IF( and that is a big if) light were to decelerate at the rate of I 13billionth of its original velocity per year , what would be the formula for computing the distance to the most distant observable objects in the universe: FRED

What a strange question. Are you asking what would be the effect if light were to decellerate from 3x10^8 to 3x10^8/13x10^9 over a year, and how we would account for that to determine the distance of objects?

By the time the Earth is created, light would be moving at practically 0, and we would never have come into existence as heat would never be transferred from the sun to Earth. Getting past that, we would only be able to see objects as they were a few years after the big bang, and a light year or 2 away.