From: sichase@csa1.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: No big crunch?
Date: 6 Nov 1992 11:43 PST
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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In article <1992Nov5.211058.25220@murdoch.acc.Virginia.EDU>, crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes...
> 
>     Though I am a bit surprised by this statement since I believe you have,
>     in the past, posited that there was very little fundamental left
>     to discover, I do agree.  However, basing one's cosmology on the 
>     *requirement* that new, unknown matter actually comprises most
>     of the known universe seems extremely speculative.  

I believe that we have built a solid foundation of fundamental physics 
which is well understood and highly unlikely to change.  We have a much
better quantitative understanding of the precise limits of our knowledge
than we used to.  Perhaps, in this context, I should have said "qualitatively
new" rather than fundamentally new, so as to avoid confusion.  The existence
of dark matter, per se, would not do any damage to the fundamentals of 
physics as we know them.  Of course, we might discover some new fundamentals
by studying dark matter in detail, assuming that it's non-baryonic.

>     I did not say that such *speculation* was outrageous, it was the
>     *production* that seems outrageous.  I would be perfectly happy
>     if all discussions of nonbaryonic dark matter were preceeded by 
>     the statement that 'this is extremely tenuous speculation'.

Several additional comments are in order here, to supplement the discussion
of the past few days.

(1) Dark matter does not only explain gravitatinal anomalies.  It also 
is necessary to get the right primorial nuclear abundances from the Big
Bang.  Essentially, the more matter in the early Universe, the longer 
the hot dense phase lasts, and the more He3, for example, gets produced.

(2) Tecent HST high-res imaging of a new gravitational lens demonstrates that
99% of the mass of the lens is not in the luminous part of the galaxy.  I 
imagine that any alternative to dark matter along the lines of GR modifications
will be highly constrained by reproducing such a lens.  Perhaps we are not
too far from ruling out the alternatives to dark matter by such observations.

>     After all, apparently HEP theorists did not do a very good job
>     extending the Standard Model to predict the top mass.  Why should
>     we take as anything but doubtful fancy the tenuous speculative
>     extentions that predict such things?

I suppose that you are entitled to take that position with regard to any
unproven scientific speculation.  On the other hand, the people who are
excited about the possibility of dark matter are not all reckless, off chasing
wild speculations.  The clues which point to the possibility of dark matter 
(not *exotic* dark matter) are as good a scientific lead as you will ever
get, IMHO, pointing towards something completely new.  What do *you* make
of all the excitement about dark matter searches, given your viewpoint
on this issue?

>      However, how easy it is to test is a matter of debate.  I suspect
>      that I could invent a dark matter distribution of various forms
>      of dark matter with different interactions that would pass any and
>      all observational tests.  I would be happy to hear differently,
>      but I'm not holding my breath.

All of the direct searches for exotic dark matter have the single assumption
that there is a measureable flux of dark matter particles in our part
of the galaxy.  If it exists at all, this is a fair assumption.  The
detectors are designed to be as generic as possible.  They are not dependent
upon the dark matter particles having any particular interaction, but merely
that they *have* an interaction with baryonic matter.  The typical apparatus
is a liquid-helium temp. bolometer which is sensitive to the small 
temperature fluctuations which result from a dark matter/atomic interaction.

>     I agree.  How about making $400 million a year available, and
>     we'll let y'all have $40 million for cosmic ray research?  I suspect
>     the concrete requirements (pun intented) for cosmic ray research
>     are somewhat lower than SSC.

Progress in HEP has often depended upon our ability to take initial 
discoveries of particles in the cosmic rays and then to automate the production
and detection of such particles in accelerators for high precision studies.
I think it quite likely that, if someone detects dark matter particles,
as soon as enough is understood about the basic characteristics of the stuff
an accerator-based production method will be devised to study it in more detail.

However, you surely know that the SSC is dedicated to a completely different
research task.  The SSC is bread-and-butter high-energy physics, without which
the field will have a hard time proceeding.  Dark matter searches are a 
more speculative avenue of research, one of many different approaches trying
to take a stab beyond the Standard Model, to point to way to whatever
lies beyond.  It is really a bummer, in a way, that the SM repeatedly 
manages to give correct answers to our best experimental tests.  It gives
is more and more confidence that the SM is basically sound, but leaves
up high and dry when we try to figure out what's next.

-Scott
--------------------
Scott I. Chase			"It is not a simple life to be a single cell,
SICHASE@CSA2.LBL.GOV		 although I have no right to say so, having
				 been a single cell so long ago myself that I 
				 have no memory at all of that stage of my 
				 life." - Lewis Thomas