Essay

WEAKLY INTERACTING MASSIVE PARTICLE (WIMP)

Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549

The name given to the elementary particles of hypothetical non-baryonic dark matter that, in some theories, are assumed to pervade the cosmos. Such a particle could account for the dark matter seen in galaxies and in clusters of galaxies (see large-scale structure), and may assist in the problem of cosmological structure formation. At least part of the dark matter could be in some baryonic form such as massive compact halo objects (MACHOs), but if the theory of cosmological nucleosynthesis of the light element abundances is correct then there cannot be enough baryonic material to provide a critical-density flat universe (see also gravitational lensing). There are many possible candidates for the hypothetical WIMPs. These are usually divided into two classes: hot dark matter (HDM) and cold dark matter (CDM).

Any relic non-baryonic particle species which has an appreciable cosmological abundance at the present epoch, and which had a thermal velocity close to the velocity of light when it was produced in the early Universe, is called hot dark matter. If a particle produced in thermal equilibrium is to have such a large velocity, it has to be extremely light. The favoured candidate for such a particle is a neutrino with a rest mass of around 10 eV (electronvolts) which is 1/500,000 of the mass of the electron (see elementary particles). It is not known whether any of the known neutrino species actually has a nonzero rest mass. But if any do, and their mass is around 10 eV, then the standard Big Bang theory of the thermal history of the Universe predicts a present-day density of relic particles close to the critical density required to make the Universe recollapse. The Universe would then be expected to have a value of the density parameter close to 1. However, HDM does not seem to be a good candidate from the point of view of structure formation theories, because the extremely high velocities of the neutrinos tend to erase structure on scales up to and including those of superclusters of galaxies. It is unlikely, therefore, that the Jeans instability of HDM can on its own be responsible for the formation of galaxies and large-scale structure.

The alternative, cold dark matter, is a more promising candidate for the cosmological dark matter. Any relic non-baryonic particle species which has an appreciable cosmological abundance at the present epoch, and which had a thermal velocity much less than the velocity of light when it was produced, would be cold dark matter. In order to be moving slowly in a state of thermal equilibrium, a CDM particle is normally (though not always) expected to be very massive. There are many possible candidates for CDM, suggested by various theories of the fundamental interactions and the physics of elementary particles (see e.g. grand unified theory). In some such theories, incorporating the idea of supersymmetry, all bosonic particles should have fermionic partners. Promising candidates for a CDM particle are therefore such objects as the photino, the supersymmetric partner of the photon. Another possible CDM candidate is the axion, which appears in certain grand unified theories. The axion actually has a very tiny mass (a mere one-hundred billionth of the mass of the electron) but interacts so weakly with electromagnetic radiation that it is never held in thermal equilibrium and therefore, paradoxically, has a very small velocity. It is even possible that primordial black holes with very small mass could behave like CDM particles. This form of dark matter has, until recently, been strongly favoured on theoretical grounds because it appears to assist in solving the problem of structure formation. Recent observational data, however, seem to suggest that the simplest versions of this picture of structure formation are not correct and some other ingredient is necessary, perhaps a smattering of HDM.

Experiments are under way to detect WIMPs experimentally using sensitive underground detectors. Hunting for particles of dark matter in this way is like looking for the proverbial needle in a haystack, because neither the mass nor the interaction rate is known.