OCR A · H556 · 5.5.3 Cosmology
Cosmology: Hubble’s law, the Big Bang and the evolving universe
How can a shifted spectral line tell us that the universe is expanding? Follow the evidence from nearby stars to distant galaxies, then travel through the universe’s history—from the first particles to the sky we see today.
Part of our free OCR A Physics resources and Module 5 revision guides.
Use the evidence, then test the explanation
1. Measure
Read distances and spectral shifts. Keep the units visible at every step.
2. Connect
Use a graph gradient to find H₀ and explain what the expanding universe was like earlier.
3. Retrieve
Close the answers. Explain the CMB, order the events and attempt the original questions.
By the end you should be able to define astronomical distance units, calculate parallax distances and redshift, use Hubble’s law in either unit system, estimate a Hubble time, explain the Big Bang evidence, and describe the sequence from particles to galaxies.
5.5.3(a)
Astronomical distances: AU, light-year and parsec
The distances between astronomical objects span too many powers of ten for metres to be convenient in every calculation. These units measure distance, including the light-year.
| Unit | Definition | Useful approximate value |
|---|---|---|
| Astronomical unit, AU | The mean Earth–Sun distance. | 1.50 × 10¹¹ m |
| Light-year, ly | Distance light travels in vacuum in one year. | 9.46 × 10¹⁵ m |
| Parsec, pc | Distance at which 1 AU subtends one arcsecond. | 3.086 × 10¹⁶ m ≈ 3.26 ly |
| Megaparsec, Mpc | One million parsecs. | 3.086 × 10²² m |
One degree contains 3600 arcseconds. A parallax of one arcsecond is therefore about 4.85 × 10⁻⁶ rad. Use the constants supplied in an examination and round at the end.
5.5.3(b–c)
Stellar parallax: a shift in viewpoint
Observe a nearby star against more distant background stars from opposite sides of Earth’s orbit, six months apart. Its apparent direction changes because the observer moves. The parallax angle p is half the full angular shift. Each half of the baseline is 1 AU.
For small p in radians, tan p ≈ p ≈ 1 AU/d. Converting radians to arcseconds and using the parsec definition gives the inverse relation above. It is a nearby-star method: a distant object has a smaller shift and a larger percentage uncertainty for the same angular resolution.
Worked example 1: do not double the distance by mistake
The full six-month shift is 0.100 arcseconds. Then p = 0.0500 arcseconds and d = 1/0.0500 = 20.0 pc ≈ 65.2 ly. Using the full shift as p would give only 10 pc.
Try two-epoch parallax in the lab: capture the two epochs, halve the difference and calculate d before revealing the model value.
5.5.3(d), 5.5.3(l)
What does “the universe is expanding” mean?
The cosmological principle describes the universe on sufficiently large scales:
- Homogeneous: its statistical properties, such as average matter density, are the same from place to place.
- Isotropic: its statistical properties look the same in every direction.
- Universal physical laws: the same laws apply throughout.
This is an average over very large regions. A planet, a star or an individual galaxy is clearly not homogeneous. The principle does not say that the universe stays the same through time.
In the expanding-universe model, separations between widely separated galaxies grow as space-time itself expands. Observers in different galaxies can each see the same distance–recession trend. There is no preferred central galaxy in this model. Gravitationally bound systems, such as the Solar System, do not simply stretch with that expansion.
5.5.3(e–f)
Redshift: read the spectral fingerprint
The Doppler effect is a change in observed frequency or wavelength associated with relative motion of source and observer. Recession gives a longer wavelength and lower frequency; approach gives a shorter wavelength and higher frequency. Identify the same emission or absorption line pattern in the source and a laboratory reference.
Take v as positive for recession. In that case Δλ is positive and Δf is negative. OCR’s fractional frequency-shift form is commonly used for magnitudes; decide whether to increase or decrease the frequency from the direction of motion. A lower frequency still satisfies c = fλ.
