����ý

In 1929, the astronomer Edwin Hubble used measurements he had taken of the speed of galaxies to show that the wavelength of light emitted (given out) by a galaxy changes depending on the direction the galaxy is travelling in. He used absorption spectra like the one below. An absorption spectrum shows which wavelengths of light are absorbed by the gas in distant galaxies.

Spectra from distant galaxies

Astronomers can observe light from distant galaxies. When they do this, they see it is different to the light from the Sun. The dark lines in the spectra from distant galaxies show an increase in wavelength. The lines are moved or shifted towards the red end of the spectrum. This effect is called red-shift. The absorption spectrum below, which shows the light from a distant galaxy, shows different lines to the one above, which shows light from the Sun – there is a red-shift.

Red-shift and expansion

Astronomers see red-shift in virtually all galaxies. It is a result of the space between the Earth and the galaxies expanding. This expansion stretches out the light waves during their journey to Earth, shifting them towards the red end of the spectrum. The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth. Scientists have discovered that the galaxies furthest away from Earth are the ones moving fastest away from us. This is evidence for the expansion of the Universe.