85: How Big is the Universe? It’s Debatable... An Essay By Brian Keating

The Great Debate

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The GREATEST Debate:

How two astronomers changed the way humanity debates

By April 1920, the Spanish flu had claimed 50,000,000 lives. The first World War ended only 17 months earlier. A polarizing presidential election was underway. On April 26, 1920, in Washington DC, two contestants took the stage in a debate that would alter the cosmos forever. Was this contest the 1920 presidential debate? No, this battle was literally for universal domination, not a mere skirmish between presidential contenders Warren Harding and James Cox. Only the wonkiest history buffs recall who won 1920’s presidential debates. But every astronomer knows the two scientists sparring on that April evening at the Smithsonian Museum of Natural History, taking sides in astronomy’s “Great Debate”: Heber Curtis, director of the Allegheny Observatory, versus Mt. Wilson Observatory astronomer Harlow Shapley. The Debate’s outcome could not have been more consequential; the span of the entire universe was at stake. This epic contest concerned the nature of the so-called “spiral nebulae” that had vexed astronomers since Lord Rosse first pointed his 54-foot long telescope, appropriately nicknamed ’the Leviathan’, toward the heavens in 1845. Seventy-five years later, Shapley claimed these diaphanous swirls of light were our neighbors within the Milky Way, a mere hundred or so light years away.

Curtis contended that some of these smudges were galaxies in their own right, assemblies of hundreds of billions of stars. If Curtis was right, these ’nebulae’ resided at truly astronomical distances from Earth. Curtis and Shapley decorously dealt philosophical punches. No mute buttons were needed. But lacking physical evidence, the Great Debate ended in a draw. The runoff would put the aftermath of the 2000 presidential election, and its ‘hanging chads’, to shame. It took over three years until Edwin Hubble discovered a curious cosmic beacon on the outskirts of the “Great Spiral Nebula” in Andromeda. This luminary, a class of star known as a Cepheid variable, periodically waxed and waned in brightness about twice per week. Years earlier, the brilliant astronomer Henrietta Leavitt showed that the rate at which Cepheid stars pulsated could be used as a sort of “cosmic ruler”, surveying distances far beyond Earth. Applying Leavitt’s Law (as we now call it), Hubble determined the Andromeda “nebula” was incomprehensibly distant—2,540,000 light-years to be exact.

The Milky Way was large, but its diameter was known to be less than 10 percent of that vast span. Andromeda was not in our galaxy, it was a galaxy, hosting 100 billion or more suns of its own. The Great Debate was settled. Shapley lost, but was humble in defeat, encouraging Hubble to widely publicize his findings. It wasn’t only the distance to the Andromeda galaxy that had grown, humanity’s perspective was forever broadened, tempered with newfound humility amidst the vastness of intergalactic space. Today, instruments like the Hubble Space Telescope, celebrating its 30th anniversary, have magnified the Great Debate’s significance, revealing that the Milky Way and Andromeda are two of as many as two trillion galaxies. What future debates will be waged in astronomy’s next century, using instruments (costing a mere fraction of modern presidential campaigns), such as the Simons Observatory, the Vera Rubin Observatory, the Xenon Dark Matter Project, and more? Will we learn that dark matter is a mysterious new particle or the effect of an unknown new force? Will we spot life on exoplanets, or discover that we truly are alone in the cosmos? Perhaps we will even find evidence that ours is not the only universe—just some of modern astronomy’s “Great Debates.”

One of the most refreshing aspects of astronomy is that it is inherently apolitical. There are no Democratic comets, no Republican asteroids. Perhaps that explains the Great Debate’s comity, despite its cosmic stakes. Great debates ennoble us, deepening our appreciation of nature's tapestry. The significance of a debate shouldn’t be judged by who “won”, for not all contests are zero-sum games. Rather, debates should be appraised by the quality of the questions we pose. Honestly debating grand mysteries, prepared to change our minds -- or even concede our opponent’s point if new evidence surfaces -- not only makes for better scientists, it makes for better human beings/citizens too. Besides, from the cosmic perspective, political victories are fleeting. Harding’s administration lasted barely two years; he died two months before Edwin Hubble settled the Great Debate for good. Time will tell if today’s political debates are remembered a century from now. But the universe, and our perceived place within it, abides forever. Or does it? It’s debatable.

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