This world has day and night. In ancient times, people didn’t know why there were day and night. But for modern people, we all understand 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, the sky we see is blue sky and white clouds, which are very beautiful.
But after night fell, the blue sky and white clouds were missing. Except for the twinkling stars, the background of the entire sky was black. The different colors of day and night may seem normal to many people, and there is nothing worth discussing. But for some wise men, they think far beyond ordinary people, and they think about things that ordinary people think are very normal.
It is believed that a few people in ancient times also realized that there is a more vast space outside the earth, and they will also think about why the sky is blue during the day and black at night? If there is cosmic space outside the earth, which color is the real cosmic background color?
In ancient times, people imagined that the universe outside the earth globe was like a heaven. It was a very beautiful world, so many ancient people thought that the colors of the universe were even more beautiful and eye-catching than the earth during the day. But with the development of human civilization and entering the age of science and technology, we finally understand that black is the inherent color of the universe, and the dark sky we see at night is the true color of the universe, not as beautiful as the ancients thought.
As for why the sky is blue during the day, I believe many friends also know it. Here is a little bit of science. We all know that the atmosphere cannot emit light by itself. Its light comes from the sun, which is transported to the earth by the sun.
The sunlight is a kind of composite light, which makes the entire surface of the earth bright up through refraction after the atmosphere. If only refraction gives the earth a daytime, then the sky we see should be a colorful sky composed of multiple colors. But the sky we see is a single blue. What is going on?
In fact, this is a glorious phenomenon, which is troublesome to explain. Simply put, it is that various gas molecules in the atmosphere scatter the sunlight, causing the blue-violet light in the sunlight to be scattered, thus permeating the entire atmosphere. . The reason why gas molecules have a stronger ability to scatter blue-violet light, while other colors of light are not displayed, is that the wavelength of light of different colors is different, and the shorter the wavelength of light, the stronger the scattering. The blue-violet light is a short wave, which is more easily scattered, so we can see the blue sky.
Of course, we all now understand that the colors of the universe are not beautiful, but as dark as our night sky. Only when night falls, we can see the true color of the universe, so why is the background of the universe so dark?
I believe many friends know that there are countless stars in the universe. The stars themselves are sources of light, and they emit light radiation all over the universe. Then the universe should be very bright, why is it black? Thinking about this issue, in 1823, German astronomer Heinrich Oberst, after observing the universe for a long time, raised this issue that has a huge impact on future generations and has plagued the scientific community for more than 200 years.
Oberst believes that if the universe is a stable space with countless stars, then no matter from which angle you look at the universe, you will see luminous stellar objects. The space between celestial bodies should be as bright as day, right? Why is the reality so dark?
At this time, many friends may say that this question is too simple, because the distance between the celestial bodies in the universe is very large, and the average is more than a few light-years, and in the space of these few light-years, there are A large amount of cosmic dust, most of the light is absorbed by cosmic dust and interstellar matter, and the unabsorbed photons are very weak, so that the entire universe and the entire night sky are black.
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 inevitably be released again. The more absorbed, the more released, so the photons emitted by stars are fixed. It is constant within the space range, and will not disappear due to the large amount of interstellar matter.
Therefore, the explanation that it is the interstellar matter that absorbs photons and causes the universe to become dark is incorrect. So what is the real reason? After Heinrich Oberst raised this question, many scientists also thought about it. At that time, most people believed that matter absorbed the light from the stars that caused the sky to turn black.
However, this explanation was quickly rejected by scientists. The existence of the law of conservation of energy tells us that this explanation is incorrect. At the time when the scientists were at a loss, there was a famous astronomer who made a major discovery, and he was Edwin Powell Hubble.
Believe that everyone is familiar with this astronomer, he proposed the theory of cosmic expansion. In 1929, when Hubble observed the galaxies in the river system, they found that the distance between galaxies was far away from each other, and the distance between them was proportional to the distance from the earth, that is, the farther away the galaxy is, the faster the distance away from us.
Such a discovery proves that the universe is expanding continuously and is still accelerating. This discovery is the famous “Hubble’s Law”. “Hubble’s law” is also the basis of the Big Bang theory. It is the observation that the universe is continuously accelerating expansion. Therefore, scientists believe that the universe may be born in a singularity big bang. After the big bang, it has its current scale through continuous expansion. .
The emergence of Hubble’s law allows scientists to find a breakthrough in the “Obers paradox”. With the theory that the universe is constantly expanding, the problem of why the universe is dark can be easily explained. The light emitted by stars continues to spread around the universe, but because the universe is accelerating and expanding, the speed of photons and the expansion speed of the universe have a red-shift phenomenon.
Redshift reduces the brightness. Even if the photon reaches us, it becomes visible light that the human eye cannot directly see. So in the naked eye world of human beings, the sky is black and the universe is black, but in fact, if we observe a certain piece of starry sky with an astronomical telescope, you will see a beautiful scenery. Under the lens of the telescope, there are countless little dots. The stars are constantly shining.
The accelerated expansion of the universe also gave us the concept of the observable universe. Because at a place far enough, the expansion of the universe’s receding speed exceeds the speed of light, and the speed of photons emitted by stars at this location is less than the regressing speed. This light will never reach the earth, and our astronomical telescopes will not be able to observe it.
Therefore, in the world of human eyes, the sky is black and the universe is pitch black, but the real universe background is not black, but a bright world. It’s just that we need to use powerful astronomical telescopes if we want to see the true state of the universe. Only then can you see the “light” of the sky.
Each ray is a star or a galaxy. It can be seen how huge the number of stellar coefficients exist in the universe, which is a data that is difficult to describe with numbers. Only by seeing the “light” in the sky, we can understand how small the earth and human beings are, and 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?