The Universe and its OriginsAQA GCSE Physics: Revision notes
Section 1
What is red-shift?
When light from most distant galaxies is analysed, its wavelengths are found to be longer than expected — this is called red-shift, because the light shifts towards the red end of the spectrum.
- The further away a galaxy is, the faster it is moving away from us
- The faster it moves away, the greater the observed red-shift
This pattern was observed across galaxies in many different directions, suggesting something fundamental about the universe as a whole, rather than something local to our galaxy.
Red-shift is like the change in pitch of a passing ambulance siren (the Doppler effect) — but for light instead of sound, and it stretches the wavelength as the source moves away.
Section 2
What does red-shift tell us about the universe?
The observed red-shift of light from distant galaxies provides evidence that:
- Space itself is expanding — galaxies are not simply moving through fixed space, but the space between them is stretching
- This expansion supports the Big Bang theory of the origin of the universe
Be precise in exams: red-shift is evidence that the universe is expanding, which in turn supports (but on its own does not fully prove) the Big Bang theory.
Section 3
What is the Big Bang theory?
The Big Bang theory states that the universe began from a very small region that was extremely hot and dense, and has been expanding ever since.
This single, simple idea explains a wide range of observations, including the pattern of red-shift seen across all distant galaxies.
Section 4
How do supernova observations affect our understanding of the universe's expansion?
Since 1998, observations of distant supernovae have shown something surprising: distant galaxies appear to be receding (moving away) at an ever-increasing rate — the expansion of the universe is accelerating, not slowing down.
This matters because:
- Each galaxy's speed of recession increases with its distance from us — this pattern is direct evidence of an expanding universe
- The changing red-shift observed over time in these supernovae supports the idea that expansion is speeding up, adding further support to the Big Bang model
Before 1998, scientists generally expected the universe's expansion to be slowing down due to gravity; supernova observations overturned this expectation.
Section 5
What is still not understood about the universe?
Despite strong evidence for an expanding universe and the Big Bang theory, much about the universe remains unexplained, including:
- Dark matter — matter that cannot be directly observed but whose gravitational effects can be detected
- Dark energy — a proposed explanation for the observed acceleration in the universe's expansion
Scientists continue to investigate these using new observations and evidence, but currently neither dark matter nor dark energy is fully understood.
When asked to evaluate scientific understanding of the universe, always mention that dark matter and dark energy remain open, unresolved questions — this shows awareness that science is an ongoing process.
Must Know
- Red-shift: light from distant galaxies shows increased wavelength; the further away, the greater the red-shift and the faster it is receding
- Red-shift is evidence that space itself is expanding, supporting the Big Bang theory
- The Big Bang theory: the universe began from a very small, extremely hot and dense region
- Since 1998, supernova observations show that distant galaxies are receding at an ever-increasing (accelerating) rate
- Much about the universe is still not understood, including dark matter and dark energy
That's the notes covered.
Carry on to the next subtopic.