In this photo you see the altar of one of the cathedral's side chapels. During the period the nine popes resided in Avignon, art and culture flourished, but so did prostitution and vice.
In 1376 Gregory XI was persuaded to return to Rome, but Avignon didn't want to lose the privilege of being the capital of Christianity and elected its own rival pope. As you may suspect, this was not only a religious rivalry, but a battle about the wealth and power of the papacy. In 1417, with the election of Martin V, the papal court in Avignon was disbanded, and Rome became the one and only capital of the Christian world.