For remote galaxies the deeper explanation is cosmological redshift: light stretches while space expands. The low-speed Doppler relation is a useful small-redshift approximation. Do not apply v = cz to large z in the early universe.
Worked example 2: line shift to speed
A line emitted at 600 nm is observed at 612 nm. z = 12/600 = 0.0200. With c = 3.00 × 10⁸ m s⁻¹, v ≈ 6.00 × 10⁶ m s⁻¹ = 6000 km s⁻¹. Wavelengths may both remain in nm because they form a ratio.
Worked example 3: approaching source
An approaching source has speed 0.00200c and emitted frequency 5.00 × 10¹⁴ Hz. The frequency increase is 0.00200 × 5.00 × 10¹⁴ = 1.00 × 10¹² Hz. fobserved ≈ 5.01 × 10¹⁴ Hz.
Change the line, predict the result
Before changing a value, predict the sign of the shift. Then double Δλ at fixed emitted wavelength. What happens to the inferred speed and distance?
At 600 nm → 612 nm: z = 0.0200, v ≈ 6000 km s⁻¹, d ≈ 85.7 Mpc for H₀ = 70 km s⁻¹ Mpc⁻¹.
The live calculator uses c = 299,792,458 m s⁻¹. Rounded worked answers use 3.00 × 10⁸ m s⁻¹. Distances are not inferred for a blueshift or for |z| > 0.1.
5.5.3(g–h)
Hubble’s law: a straight-line relationship
On large scales, more distant galaxies generally have greater recession speeds. This is the trend associated with Edwin Hubble and Georges Lemaître:
v is recession speed, d is distance and H₀ is the present Hubble constant. A graph of v on the vertical axis against d on the horizontal axis has gradient H₀. Its units depend on the axis units.
Worked example 4: keep matching units
For H₀ = 70 km s⁻¹ Mpc⁻¹ and d = 120 Mpc, v ≈ 8400 km s⁻¹. Convert this to 8.40 × 10⁶ m s⁻¹ if the next step uses c in SI units.
Galactic redshift supports expansion because known spectral patterns shift systematically towards longer wavelengths and the recession trend grows with distance. A nearby galaxy can approach us through local gravitational motion. That does not overturn the large-scale pattern.
Fit the trend before revealing the answer
Adjust the line until it follows the data rather than one convenient point. Record your gradient estimate. The model line passes through zero for this introductory exercise; the revealed statistical fit is allowed an intercept.
Choose a line and record its gradient before checking.
Read the six original data pairs
Distance / Mpc → recession speed / km s⁻¹: 20 → 1550; 50 → 3350; 90 → 6500; 130 → 9000; 170 → 12,050; 210 → 14,600. A reasonable graphical estimate is about 70 km s⁻¹ Mpc⁻¹.
Open the full Hubble investigation on this page
Open the Astrophysics & Cosmology Lab full screen
Investigation questions and expected reasoning
Why use a wide distance range? It makes the gradient less sensitive to one point. Why can the closest galaxies show the largest fractional departure? A similar peculiar velocity is a larger fraction of their smaller Hubble recession. Why not use d = v/H₀ to generate the distances and then claim to measure H₀? That would assume the relationship being tested.
5.5.3(i), 5.5.3(m)
The Hubble constant and an age estimate
A value such as 70 km s⁻¹ Mpc⁻¹ means an extra 70 km s⁻¹ of recession speed for each megaparsec of distance. It has dimensions of inverse time.
Alternatively divide by 3.086 × 10¹⁹ if distances are converted to kilometres. Do not lose the factor of 1000.
Extrapolate a galaxy’s current recession speed backwards as if that speed had stayed constant: t ≈ d/v. Substituting v ≈ H₀d gives:
Worked example 5: from a graph gradient to years
For 70 km s⁻¹ Mpc⁻¹, H₀ ≈ 2.27 × 10⁻¹⁸ s⁻¹. Hence t ≈ 4.41 × 10¹⁷ s. Dividing by 3.156 × 10⁷ s per year gives 1.40 × 10¹⁰ years ≈ 14.0 billion years.
