Why is the night sky dark?
Simple, but long troubled mankind. There is a problem that has come to the right answer since it came to modern science.
Why is the night sky dark?
For our modern people who have forgotten this night, this sounds like an absurd question.
No, of course, the night of the sun is dark.
But if you ask a countryman, you will answer this.
Why is the night dark? How many stars are there?
But those who think more deeply will have questions.
"The universe is infinitely wide, and if so, it would be infinitely many. Why is the night sky dark?"
There is a man named Albers who asked this question.
Olbers had a question.
If there is a homogeneous spread of stars in the universe, there will be more stars in proportion to the distance, and starlight will be more visible on the earth in proportion to the distance, so if the universe is infinitely wide, the sky must be infinitely bright. The universe is dark "
If you think about it, Albers is right. If the universe is infinitely wide, why is not the night sky so full?
If the universe is infinitely wide and there are infinitely many stars, the night sky will be filled with nothing but the fact that it must be bright enough to be blind.
In fact, it is normal that the stars are full not only at night but also in the daytime so the jab is not good.
Why is not the universe like this?
Several solutions have been made in the past to solve the paradox of Albers.
1. There is something in the middle to cover the starlight.
In the universe, there is not only a star, but a shining planet or gas-like debris that spreads and absorbs properly.
But the solution is no good. Light is also energy, and when we receive the sunlight, it gets warmer.
If there are infinitely many star lights, they will also shine in the end because of the energy that absorbed their energy.
In fact, most nebulae we see are nebulae that emit light by absorbing energy such as ultraviolet rays from nearby stars.
The conclusion is that if the universe is infinite and the starlight is infinite, then one day, the 'veils' that obscure the starlight can not solve the paradox because it eventually becomes energized with energy.
2. The star dies.
Because the star is dying, it can not be a source of continued starlight. So it is somewhat balanced between the stars that are properly dying and arising, so that they are moderately dark and have some starlight.
However, this can not be solved either. It takes time for the light to move. So the star we see actually dies and explodes, but we see the starlight. This is because starlight is moving through the universe before the star is dead.
Thus, even if the stars are constantly dying, the stars are constantly evolving and the starlight remaining in the universe will also increase in proportion to the size of the universe.
If so, then the universe must also become infinitely bright.
3. The starlight has not yet come to earth.
The universe is so wide that the distant starlight has not yet come to earth.
That's a pretty good answer. Then, does the universe grow brighter and infinitely brighter in the end?
solution
Olbus's paradox was conclusively concluded that modern cosmology is not a dark paradox.
We see red stars as we see distant stars. Because the universe is expanding, distant stars are farther away. (Red shift)
To be redder is that the light has a longer wavelength, and the farther away, the longer the wavelength of the star and the longer the wavelength.
The longer the wavelength, the lower the energy.
Eventually, the starlight decreases more and more as the distance increases, and the starlight of the 'observable boundary' from which the rate of expansion of the universe becomes almost equal to the starlight, will not reach the earth after that.
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