Astroparticle Physics

22911 The Cosmic MicrowaveBackground“God created two acts of folly. First, He createdthe universe in a Big Bang. Second, Hewas negligent enough to leave behind evidencefor this act, in the form of the microwaveradiation.”Paul ErdösIn this chapter the description of the early universe to coverthe first several hundred thousand years of its existence willbe presented. This leads to one of the most important pillarsof the Big Bang model: the cosmic microwave backgroundradiation or CMB. It will be seen how and when theCMB was formed and what its properties are. Most importantamong these are its blackbody energy spectrum characterizedby an average temperature of T = 2.725 K, andthe fact that one sees very nearly the same temperature independentof direction. Recent measurements of the slightdependence of the temperature on direction have been usedto determine a number of cosmological parameters to a precisionof several percent.overviewof cosmic microwaveand blackbody radiationprecision determinationof cosmological parameters11.1 Prelude: Transition to aMatter-Dominated Universe“We know too much about matter todayto be materialists any longer.”Harides ChaudhuriPicking up the timeline from the last chapter, Big Bang Nucleosynthesiswas completed by around t ≈ 10 3 s, i.e., whenthe temperature had fallen to several hundredths of an MeV.Any neutrons that by this time had not become bound intoheavier nuclei soon decayed.The break-neck pace of the early universe now shiftsgears noticeably. The next interesting event takes place whenthe energy density of radiation, i.e., relativistic particles(photons and neutrinos), drops below that of the matter (nonrelativisticnuclei and electrons), called the time of ‘matter–radiation equality’. In order to trace the time evolution of thenucleosynthesismatter–radiation equality