This graphic shows the size of the observed universe over time, highlighting differences between inflation (red line) and the standard Big Bang theory (dark gray line). In an inflationary universe, the size of the observable universe starts out small enough that regions that end up far apart after inflation (which occurs during the time shaded out by the vertical yellow bar) can have the same temperature because they were in contact beforehand. This horizon problem is one issue with a Big Bang theory that does not include inflation.