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How do we know the Universe is expanding, and what is the evidence for the Big Bang?

Cosmology: astronomical distances and parallax, the Doppler effect and redshift, Hubble's law and the expansion of the Universe, the age of the Universe, and the evidence for the Big Bang.

A focused answer to the OCR H556 cosmology content, covering astronomical distance units and parallax, the Doppler effect and redshift of light from galaxies, Hubble's law and the expansion of the Universe, the estimate of the age of the Universe from the Hubble constant, and the evidence for the Big Bang including the cosmic microwave background.

Generated by Claude Opus 4.811 min answer

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

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  1. What this dot point is asking
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What this dot point is asking

OCR wants you to use astronomical distance units and parallax, apply the Doppler effect and redshift to light from galaxies, state and use Hubble's law, estimate the age of the Universe from the Hubble constant, and describe the evidence for the Big Bang.

The answer

Astronomical distances and parallax

The Doppler effect and redshift

Hubble's law and the expanding Universe

The age of the Universe and Big Bang evidence

Examples in context

The redshift of galaxies, discovered by Hubble, transformed cosmology by showing the Universe is expanding rather than static. The cosmic microwave background, mapped in detail by satellites, is the strongest evidence for the Big Bang and lets cosmologists measure the composition and age of the Universe precisely. Type Ia supernovae used as standard candles extend the distance ladder to the farthest galaxies, and their redshifts revealed that the expansion is accelerating, attributed to dark energy.

Try this

Q1. State Hubble's law and define each term. [2 marks]

  • Cue. v=H0dv = H_0 d: recession speed is proportional to distance, with H0H_0 the Hubble constant.

Q2. A galaxy shows a redshift of z=0.010z = 0.010. Find its recession speed. Take c=3.0×108 m s−1c = 3.0 \times 10^{8}\ \text{m s}^{-1}. [2 marks]

  • Cue. v=zc=0.010×3.0×108=3.0×106 m s−1v = zc = 0.010 \times 3.0 \times 10^{8} = 3.0 \times 10^{6}\ \text{m s}^{-1}.

Q3. State two pieces of evidence for the Big Bang. [2 marks]

  • Cue. The redshift of galaxies (expansion) and the cosmic microwave background radiation (also the abundance of light elements).

Exam-style practice questions

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

OCR 20184 marksA spectral line of laboratory wavelength 6.563×10−7 m6.563 \times 10^{-7}\ \text{m} is observed from a distant galaxy at 6.598×10−7 m6.598 \times 10^{-7}\ \text{m}. Calculate the recession speed of the galaxy. Take c=3.0×108 m s−1c = 3.0 \times 10^{8}\ \text{m s}^{-1}.
Show worked answer →

Redshift: z=Δλλ=6.598×10−7−6.563×10−76.563×10−7=3.5×10−96.563×10−7=5.33×10−3z = \frac{\Delta\lambda}{\lambda} = \frac{6.598 \times 10^{-7} - 6.563 \times 10^{-7}}{6.563 \times 10^{-7}} = \frac{3.5 \times 10^{-9}}{6.563 \times 10^{-7}} = 5.33 \times 10^{-3}.

For small speeds Δλλ≈vc\frac{\Delta\lambda}{\lambda} \approx \frac{v}{c}, so v=zc=(5.33×10−3)(3.0×108)=1.6×106 m s−1v = zc = (5.33 \times 10^{-3})(3.0 \times 10^{8}) = 1.6 \times 10^{6}\ \text{m s}^{-1}.

Markers reward the redshift from the wavelength shift, the approximation Δλλ=vc\frac{\Delta\lambda}{\lambda} = \frac{v}{c}, and the speed about 1.6×106 m s−11.6 \times 10^{6}\ \text{m s}^{-1}.

OCR 20214 marksThe Hubble constant is approximately 2.3×10−18 s−12.3 \times 10^{-18}\ \text{s}^{-1}. Estimate the age of the Universe in seconds and in years, and state one assumption made. Take 11 year =3.16×107 s= 3.16 \times 10^{7}\ \text{s}.
Show worked answer →

Age estimate: t≈1H0=12.3×10−18=4.3×1017 st \approx \frac{1}{H_0} = \frac{1}{2.3 \times 10^{-18}} = 4.3 \times 10^{17}\ \text{s}.

In years: 4.3×10173.16×107=1.4×1010\frac{4.3 \times 10^{17}}{3.16 \times 10^{7}} = 1.4 \times 10^{10} years, about 14 billion years.

The assumption is that the rate of expansion (the Hubble constant) has been constant throughout the history of the Universe.

Markers reward t≈1H0t \approx \frac{1}{H_0}, the age about 4.3×1017 s4.3 \times 10^{17}\ \text{s} (14 billion years), and the constant-expansion assumption.

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