The HORIZON PROBLEM
The horizon problem results from the premise that information cannot travel faster than light . In a universe of finite age this sets a limit—the particle horizon —on the separation of any two regions of space that are in causal contact.The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature. :191–202 A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the universe at some very early period (before baryogenesis ). During inflation, the universe undergoes exponential expansion, and the particle horizon expands much more rapidly than previou...