Hubble's law
Astrophysics & Cosmology - OCR A-Level Physics
$$v = H_0 d$$
- Hubble's lawThe recessional velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of a galaxy is directly proportional to its distance from us, indicating the universe is expanding.: the recessional velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of a galaxy is directly proportional to its distance from us.
- This means the universe is expanding: galaxies are moving apart, and more distant galaxies recede faster.
- The Hubble constant $H_0 \approx 67-73$ km $s^{-1}$ $Mpc^{-1}$ (value still debated, known as the 'Hubble tension').
- Hubble's lawThe recessional velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of a galaxy is directly proportional to its distance from us, indicating the universe is expanding. does not mean we are at the centre of the universe. Every observer in every galaxy would see the same pattern of recession.
- The expansion is of space itself, not galaxies moving through static space.
Examiner Tips and Tricks
- When using Hubble's lawThe recessional velocity of a galaxy is directly proportional to its distance from us, indicating the universe is expanding., check your units carefully.
- If H_0 is in km $s^{-1}$ $Mpc^{-1}$, then d must be in Mpc and v will be in km $s^{-1}$.
- If H_0 is in $s^{-1}$, then d is in metres and v is in $m s^{-1}$.