imagine that astronomers found a galaxy that was 20 billion light years from the earth how would they have to revise the bug banf theory to account for such a discovery

Not necessary.

Explanation:

No need for the revision of big band theory because this galaxy may be a product of big band event. At the start this galaxy is shorter and hot but with the passage of millions of years, this galaxy also expands and formation of stars and planets occurs. The stars and planets that are present in the galaxy occurs due to different events that is responsible for their current conditions. This formation of stars and planets needs millions of years.