🚀 go-pugleaf

In article <1d99kkINNqu7@agate.berkeley.edu> ted@physics.Berkeley.EDU (Emory F. Bunn) writes:
>In article <1992Nov4.134438.27968@sei.cmu.edu> firth@sei.cmu.edu (Robert Firth) writes:
>>In article <1d78l7INNhu2@agate.berkeley.edu> ted@physics1 (Emory F. Bunn) writes:
>>
>(Description of the dynamical argument in favor of dark matter deleted.)
>>
>>Well, if what you say is indeed what we observe - and I'll gladly
>>take that on faith - then the "dark matter" hypothesis seems to me
>>completely bogus.  I can't believe that, the more empty space is of
>>visible matter, the more dark matter there is hiding in it.  If the
>>dark matter gravitates, as it must to close the universe, then its
>>overall distribution can't be that much different from the distribution
>>of visible matter, since at that scale gravitation is the overwhelming
>>force.
>
>This argument sounds good, but it isn't right.  The dark matter and the
>visible matter can have extremely different distributions.  To explain 
>why, I'll describe one specific cosmological model, the "cold dark 
>matter" model.  According to this theory, most of the mass of the 
>Universe is composed of massive particles that only interact via 
>gravitation and the weak force.  In particular, the dark matter doesn't
>interact electromagnetically.  (That's why it's "dark.")  Now think
>back to a time when the Universe was very young, hot, and dense.  Because
>of the high temperature, the ordinary matter was almost all ionized.
>Also, there were a lot of high-energy photons around.  That means that
>there was a large radiation pressure pushing on the baryonic matter.
>That pressure prevented small inhomogeneities in the baryons from growing
>into large ones.  

     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.

>Since the dark matter doesn't interact electromagnetically, it can't
>feel the radiation pressure.  So small inhomogeneities in the dark
>matter can start to grow from very early times, while the baryonic matter
>is forced to remain uniformly distributed until much later.  By the
>time the radiation pressure becomes small (and the protons and electrons
>combine to form atoms, which happens at roughly the same time), the 
>dark matter has already formed big clumps.  These clumps then act as
>sources that attract the baryons, and they start forming structures too.
>But because the dark matter got to start clumping much earlier, it
>winds up being clustered in ways that are very different from the
>baryonic matter.

     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.

>Standard models of quantum field theory and general relativity make
>definite predictions about what the gravitational and electromagnetic
>forces look like in a curved background spacetime, and these predictions
>are what is used in the dynamical argument in favor of dark matter.
>It is entirely possible that these theories are wrong, and that there
>is some long-range force that behaves in a different way from the
>known ones, and that it is this force, rather than dark matter, which
>explains the dynamical effects.  There are people who do research into
>this area, although it is not the preferred hypothesis among the experts.
>Apparently it's hard to come up with a force law that (a) makes sense
>in the context of relativity and field theory; (b) explains all the
>observations; and (c) doesn't conflict with existing observations.
>Dark matter, in contrast, seems qualitatively to explain what's observed
>without any completely outrageous new physics.

     Except, of course, for the completely outrageous production of 
     unknown matter.

                          dale bass



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