🚀 go-pugleaf

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.  

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.
>
>What you describe sounds to me more like a cosmological force of some
>kind, either a true fifth force or a gravitational force that, at
>large distances, obeys not an inverse square law but an inverse 1.???
>law.  Which leads to this thought: in Euclidean space, it's obvious
>why gravity and light obey the inverse square law.  But why should that
>be true in Riemannian space, where a sphere of twice the radius has less
>than four times the surface area?  Or am I on the edge of lunacy again?

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.

-Ted