Red Shift

Learning ObjectiveTo understand how Edwin Hubble provided evidence

showing the relative scale of the Universe.

Learning OutcomeTo be able to explain

Hubble’s observations and how they relate to the scale

model of the Universe.

Success CriteriaGood if: you can recall Edwin Hubble’s observations of distant objects in the sky.Excellent when: you can explain some of these observations using the idea of ‘red shift’.Exceptional if: you can use Hubble’s Law to calculate the recession velocity of a given object.

After The ‘Great Debate’

After resolving the ‘Curtis-Shapley’ debate. Hubble measured the distance to Cepheid variable stars in the Andromeda Galaxy:

2.5 million light years!

Hmm...Much further than I suspected! I also

noticed something else…..

Moving Galaxies

Hubble’s observations showed that the light from distant galaxies was ‘red-shifted’. This was due to the ‘Doppler Effect’.

By Golly, just look at that!

The dark lines in the spectrum on the right

hand side are ‘red shifted’.

Light from the Sun

Light from distant galaxies

Explaining Doppler Shift

Recession Velocity

Hubble also noticed that the further away the galaxy, the greater the red-shift. The greater the red-shift the greater the speed of recession.

My startling conclusion was that the further away the galaxy, the faster is was moving. Amazing! It also led me to develop what later became known as Hubble’s Law.

After Me!

Hubble’s Law

The red-shift of distant objects is easy to measure therefore the recession velocity can also be easily calculated.We can use the recession velocity to calculate the distance to a galaxy (or the velocity if the distance is known). This is known as Hubble’s Law.

The value of the Hubble Constant is 2x10-18 s-1 (this value is still being researched).E.g. A galaxy is 1.5 x 1020 km away. How fast is it moving?

Recession Velocity = Hubble Constant x 1.5 x 1020 = 2x10-18 x 1.5 x 1020 = 300 km/s

Speed of recession (m/s) = Hubble Constant (s-1) x distance (km)

Learning ObjectiveTo understand how Edwin Hubble provided evidence

showing the relative scale of the Universe.

Learning OutcomeTo be able to explain

Hubble’s observations and how they relate to the scale

model of the Universe.

Success CriteriaGood if: you can recall Edwin Hubble’s observations of distant objects in the sky.Excellent when: you can explain some of these observations using the idea of ‘red shift’.Exceptional if: you can use Hubble’s Law to calculate the recession velocity of a given object.