In space, the distant galaxies appear to be moving further away from us, which is supported by the red-shifting of the wavelengths of light.

This is when the wavelength of light from a light-emitting object is stretched as it moves further away from the observer. In this case, the light from distant galaxies reach Earth with longer wavelengths.

Absorption Spectrum

The image below shows the absorption spectrum for visible light that is emitted from the Sun.

On Earth, the longer, red wavelengths are on the left and the shorter, violet wavelengths are on the right. All of the wavelengths in the spectrum are emitted from the Sun towards Earth. However, there are elements in the Sun’s atmosphere that absorb certain wavelengths of light that are emitted.

This means that when the light reaches Earth, there are missing wavelengths. Different elements produce different dark line patterns. So, the absorption spectrum for visible light includes patterns of dark lines when analysed.

Spectra From Distant Galaxies

When analysing light from distant galaxies, we can observe that it looks different to the light emitted by the Sun. The spectra look more like the one below, with the black lines in exactly the same pattern. However, the dark pattern lines are all shifted towards the red end of the spectrum, increasing the wavelength.

We call this process red-shift, which happens because the light waves get stretched as they travel towards the Earth.

An explanation for this is that the distant galaxies, which were emitting the light, are travelling away from Earth. However, the galaxies themselves are not moving away from us by travelling through space. Instead, the space between Earth and the distant galaxies is expanding. This means that we end up being further away from the other galaxies over time.

Red-shift and speed

The further away from us a particular galaxy is, or the more red-shifted the galaxy’s light is, the faster it will appear to be moving away from us. This means that the more distant galaxies will be moving away from Earth the fastest.