The expansion field: The value of H_0

Understand

Any calibration of the present value of the Hubble constant requires recession velocities and distances of galaxies.

• While the conversion of observed velocities into true recession velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and cover a useful range of about 4-30 Mpc is crucial.
• A list of 279 such galaxy distances within v<2000 km/s is given which are derived from the tip of the red-giant branch (TRGB), from Cepheids, and from supernovae of type Ia (SNe Ia).
• Their random errors are not more than 0.15 mag as shown by intercomparison.