A larger H₀ gives a smaller Hubble time. The accepted cosmic age of about 13.8 billion years comes from modelling the expansion history, not simply treating 1/H₀ as an exact clock. Gravity and dark energy affect how expansion changes. The word “constant” names its present measured value; it does not say the Hubble parameter has always stayed unchanged.
For H₀ = 70 km s⁻¹ Mpc⁻¹: H₀ ≈ 2.27 × 10⁻¹⁸ s⁻¹ and 1/H₀ ≈ 14.0 billion years.
Change H₀ in the line calculator above to compare the Hubble-time estimate.
5.5.3(j–l)
The Big Bang: a hot, dense early universe
The hot Big Bang model describes a universe that was once much hotter and denser and has expanded and cooled. It explains the origin of today’s large-scale state, while the earliest instant and any “before” remain questions beyond a complete tested description.
The expansion is not an explosion from one point into surrounding empty space. Space-time is part of the evolving model. Running the expansion backwards leads to smaller separations, higher density and higher temperature.
Evidence for expansion
Known galactic spectral lines are redshifted and recession speed generally increases with distance. The pattern supports a universe whose large-scale separations are increasing.
Evidence for a hot past
The cosmic microwave background has the thermal spectrum expected for relic radiation that has stretched and cooled. It supplies a different kind of evidence from galaxy redshift.
Light-element abundances provide additional context: the hot early universe permits nuclear reactions that make helium and small amounts of other light nuclei. Detailed primordial abundance calculations are beyond this guide.
5.5.3(k)
The cosmic microwave background: why 2.7 K matters
The early ionised universe contained radiation and many free electrons. Photons repeatedly scattered, so light could not travel far without interacting. About 380,000 years after the Big Bang, nuclei could retain electrons. The free-electron density fell and the universe became much more transparent.
Expansion stretched those photons to longer wavelengths. The background now has an almost perfect black-body spectrum with mean temperature about 2.7 K, detected across the sky. Small variations are important seeds of structure; “almost uniform” does not mean perfectly identical everywhere.
Connect the evidence in a chain: hot early radiation → reduced scattering → continued expansion → longer wavelengths and lower temperature → a microwave background today. Do not just write “there is microwave radiation, so the Big Bang happened”.
Penzias and Wilson’s 1965 discovery of the background, followed by precision black-body and anisotropy measurements, helped establish the hot Big Bang model. The scientific model gained support because observations tested its predictions.
Check the explanation
Choose an explanation before checking.
5.5.3(n) · approximate times after the Big Bang
A timeline of the universe, explained
Cooling changes which particles and structures can survive. First learn the order and the reason for each transition. The dates give scale: they are estimates, and some processes overlap. OCR asks for the evolution of the universe but does not specify a table of exact event times.
Explore one event at a time
Use the event selector or the previous/next buttons. Every event is also explained below. The slider advances by event, not by equal time intervals.
The earliest phases are model-dependent; select an event to explore the sequence.
- Earlier than about 10⁻⁴³ s Context
The edge of our current theory
The hot Big Bang model traces the observable universe back to a much hotter, denser state. At the Planck time scale our tested theories do not give a complete account. A detailed description of “time zero” is not established.
Remember: Do not present the earliest extrapolation as a directly observed event.
- Roughly 10⁻³⁶–10⁻³² s Extension
A proposed period of inflation
Inflation proposes an extremely brief phase of rapid expansion. Its precise timing and mechanism depend on the model. It helps account for large-scale uniformity and the seeds of structure; OCR does not prescribe these numerical endpoints.
Remember: Inflation is useful context, not a required list of exact times to memorise.
