NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/sci/physics/18260

Text File | 1992-11-07 | 6.3 KB | 125 lines

Newsgroups: sci.physics
Path: sparky!uunet!charon.amdahl.com!pacbell.com!decwrl!concert!uvaarpa!murdoch!kelvin.seas.Virginia.EDU!crb7q
From: [e-mail] (Cameron Randale Bass)
Subject: Re: No big crunch?
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: University of Virginia
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Fri, 6 Nov 1992 04:23:06 GMT
Lines: 113

In article <[e-mail]> ted@physics3 (Emory F. Bunn) writes:
>In article <[e-mail]> [e-mail] (Cameron Randale Bass) writes:
>
>>>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

     Suppose, as I have been, that you *don't even have* GR.
     In 1906, there was no certainty that GR would be developed.
     In fact, if Einstein had died in 1906, I'm quite sure it
     wouldn't have been.

     Speculate on the ultimate develoment of GR in competition with
     present-day nonbaryonic dark matter hypotheses (ignoring for the
     instant that GR would have *never* become accepted because it
     does not fit well into QM).

>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.

     However, I doubt he would have been compelled to develop it
     in the first place.  As far as 'tweaking' goes, isn't this the
     current paradigm in modern cosmology?

>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.)

     I guess all is right with the hypothesis then?   Good show.  We can all
     pack it up and go home.

     However, I'm sure I could come up with 50 papers that 'showed'
     that maybe all is *not* right with that world.  However, I'll satisfy
     myself with the first one that comes to mind "The end of cold
     dark matter?" Davis \etal Nature, 356:489 (1992).  Though
     they certainly do not advocate the end of that particular hypothesis,
     the necessary contortions to fit mounting evidence of large-scale
     structure are amusing.  While talking about lowering omega
     within the constraint of microwave background fluctuations:

       "These  problems can be avoided by appealing to a
        cosmological constant, because a low-density universe is
        spatially flat if the cosmological constant takes the value
        Lambda = 3 H_0^2 (1 - Omega).  With such carefully chosen parameters
        it is possible to construct a CDM universe that explains large-scale
        structure, is compatable with inflation and with microwave
        background experiments, and is old enough to contain the
        oldest observed star clusters ... (even for H_2 as high as 80)...
        From the point of view of the particle theorist, the value of
        Lambda necessary to work these miracles is extraordinarily
        small, 10^120 times smaller than its 'natural' value ..."

     Drole, quite drole.  I was even more amused by the unintended
     humor in the last sentence of the paper.  Feel free to read it yourself.

>>>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.

     It fits *all* the observations?  Geez, what are we even talking
     about then?

     And I am claiming differently.  In the absence of good evidence
     that there is any of this matter floating around, it becomes
     as ephemeral as the zodiacal light hypothesis.  Unfortunately,
     ephemeral seems to take longer than it has at times in the past.

                         dale bass



--
C. R. Bass                                          [e-mail]
Department of Mechanical,
     Aerospace and Nuclear Engineering
University of Virginia                              [phone]