Hubble's Law

demonstrate an understanding of the relationship between distance and red shift of distant galaxies (Hubble’s Law) and use the formula: v = H d
describe how astronomers use the value of the Hubble Constant to determine the age of the Universe

When we observe light from distant galaxies the spectral lines are in the wrong place. They are shifted towards the red end of the spectrum, i.e. their wavelength is bigger than it is supposed to be. This is because these galaxies are moving away from us very quickly and the light waves from them are stretched.

So galaxies far away are moving away from us.

In 1929 Edwin Hubble came up with Hubble's Law. This states that the recession speed of a galaxy is proportional to how far away it is. He knew how far away many galaxies were (from observing the magnitude of standard candles in them) and we knew their recession velocities (from measuring their red shift). Hubble plotted one against the other and got a straight line. So galaxies quite close to us are moving away slowly. Galaxies very far away are moving away very quickly.

Recession velocity = H₀ x distance

The constant in this equation, H₀, is called Hubble's constant. This is equal to the gradient of the graph above, H₀ = v / d

How old is the Universe?

We can use the Hubble constant to find an age for the Universe. (If the graph above was steep then the Universe would be young. The less steep it is the older the Universe)

Assume that galaxies have been moving at the same speed since the Universe began. How long the have been moving will equal how far they have travelled divided by their velocity, i.e. if v = d / t then t = d / v which, according to the equation above, is equal to 1 / H₀

From current estimates of the Hubble constant the Universe is thought to be around 14 billion years old. ( 13.75 + or - 0.11 billion years )