Measuring the Hubble Constant:  Stress-Testing the Standard Cosmological Model

Many possible explanations have been put forward to explain the apparent 5-sigma discrepancy between the local measure of the Hubble constant (Ho) and that inferred from measurements of the cosmic microwave background (CMB). These include variation of the dark energy with redshift, early dark energy (before recombination), modifications to gravity, new particles or fields beyond the standard model. Yet to date, no convincing physical explanation for the Hubble constant tension has been found. The local Hubble constant measurements giving rise to the discrepancy are based on a calibration using Cepheid variable stars, and tying into Type Ia supernovae and give a value of Ho=73 km/s/Mpc. Adopting the standard Lambda Cold Dark Matter model, the measurements of temperature and polarization of the CMB yield a value of 67 km/s/Mpc. How serious is the current discrepancy? What are the limitations to the current measurements? How can the issue be resolved? Both Hubble Space Telescope, as well as new James Webb Space Telescope data, will be presented that are shedding new light on the issue.