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Spec links: 4.8.2 Red-shift (Physics only); 4.6.1.2 Properties of waves (wave equation) · Reading time: about 4 minutes

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A hiss from the sky

In 1964, two American scientists, Arno Penzias and Robert Wilson, were testing a large radio antenna. They kept picking up a faint hiss. It came from every direction, day and night. They even cleaned out pigeon droppings from the antenna in case those were to blame. The hiss stayed. They had found an echo of the birth of the universe itself.

Stretched waves

Think about an ambulance racing past with its siren on. As it comes towards you, the siren sounds higher. As it moves away, the pitch drops. When the source moves away, each wave is emitted from a little further off, so the waves are stretched out. The wavelength increases and the frequency decreases.

Light behaves in a similar way. Every element absorbs light at particular wavelengths, giving dark lines in a spectrum. When scientists look at light from distant galaxies, these lines are shifted towards the red end of the spectrum. This is called red-shift. It means the wavelength has increased, which shows that the galaxy is moving away from us.

Waves obey the wave equation: wave speed = frequency × wavelength, or v = f λ. All light travels through space at 300 000 000 m/s. Suppose red light has a wavelength of 0.000 000 6 m:

f = v ÷ λ = 300 000 000 ÷ 0.000 000 6 = 500 000 000 000 000 Hz (5 × 10¹⁴ Hz)

If this wavelength is stretched by red-shift, the frequency must fall, because the speed stays the same.

Further means faster

In the 1920s, astronomers measured the red-shift of many galaxies. They found that the further away a galaxy is, the bigger its red-shift. So more distant galaxies are moving away faster. The best explanation is that the whole universe is expanding. Space itself is stretching, carrying galaxies apart like dots drawn on a balloon that is being blown up.

The Big Bang

If the universe is expanding now, it must have been smaller in the past. The Big Bang theory says that the universe began from a very small region that was extremely hot and dense. It has been expanding ever since, for about 13.8 billion years.

The hiss found by Penzias and Wilson is now called the cosmic microwave background radiation. It is radiation left over from the early universe. As the universe expanded, its wavelength was stretched until it became microwaves. The Big Bang theory predicted this radiation, so finding it was strong evidence for the theory.

Still a mystery

Science is never finished. Since 1998, observations of exploding stars called supernovae suggest that distant galaxies are moving away ever faster. The expansion is speeding up. Scientists have called the unknown cause dark energy. Galaxies also spin so fast that they should fly apart, unless there is extra mass we cannot see, called dark matter (or dark mass). Nobody yet knows what either of these really is. Perhaps you will be the one to find out.

Key words

• Red-shift: an observed increase in the wavelength of light from most distant galaxies.
• Wavelength: the distance from a point on one wave to the equivalent point on the adjacent wave.
• Frequency: the number of waves passing a point each second, measured in hertz (Hz).
• Big Bang theory: the theory that the universe began from a very small region that was extremely hot and dense.
• Cosmic microwave background radiation: microwave radiation coming from all directions in space, left over from the early universe.
• Dark energy: the unexplained cause of the speeding up of the universe’s expansion.
• Dark matter: mass that cannot be seen but is needed to explain how galaxies move.

Questions

1. What is red-shift? [1 mark]
2. What did Penzias and Wilson detect in 1964? [1 mark]
3. According to the Big Bang theory, what was the universe like when it began? [2 marks]
4. Explain how the red-shift of distant galaxies provides evidence that the universe is expanding. [3 marks]
5. Explain why the siren of an ambulance sounds lower in pitch as it moves away from you. [2 marks]
6. An ambulance siren has a frequency of 680 Hz. The speed of sound in air is 340 m/s. Calculate the wavelength of the sound. Give your answer in metres. [3 marks]
7. Microwaves in the cosmic microwave background radiation have a wavelength of about 0.001 m. They travel at 300 000 000 m/s. Calculate their frequency. Give your answer in hertz. [3 marks]
8. Evaluate how well the Big Bang theory explains observations of the universe. [6 marks]

Show answers

1. An observed increase in the wavelength of light from distant galaxies (shift towards the red end of the spectrum) (1)
2. Cosmic microwave background radiation / a faint microwave hiss from every direction (1)
3. Very small region (1); extremely hot and dense (1)
4. Light from distant galaxies is red-shifted, so the galaxies are moving away from us (1); the further away a galaxy is, the greater its red-shift (1); so more distant galaxies are moving away faster, which suggests the whole universe is expanding (1)
5. As the source moves away, the sound waves are stretched out / wavelength increases (1); so the frequency heard decreases, giving a lower pitch (1)
6. λ = v ÷ f (1); = 340 ÷ 680 (1); = 0.5 m (1)
7. f = v ÷ λ (1); = 300 000 000 ÷ 0.001 (1); = 300 000 000 000 Hz (3 × 10¹¹ Hz) (1)
8. Indicative content:
   • light from distant galaxies is red-shifted, so they are moving away
   • more distant galaxies have greater red-shift, so are moving away faster
   • this fits a universe that is expanding from a small, hot, dense starting point
   • the theory predicted the cosmic microwave background radiation, which was later detected
   • CMB comes from all directions, as expected if it is left over from the early universe
   • however, since 1998 supernova observations suggest the expansion is speeding up
   • this is not fully explained; scientists propose dark energy
   • the motion of galaxies needs extra unseen mass, called dark matter, which is not yet understood
   • conclusion: the Big Bang theory is the best current explanation and is supported by strong evidence, but some observations remain unexplained
   Level 3 (5–6 marks): a balanced evaluation using evidence that supports the theory and observations it does not yet fully explain, with a justified conclusion; Level 2 (3–4 marks): describes supporting evidence with some reference to limitations, or a conclusion with limited support; Level 1 (1–2 marks): simple relevant statements about red-shift or the Big Bang.

More reading: See all 30 GCSE Physics comprehension articles