Everything Science Grade 12

CHAPTER 9. DOPPLEREFFECT 9.3

The Expanding Universe ESCEL

Stars emit light, which is why we can see themat night. Galaxies are huge collections

of stars. An example is our own Galaxy, the Milky Way, of which our sunis only one of

the millions of stars! Using large telescopes likethe Southern African Large Telescope

(SALT) in the Karoo, astronomers can measurethe light from distant galaxies. The

spectrum of light can tell us what elements are inthe stars in the galaxiesbecause each

element emits/absorbs light at particular wavelengths (called spectrallines). If these

lines are observed to beshifted from their usualwavelengths to shorterwavelengths,

then the light from the galaxy is said to be blue-shifted. If the spectral lines areshifted

to longer wavelengths,then the light from thegalaxy is said to be red-shifted. If we

think of the blue-shift and redshift in Doppler effect terms, then a blue-shifted galaxy

would appear to be moving towards us (the observers) and ared-shifted galaxy would

appear to be moving away from us.

Tip

1. If the light source
   is moving away from the
   observer (positive veloc-
   ity) then the observed
   frequency is lower and
   the observed wavelength
   is greater (red-shifted).


2. If the source is mov-
   ing towards (negative ve-
   locity) the observer, the
   observed frequency is
   higher and the wave-
   length is shorter (blue-
   shifted).

Edwin Hubble (20 November 1889 - 28 September 1953) measured theDoppler shift

of a large sample of galaxies. He found that the light from distant galaxies is red-shifted

and he discovered that there is a proportionalityrelationship between the redshift and

the distance to the galaxy. Galaxies that are further awayalways appear more red-

shifted than nearby galaxies. Remember thata redshift in Doppler terms means a

velocity of the light source away from the observer. Sowhy do all distant galaxies

appear to be moving away from our Galaxy?

The reason is that theuniverse is expanding! The galaxies are not actually moving

themselves, rather the space between them is expanding!

Chapter 9 — Summary

See the summary presentation ( Presentation: VPobj at http://www.everythingscience.co.za)

• The Doppler Effect is the apparent change in frequency and wavelengthof a wave
  when the observer andsource of the wave move relative to each other.


• The following equationcan be used to calculate the frequency of the wave ac-
  cording to the observeror listener:

fL=

v + vL

v + vS

fS

• If the direction of the wave from the listener tothe source is chosen aspositive,
  the velocities have the following signs:

Source moves towards listener vS: negative

Source moves away fromlistener vS: positive

Listener moves towardssource vL: positive

Listener moves away from source vL: negative