Who is Edwin Hubble from A Brief History of Time and what is their importance?

Hubble discovers the first galaxies outside our own. He also figures out a way to determine their distances using light spectrum shift. This leads to his observation that all the galaxies are moving away from each other—the universe is expanding. Hawking takes this observation and argues how the expansion may one day stop, after which the universe may contract back to one or a number of singularities.