Georges Lemaitre was a pudgy, pinkish Belgian Jesuit abbe—a Catholic priest—who also happened to be a skilled astronomer. Lemaitre had noticed that Einstein's general theory of relativity would have implied that the universe was expanding but for a troublesome little mathematical term called the cosmological constant that Einstein had inserted into his equations. Lemaitre saw no convincing reason why the cosmological constant should be there. In fact, Einstein himself had originally calculated that the universe was expanding, but he was a theoretician, not an astronomer. When he turned to astronomers for verification of verse existed in a steady state and there was no motion on a grand scale. So in deference to their observational experience, Einstein adjusted his general theory calculations with a mathematical "fudge factor"—the cosmological constant—that made the universe seem to be steady.

Lemaitre had independently been working off the same mathematical principles that Einstein had originally laid out, and in 1927 he wrote a dissenting paper in which he argued that the universe must be expanding, and that if it was, the redshifted light from stars was the result of this expansion. This redshift had been observed by a number of astronomers. but until then there had been no consensus on what the cause could be.

Lemaitre saw Hubble's self-evident observations and clear logic and immediately realized that it confirmed his math and refuted Einstein's general theory. Furthermore, he deduced, if the universe was exppanding equally in all directions, it must have initiated in a massive expcplosion from a single point. This meant to him that the universe is not infnfinitely old; it has a certain age, and that the moment of creation—which I British astronomer Fred Hoyle later mockingly called the "big bang";"—was analogous to God's first command at the beginning of the good abbe's most cherished book, the Bible: Let there he light.

Hubble's meticulously reported logic and observations convinced Einstein that he had been wrong about the cosmological constant. He made a pilgrimage from Germany to Mount Wilson Observatory outside of Pasadena, where he joined Hubble, Humason, Lemaitre, and others tc make a stunning public announcement. Unlike Shapley, Einstein changed his position and removed the cosmological constant from his general theory of relativity, later calling it "the biggest blunder of my life." The universe was indeed expanding.

He put the constant into his theory to keep the Universe static, but observations demonstrated it was expanding, so he changed his theory to match the evidence.

For much of the modern era, scientists followed Nicolaus Copernicus, Galileo Galilei, and Isaac Newton in believing the cosmos to be eternal and unchanging. But in 1917, when Albert Einstein applied his theory of relativity to space-time as a whole, his equations implied that the universe could not be static; it must be either expanding or contracting. This struck Einstein as grotesque, so he added to his theory a fiddle factor called the "cosmological constant" that eliminated the implication and held the universe still.

It was an ordained priest who took relativity to its logical conclusion. In 1927, Georges Lemaître of the University of Louvain in Belgium worked out an expanding model of the universe. Reasoning backward, he proposed that at some definite point in the past it must have originated from a primeval atom of infinitely concentrated energy. Two years later, Lemaître's model was confirmed by the American astronomer Edwin Hubble, who had observed that the galaxies everywhere around us were receding. Both theory and empirical evidence pointed to the same verdict: The universe had an abrupt beginning in time.

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Einstein overcame his metaphysical scruples about the big bang not long before his death in 1955, referring to his earlier attempt to dodge it by an ad hoc theoretical device as "the greatest blunder of my career."

The story of how Einstein's belief in a static Universe prompted him to introduce a fudge-factor in his Theory of Relativity, the Cosmological Constant.