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In article <25203@dog.ee.lbl.gov>, sichase@csa3.lbl.gov (SCOTT I CHASE) says:

> doctorj@slacvm says:

   [...my assertion that the Hubble constant decreases with time deleted...]

> Now *I'm* confused.  Take a galaxy G at a distance D from us.  It
> will be receeding at a speed of Ho*D up to some small peculiar velocity.
> Say that in a few billion years (by which time I might even have tenure!)
> G is now twice as far from us.  Won't its recession speed be 2*Ho*D?
> There is twice as much space expanding between us, so it should be receeding
> at twice the speed.

> Your argument seems to suggest that recession speeds are fixed so that
> in a few billion years G will be twice as far from us, but still have
> the same recession speed, causing us to conclude that Ho has shrunk
> to half its current value.

Recession speeds do not increase with time in the way that you are saying.
References:
* PJE Peebles, "Physical Cosmology,"  An old book, but still useful.
* Kolb and Turner, "The Early Universe."  A new book, but already useful.
* Sandage and Tammann, "The Dynamical Parameters of the Expanding
  Universe...," in "The Early Universe: Reprints" (edited by Kolb and Turner).

In the absence of a cosmological constant, recession speeds can only decrease
(due to mutual gravitational attraction).  If the density of the universe
were very small, there would be no deceleration, and the speeds would remain
constant.  In that case the age of the universe would be exactly T = 1/Ho.

The density is known not to be small, and is believed (on the basis of
very little evidence) to be just enough to close the universe (critical
density).  In that case, recession speeds decrease asymptotically to zero
as time goes to infinity.  The relation between the age of the universe
and the Hubble constant is then T = 2/3Ho.