Revision notes for Edexcel GCSE Physics The Big Bang, red-shift and the expanding Universe. Open the guide for explanations and worked examples. Written against the Edexcel GCSE Physics (1PH0) specification, so the content matches what's examinable rather than general Physics background.
The Big Bang, red-shift and the expanding Universe
What you'll learn
- How moving wave sources change the frequency and wavelength observed.
- What red-shift means in light from distant galaxies.
- Why red-shift is evidence that the Universe is expanding.
- How the Big Bang and Steady State theories compare, and why CMB radiation matters.
This Edexcel Astronomy content is marked P, so it is Separate Physics content. Since you are studying 1PH0 Physics, it is part of your course.
The wave ideas you need first
Light is an electromagnetic wave: a wave that can travel through empty space and transfer energy. Visible light is only one small part of the electromagnetic spectrum.
Definition
Frequency and wavelength
- Frequency, symbol fff, is the number of complete waves passing a point each second. It is measured in hertz, Hz.
- Wavelength, symbol λ\lambdaλ, is the distance from one point on a wave to the same point on the next wave, such as crest to crest. It is measured in metres, m.
- Wave speed, frequency and wavelength are linked by v=fλv=f\lambdav=fλ.
For light travelling through space, the speed is constant. So if its wavelength becomes longer, its frequency must become lower.
Example
Comparing red and blue light
- Use the wave relationship v=fλv=f\lambdav=fλ. For light in space, vvv stays the same.
- Red visible light has a longer wavelength than blue visible light.
- Since vvv is unchanged, a larger λ\lambdaλ means a smaller fff, so red light has a lower frequency than blue light.
Moving sources: the Doppler effect
A source is something producing waves, such as a star producing light. An observer is something detecting the waves, such as a telescope on Earth. Relative motion means motion of the source compared with the observer.
Definition
Doppler effect
The Doppler effect is the change in observed frequency and wavelength when a wave source moves relative to an observer.
If a source moves towards you, the waves are squashed closer together: wavelength decreases and frequency increases. If a source moves away from you, the waves are stretched out: wavelength increases and frequency decreases.
Example
Predicting a Doppler shift
- Decide the relative motion: the galaxy is moving away from Earth, so the light waves are stretched as they reach us.
- Apply the Doppler effect: the observed wavelength is longer than the wavelength originally emitted by the galaxy.
- Since light in space has constant speed, v=fλv=f\lambdav=fλ means the observed frequency is lower. In visible light, this is a shift towards the red end of the spectrum.
Red-shift in galaxy light
A galaxy is a huge collection of stars, gas and dust held together by gravity. Astronomers study galaxies by splitting their light into a spectrum, which is the range of wavelengths present. Some elements produce dark absorption lines at particular wavelengths, like a barcode.
Definition
Red-shift
Red-shift is when light received from a galaxy has its absorption-line pattern shifted towards longer wavelengths, towards the red end of the visible spectrum.
Astronomers compare absorption lines from a galaxy with the same lines measured in a laboratory on Earth. The same pattern appears, but for distant galaxies it is shifted towards longer wavelengths. More distant galaxies usually show a larger red-shift.
Common Mistake
Red-shift is not just a colour change
Red-shift does not mean the whole galaxy simply “looks red”. It means the measured wavelengths in its spectrum are longer than expected.
Example
Interpreting shifted absorption lines
A particular absorption line is measured at 500 nm in a laboratory. The same line is seen at 510 nm from Galaxy A and 540 nm from Galaxy B.
- Compare each galaxy value with the laboratory value: ΔλA=510 nm−500 nm=10 nm\Delta\lambda_A = 510\ \text{nm} - 500\ \text{nm} = 10\ \text{nm}ΔλA=510 nm−500 nm=10 nm, while ΔλB=540 nm−500 nm=40 nm\Delta\lambda_B = 540\ \text{nm} - 500\ \text{nm} = 40\ \text{nm}ΔλB=540 nm−500 nm=40 nm.
- Both observed wavelengths are longer than the laboratory wavelength, so both galaxies show red-shift and are moving away from Earth.
