This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. Feel free to rid yourself of cached thoughts by doing so in Old Church Slavonic. If a discussion gets unwieldy, celebrate by turning it into a top-level post.
If you're new to Less Wrong, check out this welcome post.
That is correct as far as the known neutrinos go. If there is a fourth generation of matter, however, all bets are off. (I'm too lazy to look up the limits on that search at the moment.) On the other hand, since neutrinos oscillate and the sun flux is one-third what we expect rather than one-fourth, you need some mechanism to explain why this fourth generation doesn't show up in the oscillations. A large mass is probably helpful for that, though, if I remember correctly.
Point of order! A massive neutrino is a WIMP. "Weakly Interacting" - that's neutrino to you - "Massive Particle".
Well, but “massive” in WIMP usually means very massive (i.e. non-relativistic at T = 2.7 K). As far as gravitational effects, particles with non-zero mass but ultrarelativistic speeds behave very much like photons AFAIK.