🚀 go-pugleaf

jbaez@riesz.mit.edu (John C. Baez) says:
>
>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!!!  So one should not think of the
>Hubble constant as being something like the electron mass or charge
>which seems to be a rather precise and unchanging constant.

This needs some clarification.  Even in the absence of deceleration,
the Hubble constant changes with time.  It is empirically
observed that the recession speed of galaxies (as measured by red shift)
proportional to the distance.  The constant of proportionality has units
speed/distance, usually given as (km/sec)/Megaparsec.  The same constant
applies (more-or-less) to all cosmological objects.  Hence the use of the
word "constant."

If one is clever, one notices that the dimensionality of the Hubble
constant is inverse time.  In fact, in a big bang cosmology it tells you
to within a numerical factor the age of the universe.  Thus, even if
there is no deceleration the Hubble constant decreases as time passes.
This is simply because the universe is getting older.  More prosaically,
receding galaxies get farther away.