Observational evidence is what is detected or measured. A model conclusion is the explanation built to fit that evidence. Questions in this topic often ask you to “explain how this observation supports the model”, and the marks depend on keeping those two parts separate and linking them.
This lesson closes the main content of space physics.
How do you sort a statement?
Ask two questions about any sentence:
- Could a telescope or detector record it directly? If yes, it is an observation. Example: “The light from distant galaxies has longer wavelengths than expected.”
- Does it say why, or describe something nobody can watch directly? If yes, it is a model conclusion. Example: “The Universe is expanding.”
Then link them with a verb of support, such as “suggests”, “is consistent with” or “supports”. The link sentence is where most of the marks are won.
The two main pieces of evidence for the Big Bang model in this topic are:
- Redshift of light from distant galaxies, which suggests galaxies are moving apart, so the Universe is expanding.
- Cosmic microwave background radiation (CMBR), detected from all directions, which is consistent with radiation left from an early, hot and dense Universe.
Check the Cambridge syllabus for the statements in your exam year, including whether the wording “Big Bang” is used.
Worked example
Question (invented): Sort these statements into observations and model conclusions, then write one sentence linking a pair.
A. Light from a distant galaxy is found to have a longer wavelength than light from the same element in a laboratory.
B. The Universe began from a very small, hot, dense state and has been expanding since.
C. Microwave radiation is detected from every direction in the sky.
D. The galaxies are moving away from us.
Sorting:
- A is an observation: measured with a spectrum.
- B is a model conclusion: no one observed the beginning directly.
- C is an observation: detected with an antenna.
- D is a conclusion from observation A. The speed is inferred from the wavelength shift, so it sits between the two.
Linking sentence: “The redshift of light from distant galaxies (A) suggests that galaxies are moving apart, which is consistent with a Universe that has been expanding since a hot, dense start (B).”
The mistake to watch for
A common slip is to write a model conclusion in the evidence slot, or to claim that evidence proves a model.
Mistaken answer: “The evidence for the Big Bang is that the Universe began as a tiny point.”
That is the model itself, so it cannot be the evidence for it.
The correction: “The evidence is that light from distant galaxies is redshifted and that microwave radiation arrives from all directions. This supports the Big Bang model.” Name the observation first, then connect it.
Check yourself
1. Is “microwave radiation arrives from all directions” an observation or a model conclusion?
Show answer
It is an observation. It can be detected directly with a suitable antenna.
2. Rewrite “Redshift proves the Universe is expanding” so that it reads as careful scientific wording.
Show answer
“The redshift of light from distant galaxies suggests that they are moving away from us, which supports the model of an expanding Universe.”
3. Give one reason why a scientific model might be revised even after many observations agree with it.
Show answer
A new observation might not fit the model. The model would then need changing or replacing, because a model is judged by how well it explains all the evidence.
Where this leads next
Test the whole topic with the space physics practice set. The habit of separating evidence from explanation also helps in physics investigations and explanations.
If your written answers read as a list of true facts without a clear chain of reasons, online one-to-one Physics tuition gives you a teacher who reads the chain with you.