Physics • Year 12 • Module 8 • Lesson 1
The Big Bang Theory
Lock in the core vocabulary, the Big Bang timeline, and the key properties of the Cosmic Microwave Background before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: Big Bang, Cosmic Microwave Background (CMB), cosmological redshift, recombination, nucleosynthesis, singularity, Hubble’s law, scale factor, inflation, lookback time. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The cosmological model describing the origin and expansion of the universe from an extremely hot, dense state approximately 13.8 billion years ago. | |
| 1.2 | Thermal radiation left over from the early universe, now cooled to 2.725 K, filling the universe uniformly in all directions. | |
| 1.3 | The stretching of light wavelengths caused by the expansion of space itself rather than by relative motion of the source. | |
| 1.4 | The epoch ~380,000 years after the Big Bang when electrons combined with nuclei to form neutral atoms, making the universe transparent. | |
| 1.5 | The period approximately 3 minutes after the Big Bang when protons and neutrons fused to form hydrogen, helium and trace lithium nuclei. | |
| 1.6 | The initial state of the universe before the Big Bang, where all matter and energy was compressed into a point of infinite density where classical physics breaks down. | |
| 1.7 | The relationship stating that the recession speed of a galaxy is directly proportional to its distance: v = H₀d. | |
| 1.8 | A dimensionless parameter, denoted a, that describes the relative size of the universe at a given time compared with today. | |
| 1.9 | A period of exponential expansion beginning around 10−35 s after the Big Bang during which the universe grew by a factor of ~1026. | |
| 1.10 | The time elapsed since the light we observe from a distant object was emitted; equal to the age of the universe minus the age of the object at emission. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 The Big Bang was an explosion of matter into pre-existing empty space. T / F
2.2 The Cosmic Microwave Background has a temperature of approximately 2.725 K and a perfect black-body spectrum. T / F
2.3 Because all galaxies appear to recede from Earth, Earth must be located at the centre of the universe. T / F
2.4 The temperature of the universe is inversely proportional to the scale factor: T ∝ 1/a. T / F
2.5 Recombination occurred approximately 380,000 years after the Big Bang, when protons and neutrons first formed. T / F
2.6 Cosmological redshift is caused by space itself expanding, stretching the wavelength of photons as they travel through it. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
13.8 billion · 2.725 · 380,000 · expansion · hydrogen and helium · inflation · neutral atoms · transparent
The Big Bang theory states that the universe began approximately ___________ years ago from an extremely hot, dense state. The Big Bang was not an explosion in space but an ___________ of space itself. About 10−35 s after the Big Bang, a brief period of exponential growth called ___________ caused the universe to grow by an enormous factor. Approximately 3 minutes after the Big Bang, nucleosynthesis produced the first nuclei of ___________. Around ___________ years after the Big Bang, electrons combined with nuclei to form ___________, an event called recombination. Before this, the universe was opaque; afterwards it became ___________. The photons released at this moment now form the CMB, which has been redshifted to a temperature of ___________ K.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 State Hubble’s law and identify the two quantities it relates.
4.2 Why does the CMB have such an important role as evidence for the Big Bang theory?
4.3 Explain why an observer in any galaxy would see all other galaxies receding, not just an observer on Earth.
4.4 State the relationship between temperature and scale factor in cosmology and use it to explain why the early universe was much hotter than today.
