7.13P Explain why the red-shift of galaxies provides evidence for the…

The idea

Red-shift is the stretching of light from a receding source towards longer wavelengths. When astronomers measure light from distant galaxies, the dark spectral lines appear at longer wavelengths than the same lines measured from a stationary laboratory source. The greater the shift, the faster the galaxy is receding. Because almost every galaxy shows this shift, and the shift grows with distance, the galaxies are moving apart from one another. Running the motion backwards places all matter close together in the past, so the whole Universe is expanding. The key reasoning is that red-shift is observed, recession is inferred, and expansion of space itself explains both.

What earns the marks

• States that red-shift means light from a receding galaxy is shifted to longer wavelengths.
• Links the measured shift of spectral lines to the galaxy moving away from the observer.
• Explains that the greater the red-shift, the faster the galaxy recedes.
• Uses the pattern that more distant galaxies show greater red-shift to conclude that the Universe is expanding.

Common mistakes and how to put them right

• Confuses red-shift with a change in the colour of the whole galaxy.

  Put it right: Keep the focus on the shift of spectral lines to longer wavelengths.

• Claims that galaxies are moving through stationary space only.

  Put it right: Add that the space between galaxies is itself expanding.

• Treats red-shift as proof of the Big Bang alone.

  Put it right: Keep the answer on why it supports an expanding Universe.

Exam tips

• Name the observation first: spectral lines from galaxies appear at longer wavelengths than expected.
• State the inference explicitly: the galaxy is receding, and greater shift means greater speed.
• Finish with the conclusion that the Universe is expanding, linking distance and shift.