The shape of the lesson
By the end of the lesson, learners can
- Listen to the background: Detect the microwave background coming evenly from every direction in space.
- Afterglow and the Hubble constant: The CMB is the cooled glow of the early universe, and the Hubble constant ties speed to distance.
- CMBR: microwaves from all directions, Big Bang evidence.
- Law: v = H0 d; H0 is the graph gradient.
- Age: about 1 / H0.
One picture every idea returns to
The cosmic microwave background is faint microwave radiation reaching us from every direction, the cooled afterglow of the early universe and strong evidence for the Big Bang. The Hubble constant links a galaxy’s recession speed to its distance and can be used to estimate the age of the universe.
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, The afterglow of everything.. Point a radio dish at the empty dark between the stars and a faint hiss comes back, from every direction at once. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Listen to the background | Detect the microwave background coming evenly from every direction in space. Project tab 5 of the lab (Cosmic microwave background) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Afterglow and the Hubble constant | The CMB is the cooled glow of the early universe, and the Hubble constant ties speed to distance. | Connect-Extend-Challenge |
| 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 Afterglow and the Hubble constant, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| The cosmic microwave background comes from every direction in space, not from a single object. | It is the cooled afterglow of the early universe and one of the strongest pieces of evidence for the Big Bang. |
The cosmic microwave background: worksheet
6 questions, 11 marks. Written in the style of the papers; no past paper question is reproduced.
State what the cosmic microwave background radiation is, and where it comes from in the sky.
State what the CMBR is evidence for.
Write down Hubble's law and state what each symbol represents.
A galaxy is at a distance of 4.0 x 10 to the 24 m and recedes at 8.8 x 10 to the 6 m/s. Calculate the Hubble constant H0.
Using H0 = 2.2 x 10 to the minus 18 per second, estimate the age of the universe in seconds.
Speeds and distances are measured for many galaxies and plotted as a graph of v against d. Explain how the Hubble constant is found from this graph.
Answer key and marking notes
- Microwave radiation left over from the early universe. ✓
- It comes from all directions in space. ✓
The Big Bang theory. ✓
- v = H0 d. ✓
- v is recession speed, d is distance, H0 is the Hubble constant. ✓
- H0 = v / d = (8.8 x 10 to the 6) / (4.0 x 10 to the 24)
H0 = 2.2 x 10 to the minus 18 per second
- age = 1 / H0 = 1 / (2.2 x 10 to the minus 18)
age = 4.5 x 10 to the 17 s (about 14 billion years)
- The points lie on a straight line through the origin. ✓
- The gradient (slope) of the line is the Hubble constant H0. ✓
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.
The cosmic microwave background (CMB) radiation is detected coming from all directions in space. What is it strong evidence for?
- Athe Big Bang theory
- Bthat the Earth is the centre of the Universe
- Cthat the Universe is shrinking
- Dthat there are no other galaxies
Astronomers find that more distant galaxies generally show a greater redshift. What does this relationship indicate?
- Agalaxies that are farther away are moving away faster, supporting an expanding Universe
- Ball galaxies move away at exactly the same speed
- Cnearer galaxies are moving away fastest
- Dthe redshift is caused by dust between the galaxies
The Hubble relationship links a galaxy's speed of recession (v) to its distance (d). Roughly how is it used to estimate the age of the Universe?
- Athe recession speed is proportional to distance, and dividing distance by speed gives a time since expansion began
- Bby adding the speeds of all galaxies together
- Cby measuring how hot the Sun is
- Dby counting the number of galaxies in the sky
Quiz key
The CMB is faint microwave radiation reaching us from every direction, interpreted as the cooled-down afterglow of the hot early Universe. It is strong supporting evidence for the Big Bang theory.
A greater redshift for more distant galaxies means they are receding faster, and the speed increases with distance. This pattern is exactly what an expanding Universe produces.
Because recession speed is proportional to distance (v is proportional to d), distance divided by speed gives roughly the time since the galaxies were all together, an estimate of the age of the Universe. Adding speeds or counting galaxies does not give an age.
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.