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ELI5 Yahaaa

Three pieces of evidence

How Do We Know the Universe Had a Beginning?

Nobody watched it happen. Three separate measurements all point back to the same moment.

1 Everything ismoving apart2 A faint warmtheverywhere3 The gas hasthe rightrecipe
Three unrelated clues, one answer.

  1. Step 1 of 6

    The question is how anyone could possibly know.

    What we can seetoday What it impliesabout the pastEvery clue below is something measurable right now.

    No one was there, and it is not the sort of thing you can repeat. So the honest version of this story is not what happened — it is what we can still measure now that only fits one explanation.

  2. Step 2 of 6

    First clue: almost everything is moving away from us.

    A nearby galaxy Moving away slowlyA distant galaxy Moving away muchfaster

    Light from distant galaxies is stretched towards the red end, which means they are receding. And the further away a galaxy is, the faster it is going.

  3. Step 3 of 6

    That does not put us at the centre.

    Dots on a balloon Every dot sees thesame thing

    Draw dots on a balloon and blow it up. Every dot sees every other dot moving away, and the far ones recede fastest. No dot is the middle. Space itself is what is growing.

  4. Step 4 of 6

    Second clue: there is a faint warmth coming from every direction.

    Point it anywhereat all The same faintglowIt was first mistaken for a fault in the equipment.

    Point a sensitive antenna anywhere in the sky and you pick up a weak hum of microwaves. It is the same everywhere, and it is exactly what a hot young universe would have cooled down to.

  5. Step 5 of 6

    Third clue: the ingredients are in the right proportions.

    4 counters Starting total ÷ 4 GROUP 1 GROUP 2 GROUP 3 GROUP 4 4 equal groups · 1 in each
    Predicted before it was measured, and it matched.

    Work out what a hot beginning would have cooked up in its first minutes, and you get about three-quarters hydrogen and a quarter helium. That is what we actually find.

  6. Step 6 of 6

    Run all three backwards and they meet.

    Expansion Leftover glow Gas recipe13.8
    Three independent measurements land within a few per cent of each other.

    Expansion, leftover warmth and the gas recipe are measured in completely different ways by different people. All three point back to the same hot, dense start about 13.8 billion years ago.

    The name was coined by an astronomer who did not believe the idea and meant it dismissively.

The short version

Three independent measurements — galaxies receding, a faint glow in every direction, and the proportions of the gas — all point back to a hot, dense beginning.

Try it yourself

Draw dots on a balloon with a marker and blow it up slowly. Watch how every dot moves away from every other one.

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