Welcome back everybody. So what we're talking about now is the moments after the Big Bang, they still seem very short relative to where we are now, but we're actually beginning to get into the more mature phase of our understanding of the early history of the universe. So we're talking about 10 to the, you know, a few seconds to a few minutes now after the end of the- or after the moment of creation. And what happened then is really quite remarkable is that, in that period of the first few minutes, the universe was actually dense enough and hot enough to act like the inside of a star. So we have what we call primordial or big bang nucleosynthesis, and so nuclei- basically protons in this case, hydrogen nuclei- were now running around the universe and slamming into each other with enough strength to allow the series of reactions to occur for heavier elements to be built up. And when we say heavier elements, really we're only talking about helium and then a few more things after that. So, but this is actually there- most of the universe is helium, the primordial helium is built up during these first three minutes. So, the universe is dense enough and hot enough to get protons to slam into each other and begin the sequence every reactions, nuclear reactions, nuclear chemistry that lead to helium. Now, if this had continued, if the universe had stayed at that density and temperature, you might be able to generate enough reactions to actually allow even more interesting things to happen. But as it was that the universe actually was cooling down and becoming less dense, so there's only a brief window here where the universe can act to produce heavier elements. So this big bang nucleosynthesis only lasts for about three minutes and then the universe gets so tenuous that basically these reactions stop, but they left their imprint in terms of the fossils- the remnants of these nuclear reactions- which we can measure today and it turns out that the universe is very delicately balanced. That if the conditions in the early universe were different, we would see a different total abundance of things like the primordial helium. So we can go out and measure primordial helium and other light elements and do a very good job then of testing Big Bang Theory. So as we'll see, this is one of the ways in which we have determined the veracity, the the truth of the Big Bang Theory. Now one question that also comes out from this period, or this period and earlier, is the balance between matter and anti-matter. The universe is made of both matter and anti-matter such that if you bring a electron and a positron, which is the anti-matter pair for an electron, together, they wipe out. And there's really- look at the laws of physics and there's fundamentally no reason why matter should be favored over anti-matter. So you'd expect that the universe should have been created with about as much matter as anti-matter, but we know that's not the case in the universe that- certainly not in the universe we- the local universe, because we don't see large chunks of anti-matter lying around anywhere. And of course, if it did bump into anything and the entire matter would annihilate, we'd see the reactions from that. So the universe seems to have been built out of- something happened to the anti-matter, we don't know what. And so this process of what we call Baryogenesis that basically- getting a universe that ends up with predominantly matter and not anti-matter, is a question that still hasn't been resolved- one that's very important because as have said the fundamental laws don't favor matter over anti-matter. So that's a question that still has to be answered. We think it has something to do with the initial conditions for the universe. However, the universe emerged out of the Big Bang, the conditions that were set at that you know, t equals zero, somehow allowed interactions to occur such that you ended up with more matter than anti-matter. OK, now, if we run time forward after this moment or after this period of big bang nucleosynthesis, what we find is the universe continues to expand, it's a mix of electrons and protons and helium nuclei, so hydrogen nuclei and helium nuclei. But everything is ionized and meaning that the electrons haven't recombined with anything yet or electrons aren't attached to any of the nuclei yet. So what this means is that the photons, the light, basically is very tightly coupled to the matter during this period. A photon cannot travel very far without being absorbed by a matter particle. And that's because when the universe is ionized this way, photons are- and given the densities- the photons are just, you know, photons and the matter interact quite often, quite readily quite easily. But as the universe continues to expand and continues to dilute and continues to cool, something remarkable happens. At about 300,000 years after the birth of the Big Bang, the protons- the hydrogen nuclei and the electrons begin- they're moving slowly enough now that the electrostatic attraction between them or their electromagnetic attraction actually allows them to combine into full hydrogen atoms. Neutral hydrogen atoms- neutral because there's a plus charge the proton and a minus charge the electron. And when that happens the universe changes. And that's because as soon as the electrons and protons find each other to make a universe of neutral hydrogen, the photons that beforehand are very closely coupled to the matter now suddenly find themselves without a dance partner. The universe becomes transparent to these photons. And so at that moment which we call stupidly recombination, because actually it's the first combination of protons and electrons, but when the universe becomes mainly neutral hydrogen, those photons that, you know, were very tightly coupled now are released and they have no one to interact with. And as the universe continues to expand, these photons just run around the universe and their wavelength expands with the expansion of the universe, and they are still around today. These photons are what we call the cosmic microwave background radiation and they are fossils leftover from this moment about 300,000 years after the birth of the universe when this recombination occurred. And they are the most important cosmological dataset we have. No matter which direction we look in the sky, if we point our microwave antennas in that direction, we will find these photons coming to us from basically you know any direction. They're coming to us from that moment 300,000 years after the birth of the universe when the recombination occurred. So they have the imprint in them of the conditions 300,000 years after the Big Bang. So they are an enormously powerful tool for exploring the early universe. Now you may say, well, why are they microwave photons? Well, essentially they weren't microwave photons, they weren't, you know, in the radio spectrum or microwave spectrum. They were much shorter wavelengths when they were emitted, but because the universe has been expanding ever since then, the wavelengths got stretched out. So as the universe cooled, this radiation cooled and stretched and so now that's why it's in the microwave region. The temperature associated with this radiation now is about 2.7 degrees above absolute zero. But when the radiation was released, the temperature- its temperature which was also the temperature of the matter was on the order of 3,000 or 4,000 degrees. So the cooling of the universe is imprinted in this microwave radiation. So most of cosmology- not most, but very much of cosmology really relies on this enormously important fossil radiation. And that's how really we've been able to tell so much about the history of the universe is from the cosmic microwave background. OK?