- About 10⁻⁶ s Core sequence
Quarks become bound in hadrons
As temperature falls, quarks become confined in protons and neutrons. Leptons, including electrons and neutrinos, and photons are also present. Particle–antiparticle creation and annihilation occur in the early universe; a small matter excess survives.
Remember: Protons and neutrons form before light nuclei; electrons are leptons, not quarks.
- About 1 s Core sequence
A hot mixture of particles and radiation
The universe contains protons, neutrons, electrons, neutrinos and radiation. Most matter and antimatter annihilate as the universe cools. It is still far too hot for neutral atoms to remain intact.
Remember: Include leptons when describing the early particle mixture.
- About 3–20 minutes Core sequence
Light nuclei form
Nuclear reactions combine protons and neutrons into light nuclei, especially helium, with traces of other light isotopes. Many protons remain as hydrogen nuclei. Cooling soon makes these reactions ineffective. Electrons are still unbound: nuclei are not yet neutral atoms.
Remember: Primordial nucleosynthesis makes light nuclei, not iron, galaxies or complete atoms.
- About 380,000 years Core sequence
Atoms form; the universe becomes transparent
At roughly 3000 K, nuclei can retain electrons as neutral atoms. With far fewer free electrons to scatter photons, existing thermal radiation travels much more freely. Expansion subsequently stretches this relic radiation into the microwave background seen today.
Remember: The CMB is predominantly earlier thermal radiation released from frequent scattering, not mainly light emitted as atoms form.
- Roughly 100–200 million years Core sequence
The first stars light up
Gravity amplifies small density differences. Gas collects in dark-matter structures and collapses until fusion starts in the first stars. Their exact first appearance is still being investigated; use an approximate range.
Remember: Stars form after neutral atoms. The first stars contain mainly hydrogen and helium.
- A few hundred million–1 billion years Core sequence
Galaxies grow and gas is reionised
Early galaxies assemble, merge and form more stars. Ultraviolet radiation progressively ionises intergalactic hydrogen. Successive generations of stars manufacture heavier elements and return material to space.
Remember: Galaxies develop over an interval, not at a single universal switch-on time.
- Roughly 8–9 billion years after the Big Bang Core context
Expansion begins to accelerate
In the standard cosmological model, the expansion changes from decelerating to accelerating several billion years ago, before the Solar System forms. Dark energy is the name given to the component associated with this acceleration; its physical nature remains uncertain.
Remember: This event precedes Solar System formation. Its date is approximate and depends on the cosmological model.
- About 9.2 billion years after the Big Bang Context
The Solar System forms
The Sun and planets form from enriched gas and dust about 4.6 billion years before the present. With a present cosmic age near 13.8 billion years, that is about 9.2 billion years after the Big Bang.
Remember: “4.6 billion years ago” and “9.2 billion years after the Big Bang” describe the same event.
- About 13.8 billion years Core sequence
The universe today
We observe galaxies in a universe that is still expanding. The mean CMB temperature is about 2.7 K. Standard-model estimates assign about 5% of the present mass–energy budget to ordinary matter, 27% to dark matter and 68% to dark energy.
Remember: These are present-day model estimates, not percentages of visible objects or a fixed recipe at every epoch.
Rebuild the sequence from memory
Explain why nuclei can survive before atoms, then choose.
5.5.3(o)
Ordinary matter, dark matter and dark energy
Stars and galaxies do not account for the whole inferred mass–energy content. In the standard cosmological model, representative present-day proportions are:
About 5% ordinary matter
Atomic/baryonic matter: stars, planets, interstellar gas and us. Some ordinary matter is faint or hard to detect; “ordinary” does not mean all of it is shining.
About 27% dark matter
Inferred from gravitational effects such as galaxy rotation and gravitational lensing. It does not emit, absorb or reflect light in the usual detectable way. Its nature is unresolved.
About 68% dark energy
The component associated with accelerating expansion. It is not another name for dark matter, and the physical explanation remains under investigation.
