Why does the night sky look dark? The first answer that comes to mind when you hear this question is that there is no sun in the night sky so the night sky looks dark. Now the question is that the sun is a star and there are many stars in the night sky. If the galaxy is evenly starred, then no matter where we look, our vision is to reach the surface of one or another star, in which case the night sky should not be dark but bright as day. So let's look at the sky, and try to find the answer to this question!
Until the early twentieth century, it was thought that the universe was essentially constant over time. Perhaps the universe has existed indefinitely. But this thought leads us to some bizarre decision. If the stars had been radiating for an indefinite period of time, they would have heated the whole universe to a temperature similar to theirs. Even at night, the whole sky shone like the sun. Because every line of our vision would end in a cloud of stars or dust. And these dust clouds would continue to heat up until they reached the same temperature as the stars.
This observation is very important that we see the night sky dark. Let's see how much can be imagined from the dark sky tonight. The dark night sky indicates that the universe we see today cannot be the same forever. There must have been something in the past that made it possible for the light of the stars to shine a finite time ago. If the stars were fixed in the same place forever, why did they suddenly become illuminated billions of years ago? Which special watch told them to light up at that very moment? Then we can imagine that the stars formed during the Big Bang could not stay in the same place and spread light indefinitely. If we could, we would see the night sky bright. So for what reason are we seeing this? Where is the evidence in our hands?
Most people in the past believed in the idea that the universe was created only a few thousand years ago, almost as it is now. Disagreements with this idea began in the second decade of the last century. This disagreement began at that time with some observers from Vesto Sliffer and Edwin Hubble. In 1923, Hubble discovered that the vast array of nebulae in the night sky, called nebulae, are actually different galaxies. That is, they are like our sun, a cluster of innumerable stars, which are far, far away from us. The reason they look small and vague is because they are so far away. So it takes millions of years, even billions of years, for light to reach us from there. This makes it clear that the universe could never have started just a few thousand years ago.
But the second thing Hubble discovered is even more remarkable. By analyzing light from other galaxies, it is possible to measure whether galaxies are moving away from us or moving away from us. In this way Hubble was surprised to see that most of the galaxies were moving away from us. Not only that, the farther away the galaxy is from us, the faster it moves away. Hubble understood the dramatic implications of this discovery. He realized that on a large scale each galaxy was moving away from all other galaxies. This means that the universe is expanding.
The discovery that the universe is expanding is one of the most remarkable intellectual revolutions of the twentieth century. Honestly it was an extraordinary event. This discovery also changed the field of discussion about the birth of the universe. If galaxies move away from each other in this way, then naturally they must have been together at some point in the past. From the current expansion rate of the universe, it can be estimated that galaxies were really close to each other 10 to 15 billion years ago. Here is an explanation of why the night sky is dark! We can only see as far as the light has reached from the Big Bang billions of years ago to the present time. No other star has been able to shed light during the 10 or 15 billion years since the Big Bang. So the light from distant lands has not reached us now.
Twentieth-century astrophysicists have proved the presence of hydrogen clouds in the universe through spectral scattering of light. As the universe expands, the energy from the stars in distant galaxies increases, so does their energy. This is why these lights can be easily absorbed by hydrogen gas. These gases are scattered in almost all galaxies, including our galaxy. The light from a distant star loses its energy due to excessive red radiation and is absorbed by the hydrogen cloud, so the light does not reach us completely. And the light from galaxies and stars farther away is still far away. We see exactly the number of sources from which light reaches the earth. And that is why we see the night sky dark despite the fact that the night sky has innumerable star.