Upper limit on the rest mass of neutrinos if. .

[UMich1344]

How can cosmology set an upper limit on the rest mass of neutrinos if we know that the universe contains about 450 neutrinos per cubic centimeter?

 

[hellfire]

The fraction of the energy density of neutrinos in the universe is constrained by models of large scale structure formation. According to the assumed thermal history and properties of the relic neutrino background, this, in turn, puts a upper limit on their mass. You can find some information about the involved physical mechanism and a very pedagogical reference about neutrinos and structure formation in this thread:
https://www.physicsforums.com/showthread.php?t=185198