Astronomy & Space · Unit 16
Cosmology
The Big Bang, cosmic expansion, the CMB, dark matter and dark energy
Cosmology asks about the universe as a single object - its origin, its contents and its fate - and it is the field where the honest answer is most often that we can measure the effect but not identify the cause.
The unit covers the Big Bang and the evidence for it, cosmic expansion, the microwave background, and dark matter and dark energy, which together make up most of the universe and remain unidentified.
This unit breaks down into 21 short steps and 120 questions, starting at difficulty 4 and building to 5. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.
- Steps
- 21
- Questions
- 120
- Difficulty
- 4-5
What this unit covers
- The Cosmic Microwave Background
- Dark Energy
- Dark Matter
- The Big Bang
- Cosmic Expansion
Where this fits
Needs Galaxies and Light, Spectra and Distances. The most abstract unit in the track.
Where people slip
The Big Bang was not an explosion in space. It was an expansion of space itself, happening everywhere at once, and there is no location you could point to as the site of it.
How the unit is structured
Cosmology runs as 21 short steps that unlock in order. 15 are practice rounds and 6 are challenge rounds that pull together everything before them. Questions start at difficulty 4 and climb to 5 as you progress.
Challenge rounds
Example questions
30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.
Cosmic Expansion
- Build the sentenceLevel 4
1. Build a true sentence about cosmic expansion.
Answer: Distant galaxies recede faster than nearby ones
Hubble's law means recession speed grows with distance, so distant galaxies recede fastest.
- Fact or fibLevel 4
2. The cosmological redshift of distant galaxies is caused by the stretching of light's wavelength as space expands.
Answer: True
As space expands, photon wavelengths stretch, shifting light toward the red end of the spectrum.
- Fill the blankLevel 4
3. Space itself stretching between galaxies is known as the ____ expansion of space.
- metriccorrect
- thermal
- magnetic
- elastic
Cosmologists call it metric expansion: the distances defined by the metric of space grow with time.
- Match the pairsLevel 4
4. Match each expansion term to its meaning.
Answer: Edwin Hubble = Distance-velocity law of 1929; Hubble constant = About 70 km/s/Mpc; Cosmological redshift = Stretched light from expansion
Hubble found v proportional to d, with slope H0 near 70 km/s/Mpc, and expansion stretches light into redshift.
- Multiple choiceLevel 4
5. Hubble's law says a galaxy's recession velocity is proportional to which of its properties?
- Its distance from uscorrect
- Its mass
- Its brightness
- Its temperature
Hubble's law, v = H0 x d, says more distant galaxies recede faster, in direct proportion to distance.
- Choose all that applyLevel 5
6. Which statements about cosmic expansion are correct? Select all that apply.
- Recession velocity is proportional to distancecorrect
- The Hubble constant is about 70 km/s/Mpccorrect
- Space itself expands between galaxiescorrect
- There is a single central point the universe expands away from
Expansion has no center; every observer sees distant galaxies receding per Hubble's law.
Dark Energy
- Build the sentenceLevel 4
7. Build a true sentence about dark energy.
Answer: Dark energy makes the universe expand faster
Dark energy drives the acceleration of cosmic expansion.
- Choose all that applyLevel 4
8. Which statements about dark energy are correct? Select all that apply.
- It makes up about 68 percent of the universecorrect
- Its evidence came from distant Type Ia supernovae in 1998correct
- It causes cosmic expansion to acceleratecorrect
- It clumps into halos around galaxies
Dark energy is smooth and repulsive, not clumpy like dark matter, and it drives accelerating expansion.
- Fact or fibLevel 4
9. Dark energy slows the expansion of the universe down over time.
Answer: False
Dark energy does the opposite: it drives the expansion to accelerate.
- Fill the blankLevel 4
10. In Einstein's equations, dark energy is often represented by the cosmological ____.
- constantcorrect
- redshift
- horizon
- gradient
The cosmological constant, symbol Lambda, is the simplest model of dark energy.
- Guess the numberLevel 4
11. Dark energy makes up about what percent of the universe's total mass-energy?
Answer: 68 percent
Dark energy accounts for roughly 68 percent of the universe.
- Match the pairsLevel 4
12. Match each component to its share of the universe's mass-energy.
Answer: Dark energy = About 68 percent; Dark matter = About 27 percent; Ordinary matter = About 5 percent
The cosmic budget is roughly 68 percent dark energy, 27 percent dark matter, and 5 percent ordinary matter.
Dark Matter
- Build the sentenceLevel 4
13. Build a true sentence about dark matter.
Answer: Dark matter is detected through its gravity
Because it does not shine, dark matter is found only through its gravitational pull.
- Choose all that applyLevel 4
14. Which are genuine lines of evidence for dark matter? Select all that apply.
- Flat galaxy rotation curvescorrect
- Gravitational lensing masses exceeding visible masscorrect
- The motion of galaxies within clusterscorrect
- The 2.7 K temperature of the CMB
Rotation curves, lensing, and cluster dynamics all imply extra mass; the CMB temperature alone is not that evidence.
- Fact or fibLevel 4
15. The flat rotation curves of spiral galaxies are strong evidence for dark matter.
Answer: True
Stars at galaxies' edges orbit faster than the visible mass allows, implying unseen dark matter.
