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The $\nu_e$ is a mixture of three mass eigenstates $\nu_1,\nu_2,\nu_3$, at least two of which are massive. The mixing coefficients form the PMNS matrix. For neutrinos, mass and flavor are not simultaneous observables, so the $\nu_e$ does not have a well-defined mass of its own.

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I would think - but could not put my hands on any evidence to back this assertion up - that such neutrinos would be unavoidably thermally coupled to the rest of the universe prior to 1 second [before this, the neutrinos are sufficiently coupled to electrons and positrons through weak interactions that they follow their temperature]. So, even though cooled ...

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In principle, the charged leptons may oscillate as well but it would need rather strange initial states that could be easily obtained in the ultrarelativistic limit only and the experimental arrangement would have to be very unusual, anyway. The reason why the neutrinos oscillate is that the initial state isn't an energy eigenstate. It is a superposition of ...

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There is such a thing as a neutrino initiated reaction and we can confidently calculate the rates for them. By that I mean that we can compute---accurately---the rate for stable particle to be transformed in a neutrino flux, and we can test those calculation with controllable sources (reactors and artificial beams). Then we can take that---well ...

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