Horizon problem
![Full-sky image derived from nine years' WMAP data](/Images/godic/202501/18/Ilc_9yr_moll40961736.png")
![When we look at the CMB it comes from 46 billion comoving light years away. However, when the light was emitted the universe was much younger (300,000 years old). In that time light would have only reached as far as the smaller circles. The two points indicated on the diagram would not have been able to contact each other because their spheres of causality do not overlap.](/Images/godic/202501/18/Horizon_problem.svg1736.png")
The horizon problem (sometimes called the homogeneity problem) is a problem with the standard cosmological model of the Big Bang which was identified in the late 1960s, primarily by Charles Misner. It points out that different regions of the universe have not "contacted" each other because of the great distances between them, but nevertheless they have the same temperature and other physical properties. This should not be possible, given that the transfer of information (or energy, heat, etc.) can occur, at most, at the speed of light.