From our modern perspective, astronomy and physics have always been intertwined. The earliest mathematical models of physical motion were developed in the context of planets and stars by astronomers like Copernicus and Kepler. Newton’s laws of gravitation were later used to explain the fundamental physics behind Kepler’s laws of planetary motion. Supporting evidence for Einstein’s General Theory of Relativity was found in its ability to explain the long-term variations in the observed motion of Mercury. In such ways, observation and theory work together to improve our understanding of the Universe.
We credit the General Theory of Relativity with laying out the mathematical logic behind the idea of an expanding universe. Yet, in spite of a long history of connections, when the General Theory of Relativity was published in 1916, observational astronomy was still learning its way around the Milky Way. Even the idea that there were galaxies other than our own was fiercely debated at the time. Because observational astronomy was focused on measuring distances within our own galaxy, cosmologists applied the tools of mathematics and geometry to explore the implications of Einstein’s new theory.
By 1922, the best work in cosmology pointed to a dynamic universe. Whether or not the Universe was expanding, contracting, or oscillating could not be determined with the existing observational evidence. This is the frame of reference from which we explore Hubble’s observational journey. From the one and only Earth in the one and only known galaxy, Hubble and others gathered information about the known universe. What they observed and reported would bring theory and observation hurtling back together due to evidence so unexpected that nearly ten years later Hubble would still refer to it as “rather startling.”
Step 1: There are other galaxies
Before scientists could even conceive questions about large distances in space and the need to measure them, they had to recognize that our own Milky Way did not occupy all known space — that something else lay beyond it. By 1926, evidence was mounting that some of the fuzzy blobs, or nebulae, observable in the sky were in reality large collections of stars — galaxies outside our own Milky Way. One of the lines of evidence was that the spatial distribution of the nebulae in the sky suggested that these same nebulae were not aligned with the plane of the Milky Way like other nearby stars and clouds. We’ll explore this in the next section!
Step 2: Galaxies are really far away
Even with confirmation that galaxies exist outside the Milky Way, testing the theory of a spatially expanding universe was not driving observational astronomy quite yet. Astronomers were very interested in understanding the structure of our own galaxy and exploring our place in it. Just as passengers on a train use distant objects to judge their speed, Hubble recognized that we could use galaxies as markers of our Solar System’s motion in the Milky Way. Knowing the distance to galaxies was an important part of understanding our place in the Milky Way (the Expedition, Leaving the Galaxy, explores distance in depth). For this Expedition, however, we look at Hubble’s effort to calculate relative galactic distances. In the end, it led to some puzzling findings.
Step 3: 1936 Redshift and Distance
Continued efforts to measure distances in extragalactic space combined with accurate measurements of redshift culminated in the 1936 paper, The Velocity-Distance Relationship Among Extragalactic Nebulae by Hubble and Humason. In this last step we revisit this landmark paper by examining the data available in the 1930s and comparing it to what is available today.