Cosmology

Hubble's Law & Constant

16.5 - Be able to use the relationship between distance and redshift of distant galaxies (Hubble’s law) including the formula:
v = H₀d
where v is the radial velocity of the recession of the galaxy, H₀ is the Hubble constant and d is the distance of the galaxy from Earth.

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16.6 - Understand the estimation of the age and size of the Universe using the value of the Hubble constant

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. Georges Lemaître was one of the most prominent of 20th century astronomers and Edwin Hubble developed his theories.

Hubble 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

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 Hubble 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.

New name for the constant

In 2018 the law was renamed as the Hubble-Lemaître to recognise the work of Belgian astronomer Georges Lemaître who derived the law and estimated the constant before Hubble.