4.6.4. Non-baryonic Dark Matter
By any measure, if = 1 then the universe is dominated by non-baryonic dark matter. Furthermore, that dark matter must be distributed more smoothly than the light. Non-baryonic dark matter comes in two basic forms: hot and cold. These terms refer to the ability for this dark matter to cool to non-relativistic velocities and clump into smaller units. A cold dark matter (CDM) universe gives rise to much different structure formation modes than a hot dark matter (HDM) universe. This is the subject of the next chapter, for now we will just take an inventory of the available candidates.
CDM consists of weakly interacting massive particles (WIMPs) that become non-relativistic at temperatures well above 104 K. As such, they are excellent candidates for producing small scale structure as prior to recombination they would have easily clumped together. This requires the rest mass of most CDM WIMPs to be extraordinarily high (up to 1016 GEV). Indeed, some WIMPs could have been created very early on through quantum fluctuations and if they did not immediately annihilate with their respective anti-WIMPS, could survive as the dominant relic mass today. Many of these particles are naturally created in the supersymmetric theories (SUSY) of particle physics. SUSY makes use of a conserved quantum number called R parity. R = +1 for particles and -1 for their SUSY partner. R parity can be linked to baryon number B and lepton number L conservation through the spin, S as
The conservation of R parity has three important implications
1. SUSY particles are always produced in pairs (hence the name of the theory)
2. Heavy SUSY particles decay into lighter SUSY particles
3. The lightest SUSY particle produced by this decay process is stable because there are no further decay modes that exist without violating R parity.
Current potential dark matter SUSY particles are the neutralino,gravitino,photino and higgsino.
Another favorite CDM particle is known as an axion. In contrast to other CDM particles, the axion is relatively light. The axion is predicted to exist as a result of a symmetry breaking associated with the strong-CP problem in quantum chromodynamics. Although the axion mass is arbitrary over the range 10-12 ev to 1 Mev, the symmetry breaking occurs at high energy scale and hence early in the Universe. Although axions are created in the very early Universe when it was quite hot, axions have very small momenta and are born cold. Axion freeze-out is also mediated through pion-to-axion conversion with nucleons acting as a catalyst. Since nucleons only come into existence after the quark-hadron transitions (energy scale 200 Mev), they are necessarily non-relativistic and so are the axions which are created in this thermal process. However, the cosmic abundance of any thermally produced axions is orders of magnitude lower than the production associated with symmetry breaking.
In contrast, HDM consists of particles that remain relativistic for significantly longer times. This requires their masses to be less than 100 ev. In contrast to the CDM case in which there are no experimentally verified candidates, HDM has a definite candidate, the neutrino. Once the total number of neutrino species is known, the density of neutrinos (as well as their cosmic temperature) can be determined. Studies of the Z0 resonances in the LEP e+e- collider at CERN strongly fixes the number of light (e.g., m << mZ / 2) neutrino species at 3 (electron, muon, and tau). This leads to a cosmic background neutrino density of 100 cm-3. Any neutrino mass above 1 eV would represent a significant contribution to the overall cosmological mass density. Values of 30 - 100 ev are required to yield = 1, depending on the value of H0. Current experiment limits on the electron neutrino are 7 eV, the muon neutrino 270 kev and the tau neutrino 31 Mev. The latter two neutrino species therefore have the potential to close the Universe and there is some experimental evidence for a non-zero neutrino rest mass (see below).