The Big Bang Theory: an interactive cosmology course

The Big Bang Theory is an interactive cosmology course about how the universe began and how we know. Its 24 short sections cover the expanding universe, why the Big Bang was not an explosion, redshift, Hubble's law, the age of the universe, cosmic inflation, the first minutes, the first light and the cosmic microwave background, the first stars, dark matter, dark energy, and the evidence for and open questions about the theory.

It opens with galaxies on a stretching grid: from any galaxy, all the others move away, and the farther they are the faster they go. That single picture explains why there is no centre to the universe.

What the course covers

Expansion and redshift

Light from distant galaxies arrives stretched to longer, redder wavelengths, because space itself expanded while it travelled. In 1929 Edwin Hubble showed that speed of recession grows in step with distance, which is Hubble's law.

Here x is a galaxy's distance in megaparsecs and y is how fast it moves away in kilometres per second. Running the expansion backwards, every galaxy arrives back together after about 1 ÷ H₀, roughly 14 billion years, close to the accepted age of 13.8 billion years.

The early universe

In the first fraction of a second, inflation stretched space enormously. Within minutes, protons and neutrons fused into the first nuclei, mostly hydrogen and helium. After 380,000 years the universe cooled enough for atoms to form and light to travel freely; that light, stretched about 1,100 times, is the cosmic microwave background at 2.7 K.

The dark universe

Galaxies spin too fast for the matter we can see, pointing to dark matter, and the expansion is speeding up, pointing to dark energy. Together they make up most of the universe, and the course is clear about what is measured and what is still unknown.

Try a question from the course

Use v = H₀d with H₀ = 70 km/s per megaparsec. A galaxy 100 Mpc away moves away at 70 × 100 = 7,000 km/s. To go the other way, divide: a galaxy moving away at 21,000 km/s is 21,000 ÷ 70 = 300 Mpc away.

Now find a speed or a distance, then press Mark to check your answer in the course.

Who this course is for

• GCSE physics students revising red-shift and the evidence for the Big Bang.
• A-level and other advanced students working with Hubble's law, the age of the universe and the cosmic microwave background.
• Anyone who has wondered what came before the stars and how scientists can know.

What you need to know first

Waves and the electromagnetic spectrum help a lot, and the Light & Energy course covers them. The calculations are short multiplications and divisions.

How long it takes

There are 24 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 12 days. A plan removes the daily limit and lets you go straight to any topic, such as dark energy.

Read next

Physics · SpaceWhat was before the Big Bang? What we know and don'tWhat the Big Bang was, the evidence for it, how we know the universe is 13.8 billion years old, and the leading ideas about what came before it.