Hot Dark Matter

The foremost HDM candidate is a particle known as the "neutrino." This elusive particle was predicted to exist as early as 1931 by physicist Wolfgang Pauli in order to account for the conservation of momentum and energy in beta decay. Beta decay is a process whereby a free neutron, after roughly 12 minutes, decays into an electron, proton, and an electron anti-neutrino. (The "beta" is just a reference to the high energy electron emitted in the process). Scientists studying beta decay were at a loss to satisfactorily explain an apparent violation of momentum and energy conservation. One prominent physicist, Niels Bohr, went so far as to suggest that the conserva- tion laws may break down at sub-atomic levels. Not satisfied with Bohr's proposal, Pauli postulated that a small, chargeless, and possibly massless particle that travelled at relativistic speeds was also emitted in beta decay. Furthermore, the particle could undergo only very weak interactions with ordinary matter. (Matter can interact according to any of nature's four forces: electromagnetic, gravitational, strong, or weak). The idea that the particle was both electrically neutral and weakly interacting indicated that it would be extremely difficult, if at all possible, to detect. (Enrico Fermi later dubbed the particle "neutrino", or "little neutral one"). The possibility that the neutrino was massless would rule out any gravitational interactions, as a particle must have mass in order to interact vis-a-vis the force of gravity.

Pauli's assertion that the neutrino would be exceptionally elusive proved correct, as it would be another 25 years from the time of its prediction that two experimentalists would finally detect a neutrino. Working with the Savannah River high-flux nuclear reactor in 1956, Fred Reines and Clyde Cowan first detected the signature of a neutrino interaction with matter. Placing a ton-sized detector near the reactor, they were able to decipher the signal of a neutrino scattering off of a proton. Since then, three different neutrino types have been found to exist. Each type, or "flavor", corresponds to a different "lepton" particle: electron, muon, or tau. The neutrino is also known to have its own anti-particle, the anti-neutrino. (In fact, it is an electron anti-neutrino that is produced in beta decay).

With the realization that neutrinos play a key role in a number of astrophysical processes, a new and massive field of research known as "neutrino astronomy" has emerged in the past two decades. Neutrino detectors have sprung up in the USA, Japan, Russia, and Canada. Of particular interest to astronomers is the "solar neutrino problem." Neutrino observatories have consistently measured only a fraction of the expected neutrino flux from the Sun as predicted from standard solar models. A number of theories have been mooted to help explain the deficiency. Some astrophysicists have come to believe that neutrinos do have some mass, albeit very tiny, thereby allowing them to interact more frequently with material as they stream out from the stell ar core. Others have theorized more extreme solutions, including a phenomenon known as "neutrino oscillations." According to this theory, neutrinos may change their flavor, or type, midway between the Sun and Earth; in so doing, they would escape detection. In order for this effect to occur, neutrinos must have some mass, no matter how small.

On the cosmological level, calculations of Big Bang nucleosynthesis indicate the production of neutrinos among the reactions which form the light elements. Therefore, just as these calculations provide some prediction of the observed light element abundances, so too they yield some indication of the neutrino abundance in the Universe. Much as the radiation field which forms today's CMB had "decoupled" from matter a few minutes after the Big Bang, so too did neutrinos decouple from matter--albeit at a much earlier time than the photon background. Hence, there is a predicted cosmic neutrino background as dense as the photons which comprise the CMB. No experiment has yet been devised with the requisite sensitivity to observe this neutrino background, but if eventually measured, this background would yield information about the Universe some 12 orders of magnitude earlier than that of the CMB.

It is now an issue of both theoretical and experimental speculation as to whether or not neutrinos indeed have any mass. Various experiments have only succeeded in setting upper-mass limits on neutrinos, but have not conclusively proven the existence of neutrino mass. Neutrinos detected from the recent explosion of SN1987A in the Large Magellanic Cloud have helped place upper-limit constraints on possible neutrino masses. Each neutrino type has a different predicted mass range, with the best estimates indicating that neutrino mass must be between 4 and 20 eV/c^2.

One of the most revealing criteria as to whether the Universe is dominated by CDM or HDM is the way that matter, in particular galaxies, are distributed throughout the sky. HDM, as represented primarily by neutrinos, does not apparently account for the pattern of galaxies observed in the Universe. Neutrinos, as aforementioned, would have emerged from the Big Bang with such highly relativistic velocities (i.e. close to light speed) that they would tend to smooth out any fluctuations in matter density as they streamed out through the Universe. In the early Universe, the neutrino density was enormous, and so most of the matter density could be accounted for by neutrinos. Given their great speeds, neutrinos would tend to free stream out of any overdense regions--that is, regions with densities greater than the average density in the Universe. This process implies that density fluctuations could appear only after the neutrinos slowed down considerably. (i.e. As the Universe expanded, its temperature decreased, thereby resulting in neutrino "cooling." This is obvious since neutrino kinetic energies are directly proportional to the temperature of the Universe).

The short and the long is that the neutrino-dominated Universe scenario would mean that there is no matter in the voids between galaxy superclusters since it would have all been swept up into filaments. However, sensitive astronomical observations indicate that the voids contain dim dwarf galaxies. This discovery tends to discredit any HDM model of the Universe.

Emad Iskander