In this blog post, we will examine whether the debate over the size and nature of the universe that took place during the Great Debate of 1920 served as the starting point for modern cosmology, and we will revisit its historical significance.
On April 26, 1920, approximately 300 scientists were invited to a National Academy of Sciences discussion held at the Baird Auditorium of the Smithsonian Museum of Natural History. There, two astronomers engaged in a debate that would later be recorded in history as the “Great Debate.” The topic of the day-long discussion was the question, “How big is the universe?” One astronomer argued that the universe consisted of a single galaxy, while the other claimed that there were many more galaxies beyond the one containing Earth. These two astronomers, renowned worldwide as the directors of Harvard College Observatory and Lick Observatory in California, were Harlow Shapley and Heber Doucure Curtis.
This debate began in 1716 with the observation of a massive nebula by the British scientist Edmund Halley. This massive light, later named Andromeda, captured the attention of many scientists. Harlow Shapley and Heber Doust Curtis also held differing opinions about Andromeda. Harlow Shapley believed that Andromeda was merely a nebula within our Milky Way, while Heber Doust Curtis argued that Andromeda was a separate galaxy existing outside our Milky Way. This debate over the scale and structure of the universe reached its climax in April 1920 during the “Great Debate.”
The Great Debate came to an end in 1923 with Edwin Powell Hubble’s discovery of Cepheid variable stars. The discovery of these variable stars and the research results published in 1924 proved the existence of Andromeda beyond the size of our galaxy as claimed by Harlow Shapley. As a result, Harlow Shapley’s claim that Andromeda existed within our galaxy was completely overturned.
As a result of the Great Debate, we realized that the universe is not limited to our galaxy, but that there are countless other galaxies beyond it. This discovery was made possible by the contributions of Heber Doust Curtis. Based on the rotational speed 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 there are various galaxies beyond our galaxy. Therefore, Heber Doust Curtis’ achievements are worthy of praise. Despite the limited observational technology and data available at the time, he made claims similar to modern cosmology.
Modern astronomy is now looking beyond our universe to a wider world. Some scientists argue that there are other universes beyond ours, while others argue that multiple universes do not exist due to their unobservability.
Just as we have discovered worlds beyond our galaxy, there may be other worlds beyond our universe. Let us consider the possibility of observing multiple universes through the Great Debate of the past. There are two ways to measure the mass of objects in the universe: using optical observations (electromagnetic waves) and using gravitational effects (gravitational waves). Scientists discovered that the mass measured by optical observations is smaller than the mass measured by gravitational effects, leading them to assume the existence of dark matter, a substance that has mass but does not interact with electromagnetic waves. However, the nature of dark matter—what particles it is made of and where it came from—remains a mystery. This situation is similar to what people thought about Andromeda in the 1920s.
At that time, people thought that Andromeda was a nebula within our galaxy, but several properties that contradicted this were discovered. A representative example is that Heber Doust Curtis observed that the number of supernovae observed in Andromeda was greater than the number of supernovae in our entire galaxy, leading him to argue that Andromeda was another galaxy outside our galaxy.
I think the same applies to dark matter. If we consider the universe to be the entirety of this world, it is difficult to explain the gravity of dark matter. However, if there are new universes outside our universe and dark matter is generated by these universes, the story changes.
In a multiverse, countless universes exist, and gravity can act between them. Therefore, dark matter could be the force acting on our universe from other universes. Of course, the multiverse has not yet been observed and may never be observed. Some people criticize the multiverse theory, arguing that believing in a theory that has not been observed and lacks definitive evidence is absurd. However, considering the example of the Great Debate, which occurred less than 100 years ago, the possibility of a multiverse cannot be completely dismissed. Heber Doust Curtis also proposed the island universe theory without definitive evidence at the time, but it was later proven correct. However, some people argue that the multiverse is observationally impossible. They believe that the case of Heber Doust Curtis and the multiverse theory are fundamentally different.
While the debate between Heber Doust Curtis and Harlow Shapley was a problem that could be resolved with the advancement of observational technology, some argue that the existence of multiple universes is not only difficult to observe but may be impossible. This argument is based on the claim that observing multiple universes would require moving beyond our current universe into another dimension, which is impossible for humans who exist in three dimensions. If multiple universes are independent of each other and cannot be observed, then it is not correct to say that they exist.
Nevertheless, I believe that the multiverse is highly likely to exist. Current cosmology remains shrouded in mystery. Even the Big Bang theory, widely accepted as the standard model, is merely an unproven hypothesis. Despite this, I support the multiverse theory because it can explain various theories. Consider string theory, proposed in the late 1960s to explain the universe. String theory posits that all matter in the universe is composed of extremely small strings. While there is a contradiction in the idea that everything is made up of strings in a four-dimensional space (three dimensions plus time), if we assume that strings exist in ten dimensions, an infinite number of universes could exist.
In this way, various theories proposed in modern science explain the possibility of a multiverse. Although the multiverse has not been observed and is unlikely to be observed in the future, this does not mean that it does not exist. A hundred years ago, a scientist successfully predicted a new world beyond what we knew, even though it had not been observed and there was no definitive evidence. We believe that through the concept of multiple universes, we may one day transcend the universe we currently know. Perhaps in the distant future, the existence of multiple universes will be widely accepted, and we may even venture into worlds beyond them.
