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Astronomy and Cosmology
Cosmology
Question on the horizon problem
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[QUOTE="PeterDonis, post: 6843461, member: 197831"] That statement is correct as a statement about General Relativity. Notice that I omitted the qualifier you put in; the statement is correct without any qualifiers at all. However, you are drawing the wrong conclusion from it. You are drawing the conclusion that we have to somehow come up with an independent assumption about "how the universe started", and then work forwards from that. That's wrong. The correct conclusion is that [I]we cannot construct a model of how the universe started at all by working forward[/I] with our current theories. We will only be able to do that working forward if we come up with a more comprehensive theory, such as a theory of quantum gravity, that [I]replaces[/I] GR in the regime in question and makes definite predictions. In the meantime, the only thing cosmologists can do is to work [I]backwards[/I]. We take the earliest state of the universe for which we have good evidence, namely, the hot, dense, rapidly expanding state that, in inflation models, occurs at the end of inflation, and try to figure out what could have preceded it. The goal is to work as far backwards as we can without running into the regime where our theories break down. [/QUOTE]
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Question on the horizon problem
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