# Would the universe be significantly different if neutrinos were massless?

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?

• 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. Jun 15, 2018 at 7:18
• 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. Jun 4, 2019 at 19:52

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