Cosmology
Edexcel International GCSE PhysicsΒ· 8.13Pβ8.18PΒ· 20 min read
1. The Big Bang Theory and Supporting Evidenceβ β βββHigher onlyβ± 5 min
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The Big Bang theory is the leading scientific model for the origin and evolution of the universe. It states that approximately 13.8 billion years ago, all matter and energy in the universe was concentrated in an extremely hot, dense, tiny point. This point then expanded rapidly, and the universe has been expanding and cooling ever since.
Big Bang Theory
Model stating the universe began from a single hot, dense point ~13.8 billion years ago and has expanded continuously since.
Two key observational pieces of evidence support the Big Bang theory: red-shift of distant galaxies, and cosmic microwave background (CMB) radiation. CMB is uniform low-energy microwave radiation detected across all directions in space, the leftover 'afterglow' from the hot early universe shortly after the Big Bang.
Cosmic Microwave Background Radiation
Uniform microwave radiation across the sky, formed when the early universe cooled enough for atoms to form ~380,000 years after the Big Bang.
A student claims CMB radiation is direct evidence the universe was once very hot. Explain why this statement is correct.
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Step 1: Recall that CMB was released when the early universe was extremely hot, at a temperature of ~3000 K.
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Step 2: At release, this radiation was high-energy visible and gamma radiation, consistent with a very hot environment.
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Step 3: As the universe expanded, the radiation stretched to longer microwave wavelengths we detect today. Its uniform presence matches Big Bang predictions, confirming the early universe was very hot.
Exam tip:
For 6-mark Big Bang evidence questions, you must name both red-shift and CMB, and explain how each supports the theory to gain full marks.
2. Doppler Effect and Galactic Red-shiftβ β β ββHigher onlyβ± 6 min
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The Doppler effect describes the change in observed frequency and wavelength of a wave when its source moves relative to an observer. If a source moves away from an observer, waves are stretched, so observed wavelength increases and frequency decreases. If a source moves towards the observer, waves are compressed, so observed wavelength decreases and frequency increases.
Red-shift
Increase in observed wavelength of light from distant galaxies, caused by the galaxy moving away (receding) from Earth.
When light from a galaxy is analysed, we compare the observed wavelength of spectral lines to their known rest wavelength (from laboratory measurements). If the galaxy is receding, the lines are shifted to longer, redder wavelengths: this is red-shift. More distant galaxies show larger red-shifts, meaning they are receding faster than closer galaxies.
Explain why larger red-shifts in more distant galaxies support the idea the universe is expanding.
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Step 1: Link red-shift to motion: a larger red-shift means a galaxy is receding at a faster speed.
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Step 2: Relate distance to speed: more distant galaxies recede faster than closer galaxies.
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Step 3: Connect to expansion: if all galaxies are moving away from each other, and more distant ones move faster, this means the entire universe is expanding, as predicted by the Big Bang theory.
Exam tip:
When explaining red-shift as expansion evidence, always make the explicit link between increasing red-shift with distance, increasing recessional speed, and universal expansion to get all available marks.
3. Red-shift Calculations using \(\frac{\Delta \lambda}{\lambda_0} = \frac{v}{c}\)β β β β βHigher onlyβ± 7 min
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You must recall the red-shift formula for your exam, as it is not provided on the formula sheet. The formula relates the change in wavelength of light from a galaxy to its recessional velocity:
Where: (\Delta \lambda) = observed wavelength minus rest wavelength (increase in wavelength), (\lambda_0) = rest wavelength of the spectral line, (v) = recessional velocity of the galaxy (m/s), (c) = speed of light (~3 Γ 10βΈ m/s, given in exam questions if required). Note that (\Delta \lambda) and (\lambda_0) must be in the same units, as their ratio is dimensionless.
A spectral line from a distant galaxy has a rest wavelength of 656 nm. The observed wavelength is 689 nm. Calculate the recessional velocity of the galaxy. Use (c = 3 \times 10^8 \text{ m/s}).
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Step 1: Calculate (\Delta \lambda): (\Delta \lambda = 689 - 656 = 33 \text{ nm})
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Step 2: Rearrange the formula to solve for (v): (v = c \times \frac{\Delta \lambda}{\lambda_0})
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Step 3: Substitute values (units for (\Delta \lambda) and (\lambda_0) match so no conversion is needed): (v = 3 \times 10^8 \times \frac{33}{656})
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Step 4: Calculate the result: (v \approx 1.51 \times 10^7 \text{ m/s}) (3 significant figures, matching given values)
Exam tip:
Always show your rearrangement of the formula, check units are consistent for (\Delta \lambda) and (\lambda_0), and use standard form for large velocities to avoid arithmetic errors.
