A-Level 9702 / Topic 25 / A2

The Universe is expanding.

Light from distant galaxies arrives stretched toward the red, and the further away a galaxy is, the faster it is fleeing. That single pattern, Hubble's law, points back to a hot, dense beginning, the Big Bang, and lets us estimate the age of the Universe itself.

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The key idea

Spectral lines from a receding galaxy are redshifted: their wavelength is stretched. For speeds well below c, the fractional shift gives the speed, Δλ/λ ≈ Δf/f ≈ v/c. Hubble found recession speed is proportional to distance, v = H₀d, evidence that the whole Universe is expanding from a Big Bang. Running it backwards gives an age of about 1/H₀.

Fig. 1 — Redshift grows with distance; plotting v against d gives a straight line through the origin whose gradient is the Hubble constant H₀

Section 01

Farther means faster.

Set a galaxy's distance and watch its spectral line slide into the red while the point climbs the straight v = H₀d line. Adjust the Hubble constant and the estimated age of the Universe, 1/H₀, changes with it. This is the evidence that space itself is expanding.

Section 02

Redshift, Hubble, Big Bang.

A short chain of ideas takes you from a shifted line to the age of the Universe.

Redshift: a galaxy moving away stretches the light it emits, so every spectral line shifts to a longer wavelength (toward the red end). A blueshift would mean approach.
The Doppler relation: for v much less than c, Δλ/λ ≈ Δf/f ≈ v/c. Measuring how far a known line has shifted gives the recession speed v.
Hubble's law: v = H₀d. Recession speed is proportional to distance, so a graph of v against d is a straight line through the origin with gradient H₀ (about 70 km s⁻¹ Mpc⁻¹).
Expanding Universe and age: every observer sees all galaxies receding, so space is expanding from a hot, dense start, the Big Bang. Assuming a constant rate, the age is roughly t ≈ 1/H₀.

Examiner trap

A redshift is a shift to longer wavelength and lower frequency. The relation Δλ/λ ≈ v/c holds only for speeds much smaller than c. To use 1/H₀ for an age you must convert H₀ into base SI units (s⁻¹) first, since 1 Mpc = 3.09 × 10²² m; the result is a time in seconds.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks on redshift, Hubble's law and the age of the Universe. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Light from a distant galaxy is redshifted. This tells us the galaxy is:

Quick check+10 XP

A galaxy shows a fractional shift Δλ/λ = 0.02 in its spectral lines. Its recession speed is about:

Quick check+10 XP

A graph of recession speed v against distance d for many galaxies is a straight line through the origin. Its gradient is:

Quick check+10 XP

The expansion of the Universe described by Hubble's law is evidence for:

Examiner trap

Hubble's law does not mean the Earth is special: in a uniformly expanding Universe, every observer sees all other galaxies receding with v = H₀d. And the age 1/H₀ is only an estimate, because it assumes the expansion rate has stayed constant throughout cosmic history.

Skill unlocked

Hubble's law and the Big Bang

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Stage 2 · Topic Paper 4 practice (all lessons)

All lessons in this topic

Original Paper 4 structured questions spanning every lesson in this topic, with full worked solutions.