Welcome back everyone. So to answer this question that is, we're obsessed with this week about the fate of the universe, we first have to understand the idea of the Big Bang. And why we even consider the Big Bang to be true. So let's start with the most important the first piece of evidence that lead to our thinking about the Big Bang, which is what's called Hubble's Law. Recall from our first week, the idea of Doppler shift, that if we see light from an object being having its light stretched, so that it be if we have something that we know the wavelength, what the wavelength should be, and we see that wavelength stretched out, we know that that is a redshift, and the object is moving away from us. If the light is compressed, and the wavelength goes down, that is a blue shift, and the object is heading towards us. Well, it was in the 1920s that Edwin Hubble went out and started making measurements of redshift or blue shift of galaxies, and what he was astonished to find was that everything was redshifted. With the exception of just one or two cases, every galaxy he looked at was moving away from us and so all the galaxies were moving away from us. And not only that, the speed at which the galaxy was moving away from us depended on how far away it was from us. So by making independent measurements of distance and combining that with the measurement of the galaxy's redshift he found what was called a linear relationship. The farther away it was, the faster it was moving, and that's called Hubble's Law. And this was one of the most remarkable discoveries in the history of astronomy, because what it implied was is that the entire universe was expanding. Not just the galaxies, but space-time itself was actually stretching and the galaxies were just sort of like you can imagine them to be marbles pinned onto a rubber sheet and as the rubber sheet was stretched, the galaxies are just going along for the ride. So this was the first indication essentially that the universe was expanding. That there was a, something we could call a Big Bang. so, you know, if you imagine putting raisins inside a loaf of raisin bread that you're or, you know or, dough. And then putting it in the oven and allowing the bread to expand, because, you know, of course, of the yeast. Every raisin will move apart from every other raisin. So, no matter which galaxy you're sitting on, you would see all the other galaxies moving away from you. So, this was our first indication that something like expan, or that the, the, it was our first indication that the universe was expanding, right. So where do, how do you get a big bang from this. Well, if you run the movie of the universe's expansion backwards, then what you'd expect is over time, things that are very far apart would end up being very close together. So it was clear from this, well it wasn't exactly clear, we took a, a, few more pieces of evidence for people to really understand this. But at least the implication with the discovery of the Hubble Law was that the universe early on must have looked different than it does now. The universe was not static, the universe was not eternal in its appearance, that there was an earlier epoch of cosmic history that was very different from our own. So using Hubble's Law, you can actually figure out in a simple way that time that it would take for everything to be very close together, and what you end up with is the naive use of Hubble's Law. You end up with a cosmic age of about 14.3 billion years. That exhumes, well, there's, there's in that, it, that calculate, it's a very simple calculation. As we'll see, that's not the actual age of the universe, but it's pretty close. So even a simple calculation gives you ten, about 10 billion years of cosmic history. What comes with the understanding that the universe has evolved, is that if we look at a galaxy 7 billion light years away, we're really looking halfway back to the beginning of the universe. Right, that's what's really remarkable. Once we come to understand that the universe has a history, when we're looking back in, when we're looking out in space, we're actually looking back in time and history. So that's the, that's what comes when we understand that the universe is not static. Alright, so the idea of the Big Bang is that the universe started from a hyper compressed state. That all the matter in the universe, was densely packed. Now to understand what this means, we have to go through, we have to understand what the Big Bang is, and what it isn't, okay? The first thing is, the universe the Big Bang is not a theory of how the universe began, how time began. It's a theory about what happens immediately after the universe began. It's a theory of after, because we're going to use the laws of physics that we understand, and we're going to track as far back into this hyper compressed, hyper, high temperature state as we possibly can. And then let time begin to run forward. But what actually happened when the universe, what actually created that state of the universe, is not science. Right now we can't answer that question. Maybe we will be able to soon, but right now we're not in a position to really answer that question definitively. So the Big Bang is a theory of what happened after The Big Bang. It also tells us how the universe evolved. It didn't tell us why the universe is there. Right, again, so this is, you see with cosmology how this is a science that's really bumping up against very deep philosophical issues and if you're so inclined even theological issues. Okay the Big Bang is our creation story right, it's our narrative of how the universe evolved and emerged to become what it is today. And it's still, because it's a scientific story, we see changes in it. the, the basic narrative has not changed now over 60 years. Our understanding of what immediately happened after the moment of creation, has not changed. But, you know, we've been adding to the story as we come to understand more more more facts. So, we have really firm observational evidence for a lot of this story, but when people begin talking about what happened before the Big Bang or or, or, your know, the idea that there is multiple universes, that stuff is still very much scientific speculation. We don't have any evidence for the existence of any other universes, or we don't have any evidence for, yet for what came, direct evidence, for what came before the Big Bang. So it's important to understand where the science is very firm, and where scientists are still, you know, try, you know, coming up with new ideas and hoping to find something that might have an observational handle on it. Okay? [BLANK_AUDIO]
