Why is space dark? © iStock

Why is space dark?

Looking up into the night sky, one has to wonder why the Universe is primarily black.

Asked by: Karen Fisher, Leicester

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This is one of those ‘silly’ questions, which has an answer that’s anything but. It’s perplexed many astronomers since the time of Sir Edmond Halley, of comet fame, in the 17th century. That’s because although starlight gets fainter with distance, a simple calculation shows the fading is cancelled out by the ever-increasing numbers of stars with distance, and should therefore leave space ablaze with light.

Clearly, there’s something wrong with the simple calculation, and a big clue about what it is came in the 1920s when astronomers discovered that the Universe is expanding after exploding from a Big Bang billions of years ago. As such, stars haven’t existed long enough to fill the Universe with their light, which is also stretched and weakened by the cosmic expansion. The result is a Universe as black as pitch.


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