The Role of Type Ia Supernovae in Constraining the Hubble Constant

In the conventional / most studied local distance ladder measurements, Type Ia supernovae (SNe Ia) are used in two of the three rungs. In the second rung, their luminosities are calibrated by standard candles like Cepheids or Tip of the Red Giant Branch (TRGB). In the third rung, the high luminosities and standardizability allow SNe to be used to calibrate the `Hubble' relation between distances and redshifts. Locally, the majority of distance ladder analyses find a high value of the Hubble Constant $H_0$ of $>70$ km/s/Mpc. Given the discrepancy with the inferred value using CMB observations, great scrutiny must be given to the role supernovae play in measuring $H_0$. Here, we review the main methodology, the many crosschecks for the supernova component of the distance ladder, and the various systematics studied. We review the important role supernovae play to explain the small disagreements seen from various local analyses. We also discuss analyses that employ an inverse distance ladder, which use similar sets of supernovae, but in the reverse direction, and yield a low value of $H_0$. We conclude given all available evidence, it is difficult to find a way that a systematic in supernovae measurements, or a non-$\Lambda$CDM component of the universe which could be measured with supernovae, can help explain the Hubble tension.