From: Ramez Naam (ramezn@Exchange.Microsoft.com)
Date: Sat Jan 29 2000 - 20:54:59 MST
From: EvMick@aol.com [mailto:EvMick@aol.com]
> Is there evidence to support the existence of DM? To me it
> appears that it
> has been decided that DM MUST exist.......
There are at least two pieces of evidence for the notion that galaxies are
more massive than we can account for by the visible stars and dust in those
galaxies. (There are probably others that I'm not aware of.)
1) Orbital dynamics. The orbital velocity of stars at the periphery of
observed galaxies is too fast. If we use estimates for galactic massed
based on the number and type of observed stars, and the mass estimates for
those stars, then the stars on the outside of the galaxy should simply fly
away. The fact that they don't suggests that these galaxies are more
massive than can be accounted for by visible objects. (Or at least, by our
estimates of the mass of those visible objects.)
2) Gravitational lensing. The new studies pointed to by Robert Bradbury's
mail shows that there is a large gravitational lensing affect around nearby
galaxies consistent with a much larger mass than can be accounted for by
visible objects. That is to say, the gravity of these galaxies is bending
light at a level consistent with a much higher mass for the galaxy than we'd
expect. These studies also demonstrate how far from the galaxy the
gravitational lensing effect extends, thus providing an estimate of the size
of the "halo".
All of this having been said, the evidence is for missing mass, not missing
matter. As the "dark energy" article in New Scientist points out, mass can
be accounted for by either matter or energy.
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