Tension Before the Hubble Tension: Consensus and the HST Program for the Luminosity Calibration of SNe Ia

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale (KP) aimed to accurately determine the Hubble constant. In the astronomy literature, the KP is often framed as producing a consensus value of the Hubble constant and settling the ‘50-or-100’ debate between Allan Sandage and Gérard de Vaucouleurs. The historical record, however, does not support this neat picture. Debate continued, with Sandage and his collaborators in the HST Program for the Luminosity Calibration of SNe Ia (SNCP) continuing to argue for a lower Hubble constant than that of the KP. Understanding this debate between the KP and the SNCP not only clarifies the historical record. It also points to shifting astronomical and cosmological methodologies around the start of the 21st century, as photographic plates and ‘by hand’ analysis gave way to CCDs and algorithmic analysis. The disagreement was not a result of intransigence on the part of the SNCP, but rather a rational disagreement regarding new methodologies and goals. Further, an examination of this earlier debate provides context for the present day Hubble tension. This present tension, too, can be seen as resulting from new methodologies and differing interests among parties to the debate.