🚀 go-pugleaf

In article <1d9lptINN94@agate.berkeley.edu> ted@physics3 (Emory F. Bunn) writes:
>In article <1992Nov4.203930.20410@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>>     This has always sounded to me like a perfectly good explanation
>>     of how pixies push galaxies around.  Lemme see, lets postulate
>>     a whole bunch of matter that we cannot see (or alternatively,
>>     invent it at an earlier time via various hypothetical processes) and
>>     has completely different properties from all the matter that 
>>     seems to be in our neighborhood.  I prefer pixies.
>
>The big difference between the two hypotheses is that the dark matter one is
>testable and the pixie one isn't.  There are at least two tests of the
>dark matter hypothesis that would distinguish it from the pixie hypothesis.
>One is internal consistency:  Only certain distributions of densities and
>velocities are consistent with the model in which structure grew from
>small initial perturbations by gravitational instability.  While no conclusive
>answer is out yet, these calculations are constantly being done, and
>so far the model seems to be withstanding the test.

     Internal consistency does not constitute a 'test'.  It is a
     necessary condition.

>The other way of testing the dark matter hypothesis is to go out and
>look for the dark matter.  There are lots of experiments going on that
>should be sensitive to the various kinds of dark matter that people have
>proposed.  If the experiments get sensitive enough, and fail to find anything,
>then the theory will have to be abandoned.

     How could this happen?  The process you have discussed will always be
     consistent with 'pixie' matter of various forms.

>>     If the dark matter is basically not in gravitational 'equilibrium'
>>     with the baryonic matter in some sense, why do we apparently see no 
>>     extremely local inhomogenities in our neighborhood?  After all, you're 
>>     going to need one buttload of non-baryonic matter, and even tiny
>>     local inhomogeneities would have a rather disconcerting effect
>>     on solar system orbits.
>>
>Whether you agree with the standard cosmological model or not, you should
>at least give the people who made it up credit for not being complete 
>fools.  The evolution of the distribution of dark matter in the theory
>has been carefully calculated by dozens of different people under
>a variety of different versions of the model.  You simply don't get large
>inhomogeneities on local distance scales in any of these models.  (If
>this did happen, then these models would have been summarily rejected as
>disagreeing with observations.)  The remarkable thing about the dark-matter
>gravitational-instability theory of the formation of structure in the 
>Universe is that it produces more or less the kinds of gravitational fields
>you need to explain how galaxies and clusters of galaxies formed, without
>producing local gravitational fields that conflict with observations.
>This doesn't mean that the theory is right, but it does mean that you're
>going to have to work harder than that to convince people it's wrong.

     At some point one should realize that the models were *created*
     so that they apparently a) explain the things that need explaining
     (rotation curves, etc.) b) don't cause too much trouble in other 
     places (backyard black holes).  However looking at the amount of 
     new matter for which we have no evidence except for a bunch of
     gravitational observations that we do not understand, there comes a 
     point at one should ask oneself if creation of a bunch of new matter 
     with rather special properties (the driving force for the development of 
     the models of which you speak) is not ridiculous.

     There is another point.  In my field, we have great difficulty calculating
     boiling in cryogenic fluids, a process for which we have a) the
     equations b) fairly good understanding of the boundary conditions
     c) limited spatial extent, d) no unknown matter mucking up the
     calculation.  It seems to me like the search for dark matter is 
     a search for a presumably very very special set of solutions to 
     the relevant system where a) we don't know the equations, b) we don't
     really know the appropriate initial/boundary conditions c) spatial
     extent is unknown, d) we have introduced great bunches of stuff
     we cannot see.  I find it extremely odd that one would get local
     inhomogeneities in baryonic matter without getting extremely local
     inhomogenities in the 'dark matter'.  Especially seeing as the
     junk is not supposed to interact via electromagnetic interactions.

     My guess is that these calculations would *never* see anything like
     turbulence or inhomogenity because these elsewhere rather ordinary
     conditions are selected against.  This, of course, is not the way
     nature appears to run elsewhere.
      
>I said:
>>>Dark matter, in contrast, seems qualitatively to explain what's observed
>>>without any completely outrageous new physics.
>>
>To which Dale replied:
>>     Except, of course, for the completely outrageous production of 
>>     unknown matter.
>>
>
>Whether you think the hypothesis that much of the matter in the Universe
>is of a kind that has so far escaped detection is an outrageous one
>is your business.  I certainly agree that it's a big one, and needs to
>be supported by substantial evidence.  It's very possible that a more
>economical theory will be discovered, and the postulate of dark matter
>will be abandoned.  However, it hasn't happened yet.  If you've got one,
>I'd love to hear it.

     I'd love to hear something compelling as well.  However, are you
     not skeptical of a hypothesis based essentially on the premise
     that most of the universe is comprised of matter we cannot see
     nor apparently detect locally?

>The hypothesis becomes a little bit more believable if you notice that
>virtually every extension of the standard model of particle physics predicts
>the existence of weakly interacting particles that have masses, interactions,
>and cosmological densities in roughly the right range to be cold dark matter,
>and predicts that neutrinos have nonzero mass, which may be in the right
>range to be hot dark matter.

     Gee, I wonder why these extensions do these wondrous things?  
     Coincidence?  Or could it be that this is one reason people feel 
     compelled to extend the standard model?

>For the moment, the theory that the Universe is dominated by dark matter
>is far from proven.  There are, however, a large number of otherwise 
>problematic phenomena that it explains.  I know of no other theory
>that can do so without introducing a much larger number of ad hoc
>hypotheses.

     Maybe, but I fear alternative hypotheses are being selected against.

     I wonder how Newton would have fared against 'dark matter'.  How about
     Einstein?  I guarantee you I can invent a local distribution of 
     'dark matter' that can explain the anomaly in perihelion precession
     of Mercury.  I might even be able to explain the other two major
     tests of GR as well.  I can do the perihelion without batting an eye
     since it is just Seeliger's zodiacal light hypothesis, with the
     added attraction of there being no stuff to 'see'.

     Pixies.

                               dale bass
-- 
C. R. Bass                                           crb7q@virginia.edu
Department of Mechanical and Aerospace Engineering
University of Virginia
Charlottesville, Virginia                            (804) 924-7926