Why is it dark at night?

Why is the night sky dark? You may think the answer is obvious because the sun goes down. However, the answer is not so.

The only reason is to leave the light in the daytime because of the light spreading out in the atmosphere. If we don't have the atmosphere, for example on the Moon, it's still dark even though the Sun is shining. So change the question to: Why is the universe dark?

It is clear that the universe has many stars, countless stars are as bright as the Sun. In a boundless universe, if you look far enough in any direction, you will see a star or a galaxy. So, in theory, the sky will be as bright as the Sun all day and night. However, the reality is not so.

Will there be a limit, but beyond which stars and galaxies will disappear? A limit between "something" and "nothing" , a "boundary" of the universe? Not really. We have all the evidence that the universe seems infinite. Even so, the universe itself has a limit, not in space but in time.

As we know, the universe has a starting point, at least about 13.7 billion years ago, when the universe was so small and twisted together that the convention of space and time is no longer true. . It is like the universe is a giant thunder and we are still waiting to hear thunder start from afar.

Because a finite amount of time has passed since this "starting point" , some stars are too far away from us, so far that their light has not yet reached Earth. Also because light takes time to move in the universe, when we look at the telescope into a very far part of the universe, we are actually seeing that part when the light is released. So when we look at the light 13.5 billion years ago, we don't see any stars, not just because the light has not yet moved, but also because we are looking at the universe before we start, before the first star formed. This seems to be a relatively reasonable reason for why we see darkness, but the truth is not so.

Of course, we can find points in the sky that don't show any stars, by looking past new stars and looking back in time. However, when we look at telescopes through new stars, we still see light, not from stars, but light from the Big Bang. We get these cosmic radiation from almost all directions, creating a layer of light that covers the stars.

Thus, perhaps the actual universe is not dark. But why does it look dark? This is a clue to the answer: when the US Space Agency's Hubble telescope takes a picture of distant stars, to get these stunning photos, people use cameras. infrared.

Why is that? The distant stars and galaxies are moving farther away from us, the universe is expanding. Just like our voices become deeper as the tapes slow down, the doppler effect makes the stars move farther away from us.The farther the star is, the faster it will move and become red, until they become . infrared. And we can't see them, at least with the naked eye. And that's why the sky seems dark at night.

In short, if we live in an infinite, unchanging universe, the sky will be as bright as the Sun. However, the reality looks dark at night because the universe has a starting point , so not every direction has a star shining; and more importantly because light from distant stars (and even further radiation) is reddened until they become infrared and we cannot see them with the naked eye.