Welcome! How can anyone know what happened 13.8 billion years ago? In this lesson you’ll look at the light from distant galaxies, plot real-style data, and weigh up the evidence like a scientist to answer the biggest question of all: where did it all begin? It takes about one hour. (Separate Physics only.)
Before you start
- You need: your exercise book or paper (graph paper if you have it), a pen, a pencil and a ruler.
- Write the title and date: Red-shift and the Big Bang – Where Did It All Begin?
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. explain what red-shift is
2. explain how red-shift shows the universe is expanding
3. explain how red-shift supports the Big Bang theory
4. describe what scientists still don’t understand.
Step 1: Starter (5 minutes)
When an ambulance drives towards you, its siren sounds higher. As it drives away, the siren sounds lower. What do you think is happening to the sound waves? Write your ideas.
Show answer
As the ambulance moves away, the sound waves get stretched out – their wavelength increases and their frequency decreases, so the pitch sounds lower. The same thing happens to light from a galaxy moving away from us: its wavelength is stretched towards the red end of the spectrum.
📝 Copy into your book
RED-SHIFT: an observed increase in the wavelength of light from most distant galaxies. It shows that the galaxies are moving away from us.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- How does the red-shift of a galaxy depend on how far away it is?
- What does the Big Bang theory say?
- What did observations of supernovae since 1998 show?
📖 Read: The expanding universe
When astronomers split the light from distant galaxies into a spectrum, they find that it has been shifted towards longer wavelengths – towards the red end. This is called red-shift. It tells us that the galaxies are moving away from us. The further away a galaxy is, the bigger its red-shift, which means the faster it is moving away. This is evidence that the whole universe is expanding.
If the universe is getting bigger, then in the past it must have been smaller. Run the clock back far enough, and everything must once have been squeezed together. The Big Bang theory says that the universe began from a very small region that was extremely hot and dense, and has been expanding ever since. Red-shift supports this theory.
But there is still a lot we don’t understand. Since 1998, observations of distant supernovae have suggested that very distant galaxies are moving away faster and faster – the expansion is speeding up. Scientists think this may be caused by something called dark energy. They also believe there is dark mass (dark matter) that we cannot see. Nobody yet knows what either of these really is.
Show answers
(a) The further away the galaxy, the bigger its red-shift (so the faster it is moving away).
(b) The universe began from a very small region that was extremely hot and dense, and has been expanding ever since.
(c) That distant galaxies are receding ever faster – the expansion of the universe is speeding up.
📝 Copy into your book
Further galaxies → bigger red-shift → moving away faster → the universe is EXPANDING → so it must once have been very small, hot and dense → the BIG BANG.
Step 3: Quick check (5 minutes)
Cover your notes and answer from memory.
Q1 Light from a galaxy is red-shifted. Is its wavelength longer or shorter than expected?
Show answer
Longer.
Q2 Galaxy A has a bigger red-shift than galaxy B. Which is further away?
Show answer
Galaxy A.
Q3 Name two things scientists believe exist but cannot yet explain.
Show answer
Dark mass (dark matter) and dark energy.
Step 4: Main task – Where Did It All Begin? (18 minutes)
You’re an astronomer who has just measured five galaxies. You’ve worked out how far away each one is, and used its red-shift to calculate how fast it is moving away from us.
| Galaxy | Distance (million light-years) | Speed moving away (km/s) |
|---|---|---|
| A | 100 | 2 100 |
| B | 200 | 4 500 |
| C | 300 | 6 400 |
| D | 400 | 8 800 |
| E | 500 | 10 900 |
Your task:
- Plot a graph of speed (y-axis) against distance (x-axis). Draw a line of best fit.
- Describe the pattern in one sentence.
- Explain what the pattern tells you about the universe.
- Evaluate: a friend says, “This proves the Big Bang happened.” Do you agree? Write a short paragraph, including what the evidence does show and what scientists still don’t know.
Stuck? Use the chain in your 📝 box from Step 2. For part 4, think about the difference between evidence that SUPPORTS a theory and evidence that PROVES it.
Show a model answer
1. Your points should lie close to a straight line going up from the origin.
2. The further away a galaxy is, the faster it is moving away from us – speed is roughly proportional to distance.
3. Galaxies in every direction are moving away, and more distant ones move faster, so the universe is expanding. If it’s expanding now, it must have been smaller in the past.
4. I partly disagree. The data supports the Big Bang theory: an expanding universe must once have been very small, hot and dense. But evidence can’t completely prove a theory – it could be explained differently, or new evidence could change it. Scientists also still don’t understand dark mass and dark energy, or why the expansion seems to be speeding up. So the Big Bang is the best explanation we have, based on current evidence.
✅ Check: did you use the words red-shift, expanding and evidence?
Step 5: Exam practice (13 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) What is meant by red-shift?
Show model answer
An observed increase in the wavelength of light (1) from most distant galaxies (because they are moving away from us) (1).
Q2 (4 marks) Explain how observations of red-shift provide evidence for the Big Bang theory.
Hint: use the chain in your 📝 box – each arrow is worth a mark.
Show model answer
Light from distant galaxies is red-shifted, so they are moving away from us (1). More distant galaxies have a bigger red-shift, so they are moving away faster (1). This shows the universe is expanding (1). So in the past it must have been very small, hot and dense – as the Big Bang theory says (1).
Q3 (2 marks) Give two features of the universe that scientists still cannot fully explain.
Show model answer
Any two: dark mass (dark matter) (1); dark energy (1); why the expansion of the universe is speeding up.
✅ Add up your marks out of 8 and write the score in your book.
Step 6: Exit ticket (8 minutes)
- Red-shift tells us that…
- Evidence can support a theory, but it can’t prove it because…
- The question about the universe I’d most like answered is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Space physics questions and the You Are Stardust lesson.
