IGCSE 0625 / Section 6.2.3 / Core

A universe that expands.

Light from distant galaxies arrives stretched, shifted toward the red. The further the galaxy, the bigger the stretch, telling us that everything is rushing apart, and hinting that it all began, long ago, from a single point.

The key idea

Light from distant galaxies is redshifted, stretched to longer wavelengths, showing the galaxies are moving away from us, with more distant ones receding faster. This expansion supports the Big Bang Theory, that the universe began from a tiny point and has been expanding ever since.

Definition · learn the exact words

Redshift is the increase in the wavelength of light from distant galaxies, showing they are moving away from us. More distant galaxies recede faster.

redshift → galaxies receding → universe expanding

This supports the Big Bang Theory, that the universe began from a single point and has been expanding ever since.

Section 01

Stretch the light.

See light from receding galaxies shifted toward the red, more so for the most distant.

Stage 1 · Learn

Check redshift big bang

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Redshift means the wavelength of light from a galaxy is...

Quick check+10 XP

Redshift of a galaxy shows that it is...

Quick check+10 XP

More distant galaxies are moving away...

Quick check+10 XP

The expansion of the universe supports the...

Section 02

From redshift to Big Bang.

Receding galaxies mean an expanding universe, which points back to a Big Bang.

Examiner trap

Redshift means the light’s wavelength is increased, stretched, which shows distant galaxies are moving away from us. The further away a galaxy is, the faster it recedes, which is the evidence that the universe is expanding.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

Light from a distant galaxy is found to be redshifted. This shows the galaxy is...

Exam style+20 XP

That more distant galaxies show greater redshift tells us that...

Exam style+20 XP

The Big Bang Theory states that the universe...

Skill unlocked

Redshift big bang, mastered.

Redshift and the Big Bang are mapped. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
Redshift as stretched wavelength showing galaxies recede, the faster-with-distance pattern, and the Big Bang Theory. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Space physics challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just redshift big bang.
Start the unit challenge →