These are approximate present-day mass–energy fractions within a model. They are not counts of stars or percentages of galaxies, and the balance between components changes over cosmic history. OCR requires the three components and their relative importance; these rounded values help visualise that ordinary matter is the smallest share.
Original practice · 12 questions · 42 marks
OCR A cosmology questions and worked answers
Attempt each question before opening its answer. These questions are original PhysicsUK practice informed by OCR’s specification and assessment guidance; they are not copied past-paper questions.
1. Three distance units [3 marks]
Define an astronomical unit, a light-year and a parsec.
Show answer and mark points
- An AU is the mean Earth–Sun distance. [1]
- A light-year is the distance travelled by light in vacuum in one year. [1]
- A parsec is the distance at which 1 AU subtends one arcsecond. [1]
2. A two-epoch parallax measurement [4 marks]
A star’s apparent position changes by 0.080 arcseconds between opposite points of Earth’s orbit. Calculate its parallax and distance in parsecs and light-years. Use 1 pc = 3.26 ly.
Show answer and mark points
- The full six-month shift is 2p, so p = 0.040 arcseconds. [1]
- d = 1/p with p in arcseconds. [1]
- d = 25 pc. [1]
- Distance = 81.5 ly (about 82 ly). [1]
3. Homogeneous or isotropic? [3 marks]
State the three elements of the cosmological principle and identify the scale on which it applies.
Show answer and mark points
- On sufficiently large scales the mean properties or matter density are the same from place to place: homogeneous. [1]
- Statistical properties are the same in every direction: isotropic. [1]
- The same physical laws apply throughout the universe; local stars and galaxies need not be uniform. [1]
4. From a line to a galaxy distance [5 marks]
A line emitted at 500 nm is observed at 510 nm. Estimate the recession speed and distance using c = 3.00 × 10⁸ m s⁻¹ and H₀ = 72 km s⁻¹ Mpc⁻¹. State one limitation.
Show answer and mark points
- z = (510 − 500)/500 = 0.0200, using the rest wavelength in the denominator. [1]
- v ≈ cz = 6.00 × 10⁶ m s⁻¹. [1]
- Convert to 6000 km s⁻¹ to match H₀. [1]
- d ≈ v/H₀ = 83.3 Mpc. [1]
- This is a low-redshift approximation; local peculiar motion or distance uncertainty can affect the inference. [1]
5. Frequency shift [3 marks]
An electromagnetic source approaches at 0.010c. Its emitted frequency is 6.00 × 10¹⁴ Hz. Find the approximate frequency change and observed frequency.
Show answer and mark points
- For a small speed, magnitude |Δf|/f ≈ v/c = 0.010. [1]
- The frequency increases by 6.00 × 10¹² Hz on approach. [1]
- Observed frequency ≈ 6.06 × 10¹⁴ Hz. [1]
6. Find the gradient [3 marks]
Two widely separated points on a best-fit v–d line are (30 Mpc, 2200 km s⁻¹) and (190 Mpc, 13,400 km s⁻¹). Find H₀ and explain why you use the fitted line rather than one raw galaxy.
Show answer and mark points
- Gradient = (13,400 − 2200)/(190 − 30). [1]
- H₀ = 70.0 km s⁻¹ Mpc⁻¹. [1]
- A fit uses the trend across many galaxies and reduces the influence of scatter or one galaxy’s peculiar motion. [1]
7. Estimate a cosmic age [4 marks]
For H₀ = 75 km s⁻¹ Mpc⁻¹, estimate 1/H₀ in seconds and years. Use 1 Mpc = 3.09 × 10²² m and 1 year = 3.16 × 10⁷ s. State the model assumption.
