What is redshift?

• The term "redshift" arises from the fact that light from more distant objects shows up on Earth more red than when it left its source
• The colour shift comes about because of the Doppler effect, which acts to "stretch" or "compress" waves from moving objects
• It is at work in the sound of a moving siren: an approaching siren sounds higher-pitched and a receding one sounds lower-pitched
• In the case of light, approaching objects appear more blue and receding objects appear more red
• The expansion of the Universe is accelerating, so in general, more distant objects are moving away from us (and each other, and everything else) more quickly than nearer ones
• At cosmic distances, the shift can profoundly affect the colour - the factor by which the wavelength is "stretched" is called the redshift