PU Gravity Group Lunch Seminar - Title Added

The value of the Hubble constant (H0) continues to be an active topic in observational astronomy and many teams are now presenting results at comparable precision. Taken together, there is evidence for a systematic discrepancy between these local measurements and measurements tied to the Cosmic Microwave Background. I will describe, in detail, the key results from the Carnegie-Chicago Hubble Program (CCHP) that has produced a calibration of the supernovae Ia using the Tip of the Red Giant Branch (TRGB) as a cross-check on Cepheid-based distances. Our first H0 measurement was published in Freedman et al. 2019, with two accompanying papers -- Hoyt et al. 2019 and Beaton et al. 2019 -- that continued to refine systematic and statistical uncertainties affiliated with the TRGB measurements. Our H0 measurement is in statistical agreement (< 2 sigma) with both the traditional Cepheid route and that from the CMB, suggesting that claims of new physics may be premature. Of note, Freedman et al. 2019 presented a new zero-point for the TRGB using the Magellanic Clouds as an anchor, which has been critiqued in the literature (e.g., Yuan et al. 2019 and Reid, Pesce, and Riess 2019). Originally the CCHP proposed to avoid the Large Magellanic Cloud, due to the complexities of measurement therein, but I will describe the struggle to use other calibration paths due to systematic uncertainties in Gaia DR2 (Neeley et al. 2019). I will counter the concerns raised in Yuan et al. with a discussion of systematics induced from stellar population variance and from nuances in photometric measurements that, when taken into account, bring our apparently disparate measurements into accord. I conclude with why moving into the infrared, where the impact of dust is reduced, is the best option, but list the current instrumentation challenges we face toward realizing that goal.