On 3/11/2025 9:26 PM, Alan Grayson wrote:
He may have shown it, but I don't believe he was the first. Friedmann, in 1922, already showed that the universe must either expand or collapse, that it has no steady state with or without a CC.On Tuesday, March 11, 2025 at 1:41:29 PM UTC-6 Brent Meeker wrote: On 3/10/2025 11:48 PM, Alan Grayson wrote:On Tuesday, March 11, 2025 at 12:33:36 AM UTC-6 Brent Meeker wrote: On 3/10/2025 11:04 PM, Alan Grayson wrote:On Monday, March 10, 2025 at 11:15:07 PM UTC-6 Brent Meeker wrote: On 3/9/2025 11:14 PM, Alan Grayson wrote:I don't think you understand my question. Without a CC, or equivalently setting it to zero, don't we get a universe which is in UNSTABLE equilibrium, like balancing a pencil of its writing tip, so the universe expands or contracts in a very short time interval? Isn't this the issue Einstein faced? If so, why wouldhe choose a positive CC? AGNo, Einstein's model with the CC=0 was static. The model when I was in grad school was an expanding universe with the CC=0 but the expansion kinetic energy was just balanced by the negative gravitational potential, so the universe would expand forever but slowing asymptotically toward static. Brent Now I am totally confused. If E's model was static with CC=0,Sorry, I miswrote. I intended to say Einstein had to make the CC>0 in order to balance the gravitational attraction. Brent OK. Does setting CC>0 result in unstable equilibrium as I think Clark claimed, and discovered by Arthur Eddington? IOW, will the universe suddenly contract if it is expanding? AGNo, it's unstable as a static universe, which was the general opinion of astronomers at the time. The Milky Way was the only known galaxy. The other smudges in the night sky were "nebula". So Einstein calculated a value for the CC that would just balance the gravitational attraction of the Milky Way, to explain why it hadn't collapsed. But this produced an unstable equilbrium. It was about 10yrs later that Hubble discovered the universe was much bigger than just the Milky Way and it was expanding. BrentIt was Arthur Eddington in 1930 who showed that a static universe with CC>0, would be in unstable equilibrium. AG
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