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Waarom is de hemel 's nachts donker?

Why is the room dark? † Olbers’ Paradox | The dark sky paradox | dark night sky

Today we will try to answer the deceptively simple question we asked in the title of this article. Let’s talk at length about the “dark sky paradox”!


Why is the dark sky called a paradox?

You might be thinking, “What are we talking about? The sky is dark at night because the sun doesn’t rise!” But think of the stars.

Our galaxy alone contains 100 to 400 billion stars and the observable universe contains 100 to 200 billion galaxies. That’s a lot of stars and that’s an understatement! Then add to this information the assumption that our universe can be infinite. Every little part of the sky you look at should contain countless stars, so the night sky should be amazingly clear.

In fact, this is not the case. This paradox is called the Olbers paradox and we will try to clarify it in a few minutes.

What is Olbers’ paradox, explained simply?

The formulation of the “dark sky paradox” is usually credited to German astronomer Heinrich Wilhelm Olbers, but many famous people (including Johannes Kepler and Edgar Allan Poe) have contributed to this problem. the question. Simply put, the paradox goes like this:

If the universe is infinite and the universe has an infinite number of stars, then The whole sky should be covered with stars† We must be able to see a star in whatever direction we look, which is why the night sky must be so brightly lit. So why is the sky dark?

To better understand Olbers’ Paradox, imagine yourself in the middle of a dense forest Situations. Everywhere you look, you will see a wall of trees, with no space between them. And it is nothing but a jungle, it can certainly be big but it is by no means limitless. Now try to apply this equation to an infinite universe full of stars.

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Possible (but incorrect) solutions to Olbers’ paradox

Over the centuries, there have been several attempts to explain the bewildering phenomenon of the dark night sky. Let’s take a closer look at some of these explanations so we can rule them out.

The stars in the universe are not evenly distributed but are irregular

Why is this not true: This hypothesis could have explained the discrepancy, but recent astronomical data indicate otherwise. The universe appears almost completely isotropic (that is, the same in every direction).

The universe has a finite number of stars

Why is this not true: Even if the universe was finite, the number of stars in the universe would still be so huge that they would have to light up the entire night sky.

Some stars that we don’t see because they are too far away and therefore shine dimly

Why is this not true: Let’s divide the infinite universe into spherical layers, and the Earth in the middle. If a layer contains a certain amount of stars, the layer will be twice as far from Earth Four times more stars It must contain because of the homogeneity of the universe. But According to the inverse square law Are the stars in the far layer too four times weaker It should shine when observed from the ground. This means that the file The overall brightness of each of these layers will be the same† So each layer of stars must provide the same amount of light, regardless of how far away that layer is. It follows that the sky should be evenly lit.

Space is full of interstellar dust that blocks the light from distant stars

Why is this not true: It is inevitable that starlight will heat up the dust. According to the law of conservation of energy, dust should soon begin to emit absorbed light and shine like stars.

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What is the best solution to Olbers’ paradox?

There are two factors that could explain the “dark sky paradox”. These two factors cause us to see an empty space between the stars in the night sky.

  • 1. The universe is not forever old.

The age of the universe is about 13.8 billion. Light needs time to move. As a result, we can only observe objects that are a maximum of 13.8 billion light-years away from us and no more. Because the universe is infinite, there are many stars and galaxies that are invisible to us because their light has not yet reached us.

  • 2. The universe is expanding.

Edwin Hubble discovered the expansion of the universe in 1929. He saw that the light from distant stars and galaxies “stretched out” as they were rapidly moving away from us. This phenomenon is called “redshift” and occurs when light acquires a longer wavelength and travels toward the red side of the electromagnetic spectrum. Light waves extend from distant objects until they become infrared rays. Human eyes cannot see infrared light, and as a result, stars and galaxies that are very far from us become invisible to us.

ConclusionEverywhere you look in the sky, there is a star or a galaxy. You can’t see much of it because its light has not yet reached us or has moved into the infrared spectrum. This makes the night sky appear dark to the human eye.