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Space · Galaxies and the universe

The Big Bang

It was not an explosion

Only spacecraft can see it

The Big Bang
Space expanding from an extremely hot state: not a blast at one point, but everywhere at once.

The numbers

How long ago
13,787,000,000yearsA little less than fourteen billion
The main proof
The background radiationThe echo of the first heat
Who proposed it
Georges Lemaître, in 1927

How it happens, step by step

  1. Galaxies move apart

    Looking far away, you can see that galaxies are moving away from one another in every direction.

  2. Go back in time

    If today everything is moving apart, yesterday it was closer. Rewinding the movie, you reach a single beginning.

  3. All was squeezed

    At the start all matter was packed together and extremely hot, and the expansion began from that state.

  4. An echo remains

    That heat left behind a faint glow that fills the sky, the same in every direction.

  5. The numbers add up

    The amounts of hydrogen and helium we measure also match the ones the theory predicts.

To get an idea

If today the galaxies are moving apart, yesterday they were closer: going back, you reach a beginning.

From the first very hot period an echo is left: a faint glow that fills the whole sky.

Fun facts

These are not in the book: there was no room on paper.

  1. Lemaître was a Belgian physicist, and at the same time a Catholic priest.

  2. He was the one who suggested that the nebulae were moving away because the cosmos was expanding.

  3. The most convincing proof is the cosmic background radiation, the echo of the first heat.

  4. That radiation fills the whole sky and is practically the same in every direction.

  5. The amounts of hydrogen and helium measured also match the ones predicted.

  6. Going very far back the theory stops working, and that limit is still open.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What was the Big Bang?

Question 2 of 5

Where can you point to as the place it began?

Question 3 of 5

What is the main proof?

Question 4 of 5

How long ago did it happen?

Question 5 of 5

Who worked out its equations?

Riddles

My name makes you think of a blast inside a room, but that is not how it went: I did not burst in a place, it is the place itself that began to grow. What am I?

I left an echo that can still be heard after billions of years, the same in every direction, and it was antennas that proved me right. What am I?

If you rewind the movie of everything that is moving apart, you reach me. I happened a little less than fourteen billion years ago. What am I?

Try it yourself

Activity 1

Try pointing on the inflated balloon to the spot where the expansion started: there is none, because it happened over the whole surface.

Activity 2

Tune an old radio or an analog TV between two channels: a tiny part of that hiss is the echo of the Big Bang.

The words

expansion
Everything moving away from everything, because space itself is growing.

Sources: Wikidata (Q323), Wikipedia (Big Bang).