Why is the night sky black? A simple question hides the secret of the universe

There are day and night in this world. In ancient times, people didn’t know why there were day and night. But for modern people, we all know that the reason why there are day and night is because of the rotation of the earth.

The sunrise and sunset we see are also due to the rotation of the earth. When the sun turns to the other side of the earth, our side is night, and the opposite side is day. Day and night are two completely different scenery, during the day, we see the sky is blue sky, white clouds, very beautiful.


But when night came, the blue sky and white clouds disappeared. Except for the twinkling stars, the background of the whole sky was black. For the different colors of day and night, it may seem very normal to many people, and there is nothing worth discussing. But for some wise people, their thinking is far more than ordinary people, they will think about things that ordinary people think are very normal.

I believe that a few people in ancient times also realized that there was more space outside the earth, and they would also think about why the sky was blue during the day and black at night? If there is space outside the earth, which color is the real background color of the universe?

In ancient times, people imagined that the universe outside the earth ball was like heaven. It was a very beautiful world, so many ancients thought that the color of the universe was more beautiful than the earth in the daytime. But with the development of human civilization, after entering the era of science and technology, we finally understand that black is the inherent color of the universe itself, and the dark sky we see at night is the real color of the universe, which is not as beautiful as the ancients thought.


I believe many friends know why the sky is blue during the day. Here is a brief introduction. As we all know, the atmosphere can’t emit light by itself. Its light comes from the sun and is transported to the earth by the sun.

The sunlight is a kind of compound light, which makes the whole earth surface bright by refraction after the atmosphere. If only refraction makes the earth have day, then the sky we see should be a colorful multi-color sky. But the sky we see is single blue. What’s the matter?


In fact, this is a kind of luster phenomenon, which is troublesome to explain. In short, various gas molecules in the atmosphere scatter the sunlight, so that the blue and purple light in the sunlight is scattered and diffuses to the whole atmosphere. The reason why the ability of gas molecules to scatter blue and purple light is relatively strong, while the light of other colors does not appear is that the wavelength of light of different colors is different, and the shorter the wavelength is, the stronger the scattering will be. And blue violet light is a kind of short wave, it is easier to be scattered, so we see the blue sky.

Of course, we all know now that the color of the universe is not beautiful, but as dark as our night sky. Only when night comes, can we see the real color of the universe, so why is the background of the universe dark?

I believe many friends know that there are countless stars in the universe. The stars themselves are light sources, and their light radiation is all over the universe. So the universe should be very bright. Why is it black? In 1823, Heinrich orbers, a German astronomer, made a long-term observation of the universe and put forward a question which had a great influence on later generations and perplexed the scientific community for more than 200 years.


Obers thinks that if the universe is a stable space and there are countless stars, then no matter from which point of view we look at the universe, we will see luminous stars and celestial bodies. The space between celestial bodies should be as bright as day? Why is the reality so dark?

At this time, many friends may say that this problem is too simple. It is because the distance scale between celestial bodies in the universe is very large, with an average of more than a few light years. In the space of a few light years, there is a large amount of cosmic dust. Most of the light is absorbed by cosmic dust and interstellar matter, and the photons that are not absorbed are very weak. Therefore, the whole universe is destroyed Chou, the whole night sky is black.


So is this explanation correct? If you think so simply, you are wrong. This explanation does not find the essence of the problem. We all know the law of conservation of energy. The energy in the universe cannot disappear for no reason. The energy absorbed by cosmic dust and interstellar matter will be released again. The more absorbed, the more released. Therefore, the photons emitted by stars will not change in a fixed space range and will not disappear because of the large amount of interstellar matter.

Therefore, the explanation that interstellar matter absorbs photons and makes the universe dark is not correct. So what is the real reason? After Heinrich orbs raised this question, many scientists also carried out thinking and research. At that time, most people believed that the sky darkened because matter absorbed the light from stars.

However, this explanation was also quickly denied by scientists. The existence of the law of conservation of energy tells us that this explanation is incorrect. When the scientists were at a loss, a famous astronomer made a great discovery. He was Edwin Powell Hubble.


This astronomer is no stranger to us. He put forward the theory of cosmic expansion. In 1929, when Hubble observed the galaxies in the river system, he found that the distances between galaxies were moving away from each other, and the speed of moving away was proportional to the distance from the earth. That is to say, the farther away the galaxy is from us, the faster it moves away from us.

This discovery proves that the universe is expanding constantly and accelerating. This discovery is known as “Hubble’s law”. “Hubble’s law” is also the basis of the big bang theory. It is precisely the observation that the universe is expanding at an accelerating speed. Therefore, scientists believe that the universe may have been born in a big explosion at a singularity. After the big explosion, the universe has its present scale only through continuous expansion.


With the emergence of Hubble’s law, scientists have found a breakthrough in the “orbs paradox”. With the theory of the expanding universe, the problem of why the universe is dark can be well explained. The light from stars is spreading around the universe. However, due to the accelerated expansion of the universe, the speed of photons has a red shift with the expansion speed of the universe.

The red shift reduces the brightness. Even if photons arrive in front of us, they become visible light that cannot be directly seen by human eyes. Therefore, in the world of human eyes, the sky is black, and the universe is black, but in fact, if we use an astronomical telescope to observe a piece of star sky, you will see a beautiful scene, under the lens of the telescope, countless stars are shining.

And the accelerating expansion of the universe also gives us the concept of observable universe. Because far enough away, the retrograde speed of cosmic expansion exceeds the speed of light, and the speed of photons emitted by stars at this location is less than the retrograde speed. These lights will never come to the earth, and our telescopes will not be able to observe them.


Therefore, in the world of human naked eye, the sky is black, the universe is dark, but the real background of the universe is not black, but a bright world. Just want to see the real appearance of the universe, we need to use a powerful telescope, only at this time, you can see the “light” all over the sky.

Every ray of light is a star or a galaxy. We can see how many galaxies exist in the universe. It’s an indescribable number. Only when we see these “lights” all over the sky, can we understand how small the earth and human beings are. In the endless starry sky, I don’t know how many intelligent civilizations like human beings are also looking up at the starry sky and Thinking: why is the night sky black?

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