Science shelf · Class 10
Sound waves from the early universe
There is no sound in space today. So how can astronomers say that sound waves once rippled through the whole universe, and that we can still find their pattern?
What you will learn: Explain how sound could travel in the young universe, how astronomers find its "fossil" pattern by counting pairs of galaxies, and why computers and averaging are needed to see it.
What should I know first?
If this feels new, try these first. It is fine to go back.
- Looking back in time: how old is the light you see? Class 9
- Speed of Sound and Reflection (Echo) Topic ladder, Classes 8–10
Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓
Jump to: Check yourself · For grown-ups · For teachers
Safety first
The water test is a shallow tray or bucket on the floor. Wipe up spills so nobody slips.
Keep it safe:
- Use a shallow tray or a bucket of water on the floor, away from electric sockets. Wipe up spills so nobody slips. Empty it afterwards.
- Everything else is reading a chart and doing sums. No telescope is needed; never look at the Sun through anything.
- Keep the pebbles and the die away from babies: nothing goes in the mouth.
You will need
- A tray or bucket of water and two small pebblesTo see ripples spread.
- A die (optional)For the averaging test.
- Paper, a pencil and a calculatorFor the pair-counting sums.
Time: About 45 minutes.
What do you think?
Sound needs something to travel through. Space today is almost empty. So how could sound waves have travelled in the early universe?
Make a prediction and write one reason for it. Then tap the guess closest to yours to see how to test it. It is fine to change your mind after checking.
The early universe was not empty
Write your reason. Then find out: what filled the young universe, and could pressure waves travel through it?
Sound does not need anything
Write your reason. Then think about the bell in a jar: what happens to its sound when the air is pumped out?
It must be a mistake
Write your reason. Then ask: what would sound need to travel, and was that there in the early universe?
Not sure
That is fine. Start with what sound is: what does a sound wave push on as it travels?
Need a hint?
Think about what filled the universe when it was very young and very hot.
The story
Dhruv has been asking an AI chatbot about space. "It says there were sound waves after the Big Bang," he tells Tara. "But sound needs air. Space is empty!" Tara thinks. "Maybe space was not empty then." Dadi drops two pebbles into a bucket of water, and rings spread out. "Rings like these, but in the young universe," says Tara. "How would anyone find them now?"
The idea
Sound is a pressure wave: particles push on their neighbours. It cannot cross empty space, but it can travel through any gas, liquid or solid.
The young universe was filled with hot gas and light, pressed tightly together. Sound waves spread through it at more than half the speed of light.
When atoms formed, about 3,80,000 years after the start, light flew free (we still detect it as the cosmic microwave background) and the waves froze in place.
Their trace is in where matter, and so galaxies, ended up: a few more pairs of galaxies sit about 50 crore light-years apart than chance alone would give. This was first measured in 2005 by large galaxy surveys.
Finding it means measuring the distance between every pair of galaxies. With n galaxies there are n × (n − 1) ÷ 2 pairs: for 10 lakh galaxies, about 5 × 10¹¹ pairs. Only computers can do that.
Telescope signals are faint and noisy. A main trick is to take many measurements and average them (or add many images together): random noise cancels out, and the real signal stays. Computers handle these huge sets of numbers as grids called matrices.
Worked example
Why computers?
A survey has 10 lakh (10⁶) galaxies. How many pairs are there?
- Pairs = n × (n − 1) ÷ 2.
- n × (n − 1) ≈ 10⁶ × 10⁶ = 10¹².
- Halve it: about 5 × 10¹¹ pairs.
About 5 lakh crore pairs: a computer at a million pairs a second would still need about 6 days.
Try it
Make ripples.
Drop two pebbles into still water, a little apart. Watch the rings spread and cross. Draw what you see.
Read the chart.
Look at the made-up pair chart. Where are there more pairs than the smooth fall would give? What distance is the bump at?
Count pairs by hand.
Draw 4 dots and join every pair. Count them. Check with n × (n − 1) ÷ 2. Try 10 dots with the formula.
Scale it up.
Work out the pairs for 10 lakh galaxies. How long would a computer take at a million pairs a second?
Beat the noise.
Roll a die once and write the number. Then roll 10 times and find the average. Which is closer to 3.5, the true average? Why?
Explain it.
Write 4 sentences: why sound could travel then, why it stopped, what pattern it left, and how computers find it.
What you make (paper or offline)
Your pair-counting table
Copy the table and show the working for 10 and 100.
Add a row for 1,000 galaxies.
Write one sentence on why surveys need computers.
Make it on paper. Keep it with you. Nothing to upload.
What did we find out?
Read this after you have tried it. Look back at your guess and your reason first.
The young universe was a hot, dense gas of particles and light. Sound waves (pressure waves) rippled outwards through it.
