Increasing Accuracy in Measurements of the Hubble Constant: What is the Evidence for New Physics?*

An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. Recent measurements of the Hubble constant (Ho), which are based on Cepheids and Type Ia supernovae (SNe), appear to be discrepant at the 5-sigma level with values of Ho inferred from measurements of fluctuations in the cosmic microwave background (CMB). The latter assumes LCDM, and the former assumes that systematics have been fully accounted for. If real, the current discrepancy could be signaling a new physical property of the universe. I will present results based on Hubble Space Telescope Advanced Camera for Surveys data resulting in an independent calibration of SNe Ho based on measurements of the Tip of the Red Giant Branch (TRGB). The TRGB marks the luminosity at which the core helium flash in low-mass stars occurs, and provides a high-precision and accuracy standard candle. Moreover, the TRGB method is less susceptible to extinction by dust, to metallicity effects, and to crowding/blending effects than Cepheid variable stars.  I will  address the current uncertainties in both the TRGB and Cepheid distance scales, as well as discuss the current tension in Ho and the evidence for additional physics beyond the standard LCDM model.