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Hubble Red Shift

Imagine a you are by the side of the road and a very noisy motorbike is travelling towards you. How does the sound from the motorbike change as it nears you and then passes you?

When the bike is moving away from you the pitch of the noise it makes is falling. The wavelength of the sound reaching your ears is dropping. We call this the Doppler effect.

To see an applet demonstrating this have a look at

 http://webphysics.davidson.edu/Applets/Doppler/Doppler.html

http://lectureonline.cl.msu.edu/~mmp/applist/doppler/d.htm

Notice how the effect is more extreme the faster the object is travelling.

It is not just sound waves which are effected like this. When elements are very hot, as they are in stars, they emit light with very definite wavelengths. We call these spectra. When we look at the light from distant galaxies we notice that these spectral lines are shifted towards the red

.

This must mean that these galaxies are moving away from us very quickly.

It was known that light from distant galaxies was red shifted. Hubble found that there was a relationship between the distance of a galaxy and the amount that the light from it has be red shifted. galaxies which are further away show a bigger red shift.

For an excellent website on this go to

http://www.nyu.edu/classes/gspscience/hou/html/hubble/hubble.html