The shape of the lesson
By the end of the lesson, learners can
- Stretch the light: See light from receding galaxies shifted toward the red, more so for the most distant.
- From redshift to Big Bang: Receding galaxies mean an expanding universe, which points back to a Big Bang.
- Redshift: light stretched toward red.
- Link: more distant galaxies recede faster.
- Conclusion: expanding universe, Big Bang.
One picture every idea returns to
Light from distant galaxies is redshifted, stretched to longer wavelengths, showing the galaxies are moving away from us, with more distant ones receding faster. This expansion supports the Big Bang Theory, that the universe began from a tiny point and has been expanding ever since.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, A universe that expands.. Light from distant galaxies arrives stretched, shifted toward the red. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Stretch the light | See light from receding galaxies shifted toward the red, more so for the most distant. Project tab 4 of the lab (Redshift and the Big Bang) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: From redshift to Big Bang | Receding galaxies mean an expanding universe, which points back to a Big Bang. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 11 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in From redshift to Big Bang, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Redshift means the light’s wavelength is increased, stretched, which shows distant galaxies are moving away from us. | The further away a galaxy is, the faster it recedes, which is the evidence that the universe is expanding. |
Redshift and the Big Bang: worksheet
6 questions, 11 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by redshift, and what it tells us about a galaxy.
State the link between how far away a galaxy is and how fast it moves away.
Explain how redshift provides evidence that the universe is expanding.
Galaxy P has a larger redshift than Galaxy Q. State which is further away and which is moving faster.
State the name of the theory that the expanding universe supports.
A student thinks the redshift proves the Earth is at the centre of the universe. Explain why this is not correct.
Answer key and marking notes
- The galaxy's light is shifted toward the red (longer wavelength) end of the spectrum. ✓
- It tells us the galaxy is moving away from us. ✓
- The further away a galaxy is, the faster it is moving away. ✓
- So more distant galaxies have a greater redshift. ✓
- Distant galaxies are all moving away from us. ✓
- Galaxies moving apart in every direction means the universe is expanding. ✓
- Galaxy P is further away. ✓
- Galaxy P is moving away faster. ✓
The Big Bang theory. ✓
- Space itself is expanding, so every galaxy sees all the others moving away. ✓
- An observer anywhere would see the same thing, so there is no special centre. ✓
3 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Light from distant galaxies is shifted toward the red end of the spectrum (redshift). What does this tell us?
- AThe distant galaxies are moving away from us.
- BThe distant galaxies are moving toward us.
- CThe galaxies are getting hotter.
- DThe light is slowing down as it travels.
Galaxies in all directions are found to be moving away from us. What does this suggest about the Universe?
- AThe Universe is expanding.
- BThe Universe is shrinking.
- CThe Earth is at the exact centre of the Universe.
- DThe Universe is staying exactly the same size.
What does the Big Bang theory state about the beginning of the Universe?
- AThe Universe began from a very hot, dense point and has been expanding ever since.
- BThe Universe has always existed at its current size.
- CThe Universe formed when two galaxies collided.
- DThe Universe is collapsing back to a single point.
Quiz key
Redshift means the wavelengths are stretched, which shows the galaxies are moving away from us. A shift toward blue would indicate approach; the size of the shift does not measure temperature.
Galaxies receding in every direction show that the Universe as a whole is expanding, with the space between galaxies growing. This does not mean the Earth is special; an observer in any galaxy would see the same.
The Big Bang theory states that the Universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding ever since. The observed expansion is the evidence that supports running this picture backward to a single origin.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.