On Saturday, July 19, 2025 at 6:48:18 PM UTC-6 Brent Meeker wrote:
On 7/19/2025 10:54 AM, Alan Grayson wrote: On Friday, July 18, 2025 at 11:34:17 PM UTC-6 Brent Meeker wrote: On 7/18/2025 6:49 PM, Alan Grayson wrote: *And the farther away it is, the greater is its red shift and recessional velocity. So the recessional velocity seems to be DECREASING with time as the universe expands. * The second does not follow from the first. Further away means later in time. Further away is receding faster. Brent *Let me rephase that; first, consider the model of the universe as an expanding sphere and two separated galaxies on that sphere, say on the equator. As we discussed, it's an effect of geometry that the rate of the separation distance increases depending on the initial separation distance, and the red shift increases as well as the separation velocity. * Why do you write "as well as"? The red shift *is due to* the separation velocity. *OK, no problem. AG* *That's as time moves forward. Now we get the same result when we consider time moving backward, as we look backward in time and see the red shift and recessional velocity increasing. * First you write "consider time moving backward" and then you write "as we look backward"?? Are you considering time reversed motion; bodies moving closer together? Or are you just thinking of how things must have been ten or so billion years ago? Or are you saying you're going to look at some distant galaxies, which are implicitly far back in time from the "now" defined by distance from the CMB? What are you measuring the recessional velocity from? From some specific galaxy? Or from galaxies at some specific distance? If you mean a specific galaxy then in the past we were closer to it and therefore our recession from it was slower. If you mean galaxies at a specific distance then the Hubble parameter being constant means that recession velocity was the same in the past. *I'm considering galaxies in varying distances from our galaxy, so I'm looking at recessional velocities in the past, and they are all increasing. So, even though a now distant galaxy was close to the point in spacetime which was where our galaxy came into existence, wasn't it still receding rapidly from that location? How then can anyone conclude that the separation velocity was small? AG* Try thinking in terms of the Hubble parameter as constant. That means space has been expanding by a constant multiplier, e>1, per unit time. So our recessional velocity relative to galaxies at a given distance D, is and always has been De. Once you've got that straight you can consider models in with e changes with time from the Big Bang, which is modeled by all those nice colored curves I posted. *There's my dilemma. Whether we go backward or forward in time, two separated galaxies exhibit increasing red shift and increasing recessional velocity. AG* No, e is a constant. So if you consider two specific galaxies their separation in the past was less and a smaller D means a lesser recessional velocity De. Brent -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/everything-list/3843398a-9e8b-46d0-8e2e-44faa60ae118n%40googlegroups.com.
