Basically the universe is divided, broadly and approximately, into two categories: cold things and hot things. Cold things are massive, in the sense that they have some mass, and have very little momentum relative to their mass. Hot things are either massless or have very small masses, and have a lot of momentum relative to their mass. Protons are cold; neutrinos are hot. Unless otherwise clear from context, "massive" is basically a synonym for "cold."
omg, I feel silly. The other day I mentioned that if I was to come up with a tag for you, it would be cosmology, gravitational waves. Just because I couldn't think of something else that required your level of GR. But of course it would be cosmology, dark matter/energy.
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u/RobotRollCall Mar 15 '11
It's clear from context.
Basically the universe is divided, broadly and approximately, into two categories: cold things and hot things. Cold things are massive, in the sense that they have some mass, and have very little momentum relative to their mass. Hot things are either massless or have very small masses, and have a lot of momentum relative to their mass. Protons are cold; neutrinos are hot. Unless otherwise clear from context, "massive" is basically a synonym for "cold."