If we can find a star in each direction of the sky that we point to, then why is the night dark? Have you ever heard of the paradox of Olbers, also known as the Dark Night paradox? He was first described by German astronomer Heinrich Wilhelm Olbers in 1826, and asks the following question: "If the universe is filled with stars (in any direction we point) then why is the night sky not as clear as the sun?" Sounds like a simple, harmless question… But his answer contradicted the assertion that the universe would be infinite and static, after all, the darkness of the night sky is one of the proofs that the universe is dynamic. And believe me if you can: the simple darkness of the night is one of the evidences of the Big Bang itself. Understand… The expansion of the universe if the universe were static, populated by an enormous amount of stars divided in a homogeneous way, any point in the sky should have a visible star. Therefore, the night sky should be as clear as our sun, as shown in the animation below:
Just look at the night sky to know that this is not reality. And this is mainly due to the fact that the universe is in continuous expansion. It is worth remembering that all the stars that we see in the night sky, without exception, are contained in our own galaxy, the Milky Way. Other diffuse light points that we see are nebulae (also within the Milky Way) and galaxies, which have their own grouping of stars. Millions of them… Although space seems to have no limits, the universe we know has an end, and a beginning. And not just a space boundary, as well as temporal. As far as we know, the universe had its start with the Big Bang, about 13.7 billion years ago. And the light of most of the stars (and their galaxies) did not have enough time to reach us. It's as if we were observing a lightning storm, and we were waiting for the sound of the thunder that hasn't arrived yet. When we point to a super-potent telescope for a region far away from the universe, we're observing the past. For example, if you look at a distance of 13.5 billion light years, you're looking at the beginning of the universe, and you don't see stars, because they haven't had enough time to graduate. But wait a minute! There is something wrong, after all, if we count only on the galaxies we know in the observable universe (and recent studies show that their number reaches 2 trillion) we should see the brightness of a star at any point in the sky. And why doesn't that happen? First of all, because we don't have infrared vision.