Inflation can explain why the Cosmic Microwave Background Radiation (CMBR) is so isotropic to a high degree despite that many regions of the universe were never in contact with each other. According to it, regions that are not in causal contact with each other right now were in the past, and could communicate. Then the universe expanded, the physical scales increased in size, and photons from those regions never came into causal contact again.
In slow roll models, inflation is driven by the potential energy of a scalar field, the inflaton, which slowly rolls down the potential. When the slow-rolling stops, inflation comes to an end, and the inflaton field oscillates around the minimum of potential energy. In this period, it produces the elementary particles known to us, in a process called reheating.
Here's what I don't understand - if photons are only produced in this period, after inflation ends, how can they have "talked" before when the universe was expanding, if only the inflaton field was present?