The Universe – WJECCosmological red shift of the light from galaxies

Wavelength of sound and light can be stretched or compressed by movement.

For example, when an ambulance is travelling towards an observer, he will hear a higher pitch – a higher frequency note. The sound wavelength is compressed by the movement of the ambulance towards the observer.

However, when the ambulance is moving away from the observer he hears a lower pitch - a lower frequency note. The sound wavelength is stretched by the movement away from the observer.

The ambulance driver will not hear a difference in the sound pitch. There is no relative movement between them.

When an object that emits waves begins to move, the movement can cause the stretching or compressing of the waves. This is called the Doppler shift.

Similarly, light wavelengths from an object emitting light are stretched by movement away from the observer. This is called the red shift.

Light wavelengths from an object emitting light are compressed by movement towards the observer. This is called the blue shift.

Edwin Powell Hubble (1889 – 1953)

Hubble noticed that the absorption spectrum lines from galaxies showed the same pattern as the Sun’s absorption spectrum – showing that the same elements were present, but the lines had been shifted towards the red side of the visible spectrum.

He realised that this red shift meant that the wavelengths of the absorption dark lines had been stretched or made longer.

He concluded that this shift or stretch was caused by the galaxy moving away from us. This cosmological red shift occurs because of the increasing space between the distant galaxy and the observers on Earth, and is evidence for the expansion of the Universe.