1. The problem statement, all variables and given/known data At the time of decoupling the universe was 1/1000 of its present size. How for could light have travelled in the time up to decoupling? (assume that the universe was dominated by radiation until then) 2. Relevant equations 3. The attempt at a solution I'm a little confused about how to include the fact that the universe was expanding while the light was travelling up to the time of decoupling. I have the age of the universe at decoupling(calculated earlier) and I have calculated the scale factor as a function of time for a radiation dominated universe. I assume that the scale factor means that light travels less distance with time as distance is expanding but i'm not sure about this.....and how would I calculate this distance?