P1.5.4 Red-shift

Question 1

What is the doppler effect?

Answer 1

If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency. This is known as the Doppler effect.

Question 2

What type of wave could the wavelength be? Give three examples.

Answer 2

The Doppler effect happens to both longitudinal waves(e.g. sound) and transverse waves (e.g. light and microwave).

Question 3

What happens to the wavelength and the frequency when the source moves away from the obserever?

Answer 3

When the source moves away from the observeer, the observed wavelength incerases and the frequency decreases.

Question 4

What happens to the wavelength and the frequency when the source moves towarrds from the obserever?

Answer 4

When the source moves towards the observer, the observed wavelength decreases and the frequency increases.

Question 5

What is red-shift?

Answer 5

• There is an observed increase in the wavelength of light from most distant galaxies.
• The further away the galaxies are, the faster they are moving, and the bigger the observed increase in wavelength.
  This effect is called red-shift.

Question 6

Explain red-shift, in regards to light from other galaxies.

Answer 6

• Different chemical elements absorb different frequencies.
• Each element produces a specific pattern of dark lines at the frequencies that it absorbs in the visible spectrum.
• When looking at light from distant galaxies, the same pattern is visible, however the pattern is at slightly lower frequencies - they’ve shifted towards the red end of the spectrum,
  This is called red-shift.

Question 7

‘The further away a galaxy is…’

Finish this statement about red shift, explaining what this signifies about the whole universe.

Answer 7

• The further away a galaxy is, the greater the red-shift.
• This provides evidence that the whole universe is expanding:
  1. Measurements of the red-shift suggest that that all the galaxies are moving away from us very quickly - and it’s the same result whichever direction you look in.
  2. More distant galaxies have greater red-shifts than nearer ones.
  3. This means that more distant galaxies are moving away from us faster than nearer ones.
  i. e. this is evidence for the expansion of the universe.

Question 8

What is the ‘Big Bang’ theory?

Answer 8

According to the ‘Big Bang’ theory, all the matter and energy in the universe must have been compressed into a very small space. Then it exploded from that very small initial point, and started expanding.

Question 9

How long ago was the ‘Big Bang’ theory estimated to have happened - how is this number calculated?

Answer 9

The expansion in the ‘Big Bang’ theory is still going on. Therefore, the current rate of expansion of the universe can be used to estimate its age: 14 billion years ago.

Question 10

What is CMBR?

Answer 10

Scientists have detected low frequency electromagnetic radiation coming from all parts of the universe: cosmic microwave background radiation (CMBR) is a form of electromagnetic radiation filling the universe. It comes from radiation that was present shortly after the beginning of the universe.

Question 11

Which theory for the beginning of the universe can explain CMBR and how?

Answer 11

The ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR:

• Just after the ‘Big Bang’, while the universe was extremely hot, everything in the universe emitted very high frequency radiation.
• As the universe expanded it has cooled, and this radiation has dropped in frequency and is now seen as microwave radiation.

Question 12

Give two limitations of the ‘Big Bang’ theory.

Answer 12

• The ‘Big Bang’ theory doesn’t actually give the whole explanation of the universe - there are observations that the theory can yet explain. E.g. the ‘Big Bang’ theory predicts that the universe’s expansion should be slowing down - however it actually appears to be speeding up.
• The ‘Big Bang’ explains the universe’s expansion well but it’s isn’t an explanation for what caused the explosion in the first place, or what the conditions were like before the explosion.