Show answer and mark points
- H₀ = 75,000/(3.09 × 10²²) = 2.43 × 10⁻¹⁸ s⁻¹. [1]
- 1/H₀ = 4.12 × 10¹⁷ s. [1]
- About 1.30 × 10¹⁰ years, or 13.0 billion years. [1]
- Assumes the present recession speed/expansion rate can be extrapolated back without accounting for changes in expansion history. [1]
8. Explain the CMB evidence [4 marks]
Explain why a microwave background with a black-body spectrum at about 2.7 K supports a hot Big Bang.
Show answer and mark points
- The early universe was hot and contained thermal radiation. [1]
- After neutral atoms formed, photons scattered much less and travelled freely. [1]
- Expansion stretches the radiation to longer wavelengths and lowers its black-body temperature. [1]
- A nearly all-sky black-body background at about 2.7 K agrees with this relic-radiation prediction. [1]
9. Put the universe in order [5 marks]
Outline the evolution from an early quark/lepton mixture to galaxies. Distinguish nuclei from atoms and include approximate times for the two formation stages.
Show answer and mark points
- Expansion and cooling allow quarks to become bound in protons and neutrons; leptons such as electrons are also present. [1]
- During the first few minutes, protons and neutrons combine into light nuclei, especially helium. [1]
- About 380,000 years later, nuclei retain electrons and neutral atoms form. [1]
- Photons scatter much less; the freely travelling relic radiation becomes the CMB as expansion continues. [1]
- Gravitational growth of density variations forms stars and galaxies over hundreds of millions of years. [1]
10. Use the right time origin [2 marks]
The universe is about 13.8 billion years old and the Solar System formed about 4.6 billion years ago. When did the Solar System form after the Big Bang?
Show answer and mark points
- Subtract the lookback time from the present age: 13.8 − 4.6. [1]
- About 9.2 billion years after the Big Bang. [1]
11. Matter and energy [3 marks]
Distinguish ordinary matter, dark matter and dark energy. Explain why “95% of galaxies are invisible” is not a valid reading of a 5:27:68 composition chart.
Show answer and mark points
- Ordinary matter is atomic/baryonic matter, including gas, stars and planets; dark matter is inferred from extra gravitational effects. [1]
- Dark energy is associated with accelerating expansion and is distinct from dark matter. [1]
- The chart describes the present inferred mass–energy budget, not the percentage of galaxies that can be seen. [1]
12. An observer in another galaxy [3 marks]
An observer in a distant galaxy also finds recession speeds increasing with distance. Explain how this can agree with observations from Earth and why a nearby galaxy may instead be blueshifted.
Show answer and mark points
- Uniform expansion stretches separations between widely separated galaxies, so different observers obtain the same general trend. [1]
- No special central galaxy is needed in a homogeneous expanding model. [1]
- A nearby galaxy’s local gravitational motion can overcome the small recession due to expansion, producing a blueshift. [1]
Before you move on
- Define AU, light-year and parsec without confusing distance and time.
- Take half the six-month shift for p; use arcseconds and parsecs in p = 1/d.
- Distinguish homogeneous from isotropic and specify large scales.
- Use the emitted wavelength in Δλ/λ; apply the correct shift direction.
- Match km s⁻¹ with Mpc, or convert everything to SI.
- Take a v–d gradient for H₀; convert to s⁻¹ before taking 1/H₀.
- Explain why 1/H₀ is a model estimate.
- Link the CMB’s spectrum and temperature to a hot, cooling universe.
- Order quarks, hadrons, light nuclei, neutral atoms, stars and galaxies; mention leptons in the early mixture.
- Separate dark matter’s gravitational role from dark energy’s association with acceleration.
OCR A 5.5.3 coverage checklist
Mapped to OCR H556 version 3.0 (March 2026), printed pages 39–40. Select a statement to revisit its teaching section.
