Ann wrote:Anthony Barreiro wrote:
If there were cosmologists more than five billion years ago, they would have observed that the expansion of the universe was decelerating, and would have reasonably predicted an eventual "big crunch."
Sorry about standing in for Nitpicker here, but since I am so relieved that we are apparently
not headed for a Big Crunch, I have to defend the open universe. (I was about to write "the open university"!)
If the universe was braking hard enough, then we would undoubtedly be headed for a Big Crunch. But the universe has never been slowing down that sharply. The expansion of the universe that was slowing down for a while, but the universe kept expanding. When the two teams headed by Saul Perlmutter and Brian Schmidt and Adam Riess were chasing distant supernovas, their aim was to find out by how much gravity was slowing down the expansion of the universe. By finding out how much the expansion of the universe was being slowed down by the effects of gravity, the teams wanted to find out if the universe was closed or not. They took it for granted that the expansion of the universe was slowing down, but they didn't take it for granted that the universe was closed. It depended on how much the universe was slowing down. Of course, what they found was that the expansion of the universe was speeding up.
Now imagine that there were astronomers and cosmologists more than five billion years ago. If they measured the expansion of the universe at that time, they would indeed find that the expansion was slowing down. But they would also find that the slowing down of the expansion was not enough to ever "turn the universe around" and eventually make it collapse. They, too, would have concluded that they lived in an open universe.
Ann