The Tip of the Red Giant Branch Distance Ladder and the Hubble Constant

TL;DR

This paper reviews the use of the tip of the red giant branch as a primary distance indicator for measuring the Hubble Constant, highlighting recent developments, challenges, and future prospects with JWST and Gaia.

Contribution

It provides a comprehensive review of TRGB-based Hubble Constant measurements, including recent calibrations, algorithm improvements, and future observational prospects.

Findings

Recent calibrations improve TRGB distance measurements.

Challenges in TRGB measurement are identified and discussed.

Future JWST and Gaia observations will enhance Hubble Constant estimates.

Abstract

While the tip of the red giant branch (TRGB) has been used as a distance indicator since the early 1990's, its application to measure the Hubble Constant as a primary distance indicator occurred only recently. The TRGB is also currently at an interesting crossroads as results from the James Webb Space Telescope (JWST) are beginning to emerge. In this chapter, we provide a review of the TRGB as it is used to measure the Hubble constant. First, we provide an essential review of the physical and observational basis of the TRGB as well as providing a summary for its use for measuring the Hubble Constant. More attention is then given is then given to recent, but still pre-JWST, developments, including new calibrations and developments with algorithms. We also address challenges that arise while measuring a TRGB-based Hubble Constant. We close by looking forward to the exciting prospects from telescopes such as JWST and Gaia.