Qualitative treatment of Big Bang theory including evidence from…

The idea

The Big Bang model describes an expanding Universe from a hot, dense early state. Qualitatively, expansion explains cosmological redshift: as space stretches, photon wavelengths increase, so distant galaxies appear redshifted. The cosmological microwave background radiation (CMBR) is thermal radiation at about 2.7 K filling all space; it is relic radiation from when the early Universe became transparent, and its near-uniform temperature with tiny ripples supports an initially hot, dense phase. The observed relative abundance of hydrogen and helium, roughly three parts hydrogen to one part helium by mass, matches predictions from Big Bang nucleosynthesis in the first minutes, when protons and neutrons fused mainly into helium. Together, redshift, the CMBR and light-element abundances provide independent evidence for the model.

What earns the marks

• States that the Universe is expanding and that the early Universe was much hotter and denser than it is now.
• Explains cosmological redshift qualitatively as stretching of photon wavelengths by expansion of space, not as a Doppler shift through a stationary medium.
• Identifies the CMBR as microwave radiation at about 2.7 K coming from all directions and interprets it as relic thermal radiation from the early Universe.
• Links the near-uniform temperature and small ripples in the CMBR to an initially hot, dense state that later cooled and clumped to form galaxies.
• Explains that Big Bang nucleosynthesis in the first minutes produced mainly hydrogen and helium, giving the observed relative abundance of about three parts hydrogen to one part helium by mass.
• Uses at least two independent pieces of evidence to support the Big Bang model rather than relying on a single observation.

Common mistakes and how to put them right

• Treating cosmological redshift as a simple Doppler shift through space.

  Put it right by describing expansion of space stretching photon wavelengths.

• Describing the CMBR as coming only from the centre of the Universe or from a single direction.

  Put it right by stating it is isotropic, arriving from all directions.

• Claiming that all helium was made in stars.

  Put it right by explaining that the observed primordial abundance is too large for stellar production alone and matches Big Bang nucleosynthesis.

• Stating that the CMBR is the same as the light from the first stars.

  Put it right by identifying it as thermal radiation released when the early Universe became transparent, long before stars formed.

Exam tips

• When asked for evidence, name the observation, state what is measured and explain how it supports the Big Bang model in one linked chain.
• Use the phrase 'expansion of space' rather than 'galaxies moving through space' when explaining cosmological redshift.
• For the CMBR, quote the approximate temperature 2.7 K and mention its near-uniformity and small ripples.
• For hydrogen and helium abundance, give the approximate mass ratio three to one and link it to nucleosynthesis in the first minutes.