Whatever Happened to Hot Dark Matter?

TL;DR

This paper reviews the role of neutrinos as hot dark matter in cosmology, discussing their impact on matter distribution, experimental evidence for neutrino mass, and current density constraints.

Contribution

It provides a comprehensive summary of neutrino cosmology, highlighting recent experimental findings and their implications for hot dark matter models.

Findings

Neutrinos are as numerous as photons in the universe.

Neutrino mass implies a minimum hot dark matter density.

Upper limit on neutrino contribution to cosmology is about 0.1.

Abstract

This article summarizes the possible roles of neutrinos in cosmology, from the first three minutes onward. The fact that primordial neutrinos are about as numerous as the photons of the cosmological background radiation means that neutrino (or ``hot'') dark matter can have a significant impact upon how matter is distributed in the universe. Over the past two decades, the liklihood of this has risen and fallen as more and more data has become available from laboratory experiments and the latest telescopes. Hot dark matter models were studied in the late 1970s and early 1980s, and there was considerable interest in cold plus hot dark matter models for a few years in the mid-1990s. We now know from Super-Kamiokande and Sudbury Neutrino Observatory data that neutrinos have mass, and that the total neutrino contribution to the cosmological density must be at least as large as $\Omega_\nu \sim 0.001$, a little less than that of the visible stars in the universe. But the upper limit on the hot dark matter cosmological density is $\Omega_\nu < 0.1$; correspondingly, the sum of the masses of all three active neutrino species is not more than about 5 eV.