Cosmology: (j) the Big Bang theory

The idea

The Big Bang theory states that the universe began from an extremely hot, dense state about 1.4 × 10¹⁰ years ago and has been expanding and cooling ever since. It is not an explosion of matter into pre-existing space; rather, space-time itself expands, carrying galaxies apart. Evidence includes Hubble's law, the cosmic microwave background radiation at 2.7 K, and the observed abundances of light elements such as hydrogen and helium. A useful check is that the theory predicts a hot, dense early universe, so a faint uniform microwave glow should fill the sky, which is exactly what Penzias and Wilson detected in 1965.

What earns the marks

• The universe began from a very hot, very dense state at a finite time in the past.
• Space-time itself has been expanding since that origin, so galaxies move apart.
• The theory predicts a hot early universe that has cooled as it expanded.
• Key evidence: Hubble's law, the 2.7 K cosmic microwave background, and light-element abundances.
• The age of the universe is roughly 1.4 × 10¹⁰ years, consistent with 1/H₀.

Common mistakes and how to put them right

• Describing the Big Bang as an explosion of matter into empty space.

  Put it right: The correct picture is expansion of space-time itself, with no centre or edge.

• Claiming the theory explains only the motion of galaxies.

  Put it right: It also predicts the microwave background and light-element abundances.

• Saying the universe has a centre.

  Put it right: In the standard model every observer sees galaxies receding, so there is no privileged centre.

Exam tips

• Link each piece of evidence explicitly to a prediction of the theory rather than listing evidence loosely.
• Use the phrase 'expansion of space-time' rather than 'expansion through space' to show precise understanding.
• Quote the age of the universe as about 1.4 × 10¹⁰ years and connect it to 1/H₀.