 Greetings and welcome to the Introduction to Astronomy. As we continue to look at the very early universe, we are going to talk today about the Big Bang which created our universe nearly 14 billion years ago. So what is the beginning of the universe? Well, let's look at the beginnings and the first ideas of the concept of a Big Bang were proposed by Georges Lematra in the 1920s. And pictured here from the 1920s in the center there, and you may recognize Albert Einstein on the right side of this image as well, he had considered a primeval atom that broke apart forming the present atoms in the universe. So something similar to the idea of fission that we've talked about previously with nuclear reactions, larger things breaking down into smaller ones. And it was compared to the very end of a firework show where you're watching just the fading embers of the universe where we happen to live. Of course this was prior to really our knowledge of nuclear physics expanding, and the universe still must have been hotter and denser in the past. As we go forward a little bit, in the 1940s we had the idea of nuclear fusion, and we learned that nuclear fusion was the source of energy for the sun. Georges Gamal suggested that the universe built the heavy elements through nuclear fusion. And this is rather similar to what we believe today, although today we know that only the lightest elements formed in the Big Bang, that is hydrogen, helium, and a small amount of lithium. Everything else was created through other methods. So how can we try to understand the Big Bang? Well it's really difficult because the temperatures and the densities and the time scales and sizes we're thinking about are so incredibly small. On our graph here we see that we are talking about very small distances, times. This is tiny fractions of a second. So even as we get all the way out here we're talking about one one thousandth of a second, ten to the minus third seconds. So even at that point we are talking about a tremendous number of, I mean a tiny fraction of a second, and we're going to be looking at the actual Big Bang is really a lot of this earliest, this first tiny fraction of a second. So as what we see from this graph is that the temperatures decreased very rapidly. After only .01 seconds we were down to a hundred billion Kelvin. Now a hundred billion Kelvin is extremely high temperatures, much hotter than the temperature at the center of a star like our sun, but it was much much less than the unimaginable temperatures of say ten to the thirty first Kelvin, a one followed by thirty one zeros in the very earliest history here. After three minutes we were down to one billion Kelvin, still again much hotter than our sun, and after nearly a hundred thousand years it had gotten down to three thousand Kelvin, about half the temperature of our sun and about the surface temperature of a very cool red dwarf star. In this very early universe radiation dominated everything. And as we know matter can be created into energy, but also energy can be created to matter under these extreme circumstances. The energy and matter are interchangeable, matter can become energy, energy can become matter. However in order to convert energy into matter very high temperatures are needed. Six billion Kelvin will create an electron and a positron pair, an electron and an anti-electron. You need even higher temperatures of a hundred trillion to create the much more massive proton and anti-proton pair. So they were able to form at one stage here, that's where protons and anti-protons formed, electrons and positrons a little bit later, but still very high high temperatures in that very earliest fraction of an instant of the universe. Now, so what was going on here? On the earliest time we don't know. I know it seems like a tiny fraction, it's ten to the minus forty-third seconds, so put that decimal place and all those zeros there and followed by a one and get that tiny fraction of a second. At this point the concept of time is our understanding breaks down. Now a little easier to understand when we get later on, when we get to the points of say a hundredth of a second. At this point we can have, we have the production of electron-positron pairs as we see in the first frame here. So gamma rays, release energy can all of a sudden produce an electron and a positron. And of course they would be able to then almost immediately reannihilate each other producing gamma rays. It was just a constant conversion, gamma rays became particles, particles became gamma rays until the universe had expanded enough that there was that the particles had time to separate. But in this very early soup it was just gamma rays matter and energy would just becoming completely interchangeable. At about one second the neutrinos were able to escape. So we could see this would be something that would be nice to be able to detect. And remember neutrinos are really hard to detect, but at less than one second the universe was so dense that even neutrinos couldn't get through it. And the neutrinos would be able to escape if we could, this would be some good evidence for the Big Bang, unfortunately that is a very, very hard thing to detect. Now as we go through into the first few minutes, after three minutes we start to form atomic nuclei. So hydrogen and hydrogen fuses to helium. So we have something what we looked at similar to the proton-proton chain, not quite the same, but somewhat similar. Here a proton and a neutron combine to form deuterium and then that combines with a proton to form helium-3 and then you can either have helium-3 and a proton to give you helium-4 or two deuterium atoms to give you helium-4. This was going on in the first few minutes. This is when hydrogen fused to helium and this is