The Expanding Universe (CIE IGCSE Physics)

• Usually, when an object emits waves, the wavefronts spread out symmetrically
• If the wave source moves, the waves can become squashed together or stretched out

Diagram showing the wavefronts produced from a stationary object and a moving object

• A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
  • The wavelength of the waves in front of the source decreases and the frequency increases
  • The wavelength behind the source increases and the frequency decreases
  • This effect is known as the Doppler effect

 

• The Doppler effect also affects light
  • 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

• Redshift is:

An increase in the observed wavelength of electromagnet radiation emitted from receding stars and galaxies

Light from a star that is moving towards an observer will be blueshifted and light from a star moving away from an observer will be redshifted

The observer behind observes a red shift

• The Milky Way is just one of billions of galaxies that make up the Universe
• Light emitted from distant galaxies appears redshifted when compared with light emitted on Earth

• The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to us from a distant galaxy

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

• The diagram also shows that the light coming to us from distant galaxies is redshifted
  • The lines on the spectrum are shifted towards the red end

• This indicates that the galaxies are moving away from us
• If the galaxies are moving away from us it means that the universe is expanding
• The observation of redshift from distant galaxies supports the Big Bang theory

• Another observation from looking at the light spectrums produced from distant galaxies is that the greater the distance to the galaxy, the greater the redshift
  • This means that the further away a galaxy, the faster it is moving away from us

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