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The Big Bang & expanding universe – Cosmology
Image: NASA/WMAP Science Team · Public domain
💥 Cosmology · The Whole Universe

The Big Bang & expanding universe

Cosmology

The universe began in an extremely hot, dense state about 13.8 billion years ago and has been expanding and cooling ever since. Galaxies are moving apart not because they fly through space, but because space between them is stretching.

  • EverywhereIt is all around you, in every direction.
  • 13.8 billion yearsAge of universe
  • Space itself expandsKey idea
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The First Ingredients

In its first few minutes, the young universe worked like a giant nuclear oven, cooking up hydrogen and helium – about three quarters hydrogen and one quarter helium by weight. Almost everything heavier, like the carbon in your body, was made much later inside stars.

The Raisin Bread Universe

Picture raisins in a loaf of bread as it rises in the oven. Every raisin moves away from every other one, and the farthest move fastest, but none is the centre. The raisins themselves don’t grow – just as you, Earth and the Milky Way are held together and don’t get stretched.

Measuring the Stretch

Astronomers measure how fast space is growing by comparing galaxies’ distances with how quickly they move away. Using nearby stars and the Big Bang’s afterglow gives slightly different answers – a puzzle called the “Hubble tension” that nobody has solved yet.

How we found out

  1. 1912

    Vesto Slipher finds that most galaxies are moving away from us.

  2. 1922

    Alexander Friedmann shows Einstein’s equations allow an expanding universe.

  3. 1927

    Georges Lemaître, a Belgian priest and physicist, proposes the expanding universe; in 1931 he imagines it began from a “primeval atom”.

  4. 1929

    Edwin Hubble shows that the farther away a galaxy is, the faster it moves away.

  5. 1949

    Fred Hoyle, who disliked the idea, calls it the “Big Bang” on BBC radio – and the name sticks.

🤔 Something to wonder about

What is the universe expanding into?

See it yourself

Every direction you point is “towards the Big Bang”.

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