Cosmology

Question 1

what evidence is there for the past evolution and main arguments in favour of the Big Bang theory?

Answer 1

• red shift suggests the universe is expanding
• CMBR is strong evidence for an initial Big Bang
• the galaxies are moving away from each other at great speed, suggesting something must have got them going from a single starting point. that something was probably a big explosion:
  1. initially, all the matter in the universe occupied a single point
  2. this tiny space was very dense and very hot
  3. this single point then “exploded” - the big bang
  4. space started expanding, and the expansion is still going on.
• according to the Big Bang model, CMBR is the leftover energy of this initial explosion.

Question 2

what are waves affected by?

Answer 2

the motion of the source.

• when a source of waves is moving relative to the observer, the waves will undergo an apparent change in frequency and wavelength when they are observed, compared to when they were emitted (Doppler effect)
• this happens with all types of waves, including light

Question 3

what is redshift?

Answer 3

• if the light source is moving away from you, the light it emits will appear to be shifted towards the red end (i.e. the lower frequency end) of the visible part of the EM spectrum - this is red shift.
• astronomers see this happening with light from stars. the light from distant stars is red-shifted, we observe light with a longer wavelength (lower frequency) than we would expect the stars to emit. the stars must be moving away from the earth.

Question 4

what evidence is there for redshift?

Answer 4

• different elements absorb different frequencies (or wavelengths) of light.
• when light is passed through a sample of an element, a pattern of dark lines is produced, with a dark line at each of the frequencies in the visible part of the EM spectrum that the element absorbs.
• when we look at light from distant galaxies we see the see patterns but at slightly lower frequencies (and so longer wavelengths) than they should be
• the patterns have been shifted towards the red end of the spectrum = redshift.

Question 5

how do you calculate red-shift?

Answer 5

• the amount of light from a galaxy is red-shifted is determined by the following formula:

change in wavelength. velocity of a galaxy
——————————– = ——————————
reference wavelength speed of light

• the reference wavelength means the wavelength of the light when it was emitted, before it was red-shifted.

Question 6

what does red-shift suggest?

Answer 6

• measurements of the red-shift suggest that all the disant galaxies are MOVING AWAY from us very quickly, and its the same result whichever direction you look in.
• more distant galaxies have greater red-shifts than nearer ones, they show a bigger observed increase in wavelength
• this means that more distant galaxies are moving away faster than nearer ones.
• the inescapable conclusion appears to be that the whole universe is expanding.

Question 7

example to demonstrate redshift

Answer 7

• imagine a balloon covered with pompoms
• as you blow into the balloon, it stretches. the pompoms move further away from each other
• the balloon represents the universe and each pompom is a galaxy. as time goes one, space stretches and expands, moving the galaxies away from each other.
• this is a simple model (balloons only stretch so far, and there would be galaxies “inside” the balloon too) but it shows how the expansion of space makes it look like galaxies are moving away from us.

Question 8

what direction does CMBR act in?

Answer 8

ALL DIRECTIONS.

• scientists can detect low frequency microwave radiation coming from all direction and all parts of the universe.
• its known as Cosmic Microwave Background Radiation.
• this background radiation is strong evidence for an initial Big Bang.
• as the universe expands and cools, this background radiation “cools” and drops in frequency.