I love it when a seemingly simple question—so simple it seems silly to ask—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 it deep?”
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yes Yes, it is. I can even imagine you thinking, “But the answer is obvious! There are just so many stars around. Those further away are more modest. So there is a lot of blackness between them and nothing they would lighten up. Of of course the sky is dark at night.’
But that answer is wrong or, at best, incomplete, and the real situation was not apparent even to eminent scientists a century or more ago. And the first person to solve this mystery—or at least the first to write down the correct scientific answer—is almost certainly not who you might think!
To be clear, there is no heaven perfectly dark at night Earth’s atmosphere has a faint glow from even the darkest zone. Here, however, we are talking cosmologically dark: the universe itself is without light. There are some background light sources even then—distant stars and galaxies too small and faint to see individually—yet the sky appears quite black compared to, say, the surface of the sun.
That may seem like an extreme comparison, but it’s not. That is the crux of the problem.
Historically, astronomers believed that the universe was infinite in time and space, stretching on forever. It always was, and always would be. It was static, unchanging.
There was reason to believe that, especially if you were an astronomer in, say, the 19th century. You would think that the Milky Way was the entire universe and, according to Ptolemy’s ghost, did not change from night to night. Oh, the moon certainly went through phases, you might say; each planet True to Greek etymology from that word, he wandered in the sky, and so on. But the stars were always there and always had been.
There is a problem with this idea, however: if the universe is infinite in space, with stars evenly distributed, the sky should be clear.– very bright, based on its geometry.
Suppose you count all the stars in a thin spherical shell centered on Earth, with walls one light-year across and 1,000 light-years away. If you look at another shell twice the distance (2,000 light years) and the same thickness, the math says it will have four times the volume and therefore four times the number of stars. The volume of such a thin shell increases with the square of its distance.
It’s farther away, though, which means the stars will be fainter. This continues the reverse square law: brightness decreases with the square of the distance. So a star twice as far away will be a quarter as bright.
This means that any shell you choose looks as bright as any other, regardless of distance. Near or far, volume up and brightness down are exactly canceled out.
OK, that’s fine. But remember, in our 19th century way of thinking, the universe is infinite; this means that there are an infinite number of shells and an infinite number of stars. In this scenario, no matter how far you look, it will be your vision always hit a star, no matter what small area of the sky you observe. That means the whole sky should be as bright as a star!
Not too hard to break, but not like that. The sky is dark. How can this be? Well, you will argue that there are nebulae, dust and gas clouds in space. These can block the light of more distant stars, keeping the sky dark.
Unfortunately, this doesn’t work. Those clouds heat up from absorbing all that radiation and soon become as bright as any star. You didn’t solve the problem by adding nebulae; You’re a little late. And the infinite universe is patience. Soon, it will be bright all the time everywhere, and you’re back to square one.
We call this apparent contradiction in the dark sky the Olbers paradox, after the German astronomer Heinrich Wilhelm Matthias Olbers, who wrote in 1823 in his treatise “Über die Durchsichtigkeit des Weltraums” (“On the transparency of space”). I’ll note that in the great tradition of Western nomenclature, this is what it’s called, even though Olbers wasn’t the first person to think or write about it—the question had been around for centuries and dates back to the 16th century. It was discussed by the 19th century English astronomer Thomas Digges. when he proposed that the universe could be infinite in nature.
Several scholars tackled the issue, but none provided a scientific solution to Olbers’ paradox. That changed in 1848, however, when an American writer published the essay of the title Eureka: A Prose Poem. In it, he proposed that it was the universe no infinite in extent and, being the speed of light also finite, there was not enough time for all this light to reach us:
The only way, therefore, we could understand under such a situation the gaps what our telescopes find in innumerable directions, would be to suppose that the distance of the invisible background is so great, that no ray coming from it has been able to reach us.
Although the essay does not provide a quantitative mathematical solution – it would arrive decades later through the famous British physicist Lord Kelvin (William Thomson) – it is correct in principle, including one of the two main parts of the correct explanation.
Who was the author of the essay? Edgar Allan Poe. I bet you’ll never look at it the same way again.
The other part of the answer is similar, because the universe has not always existed. At the beginning of the 20th century, astronomers already suspected that the universe was much bigger than the Milky Way. they were more and more secure. The discovery of the cosmic expansion rejected the static cosmos hypothesis, the view that the universe is unchanging, and, along with other observations, paved the way for the current big bang model. The Big Bang model does not preclude a universe that is infinite in space, but, in support of Poe’s conjecture, it predicts a beginning, a cosmic First Day. In the end, we can see so far that the light from stars that are too far away cannot reach us.
Interestingly, the expansion of the cosmos would still explain the dark night, even if it occurred in a steady state universe. If the entire universe were if it expands, at a certain distance from Earth, that expansion would carry the stars too fast for the light to reach us. So it’s not so much that the dark sky proves the big bang model, but that the big bang is a key aspect to understanding what’s going on.
We now know that the universe was formed 13.8 billion years ago, putting a limit on how far we can see objects. The expansion also extinguishes their light, because they lose the energy that reaches us (creating cosmic redshift). Also, stars have a finite lifespan, and when all the nebular gas to make them is consumed, the universe will darken, making the dark sky even darker over time.
Lord Kelvin himself said that there are no paradoxes in science; they are illusory conflicts that arise because of our limited understanding. The more we learn about the universe, the more the so-called paradoxes become obscured and disappear.
