Welcome back, everybody. So, you know, we've been going through this really remarkable story about the birth of the universe, and you may wonder how do we possibly have, how can we tell this story is true, right? because this is science. You know we're not doing mythology. We think that we should have observational evidence for any part of a story in science that we believe. And what's remarkable about cosmology is we actually have quite strong evidence that this story of the universe beginning as a small or beginning compressed and high temperature, and then expanding actually is true. So, there are actually three pillars of what we call three pillars of evidence for the classic Big Bang model. Let's run through them. We've already talked to them about them before, but let's just touch on them again. The first is cosmic expansion. We, Hubble discovered this in 1928, that all the galaxies that we see are all expanding way from us. And from the symmetry of the situation, we know that all the galaxies are expanding from all the other galaxies. So if the galaxies are just pinned onto space time, that tells us that space is expanding. So that's the first part. That, that space time is expanding and if you run the movie backwards, that means that everything must have been compressed at earlier times. We've already talked about Big Bang nucleosynthesis. The idea that during the first three minutes or around three minutes after the Big Bang, the universe was hot enough and dense enough to allow nuclear fusion to occur. We can actually go out and measure the abundances of the products of that Big Bang nucleosynthesis, things like primordial helium, some isotopes of helium like helium 3, and by going out and finding places where it's convenient or easy to measure these light elements. We can get their abundances, and compare what we observe about how much primordial helium there is versus how much is predicted by the models. And see whether or not the models work. And it turns out that the models work exquisitely. So there's a very, very strong agreement between observation and theories. And that tells us not only does it tell us that the Big Bang actually happened, it actually puts pretty severe constraints on the variations in the Big Bang models. So it actually eliminates classes of Big Bang model for us. The other third, the third pillar that I've already mentioned is the cosmic microwave background radiation, which is, as I said, perhaps the most important evidence we have that the universe must have been hotter at one point, and must have been denser. Because there's really no way to have this fossil radiation existing all throughout the entire universe, no matter which direction you point in, without having a period, an earlier period when space must've looked much different than it does today. We go out and we look at space now, and it's pretty much empty. Matter is, is very, very clumped, with, you know, large gaps or voids between it, and that's just not the case if there was for the, if there. That couldn't have been the case if you were to be able to be be able to produce the cosmic microwave background. So that's clear evidence that the universe must have been much much hotter and much much denser earlier on. Now we also talk about inflation, which is this addition to the standard Big Bang model. That early on a chunk of the universe, or not a chunk, a tiny speck, actually went through a huge period of expansion. A very rapid period of expansion. And that little sliver of the universe is what we called the observable universe today. And there's actually some evidence for inflation, it's not a huge amount of evidence but there is some evidence for inflation. And in particular it comes out of the fluctuations that must have occurred during inflation, the quantum mechanical ups and downs, jumps and you know, regions of slightly over density or under density that must have been imposed on the universe during that era of inflation. And when we look today, when we go out and we look at the distribution of galaxies in the universe today, or we look at the cosmic microwave background and look at the tiny fluctuations in temperature or density we can tell the spectrum, the range of perturbations. You know how much how large or how many perturbations were there that were you know larger than a few light years across and, how many that were larger than ten light years across, et cetera. And those actually compare very well to the models of inflation. So, there is actually some evidence that this very early epoch of insane expansion, that went on to inflate the bubble of this space time that we call our universe actually occurred, as well. So, we have three enormously solid pieces of evidence for the Big Bang and, you know, some evidence for this addition to the Big Bang that we call inflation. And if you want to come up with another model, if you're like the Big Bang's crazy, then you need to address these three pieces of evidence and if you cannot simultaneously account for the expansion, account for the distribution of light elements and the cosmic microwave background, then your model just isn't going to go head to head with the the Big Bang. So, we really have excellent evidence for what happened after the universe began, however it began. You know, running the clock forward from there, expansion, cosmic microwave background, et cetera. So we have really a, a, a pretty coherent story that is backed up by evidence, and that's the important thing to understand. Okay? [BLANK_AUDIO]
