Why is the night sky dark?
answer:
The more obvious answer to this question is: during the day, sunlight scatters through the atmosphere and then enters our eyes, so it is bright during the day. At night, due to the rotation of the Earth, we are on the back side of the Earth, and the sun does not shine, so the night sky is dark.
But in fact, it is not so simple to really solve this problem. We are in a vast universe, there are countless luminaries like the sun in the universe, if the earth receives all the luminous bodies from the infinite universe of the photons accumulated, through the integral calculation, it is not difficult to find that the result tends to infinity, that is, the earth and any other planet in the universe should be always lit. The problem is the Orbersian paradox.
But this is clearly not the case, and the crux of the matter is the expansion of the universe. Due to the expansion of the universe, the density of luminaries throughout the universe is actually decreasing, and some of the luminaries are moving away from us, which will lead to the redshift of light waves, that is, the energy of photons decreases. On the other hand, according to the theory of cosmic expansion, the universe has an initial point, about 13.7 billion years ago, which means that we can only see luminous bodies within 13.7 billion light-years of us, and the photons in the depths of the universe have no time to spread to Earth. Therefore, the photons received by the earth are actually from a part of the universe that is relatively close to the earth, and the energy is constantly decreasing, these energy is limited, and the number of photons is indeed not enough to illuminate the earth, in fact, the universe has microwave background radiation, it is also derived from these low energy photons, the wavelength of microwave background radiation is at the centimeter level, which has exceeded the perceptual range of the human eye, so the night sky we see is black.
Resources:
Why is the night sky black? | Heaven asks the column
By there is a balance
