Astroparticle Physics

246 12 Inflation12.1 The Horizon Problem“The existing universe is bounded innone of its dimensions; for then it musthave an outside.”Lucretiuscosmic microwavebackgroundregions in causal contacthorizon problemIn the previous chapter it was shown that, after correcting forthe dipole anisotropy, the CMB has the same temperature towithin one part in 10 5 coming from all directions. This radiationwas emitted from the surface of last scattering at a timeof around t ls ≈ 380 000 years. In Sect. 11.6 it was calculatedthat two places separated by the particle-horizon distance att ls are separated by an angle of around 1.4 ◦ as viewed today.This calculation assumed R ∼ t 1/2 for the first 50 000 yearsduring the radiation-dominated era, followed by a matterdominatedphase with R ∼ t 2/3 fromthenuptot ls .Foranymixture of radiation and matter domination one would findvalues in the range from one to two degrees.So, if regions of the sky separated in angle by more thanaround 2 ◦ are considered, one would expect them not to havebeen in causal contact at the time of last scattering. Further-more, the projected entire sky can be divided into more than10 4 patches which should not have been in causal contact,and yet, they are all at almost the same temperature.The unexplained uniform temperature in regions that appearto be causally disconnected is called the ‘horizon problem’.It is not a problem in the sense that this model makesa prediction that is in contradiction with observation. Thedifferent temperatures could have perhaps all had the sametemperature ‘by chance’. This option is not taken seriously.The way that systems come to the same temperature is by interacting,and it is hard to believe that any mechanism otherthan this was responsible.12.2 The Flatness Problem“This type of universe, however, seemsto require a degree of fine-tuning ofthe initial conditions that is in apparentconflict with ‘common wisdom’.”Idit Zehavi and Avishai DekelΩ ≈ 1It was found in Sect. 11.6 that the total energy density ofthe universe is very close to the critical density or Ω ≈ 1.