Hubble's Law Recession Velocity Calculator
Calculate galaxy recession speed using Hubble's Law.
Choose from Planck 2018, standard, or local H₀ values.
Includes Hubble sphere check.
Hubble’s Law states that galaxies recede from us at a speed proportional to their distance. The farther away a galaxy is, the faster it moves away from us.
v = H₀ × d
Where:
- v = recession velocity (km/s)
- H₀ = Hubble constant (km/s per Megaparsec)
- d = distance to the galaxy (Megaparsecs or Gigalightyears)
The Hubble constant controversy: There is an ongoing debate about the precise value of H₀:
- Planck 2018 (CMB method): H₀ = 67.4 km/s/Mpc
- Standard value (widely used): H₀ = 70 km/s/Mpc
- Local measurements (distance ladder): H₀ = 73 km/s/Mpc
This discrepancy, known as the Hubble tension, may point to new physics beyond the standard cosmological model.
The Hubble sphere: At a certain distance, the recession velocity equals the speed of light (c). This distance is called the Hubble radius or Hubble sphere:
d_H = c / H₀
Galaxies beyond this radius recede faster than light — they are outside our Hubble sphere. However, light from some of these galaxies can still reach us due to the geometry of spacetime.
Note on lookback time: For nearby galaxies (d « Hubble radius), lookback time ≈ d/c in light-years. For distant galaxies, a full cosmological calculation is needed. The observable universe has a radius of about 46 billion light-years.