- Galaxy B has the larger wavelength shift, so it has the larger red-shift. At GCSE, this is taken as evidence that Galaxy B is more distant and receding faster than Galaxy A.
Red-shift as evidence for an expanding Universe
The Universe means all of space and everything in it. Observations show that light from most distant galaxies is red-shifted, and the red-shift is larger for galaxies farther away. The best explanation is that space itself is expanding, so the distances between galaxies increase over time.
Key Idea
Why red-shift suggests expansion
If galaxies in all directions are moving away from us, Earth is not necessarily at the centre. Instead, space is expanding, so distant galaxies become farther apart from every viewpoint.
You can think of the light waves being stretched as space expands while the light travels across the Universe. Longer wavelength means red-shift.
Common Mistake
Nearby galaxy exceptions
A few nearby galaxies can be blue-shifted because local gravitational motion can make them move towards us. The GCSE evidence for expansion is about the overall pattern for distant galaxies.
Big Bang theory and Steady State theory
A scientific model is an explanation that can be tested against evidence. Two important models for the Universe are the Big Bang theory and the Steady State theory.
Definition
Big Bang and Steady State
- The Big Bang theory says the Universe began in a very hot, dense state and has expanded ever since.
- The Steady State theory says the Universe has always existed and, on a large scale, looks the same over time. It includes expansion, but suggests new matter is continuously created to keep the average density constant.
| Feature | Big Bang theory | Steady State theory |
|---|---|---|
| Beginning | Universe began hot and dense | Universe has no beginning |
| Expansion | Space has expanded since the beginning | Space expands continuously |
| Matter | Matter was present in the early Universe | New matter is continually created |
| Red-shift | Explained by expansion of space | Also explained by expansion of space |
| CMB radiation | Expected as leftover radiation from early Universe | Not naturally explained |
Both theories can account for red-shift because both include an expanding Universe. Red-shift alone supports expansion, but it does not by itself prove which origin model is correct.
Example
Explaining why red-shift alone is not enough
- Start with the observation: distant galaxies show red-shift, so their light has been stretched to longer wavelengths.
- Link this to motion and expansion: the galaxies are receding, which supports the idea that the Universe is expanding.
- Compare the models: Big Bang theory includes expansion, but Steady State theory also includes expansion, so red-shift alone cannot choose between them.
Cosmic microwave background radiation
Definition
CMB radiation
Cosmic microwave background radiation, or CMB radiation, is microwave electromagnetic radiation arriving from all directions in space. It is evidence of leftover energy from the early Universe.
In the Big Bang model, the early Universe was extremely hot and dense, so it was filled with radiation. As the Universe expanded, this radiation was stretched to much longer wavelengths and cooled until it became microwave radiation. The discovery of the CMB was strong evidence for the Big Bang theory.
The Steady State theory did not naturally predict a Universe filled with this leftover microwave radiation. Because there is more evidence supporting the Big Bang theory than the Steady State theory, the Big Bang is the currently accepted model for the origin of the Universe.
Example
Explaining why CMB favours the Big Bang
- Identify what CMB radiation is: microwave radiation detected from all directions in space.
- Link it to the Big Bang model: a hot, dense early Universe would leave behind radiation, which would be stretched and cooled as space expanded.
- Compare with Steady State: Steady State can explain red-shift through expansion, but it does not naturally explain the CMB, so the CMB makes the Big Bang model better supported.
Tip
Best evidence pair
For this topic, remember: red-shift shows expansion, while CMB radiation supports a hot, dense beginning. Together, they support the Big Bang theory.
Exam technique
In the exam
- For red-shift, always state both parts: observed wavelength increases and observed frequency decreases.
- To explain expansion, link red-shift from distant galaxies to galaxies receding, then to space itself expanding.
- If comparing Big Bang and Steady State, say that both can explain red-shift because both include expansion.
- To explain why Big Bang is accepted, add the CMB evidence: it is leftover microwave radiation from the early hot Universe.
Self review
Check yourself
- If a galaxy’s absorption lines are shifted to longer wavelengths, what does that tell you about its motion relative to Earth?
- Why does red-shift support expansion but not, by itself, prove the Big Bang theory?
- How does CMB radiation support the idea that the Universe was once very hot and dense?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
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