Re: Cosmology

["Moore, David" <DMoore@GDA.ORG>]

Title: RE: Cosmology

According to Robert Kirshner's its possible that the expansion of the universe is accelerating. That is, space itself is expanding along with the distances between the galaxies increasing due to the initial Big Bang, and such an expansion of space may be described by Einstein's cosmological constant. If that argument has any merit, then to me, the expansion of space should be noticed locally.  That is, the space between the local galaxies (and between me and the computer that I am writing this on) should be expanding and not held in check by local gravitational forces.  Can someone correct my dubious logic here?

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-----Original Message-----
From: Robert B Zannelli [mailto:Spinoza321@AOL.COM]
Sent: Sunday, February 11, 2001 11:34 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Cosmology

In a message dated 2/11/01 8:13:29 PM Eastern Standard Time,
JMGreen@SISNA.COM writes:

<< You (and someone else) say that neither the local nor even small groups of
 local clusters expand, but the super clusters do.   Why???  Well yes, if
 the Universe is expanding something has to spread out a bit, but why super
 clusters and not our galaxy?  What makes a local cluster "gravitationally
 bound" and not a super cluster?
  >>
Jim Your questions are really quite good. Even though there are eminent
cosmologists on this list who no doubt could do much better than I in
answering these questions, I will give your question a try. First the whole
theory of the structure of the cosmos and how it that structure evolved, is
very much cutting edge science where very little is settled.
     One way to look at it, is that in an explosion there are always some
pieces which remain whole. This is because the forces holding them together
exceed the forces pulling them apart. Of course as the pieces get bigger
those forces holding the structure together get weaker due to distance
effects. Of course with regard to our Universe there were no "pieces" in the
beginning since all the cosmic structure evolved after the big bang, but the
same basic principals were at work once those structures were formed.
      How exactly our Universe got it's present structure is really not very
well known at this time. However we do know, thanks to the data provided by
NASA's COBE probe, that there were Quantum fluctuations in the energy density
of the earlier Universe which became the seeds for our present cosmic
structure.
     There are two main classes of theories to explain all this. There is the
bottom-up theories which maintains that that these primordial seeds formed
proto galaxies and that these objects subsequently got together to form all
the larger structures. There is also the top-down class of theories which
maintains that the large structure formed first, then divided into all the
other substructures. As of this time, as far as I know, there really is not
much evidence available to decide this one way or the other.
      Nevertheless, whichever way it happened, all of this structure building
took place in an expanding space time. Obviously for this structure to exist,
it must overcome the expansion of the universe locally. Of course the force
that built all this structure is gravity. This is because unlike the other
forces of nature, the effects of gravity are cumulative as there is no
balancing anti gravity force except possibly at very large scales.
      The whole topic of what were the agents of these gravitational forces,
cold dark matter, or hot dark matter, etc., is a fascinating subject. The
fact that we are now pretty sure that the subatomic particles called
neutrinos have mass, may provide part of the answer to this puzzle.
     So basically those cosmic structures the size of clusters or smaller
have sufficient gravitational attraction to maintain their structure despite
the forces of expansion while larger structures are being stretched or pulled
apart because at their scale, gravity is too weak to hold them together. I
hope all this helps.

Bob Zannelli