Cosmology questions

Cosmology syllabus

Questions on cosmology can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 8.13P describe the past evolution of the universe and the main arguments in favour of the Big Bang theory
• 8.14P describe evidence that supports the Big Bang theory (red-shift and cosmic microwave background - CMB - radiation)
• 8.15P describe that if a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength
• 8.16P use the equation relating to change in wavelength, reference wavelength, velocity of a galaxy and the speed of light: change in wavelength / reference wavelength = velocity of a galaxy / speed of light (λ − λ₀) / λ₀ = Δλ / λ₀ = v / c
• 8.17P describe the red-shift in light received from galaxies at different distances away from the Earth
• 8.18P explain why the red-shift of galaxies provides evidence for the expansion of the universe

How to answer cosmology questions

1. Identify the scale and object involved before choosing a gravitational or orbital relationship.
2. Keep the life cycles of Sun-like and high-mass stars separate and in the correct order.
3. Use observations such as red-shift or background radiation as evidence, then state the conclusion.
4. Show conversions for orbital radius, period and speed before calculating.

Cosmology example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Locating maximum red-shift in an orbit

November 2023 · Paper 2P · Question 8(a)

Question 1

This question is about red-shift.

The diagram shows two identical stars, A and B, orbiting each other.

Light from each star arrives at Earth.

As the stars move in their orbit, the wavelength of the light observed at Earth changes due to the Doppler effect.

Add an X to the diagram to show the position of star B when the light emitted from it shows maximum red-shift when detected on Earth.

Final answer

Place X at the rightmost point of the orbit—the 3 o’clock position—as shown.

Mark scheme points

1. M1 X at the 3 o’clock position on the orbit. Any indicative mark within one star’s radius of the correct position is accepted.

Explanation

Maximum red-shift occurs when star B has its greatest speed away from Earth. At the rightmost point, the direction of orbit makes star B move upwards, directly away from Earth below the diagram.

• Check that X is on the orbit, directly to the right of its centre.

Common mistakes

• Very few candidates linked maximum red-shift to the highest speed away from Earth. Check the star’s direction of motion, rather than choosing the point furthest from Earth.

Example 2: Drawing a Hubble–Humason line of best fit

January 2023 (R) · Paper 2P · Question 8(b)(ii)

Question 2

Between 1929 and 1931, physicists Hubble and Humason investigated the red-shift of light from galaxies at different distances from the Earth.

The distance unit they used is the megaparsec (Mpc).

The graph shows some of the results of their investigation.

Draw the line of best fit on the graph.

Final answer

Draw the straight line of best fit shown below, excluding the anomalous point.

Mark scheme points

1. M1 Draw a straight line within one small square of each non-anomalous data point. Follow-through is allowed from the anomalous point identified in the previous part.

Explanation

A line of best fit shows the overall trend, rather than joining the points individually. Check that:

• The line is straight: use a ruler.
• The line passes within one small square of the non-anomalous points, including the origin.

Common mistakes

• Candidates generally drew the straight line correctly. Keep this approach: do not draw a point-to-point zigzag.
• Do not force the line towards the anomalous point near 0.5 Mpc; this would make it miss the main trend.

Practise Cosmology questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.