The universe
Where did all the atoms in your body actually come from?
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Explain
The universe began about 13.8 billion years ago in the Big Bang. Space has been expanding ever since.
At first there was only hydrogen, helium and a tiny amount of lithium. Then gravity pulled gas together to make stars.
Stars made heavier atoms and scattered them when they died. So most of your body is star stuff!
Hubble's law: v = H₀ × d, where H₀ ≈ 70 km/s per megaparsec. Further galaxies are moving away faster.
This suggests space itself is expanding. The light left over from the Big Bang still fills space as the cosmic microwave background.
Worked example: a galaxy at 100 Mpc is moving away at about 70 × 100 = 7000 km/s.
- Big Bang
- Stars form and shine
- Atoms make planets and you
Want the full story?
The Big Bang was not an explosion in space, but an expansion of space itself. The exact value of H₀ is still debated. Most of the universe seems to be made of unknown dark matter and dark energy. The hydrogen in your body is even older than the stars, as it formed soon after the Big Bang.
The Formula
speed away from us = Hubble constant × distance
- the speed away from us, measured in kilometres per second (km/s)
- the Hubble constant, about 70, measured in kilometres per second per megaparsec (km/s/Mpc)
- the distance, where 1 Mpc is about 3 million light years, measured in megaparsecs (Mpc)
Try the numbers
Speed away from us: 7,000 km/s
Try it at home
Ask a grown-up to help
You need a balloon, a pen.
You need: a balloon, a pen.
- Draw dots on a part-blown balloon.
- Blow it up further.
- Measure how far apart some dots move.
What's happening: every dot moves away from every other dot. Further dots move further, as in Hubble's law.
Balloons are a choking hazard. Keep burst pieces away from small children.
Physics is life
Your body
The iron in your blood came from stars.
Daily life
Gold in rings came from space.
Nature
Our planet formed from star dust.
Space
There are countless galaxies.
Quick check
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Words to know
Big Bang: The event about 13.8 billion years ago when our universe began expanding.