The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a >4.0-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2019) and that determined from modeling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2018). At the heart of the tension, is not only a difference in method but also a fundamental difference in how the techniques are anchored either in very local measurements (distance ladder at z~0) or in the standard cosmological model (CMB at z~1100).
As part of the Carnegie-Chicago Hubble Program, I participated in a 3-year project to measure the Hubble constant using the distance ladder with techniques independent of the classical Cepheid path by using the Tip of the Red Giant Branch (TRGB). I will motivate this form of the distance ladder using the history of the Hubble constant as a guide. I will discuss how we make the TRGB-measurements and the challenges that we solved in our program. Ultimately, our program produced a value intermediate between that of Cepheids and the CMB, suggestive of unresolved systematics in the Cepheid distance scale. I will address recent controversy over the zero-point of the TRGB-ladder that suggests there are real differences between the Cepheid and TRGB-based calibrations of Supernova Type-Ia. To close, I will show how the TRGB, especially used at infrared wavelengths, presents a uniquely powerful long-term distance-measurement technique in application to a number of cosmological problems. I will summarize the progress of my team to address the short-term challenges of the IR-TRGB in anticipation of JWST, the Vera Rubin Observatory, Roman Space Telescope, and ELTs.