What does Edexcel GCSE Astronomy Topic 16 cover?

Topic 16 covers the redshift of distant galaxies and that it is caused by recession, the redshift formula, Hubble's law relating recession velocity to distance, the estimation of the age and size of the Universe, how expansion supports both the Big Bang and Steady State theories, the observational evidence for the Big Bang (quasars, the CMB, the Hubble Deep Field), the CMB fluctuations, and dark matter, dark energy and the fate of the Universe.

What is Hubble's law?

Hubble's law states that a galaxy's recession velocity is directly proportional to its distance: v = H0 d, where H0 is the Hubble constant (about 68 km/s/Mpc). So more distant galaxies recede faster. On a graph of velocity against distance, the points lie on a straight line whose gradient is the Hubble constant.

What is the strongest evidence for the Big Bang?

The cosmic microwave background (CMB) radiation is the strongest evidence. The Big Bang theory predicts it as the cooled-down afterglow of the hot, dense early Universe, stretched to microwaves by the expansion. The rival Steady State theory cannot explain it, so the CMB is what makes the Big Bang the accepted model.

What are dark matter and dark energy?

Dark matter is matter that emits no light and is detected only by its gravity, such as the way galaxies rotate faster than their visible mass allows. Dark energy is a mysterious energy thought to be causing the expansion of the Universe to accelerate. Both are hard to detect, yet together they make up most of the Universe.

How should I revise Topic 16 for Edexcel GCSE Astronomy?

Drill the redshift formula and Hubble's law (and reading H0 as a graph gradient), and the age estimate (about 1 over H0). Remember redshift is increased wavelength from recession, and that the CMB (not redshift alone) makes the Big Bang the accepted model. Distinguish dark matter (gravity) from dark energy (acceleration).

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Edexcel GCSE Astronomy Topic 16 Cosmology: a complete overview of redshift, Hubble's law, the Big Bang and the fate of the Universe

A deep-dive Edexcel GCSE Astronomy guide to Topic 16 Cosmology. Covers redshift and the redshift formula, Hubble's law and the age of the Universe, the evidence for the Big Bang, the CMB fluctuations, and dark matter, dark energy and the fate of the Universe, with the exam patterns Pearson repeats.

Generated by Claude Opus 4.813 min read1AS0 Topic 16Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What Topic 16 actually demands
Redshift and Hubble's law
The Big Bang and the fate of the Universe
How Topic 16 is examined
Check your knowledge

What Topic 16 actually demands

Cosmology closes Paper 2 with the expanding Universe and its origin. It is calculation-rich (the redshift formula and Hubble's law) and conceptually demanding (the evidence for the Big Bang, dark matter and dark energy). It rewards confident formula work and careful nuance about what the evidence shows.

This guide walks through the dot points, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Redshift and Hubble's law

Light from distant galaxies is redshifted (wavelength increased) because they are receding. The redshift formula $\dfrac{\lambda - \lambda_0}{\lambda_0} = \dfrac{v}{c}$ gives the recession velocity. Hubble's law $v = H_0 d$ states that recession velocity is proportional to distance (more distant galaxies recede faster), with $H_0 \approx 68\,\text{km/s/Mpc}$ . The age of the Universe is roughly $\dfrac{1}{H_0}$ , about 14 billion years.

The Big Bang and the fate of the Universe

The expansion supports both the Big Bang and the Steady State theory, so the decisive evidence is the cosmic microwave background (CMB) (the cooled afterglow of the hot early Universe), with quasars and the Hubble Deep Field. The CMB fluctuations (mapped by WMAP and Planck) seeded today's structure. Dark matter (found by gravity) and dark energy (driving accelerating expansion) dominate the Universe, and its fate depends on their balance.

How Topic 16 is examined

A typical Edexcel profile for cosmology:

Calculation. The redshift formula and Hubble's law (and H0 as a graph gradient).
Explanation. Why the CMB, not redshift alone, makes the Big Bang the accepted model.
Description. Dark matter and dark energy, and why they are hard to detect.
Reasoning. The age of the Universe from the Hubble constant.

Check your knowledge

A mix of calculation and explanation questions covering Topic 16. Attempt them under timed conditions, then check against the solutions.

State what the redshift of a distant galaxy tells us about its motion. (1 mark)
A galaxy line at 500 nm is observed at 510 nm. Calculate the recession velocity (use $c = 3.0 \times 10^8\,\text{m/s}$ ). (3 marks)
State Hubble's law in formula form. (1 mark)
A galaxy recedes at 2000 km/s. Calculate its distance (H0 = 68 km/s/Mpc). (1 mark)
State the strongest evidence that makes the Big Bang the accepted model. (1 mark)
Explain why redshift alone does not prove the Big Bang. (2 marks)
State how dark matter is detected. (1 mark)

Solutions

Q1: It is moving away from us (receding); a greater redshift means a faster recession.
Q2: $\lambda - \lambda_0 = 510 - 500 = 10\,\text{nm}$ , so $\dfrac{10}{500} = \dfrac{v}{3.0 \times 10^8}$ , giving $v = 0.02 \times 3.0 \times 10^8 = 6.0 \times 10^6\,\text{m/s}$ .
Q3: $v = H_0 d$ .
Q4: $d = \dfrac{v}{H_0} = \dfrac{2000}{68} \approx 29\,\text{Mpc}$ .
Q5: The cosmic microwave background (CMB) radiation.
Q6: Both the Big Bang and Steady State theories explain the expansion (and so redshift), so redshift cannot decide between them; the CMB is the deciding evidence.
Q7: By its gravitational effects (for example, galaxies rotating faster than their visible mass allows).

Sources & how we know this

Pearson Edexcel Level 1/Level 2 GCSE (9-1) in Astronomy (1AS0) specification — Pearson (2017)