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ELI5 Yahaaa

A very old puzzle

Why Is the Night Sky Dark?

With endless stars, the sky should blaze. It doesn't — and the reason is startling.

If the universe wereinfinite and eternal Every line of sightends on a star — thesky blazesWhat we see Mostly black
The darkness itself is evidence.

  1. Step 1 of 5

    Start with a simple thought.

    Stars in everydirection Like a forest — everysightline hits something

    Imagine stars scattered endlessly in every direction. Look anywhere and eventually your line of sight must land on one, just as looking into a thick forest always ends on a trunk.

  2. Step 2 of 5

    So the whole sky should glow.

    Prediction Sky as bright as theSun everywhereObservation Dark, with scattereddots

    If every sightline ends on a star, the night sky should be as bright as the surface of the Sun in every direction. It obviously is not. That contradiction is called Olbers' paradox.

  3. Step 3 of 5

    The first answer: the universe is not old enough.

    How far light has travelled13.8 billionlight yearsThe Milky Way's width100,000 lightyearsTo the nearest star4 light years

    Light travels fast but not instantly. The universe is 13.8 billion years old, so light from anything further than 13.8 billion light years has not arrived yet.

  4. Step 4 of 5

    The second answer: space is stretching.

    1 Light leaves adistant galaxy2 Space stretcheson the way3 Arrives toostretched tosee

    The universe is expanding, so distant light gets stretched on its way to us. Stretched enough, visible light turns into infrared and radio that your eyes cannot see.

  5. Step 5 of 5

    The glow is actually there — just invisible.

    Radio telescope A faint hum fromevery directionThe cosmic microwave background

    Point a radio telescope anywhere and you find a faint hum filling the whole sky. It is the stretched-out light of the young universe, and it really does come from every direction.

    A dark night sky was one of the first hints that the universe had a beginning.

The short version

The night is dark because the universe has a finite age and is stretching light beyond what eyes can see.

Try it yourself

Stand at the edge of a small wood and you see through it. Walk into a big one and every direction is trunk. Our universe is the small wood.

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