Why Is the Night Sky Dark? Olbers’ Paradox and the Age of the Universe

Step outside on a clear night far from city lights and you will see thousands of stars. Through a telescope there are billions more, and beyond them hundreds of billions of galaxies. With so many suns out there, a simple question turns out to be surprisingly deep: why is the sky between the stars dark at all?

Astronomers call this puzzle Olbers’ paradox, and its answer tells us something fundamental about the universe: it has not existed forever.

The paradox in one picture

Imagine standing in the middle of a huge forest. Near you, you can see gaps between the trunks. But if the forest goes on forever in every direction, every line of sight will eventually hit a tree. The view would become a solid wall of wood.

Now replace the trees with stars. If the universe were infinite in size, infinitely old and unchanging, every direction you looked would end on the surface of a star. Distant stars look dimmer, but there are more of them at greater distances, and the two effects cancel out exactly. The result would be a sky as bright as the surface of the Sun, day and night.

The fact that we sleep in darkness means at least one of those assumptions is wrong.

An old question with a famous name

The puzzle is much older than its name. Johannes Kepler used the dark sky in 1610 to argue that the universe of stars could not be infinite. In the 18th century, the Swiss astronomer Jean-Philippe Loys de Chéseaux worked out how bright an infinite sky should be. In 1823 the German physician and astronomer Heinrich Wilhelm Olbers described it again, and his name stuck.

For a long time, the favourite explanation was dust: perhaps clouds of gas and dust between the stars absorb the light. It sounds reasonable, but it fails. In an eternal universe the dust would keep absorbing energy until it became as hot as the stars, and then it would shine just as brightly.

The real answer: the universe has an age

The key insight came from modern cosmology. The universe began about 13.8 billion years ago in the hot, dense state we call the Big Bang. Light travels fast, but not infinitely fast. That means we can only see objects whose light has had time to reach us.

Looking out into space is therefore also looking back in time. Beyond a certain distance there is simply no starlight that has arrived yet. Most lines of sight do not end on a star; they end in the dark, early universe before the first stars formed.

Curiously, one of the first people to suggest something like this was not an astronomer but the writer Edgar Allan Poe. In his 1848 prose poem Eureka, he suggested that the dark gaps might be regions so distant that their light has not yet had time to reach us. It was a lucky guess, but a remarkably good one.

Stars do not shine forever

There is a second limit. Even in a very old universe, stars are not eternal lamps. A star like the Sun lives for roughly 10 billion years; massive stars burn out in a few million. There is simply not enough matter, or enough time, for stars to fill the whole sky with light.

Physicist Edward Harrison showed in the 1980s that the total energy stars can produce is far too small to light up the sky like the Sun’s surface. Even if all the matter in the universe were turned into starlight, the night would still be dark.

Expansion stretches the light

The universe is not only finite in age; it is also expanding. As light travels through expanding space, its wavelength is stretched. Astronomers call this redshift.

Stretched light carries less energy. Light from the most distant galaxies has been shifted out of the visible range into the infrared. So even the light that does reach us from far away is largely invisible to our eyes. Calculations, such as those by Paul Wesson in 1991, show that expansion makes the sky darker, but the finite age of the universe is the bigger factor.

The sky is not perfectly dark

If you could see in microwaves, the night sky would glow everywhere. This is the cosmic microwave background: light released about 380,000 years after the Big Bang, when the universe first became transparent. At that time it was the glow of a hot gas at around 3,000 kelvin. Expansion has since stretched it into microwaves, corresponding to a temperature of only about 2.7 degrees above absolute zero.

In a way, this is the ghost of Olbers’ bright sky. The early universe really was bright in every direction. We just see that brightness redshifted into darkness.

There is also a faint extragalactic background light from all the galaxies that have ever shone, but it is far too dim for the human eye to notice.

Why this matters

The dark night sky is one of the simplest pieces of evidence that the universe had a beginning. Every time you look up and see black between the stars, you are seeing the edge of time, not the edge of space.

In short

• In an infinite, eternal, unchanging universe every line of sight would end on a star and the night would be as bright as the Sun.
• The sky is dark mainly because the universe has a finite age of about 13.8 billion years, so light from beyond a certain distance has not reached us yet.
• Stars have limited lifetimes and could never produce enough light to fill the sky.
• The expansion of the universe redshifts distant light out of the visible range.
• The darkness is filled with the faint cosmic microwave background, the stretched-out glow of the early universe.