that if two Cepheid variable stars had similar periods but one was dimmer than the other, it was probably farther away. He then searched for and found a Cepheid variable close enough to Earth to measure the distance to it using parallax. Knowing this distance, he measured its apparent brightness and used Leavitt’s graph to reverse engineer its actual brightness. The resulting formula, called the Distance-Luminosity relation, allowed scientists to measure the distances to all Cepheid variables, and thus also to the stars and other nearby formations. The blinking stars became “standard candles” throughout the heavens. Leavitt was paid a premium rate of thirty cents per hour over the usual two bits because of the high quality of her work.
The standard candle measurement of space was an immense discovery. In 1915, American astronomer Harlow Shapley, a Democrat, used it and Mount Wilson’s sixty-inch telescope to map the Milky Way in three dimensions. Shapley’s measurements expanded the known size of the Milky Way severalfold and showed that the sun was not at the center of the galaxy, as had been thought until that moment, but was in fact located in a distant outer arm. This overturned the concept of the centrality of humans yet again, and Shapley was celebrated as the greatest astronomer since Copernicus—a title he himself helped promote—for having achieved the “overthrow” of the heliocentric universe.
The Great Debate: A Cautionary Tale
In what would become an important cautionary tale for science—and, by extension, democracy—Shapley became blinded by his belief that the Milky Way was the entire universe—or maybe by his hubris, in wanting to believe that he had mapped the whole shebang. He argued that the spiral nebulae seen in the heavens were simply wisps of gas and clouds of dust within the Milky Way, rather than entire “island universes”—that is, galaxies—of their own, as fellow astronomer Heber Curtis posited. Shapley debated this point with Curtis at a meeting of the US National Academy of Sciences in April of 1920, in an event famously called the Great Debate.
The debate ended in a draw because there wasn’t yet enough observational data to draw firm conclusions. But this was partly because Shapley, without realizing it, had stopped using Locke’s and Bacon’s inductive reasoning to build knowledge from observation. Instead, he was trying to prove his point with a rhetorical argument—an a priori, top-down, Cartesian approach of first principles that had him arguing more like an attorney than a scientist. When his assistant, Milton Humason, showed him a photographic plate that seemed to indicate the presence of a Cepheid variable in Andromeda, Shapley shook his head and said it wasn’t possible. Humason had unimpressive formal credentials—he had been elevated to assistant from a mule driver and had only an eighth-grade education—but it was Shapley’s a priori ideas that occluded his vision. He took out his handkerchief and wiped the glass plate clean of Humason’s grease pencil marks before handing it back. No one realized it at the time, but Shapley’s career as a major scientist ended in that moment.
Soon after, Shapley moved on to run the Harvard College Observatory while his tale-telling rival, Edwin Hubble, took over the telescope. Adopting the uncredentialed but brilliant Humason as his assistant and adhering to his strict Baconian observational methodology, Hubble soon identified Cepheids in Andromeda and used them to show that the spiral “nebula,” as he called the galaxy, was not part of the Milky Way at all. In fact, it was nearly a million light-years distant—more than three times farther away than the diameter of Shapley’s entire known universe. The Great Debate was settled, and Hubble became an overnight sensation.
A Republican Expansion
In 1929, Hubble and Humason followed this accomplishment up by showing that there is a direct correlation between how far away a galaxy is and its redshift—the degree to which its light waves are shifted to the red end of the spectrum. Light waves are emitted at known frequencies. Redshift is caused by a star’s light waves stretching out, apparently due to the star’s rushing away from Earth, making them lower in frequency and thus shifted toward the red end of the visible-light spectrum.
Scientists had already established that the redshift suggested that light waves were subject to the Doppler effect. We notice the Doppler effect in everyday life when the sound of a train’s whistle or a police siren lowers in pitch as it races away from us. Astronomers believed redshift could be used as a measure of the speed at which a star appears to be moving away from us. Hubble correlated that redshift with distance, and then showed via painstaking observation—performed mostly by Humason, whom Shapley had recommended for promotion to the scientific staff—that the farther away a star is, the greater the redshift. The odd couple of the liar and the mule driver found this to be uniformly true in every direction of the sky. This suggested that the universe itself was probably expanding at an even rate.
To picture this, imagine blowing up a perfectly round balloon until it is no longer flaccid but not yet taut. Now take a marker and mark spots in a grid pattern over the entire surface of the balloon, each spot exactly one inch from the next. Now finish blowing up the balloon and watch what happens. As the balloon expands, every dot moves farther away from every other dot. The space between each pair of dots expands. In addition, the dots that are, say, five dots apart from each other move apart five times faster than the dots that are only one dot apart do because the surface is expanding uniformly. Five times the distance, five times the expansion. This is a close analogy to what Hubble and Humason saw happening in three dimensions in the universe.
This fundamental velocity-distance relationship came to be known as Hubble’s law, and it is recognized as one of the basic laws of nature. It was a liar who, ironically, using the tools and methods of science, discovered some of the universe’s most fundamental truths. But his work also implied something even more momentous.
A Catholic Priest’s Big Bang
Georges Lemaître was a pudgy, pinkish Belgian Jesuit abbé—a Catholic priest—and also a skilled astronomer. Like the Puritans, Lemaître was interested in reading the Book of Nature, something the Catholic Church has come to support in the ensuing centuries, funding major astronomical observatories. He was also a reasonably good mathematician, and he 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, which Einstein had inserted into his equations. Lemaître saw no convincing scientific or mathematical 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 his theory, he found that almost all of them held the notion that the universe existed in a steady state, and that there was no motion on a grand scale. 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.
Lemaître worked independently off the same mathematical principles that Einstein had originally laid out. Nine years later, 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. This redshift had been observed by a number of astronomers, but until then there had been no consensus on the cause.
Lemaître saw Hubble’s self-evident observations and clear logic and immediately realized that Hubble’s work confirmed his math—and refuted Einstein’s general theory of relativity. Furthermore, he deduced, if the universe was expanding equally in all directions, it must have initiated in a massive explosion from a single point. This meant that the universe is not infinitely old; it has a certain age, and that the moment of creation—which British astronomer Fred Hoyle later mockingly called the “big bang”—was analogous to God’s first command: Let there be light.
Hubble’s meticulously reported observations and ironclad, self-evident conclusions convinced Einstein that he may have been wrong to insert the cosmological constant. He made a pilgrimage from Germany to Mount Wilson Observatory outside of Pasadena, where he joined Hubble, Humason, Lemaître, and others to speak with Hubble and examine the stars through the one-hundred-inch telescope. Then he held a news conference. Standing with Hubble, Humason, and the other scientists, he made a stunning public announcement. Unlike Shapley, Einstein changed his mind based on the evidence, and removed