 Greetings and welcome to the introduction to astronomy. In this lecture we are going to talk about the Big Bang, which is our model for the origin of the universe that explains how it formed. So we want to take a little bit of a look at the history of that and how we come to understand how the Big Bang might work. So let's go ahead and get started here and what we see is that some of the very earliest thoughts of this were given quite a while ago, in fact back in the 1920s by Georges Slamatre. And what he found was, what he suggested was that the idea of the Big Bang was this primeval atom that broke apart and forming the present atoms in the universe. So this was an idea of fission that things had split apart. And it was compared to the end, we were looking at the end of a firework show, just watching the fading embers of the universe, which is all that was left after this primeval atom broke apart. So the suggestion was that it started as something big and then broke apart. Now we have to think that this was long before we understood nuclear physics. At this time we really didn't even understand what powered the sun, we were just getting a handle on that. So what we know now is that the universe must have been, even with that the universe must have been hotter and denser in the past. So we do know that that is the case. But some of the very ideally ideas of the formation of the universe even go back just about a hundred years. Now what we look now more recently in the 1940s, we have nuclear fusion was beginning to be understood. We had now learned that nuclear fusion was the source of energy for the sun. So this is where the sun got all of its energy. George Gamma said that the universe built the heavy elements through nuclear fusion. And that's what happens in the stars, and he suggested that the universe did the same kind of thing. This is actually relatively similar to the ideas that we have today, except that now we say that only the very lightest elements formed in the Big Bang, hydrogen and helium and maybe small amounts of lithium, and that all of the heavier elements had to be formed within the stars. So let's look a little bit about how we can understand what was going on in the very earliest part of the universe, very earliest times. And what we see is that the temperature must have dropped very rapidly. In fact, going from one one-hundredth of a second after the Big Bang, being one hundred billion Kelvin, and after three minutes dropping to just one billion Kelvin, and then after just one hundred thousand years, a very short time compared to the fourteen billion year age of the universe, it was down to three thousand Kelvin. And what dominated early on is that radiation dominated the universe for that very earliest stretch of the universe, when things were happening here, and that goes out to about, probably into about this range, we can see radiation dominates, and it isn't until we start to begin to form particles and atoms here that radiation becomes less and less important in the universe. But we also know, because radiation dominated, we have to remember also that energy can be converted to matter by Einstein's equation, E equals MC squared. And what that means, we usually think of it in one direction, that we can take a small amount of mass and make a large amount of energy. But it works the other way around as well. We can take large amounts of energy and make small amounts of mass as well. It requires very high temperatures because it requires lots of energy to do this. But when the temperature is six billion Kelvin, that means that you could create electron-positron pairs, and you would get this primordial soup that would form that would have mixtures of energy and electron-positron pairs that are just popping into and out of existence. You have to go even higher temperatures because the electron-positron are a lower mass. You need even higher temperatures of 100 trillion Kelvin to be able to produce proton-antiproton pairs. But this was something quite achievable. We were getting temperatures like that in the very first fraction of a second of the universe. Now, how would this have evolved then? What would have changed? And what we can learn as we look at the very earliest time that we know of is this 10 to the minus 43rd seconds. And if you can imagine drawing those 43 zeros after the decimal point to get to this very first instant of the universe. And the problem is, because this is where everything started from, but at this point, even time, our concept of time starts to break down. So our equations become meaningless at this, and we simply cannot know anything that early. But it's important to try to understand because it's when everything was happening. Later times, when we get down to about one-one-hundredth of a second, we have a little bit easier understanding. And in fact, this is the time when we would have had electron-positron pairs forming. So you could have two gamma rays, lots of energy come together, and that would create an electron and a positron. And you could then have that positron and that electron meet up with another one, and then form a... and then form two gamma rays. So it was a constant changing between matter and energy at this point. So we were switching back and forth between... you could switch back and forth between matter and energy. And this is what was happening with the positron-electron-positron pairs. They were able to form these constantly. Now, as we got a little further along, about one second into this, neutrinos were able to escape. So we're finally getting to the point where the universe is low enough density that neutrinos were able to escape. And they, if you recall, neutrinos can travel through just about anything, but they do not interact with a lot of stuff, but the universe was so dense before one second that even neutrinos were trapped and unable to escape. They would be unfortunately extremely hard to detect, but would give us some evidence of the very early history of the universe. Now, as we go into later times, we can understand a little bit more. And we're getting into a little bit later, up to three minutes, we're actually starting to form atomic nuclei. That's what we see in the second image here. We can have protons and neutrons combining to form deuterium, a form of heavy hydrogen, which could then help form helium, and finally form helium. Four, which is the standard helium that we see? This is what was going on in those first few minutes after the Big Bang. We took hydrogen and converted it to helium. Essentially, the universe was like a gigantic star fusing hydrogen into helium. But by the time we reached four minutes, the universe had cooled too much that we were no longer able to fuse elements together. So this gives us, again, a very early history. That's why the early universe only formed hydrogen and helium. The temperatures cooled off too rapidly before there were sufficient quantities of helium for helium to fuse into carbon. So everything beyond that on the periodic table had to be formed after the Big Bang in stars. By studying deuterium, we can look at the density of the early universe. We can tell us about the density of the ordinary matter in the universe. It does not tell us about dark matter. And we can then see what else... talk about what else is beginning to form there. So what we see here is that we kind of went in stages. We first formed particles. Even before this, we would have formed proton and antiprotons. But then after this, we formed the electrons and positrons. And then we began to form simple atoms, actually nuclei. And then finally, after a few hundred thousand years, we were beginning to form the larger, large actual elements. Now what this leads to is what is the evidence for any of this with the Big Bang? And what we want to see is, you know, what kind of evidence? What can we see that shows that the Big Bang is correct? We say that it is our current model. But any good theory has to make predictions. So