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(2) Explaining the Doppler effect with reference to cosmology and models of the universe

• Know that current evidence suggests that the universe is expanding and that matter and space expanded violently and rapidly from a very small initial ‘point’, ie the universe began with a ‘big bang’.

• The universe is big, but seems to be getting bigger and all the galaxies seem to be moving away from each other - so how are we going to explain this cosmological expansion?

• First, a bit of classical physics to help you understand this page on the red-shift and expanding universe.

• If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency. This is called the Doppler effect.

• To observe the Doppler effect, the wave source could be sound, light, microwave or any other part of the electromagnetic spectrum.

  • When the source moves away from the observer, the observed wavelength increases and the frequency decreases because the waves get stretched out.

  • When the source moves towards the observer, the observed wavelength decreases and the frequency increases because the waves become compressed.

  • You experience the Doppler effect quite clearly when a loud racing car or train passes by you.

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    • As the loud moving object approaches you, the frequency (pitch) of the sound waves increases as oncoming sound waves are compressed closer together (increasingly shorter wavelength).

    • As the object moves away from you, the frequency (pitch) decreases as the waves stretch out (wavelength becoming increasingly longer).

    • This is an effect, quite distinct from the fact that the sound of the moving object becomes louder then softer as the object passes you.

    • In the case of a bright object like a bright galaxy emitting white light however near or far away ...

      • (i) if there is no great difference in speed relative to planet Earth, the object will look white,

      • (ii) if the object was hurtling towards us at great speed the light waves would be compressed, wavelength decreases, frequency increases, the light shifts towards the blue end of the spectrum (a blue-shift), so the object would look a little more bluish than it really is,

      • (iii) if the object is moving away from us at great speed the light waves are stretched, wavelength increases, frequency decreases and the light shifts towards the red end of the spectrum (red-shift), so the object would look a little more reddish than it really is.

  • The Doppler effect on light waves is used to measure the speed at which the galaxies seem to be moving away from us and our galaxy in all directions!

  • This 'astronomical' Doppler effect (quite literally and metaphorically!) is one of the main pieces of evidence for the 'Big Bang' theory of the expansion of our universe from some 'point' at 'time zero' around 14 billion years ago!

  • This piece of evidence for the big bang theory is called the red shift and is explained in more detail Part 4 after you have a learned little spectroscopy of how elements are detected using emitted light from the very hot atoms of stars.