 Greetings and welcome to the Introduction to Astronomy! In this lecture we are going to start talking about our understanding of the universe and the Big Bang, looking at our model for the universe as to how the universe formed. So let's go ahead and start with what is the age of the universe? Well, we know from previous lectures that we talked about we know we are in an expanding universe. We have observed the universe expansion. That's right now assumed that it's expanding at a constant speed. So what happens to an expanding universe in the distant past? Well, the universe will have been smaller. If it's been expanding to a large size now, it must have been smaller in the past. So it would have been compressed to a small size. If you trace this back literally, it means that all matter and all energy in the universe would have been compressed into an infinitesimally small volume. And this is what we call the Big Bang. So the Big Bang, when did this occur? We can calculate it based on these assumptions. We can calculate it using Hubble's law. So Hubble's constant is going to tell us the age of the universe. Time equals distance divided by velocity. So if we write that as an equation, but we know from previous lectures that the velocity is just Hubble's constant times the distance. The distances will cancel, meaning that the time or age of the universe is just one divided by Hubble's constant. So Hubble's constant will tell us what the age of the universe is. Now that again, that assumes that everything is expanding at a constant speed. What happens if the velocity of expansion is changing? Well, we think the universe should be decelerating. Why? Well, gravity is going to slow the expansion over time. Every galaxy pulls on every other galaxy that would cause them to slow down. The amount of slowing would depend on how much matter there is in the universe. So we would expect that over time things are moving slower now than they were in the distant past. But what we're finding by looking at type 1a supernovae, remember those are standard bulbs or candles that tell us the brightness, that means that based on this they should appear brighter than expected if the universe were decelerating again as we expect. So we looked for type 1 supernovae. They should be brighter because the expansion has slowed and things were expanding faster in the past. However what we found is that they are fainter than expected. That's a big difference. What that means is because they're fainter that the universe is not slowing down but is instead accelerating and it's going faster now than it was in the past. So what can be at work here to overwhelm gravity? Gravity has to slow things down. Gravity never is a repulsive force. So it always slows things down. What else could be involved? Well we come to the concept we call dark energy. Energy, some kind of energy is required for the universe to accelerate. This becomes the energy of the vacuum of space. It really was not important early on when the universe was first forming. This was not an important portion of the universe. However as the universe expanded it became more important and it finally forces the expansion to accelerate. So originally we had the big bang and we had expansion and it was slowing down. However once we got to the point where the dark energy dominates then it began to accelerate and here we are right now in this section but in the future things will accelerate even faster outward. So the acceleration will make the universe appear younger than the value given by Hubble's constant. Remember Hubble's constant is then based on the universe always expanding at the same rate. If it's now expanding faster we are going to make the universe appear younger than it truly is. So how can we figure out these ages? How can we figure out the age of the universe? Well there's a few things we can look at. We can look at stars in globular clusters remember these were some of the first things to form after the big bang and they are 12 to 13 billion years old. We can look at some of the oldest stars we find and based on decay of uranium then we find that they are about 12 and a half billion years old. Again both of these are consistent with the age that is determined from the expansion. So we are not finding anything that is really out of whack here we can come up with an age of the universe of very close to 14 billion years. It's not like we are finding things that are older that would be a great concern if we started finding globular clusters that were 15 or 20 billion years old then something would be very very wrong with our models. So what does this do for the expanding universe? Well we can have various different models but they must include expansion. We know that the universe is expanding. We also know that the universe is homogeneous and isotropic on those very large scales. So based on this there can be three possibilities for the universe. One the universe can be decelerating that means it's slowing down. It can be coasting just going at a constant speed that would be the same Hubble constant all the time. Or it can be accelerating. So in a decelerating universe you have two cases it's slowing down. It can eventually slow down enough to stop and come back down and collapse down to a single point again. It can just barely expand outward forever in the second model here. Or you can have the coasting universe which does expand forever and always has the same value for Hubble constant. But where we appear to be is the accelerating universe that the universe started out and it's been going faster and faster and faster in its acceleration over time. Now this tells us something about how much material is in the universe and we call this the density of the universe. In order to determine the density we imagine that the material is spread uniformly throughout the universe. More matter means more gravity which means a slower expansion rate. So if you have a very high density then you have a completely closed universe where it goes out expands to a certain size, comes back down and collapses again and maybe that's a cyclical thing and will continue over and over again. That's line one. Lines two and four are an open universe where the universe continues to expand forever. So in line two it continues to expand and in line four it continues to expand. Line two it expands but it's slowing down. Line four it expands but it's feeding up. That is where the dark energy comes in. The critical density number three is the boundary between open and closed. So it expands but it just barely keeps expanding and the particles are moving fast enough to just reach an infinite distance before they are stopped. So you have these four models and again where does it look like we are? It looks like the universe is accelerating and that would be model four here where the universe gets larger and larger over time. So what then is the future of the universe? Well current evidence says that the universe is going to expand forever. Dark energy improves this and the universe will eventually get cold and dark with galaxies then disappearing as they expand outward at faster and faster rates. So in the long run that means we would see no distant galaxies. Now the acceleration would not rip apart things that are gravitationally bound so our galaxy would still be there. So the night sky being dark here is really referring to the galaxies that we're not seeing those distant galaxies that we see today and we would not see those as the universe continues to accelerate. However, the stars within our galaxy would remain bound together, the galaxy itself would not be torn apart by the universal expansion. So let's go ahead and finish up with our summary and what we've looked at today is that Hubble's constants let's us estimate the age of the universe. And again that assumes a constant rate of expansion. Our current observation suggests that the universe is accelerating because of this dark energy which 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.