 Greetings and welcome to the introduction to astronomy. In this video we are going to talk about a model of the universe and how we can put together some of our understanding as to how the universe has formed. So let's go ahead and get started here. And what we have is, first of all, let's look at the age of the universe. How do we figure out how old the universe is? Well, one of the easiest ways to try to get an estimate of this is to consider an expanding universe. What happens, what has happened to a universe that was expanding now at, for example, we're going to say a constant speed, in the distant past? Well, if it's expanding, that means that you can imagine that layers that are out here now had to have been in closer and in closer and closer. And eventually you can trace everything back down to a single point and that everything is compressed. And that would mean that all matter, everything that we have, and all energy, compressed into a very tiny volume. And at the beginning of the expansion, we can then use this to determine the Big Bang. And the Big Bang, we can calculate when that occurred based on these assumptions. Now, note that these assumptions are not completely correct. For example, constant speed is not necessarily a good assumption. But let's go ahead and start with that and we can use Hubble's law. Now, we know that if we're trying to figure out how long something took, how long it took to get someplace, we take the distance that we traveled, divide it by our velocity, and that will tell us how long it took to get there. And so if we traveled, for example, 500 miles at 50 miles an hour, we could take the 500, divide it by the 50, and find out that it took us 10 hours. Well, we can do the same thing with the universe. So time is equal to distance divided by velocity, but we know from Hubble's law that velocity is equal to Hubble's constant times the distance. Distance divided by distance cancel, and that leaves us that the amount of time is just equal to 1 divided by Hubble's constant. So if you take Hubble's, take 1 and divide it by Hubble's constant, it gives you the age of the universe. Now, there are some conversions that have to go through there because we measure Hubble's constant in kilometers per second, per million light years, or kilometers per second, per megaparsec, million parsecs. So the units have to be converted. You have to do a conversion to convert kilometers into megaparsecs to actually get the age of the universe by doing this. So just taking a value of Hubble's constant and dividing and taking it and 1 divided by that will not directly give you the age without doing this extra conversion. But it can be done and does give us the age of the universe. But what we also have to look at is that there are some assumptions that we made there that were not correct. And one of those was that the velocity of expansion is changing. So what we would expect is that there would be a deceleration of the universe. Why? Well, gravity. Gravity is pulling on everything. So if you have an elliptical galaxy here and another elliptical galaxy here away, there is a gravitational force between the two. It may never pull them together, but it will be, it is always an attractive force. And if this one is moving in this direction, that's force will serve to slow it down and make it move a little bit slower. So over time, gravity will slow the expansion of the universe. And that depends on how much material there is in the universe, how much matter is there. However, what we find now, this is what we would expect, and this is what we expected for the longest time, is that there is actually a universal acceleration. That things are getting further apart and are getting faster, apart faster. And one of the studies that has shown us this is studying the type 1A supernovae. These are what we called standard bulbs, meaning that they're all the same type of object and allow us to give an estimate of distances to very far galaxies. Now, what this means is that based on the assumption of deceleration, these should appear brighter than expected. So they should appear brighter than we expect them to do if the universe was decelerating. However, they are found to be fainter than we expect, which means that the universe is actually accelerating. And that means that we're moving faster now than we were billions of years ago. That's a big issue, confusion. How does that work? Because how do we explain that things are accelerating when gravitational forces are always attractive? And this leads us to the concept that we have come up with to explain this, and that is what we call dark energy. So dark energy, in order to get the universe to accelerate, we need some kind of energy. And the dark energy is just the energy of the vacuum of empty space. Early on in the history of the universe, it was not very important. So when we started off, we had the Big Bang, we had an expansion out here, and then as the universe expands, this dark energy becomes more and more important, and the acceleration begins to take over. So early on we did have what we expected. Early on we had this slowing expansion due to gravity, but as the universe continued to accelerate and continued to grow in size, eventually the dark energy took over and caused the universe to now accelerate, and accelerate at an increasing, so the velocities are increasing at an increasing rate over time. And that makes the universe appear younger than the value given by Hubble's constant. So the fact that it is accelerating means that this will make our universe look younger. Now this is a sketch showing what is happening here. There's the Big Bang that started out, and we can see back to the further supernova about here, so we have less information when we try to get back to this distance beyond that, but we can look at the supernovae, the galaxies, and we can find out what they were doing here, and then we can find out what has happened since then. And what we're seeing is, again, this slowing expansion early on, and then the acceleration, this great acceleration that is going