From: ted@physics3 (Emory F. Bunn)
Newsgroups: sci.physics
Subject: Re: No big crunch?
Followup-To: sci.physics
Date: 6 Nov 1992 03:16:19 GMT
Organization: /etc/organization
NNTP-Posting-Host: physics3.berkeley.edu

In article <1992Nov6.020022.29066@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>In article <1dcf7vINNfi0@agate.berkeley.edu> ted@physics.Berkeley.EDU (Emory F. Bunn) writes:

(stuff deleted.)
>>cosmology.  Grand unified theories require new massive particles.  This

>      How can one make definitive statements about GUT's *requiring*
>      new massive particles without there existing a tenable GUT?
>      After all, I am reasonably sure that there are infinite possible
>      theories that do not require new particles.  In fact, there are 
>      probably potential GUTs that do not even use the concept of particles.
>      I am not so confident as to the suitability nor the uniqueness of
>      our representations as you appear to be.

When I spoke of GUTs, I meant theories of the sort that particle physicists
currently think are likely.  Specifically, these are theories in which the
electroweak and strong interactions by viewing them as the gauge interactions
of a single larger gauge group.  I'm not asserting that such a theory is
certainly right, but rather that there are good, noncosmological reasons
for thinking that such theories are not improbable, and that such
theories quite generically produce nonbaryonic matter of the sort that
people hypothesize.  This is not a proof of the existence of nonbaryonic
matter:  There is no such proof.  It's simply an argument that nonbaryonic
matter is more plausible than it might initially appear.


>>is an argument in favor of nonbaryonic dark matter which is completely
>>independent of the usual cosmological ones.
>
>      In the absence of a tenable theory, this is not an argument for
>      much of anything.
>
>      I repeat my question.  How would Einstein have fared against
>      an army of 'believers' in the nonbaryonic dark matter hypothesis?
>      Thank goodness the particle zoo was rather barren at the time.
>      I suspect that he wouldn't have wasted his time because one
>      of his primary motivations would have been gone.  Even if he
>      did, I suspect no one would have listened.  Seelinger's hypothesis
>      of intra-Mercurial matter would have been greatly strengthened
>      if he had just known about the nonbaryonic dark matter that was 
>      everywhere in great abundance, especially seeing as you didn't
>      actually have to *see* anything.

You are discussing two competing theories of gravity:  Einsteinian gravity
and Newtonian gravity plus dark matter.  Suppose that you don't know
which of these theories is right.  What do you do?  You make lots of 
observations of gravitating objects, and see which theory does better
at predicting their behavior.  You'll find that Einsteinian gravity
makes the correct predictions without constant tweaking of the parameters,
while the dark matter theory requires continual modifications in the
distribution of dark matter for each new observation.  It wouldn't take
you long to settle on Einstein's explanation as a better one.

The amazing thing about the cosmological dark matter hypothesis is that you 
_don't_ have to keep changing the parameters as new observations come in.
If you read the actual papers on the subject, you find that in the 
cold dark matter theory there is only one free parameter, with which
one can fit a variety of observations.  (The shape of the galaxy-galaxy 
correlation function and the recent microwave background anisotropies
among others.)
>
>>The most important point is simply this:  If you want to decide whether
>>or not to believe in nonbaryonic dark matter, you'd best study the evidence,
>>and see whether or not you find it compelling.  It's not good enough just
>>to say, "It sounds pretty strange to me, so I don't buy it."
>
>      Nor is it good enough to say 'everyone accepts it, so it must
>      be okay'.   I am reminded of the emperor's clothes.
>

Agreed.  I never intended to make any such assertion, and I don't believe
I did.  All I'm claiming is that the dark matter hypothesis is a solid
one by the usual standards of science:  It is a simple theory, with
few free parameters, which fits all of the known observations.  Not
only that, but the agreements with theory are genuine predictions;
that is, the predictions were made before the observations.

-Ted