Hubble's Law
Hubble's law relates a galaxy's recession velocity to its distance.
The foundation of the expanding universe model.
The Formula
v = H₀ × d
Hubble's law states that distant galaxies move away from us at speeds proportional to their distance. The farther away a galaxy is, the faster it recedes. This is key evidence for the expanding universe.
Variables
| Symbol | Meaning |
|---|---|
| v | Recession velocity of the galaxy (km/s) |
| H₀ | Hubble constant (approximately 70 km/s per Megaparsec) |
| d | Distance to the galaxy (Megaparsecs, where 1 Mpc ≈ 3.26 million light-years) |
Example 1
A galaxy is 100 Mpc away. How fast is it receding?
v = 70 km/s/Mpc × 100 Mpc
v = 7,000 km/s
Example 2
A galaxy has a recession velocity of 21,000 km/s. How far away is it?
d = v / H₀ = 21,000 / 70
d = 300 Mpc ≈ 978 million light-years
When to Use It
Use Hubble's law when:
- Estimating distances to faraway galaxies using their redshift
- Calculating the recession speed of distant objects
- Estimating the age of the universe (1/H₀)
- Studying the rate of expansion of the universe