What is redshift in astronomy?

This NASA animation shows how light from distant galaxies is stretched by the expansion of the Universe: a phenomenon known as 'redshift'.

The Universe is expanding in the aftermath of the Big Bang explosion 13.8 billion years ago.

During the time that light from an astronomical object has been travelling across space to Earth, the Universe has grown in size.

The effect of this is to stretch the ‘wavelength’ of the light, much as a wave drawn on the fabric of a balloon would be stretched if the balloon were inflated.

The wavelength of red light is about twice that of blue light, so the stretching of visible light shifts it towards the red end of the spectrum: thus the term ‘redshift’.

Credit: NASA's Goddard Space Flight Center, NASA/JPL-Caltech//R. Hurt (Caltech-IPAC)

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theory! No experiment has ever demonstrated redshift