Students should be able to explain: qualitatively the red-shift of light…

The idea

Light from galaxies is red-shifted; the shift is larger for more distant galaxies; so the further away a galaxy is the faster it recedes; so space is expanding everywhere; so running the expansion backwards gives a hot dense beginning, which is the Big Bang. Then the honest ending. Theories are built from observations and are changed when new observations do not fit, which is what happened in 1998. Much of the universe is still unaccounted for: dark mass, also called dark matter, is needed to explain the way galaxies rotate, and dark energy is needed to explain the accelerating expansion. Neither has been detected directly.

What earns the marks

• Red-shift showing that galaxies are receding, with a larger shift for more distant galaxies.
• Interpreting this as evidence that the universe is expanding.
• Tracing the expansion back to a hot dense start as evidence for the Big Bang model.
• Stating that theories are changed when new observations do not fit the current model.
• Naming dark mass or dark energy as evidence that much about the universe is not understood.

Common mistakes and how to put them right

• Presenting the Big Bang as a proven fact rather than as the theory that best fits the current observations.

• Confusing dark matter, which is about missing mass, with dark energy, which is about the accelerating expansion.

• Saying scientists change theories because they made a mistake, rather than because new evidence appeared.

• Listing observations without drawing a conclusion from any of them.

Exam tips

• Write the argument as a chain of so statements, because each link is a mark.
• Name dark mass and dark energy separately and say what each was proposed to explain.
• Use the phrase the current model to show that you understand science is provisional.