Neutrinos are the most elusive particles among those currently known. They are tiny, they interact very feebly with other particles, they are theoretically puzzling. When in 1930 the scientist Wolfgang Pauli proposed their existence as a way to explain energy conservation in the interactions ruling neutrino decay, he also confessed that “I have done a terrible thing today, something which no theoretical physicist should ever do. I have suggested something that can never be verified experimentally”. This statement highlights the skepticism of those times, although history made Pauli wrong, luckily. Since late ‘50s, cutting-edge experiments have detected neutrinos, so that we are very confident about their existence today.
However, neutrino challenges have not stopped. Perhaps the most intriguing challenge is represented by the experimental evidence of neutrino oscillations. We know that three different types of neutrinos exist. Scientists observe neutrinos changing types (flavor) along the path from the neutrino source and the neutrino detector. This phenomenon is possible only if neutrinos have a mass. The challenge comes from the fact that the current model of particle physics, although very successful in predicting and explaining many other observations, is unable to explain why neutrinos are massive and how their mass is created. What is more, even though we know from oscillation experiments that neutrinos must have a mass, we still miss an experimental measurement of the neutrino mass.
Cosmology is one of the most promising avenues that would lead to a detection of the neutrino mass. Current cosmological surveys already tell us that the sum of the neutrino masses should be very tiny, almost 10 million times smaller than the electron mass. Neutrinos were one of the components of the primordial Universe and almost 100 relic neutrinos per type per cubic cm are still around today. Their peculiar properties make neutrinos one of the main characters in shaping the cosmological structures we observe today. Neutrinos were relativistic in the early times, therefore they counted as a radiation contribution that modified the expansion rate, among other things. As time passed by, neutrinos became less energetic and eventually non-relativistic. At that point, their mass played a non-negligible role in governing the evolution of matter structures forming in the Universe.
With Simons Observatory, we will be able to observe all these effects in three different ways, therefore providing a robust handle to neutrino masses. In particular, SO will be very sensitive to the deflection of CMB light due to photons passing by very massive collections of cosmological objects (gravitational lensing), whose structures and evolution depend on how massive neutrinos are. In addition, Simons Observatory will also detect a huge number of clusters of galaxies of various masses and at various distances from us, whose counting also depends on how massive neutrinos are. Finally, Simons Observatory will map the distortions in the energy carried by CMB photons when they pass through galaxy clusters (Sunyaev-Zeldovich effect), and the detailed texture of these maps is again dependent on how massive neutrinos are.