Why Is the Night Sky Dark? You Can Thank the Big Bang

I love it when a seemingly simple question—one so simple that it feels silly to even ask it—leads to profound, even cosmic, insight.

For example: Why is the sky dark at night?

I can imagine you reading this and thinking, “Seriously? That’s profound?”

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Yes. Yes, it is. I can also imagine you thinking, “But the answer’s obvious! There are only so many stars to go around. Ones farther away are fainter. So there’s lot of black between them and nothing out there that they would illuminate. Of course the sky is dark at night.”

But that answer is wrong or, at best, incomplete, and the actual situation wasn’t obvious even to eminent scientists a century or so ago. And the person who first solved this mystery—or, at least, the first to give a correct scientific answer in writing—is almost certainly not who you’d think!

To be clear, the sky isn’t perfectly dark at night. Earth’s atmosphere glows faintly even from the darkest site. Here we’re talking cosmologically dark, however: the universe itself is in a state of being unlit. There are some sources of background light even then—distant stars and galaxies too small and faint to be seen individually—yet the sky looks pretty black compared with, say, the surface of the sun.

That may seem like an extreme comparison, but it’s not. That’s the heart of the problem.

Historically, astronomers held to the idea that the universe was infinite in time and space, stretching on forever. It always had been, and it always would be. It was static, unchanging.

There was reason to believe this, especially if you were an astronomer in, say, the 19th century. You’d think that the Milky Way was the whole universe and that, by Ptolemy’s ghost, it didn’t change from night to night. Oh, certainly the moon underwent phases, you’d say; each planet, true to the Greek etymology of that word, wandered the sky, and so on. But the stars were always there and always had been.

This idea leads to a problem, though: if the universe is infinite in space, with stars evenly distributed throughout, then the sky should be bright—very bright, based on its geometry.

Let’s say you count up all the stars in a thin spherical shell that’s centered…