IGCSE 0625 / Section 6.2.3 / Extended
Supplement (Extended) content

The afterglow of everything.

Point a radio dish at the empty dark between the stars and a faint hiss comes back, from every direction at once. It is the cooled afterglow of the young universe, and the strongest clue we have that the Big Bang really happened.

The key idea

The cosmic microwave background is faint microwave radiation reaching us from every direction, the cooled afterglow of the early universe and strong evidence for the Big Bang. The Hubble constant links a galaxy’s recession speed to its distance and can be used to estimate the age of the universe.

Definition · learn the exact words

The cosmic microwave background (CMB) is microwave radiation coming from all directions in space, left over from the early universe. It is strong evidence for the Big Bang Theory.

recession speed = Hubble constant × distance

The Hubble constant relates a galaxy’s speed to its distance, and its value can be used to estimate the age of the universe.

Section 01

Listen to the background.

Detect the microwave background coming evenly from every direction in space.

Stage 1 · Learn

Check cosmic microwave background

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

Quick check+10 XP

The cosmic microwave background is...

Quick check+10 XP

The cosmic microwave background is strong evidence for the...

Quick check+10 XP

The CMB is the leftover radiation from...

Quick check+10 XP

The Hubble constant relates a galaxy’s recession speed to its...

Section 02

Afterglow and the Hubble constant.

The CMB is the cooled glow of the early universe, and the Hubble constant ties speed to distance.

Examiner trap

The cosmic microwave background comes from every direction in space, not from a single object. It is the cooled afterglow of the early universe and one of the strongest pieces of evidence for the Big Bang.

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

A key feature of the cosmic microwave background is that it comes...

Exam style+20 XP

The cosmic microwave background supports the Big Bang because it is...

Exam style+20 XP

The Hubble constant can be used to estimate the...

Skill unlocked

Cosmic microwave background, mastered.

The cosmic microwave background completes the Universe strand and Unit 6. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
The cosmic microwave background as evidence for the Big Bang and the Hubble constant relating recession speed to distance. 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 cosmic microwave background.
Start the unit challenge →