Dark Matter

tivol at tethys.ph.albany.edu tivol at tethys.ph.albany.edu
Sun Jun 5 13:33:41 EST 1994

Bill Jefferys wrote that 90+% of matter is H or He, to which Peter Walker re-
plied "Nope.  90-99% of the matter (by mass) in the universe is dark matter."
If another Bill can comment, there is no _a priori_ reason why H and/or He
cannot be dark if they are cold.  As far as the distinction between dark and
bright matter being observable, in a sense both are only indirectly observ-
able.  We can see the light emitted by bright matter (and propagated through
the intervening part of the universe to our instruments), and we "see" the
gravitational effects etc. of the matter which doesn't emit visible light.
As our abilities to detect various signals expand--e.g. detection of electro-
magnetic radiation at wavelengths not now detectable or gravitational waves--
who's to say that dark matter will continue to be invisible?  Not being an
astrophysicist, I may be unaware of reasons why H and He can be excluded as
significant components of dark matter.  Finally, the observability, or lack
thereof, of a fraction of the stuff referred to in a theory does *not* deter-
mine the scientific (or lack thereof) nature of the theory.  As has been pos-
ted here many times, it is the process of construction of predictions out of
a hypothesis generated by fitting the known experimental data that defines a
theory--nowhere does it say that the experimental data must be complete.


					Bill Tivol

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