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The Question
(Submitted October 21, 1997)
I am a real novice (advance apology for stupid question) but I first heard
about dark matter last week and have been transfixed by this mystery. I
basically understand (1) that particle physicists are trying to establish
the existence of WIMP's in cryogenic crystals and in the ice shelf and
(2) that part of the key to the dark matter theory is that as you move
out from the center of a galaxy, the speed of the rotation stabilizes which
implies that there is mass there that does not emit light. My question
is do particle physicists believe that WIMP's are dispersed throughout
the galaxy? And if so, why wouldn't WIMP's affect the orbit of other
bodies within the galaxy (i.e. our own solar system)?
The Answer
Two likely possibilities for the dark matter in our own galaxies are
MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive
Particles). MACHOs are low mass
stars, brown dwarfs, neutron stars and white dwarfs. If MACHOs make
up most of the dark matter, the distribution is not smooth on the
scale of the Solar system, but it is smooth on a much larger scale.
If the Galactic dark matter consists of WIMPs, then they are dispersed
throughout the Galaxy, with a distribution somewhat different from that
of the stars that we can see. Since gravitational pulls of WIMPs from
different directions tend to cancel out, the orbit of planets in our
solar system is not affected by the presence of WIMPs. However, since
there are more WIMPs towards the center of our Galaxy than away from it,
the motion of the Solar system (and other stars) in the Galaxy is strongly
affected --- this is how astrophysicists infer the presence of the dark matter.
Best wishes,
Koji Mukai
for Ask a High-Energy Astronomer
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