<Moderator's note: Spin-off from another thread: https://www.physicsforums.com/threads/does-red-shift-approach-infinite-near-time-zero.948696/.>
We do, however, have a lower limit, because of the solar neutrino experiment: too light and the neutrinos wouldn't have enough proper time to oscillate among the 3 flavors before reaching the Earth. (And, of course, an upper limit: WP lists ~ .1 eV as the upper limit. But that only gives us a lower limit on clustering.)Orodruin said:This depends on the absolute neutrino mass, which we do not know.
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