansion as an expansion of a space it is not a direct motion of galaxies it is the space between the galaxy clusters that is expanding and the galaxies are dragged along with this expanding space 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.