By applying the maths that flow from the Theory of Relativity, he was the first to propose the expanding universe, and later what we call the Big Bang. He came up with Hubble’s Law, and calculated an approximation of Hubble’s Constant. Einstein was skeptical about his theories and it was Einstein that was wrong . (Einstein, being a good scientist, changed his views in the light of the evidence).
Yet his name’s not Hubble, it’s Georges Lemaître.
Yet, for a theory that implies the Universe is billions of years old, and for developing a mathematical model of how it all happened, look at the sanctions the Church applied to Mgr Lemaître. (The clue’s in his title- he was made a “Monsignor”. )
He’s also a famous Belgian. Quite a bloke. He was also a Cambridge man but, let’s face it, nobody’s perfect.