It's a matter of course that the day is bright and the night is dark, but in reality it's not as natural as we feel it.
In the places we can see, the universe is the most mysterious, about the universe, we have too many questions in our minds, such as the universe is not boundless? Has the universe been around for an infinite amount of time? In fact, it is not impossible to answer these questions, as long as we figure out why the night is dark, these questions will naturally have answers. Isn't it normal that the night is dark? It is true that there is no sun shining at night, but although there is no sun, there are stars in the sky, and most of these stars come from distant stars, which are as luminous as the sun, but they are too far away from us, so they seem to be just a small bright spot.
Can you rely on the stars to illuminate the entire night sky and make the night as bright as day?
Before answering this question, let's count how many stars there are in the night sky. It's not easy to count, but as long as we are willing to spend effort, we can still count roughly, maybe the number you and I counted are very different, but it is certain that there are certainly not a hundred million stars in the sky. That's the problem, there are also a conservative estimate of 100 billion stars in the Milky Way, and to be bold, it could be as many as 400 billion. A little farther away, outside the Milky Way, there are many galaxies like the Milky Way, and the number of stars in the entire universe is incalculable, although they are far away, and the light that can reach the earth is extremely weak, but their number is too large, and it is easy to illuminate the night sky of the earth.
But in fact nothing happened, the night on Earth was still pitch black, and there was only a little starlight in the sky, what did that mean? This means that the universe is not boundless, and the universe has not existed for an infinite amount of time.
If the universe is boundless, it means that there are infinite stars in the universe, and if the universe exists for an infinite period of time, the light of all stars will eventually reach the earth, no matter how weak the light of each star is, they will converge far beyond the light of the sun, so the night of the earth will never be dark. Just by virtue of the dark night on the earth, it negates the permanence of the universe, is it too hasty? It's a bit sloppy, but it doesn't matter, because we have other evidence.
When a star is farther and farther away from The Earth, the wavelength of the light it emits when it reaches the Earth will become longer and the frequency will decrease, which is called redshift.
Conversely, if a star gets closer and closer to Earth, the wavelength of the light it emits will become shorter and the frequency will increase when it reaches Earth, a phenomenon known as a blue shift. After analyzing the spectra of various galaxies in the universe, the astronomer Hubble came to the world-shaking conclusion that almost all galaxies have redshifts, which means that all galaxies are moving away from us. Why are all galaxies moving away from Earth? Why is the earth so rejected? There is only one explanation, not all galaxies are moving away from Earth, but that the universe itself is expanding.
The universe is a space, how does space expand?
The analogy of a balloon is the most appropriate. We draw two points on a balloon, and when we start blowing air into the balloon, the balloon begins to expand, which is like the expansion of space, and the two points drawn on the balloon are also far away from each other because of the balloon's expansion, and if the distance between the two points is farther away, the greater the increase in the distance between the two points with each breath. In other words, the farther away the points, the faster they can move away from each other due to the expansion of space. So not all galaxies are moving away from Earth, but all galaxies in the universe are moving away from each other due to the expansion of space, and it is precisely because of this that almost all celestial bodies have redshifts.
Because the universe has been expanding, and the farther away the faster the speed, beyond a certain distance, the galaxy is farther away than the speed of light, so the star light there can never reach the earth, and the stars we can see on the earth are naturally limited.
Since the universe has been expanding, we can also reverse the deduction according to the rate of expansion of the universe, and finally get a number, 138, that is to say, the universe was only an infinitesimally small singularity 13.8 billion years ago, this singularity is very hot and hot, at some point, for some reason, the singularity exploded, the universe was born, and the residual temperature of the original universe was left behind, which we call "cosmic microwave background radiation". The truth of the universe as we know it makes us feel even smaller, and the thought that eventually all the matter in the universe will cease to see each other due to the expansion of space makes us feel infinitely desolate.