Which theory is attributed to the Belgian Catholic priest and scientist Georges Lemaitre?

The archival picture shows the Belgium scientist Georges LeMaitre (1894-1966), a Belgian Catholic priest, theoretical physicist, mathematician, astronomer and professor of physics. He established the scientific theory that the universe is expanding because space is stretching, which was observationally confirmed soon afterwards by Edwin Hubble (1889-1953), an American astronomer.

He was also the first scientist to propose that the universe and time itself began in a single instant, later called the ‘Big Bang Theory’ by subsequent scientists. He advanced the theory of an explosion of a ‘primeval atom’ whose matter spread and evolved to form the galaxies and stars we observe in the night sky.

His parents were devout Catholics and at the age of 9, Georges announced that he wanted to become a priest. The following year he enrolled at a Jesuit High School where he excelled in chemistry, physics, and mathematics. When his family moved to the capital city of Brussels, George’s, now age 16, enrolled at another Jesuit school, the College Saint Michel.

His studies were interrupted with the onset of WWI in July 1914. He immediately enlisted in the Belgian Army and 3 months later saw action on the front line. He survived the war and changed his pursuit of engineering to enroll at the Catholic University of Leuven to study for an advanced degree in mathematics. After studying Albert Einstein’s theory of relativity, he went on to be ordained as a priest in 1923.