But in Germany in the early19th century, there was such a stuffed astronomer. He pointed out that if the universe is infinite and the stars are evenly distributed, then the night sky we see should be bright. The astronomer with a long and complete name is Heinrich Wilhelm Matthias Olbers.
This question put forward by Olbers has puzzled the scientific community 100 years, which is called Olbers Paradox. But in fact, the German astronomer Kepler (who discovered the laws of planetary motion) raised a similar question 400 years ago, which shows that this is really not a completely supported question.
The soul torture of Olbers's paradox: Why isn't the night sky dark? You are not mistaken. Since we take it for granted that the night sky should be black, we should first ask Comrade Olbers why the night sky is not black. ……
In fact, Olbers raised this question on the premise that 1, the universe is eternal and infinite. 2. The stars are evenly distributed. These two conditions were taken for granted two hundred years ago. At that time, scientists thought that the universe was eternal and infinite, so we didn't need to consider the creation of the universe, and the uniform distribution of stars was also in line with the gravitational balance necessary to maintain Newton's theory of universal gravitation.
Assuming that the above two conditions hold, we will get the result that the universe is infinite, so are the evenly distributed stars, and every ray of light emitted by each star can and has spread to infinity. From this, we will get the same conclusion from Olbers: If we look in any direction, our sight will eventually stay on a bright star with a limited distance, so the night sky should be bright no matter what direction we look.
The argument in the eternal universe seems impeccable, but in fact, even under the premise of infinite stars in the eternal universe, there is still controversy. One of the most important points is that although the universe is infinite, the life of a star is not infinite, which means that it can only emit limited light in a limited life cycle and spread to a limited distance. For example, a star with a life span of 5 billion years can only emit light in the radial range of 5 billion light years, and it is invisible outside this range. In this way, even if such a star blocks our view at a certain distance, it may be a dark planet that doesn't emit any light.
Is Olbers's Paradox so easily broken? Of course not! In the eternal universe, infinite stars emit infinite light and carry energy, which will heat any planet it illuminates. Therefore, in the infinite universe, each planet absorbs more heat than it emits, and eventually each planet will be lit up, whether it emits light or not.
Another controversial point is that distant starlight may be blocked by interstellar dust, but for the same reason, under infinite starlight, the dust will eventually be heated and illuminated until generate emits bright light. ...
That is to say, under the infinite starlight of the eternal universe, Olbers's paradox has no solution! The night sky should be bright ... of course, this is impossible in reality. Otherwise, not only will the night sky be as bright as day, but we will be scorched by the infinite starlight and shine brightly, and no one will think about this strange problem. ...
Solve Olbers's Paradox —— The universe is expanding. About one hundred years after Olbers's paradox was put forward, the great German physicist Einstein published his general theory of relativity, predicting that the universe is not a constant state, and it is either expanding or contracting. If it is the former, Olbers's paradox can be solved perfectly. But as a supporter of the steady-state universe, Einstein gave up this great discovery and chose to add a parameter-cosmological constant to the gravitational field equation of general relativity to keep the universe in a steady state.
But he soon regretted making this decision to gild the lily. Ten years after he added the cosmological constant to the gravitational field equation, American astronomer Hubble (finally one of them is not German ...) found that the universe is expanding through the observation data of galaxy redshift!
Why the expansion of the universe can solve Olbers's paradox? According to the fact that the universe is expanding, all the galaxies in the universe must gather at one point, which means that the universe must have a beginning, and the universe is not eternal and infinite. Because at least in the observable range, it is limited in time. Now scientists speculate that the universe has only been 654.38+03.82 billion years since its birth. According to the principle that the speed of light is constant, this means that the farthest starlight only travels 65.438+0.382 billion light years. Of course, with the expansion of the universe, this number is actually a little larger. According to the calculation, the radius of Hubble volume is about 46.5 billion light years, and our Hubble volume is about 93 billion light years. Any starlight outside this sphere has not spread to us at the moment.
Because the universe we can observe is limited in time and space, and the number of natural stars is also limited, the precondition of Olbers's paradox is not established, and Olbers's paradox is not established. If we look at the universe in any direction, we may not be blocked by any starlight, but we will still see a dark universe, just like what we see now.
Postscript Although there is only a little starlight in the visible light range in the night sky, it is actually full of faint light that we can't detect in the dark background of the universe. This is the first ray of light that shoots into the distance after the birth of the universe-cosmic microwave background radiation. It's all over the universe. It emits bright orange-red light at a high temperature of 3000K, but with the expansion of the universe, its frequency has been declining, and now only 2.7K low-temperature microwave radiation below the human eye can be recognized.
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