Georges Lemaître

Monsignor Georges Lemaître (Georges Henri Joseph Édouard Lemaître, 17 July 1894 – 20 June 1966) was a Belgian priest, astronomer , mathematician and professor of physics at the Catholic University of Louvain. 
He was the first person to say the Universe is growing. Some people think it was Edwin Hubble, but that is not correct. Lemaître was also the first to make the Hubble's law and the Hubble constant.
Lemaître also started what became known as the Big Bang theory of the origin of the Universe. He called it his 'hypothesis of the primeval atom'.