Red-Shift (Oxford AQA IGCSE Physics)

Red-Shift

• There is an observed increase in the wavelength of light from most distant galaxies

  • The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength

  • The wavelength of light shifts towards the red end of the spectrum

  • This effect is called redshift

• The observed redshift suggests that space is expanding and supports the Big Bang model (that the universe began from a very small initial point)

• Astronomers study distant objects, such as galaxies, by using absorption spectra

  • The distant objects emit spectra of light (this is the plural of spectrum)

  • There are dark lines in these spectra, at specific wavelengths, where elements (e.g. hydrogen) in the objects absorb light of this wavelength

• When astronomers compare the absorption spectra of light from distant galaxies with spectra from nearby objects, such as the Sun, the observed dark lines are shifted towards the red end of the spectrum

Redshift of light from a distant galaxy

• Through observations, astronomers realised that the galaxies further from Earth are more red-shifted

  • This means they are moving away from Earth at a greater speed

Relationship between redshift and galaxy distance

• The following evidence supports the theory that the Universe is expanding:

  1. Light from distant galaxies is red-shifted

  2. The greater the distance from Earth, the greater the red-shift

• If all distant galaxies are moving away from Earth, they are all moving away from each other

• Therefore, the space between the galaxies, and the whole Universe, is expanding

  • If the Universe is expanding in all directions now, it must have been smaller in the past

  • Therefore, this evidence supports the Big Bang model, which states that the Universe began from a small point

Blueshift

• If a source of light moves towards an observer, the observed wavelength decreases, as per the Doppler effect

  • The wavelength of light shifts towards the blue end of the spectrum

  • This is called blueshift

Redshift and blueshift of light