Welcome back. Now, we're going to look at one of the really amazing features of the cosmic expansion. Scientists estimate that humans have lived for something like 200,000 years, beginning in Africa. And it's easy to imagine that from the very beginning we have been fascinated by the stars, we and other animals. But because we have language, we are able to pass down the knowledge we've learned about the configuration of the stars. And so as we learn where the constellations are and how they move, we also begin to notice certain things that are maybe outside of our knowledge. And these were little smudges, little fuzzy areas that in the 19th century, astronomers began to wonder about. So it was that William Herschel in England, he imagined that possibly these little fuzzy areas were entire star systems outside of the Milky Way galaxy. Herschel had actually begun to map out the Milky Way, and he realized it was a great, flat disc of stars. We now know there are something like 300 billion stars in our Milky Way galaxy. And so during the 19th century and before, the Milky Way was taken as the entirety of the universe. But, there was this surmise, maybe, there are other galaxies. Then, as our technology improved in the 20th century, Edwin Hubble, using the ideas of Henrietta Leavitt, as well as some improved technology and telescopes, actually determined that these tiny smudges were entire galaxies. The Andromeda galaxy wasn't just a little smudge, it was a galaxy of 400 billion stars, over 2 million light years away. So, Hubble was the one who enabled us to break out of the idea that we live just in the Milky Way galaxy and that the universe entirely. No, the universe consisted of billions of galaxies possibly even a trillion galaxies. Hubble discovered something else. Not just the existence of galaxies, he discovered that the galaxies were all moving away from us. This was the greatest discovery in the history of astronomy, without question. And the galaxies were moving in a very particular way. It's easy to say that the distance determined how fast they were moving. So that a galaxy that was twice as far away as another galaxy was moving twice as fast. Just imagine, I'm Edwin Hubble. I'm there on the top of Mount Wilson and I'm looking out at two galaxies. There's galaxy one, here's galaxy two, twice as far away. And I determined that galaxy two is moving twice as fast as galaxy one. So then you do a thought experiment. Let's go back in time. If we go back in time they're moving toward us, but because this galaxy is moving twice as fast, it overcomes the first galaxy and arrives here at the same time. Hubble looked at other galaxies. A galaxy that was ten times as far away. And found out that, that galaxy was moving ten times as fast. So if you go back in time, it would overcome the first and arrive here at the same time. It's because of this empirical discovery of the movement of the galaxies that we came to the realization that all of the universe had its origin in one place, in one place, here where Edwin Hubble was looking out. Now it sounds strange to say that we are at the center of the universe and indeed, what was discovered was something different than that. Because following the equations of Einstein about the large-scale structure of the universe, scientists realize that every place in the universe was at the center of the expansion of the universe. Now I know that's hard to understand. I understand, it's hard. But this was the breakthrough in understanding that in our universe every place is at the very center of the expansion. How can we even begin to make sense of this? If you can just imagine the universe consisting of a raisin loaf that's rising, and if you can imagine each raisin as being a galaxy, then no matter where you position yourself in this raisin loaf, and if you look about, you will see all of the other raisins moving away from you. So we have discovered we live in a universe that's expanding, that is an omnicentric expansion. We live in a universe where every place in the universe is at the very center of the expansion. And next week, we are going to discuss a discovery concerning Earth that is just as amazing.
