The objective of the workshop is to reach a community consensus view on the state-of-the-art knowledge of the local distance ladder using the broadest possible variety of methods available today, in order to obtain the most robust determination of the local value of the Hubble Constant to date, and to strengthen our understanding of the “Hubble Tension”.
The Hubble Tension is the apparent conflict between the value of the Hubble constant measured locally and the value inferred from the favored cosmological model of the Universe, ΛCDM. The ΛCDM has provided for decades a successful explanation of the observed properties of galaxies and the structure of the Universe itself. With the extremely detailed measurements of the Cosmic Microwave Background now available, coupled with the physics of the early Universe, this model predicts with great precision the value of the Hubble constant, the rate of expansion of the Universe at the present time. But the Hubble constant can also be measured directly in the local Universe, from distance estimates to individual objects and their measured redshift. Distance estimates are often based on a series of steps involving multiple distance indicators, forming the so-called “distance ladder”, which ultimately measure the distance of Supernovae or other far-reaching distance indicators in the expansion flow. It is now clear that the measured value of the Hubble constant, using multiple versions of the distance ladder, does not agree with the value predicted by the ΛCDM model, with the discrepancy estimated to be at least 5-6σ. Resolution of the Hubble tension promises to either identify significant, undiscovered anomalies in a large amount of collected data, or uncover new physics affecting the expansion and evolution of the Universe. Despite several years and dozens of attempts, no conclusive explanation or convincing interpretation has emerged for this discrepancy. The recently published book “The Hubble Constant Tension”, edited by Di Valentino and Brout (Springer, July 2024), collects the most recent advances and discusses the multiplicity of methods that can be used to measure the Hubble constant.
The ability to constrain the local distance ladder has evolved significantly over the last several years. Different distance indicators have been studied in greater detail, and now provide partially independent, comparable paths to distance estimates of the same objects; these include, for example, the period-luminosity relation for Cepheids; the luminosity of the tip of the red giant branch; the luminosity of the J-region of the asymptotic giant branch; the luminosity of oxygen-rich Mira variables; the angular size of Type II supernovae; and the amplitude of surface brightness fluctuations in galaxies. Multiple classes of distance indicators reach into the Hubble flow and provide independnt ways to sample the expansion of the Universe; these include, for example, the standardized luminosity of Type Ia supernovae; the fundamental plane of elliptical galaxies; and megamasers. Together, these indicators have led to the development of a complex “distance network”, in which separate or partially overlapping paths can be followed to arrive at an estimate of the Hubble constant. However, in many cases different indicators share common elements, such as anchors, distance calibrations, or other types of common uncertainties, and thus cannot simply be averaged together. The only way to properly combine all these measurements into a coherent, single estimate of the local value of the Hubble constant is to fully understand the extent to which they share uncertainties and, conversely, the extent to which they provide independent constraints for the same quantity.
The goal of this workshop is to bring together experts in each of these methods in order to fully address and answer these underlying questions. Through detailed discussions and a thorough analysis, supported by expert knowledge and deep understanding of the details of each approach, the participants will quantify the interrelations among all methods and build a full statistical model of the diverse constraints on the local distance scale. Such a comparison has never been carried out with full information and rigor; having the world’s experts together in the same room, in a neutral and welcoming environment, working to confirm each other’s methodology, will provide the answer to the question: “What is the best local value of the Hubble constant based on all evidence available today?” As a result, this analysis will yield a distance network and a more robust, more accurate unified determination of the distance scale. This answer will strongly inform the Hubble Tension debate and provide the best information on which to base theoretical advances that might explain the tension itself.
Achieving the best possible consensus bounds on the Hubble constant is especially timely, in view of the major improvements in accuracy and systematics offered by JWST. A possible next step could then be a workshop to directly address the nature of the tension, but this cannot be done without this first necessary step.