The Expanding Universe (CIE IGCSE Physics)
Revision Note
Author
LeanderExpertise
Physics
Galaxies & Redshift
- 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
- If an object moves away from an observer the wavelength of light increases
- 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
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