Halton Christian Arp was an American astronomer quite unlike any other. His lifetime of work cataloging astronomical curiosities continues to provide a valuable resource for modern researchers, although at one time his theories caused quite an upset in the astronomical community. The resulting debate was, for a time, even recognized as a potential challenge to long-held theories about our universe that included the Big Bang, the event which marks the beginning of the universe as we know it.
Born in 1927, Arp received his bachelor’s degree from Harvard University at age 22 and went on to complete his Ph.D. at Caltech four years later. Almost immediately following his graduation, he became a Fellow of the Carnegie Institution of Washington, where he began research at the Mount Wilson Observatory and Palomar Observatory, where he would eventually become a staff member after a brief stint at Indiana University in the middle 1950s. Arp would remain there for close to three decades, where he manned a 200-inch telescope positioned atop Palomar Mountain before finally joining the Max Planck Institute for Astrophysics, remaining in Germany until he died in 2013.
Arp began to observe several strange galaxies, which included one he labeled Arp 230 (the eponymous title of which references a classification system of “Arp numbers” the astronomer began to use). This galaxy possesses a most unusual shape which astronomers believe to have been caused by a violent collision between it and another galaxy at some point in the distant past, which also resulted in the unique polar ring Arp 230 possesses. Curious features like this one, involving a galaxy encircled by an outer ring that was likely composed of gas and stars that appear to rotate over the poles of the galaxy, had intrigued Arp enough that he began to compile an entire catalog of similar unusual galaxies, resulting in the aptly (or perhaps arptly?) named Atlas of Peculiar Galaxies in 1966.
Like other chroniclers of scientific anomalies that include American physicist William R. Corliss, Arp understood that observations of the wide diversity represented by these “peculiar” galaxies showcased the limitations of knowledge scientists possessed on their formation and nature, as well as the changes they underwent over time. With his Atlas, Arp hoped to provide a resource for other astronomers who studied the way galactic evolution occurred, although with time his observations of the peculiarities of the universe led to deeper, and even more potentially heretical insights.
Through his observations, Arp's skepticism increased over time. In some of the peculiar galaxies that he observed, Arp noticed that they appeared to be positioned relatively close to quasars, the remote and extremely massive celestial objects that some astronomers believe may harbor black holes, and even offer clues about the formation of galaxies.
Why had this been significant? Quasars could not be extremely remote celestial objects, but at the same time be near certain galaxies. Fundamentally, this appeared to contradict the distance estimates astronomers used based on the pioneering work of Edwin Hubble in 1929, which resulted in our current notion of an ever-expanding universe. Based on the steady stretching of light waves over great distances (recognized since the time of Hubble’s discovery as that object’s redshift) celestial objects that are farthest away from Earth are also moving faster. Thus, a galaxy possessing a low redshift and a quasar possessing a higher one could not actually be close together, based on the nature of the light astronomers can observe them producing.
However, Arp’s observations appeared to conflict with this idea. Not only did certain quasars and galaxies appear to be closer than many astronomers had imagined, but they also appeared to have been connected by what Arp speculated to have been bridges of gas, which to him clearly showed that the objects were closer than previously thought. Based on this, Arp conceived that in instances where the nucleus of a galaxy explodes, the result could be the ejection of a quasar, which would be moving at such a great velocity as it exited the galaxy that this, rather than distance, could account for its redshift. Quasars weren’t necessarily far away, in other words; Arp believed they were just moving really fast.
Arp’s theories, which resulted in what became known as the “redshift controversy”, caused quite a stir at the time. However, the debate only lasted until the early 1980s, and most astronomers during the ensuing years rejected his ideas based on a lack of evidence. This, paired with mounting support for the idea that quasars are indeed distant galaxies, although highly energetic ones, all having accumulated over time as telescopes and other technologies astronomers rely on have improved.
Always the contrarian, Arp never backed away from his theory, as he saw its deeper implications: if the position of quasars in proximity to certain galaxies did in fact represent the speed of the movement of those objects, rather than their extreme distance, this also challenged Hubble’s Law, and the idea of the Big Bang itself. Until his death, Arp continued to champion his critiques of the Big Bang theory, remaining a curious thorn in the side of mainstream astronomy.
Although his theories weren’t widely accepted after the 1980s, Arp’s famous Atlas of Peculiar Galaxies is still recognized as a useful resource for astronomers, and some of his most curious discoveries remain standards in the study of high-redshift galaxies, despite being understood to behave much differently from the way Arp imagined.