From: metares@well.sf.ca.us (Tom Van Flandern)
Subject: Re: Expanding space, or not?
Organization: Whole Earth 'Lectronic Link
Distribution: na
Date: Fri, 22 Jan 1993 07:27:37 GMT


zellner@stsci.edu (Ben Zellner) writes:

>> So what is all this talk about "expanding space"?

and Doctor J (Email: DoctorJ@UIUC.Edu) replied:

> I don't know.  It is probably a result of taking the balloon analogy too
> seriously.

     In the big bang theory, space itself expands.  Galaxies remain
motionless (ignoring their small, peculiar velocities) in space, and move
apart because new space is continually appearing between them.

     If the galaxies themselves moved, that would require a center and an
edge to the big bang explosion.  Then the universe would not look
everywhere the same, as the Friedmann postulates require.

     Here are some references.  In the new Sky & Telescope article (Feb.
1993, pp. 31-35), Odenwald & Fienberg say: "This [cosmological] redshift,
which again is *not* a Doppler shift, arises from the expansion of space-
time itself.  Light waves literally stretch as the universe expands between
the time the light was emitted and today, when it finally reaches us."
"Now galaxies are located at fixed positions in space.  They might perform
small dances about these positions in accordance with special relativity
and local gravitational fields, but the real "motion" is in the literal
expansion of the space between them."  "This is not a form of motion that
any human being has ever experienced, in that it does not involve travel
*through* space.  So it is not surprising that our intuition reels at its
implications and seeks less radical interpretations."  [See illustrations.]

     "Space, Time, and Gravity" by R.M. Wald (2nd ed., p. 48), U. Chicago
Press, 1992, has an easy-to-follow mathematical description of this
property using Einstein's equation.

     "Introductory Astronomy & Astrophysics" by Zeilik, Gregory & Smith
(3rd ed., p. 432), Saunders College Publ., 1992 has an easy, general
discussion that makes these same points.

     Of course, if you are looking for a comprehensive, authoritative
treatment, see "Gravitation" by Misner, Thorne & Wheele (p. 719), W.H.
Freeman & Co., 1973.

     All of these references say essentially just what I said above.

>> do the standard, General-Relavistic models of the expanding universe
>> include some mystical "expansive force" and some "expansion of space",
>> whatever that means, or just inertia and gravity?

> There is no force of expansion of space (as a physical entity) in
> associated with the expansion of the universe in GR.

     There is a mystical "expansion force" only in the so-called
"inflationary" version of BB, and only during the early stages.  The
expansion of space proceeds today from the momentum of the original
explosion of space.  No force drives it.  It is slowing due to the gravity
of all matter in the universe.

>> Suppose you had two test particles a few megaparsecs apart, feeling the
>> gravity of the universe as a whole but not significantly from each
>> other, and started them AT REST with respect to each other (NOT
>> co-moving with the Hubble flow).  Would they subsequently remain at rest
>> with respect to each other or would they move apart at the Hubble
>> expansion rate and show a red-shift to each other?

> There is nothing special about comoving wrt the Hubble flow.  An object
> moving at some other speed will not feel any force due to that fact.
> Therefore, two particles at rest (wrt each other) will remain at rest
> unless some conventional force moves them.

     This contains a confusion of terms.  In the big bang, galaxies are
considered at rest in space, though they move apart because new space
appears between them.  As I understood Ben's question, if two particles
started at rest with respect to each other (NOT co-moving with the Hubble
flow), then one of them must be moving through space rapidly toward the
other to overcome the Hubble flow carrying it away from the other.  No
forces act.  Bodies in motion remain in motion until forces cancel that
motion.

>> Unless a real, standard-model cosmologist convinces me different, the
>> bottom line seems to be that the galaxies move apart because they have
>> kinetic energy and momentum relative to each other, left over from the
>> big bang, and for no other reason.

     In E-mail, Ben made this same point: he wants to hear the words from
someone other than me, because I am a big bang critic.  So my citations and
quotations above may not be sufficient.  Therefore let me amplify Ben's
call for a standard-model cosmologist to comment on whether the big bang is
being accurately portrayed or not in this message.  -|Tom|-

-- 
Tom Van Flandern / Washington, DC / metares@well.sf.ca.us
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