7.14P Explain how both the Big Bang and Steady State theories of the…

The idea

Both theories accept the observed red-shift: galaxies recede and the shift grows with distance. The Big Bang theory explains this by an initial hot, dense state followed by expansion of space, so galaxies move apart and their light is stretched. The Steady State theory also accepts an expanding Universe but proposes that matter is continuously created so the overall density stays constant, with no beginning. Both therefore account for red-shift, but they differ on origin and on whether the Universe had a start. Later evidence, such as the cosmic microwave background, favoured the Big Bang.

What earns the marks

• States that both theories accept that galaxies are receding and show red-shift.
• Explains that the Big Bang theory attributes red-shift to expansion from an initial hot, dense state.
• Explains that the Steady State theory accepts expansion while proposing continuous creation of matter to keep density constant.
• Distinguishes the theories by noting that only the Big Bang proposes a definite beginning.

Common mistakes and how to put them right

• Claims the Steady State theory denies expansion.

  Put it right by noting it accepts expansion but adds continuous creation of matter.

• Assumes red-shift alone decides between the theories.

  Put it right: Note that both explain it and other evidence is needed.

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

  Put it right: Describe expansion of space from a hot, dense state instead.

Exam tips

• State the shared observation first, then show how each theory explains it.
• Use the words 'continuous creation' and 'constant density' when describing Steady State.
• Point out the difference in origin: a beginning for Big Bang, no beginning for Steady State.