(4) In cosmological terms what is the red-shift?

• You should have read Parts 2 and 3 before studying Part (4)

• (2) Explaining the Doppler effect in a cosmological context

• (3) Why do we get light from stars? How do we analyse this light?

• What is the red shift phenomena?

• The light from distant galaxies seems to be of lower frequency than expected.

• Hydrogen gives a series of specific spectral lines eg one in the red, one in the green, several in the blue and many in the indigo and violet region (which are not numbered in the diagram below).

  • The vertical black lines in the diagram below represent the visible light frequencies absorbed.

  • Now, if we bring in the idea of the Doppler effect, we can use this stellar (stars, galaxy, nebulae) absorption spectrum as evidence to show that the universe is expanding.

  • So instead of racing cars or trains, think stars, if the galaxies are moving away from us, then the light waves will be stretched out over the billions of miles so that the wavelengths get longer - which is in the red direction of the visible spectrum!

  • When the spectra from galaxies from a variety of huge distances away from Earth, a pattern was noticed, first recognised by American astronomer Walter S. Adams in 1908.

  • The pattern of spectrum of lines of elements like hydrogen seemed to be the same, BUT, the frequencies were smaller, wavelengths longer, than what you observe on Earth e.g. from the Sun, our nearest star.

  • The shift to lower frequencies called the red-shift because the 'shift' was towards the lower frequency red end of the visible spectrum.

  • It is an example of the Doppler Shift described in section 1.

  • The astronomer Edwin Hubble in 1929, recognised the spectral pattern for hydrogen, but realised all the wavelengths were shifter to longer values.

  • He further analysed the red shifts and related the increasing value of the shift to faster more distant galaxies appearing to move away from us at great speed.

• The red-shift to longer wavelengths and lower frequencies is indicated by the white arrows on the diagram below.

  • I've only indicated the shift for the first two lines in the spectrum of hydrogen (a fragment of the spectral patter for hydrogen, but enough to illustrate the red shift).

  • 1. goes deeper red and 2. goes from green to yellow-green to yellow (still described as a 'red shift').

  • Notice that the 'pattern of lines', the hydrogen spectral 'fingerprint' remains the same.

  • Also note that the more distant the galaxy, the greater the red shift in frequencies/wavelengths.

• The vertical black lines represent the emission or absorption spectrum of hydrogen.

• The indigo should a dark blue and violet, but on saving the graphic image, a few curious effects happened, sorry about that, but it doesn't detract from the explanation of the 'red shift'!

• As you can see from the diagram, the more distant the galaxy, the bigger the red shift - the more the waves are stretched out with a longer wavelength and lower frequency.

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• The spectral lines observed from Earth of: our Sun ==> distant galaxy  ==> very distant galaxy, and this change, the 'red shift' in wavelengths and frequencies applies to the whole pattern of spectral lines of any element, but is best observed, and first discovered, by studying the light from very hot hydrogen atoms.

• What Hubble and other astronomers found that the further a galaxy is from us (the observer) the faster the galaxy seems to moving away from as.

• These speeds are calculated from the red-shift.

• Cosmologists have therefore concluded that the whole of the universe is expanding (See Part (5).

• INDEX for physics notes on COSMOLOGY

In cosmology know what is meant by the red-shift (e.g. in the spectral lines of hydrogen).

Know how and why the spectral lines of hydrogen light from distant stars galaxies are shifted to longer wavelengths and lower frequencies by the Doppler effect.

Appreciate how the observed red-shift provides evidence that the universe is expanding and supports the ‘Big Bang’ theory (that the universe began from a very small initial point), rather than the 'steady state' theory.