This blog post examines whether the debate over the size and nature of the universe during the 1920 ‘Great Debate’ served as the starting point for modern cosmology, and reflects on its historical significance.
On April 26, 1920, approximately 300 scientists were invited to a National Academy of Sciences debate held in the Baird Auditorium of the Smithsonian National Museum of Natural History. Here, two astronomers engaged in a debate later recorded in history as the ‘Great Debate’. The topic of this all-day discussion was the question: “How big is the universe?” One astronomer argued that the universe consisted solely of a single galaxy, while the other maintained that more galaxies existed beyond the one containing Earth. These two astronomers, Harlow Shapley and Heber Doust Curtis, were world-renowned directors of the Harvard College Observatory and the Lick Observatory in California, respectively.
This debate originated from the observation of a vast nebula by the British scientist Edmund Halley in 1716. The nature of this enormous luminous object, later named Andromeda, captured the attention of many scientists. Harlow Shapley and Heber Doust Curtis also held differing opinions about Andromeda. Harlow Shapley believed Andromeda was merely a nebula within our own Milky Way galaxy, while Heber Doust Curtis maintained it was a separate galaxy existing outside our Milky Way. This debate over the scale and structure of the universe culminated in the ‘Great Debate’ of April 1920.
The Great Debate concluded in 1923 with Edwin Powell Hubble’s discovery of Cepheid variables. The discovery of these variables and the research results published in 1924 proved the existence of Andromeda beyond the size of our galaxy as claimed by Harlow Shapley. Consequently, Harlow Shapley’s assertion that Andromeda existed within our galaxy was completely overturned.
The outcome of the Great Debate revealed that the universe is not confined to our galaxy alone, but that countless other galaxies exist beyond it. This discovery owed much to the work of Heber Doust Curtis. Based on the rotational velocity of the Pinwheel Nebula observed by Van Maanen at the time, he presented an opposing view to Harlow Shapley’s claim. Today, we take it for granted that diverse galaxies exist beyond our own. Therefore, Heber Doust Curtis’s achievements deserve praise. He made claims similar to modern cosmology even with the underdeveloped observational technology and data available at the time.
Modern astronomy is now turning its gaze beyond our universe to a wider world. Some scientists argue that other universes exist beyond ours, while others claim that a multiverse does not exist due to its unobservability. Just as we discovered worlds beyond our galaxy, another world may exist beyond our universe. Let us then consider the observability of a multiverse through the lens of the Great Debate of the past.
There are two primary methods for measuring the mass of objects within the universe: utilizing optical observations (electromagnetic waves) and utilizing observations of gravitational effects (gravitational waves). Scientists discovered that the mass determined by optical observations is smaller than the mass determined by gravitational effects. This led to the assumption that a substance exists which possesses mass but does not interact with electromagnetic waves—dark matter. However, what particles dark matter is composed of and where it came from remain a mystery.
This situation is similar to the thinking about Andromeda in the 1920s. At that time, people thought Andromeda was a nebula within our Milky Way, but several properties contradicting this were discovered. A prime example is the observation by Heber Doust Curtis that the number of supernovae observed within Andromeda exceeded the total number of supernovae in our entire Milky Way, leading him to argue that Andromeda was another galaxy existing outside our Milky Way.
I think the same applies to dark matter. If we consider the universe to be the entirety of this world, it becomes difficult to explain the gravity possessed by dark matter. However, the story changes if a new universe exists beyond our own, and dark matter arises due to this universe. In a multiverse, countless universes exist, and gravity can act between them. Therefore, dark matter could be considered the force exerted by other universes upon our own.
Of course, the multiverse has not yet been observed and may never be. Some critics argue that believing in a theory that is neither observed nor supported by solid evidence is absurd. However, considering the example of the Great Debate, which occurred less than a century ago, the possibility of a multiverse cannot be dismissed. Heber Doust Curtis also advocated the island universe theory without definitive evidence at the time, yet modern science has proven that theory correct.
Nevertheless, some argue that the multiverse is fundamentally unobservable. They view the case of Heber Doust Curtis as intrinsically different from multiverse theory. While the dispute between Heber Doust Curtis and Harlow Shapley was a problem solvable with advancing observational technology, some argue that observing a multiverse may be not just difficult, but fundamentally impossible. This argument is based on the claim that it would require moving into another dimension beyond our own universe, something impossible for humans existing in three dimensions. If multiverses are independent and cannot observe each other, it is argued that it is incorrect to say they exist.
Nevertheless, I believe the multiverse is highly likely to exist. Current cosmology remains mired in mystery. Even the Big Bang theory, widely accepted as established fact, is merely a hypothesis not yet fully proven. Yet I support multiverse theory because it explains various theories. Consider string theory, proposed in the late 1960s to explain the universe. String theory posits that all matter in the world is composed of extremely small strings. While constructing everything from strings in a four-dimensional space (three dimensions plus time) creates contradictions, assuming these strings exist as matter in ten dimensions allows for the possibility of countless universes.
Thus, various theories proposed in modern science explain the possibility of a multiverse. Although the multiverse has not been observed and likely never will be, this does not mean we can definitively conclude it does not exist. A century ago, a scientist succeeded in predicting a new world beyond the one we knew, without observation or definitive evidence. We too believe we can transcend the universe we know through the multiverse. Perhaps in the distant future, the multiverse will be accepted as established fact, and we may even venture beyond it into yet another world.
