UK GCSE level age ~14-16, ~US grades 9-10 PHYSICS revision notes re-edit 25/05/2023 [SEARCH]

Cosmology: 4. In cosmological terms what is the red-shift?

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

  • 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 millions/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 than what you observe on Earth or from the Sun.

    • The shift to lower frequencies was 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, further analysed the red shifts and related the increasing value of the shift to faster more distant galaxies.

  • 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.

    • 1. goes a deeper red and 2. goes from green to yellow-green to yellow.

    • 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, 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.

  •   ==> ==>

  • 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.


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