🚀 go-pugleaf

People:

The following is a log of some e-mail that I exchanged with particle
physicist "Doctor J (Email: DoctorJ@UIUC.Edu)", slightly edited in the
interest of saving bandwith.

Ben Zellner


----BEGIN QUOTES---------

  From:	IN%"JJT@UIHEPA.HEP.UIUC.EDU"  "Doctor J (Email: DoctorJ@UIUC.Edu)" 
  To:	IN%"ZELLNER@stsci.edu"
  Subj:	RE: "Modeling" the Expanding Universe?

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

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


From:	IN%"JJT@UIHEPA.HEP.UIUC.EDU"  "Doctor J (Email: DoctorJ@UIUC.Edu)" 
To:	IN%"ZELLNER@stsci.edu"
Subj:	Cosmology

I'm an experimental particle physicist.  My knowledge of cosmology is limited to
what I have learned from reading Kolb & Turner (a very good book) and scanning
the preprints as they whiz by. 

  [omitted discussion of dark matter, microwave background]



From:	STSCIC::ZELLNER      "BEN" 20-JAN-1993 17:40:55.35
To:	IN%"JJT@UIHEPA.HEP.UIUC.EDU"
CC:	ZELLNER
Subj:	RE: Cosmology

Thanks for all of that!  Actually my question was much narrower.  

Regardless of whether they describe our real universe or not, 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?  

I realize that I am using Newtonian terminology, but it's hardly possible to 
be more rigorous without getting into Christoffel symbols and all that, right? 

Is this a good question?  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? 

Ben


From:	IN%"JJT@UIHEPA.HEP.UIUC.EDU"  "Doctor J (Email: DoctorJ@UIUC.Edu)" 
        20-JAN-1993 18:28:55.16
To:	IN%"ZELLNER@stsci.edu"
Subj:	RE: Cosmology

  > Regardless of whether they describe our real universe or not, 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.

  > Is this a good question?  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.

----------- END QUOTES

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.

Cheers, Ben