we need some kind of predictions that are made. And one of the predictions that the Big Bang makes is that at about 400,000 years, electrons and protons would combine into atoms. And at that point, the universe became transparent. Before this time, no radiation could travel through the universe. It was constantly being absorbed and re-emitted. So just as we cannot see into the center of the sun, it was too dense, there's too much absorption and re-emission of the radiation, until it actually reaches an area where the opacity is small enough that it can escape, we can't see anything. And at this point, matter and radiation decoupled from each other, and electromagnetic radiation like light could travel through the universe. This is the key because this can still be detected today. So detecting this radiation from the early universe would be a very important key to... a very important piece of evidence for the Big Bang. So how can we detect this background radiation? Well, first of all, let's look a little bit about the history of it. In the 1940s, Alfred, Herman, and Gamal got together and were thinking that the universe would have had a temperature of 3,000 Kelvin when it became transparent, as we've just talked about. They had determined this in the 1940s, and what they knew was that the universal expansion would have stretched this radiation. Something at 3,000 Kelvin would be a very cool star and is emitting primarily in the infrared part of the spectrum. So at that point, the universe would have been glowing in the infrared very brightly and would have had even a good amount of visible light just throughout the entire universe, even the empty space, would have been filled with this background radiation. However, expansion has now increased this by a factor of 1,000. So it has stretched these wavelengths that used to be in the infrared by a factor of 1,000 and would now be more comparable to a temperature of 3 Kelvin. And that would be in the radio part of the spectrum. However, at this time, we didn't have the sensitivity of the instruments, and we were just beginning to really put good radio telescopes together. So we were able to then be able to see what was going on. So with Penzias and Wilson in the 1960s, actually did detect this. And in their thing here, in their telescope here, they were trying to detect not the cosmic background radiation. They were actually looking for other radio signals, and were trying to detect those signals and get the best signals they possibly could. And what they found with their telescope is that they found radio waves that were coming from every direction that they looked. And they looked for sources of interference and were trying to eliminate all of those and eventually found that they simply could not, that there was this little bit of a glow that was coming from everywhere in the universe. And this was actually the detection of that background radiation. So how can we measure this? We've got better instruments that can measure it in more detail now. And in fact, in the 1990s, COBE, the Cosmic Background Explorer, measured the radiation spectrum and found it to be a perfect fit to a black body of 2.73 Kelvin. If you look here, the green line is the black body spectrum and the COBE data are shown in the red. And you can see that they match up almost perfectly. We don't generally get experiments that match up that perfectly. So this really fits it and fits it extremely well and is a great to confirms the predictions of the Big Bang model. So any other model is going to have to also be able to explain this background radiation. Then are there variations? We see it as fitting very well, but what are the variations in the microwave background? Well, we actually do see that there are some variations. And in fact, when we map out the entire sky, we see that we have some here areas in the red, which are warmer areas and some areas in the blue that are very cool areas. So there are some slight variations, but these are measuring tiny fractions of a degree. These are not measuring tenths or hundreds of a degree. We're talking tens of thousands to hundreds of thousands of a degree in variation. So while there is some slight variation, it is not very large, but it is important because this is what gives rise to the structure we see in the universe today comes from these denser areas and these less dense areas that we see. And that has given rise to the structures that we actually see today. Now, what this leads to is we have to try to understand the density of the universe and what we mean by the density. And what the density shows is that there are then possible geometries for the universe. So when we look at these variations that we see by the Planck satellite as we see here, we can see that there are very significant variations. So Kobe gave us our first measurements of the original background. Now we're able to get some here, but we're seeing the Planck satellite has given us maps of the entire sky. So this is an entire sky picture going around the sky, looking at all the different places. So looking for the background radiation from all the different areas. And what we find is that this helps tell us the shape of the universe because it is related to the density. And there are three possible geometries for the universe. And what we can have is one of three things. We can have a spherical or closed universe which is like this, where parallel rays, things that start out parallel, will eventually meet off in the distance. There is a flat universe which is what we're used to thinking about here, where parallel rays will go on forever. And there is a hyperbolic open universe where parallel rays will diverge and get further and further apart over time. So we will have to come back and look at these to try to understand what the shape of the universe actually is. So the results that we get from this are, first of all, that we find that the density is equal to the critical density. So that tells us that there is a flat universe, that the universe is very flat based on our measurements. We also find that the age of the universe to a very accurate amount is close to 14 billion years, about 13.8 billion years with only a very small error estimate. We also know that dark energy accounts for a large percentage of the universe, being 68.5%, and that the matter content, including ordinary and dark matter, is only 31.5%. These are all consistent with the values that we have found and the information we have found from the microwave background. And do recall that only 4% of the universe is ordinary matter, so most of this is actually dark matter. Most vast majority of this is dark. So in terms of dark matter and dark energy, that makes up about 96% of the mass energy in the universe. So this is kind of what we found. We have learned. We know the age of the universe. We learned something about the density. But of course, how does this tie in with the fact that the universe appears to be accelerating? And that is very interesting and some things that we will look at coming up in a future lecture. Because if the universe is accelerating, we would expect it to be open. So there are some other things that have to come in here, so that the Big Bang is not the complete explanation for the universe, but is a very good start and does match a lot of things very well. So let's finish up, as we do, with our summary. And what we find is the Big Bang model explains the observations that we see of the universe today. In that early universe, we formed hydrogen, helium, and maybe a small amount of lithium, and that was it. The rest was formed in stars. The cosmic background radiation provides evidence for the Big Bang. And variations in this background will tell us about the early history of the universe, how the structures that we see today are formed, and can also tell us the ultimate fate of the universe. So that concludes this lecture on the Big Bang. 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.