- 5.5.3(a) AU, light-year and parsec
- 5.5.3(b) Stellar parallax
- 5.5.3(c) The inverse parallax relation
- 5.5.3(d) The cosmological principle
- 5.5.3(e) Electromagnetic Doppler shift
- 5.5.3(f) Fractional wavelength and frequency shifts
- 5.5.3(g) Hubble’s law
- 5.5.3(h) Galactic redshift as expansion evidence
- 5.5.3(i) Astronomical and SI Hubble units
- 5.5.3(j) The hot Big Bang model
- 5.5.3(k) The 2.7 K microwave background
- 5.5.3(l) Expansion of space-time
- 5.5.3(m) Estimating the age of the universe
- 5.5.3(n) Evolution from the early universe to today
- 5.5.3(o) Ordinary matter, dark matter and dark energy
Inflation times and the Solar System date are labelled context/extension. Critical-density formulas, open/closed-universe calculations and detailed cosmological integration are outside the required 5.5.3 scope used here.
Common cosmology revision questions
What is Hubble’s law?
For galaxies in the large-scale expansion, recession speed is approximately proportional to distance: v ≈ H₀d. On a velocity-against-distance graph the gradient is H₀. Nearby galaxies can depart from the trend because of local gravitational motions.
How do I convert the Hubble constant to SI units?
Multiply a value in km s⁻¹ Mpc⁻¹ by 1000 and divide by 3.086 × 10²². The result is in s⁻¹. For 70 km s⁻¹ Mpc⁻¹ this gives about 2.27 × 10⁻¹⁸ s⁻¹.
Why is 1/H₀ only an estimate of the age of the universe?
It extrapolates the present expansion rate as if recession speed had been constant. Gravity and dark energy change the expansion history, so a full age calculation requires a cosmological model.
How does the CMB support the Big Bang?
A hot early universe should contain thermal radiation. As the universe expands, that radiation stretches and cools. The nearly uniform, near-perfect black-body microwave background at about 2.7 K matches this prediction.
When did the first atoms form?
Neutral atoms formed about 380,000 years after the Big Bang, when the universe cooled enough for nuclei to retain electrons. Light nuclei had formed much earlier, during the first few minutes.
Does the universe have a centre of expansion?
In the homogeneous expansion model there is no special centre within space. Observers in different galaxies see distant galaxies receding. The Big Bang describes expansion of space-time, rather than debris moving away from one location in pre-existing space.
What is the difference between dark matter and dark energy?
Dark matter is inferred from gravitational effects beyond those explained by ordinary matter. Dark energy is associated with accelerating cosmic expansion. They are different components and their underlying nature is still uncertain.
Must I memorise every date in the cosmic timeline?
OCR requires the evolution of the universe after the Big Bang, but does not give a fixed table of event times in 5.5.3(n). Learn the sequence and physical reasons. The approximate dates here provide scale; inflation endpoints are labelled extension.
Connect this topic to the rest of physics
- Stellar evolution and star life cycles: how later stars manufacture heavier elements.
- Gravitational fields: the interactions that help matter form structure.
- Thermal physics: temperature, particle energy and cooling.
- Photons and electromagnetic radiation: connect frequency, wavelength and energy.
- Astrophysics & Cosmology Lab: parallax, galaxy spectra, expansion and the CMB.
Sources and review
The teaching sequence and questions were written for PhysicsUK. OCR defines the assessed scope; NASA and ESA supply the additional chronology and cosmological context.
- OCR A Level Physics A specification, version 3.0, 5.5.3, printed pages 39–40.
- OCR H556/01 Summer 2018 examiner report, Q23: precise cosmological-principle language, wavelength shifts, units and evolution order including leptons.
- OCR H556/01 sample assessment, Q23: explaining background radiation as evidence.
- NASA: the universe’s history, approximate dates and the later formation of structures.
- ESA: cosmic structure formation, the early particle mixture and growth of structure.
- ESA: the CMB and inflation, model context for the earliest interval.
- NASA: cosmological redshift, expansion and the late acceleration era.
- NASA: dark matter, approximate present-day component fractions.
Reviewed 19 October 2026. Illustration generated for PhysicsUK; scientific diagrams and teaching data authored for this resource. Report a correction.