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When we look up at the starry sky, why do we see that the whole universe is black?
First of all, I want to say that for us ordinary people, this is a seemingly simple question, but for some scientists, this is an incredible question, at least in the past. Assuming that the universe is stable and infinite, and the stars are evenly distributed, there are infinitely many luminous bodies in the universe, which is theoretically enough to make the night sky much brighter than above. On the contrary, the night sky we see is almost dark except for the brighter stars, as shown in the figure below. The person who first put forward why the sky is always dark can be traced back to 1576. Thomas Digges, an Englishman, mentioned this problem in his father's book "Predicting the Future". It was the German astronomer Olbers who put forward and wrote a paper on cosmology in 1823.

This is the so-called Olbers Paradox, which points out that if the universe is stable and infinitely flat, the distribution of many stars will be very uniform, the light of each star will reach the earth at the same time, and our night sky will look bright. But what we see in the night sky today is just the opposite, which creates a paradox. At that time, many people didn't pay enough attention to this simple question and wanted to know what these scientists were saying and why. It was not until the 1950s that we got a perfect answer. Before this, there were many explanations for this phenomenon, but they were all denied.

For example, oberth himself explained that light from a star will never reach the earth because it is blocked or absorbed by distant cosmic dust particles. But his explanation violates the first law of thermodynamics, which points out that energy must be conserved, so the absorbed light must radiate photons in all directions when it is transmitted to dust. The first person to put forward a correct explanation was edgar allan poe. To our surprise, he is not a so-called astronomer but a romantic American poet. In his prose poem Eureka (1848), he thinks that the reason why we see the dark background of the night sky is mainly because some invisible stars are too far away from us and their light cannot reach us.

In the 20th century, we finally got a fairly good answer, that is, the universe is too young and still expanding. Now we see a phenomenon called "red shift". Almost everything far away is far from the earth. So what we need to know is that the wavelength of the light emitted by distant stars is pulled too long, so that the energy of the corresponding light becomes very weak, so the light emitted by many distant stars is so weak that we can hardly see it when it reaches us. The most important thing here is that the speed of light propagation is limited, and there are too few stars in the universe to illuminate the whole universe.