Hubble's Law

Hubble's law says that, on very large scales, galaxies are observed to move away from us faster the farther away they are. The reason is not that we are at a special center, but that space itself is expanding, so light from more distant galaxies has had more space to stretch across and the recession speed accumulates with distance. The constant H_0 sets the current expansion rate of the Universe, so multiplying it by distance gives the approximate recession velocity for galaxies far enough away that local gravitational motions are small compared with the expansion.

Use this relation when estimating the recessional speed of distant galaxies from their measured distance, or when turning an observed redshift into an approximate distance via the current Hubble constant. It is most reliable on cosmological scales, not for nearby objects bound by gravity or for highly precise local motion studies.

• Using the formula for nearby galaxies whose motions are dominated by local gravity rather than expansion
• Forgetting that H_0 must match the distance units, so Mpc and km/s/Mpc go together
• Interpreting v as an ordinary Doppler speed through space instead of recession due to expanding space