After about 3,80,000 years it cooled enough for atoms to form. Light escaped, the pressure vanished, and the waves stopped where they were.
This left a slight preference for matter, and later galaxies, to be about 50 crore light-years apart. Astronomers find it by counting pairs of galaxies at each distance: there are a few more pairs at that distance than chance would give.
Check yourself
Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.
How do you want to answer? You choose, and you can change at any time.
See the answer and why
- A. The early universe was full of hot gas Correct. Sound needs particles to push on each other; the young universe was packed with them.
- B. Sound was faster in the past Not this one. Speed is not the issue: today space has almost nothing to carry sound.
- C. There was no light in the early universe Not this one. There was plenty of light; it was trapped in the hot gas.
- D. Sound waves only travel near stars Not this one. Sound needs a medium, not a star.
See the answer and why
- A. They reached the edge of space Not this one. The waves did not hit an edge; the conditions changed everywhere.
- B. The universe ran out of sound Not this one. Sound is not used up; the medium changed.
- C. Atoms formed and light escaped Correct. With light free, the pressure that carried the waves was gone, so they froze in place.
- D. The first galaxies absorbed them Not this one. Galaxies formed much later, partly where the waves had left extra matter.
See the answer and why
- A. Counting pairs of galaxies at each distance Correct. A few more pairs sit about 50 crore light-years apart than chance gives.
- B. Listening with very big microphones Not this one. There is no sound to hear now; the pattern is in where galaxies are.
- C. A close-up photo of one galaxy Not this one. The pattern shows only across huge numbers of galaxies.
- D. Measuring the temperature of the Sun Not this one. The Sun has nothing to do with this pattern.
See the answer and why
- A. 200 Not this one. That doubles 100; the number of pairs grows much faster.
- B. 4,950 Correct. 100 × 99 ÷ 2 = 4,950.
- C. 99 Not this one. That is the pairs that include one particular galaxy, not all of them.
- D. 10,000 Not this one. That is 100 × 100: it counts each pair twice and pairs each galaxy with itself.
See the answer and why
- A. One measurement is never allowed Not this one. It is allowed, but it is noisy; averaging makes it more reliable.
- B. Computers cannot store one measurement Not this one. Storage is not the reason; reliability is.
- C. Random noise cancels out; the signal stays Correct. Like averaging many dice rolls: the random ups and downs cancel.
- D. Averaging makes the galaxies brighter Not this one. The galaxies do not change; the noise in the measurement shrinks.
Think back
What surprised you most about sound in the early universe?
Why do some questions in science need computers, not just careful people?
Investigate further
- Find out what the cosmic microwave background is and how it was discovered (by accident, in 1965).
- Ask an AI chatbot about baryon acoustic oscillations, then check two of its claims against a trusted source such as a space agency or university website.
Can you use it here?
A survey maps 10 lakh galaxies. How many pairs must a computer measure? Why would nobody do this by hand?
Ask someone at home
Ask someone at home: "If you could ask a scientist one question about the early universe, what would it be?" Try to find the answer together from a trusted source.
No one free to join in? That's fine. You can do it by yourself with drawings, and ask a grown-up to help with the materials.
For grown-upsTap a heading to open it. The safety notes are at the top of the page.
Before you start
- A shallow tray or bucket of water on the floor, away from sockets; wipe spills. A die and a calculator are optional.
- The chart is made up to show the shape of the real result; the numbers in the text are rounded.
Questions that help
- "What did the universe contain when it was young?"
- "Why are pairs counted, not single galaxies?"
- "How would you check something a chatbot told you?"
What's really going on
Big ideas like this are fine for curious children if the basics are right: sound is a pressure wave and needs a medium.
The young universe was a hot gas, so sound could travel. When it cooled, the waves froze into the pattern of where matter ended up.
The pattern is tiny and spread over millions of galaxies, which is why counting pairs and averaging with computers is essential.
Other ways to do it
- No tray? Watch rain ripples in a puddle, from a safe spot.
- Skip the dice and do the pair sums only.
For teachersLearning aims, classroom ideas and what to look for.
Learning aims
- Explain that sound needs a medium and that the early universe provided one.
- Describe how a statistical pattern (extra pairs at one distance) is detected.
- Use n(n − 1)/2 and explain why averaging reduces random noise.
Subject links
- Science: sound needs a medium; the universe.
- Class 9 "Looking back in time" (light-time).
- Maths: combinations and averages.
These links are general. They have not been checked against a specific board syllabus.
In the classroom
- Pairs draw dot maps and count pairs at two distances, then compare with a random map.
- Dice averaging: each group rolls 1, 10 and 50 times and plots how close the average gets to 3.5.
What to look for
- Says why sound could travel then and not now.
- Uses n(n − 1)/2 correctly.
- Explains averaging in terms of noise cancelling.