Galactic Red-shift (Edexcel GCSE Physics)

• The Doppler effect affects all types of waves, including light
• Light emitted from stars and galaxies will be at a certain wavelength in the visible part of the electromagnetic spectrum
• If an object moves away from an observer the wavelength of light increases
  • This is known as redshift as the light moves towards the red end of the spectrum

• If an object moves towards an observer the wavelength of light decreases
  • This is known as blueshift as the light moves towards the blue end of the spectrum

Light from a star that is moving towards an observer will be blue shifted and light from a star moving away from an observer will be red shifted

• An increase in wavelength is a decrease in frequency and vice versa

The observer in front observes a blue shift, the observer behind observes a redshift

• Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe
• The diagram below shows the light coming to the Earth from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy

Comparing the light spectrum produced from the Sun and a distant galaxy

Red-shift provides evidence that the Universe is expanding because:

• Red-shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum
  • This is because light waves are stretched by the expansion of the universe so the wavelength increases (or frequency decreases)
  • This indicates that the galaxies are moving away from us

• Light spectrums produced from distant galaxies are red-shifted more than nearby galaxies
  • This shows that the greater the distance to the galaxy, the greater the redshift
  • This means that the further away a galaxy is, the faster it is moving away from the Earth

• These observations imply that the universe is expanding and therefore support the Big Bang Theory

• Both the Big Bang and Steady State theories of the origin of the Universe account for the red-shift of galaxies
• The fact that light from distant galaxies is redshifted shows that the galaxies are:
  • Moving away from the Earth
  • Moving away from each other

• This is a predicted consequence of the universe expanding from some initial point (as implied in the Big Bang Theory):
  • Suppose the galaxies were originally all grouped together and then started to spread out at different speeds
  • The galaxies that are moving fastest would move the furthest – the distance they move would be proportional to their speed - exactly the sort of relationship shown in the following graph

Graph showing the greater the distance to a galaxy, the greater the redshift

• The Steady State Theory, however, does also account for the redshift of galaxies
• It suggested that as galaxies moved apart, new ones were created in the space in between, resulting in a universe that remains the same over time
  • This means that more distant galaxies are still seen to have a greater redshift

• However, the Steady State Theory does not support the evidence from the Cosmic Microwave Background radiation, and hence is no longer a supported theory