From: metzler@pablo.physics.lsa.umich.edu (Chris Metzler)
Newsgroups: sci.physics
Subject: Re: Request info on Cold Dark Matter
Date: 8 Jan 1993 21:56:44 GMT
Organization: University of Michigan Department of Physics
Distribution: world
NNTP-Posting-Host: pablo.physics.lsa.umich.edu
Keywords: cold dark matter, cosmology

In article <1993Jan8.151056.14346@ulrik.uio.no>, jarleb@athena.uio.no (Jarle Brinchmann) writes:
|> 
|> Chris Metzler writes:
|> 
|> > 2.  Hi Mike.  Hope things have gone well since Varenna.  I would take a
|> > small amount of exception with your statement that "COBE results have
|> > shown that minimal CDM is not correct, since it creates too much structure
|> > at small scales if one normalizes the distribution to the large (COBE)
|> > scales."  I would take exception to this.  Remember that the simulations
|> > people have done which have told them that there's too much structure on
|> > small scales assume that the collisionless particles in the N-body
|> > simulations are accurate tracers of galaxies.  I am NOT talking about
|> > spatial bias (biased CDM) here.  Galaxy formation simulations which include
|> > both N-body and hydro suggest that there exists velocity bias even in
|> > unbiased CDM models, whether from dynamical friction or from the
|> > nature of the environment galaxies are born into.  Thus, the whole
|> > "pairwise-velocity-dispersions are too high" problem may not even exist.
|> > You may be right, but I think the jury is still out; the impression
|> > I'm starting to get from the CDM gurus is 'hey, maybe unbiased CDM
|> > wasn't so bad after all.'
|>   
|> Maybe this is because they are _CDM_ gurus :-) ? I recently read (or is
|> reading) a preprint from A. Kashlinsky, in which he argues that with our
|> current observations of high-z objects, there is not easy to get enough
|> power on small scales with CDM. Not easy means virtually impossible in
|> his opinion, but then he is not very supportive of CDM.....
|> 

Well, I'll have to look at the preprint.  We may have it around; we get
Kashlinsky's stuff.  The volume of published results, let alone preprints,
is way above what one can keep up with any more.  But I suspect that's true
of any field.

Anyway, this would indeed be an interesting result if Kashlinsky got it
with unbiased CDM, since other results suggest too much small-scale power.
Hmmmmm.

|> I think very many wants CDM to survive because it is quite well understood
|> and it has survived comparatively long. But if we shall start to use models
|> with mixed CDM and HDM or a non-zero cosmological constant, I see no 
|> aestethic reason for not having a tilted spectrum. IMHO both a mixed DM 
|> universe and a tilted spectrum contains some a priori unknown constants, 
|> and therefore I guess we can't say much. Too bad COBE gave us such results
|> it would have been much more fun if we could make everything as simple as
|> possible :-)
|> 
|> 

Well, the primary sexiness about mixed dark matter (MDM) is that the simplest
extensions to the Standard Model of Elementary Particle Physics seem to
predict both a cold dark matter candidate particle and a massive neutrino.
And despite the rousing success of the Standard Model at accessible energies,
very few particle physicists believe we know the whole story.  So there is
some theoretical motivation for MDM that isn't quite there for a tilted
Harrison-Zel'dovich spectrum (it's a question of "should be true" vs. "could
be true").  Don't interpret that as my saying that I think tilted spectra
are necessarily wrong -- I don't know enough about the issue to even THINK
about making a statement like that.

And re: your COBE statement . . .well, another way of looking at all this is
that as long as observations are pushing theorists to the limit, then
theorists have jobs.  Important to those of us who'll be going into the
job market in the not too distant future.


-- 
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Chris Metzler                                    313-764-4607 (office)
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