The fact that the black body distribution of the cosmic microwave background radiation is that of a black body, implies that during recombination the universe was in thermal equilibrium. My question is how this almost perfect thermal equilibrium was reached... During inflation, space was expanded exponentially, inflating a small causally connected patch. This provides a initial causal connection from which the whole universe within our current particle horizon evolved. But, although inflation provides the causal connection, it does not provide a thermal equilibrium. The expansion of space makes difficult the establishment of thermal equilibrium and, additionally, the reheating (the process which takes place after inflation) drives the content of the universe far from equilibrium. This disequilibrium is needed in order to satisfy the requirements for baryogenesis and matter-antimatter asymmetry (Sakharov conditions). Afterwards, the plasma filling the universe must thermalize and reach equilibrium before recombinantion (actually, equilibrium is reached more earlier before nucleosynthesis). If thermal equilibrium is reached due to collisions, is it ensured that the whole plasma reaches the same temperature (no causal connection is possible now) leading to the homogeneity and isotropy observed in the CMB?