From: alex@csn.org (ALEX MATTHEWS)
Subject: Re: "Modeling" the Expanding Universe? 
Organization: University of Colorado, Boulder
Date: Thu, 21 Jan 1993 15:26:02 GMT

In article <727571532snz@chrism.demon.co.uk> chris@chrism.demon.co.uk writes:
>In article <93020.102708DOCTORJ@SLACVM.SLAC.STANFORD.EDU> DOCTORJ@SLACVM.SLAC.STANFORD.EDU writes:
>>
>>If two test particles are placed at rest wrt each other, the Hubble
>>expansion will not cause them to begin to move apart.  That is, the
>>expansion of the universe does not induce any effects that would be
>>interpreted as forces.  Thus, there is no observable efect on bound
>>systems such as the solar system, hydrogen atoms, galaxies, etc.
>>
>
>I'm probably being dense, but if this is the case, why do the galaxies
>move apart, then?  Surely, if two galaxies were initially of rest wrt to
>one another, but were sufficiently far apart that they didn't interact
>gravitationally in such a way as to "prevent" the expansion of space,
>then the expansion of the universe *would* make them recede from one
>another, wouldn't it?

The "universe" is defined by the galaxies we observe.  Therefore, the
"universe" cannot exert forces on those galaxies to accelerate them
away from each other.  The gravitational forces originating from the
galaxies *can* accelerate them towards each other -- in the framework
of the Big Bang this contributes to the formation of clusters of 
galaxies and other large-scale structures.  

Picture the universe as an adiabatically expanding gas (a gas 
expanding via its own internal pressure).  The initial 
pressure is high and dominates the gravitational attraction, and the
particles are forced away from one another.  At later times the pressure
and density are low, collisions are rare, and the bulk motion is
everywhere outwards (i.e. all particles move away from one another
in a spatially infinite gas).  This relict momentum is nibbled away
by gravitational attraction between particles, which now dominates
because the pressure has decreased so drastically.  At small separations
the relict momentum is smallest and the gravitational forces are 
greatest, so these regions "turn around" and fall into each other
soonest.  Once thus collapsed, they "virialize", or balance their
kinetic and potential energies, and form relatively stable, bound 
objects like solar systems and galaxy clusters.

Try not to be confused by talk of general relativity, curvature,
departure from Hubble flow, etc.  The mathematical framework is
opaque to most of us until we understand the simple physical 
behavior of the observed universe.  I am a proponent of the "raisins
in a loaf of rising bread" analogy, with one caveat: in the context 
of the previous paragraph, the bread dough is analogous to the 
gas pressure, and is only important early on; at later times the
dough is so diffuse that it ceases to accelerate the raisins 
outward and the only important force is the gravitational attraction 
between raisins.  Picture, if you will, an infinite loaf of invisible 
bread studded with brightly glowing raisins in a cosmic microwave 
oven....  :-)

- Alex
-- 
- Alex Matthews  (new address: alexm.csn.org)

  "A typical Grand Prix race.  High noon.  Tension mounts.  Throttles clench."
                        Patricia Zonker, _Murdercycles_