Galactic Redshift as Evidence for an Expanding Universe — Cosmology | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Red-shift, the Big Bang and expansion of the universe regularly tested as explain/describe questions.

What galactic redshift tells us

Two key observations from looking at galaxies all across the sky:

Every distant galaxy is redshifted; they are all moving away from the Earth

The further away a galaxy is, the greater the redshift, so the faster it is moving away

This relationship, distance proportional to recession speed, is known as Hubble's law
It tells us that the Universe is expanding uniformly. Each galaxy sees every other galaxy receding, just like dots on the surface of an inflating balloon

Why this is evidence for the Big Bang

The logic chain runs as follows:

Distant galaxies show redshift → they are moving away from us
The further away a galaxy, the bigger its redshift → distant galaxies are moving away faster
If you rewind time, all galaxies must once have been very close together
Rewind enough, and the entire Universe was concentrated in a single small region
That fits the Big Bang picture: a hot dense beginning, followed by 14 billion years of expansion

The galactic redshift observations were one of the first major pieces of evidence for the Big Bang, first measured by Edwin Hubble in the 1920s, and refined ever since with better telescopes and supernova observations. The discovery of the CMB in 1964 then provided the second, independent piece of evidence.

What galactic redshift does NOT mean

The galaxies are not moving through space away from a centre. Instead, space itself is expanding between them, carrying them apart (the balloon analogy)
There is no single point in space that the Universe is expanding from; every point is moving away from every other point
The Earth is not at the centre of the Universe. Any observer in any galaxy would see all the other galaxies redshifted by the same amount, just as every dot on the inflating balloon sees all the other dots receding

Exam tip

Explaining how redshift supports the Big Bang (3–4 marks)

What comes up: "Explain how these observations give evidence for the Big Bang theory" — typically 3–4 marks, using data about galaxies at different distances showing different amounts of redshift.

Write: work through the chain — (1) light from galaxies is redshifted, so galaxies are moving away from us; (2) galaxies that are further away show greater redshift, meaning they are receding faster; (3) all galaxies receding in this pattern means the universe is expanding; (4) tracing that expansion back in time means all matter must once have been concentrated in a single point — consistent with the Big Bang.

Watch out: for a question specifically about redshift observations, the mark scheme does not award credit for mentioning the cosmic microwave background — keep the answer focused on the redshift evidence chain. Also, stating that "the universe is expanding" alone is not enough for the final mark; you must link it back to the idea that the universe had a single origin point.