Forget that the universe is expanding for a second. Imagine a photon was released very early in the Universe and travelled freely until it hits the North Pole of the Earth. Now imagine another photon was released at the same time, but “opposite” to the first one. It will hit the Earth at the South Pole. Could these two photons exchange any information from the time when they are released? Clearly not, because the time required to send information from one photon to the other would be two times the age of the Universe. The photons are causally disconnected. They are outside of each other’s horizon.
These photons could not have communicated with each other unless inflation took place during the very early Universe
However, we observe that photons from opposite directions must have communicated somehow, because the cosmic microwave background radiation has almost exactly the same temperature in all directions over the sky.
This problem can be solved by the idea that the Universe expanded exponentially for a short time period after the Big Bang. Before this period of inflation, the entire Universe could have been in causal contact and equilibrate to a common temperature. Widely separated regions today were actually very close together in the early Universe, explaining why photons from these regions have (almost exactly) the same temperature.
A simple model for the expansion of the Universe is to consider the inflation of the balloon. A person at any point on the balloon might consider themselves to be at the centre of the expansion, as all neighbouring points are getting further away.
Analogy of inflation using a balloon as the balloon inflates, the distances between objects on the surface of the balloon increases.
During inflation, the Universe expanded by a factor of about e60=1026. This number is a one followed by 26 zeros.