The Big Bang and the Expanding Universe
- Earth & Space
- Ages 14–18 · Grades 9–12
What you’ll find out
- Redshift and Hubble’s law show that space is expanding, carrying galaxies apart.
- The universe began about 13.8 billion years ago in a hot, dense state and has cooled as it expanded.
- The cosmic microwave background and the hydrogen–helium ratio support the Big Bang theory.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
An expanding universe
In the 1920s, the astronomer Edwin Hubble made a surprising discovery. Almost every distant galaxy is moving away from us. The more distant a galaxy is, the faster it moves away. The simplest explanation is that the universe itself is expanding. The space between galaxies is stretching. A useful model is a balloon covered in dots. As it inflates, every dot moves away from every other, and dots farther apart separate faster. Astronomers detect this motion through redshift. Light from galaxies moving away is stretched to longer, redder wavelengths. The greater the redshift, the faster the galaxy is moving away.
Evidence for the Big Bang
The Big Bang theory says that the universe began about 13.8 billion years ago. It started in an extremely hot, dense state and has been expanding and cooling ever since. It was not an explosion into empty space. Space itself expanded everywhere at once. As it cooled, tiny particles formed. After about 380,000 years, the first atoms formed, mostly hydrogen and helium. Three main pieces of evidence support the theory. Distant galaxies show redshift. A faint glow of microwaves fills the whole sky; scientists call it the cosmic microwave background. And the amounts of hydrogen and helium match what the theory predicts.
Curriculum links: NGSS HS-ESS1-2 · England KS4: Physics · Cambridge IGCSE: Physics