Red-Shift & the Expanding UniverseOxford AQA IGCSE Physics: Revision notes
Section 1
What is the Doppler effect?
The Doppler effect occurs when a wave source moves relative to an observer, causing the observed wavelength and frequency to change.
- When the source moves away from the observer: the observed wavelength increases and the frequency decreases
- When the source moves towards the observer: the observed wavelength decreases and the frequency increases
This effect applies to any wave, including sound and light.
Think of a siren on a passing ambulance: the pitch sounds higher (shorter wavelength) as it approaches and lower (longer wavelength) as it moves away — the same principle applies to light from galaxies.
Section 2
What is red-shift?
Red-shift is the Doppler effect applied to light from distant galaxies.
- Light from most distant galaxies shows an increase in wavelength — shifted towards the red end of the spectrum
- This means those galaxies are moving away from us
- The further away a galaxy is, the faster it is moving away, and the greater the red-shift observed
Red-shift is strong evidence that the universe is expanding, and it supports the Big Bang model of the universe's origin.
Always connect red-shift to two conclusions in your answer: (1) the universe is expanding, and (2) this supports the Big Bang model.
Section 3
What is cosmic microwave background radiation?
Cosmic microwave background radiation (CMBR) is electromagnetic radiation that fills the entire universe.
- It originates from shortly after the beginning of the universe
- It can be detected coming from every direction in space
- Its existence is strong evidence for how the universe began
Section 4
What is the Big Bang model?
Scientists believe the universe began with a Big Bang approximately 14 thousand million years ago — an event in which all matter and energy in the universe began expanding from an extremely hot, dense point.
The Big Bang theory is currently the only theory that can explain the existence of the cosmic microwave background radiation, which is why it is the accepted model for the origin of the universe.
Students often only mention red-shift as evidence for the Big Bang. CMBR is equally important evidence — and is described as the only phenomenon the Big Bang theory alone can explain.
Must Know
- The Doppler effect: a moving source causes a change in observed wavelength/frequency — wavelength increases and frequency decreases when the source moves away
- Red-shift is an increase in wavelength of light from distant galaxies, showing they are moving away from us
- The further away a galaxy, the greater its red-shift and the faster it is moving away
- Red-shift is evidence the universe is expanding and supports the Big Bang model
- Cosmic microwave background radiation (CMBR) fills the universe and originates from shortly after the universe began
- The Big Bang theory is the only theory that explains the existence of CMBR; the universe began approximately 14 thousand million years ago
That's the notes covered.
Carry on to the next subtopic.