The Big Bang and its Evidence

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 8.13 — The past evolution of the universe and the main arguments for the Big Bang theory (bold P statement — Paper 2 only)
• 8.14 — The evidence supporting the Big Bang: red-shift and cosmic microwave background radiation (bold P statement — Paper 2 only)

Specification section 8(c) Stellar evolution — also taught in The Life Cycle of Stars, The Hertzsprung–Russell Diagram.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The BIG BANG THEORY states that the universe began about 13.8 thousand million years ago from an extremely small, extremely hot and extremely dense state, and has been EXPANDING and COOLING ever since. It is not a theory about an explosion IN space — it is a theory about SPACE ITSELF expanding, everywhere at once.

That distinction sounds pedantic and it is not. An explosion has a centre and things fly outwards from it into surrounding space. The expansion of the universe has no centre: every galaxy sees every other moving away from it, and there is no place you could stand and say the expansion began here. Asking where the Big Bang happened is like asking which point on the surface of an inflating balloon the inflation started from — the answer is everywhere on it at once.

Think of it like currants in a rising loaf

Put currants into dough and leave it to rise. As the dough expands, every currant moves away from every other, and a currant twice as far away recedes twice as fast — because there is twice as much dough between it and you, and all of that dough is expanding. Now notice two things the picture gets right. Whichever currant you sit on, every other currant appears to be moving away from YOU, so no currant is at the centre. And the currants themselves do not swell: it is the dough between them that grows. Galaxies do not expand — space does, and it carries them apart.

The theory rests on two independent pieces of observational evidence, and statement 8.14 names both. The first is RED-SHIFT. Light from distant galaxies arrives with its wavelength stretched towards the red end of the spectrum, and the further away the galaxy the greater the shift. That means the galaxies are moving away from us, and the more distant ones are moving faster — which is exactly the pattern expansion produces.

The second is COSMIC MICROWAVE BACKGROUND RADIATION, usually shortened to CMB. Faint microwave radiation arrives from every direction in the sky, at almost exactly the same intensity whichever way a telescope points. It is the light released when the young universe first became transparent — enormously hot at the time, and stretched by the expansion over 13.8 thousand million years into the microwave region. It is the strongest evidence there is, because it was PREDICTED before it was found.

Why the CMB is the decisive evidence

1. Red-shift shows the universe is expanding now, and running that expansion backwards suggests everything was once close together.
2. But the rival Steady State theory, which held that the universe had always existed and that new matter was created continuously as it expanded, could also account for red-shift.
3. The Big Bang theory made a prediction the Steady State theory could not: if the universe began hot, radiation left over from that period should still be arriving from every direction, cooled by the expansion.
4. In 1964 Penzias and Wilson found exactly that, by accident, while trying to eliminate a persistent hiss from a radio antenna they could not account for.
5. Its temperature and its uniformity across the whole sky match the prediction, and the Steady State theory has no explanation for it at all.
6. That is why one piece of evidence settled a debate the other had left open: a theory that PREDICTS something before it is found is far more strongly supported than one that explains it afterwards.

It is worth being clear about what the theory does and does not claim. It describes the universe from a tiny fraction of a second after the beginning onwards, and it accounts very successfully for the expansion, the CMB, and the proportions of hydrogen and helium observed. It does not explain what caused the beginning, or what if anything came before it, and the physics on this page does not apply at the very first instant. Saying so is not a weakness of the theory but an accurate statement of its scope.

One more thing follows from the finite speed of light and is genuinely useful. Looking at a distant galaxy means seeing light that left it millions or thousands of millions of years ago, so looking further away is looking further back in time. Astronomers can therefore observe the young universe directly rather than inferring it, and galaxies at great distances really do look different from nearby ones — which is another observation the Steady State theory could not accommodate.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · The Big Bang theory

The theory that the universe began about 13.8 thousand million years ago from an extremely small, hot and dense state, and has been expanding and cooling ever since. Space itself expands; it is not an explosion into surrounding space.

Learn this definition · Cosmic microwave background radiation

Faint microwave radiation arriving almost uniformly from every direction in the sky. It is radiation from the hot early universe, stretched to microwave wavelengths by the expansion.

The vocabulary of this page

Expansion of space

Space itself growing everywhere at once, carrying galaxies apart. It has no centre, and galaxies themselves do not expand.

Steady State theory

The rival theory that the universe had always existed, with new matter created continuously as it expanded. It cannot explain the microwave background.

Age of the universe

About 13.8 thousand million years, obtained by tracing the observed expansion backwards.

Statement 8.14 — the two pieces of evidence

• RED-SHIFT — light from distant galaxies is stretched towards the red end, and the further the galaxy the greater the shift, so the more distant galaxies are receding faster
• COSMIC MICROWAVE BACKGROUND — faint microwaves from every direction, at almost the same intensity whichever way you look
• The CMB is radiation from the hot early universe, stretched by the expansion into the microwave region
• The CMB is the STRONGEST evidence because it was PREDICTED before it was found, in 1964, by Penzias and Wilson
• The rival Steady State theory could explain red-shift but has no explanation for the CMB

The argument from observation to theory

1. Observe that distant galaxies are red-shifted, and more so the further away they are.
2. Conclude that the universe is expanding, with more distant galaxies receding faster.
3. Run the expansion backwards in time: everything was once much closer together, hotter and denser.
4. Predict that radiation left over from that hot early period should still arrive from every direction, cooled by the expansion.
5. Observe the cosmic microwave background, matching that prediction.
6. Conclude that the universe began in a hot dense state about 13.8 thousand million years ago.