4. Key Concept Summary and Checkβ β βββHigher onlyβ± 2 min
Which of the following is NOT evidence for the Big Bang theory?
A: Red-shift of distant galaxies
B: Cosmic microwave background radiation
C: The existence of black holes
D: Uniform temperature of CMB across the sky
Reveal answer
C βBlack holes are a prediction of general relativity but are not direct evidence for the Big Bang. The other three options all support the model.
A galaxy shows a (\Delta \lambda) of 12 nm for a rest wavelength of 480 nm. What is its recessional velocity? Use (c = 3 \times 10^8 \text{ m/s})
A: 7.5 Γ 10βΆ m/s
B: 1.2 Γ 10β· m/s
C: 3 Γ 10βΈ m/s
D: 4 Γ 10β· m/s
Reveal answer
A β(v = c \times \Delta \lambda/\lambda_0 = 3 \times 10^8 \times 12/480 = 7.5 \times 10^6 \text{ m/s})
5. Common Pitfalls
Wrong move:
Confusing red-shift as galaxies moving towards Earth
Why:
Red-shift is an increase in wavelength, which only occurs when a wave source moves away from an observer, not towards them.
Correct move:
Remember red = longer wavelength, so stretched light waves = the galaxy is receding from Earth.
Wrong move:
Using different units for (\Delta \lambda) and (\lambda_0) in calculations
Why:
The ratio of wavelengths is dimensionless, so mismatched units will produce an incorrect value for recessional velocity.
Correct move:
Convert both wavelengths to the same unit (e.g. nm or m) before substituting values into the red-shift formula.
Wrong move:
Only naming one piece of evidence for the Big Bang in 6-mark extended response questions
Why:
Full marks require you to explain both red-shift and CMB radiation, not just one evidence type.
Correct move:
Structure answers to cover both evidence types, with clear explanations of how each supports the Big Bang model.
Wrong move:
Expecting the red-shift formula to be provided on the exam formula sheet
Why:
The (\Delta \lambda/\lambda_0 = v/c) formula is not included on the formula sheet for 4PH1 exams.
Correct move:
Memorize the formula and its rearrangement for (v) as part of your Paper 2 revision.
Wrong move:
Stating red-shift directly proves the Big Bang without linking to expansion
Why:
Red-shift first proves the universe is expanding, which is a core prediction of the Big Bang model.
Correct move:
Explicitly follow the logical chain: red-shift β recessional motion β universal expansion β supports Big Bang in explanation questions.
6. Quick Reference Cheatsheet
Concept | Key Details | Exam Reminder |
|---|---|---|
Big Bang Theory | Universe began as hot dense point ~13.8bn years ago, expanded ever since | Name both evidence types for 6-mark questions |
CMB Radiation | Uniform microwave 'afterglow' from early hot universe | Links directly to the hot origin of the universe |
Red-shift | Increase in observed wavelength of light from receding galaxies | Larger red-shift = more distant = faster recession |
Red-shift Formula | (\frac{\Delta \lambda}{\lambda_0} = \frac{v}{c}), (c = 3 \times 10^8 \text{ m/s}) | Must recall this formula, not provided on formula sheet |
Calculation Rules | (\Delta \lambda) and (\lambda_0) must have matching units, (v) in m/s | Use standard form for large values to avoid errors |
7. Frequently Asked
Do I need to remember the red-shift formula for the exam?
Yes, the (\frac{\Delta \lambda}{\lambda_0} = \frac{v}{c}) formula is not provided on the exam formula sheet, so you must recall it for Higher Paper 2 assessments.
Is cosmology included in Foundation tier or Double Award Physics?
No, this entire topic is only assessed in Higher Paper 2 of the Single Award Physics (4PH1) specification, and is excluded from Foundation and Double Award (4SD0) content.
Going deeper
- official_specEdexcel 4PH1 2017 SpecificationRefer to section 8 for Astrophysics content
- practice_setPaper 2 Astrophysics Practice QuestionsContains exam-style red-shift and Big Bang questions
What's Next
Now that you have mastered cosmology content, you are ready to tackle full Paper 2 Astrophysics practice questions, which often combine this topic with content on galaxies, orbital motion, and stellar evolution. This topic is consistently tested in Higher Paper 2, so make sure you practice red-shift calculations and 6-mark extended response questions on Big Bang evidence to maximize your marks. You can also review wave basics to reinforce your understanding of wavelength and frequency, which underpins the Doppler effect and red-shift calculations.