Cosmological Red Shift (WJEC GCSE Physics)

• Usually, when an object emits waves, they spread out symmetrically
• If the source of the waves moves, the waves can become squashed together or spread out

• If a source of light moves towards an observer, the waves become squashed, causing the wavelength to decrease
  
  • The wavelength of light shifts towards the blue end of the spectrum
  • This is called blueshift

• If a source of light moves away from an observer, the waves spread out, causing the wavelength to increase
  
  • The wavelength of light shifts towards the red end of the spectrum
  • This is called redshift

Redshift and Blueshift of Light

When light moves away from an observer, they observe redshift and when light moves towards an observer, they observe blueshift

• When astronomers compare the absorption spectra of light from distant galaxies with spectra from nearby objects, such as the Sun, they observe the dark lines are shifted towards the red end of the spectrum
• This means the wavelength has increased as the light travelled from the galaxy to the Earth
• This shows that distant stars and galaxies are moving away from the Earth

Redshift of Light from a Distant Galaxy

Comparing the absorption spectra produced by the Sun and a distant galaxy shows the light from the distant galaxy has been shifted towards red wavelengths (it has stretched) due to it moving away

• Cosmological redshift indicates that distant galaxies are moving away from us
• If galaxies are moving away from us, this means the Universe must be expanding

Expansion of the Universe

Observations of light from galaxies show they are moving away from us which means the Universe is expanding

• When Edwin Hubble looked at the absorption spectra of distant galaxies, he determined a relationship between the speed of a galaxy and its distance from Earth

Comparing Redshifts of Galaxies

Hubble discovered that all galaxies show redshift, but the galaxies that are further away show a greater increase in redshift

• This is Hubble's law, which states

The speed of recession is proportional to the distance of the galaxy away from Earth

• 'Recession' speed means the speed at which something is moving away
• This means that the further away a galaxy is from Earth:
  • The faster it is moving away
  • The greater the increase in redshift

Relationship between Redshift and Galaxy Distance

Hubble's law tells us the greater the distance to a galaxy, the greater the redshift, or the speed it moves away from Earth