Hot Dark Matter in Cosmology

Cosmological dark matter in the form of neutrinos with masses of up to a few electron volts is known as hot dark matter. After an historical review of the subject, this article considers constraints on hot dark matter from current data on neutrino oscillations and on cosmology. The atmospheric neutrino oscillation data imply a lower limit on the HDM contribution to the cosmological density $\Omega_\nu \gsim 0.001$. The possible improvement of low-$\Omega_m$ flat ($\Lambda$CDM) cosmological models with the addition of light neutrinos appears to be rather limited, but$\Lambda$CDM models with $\Omega_\nu \lsim 0.1$ may be consistent with presently available data. Data expected soon may permit detection of such a hot dark matter contribution, or alternatively provide stronger upper limits on $\Omega_\nu$ and neutrino masses.