(h) model of an expanding universe supported by galactic red shift

The idea

The model of an expanding universe states that space itself is expanding, causing galaxies to move apart from one another. This is supported by the observation of galactic red shift: light from distant galaxies is shifted towards longer wavelengths, indicating that they are receding. Hubble’s law shows that recessional speed is proportional to distance, which is consistent with a uniform expansion. The red shift is not due to galaxies moving through space but to the expansion of space between them. You should be able to explain how red shift data supports the expanding universe model and distinguish it from alternative explanations.

What earns the marks

• The expanding universe model proposes that space itself is expanding.
• Galactic red shift is the observation that spectral lines from distant galaxies are shifted towards longer wavelengths.
• Red shift indicates that galaxies are receding from the observer.
• Hubble’s law (v ≈ H₀ d) shows that recessional speed increases with distance, supporting uniform expansion.
• The red shift is interpreted as evidence for the expansion of space, not motion through space.

Common mistakes and how to put them right

• Thinking that galaxies are moving through space away from a central point.

  Put it right: Space itself is expanding, so galaxies are carried apart.

• Believing that red shift is caused by dust or other absorption.

  Put it right: Red shift is a Doppler shift due to relative motion, not absorption.

• Assuming that the universe has a centre.

  Put it right: The expansion is uniform and has no centre.

• Confusing red shift with redshift due to gravitational effects.

  Put it right: Galactic red shift is primarily cosmological, due to expansion.

Exam tips

• Link red shift to the Doppler effect and Hubble’s law when explaining the expanding universe.
• Use the phrase ‘space itself is expanding’ to distinguish from motion through space.
• In multiple-choice questions, look for options that correctly describe red shift as evidence for expansion.