What the theory does not claim

• It does NOT say the Big Bang happened at a particular point in space — the expansion has no centre
• It does NOT describe what caused the beginning, or what came before it
• The physics does not apply at the very first instant
• GALAXIES do not expand — the space between them does, and carries them apart

Model answer [4 marks]

State the two main pieces of evidence for the Big Bang theory and explain what each shows. [4]

The first is red-shift: light from distant galaxies is shifted towards the red end of the spectrum, and the further away a galaxy is the greater the shift, which shows that galaxies are moving away from us and that more distant ones are receding faster — the pattern produced by an expanding universe. The second is cosmic microwave background radiation, faint microwaves arriving almost uniformly from every direction in the sky. This is radiation released when the young hot universe first became transparent, stretched to microwave wavelengths by the expansion since, and it shows that the universe was once extremely hot and dense.

Model answer [4 marks]

Explain why the discovery of the cosmic microwave background was considered stronger evidence than red-shift alone. [4]

Red-shift shows that the universe is expanding, but the rival Steady State theory could also account for expansion by proposing that new matter was continuously created as the universe expanded, so red-shift alone did not decide between the two theories. The Big Bang theory made a prediction the Steady State theory could not: if the universe began in a hot dense state, radiation left over from that period should still be detectable from every direction, cooled by the expansion. That radiation was found in 1964, with the temperature and uniformity predicted, and the Steady State theory offers no explanation for it. Evidence that confirms a prediction made in advance supports a theory far more strongly than evidence explained after the fact.

Not this: The Big Bang was an explosion at a particular point, and the galaxies are debris flying outwards from it.

This: There is no centre. SPACE ITSELF expands, everywhere at once, and carries the galaxies apart — which is why every galaxy sees every other receding from it. The galaxies themselves do not expand.

Mark-losing trap. The Big Bang has NO CENTRE — space expands everywhere at once, it is not an explosion into space.

Mark-losing trap. The CMB is the STRONGEST evidence because it was PREDICTED before it was found.

Mark-losing trap. Space expands; galaxies do not. The dough rises, the currants do not swell.

Mark-losing trap. The theory does not explain what caused the beginning. Saying so is accurate, not a weakness.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — According to the Big Bang theory, what has the universe been doing since it began?
2. Grade 7 · Explain [3 marks] — A student asks where in the sky they should look to see the point where the Big Bang happened. Explain the answer.
3. Grade 8 · Explain [3 marks] — Explain what the cosmic microwave background radiation is and why it arrives from every direction in the sky.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why the cosmic microwave background is regarded as stronger evidence for the Big Bang than red-shift alone.
5. 9+ · Evaluate [6 marks] — A student argues: 'The Big Bang theory cannot be scientific, because it says nothing about what caused the Big Bang or what came before it. A theory with an admitted gap at its foundation explains nothing.' Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one whose measurements made the theory necessary and who would not endorse it, and the one who asked what the theory says about the beginning itself. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Edwin Hubble — the one whose measurements made the theory necessary and who would not endorse it

The red-shift evidence on this page is Hubble's, and what makes him interesting here is his refusal to draw the conclusion everyone else drew from it. He plotted recession speed against distance, found the straight line the theory rests on, and then remained publicly non-committal about the expanding universe for the rest of his life, preferring to present the observation and leave the cosmology to others. He is the right person to ask about the difference between what a measurement shows and what a theory claims, and about how much caution is appropriate when your own data implies something extraordinary.

• “What did your plot of speed against distance actually show?”
• “Why would you not say outright that the universe is expanding?”
• “Are we at the centre of the expansion?”
• “How certain were you of the distances you measured?”
• “What would have convinced you?”

Stephen Hawking — the one who asked what the theory says about the beginning itself

This page is careful to say that the theory does not describe the very first instant, and Hawking spent much of his career on exactly that boundary. With Roger Penrose he proved that under general relativity the expansion must trace back to a point where the equations break down entirely, and he later argued that asking what came before it may be as meaningless as asking what lies north of the North Pole. He is the right figure to ask what a scientific theory is entitled to say about its own starting point, and what it means for a question to have no answer rather than an unknown one.

• “What happened before the Big Bang?”
• “Why do the equations break down at the beginning?”
• “Can a question be meaningless rather than merely unanswered?”
• “What does it mean for a theory to have a boundary?”
• “How confident should we be about the first instant?”

Carry on through the course

• Previous lesson: The Hertzsprung–Russell Diagram
• Next lesson: Red-Shift
• The Universe and Gravity — From a moon to the whole universe in five steps, and why your weight would change on the way.
• Orbits — Why anything in orbit is falling, why comets speed up as they arrive, and how to find an orbital speed.
• Star Colour and Temperature — Why blue stars are the hot ones, and how a colour tells you a temperature across the whole galaxy.
• The Life Cycle of Stars — Two endings from one beginning — and the mass a star is born with decides which it gets.
• The Hertzsprung–Russell Diagram — Plot every star by colour and true brightness and they do not scatter — they fall into groups.
• All of Physics · Physics key terms · required practicals · equations · Incandio Science · The 4PH1 specification