where the vast majority of the helium in the universe was formed. That was at about three minutes, by four minutes, we had a whole minute to do this, the universe had cooled too much. Fusion could no longer occur, we were at too low temperatures, too low densities for this to occur. So studies of deuterium, how much is left over, tells us about the density of the early universe, how dense was this early universe. What they do not tell us about is dark matter. Now so how can we detect, how do we test a theory? Well we look for what predictions they make and we try to test those predictions. Well one prediction of the Big Bang Theory is that after 380,000 years electrons and protons could combine into and form atoms. So hydrogen atoms can form. At this instant the universe became transparent, matter and energy are no longer coupled together. So we no longer have matter and energy together, they are now separate and the universe becomes transparent and now the electromagnetic radiation can travel through the universe. This can still be detected today. And in fact it was detected back in the 1960s, although it was by Penzias and Wilson here who detected radio waves coming from every direction in the sky. However this was predicted in the 1940s when Alpha, Hermann and Gamma found out that the universe would have had a temperature of 3000 Kelvin when it became transparent. Now 3000 Kelvin is, gives off some red light but mostly infrared light. However the universe is expanding. That expansion would have increased this, would have increased the wavelength of this light. So if we increase the wavelength of radio waves, sorry of infrared waves, guess what? They become radio waves. So this would have effectively a temperature now of about 3 Kelvin. They knew it should exist as a radio glow but at the time did not have the instrumentation to be able to detect it. And that was Penzias and Wilson were able to detect this when they were looking at getting rid of as much noise as they could from their antenna and still found these radio waves at this specific wavelength coming from every direction. So how do we measure the background radiation? Well in the 1990s we had Koby, the Cosmic Background Explorer, which measured the background radiation spectrum. And it was measured here and here we see the data. The black body spectrum to a fit to this is in the green. The data recorded by the satellite are in red and as you can see there are an almost exact fit. It is a perfect fit to a black body at 2.73 Kelvin, confirming the predictions of the Big Bang model. Now we do see some variations. When we've looked at this in more detail we've been able to see that there are variations. So this was measured by the Planck satellite that was set up to measure the background in more detailed details. So it's very smooth, but not completely smooth. And this is what gives rise to the structures that we see in the universe today. Now remember that's 2.73 degrees, the variations we're seeing here from the brightest reds to the darkest purples here are only a tiny fraction of that. We're talking, you know, thousands, ten thousands of a degree in variation of temperature. So there are some variations they are extremely small. Now as this has been studied, and again let's leave this up here, what those variations tell us is the density of the universe and the shape of the universe. What does the universe look like? Well there are three possibilities that this can tell us. And the universe can be a spherical universe. It can be a flat universe or it can be a hyperbolic universe. Now you'll have to remember that we are changing the dimensions here. Our universe has three dimensions of space and one time dimension. For just considering the three spatial dimensions here, really when we're looking at a spherical we're looking at a two-dimensional version of that three-dimensional space. Because we can't picture a hyper sphere with an extra dimension. So we could have a flat universe, which is what we're used to. That's the geometry we're used to studying in Euclidean geometry. That would then be the parallel lines never meet. In a spherical universe parallel rays eventually meet. And in a hyperbolic universe the parallel rays diverge. Well where we are here depends on the density. If we have a high density it's a spherical universe, a low density is a hyperbolic universe, and right in between is the flat space that would be a perfectly critical universe, right at the border between those two. So what do we know? What have measurements shown us? Well we find that the density is equal to the critical density or a flat universe. That's interesting in comparison to what we found with the expansion of the universe. The universe is expanding and accelerating. How can it be flat, which is just barely expanding forever? So this is a question and we will try to look at some solutions to this in a future lecture. How old is the universe? Well about 13.8 billion years old based on all of our measurements. Work energy consists of about 68.5% of the universe, matter content of the universe is 31.5% but only 4% of that is the ordinary matter that you and I and Earth and the stars and the galaxies and everything else that we are studying is made up of. So let's go ahead and finish up with our summary and what we've looked at here is that the Big Bang model can explain observations of the universe today. The only things formed in the Big Bang were hydrogen, helium and a small amount of lithium. Evidence for the Big Bang includes the cosmic background radiation detected in the 1960s and variations in that background can tell us about the early history of the universe and how the structures we see today were formed. 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.