As far as I know, neutrino oscillation experiments have fairly conclusively demonstrated that neutrino states have mass. However, neutrinos are so frequently taken to be massless in cosmology that I began to wonder whether it would have made an appreciable difference in the early universe after the big bang if neutrinos were indeed massless.

Therefore, would the universe have turned out significantly differently if neutrinos were massless? If so, would that mean that there is some anthropic argument for neutrinos to be massive?

  • $\begingroup$ I think any effects would be more apparent in the later universe. The higher the energy of a particle, the more effectively "massless" (i.e ultrarelativistic) it becomes. $\endgroup$ – ProfRob Jun 15 '18 at 7:18
  • $\begingroup$ Given that the sum of the neutrino masses and number of neutrino types can be discerned from cosmic microwave background radiation patterns, such as those observed by Planck, it follows that there would be some discernible difference, albeit a quite modest one that might not be readily apparent to the average person. Also, the fact that all fundamental fermions in the SM (and for that matter all particles that interact via the weak force) have a non-zero rest mass might be important in a deeper or unified theory in ways that we don't fully understand. $\endgroup$ – ohwilleke Jun 4 '19 at 19:52

This is the timeline of the present Big Bang model of the universe:


Note that before weak symmetry breaking , $10^{-10}$ seconds all elementary particles are massless, thereore it would not make a difference to initial modeling.

As the masses are still very small the observation that neutrinos have mass does not make them candidates for dark matter , so I do not see any difference, except that the standard model of particle physics would not have to be extended to fit the experimental appearance of neutrino masses.

  • $\begingroup$ Isn't neutrino mass unrelated to the Higgs field? $\endgroup$ – Slereah Jun 15 '18 at 6:34
  • $\begingroup$ @Slereah The neutrino is part of the assumptions building the standard model, i.e. masses in the table are assumed as axioms, and they appear after symmetry breaking. the neutrino couples with the weak coupling $\endgroup$ – anna v Jun 15 '18 at 7:20

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