NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/astro/14529

Internet Message Format | 1993-01-28 | 3.2 KB

Path: sparky!uunet!ulowell!m2c!bu.edu!weyl.bu.edu!sbs
From: [e-mail] (Stephen Selipsky)
Newsgroups: sci.astro
Subject: Re: "Modeling" the Expanding Universe?
Message-ID: <[e-mail]>
Date: 28 Jan 93 21:26:54 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail]
Organization: Boston University Physics Department
Lines: 58


In article <[e-mail]>,
 [e-mail] (Tom Van Flandern) writes:
 ...
|>     Distant objects do not move through space at all, much less slow due
|>to the gravity of other objects.  Since the universe is supposed to be
|>homegeneous and isotropic on the large scale as seen by every observer, in
|>what direction could a "slowing" possibly act?
 ...

sbs:
   The "slowing" acts in the direction connecting each observed moving
object to the observer watching it.  You can use Newtonian intuition;
every pair of points defines center and surface of a sphere (or two),
and Birkhoff's theorem eliminates dynamical effects of matter outside
the sphere, so the matter on the surface of the sphere is slowed by the
attraction of matter inside the sphere.  This viewpoint is symmetrical
between which point is the center and which is on the surface, so either
way separation velocities decrease.

|>
|>and [e-mail] (Stephen Selipsky) writes:
|>
|>> nothing weird and nonlinear happens; the Solar system just gets crushed
|>> as all that external matter crashes in.  "The fall doesn't kill you, it's
|>> when you hit the bottom".
|>
|>     If all of space is uniformly contracting when the matter density is
|>uniform, and the only exception is a spot of nearly infinite matter
|>density, the stress-energy is surely higher there than elsewhere.  Why
|>would this operate to *nullify* an ongoing general contraction?  The sign
|>is wrong.
|>
|>     The extra space is supposed to be continuously created out of the big
|>bang.  Why does the new space that tries to appear in, e.g., the solar
|>system get suppressed so that it never appears at all, or appears somewhere
|>else?  -|Tom|-

sbs:
As you say, space has *much more curvature* near the local extra mass
than it does in the rest of the universe.  (Gravitational deceleration
of an object leaving the Sun is indeed much greater than the Hubble
deceleration.)  And that's the point; the curvature of local space is
dominated by the amount of local collapsed matter, which is pretty much
*constant*, so the metric nearby is also *constant*.  No expansion or
contraction.  In the rest of the universe, density is *changing* due to
the contraction or expansion, which changes the metric, equivalent to
more expansion/contraction.  This is why the density determines the
Hubble parameter ("velocity"), not only the "acceleration" q, a point
that sometimes confuse people.
   When the rest of the universe (*) gets so dense that its changing
density is significant relative to the density of the Solar system
(or local supercluster), *then* you'll see the local metric start to
change with time!  Not otherwise; GR is a local theory.

     Regards,  -- stephen.

(*)The boundary condition for the local Schwarzchild metric.