- Fill the blankLevel 4
16. Dark matter is thought to form invisible ____ that surround and extend beyond visible galaxies.
- haloscorrect
- rings
- jets
- belts
Galaxies sit inside extended dark matter halos that dominate their total mass.
- Multiple choiceLevel 4
17. Roughly what fraction of the universe's total mass-energy is dark matter?
- About 27 percentcorrect
- About 5 percent
- About 68 percent
- About 90 percent
Dark matter makes up about 27 percent of the universe, ordinary matter about 5 percent, and dark energy about 68 percent.
- Guess the numberLevel 5
18. Ordinary (baryonic) matter makes up only about what percent of the universe's total mass-energy?
Answer: 5 percent
Ordinary matter is just about 5 percent; dark matter is about 27 percent and dark energy about 68 percent.
The Big Bang
- Build the sentenceLevel 4
19. Build a true sentence about the Big Bang.
Answer: The early universe was hot and dense
The Big Bang model begins with an extremely hot, dense state that expanded and cooled.
- Choose all that applyLevel 4
20. Which of these are predictions or features of the Big Bang model? Select all that apply.
- A hot, dense early universe
- The abundance of primordial hydrogen and heliumcorrect
- The cosmic microwave backgroundcorrect
- A universe that is unchanging and eternal
The Big Bang predicts an evolving universe with a hot start, light-element abundances, and the CMB, unlike the old steady-state idea.
- Fill the blankLevel 4
21. By mass, Big Bang nucleosynthesis produced roughly 75 percent hydrogen and about 25 percent ____.
- heliumcorrect
- carbon
- oxygen
- iron
The early universe ended up about three-quarters hydrogen and one-quarter helium by mass.
- Multiple choiceLevel 4
22. About how long ago did the Big Bang occur, according to current cosmology?
- About 13.8 billion years agocorrect
- About 4.6 billion years ago
- About 46 billion years ago
- About 100 million years ago
Current measurements put the age of the universe at about 13.8 billion years.
- Odd one outLevel 4
23. Three of these are light elements made in Big Bang nucleosynthesis. Which was NOT made then in any significant amount?
- Carboncorrect
- Hydrogen
- Helium
- Lithium
Nucleosynthesis made hydrogen, helium, and a little lithium; carbon formed much later inside stars.
- Fact or fibLevel 5
24. Cosmic inflation was proposed partly to explain why the universe appears so flat and uniform.
Answer: True
Inflation naturally addresses the flatness and horizon problems of the standard Big Bang.
The Cosmic Microwave Background
- Fact or fibLevel 4
25. The cosmic microwave background is often described as the afterglow of the hot early universe.
Answer: True
The CMB is relic radiation released about 380,000 years after the Big Bang, the leftover glow of that hot era.
- Fill the blankLevel 4
26. The moment the universe became transparent and released the CMB is called the surface of last ____.
- scatteringcorrect
- fusion
- collapse
- redshift
At the surface of last scattering, photons stopped scattering off free electrons and streamed freely.
- Match the pairsLevel 4
27. Match each CMB fact to its description.
Answer: Penzias and Wilson = Discovered the CMB in 1965; CMB temperature = About 2.7 K; Surface of last scattering = About 380,000 years after the Big Bang
The CMB, at about 2.7 K, was released at last scattering roughly 380,000 years after the Big Bang and found by Penzias and Wilson.
- Multiple choiceLevel 4
28. The cosmic microwave background has a blackbody temperature of about:
- 2.7 Kcorrect
- 27 K
- 270 K
- 0 K
The CMB is an almost perfect blackbody at roughly 2.7 K.
- Odd one outLevel 4
29. Which of these is NOT a property of the cosmic microwave background?
- A line-emission spectrum from hot gascorrect
- A near-perfect blackbody spectrum
- A temperature near 2.7 K
- Tiny temperature anisotropies
The CMB is a smooth blackbody, not a line-emission spectrum from a specific hot gas.
- Choose all that applyLevel 5
30. Which are true of the cosmic microwave background? Select all that apply.
- It has a near-perfect blackbody spectrumcorrect
- It was released at the surface of last scatteringcorrect
- It was discovered in 1965correct
- It is emitted by nearby stars in the Milky Way
The CMB is relic radiation from the whole early universe, not light from nearby stars.
Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.
How you practise
This unit mixes 14 different question formats, so you are recalling and applying rather than recognising the same layout every time.
- Build the sentence
- Choose all that apply
- Fact or fib
- Fill the blank
- Guess the number
- Match the pairs
- Multiple choice
- Odd one out
- Picture question
- Put in order
- Sort into groups
- Spell it
- True or false
- Type the answer
Practise Cosmology
120 questions across 21 steps. Start with step one and crawl at your own pace.
Play this unitRead about Cosmology
Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.
- What Is Dark Matter?Why galaxies spin too fast to hold together, the evidence for invisible mass, and the honest state of the search.August 28, 2026 · 6 min read
- What Is the Big Bang Theory?What it actually claims, the three main lines of evidence for it, and the common misunderstandings worth clearing up.August 28, 2026 · 6 min read
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