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years. “The evolution of the world can be compared to a display of fireworks that has just ended: some few red wisps, ashes, and smoke,” the Belgian cleric would later write. “Standing on a well-chilled cinder, we see the slow fading of the suns, and try to recall the vanished brilliance of the origin of the worlds.” This idea would later be revised by others to show how our universe evolved, not from a super-atom, but from a cosmic seed of pure energy. From Lemaître's poetic scenario arose today's vision of the Big Bang, the cosmological model that shapes and directs the thoughts of cosmologists today as strongly as Ptolemy's crystalline spheres influenced natural philosophers in the Middle Ages.

Though ordained as an abbé, later rising to the rank of monsignor, Lemaître did not endure the fate of Galileo in contemplating a scientific explanation for heaven's workings, in this case the universe's creation. As Helge Kragh has noted, “Lemaître believed that God would hide nothing from the human mind, not even the physical nature of the very early universe.” Times had assuredly changed—while Galileo was condemned by church officials to house arrest for his defense of a Sun-centered universe, Lemaître was lauded by the Church for his cosmological breakthrough. However, nothing could upset Lemaître more than assuming his cosmological model had been inspired by the biblical story of Genesis. His contemplation of the origin of space and time, he persistently asserted, arrived exclusively from the equations before him. As a scientist/priest, Lemaître religiously kept his physics and theology in separate, unattached compartments.

But the Big Bang model faced a number of challenges before it could be fully accepted. The biggest hurdle was the estimate of the universe's age, based on early (and incorrect) measurements of the rate of cosmic expansion. Hubble's initial rate, calculated from a relatively small sample of galaxies, suggested that the universe originated just two billion years ago, but astronomers already knew of stars around ten billion years old. Looking closer to home, it was also less than the estimated age of Earth. Geologic evidence at the time indicated that Earth's crust was at least three billion years old, likely more. This paradox posed a dilemma for the model for quite a while. How could Earth possibly be older than the universe?

There were other loose ends. For one, the Milky Way still appeared to be far larger than the other galaxies. The Andromeda galaxy, the closest spiral to us, shared so many features with the Milky Way—the same disk of stars, the same system of globular clusters arranged in a halo around it, the same variable stars blinking on and off—and yet all these objects appeared fainter than those in the Milky Way, based on Hubble's initial distance measurement. More than that, Andromeda was smaller. This greatly bothered astronomers, who were now readily applying the Copernican rule to the entire universe: It is unlikely that we occupy a privileged place in the cosmos.

This puzzle persisted until 1952, just when Hubble's long reign as the emperor of cosmology was coming to an end. Having gone into military work during World War II, Hubble had a lot of catching up to do at the war's end, but ill health prevented him from getting back on top. By then Walter Baade, a gifted observer, was beginning to overshadow Hubble with his revelatory work that at last put the universe (and the Big Bang) in better shape. Using both the 100-inch telescope during the war and later the 200-inch launched in 1948 on Palomar Mountain in California, Baade was able to prove that there were two distinct kinds of Cepheid stars. The Cepheids that Hubble used to determine the distance to Andromeda and other galaxies were actually more luminous than the Cepheids that Shapley used to determine his distances to the globular clusters surrounding the Milky Way. Consequently, Hubble had been underestimating his distances to Andromeda and the other galaxies. Hubble's distances had to be completely reworked. Andromeda, for example,