Who was Georges Lemaître, founder of modern Big Bang theory?

Born on July 17, 1894, Belgian astronomer Georges Lemaître is regarded as the founder of the modern Big Bang theory, which says the observable universe began with an explosion of a single particle. Based on Einstein's theory of general relativity, Lemaître also proposed the universe is expanding, which was observationally confirmed in 1929 by Edwin Hubble and named Hubble's Law.