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Frontiers and Controversies in Astrophysics: Lecture 18 Transcript April 5, 2007 << back Professor Charles Bailyn: We've been talking about the expansion of the Universe and in answer--I want to follow up on something I said kind of in answer to a question last time, which is that it turns out that the expansion of the Universe alone doesn't require you to have a Big Bang. There are other explanations, or in particular, one other class of explanation, which is compatible with the observation of the expanding Universe, but doesn't exactly lead to a Big Bang, in the sense that we think of it today. And so, I want to talk about these alternatives. This is Frontiers and Controversies as of 1950. In 1920, you'll recall, they were worried about whether the spiral nebulae were island galaxies of their own, or not. That was settled by Hubble's observation. In the 1950s, it had become clear that it really is true that the Universe is expanding. But there were two categories of explanation that were being put forward to explain that, and deciding which is which was the current big topic of the day. So, the Universe expands. And what do we make of this fact? Well, one option is the Big Bang, what we now call the Big Bang, which, as I've mentioned, implies that in the past, everything was closer together. It was denser, and that creates other changes. If things get denser, they also get hotter, you may remember from chemistry. So, this implies in the past things were different from how they are now. They were denser, hotter, and that in the future, it'll go the other way. Things will become sparser and cooler. And there may be other changes associated with this. There may be different kinds of galaxies in the past from now and different kinds of galaxies now, as compared to the future, and so forth. But the Universe is a place whose bulk properties can change in time. So, things change in time. And the implication is that you can extrapolate this back to an initial singularity--that there was a moment, at some point in the past, where all currently existing space was piled up in a single point. The way I talked about this last time was that the scale factor was equal to zero, and so, there is this implication of an initial singularity. Now, the initial singularity is not the kind of thing that can be verified scientifically, because all the physical laws break down the same way they do in the singularity inside a black hole, or in an event horizon, or something like that. But nevertheless, that's the implication. That's the extrapolation of this set of ideas. But, at least, at the time, that was not the only set of ideas that could explain this expansion. There was an alternative, which was described as the "steady state." I should say that both of these names were given to these ideas by people who supported the "steady state." Big Bang was--that phrase was used as an insult by the "steady state" people to make fun of the ideas of the Big Bang, which we now actually know to be correct. And that's why that particular phrase is, in a way, so misleading--that they were deliberately trying to
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This note was uploaded on 02/06/2012 for the course BENG 100 taught by Professor Marksaltzman during the Spring '08 term at Yale.

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