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The Question

(Submitted July 20, 2003)

Neutrino oscillations are real, ergo they have rest mass. Can they be slowed down, to be stationary relative to galaxy clusters? If so, could they comprise at least a significant fraction of (cold) dark matter (cf the hot dark matter they were always considered to be)? If not, why not?

The Answer

All non-baryonic dark matter candidates interact very weakly with each other and with ordinary matter: They cannot have strong nuclear or electromagnetic interaction, by definition. They can interact via weak nuclear force, which is the case with neutrinos. However, this force is so weak a neutrino can go through a chunk of lead a light year thick without being stopped. So, non-baryonic dark matter particles cannot be slowed down, except by gravity. (The general expansion of the universe can slow them down, too, but this is another manifestation of gravity.)

The measurements indicate the neutrinos can only have a small rest mass. This makes it rather hard for gravity to slow them down. In fact, cosmologists believe neutrinos (even assuming they are massive) are moving very fast, not much below the speed of light. In general, particles with near-zero rest masses (including neutrinos) can only be a constituent of hot dark matter:
http://csep10.phys.utk.edu/astr162/lect/cosmology/darkmatter.html

See also:
http://www.astro.princeton.edu/~dns/MAP/Bahcall/node6.html

Best wishes,

Koji & Scott
for "Ask an Astrophysicist"

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