Hubble's Law & Constant

Demonstrate an understanding of the relationship between distance and redshift of distant galaxies (Hubble's Law) and use the formula: v = Hd

Describe how astronomers use the value of the Hubble Constant to determine the age of the Universe

So far we have looked at evidence for the expanding Universe. Not until the early 20th century did scientists realise that 'spiral nebula' were actually different galaxies and not part of ours. One of the most prominent of modern astronomers was Edwin Hubble.

He proposed there was a relationship between the distance to galaxies to their redshift (or receding velocity). In other words how fast a galaxy moved was in proportion to its distance.

This is Hubble's Law -

v=H0D (the specification states V=hD, ignore the difference between the two for now).

v = recession velocity
H0 = Hubble constant
D = distance to galaxy (mega parsec - Mpc)

Velocity is taken by measuring the galaxy over a period of time

We can calculate the distance to nearby galaxies by knowing their apparent and absolute luminosity.

To do this Hibble needed a constant of proportionality - the Hubble constant.

This is a special measurement to astronomers, as it means they can measure the age of the Universe.

Different measurements have been made, notably by the Hubble Space Telescope (HST) and Wilkinson Microwave Anisotropy Probe (WMAP). Most past measurements have been values between 50 and 100 Mpc. HST measured 74.2 ± 3.6 (km/s)/Mpc in 2009.

If the number is too high we find stars older than the Universe, and if it is too low there is not enough matter in the Universe to account for it. The constant therefore supports the Big Bang theory.

There are two areas on which scientists cannot yet agree:
1. An accurate measurement everyone can agree on
2. Knowing if the constant is constant and always has been that value.

The implication for the Law is that the Universe is expanding. Remember however that groups of galaxies can be gravitationally bound (like ours), so this does not necessarily apply.


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University of Oregon - Cosmic Edge


  • Why is the value of the Hubble constant significant?
  • In a sentence, describe Hubble's Law
  • What is the formula for Hubble's Law?

Edwin Hubble