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Simply hot dark matter is not allowed due to free streaming. So do gravitational waves:

a) free stream? If not why not? Surely they can since they are relativistic and weakly interacting.

b) if they free stream, is it allowed because they don't dominate the dark matter content?

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1. Gravitational waves are a miniscule part of the energy density of the universe. We know this because we haven't detected them yet. 2. Since they travel at the speed of light gravitational waves would be a part of the radiation component of the universe, not matter. They behave differently cosmologically and the radiation component is unimportant at late times. – Michael Brown Mar 12 '13 at 1:59
@MichaelBrown irrelevant details: the lack of detection has more to do with the weak coupling to matter than the actual energy content --- which is fairly large (e.g. enough to merge a binary system). – DilithiumMatrix Mar 12 '13 at 4:20

Gravitational waves do free-stream for just the reasons you give: because they are always traveling at the speed of light, and they have an extremely low probability of scattering. I don't see anyway in which it is connected to the relative contribution of dark matter.

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well i was just wandering why they aren't considered as HDM since they are weakly interacting. But i guess since they make up a tiny proportion of the energy density it wouldn't matter anyway. – user21119 Mar 12 '13 at 8:37
@user21119 ah, I see. The term 'dark matter' doesn't apply to something like gravitational waves. – DilithiumMatrix Mar 12 '13 at 17:35

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