Why the sky is dark at night (and what it tells us about the universe)

At first glance, there’s no puzzle. The sun is below the horizon, so the sky is dark. End of story.

But sit with the question for a minute and it stops feeling obvious. The universe has a lot of stars. Our own galaxy has a few hundred billion. There are an estimated two trillion galaxies in the observable universe. If the universe were truly infinite, and filled uniformly with stars, then in whatever direction you looked at night, your line of sight would eventually land on the surface of a star. Every point in the sky should be as bright as a star.

The sky, in any direction, should be dazzling.

It isn’t. It’s mostly black. Why?

This question is known as Olbers’ paradox, after the 19th-century astronomer Heinrich Olbers. And the answer, worked out in full only in the 20th century, turns out to tell us something extraordinary about the universe itself.

Setting up the paradox

The argument goes like this. Assume three things:

1. The universe is infinite in size.
2. The universe is infinitely old.
3. The universe is roughly uniformly filled with stars (or galaxies, which comes to the same thing).

Now picture standing on Earth and looking up. Draw an imaginary thin shell around Earth at, say, one light year away. It contains some number of stars. Draw a second shell at two light years. It contains more stars, because its surface area is larger. The number of stars in each shell grows with the square of the distance.

But light from each star fades with the square of the distance. The inverse-square law. So the amount of light reaching you from each shell is, roughly, constant. The faintness of each individual star in the more distant shell is exactly balanced by there being more of them.

Now sum all the shells, going out to infinity. You get an infinite amount of light. Every line of sight, in a truly infinite, uniformly filled, eternal universe, ends at a star. The sky should glow.

It doesn’t. So at least one of those three assumptions must be wrong.

Why the old answers don’t work

For a long time, people suggested that interstellar dust might absorb distant starlight and explain the darkness. Doesn’t work. If dust were absorbing all that starlight, it would heat up and eventually re-radiate the energy as infrared light. You’d still be bathed in a glowing sky, just at a different wavelength.

Others suggested the universe wasn’t uniformly filled. That stars thinned out at large distances, or clumped in some pattern. Possible, but without evidence, it felt like special pleading.

The real answer took a different form entirely.

The answer: the universe has a beginning

The resolution of Olbers’ paradox is that the universe is not infinitely old. It had a beginning, roughly 13.8 billion years ago. That beginning profoundly limits how far back, and therefore how far away, we can see.

Here’s why. Light travels at a finite speed, about 300,000 km per second. If a star is 10 light years away, its light takes 10 years to reach us. If the universe is 13.8 billion years old, then the most distant light that could possibly reach us would have set out nearly 13.8 billion years ago.

That means there’s a horizon. Beyond a certain distance, light simply hasn’t had time to reach us yet. It doesn’t matter how many stars exist beyond that horizon. Their light is still on the way. Some will arrive in the far future. But right now, those regions of the universe are effectively invisible.

So the total starlight we can receive isn’t infinite. It’s the sum of starlight from everything close enough that its photons have had time to get here. That’s a finite amount. A finite amount of light, spread thinly across the entire sky, leaves most of the sky very, very dark.

The expanding universe makes it darker still

There’s a second factor. The universe isn’t just finite in age. It’s also expanding. Every galaxy beyond our local neighbourhood is moving away from us. More distant galaxies are moving faster.

As light from those distant objects travels through expanding space, its wavelengths stretch. Visible light from extremely distant galaxies shifts toward the infrared, eventually into microwaves. It stops being visible to human eyes even when it does reach us.

This is why the most distant observable structure in the universe, the afterglow of the Big Bang, is visible not as light but as microwave radiation. If you could see microwaves, the entire sky would, in fact, glow. This is the Cosmic Microwave Background, and it exists in every direction. You’re being bathed in it right now.

When you look up at a dark night sky, you’re looking at a universe that has a beginning and is stretching. Two profound cosmological facts, visible in the simplest observation any child can make.

What the dark is telling you

Summed up simply: the sky is dark at night because

1. The universe isn’t infinitely old. Light reaches us from only a finite volume of space.
2. The universe is expanding, which dims and redshifts the light from the most distant galaxies out of visible range.

If the universe were eternal and static, Olbers was right. The sky would be uniformly bright, blinding, and life as we know it wouldn’t exist. The fact that we can look up at night and see distinct stars against a black background is a direct consequence of the fact that our universe is young (by cosmic standards), finite in accessible size, and still growing.

Look up tonight

Next time you step outside into a dark sky, notice the blackness between the stars. That darkness isn’t empty of meaning. It’s evidence, visible from any back garden, that the universe had a beginning. The most important cosmological measurement you’ll ever make without leaving your doorstep.

The darkness between the stars is telling you about time itself.