🚀 go-pugleaf

John Baez writes:
>In article <1468500004@igc.org> anlyyao@igc.org (An-Ly Yao) writes:
>>Why and in relation to what is the "Hubble constant" constant?
>
>It's not constant in any profound sense.  To the extent that the
>universe is expanding at a constant rate, the Hubble constant is that
>constant.  However GR predicts that with some matter around the rate
>of expansion of the universe is decreasing; so in addition to Hubble's
>constant people often use an extra "deceleration fudge-factor" (sorry, I
>forget the correct name).  Moreover, when it comes to actually MEASURING
>the Hubble constant there are two schools of thought which differ by a
>factor of 2 as to what it equals!!!  ...

Now for the astronomy viewpoint:
The Hubble constant (H_0) has dimensions velocity/distance (so is an inverse
time, really) and is currently thought to be somewhere in the range
50-100 km/s/Mpc.  It's supposed to be the ratio of the speed at which
distant objects are receding over their distance (so it's really a
redshift over distance) AT THE PRESENT TIME (disregarding deviations
from the universal expansion).  It expresses a linear relationship, 
but a little dash of special relativity will convince you that the
linearity is an approximation good only for low z (redshift). 

Additionally there is the deceleration parameter q_0 (no fair calling
it a fudge John.  The cosmological constant ... now there's a fudge)
which describes how the Hubble "constant" changes over time.  That is,
at some point in the past the universe was expanding faster and H_0
was bigger, and at some later date H_0 will be smaller.  Thus H_0
depends on the time, describes a linear relationship which is only
an approximation, and describes a universal expansion from which the
deviations (peculiar motions) are now considered significant.  Some
constant, eh?