Standard neutrinos were once relativistic and contribute to the radiation density, but massive neutrinos later become nonrelativistic and affect both the expansion rate and the growth of structure. Their large thermal velocities suppress clustering below a free-streaming scale.

The parameter $N_{\rm eff}$ captures the energy density in relativistic species beyond photons. It is sensitive to neutrino decoupling physics, extra light particles, and some early-universe reheating scenarios.

Cosmology is most sensitive to the sum of masses $\sum m_\nu$, rather than the individual mass eigenstates. This makes it complementary to oscillation experiments, which measure mass splittings but not the absolute mass scale.

Cosmology constrains sterile neutrinos, majorons, axion-like particles, and other light relics through their effects on $N_{\rm eff}$, free-streaming, and structure formation. These constraints often depend on production mechanism and thermal history.