5. Sequence the Big Bang timeline
The eight events below are out of order. Write the numbers 1–8 in the “Order” column to place them in chronological sequence from earliest to latest. 8 marks (1 each)
| Order | Event |
|---|---|
| Electrons combine with nuclei to form neutral hydrogen (recombination); universe becomes transparent; CMB is released. | |
| First stars begin to form from gravitational collapse of hydrogen gas clouds. | |
| Quarks combine to form protons and neutrons. | |
| Singularity — all energy and space compressed to a point; classical physics breaks down. | |
| Nucleosynthesis — protons and neutrons fuse into hydrogen, helium, and trace lithium nuclei. | |
| Exponential inflation — universe grows by a factor of ~1026 in ~10−32 s. | |
| Universe reaches its current age of ~13.8 billion years; expansion continues to accelerate. | |
| Gravity separates from other fundamental forces at the Planck time (~10−43 s); temperature ~1032 K. |
6. Label the balloon analogy
The diagram below shows the balloon analogy for the expansion of the universe. Two balloons represent the universe at an early time (left) and the present day (right). Dots represent galaxies. Write the label for each lettered arrow or feature in the table. 6 marks (1 each)
| Label | Description |
|---|---|
| A | |
| B | |
| C | |
| D | |
| E | |
| F |
Q1 — Term–definition match
1.1 Big Bang • 1.2 Cosmic Microwave Background (CMB) • 1.3 cosmological redshift • 1.4 recombination • 1.5 nucleosynthesis • 1.6 singularity • 1.7 Hubble’s law • 1.8 scale factor • 1.9 inflation • 1.10 lookback time.
Q2 — True / false with correction
2.1 False. The Big Bang was NOT an explosion into pre-existing space. It was an expansion of space itself — there was no pre-existing empty space “outside” for matter to explode into.
2.2 True.
2.3 False. Earth is NOT at the centre of the universe. From any galaxy, all other galaxies appear to recede because space itself expands uniformly. The expansion has no privileged centre.
2.4 True.
2.5 False. Recombination occurred ~380,000 years after the Big Bang. The formation of the first protons and neutrons from quarks occurred much earlier, at approximately 10−6 s. Nucleosynthesis (fusion of nuclei) occurred at ~3 minutes.
2.6 True.
Q3 — Cloze paragraph
In order: 13.8 billion / expansion / inflation / hydrogen and helium / 380,000 / neutral atoms / transparent / 2.725.
Q4.1 — Hubble’s law
Hubble’s law states that the recession speed of a galaxy is directly proportional to its distance from the observer: v = H₀d, where v is the recession speed (km/s), d is the distance (Mpc), and H₀ ≈ 70 km/s/Mpc is the Hubble constant.
Q4.2 — CMB as evidence
The CMB is a relic of the hot, dense early universe. Its uniform temperature (~2.725 K) and perfect black-body spectrum match the prediction that, after recombination, the universe released thermal radiation that has since cooled due to cosmic expansion. This is direct observational evidence that the universe began in a hot state and has been expanding and cooling — consistent with the Big Bang model and not predicted by any static-universe model.
Q4.3 — No privileged centre of expansion
The expansion of space is uniform — every point in space moves away from every other point. Using the balloon analogy: no dot on the balloon surface is the “centre.” Any observer, from any galaxy, sees the same pattern of recession. This means the observation that all galaxies recede from Earth does not imply Earth is at the centre; any observer anywhere would see the same thing.
Q4.4 — Temperature and scale factor
Temperature is inversely proportional to the scale factor: T ∝ 1/a. When the universe was younger, the scale factor was much smaller, so the temperature was much higher. As the universe expands (scale factor increases), the temperature decreases. The early universe had a scale factor close to zero and temperatures of 1032 K or higher; today T = 2.725 K.
Q5 — Big Bang timeline (chronological order)
1. Singularity • 2. Gravity separates (Planck time) • 3. Inflation • 4. Quarks → protons/neutrons • 5. Nucleosynthesis (~3 min) • 6. Recombination/CMB (~380,000 yr) • 7. First stars (~200 million yr) • 8. Present day (~13.8 Gyr).
Q6 — Balloon analogy labels
A: Early (small) universe — soon after the Big Bang, the universe was much smaller. B: Expansion of space (the process by which the balloon/universe grows). C: Present-day (large) universe. D: A galaxy (each dot represents a galaxy embedded in the surface of space). E: The surface of the balloon represents space itself (a 2-D analogy for 3-D expanding space). F: No centre — no dot on the balloon surface is at a special central position; all dots see all others receding.