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.
*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.
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
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