WASHINGTON (Reuters) - Astronomers said on Monday they have detected a cosmic explosion at the very edge of the visible universe, a 13-billion-year-old blast that could help them learn more about the earliest stars. The brilliant blast -- known as a gamma ray burst -- was probably caused by the death of a massive star soon after the Big Bang, but was glimpsed on September 4 by NASA's new Swift satellite and later by ground-based telescopes. The explosion occurred soon after the first stars and galaxies formed, perhaps 500 million to 1 billion years after the Big Bang explosion that...