In space, distant galaxies appear to be moving further away from us, a phenomenon supported by the red-shifting of light wavelengths.
This occurs when the wavelength of light from an object is stretched as the object moves further away from the observer. In this case, the light from distant galaxies reaches Earth with longer wavelengths.
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 the wavelengths in the spectrum are emitted by the Sun and travel 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 distinct dark line patterns. Therefore, the absorption spectrum for visible light displays patterns of dark lines upon analysis.
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 stretch as they travel towards the Earth.
One explanation is that the distant galaxies emitting the light are moving 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.
The farther a particular galaxy is from us, or the more its light is red-shifted, the faster it appears to be moving away. This means that the more distant galaxies will be moving away from Earth the fastest.