Astroparticle Physics

13.2 Motivation for Dark Matter 279Based on theoretical considerations in the framework ofQCD the electric dipole moment of the neutron should be onthe same order of magnitude as its magnetic dipole moment.Experimentally one finds, however, that it is much smallerand even consistent with zero. This contradiction has beenknown as the so-called strong CP problem. The solution tothis enigma presumably lies outside the Standard Model ofelementary particles. A possible solution is offered by the introductionof additional fields and symmetries, which eventuallyrequire the existence of a pseudoscalar particle, calledaxion. The axion is supposed to have similar properties asthe neutral pion. In the same way as the π 0 it would havea two-photon coupling and could be observed by its twophotondecay or via its conversion in an external electromagneticfield (Fig. 13.10).Theoretical considerations appear to indicate that the axionmass should be somewhere between the µeV and meVrange. To reach the critical density of the universe with axionsonly, the axion density – assuming a mass of 1 µeV –should be enormously high (> 10 10 cm −3 ). Since the conjecturedmasses of axions are very small, they must possessnon-relativistic velocities to be gravitationally bound to agalaxy, because otherwise they would simply escape fromit. For this reason axions are considered as cold dark matter.As a consequence of the low masses the photons generatedin the axion decay are generally of low energy. Foraxions in the preferred µeV range the photons produced byaxion interactions in a magnetic field would lie in the microwaverange. A possible detection of cosmological axionswould therefore involve the measurement of a signal in a microwavecavity, which would significantly stand out abovethe thermal noise. Even though axions appear to be necessaryfor the solution of the strong CP problem and thereforeare considered a good candidate for dark matter, all experimentson their search up to date gave negative results.strong CP problem(a)(b)axionfexternalfieldfFig. 13.10Coupling of an axion to twophotons via a fermion loop (a).Photons could also be provided byan electromagnetic field for axionconversion (b)gravitational bindingto the galaxycold dark matteraxion decayfff13.2.5 The Rôle of the Vacuum Energy DensityTo obtain a flat universe a non-zero cosmological constantis required, which drives the exponential expansion of theuniverse. The cosmological constant is a consequence of thefinite vacuum energy. This energy could have been storedoriginally in a false vacuum, i.e., a vacuum, which is notat the lowest-energy state. The energy of the false vacuumflat universeand cosmological constantfalse vacuum