Skip to main content
Northern IrelandPhysicsSyllabus dot point

How do we measure the universe, and what is the evidence for the Big Bang?

Stellar distances and luminosity, the Doppler effect and redshift, Hubble's law and the expanding universe, and the evidence for the Big Bang.

A CCEA A-Level Physics answer on measuring stellar distances and luminosity, the Doppler effect and redshift of galaxies, Hubble's law and the expanding universe, and the main lines of evidence for the Big Bang.

Generated by Claude Opus 4.811 min answer

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

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

CCEA wants you to describe how stellar distances and luminosity are measured, explain the Doppler effect and redshift, state and use Hubble's law and the idea of an expanding universe, and outline the evidence for the Big Bang. Expect a numerical redshift-to-distance calculation and a structured "evidence" question.

The answer

Measuring stars

A "standard candle" is an object of known luminosity (such as a Cepheid variable or a type Ia supernova); measuring how faint it appears reveals how far away it is, which is how the cosmic distance ladder is built up.

The Doppler effect and redshift

Hubble's law and the Big Bang

The main evidence for the Big Bang is the cosmic microwave background radiation, a near-uniform microwave glow at about 2.7 K2.7\ \text{K} filling all of space, the cooled remnant of the early hot universe, and the observed abundance of light elements (about three-quarters hydrogen and one-quarter helium by mass) predicted by Big Bang nucleosynthesis.

Worked example: recession speed and distance

Examples in context

Example 1. Type Ia supernovae and dark energy. Because every type Ia supernova explodes at nearly the same luminosity, astronomers use them as standard candles to measure distances to very faraway galaxies. In the late 1990s these measurements showed distant galaxies are fainter (further) than a steadily expanding universe predicts, evidence that the expansion is accelerating, attributed to dark energy.

Example 2. The Planck satellite map of the CMB. The cosmic microwave background was mapped in fine detail, revealing tiny temperature ripples of about one part in 10510^{5}. These ripples are the seeds of today's galaxies and let cosmologists pin down the Hubble constant and the age of the universe at about 1.4×10101.4 \times 10^{10} years.

Try this

Q1. A spectral line of wavelength 500 nm500\ \text{nm} from a galaxy is observed at 505 nm505\ \text{nm}. Find the recession speed. Take c=3.0×108 m s−1c = 3.0 \times 10^{8}\ \text{m s}^{-1}. [3 marks]

  • Cue. Δλλ=5500=0.01\frac{\Delta\lambda}{\lambda} = \frac{5}{500} = 0.01, so v=0.01×3.0×108=3.0×106 m s−1v = 0.01 \times 3.0 \times 10^{8} = 3.0 \times 10^{6}\ \text{m s}^{-1}.

Q2. State one piece of evidence that supports the Big Bang theory. [1 mark]

  • Cue. The cosmic microwave background radiation (or the hydrogen-to-helium abundance).

Q3. A star has the same luminosity as the Sun but appears 1/4001/400 as bright. How many times further away is it? [2 marks]

  • Cue. Brightness ∝1/d2\propto 1/d^2, so d∝1/bd \propto 1/\sqrt{b}; the star is 400=20\sqrt{400} = 20 times further away.

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA 20205 marksA hydrogen spectral line normally at 656.3 nm is observed at 661.2 nm in the spectrum of a distant galaxy. Calculate the recession speed of the galaxy and, using a Hubble constant of 2.3 times 10 to the minus 18 per second, estimate its distance from Earth. Take c as 3.0 times 10 to the 8.
Show worked answer →

The fractional change in wavelength gives the recession speed through the Doppler relation:

Δλλ=661.2−656.3656.3=4.9656.3=7.47×10−3\frac{\Delta\lambda}{\lambda} = \frac{661.2 - 656.3}{656.3} = \frac{4.9}{656.3} = 7.47 \times 10^{-3}.

So v=c×Δλλ=3.0×108×7.47×10−3=2.24×106v = c \times \frac{\Delta\lambda}{\lambda} = 3.0 \times 10^{8} \times 7.47 \times 10^{-3} = 2.24 \times 10^{6} m per second.

Using Hubble's law v=H0dv = H_0 d:

d=vH0=2.24×1062.3×10−18=9.7×1023d = \frac{v}{H_0} = \frac{2.24 \times 10^{6}}{2.3 \times 10^{-18}} = 9.7 \times 10^{23} m.

Markers reward the fractional shift, the recession speed, and the rearrangement of Hubble's law giving a distance of the order of 10 to the 24 m.

CCEA 20184 marksState Hubble's law and explain how it leads to the idea of an expanding universe. Describe two pieces of observational evidence that support the Big Bang model.
Show worked answer →

Hubble's law states that the recession speed of a galaxy is directly proportional to its distance from us, v=H0dv = H_0 d, where H0H_0 is the Hubble constant.

Because more distant galaxies recede faster, every observer sees all galaxies moving away, which is exactly what is expected if space itself is expanding uniformly. Running the expansion backwards in time brings all matter together to a single hot, dense origin, the Big Bang.

Two pieces of evidence: (1) the cosmic microwave background radiation, a near-uniform microwave glow at about 2.7 K filling all of space, the cooled remnant of the hot early universe; (2) the observed abundance of light elements, roughly three-quarters hydrogen and one-quarter helium by mass, matching the prediction of Big Bang nucleosynthesis.

Markers reward the proportionality statement, the uniform-expansion reasoning, and two distinct, correctly described pieces of evidence.

Related dot points

Sources & how we know this