faster and faster and faster, and will eventually cause the universe to become very cold and dark. So how can we compare, what other ways can we use to look at various ages of things in the universe? Well, we have a couple of ways to determine ages. One is looking at stars in globular clusters. We can look at the oldest stars that are just finishing up their lives, and we can find that globular clusters tend to be in the range of 12 to 13 billion years old. We can look at the very oldest stars through a process called uranium decay, looking at the decay of uranium, and that will give us ages of about 12 and a half billion years. So as we can see, these tend to be quite consistent. We're not getting wildly differing ages, which is good, and they are both consistent with the age that has been determined from the expansion of the universe. So we can get a very good estimate of our universe being in the range of 14 billion years old. Now, for the expanding universe, what do our models have to show? Well, our model has to show expansion. We know that the universe is expanding. We also have to be able to show why the universe is homogeneous and isotropic on the largest scales, going back to our cosmological principle. So there are various models that can occur depending on this, depending on the amount of material in the universe. We can have a decelerating universe, a coasting universe, or an accelerating universe. In a decelerating universe, you can have a couple of options here where the Big Bang occurred and things expanded outward, and then eventually there was enough gravity, enough material that things slowed down and stopped, and once this expansion stops, then gravity is still pulling on things, so it will continue to go down. So you'll start with a Big Bang here and end with a big crunch on the other side. You can also have cases where it continues to decelerate and go slower and slower and slower, but never quite stops. So it will continue expanding forever, not at a very fast rate. The universe's velocity will continually slow down, but it will never actually stop. It would take an infinite amount of time. A coasting universe is what we consider with a constant value for Hubble's constant. If Hubble's constant does not change, then the universe has always been expanding at the same rate and would continue to do so, and then we have the accelerating universe, where the universe started out expanding slowly, and then over time continues to accelerate faster and faster. So any of our models have to be able to explain these things that we see. So these are some of the different models that we look at, but they have to be able to explain all of these different aspects that we see for the universe. Now one of the other things that we look at is the density of the universe. So if we imagine that all the material that is in the universe is spread out uniformly, what it isn't, but it gives us a matter of determining the density. So the more matter we get, the more gravity there's going to be and the slower the expansion. So you can imagine we can have what we call a closed universe. This is a high-density universe where the universe expanded, reached a peak, and then started to collapse. That is what we call a closed universe because there will eventually be a ending to the universe. We have a start at the Big Bang, and we could have a big crunch here later on. A low-density universe, which is lines 2 and 4 in the graph here, will mean it's an open, it means it's open, and it continues to expand forever. So here you have the open universe without using dark energy. That's number 2, it just continues to expand, the universe will get larger and larger. But it is also still expanding at a decreasing rate. So it's not accelerating here, it is expanding at a decreasing rate, but will never stop, it's expanding too fast for the amount of material in it to ever pull that gravity, pull the materials down to make them stop. 4 is what adds in the dark energy. So this one shows the acceleration in that the universe is getting bigger and bigger and bigger. The critical density, number 3, is the boundary. So this is the edge, this is the limit where it just barely expands forever. It will never quite come to a stop. This one easily expands forever, anything less than this would eventually turn around and collapse back down. This is the boundary right between the two, which is what we call the critical density. Now one thing we see in this chart is that it also tells us the age. So the times from T1, T2, T3 and T4 tell us how old the universe is. So based on the models, Model 4 gives us an older age for the universe and Model 1 gives us a much younger age that the universe began much shorter time ago. So the age of the universe is also important and is also tied in to this density, how much material there is in the universe. So what is the future fate? What is going to happen to the universe? Well, what is the current evidence right now? The current evidence shows that the universe is going to expand forever and dark energy really strengthens this, means that the universe is going to continue to accelerate. So what we see for the universe today, as we can look out to clusters and clusters of galaxies, that is eventually going to change. And what we would expect to see many and many billions of years from now is that the universe will get very cold and dark. So let's finish up here as we do with our summary and what we find is that what we've learned is that Hubble's constant allows us to get an estimate of the age of the universe. So we can estimate the age from Hubble's constant, but it does assume a constant rate of expansion which we know is not true. Current observations suggest that the universe is accelerating. We know this because of supernova observations and the reason that we're giving that is the presence of this dark energy, this vacuum pressure that is pushing the universe apart. And this implies that the universe will die a cold, dark death. So that concludes this lecture on a model 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.