Neutrinos and Mass...

Comparing the value of mass in the universe derived using the CMB as compared with counts of galaxy clusters and gravitational lensing. (Courtesy: Adam Moss, Planck Collaboration)

Scientists know that when they measure the total amount of matter in the universe using two competing methods, one will give a higher value for the total density of matter than the other. To resolve this measurement discrepancy, two separate research groups have now proposed that the missing mass might be in the form of neutrinos. Accurately measuring the total amount of matter in the universe is a crucial cosmological parameter for interpreting a vast number of astrophysical phenomena.

Neutrinos are difficult to study because they interact only by the weak nuclear force, which acts only over very short distances, and by gravity, which is extremely weak. In the Standard Model of particle physics, neutrinos come in three flavours – electron, muon and tau. They were once thought to be massless, but the discovery of neutrino oscillations – whereby neutrinos change flavour, requiring that their masses be different – implies at least two of them have mass, with the rate of oscillation depending on the mass difference of each flavour. The rate has been measured in particle-physics experiments such as Super-Kamiokande in Japan, which has allowed particle physicists to place a lower bound on the sum of the neutrino masses of 0.06 eV, but the absolute values remain unknown.

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