Olbers’ Paradox: Why Is the Sky Dark at Night?
The night sky is dark, but have you ever wondered why? With countless stars scattered across the Universe, shouldn’t their combined light make the entire sky glow? This question, known as Olbers’ paradox, has puzzled astronomers for centuries. While the answer may seem simple at first, the real explanation reveals surprising truths about the nature of our Universe.
Contents
- Why is the dark sky considered a paradox?
- What is Olbers’ paradox in simple terms?
- Possible (but wrong) solutions to Olbers’ paradox
- Olbers’ paradox solution: The mystery unraveled
- Dark sky paradox: Bottom line
Why is the dark sky considered a paradox?
You might be thinking: “What’s there to discuss? The sky is dark at night because the Sun is down!”. But think about the stars.
Our Milky Way galaxy alone contains 100 to 400 billion stars, and the observable Universe contains 100 to 200 billion galaxies. That’s a huge number of stars, to say the least! Add to this the assumption that our Universe might actually be infinite. Any small patch of the sky you choose to look at should contain myriads of stars, and so the night sky should be dazzlingly bright. However, in reality, this is not the case.
This puzzling contradiction is known as Olbers’ paradox, named after German astronomer Heinrich Wilhelm Olbers (1758-1840). Stay with us — we’re planning to solve the paradox in just a few minutes!
What is Olbers’ paradox in simple terms?
Here’s a simple way to state Olbers’ paradox:
If the Universe is infinite, and there’s an infinite number of evenly distributed stars in it, then the entire sky should be covered with stars. We should be able to see a star in any direction we look, and so the night sky should be brightly lit. Why is it dark then?
To understand Olbers’ paradox better, imagine yourself in the midst of a thick forest. Anywhere you look, you’d see a wall of trees with no gaps between them. And it’s only a forest, which can be big but is by no means infinite. Now try to extend this analogy to an infinite Universe filled with stars!
Possible (but wrong) solutions to Olbers’ paradox
Over centuries, there have been several attempts to explain the puzzling phenomenon of the dark night sky. Let’s consider some of these explanations and rule them out.
The stars in the Universe are distributed fractally, not uniformly
Why it’s wrong: This hypothesis could have explained the paradox, but modern astronomical data don’t support it. The Universe seems to be almost perfectly isotropic (i.e., the same in all directions).
The Universe has a finite number of stars
Why it’s wrong: Even if it’s finite, the number of stars in the Universe is still enormous and would be enough to light up the entire night sky.
We don’t see some stars because they are too far away and thus too faint
Why it’s wrong: Let’s divide the infinite Universe into even spherical layers with the Earth in the center. If one layer contains a certain amount of stars, then the layer positioned two times further away from the Earth would contain four times more stars due to the Universe’s homogeneity. However, according to the inverse-square law, the stars in the further layer would also shine four times dimmer when observed from the Earth. This means that the total luminosity of each of these layers would be the same. So, every layer of stars would produce the same amount of light, no matter how far away it is. As a result, the sky would be uniformly lit.
Space is filled with interstellar dust that blocks the light from distant stars
Why it’s wrong: The starlight would inevitably heat up the dust. According to the law of conservation of energy, the dust would soon start to reradiate the absorbed light and shine as brightly as the stars.
Olbers’ paradox solution: The mystery unraveled
There are two main factors that explain the “dark sky paradox”. Due to both of them, we see empty spaces between the stars in the night sky.
- 1. The Universe is not infinitely old
Our Universe is about 13.8 billion years old. Light takes time to travel, so we can only observe objects that are located 13.8 billion light-years away — not further. As the Universe is infinite in size, many stars and galaxies are invisible to us simply because their light has not reached us yet.
- 2. The Universe is expanding
Edwin Hubble discovered in 1929 that the Universe is expanding, causing light from distant galaxies to stretch as they move away from us. This effect, known as redshift, shifts light to longer wavelengths and eventually — into the infrared spectrum. Human eyes can’t see infrared light, so very distant stars and galaxies become invisible to us.
So, the darkness of the night sky is due to the limits of time, space, and human vision.
Dark sky paradox: Bottom line
Everywhere you look in the sky, there is a star or galaxy. You simply can’t see many of them because either their light hasn’t reached us yet or it has shifted into the infrared spectrum. That’s why the night sky appears dark to the human eye. Want to explore the stars you can see? Use the Sky Tonight app to identify any bright dot in the sky in real-time!
