American astronomer noted for challenging the theory that red shifts of quasars indicate their great distance.

Arp received a bachelor's degree from Harvard University in 1949 and a Ph.D. from the California Institute of Technology in 1953. He subsequently accepted a research fellowship to conduct postdoctoral studies at the nearby Hale Observatories (now the Mt. Wilson and Palomar Observatories). After he worked as a research associate at Indiana University from 1955 to 1957, Arp returned to Mt. Wilson, securing a post as an assistant astronomer on the observatory staff. He was appointed astronomer there in 1965.

Arp became skeptical about the distance of quasars (quasar) when he noticed that some of the galaxies that he had included in his Atlas of Peculiar Galaxies (1966) seemed to lie in the vicinity of quasars. Using photographic evidence, Arp tried to prove that the low-red-shift galaxies and the high-red-shift quasars not only appear close together but also actually are connected by gaseous bridges, an impossibility if the quasars are billions of light-years further away than the galaxies. Arp theorized that the nuclei of galaxies may explode, ejecting quasars with a velocity great enough to account for their red shifts.