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CosmologyEdexcel IGCSE Physics: Revision notes

Section 1

How did the universe begin?

The Big Bang theory states that the universe began from an extremely hot, dense point and has been expanding and cooling ever since. This is the accepted scientific model for the origin and evolution of the universe.

  • Before the Big Bang, all matter and energy were concentrated in a single point
  • Since then, space itself has been expanding, carrying galaxies apart
  • The universe has been cooling as it expands
Key termsBig Bang theory

Section 2

What is red-shift and why does it matter?

When a wave source moves relative to an observer, the observed frequency and wavelength change. This is called the Doppler effect.

  • If a light source moves away from an observer, the observed wavelength increases (shifts towards red) — this is red-shift
  • If a source moves towards an observer, the wavelength decreases (shifts towards blue) — blue-shift
  • Light from almost all distant galaxies shows red-shift, meaning they are moving away from us

The equation relating the change in wavelength to velocity is:

Δλ/λ0 = v/c

where Δλ = λ − λ0 is the change in wavelength, λ0 is the reference (unshifted) wavelength, v is the velocity of the galaxy, and c is the speed of light.

Key termsDoppler effectred-shiftblue-shift
Example

A galaxy emits light with a reference wavelength of 500 nm. The observed wavelength is 505 nm. Δλ = 5 nm, so v/c = 5/500 = 0.01, giving v = 0.01 × c ≈ 3,000,000 m/s away from Earth.

Section 3

How does red-shift relate to distance?

Observations show that:

  • Almost all galaxies show red-shift, meaning almost all galaxies are moving away from Earth
  • The further away a galaxy is, the greater its red-shift, meaning the faster it is moving away

This relationship between distance and speed of recession is direct evidence that the universe is expanding — space itself is stretching, carrying galaxies further apart over time.

Common mistake

Do not say galaxies are moving through space away from us like an explosion — it is space itself that is expanding, carrying galaxies apart.

Section 4

What other evidence supports the Big Bang?

Two main pieces of evidence support the Big Bang theory:

  1. Red-shift — nearly all galaxies show red-shifted light, and more distant galaxies show greater red-shift, showing the universe is expanding from a single point in the past
  2. Cosmic Microwave Background (CMB) radiation — a faint, near-uniform microwave radiation detected coming from all directions in space. It is the leftover, cooled radiation from the very early, hot universe, and its existence was predicted by the Big Bang theory before it was observed

Together, red-shift and CMB radiation provide strong evidence that the universe began in a hot, dense state and has been expanding and cooling ever since.

Key termsCosmic Microwave Background (CMB) radiation
Exam tip

In exam answers, name BOTH pieces of evidence (red-shift AND CMB) — a question asking to describe evidence expects both, not just one.

Must Know

  • The Big Bang theory says the universe began from a hot, dense point and has been expanding and cooling since
  • Red-shift: the observed wavelength of light increases when its source moves away from the observer
  • Δλ/λ0 = v/c relates the fractional change in wavelength to the velocity of a galaxy
  • More distant galaxies show greater red-shift — this shows the universe is expanding
  • CMB radiation is uniform microwave radiation from all directions, left over from the early universe
  • Red-shift and CMB radiation together are the two key pieces of evidence for the Big Bang theory

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