Investigate the evidence that led to the discovery of the expansion of the Universe by Hubble
- Henrietta Leavitt observed a relationship in the luminosity and the period of cepheid-variable stars.
- Using this relationship, Harlow Shapley was able to measure the distance between Earth, groups of stars and nebulae.
- Edwin Hubble then examined the Andromeda nebula using the large telescopes at Mount Wilson, California. He studied numerous novae and cepheid-variable stars in Andromeda and was able to establish the size and distance of Andromeda from Earth.
- This led to the realisation that Milky Way was just a tiny portion of the universe, and that there were numerous other galaxies out there.
- Shapley’s studies of velocities of nebulae (later found to be galaxies) in 1919 showed that nearly all of them were moving away from the Earth.
- Hubble explored this further in 1929 and showed that the further the galaxies were away from us, the faster they moved away.
- This showed us that the Universe or space itself was probably expanding. Hubble was also able to conclude that if universe is currently expanding, then if we ran time backwards, it might have originated at single point.
- Hubble measured the expansion of space using a phenomena called Doppler effect.
- Doppler effect is an increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.
- Light emitted by a star or galaxy that is moving away from us is shifted towards the red end of the spectrum. This is commonly called the red shift. (If the object was moving closer , there would be a blue shift)
- Observed spectrum from stars contains lines characteristic of the gases they are made from. When these lines are compared with the spectrum observed in laboratory, we can identify and measure the redshift.
- The Doppler effect is the same irrespective of the distance of the source. Only the speed of the source determines the amount of redshift (or blue shift) .
- Hubble realised that the redshift in the spectrum was greater for galaxies which were further away.
- Using the doppler effect one can conclude, the farther the galaxy is , the faster it is moving away.
- This led to the discovery of expansion of the universe.
Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)