Astroparticle Physics

172 8 CosmologyBig Bang Nucleosynthesiscosmic microwavebackground radiationcritical density‘Big Crunch’vacuum energy densitydark matterneutralinosthe universe as it is now. Essentially all of the positrons hadannihilated with electrons within the first couple of seconds.Around three minutes after the Big Bang, the temperaturehad dropped to the point where protons and neutronscould fuse to form deuterons. In the course of the next fewminutes these combined to form helium, which makes up aquarter of the universe’s nuclear matter by mass, and smallerquantities of a few light elements such as deuterium, lithium,and beryllium. The model of Big Bang Nucleosynthesis(BBN) is able to correctly predict the relative abundancesof these light nuclei and this is one of the cornerstones ofthe Hot Big Bang model.As the universe continued to expand over the next severalhundred thousand years, the temperature finally droppedto the point where electrons and protons could join to formneutral atoms. After this the universe became essentiallytransparent to photons, and those which existed at that timehave been drifting along unimpeded ever since. They can bedetected today as the cosmic microwave background radia-tion. Only small variations in the temperature of the radiation,depending on the direction, at a level of one part in 10 5are observed. These are thought to be related to small densityvariations in the universe left from a much earlier period,perhaps as early as the inflationary epoch only 10 −36seconds after the Big Bang.Studies of the cosmic microwave background radiation(CMB) also lead to a determination of the total density ofthe universe, and one finds a value very close to the so-calledcritical density, above which the universe should recollapsein a ‘Big Crunch’. The same CMB data and also observationsof distant supernovae, however, show that about 70%of this is not what one would call matter at all, but rather asort of energy density associated with empty space – a vacuumenergy density.The remaining 30% appears to be gravitating matter, butof what sort? One of the indirect consequences of Big BangNucleosynthesis is that only a small fraction of the matter inthe universe appears to be composed of known particles. Theremainder of the dark matter may consist of neutralinos,particles predicted by a theory called supersymmetry.The framework in which the early universe will be studiedis based on the ‘Standard Cosmological Model’ or ‘HotBig Bang’. The basic ingredients are Einstein’s theory ofgeneral relativity and the hypothesis that the universe is