Image 1 Caption: Hubble's law illustrated by a Hubble diagram of galaxies for the very local universe out to ∼ 20 Mpc (i.e., cosmological redshift z ≅ 0.0047) and a cartoon Hubble diagram out to ∼ 300 Mpc (i.e., cosmological redshift z ≅ 0.070).

  v=Hr  ,

where v is recession velocity nowadays measured in km/s, r is distance nowadays measured in megaparsecs, and H is a constant, now called the Hubble constant.

The Hubble constant circa 2021 is thought to be either H_0 ≅ 67 (km/s)/Mpc or H_0 ≅ 73 (km/s)/Mpc. The discrepancy between these two values is the Hubble tension which may require a revision or replacement of the Λ-CDM model of the observable universe. For more on the Hubble tension, see big_bang_cosmology_limitations.html. A fiducial value for the Hubble constant 70 (km/s)/Mpc.

Note the Hubble constant is a constant in space (at least within the observable universe), NOT in cosmic time and it is the rate of expansion of the universe per unit length or the relative rate of expansion of the universe.