The idea that there are galaxies that surround us, and that they stretch out to the edge of the universe, is a relatively new one in the historical context. It is an idea that was perhaps first suggested by the philosopher Immanuel Kant, who proposed that certain nebula (cloudy smudges of light) seen in telescopes were outside of our own local sphere of space and existed in their own right as what he called ‘island universes.’ The resolution of this issue would have to wait until the beginning of the 20th century, when new astronomical observations brought, quite literally, new light to the discussion.
The nature of these nebula was the subject of a serious discussion that took place at the meeting of the National Academy of Sciences in 1920, an event now known as the Great Debate. On one side of the debate was Harlow Shapley, who argued that the nebula were contained within our own local area and that the Milky Way encompassed the entirety of the universe. Arguing against this was Heber Curtis, who proposed that the number of nova observed within the Andromeda nebula indicated that it was an entirely separate galaxy from our own.
One of the definitive pieces of evidence in favor of the ‘island universe’ hypothesis came in 1924, when Edwin Hubble (the namesake of the famous space telescope) published his observation of Cepheid variables (a certain class of star known to have specific relationships between brightness, fluctuation period, and distance) in the Andromeda nebula, establishing it at a distance on the order of a million light years, well beyond the acknowledged limits of the Milky Way. Andromeda was not a mere nebula, but an entire galaxy.
These results build on earlier work by Shapley, who had undertaken a mapping of the Milky Way and found that the Sun occupied an unremarkable location towards the edge. Just as Shapley’s work had displaced mankind from the center of our own galaxy and reinforced the possibility that our own solar system was one of many contained within that galaxy, Hubble’s results now suggested that even our galaxy was only one of many.
Shapley recognized the implications of this continued progression. He remarked,
“The physical universe was anthropocentric to primitive man. At a subsequent stage of intellectual progress it was centered in a restricted area on the surface of the earth. Still later, Ptolemy and his school, the universe was geocentric; but since the time of Copernicus the Sun, as the dominating body of the solar system, has been considered to be at or near the center of the stellar realm. With the origin of each of these successive conceptions, the system of stars has ever appeared larger that was thought before. Thus the significance of man and the earth in the sidereal scheme has dwindled with advancing knowledge of the physical world, and our conception of the dimensions of the discernible stellar universe has progressively changed.”
As Shapley found, both in his own work and in the history of scientific progress that came before him, the more knowledge that we gain about the way that the universe works, the further from the center of things we move, and the more apparent it becomes that we occupy a small place in the cosmos. And yet, as conscious minds, our small place is a very special place. As Carl Sagan remarked, “we are a way for the cosmos to know itself.” Perhaps we may discover other life out in the universe somewhere, potentially even conscious, intelligent life. As enormous as the universe is, anywhere that conscious life arises is itself a very remarkable place.