Contrary to this expectation, the universe is observed to be very close to isotropic, which also implies homogeneity.[1] The cosmic microwave background radiation (CMB), which fills the universe, is nearly the same temperature everywhere in the sky, about 2.728 +/- 0.004 K. The differences in temperature are so slight that it has only recently become possible to develop instruments capable of making the required measurements. This presents a serious problem; if the universe had started with even slightly different temperatures in different areas, then there would simply be no way it could have evened itself out to a common temperature by this point in time.
According to the Big Bang model, as the density of the universe dropped (while it expanded) it eventually reached a point where photons in the "mix" of particles were no longer immediately impacting matter; they "decoupled" from the plasma and spread out into the universe as a burst of light. This is thought to have occurred about 300,000 years after the Big Bang. The volume of any possible information exchange at that time was 900,000 light years across, using the speed of light and the rate of expansion of space in the early universe. Instead, the entire sky has the same temperature, a volume 1088 times larger.