Hubble's Law
Redshift gives a galaxy's recession speed. A standard candle gives its distance. Hubble's law compares those two measurements for many galaxies.
More distant galaxies recede faster
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Definition
2 marksWhat is meant by Hubble's law?
Model answer: The recession speed of a galaxy is directly proportional to its distance from the observer.
- On a graph of recession speed against distance, the best-fit gradient is the Hubble constant.
- Individual data points scatter, but the overall trend is what the law describes.
Symbols
- = Hubble constant (s⁻¹)
- = recession speed (m s⁻¹)
- = galaxy distance (m)
A galaxy is 1.2 × 1024 m away and recedes at 2.8 × 106 m s−1. Calculate the Hubble constant. [2]
Show worked answer
The Hubble constant is 2.3 × 10⁻¹⁸ s⁻¹.
One galaxy supplies one point on the graph
Build one Hubble-law data point
Treat the standard candle and spectral lines as observations of the same galaxy.
1. Infer distance
Known luminosity: 1.90 × 10^36 W
distance = 1.34 × 10^25 m
2. Infer recession speed
Laboratory wavelength: 658 nm
recession speed = 3.10 × 10^7 m s⁻¹
3. Put distance and speed together
redshift z
0.1033
H₀ = v / d
2.31 × 10^-18 s⁻¹
simple age 1 / H₀
1.37 × 10^10 years
The speed relation assumes a small redshift. The age is only a simple constant-expansion estimate.
Lower measured flux means a greater inferred distance. A larger fractional wavelength shift means a greater inferred recession speed. Combining both for the same galaxy gives one value of .
Common mistake