Why the Sky Is Dark at Night

Step outside on a clear night, away from city lights, and look up. The sky is mostly black, with scattered points of light. This seems so ordinary that nobody asks about it. But it is actually a deep puzzle, and the answer tells us something strange about the universe.

Here is the puzzle. Imagine the universe is infinitely large, has existed forever, and is filled roughly evenly with stars. Then no matter which direction you look, your line of sight should eventually hit the surface of some star. Maybe it takes a long way before it lands on one, but in an endless universe full of stars, every line eventually arrives somewhere bright. If that were true, the whole sky should blaze like the surface of the Sun. Day and night should look the same.

You might guess that faraway stars look dim, so they cannot fill the sky with light. That is true for one star, but it does not save us. When you look farther out, you also see more stars packed into that direction, because the volume of space grows. The extra stars exactly cancel out the dimming. So distance alone does not explain the darkness. This problem is called Olbers' paradox, after a nineteenth-century astronomer who puzzled over it.

The real answer has two parts, and both were discovered in the twentieth century.

First, the universe has not existed forever. It began about 13.8 billion years ago in what we call the Big Bang. Light travels fast, but not infinitely fast, so light from a star can only reach us if the star has been shining long enough for the light to make the trip. Stars beyond a certain distance simply have not had time to send their light to Earth yet. We are looking out at a finite bubble of sky, not an endless one. Most of the lines of sight from your eyes never reach any star at all, because the universe is not old enough to fill them in.

Second, the universe is expanding. Space itself is stretching, carrying distant galaxies away from us. When light travels through stretching space, its waves get stretched too. Visible light from very distant, very early sources gets pulled into longer wavelengths, sliding out of the colors our eyes can see and into infrared and radio waves. Some of the oldest light in the universe is still arriving at Earth right now, but it has been stretched so much that it is no longer light we can see. Telescopes tuned to microwaves detect it as a faint glow coming from every direction, called the cosmic microwave background. In a sense, the sky really is glowing everywhere, just not in colors your eyes were built for.

So the darkness of the night sky is not a small fact about lamps and shadows. It is evidence that the universe had a beginning and is still changing. A child who asks why it gets dark at night is asking, without knowing it, one of the largest questions in science. The answer is that we live inside a universe with an age and a stretching shape, and the blackness between the stars is what that looks like from the inside.