Red-Shift and the Big Bang Theory — AQA GCSE Physics (Separate)

Red-shift is evidence that the universe is expanding, which supports the Big Bang theory.

Red-shift

When we observe light from distant galaxies, we find that the wavelengths are longer than expected — the light is shifted towards the red end of the spectrum. This is called red-shift.

Red-shift happens because the galaxies are moving away from us. As the source moves away, the wavelength of its light is stretched (increased).

Evidence for an expanding universe

Two key observations:

• All distant galaxies are moving away from us (they show red-shift).
• The further away a galaxy is, the greater its red-shift — meaning more distant galaxies are moving away faster.

This shows that the whole universe is expanding.

The Big Bang theory

The Big Bang theory states that the universe began from a very small, hot, dense region that expanded (and is still expanding). The red-shift evidence supports this idea.

Limits of current understanding

There is still much we do not understand. Observations suggest the expansion of the universe is accelerating, which led to ideas about dark matter and dark energy — areas of ongoing research.

Exam tips

• Red-shift = light from distant galaxies shifted to longer (redder) wavelengths.
• It shows galaxies are moving away → the universe is expanding.
• More distant galaxies have greater red-shift (moving faster).
• This is key evidence for the Big Bang theory.