Why is the night sky dark?

If you ponder the question of why the night sky is dark, it may seem that all this does not make sense. While anyone who has ever looked into the night sky, there was no doubt that it is very dark. The atmosphere on our planet are largely transparent to visible light, allowing us to peer into the vast cosmic ocean. During the day sunlight floods the atmosphere in all directions, and both direct and reflected sunlight enters from everywhere. Night, sunlight does not penetrate the atmosphere, so the sky looks dark. But that is only part of a complex response to the question of why we generally believe that the sky is and including space – black.

If the universe is infinite?

The universe is full of stars and galaxies that are large distances from each other: millions, billions, or even tens of billions of light years. Star light travels through the Universe and reaches our telescopes, revealing secrets of the universe. It is not excluded that the universe is infinite, and the number of stars and galaxies it is impossible to count. In fact, scientists are still undecided, the ultimate universe or not; we just don’t know. But we know that that part of the Universe that we can observe, should be finite.

In 1800-ies Heinrich Olbers drew attention to one mathematical paradox. If our universe was infinite with a constant density of stars and/or galaxies, then we would see an infinite amount of light from all sides, everywhere I look. First we would see the stars in the area, and then in between them – could see more distant stars. Thus, regardless of the distance, millions, billions, trillions of quadrillions of light years, etc. — in the end, wherever we looked, we would run into a star.

It is important to understand that the stars can be of different colors, shapes and sizes. So, in the vastness of the Universe there are stars many times exceeding the mass of our Sun. This best illustrates the star cluster NGC 3766 in the constellation Centaurus. If the universe was infinite, even in such a cluster would not be “gaps” between the stars, as a more distant star in the end, would fill these gaps.

The Paradox Of Olbers

But the total number of stars that can be seen at a certain distance, due to the surface area of the sphere, which increases with the distance squared. Multiply the number of stars the brightness of each star and you will get a constant value. But the brightness at a distance is a special value: let’s call it B. But what happens if the star is twice as far, it is the same brightness B. three times? Still B. four? And again, B. If you put everything together, we get b + B + b + B + ….. and so on. The answer, as usual, lies in the direction of infinity.

German astronomer, physicist and physician Heinrich Olbers in the nineteenth century used this line of reasoning, which led him to the conclusion that the observable universe cannot be infinite. But sure this is all it was. In the end, there were other astronomical problems. One of the most common objections was that this naïve analysis does not take into account all opaque dust, which can be seen just by looking at the plane of the milky Way. Even today, according to Forbes, many of the most famous astronomical sites filled with light-blocking dust.

Dark, dusty molecular clouds, similar to what within the milky Way, eventually collapse and give rise to new stars, and in the most dense areas are formed, the most massive stars. But the light of the stars can not break through the dust – he is absorbed by it. In the ultimate Universe this dust can compete with starlight, since visible light entering the dust is absorbed and re-emitted at lower energies. But if the universe were truly infinite, then the problem of the paradox of Olbers had for each speck of dust each speck of dust would have to absorb an infinite amount of starlight, yet she did not would radiate at the same temperature all the light absorbed by it!

In other words, something was wrong. Our universe cannot be static, infinite and forever filled with shining stars. If this were so, then the night sky would be bright in all places and in all directions. Obviously, there is something else. From our point of view, the observable universe can be 46 billion light years in all directions but is, of course, another unobservable universe, perhaps even an infinite number of them, of which more can be read in this article.