When we look at the night sky we are presented with a paradox: given the unfathomable size of the universe and the inconceivable number of stars that make it up, how can it be that the night sky is dark, instead of bright as a star? In an infinitely large universe homogeneously populated by infinitely many stars , any point in the night sky should be occupied by a star, be it relatively close, or on the other side of the cosmos. And yet what we observe is a night sky where we can observe several thousand stars with the naked eye and several billion with our most sensitive instruments, but without a doubt not infinite of them and not enough to fill the absolute totality of the firmament with its light.
This is what is known as the Olbers paradox , named after the amateur German astronomer Heinrich Wilhelm Olbers, who described it in 1823 . Although he did not describe it that way, nor was he the first to do so, we will consider the paradox in the following terms. Imagine that we divide the entire universe into concentric spheres with their origin on Earth and, for example, a light year thick. Any sphere, located ten thousand light years away, for example, will contain a certain number of stars . If we now take into account a sphere twice as large , located twenty thousand light years from Earth, we will see that it will contain, if the distribution of stars in the universe is homogeneous, about four times as many stars as the first sphere.
This homogeneous distribution will mean that, broadly speaking, the entire universe will be equally full of stars . Stars from less than one place will be offset by stars from more from another, but in general there will be the same number of stars in any piece of the universe we choose. Well, those 4 times more stars in the second layer , because they are further away, will shine about 4 times less seen from Earth than those in the first layer. Consequently, the light that we will receive from them will be approximately the same as what we received from the first layer. This means that each of these light-year-thick layers will contribute the same amount of light, and with infinitely many of them, we should receive an infinite amount of light , or at least enough to illuminate the night sky. Since the night sky does not look like that and is dark, one of the premises must be incorrect.
Even if some cloud of gas and dust could obstruct the view of a portion of the sky, as it happens for example in our own galaxy, in the direction of the constellation of Sagittarius, that the clouds of gas and dust prevent us from seeing the galactic nucleus directly. In an infinitely large universe filled with infinite stars, this gas would end up heating up by radiation and emitting as much light as any star , returning the brightness to that region of the sky.
So what is wrong? The size of the universe we do not know , because we cannot deduce anything about what is beyond the most distant that we are able to see. But even if the universe were infinitely large and full of infinite stars (we have no evidence for or against this) the reality is that the observable universe is not infinite . Since the universe had a beginning, and therefore it does not have an infinite age, and since the speed of light is not infinite either, despite having a very large value, only light from a finite region of the universe has been able to reach us. universe , which we know as the observable universe.
The fact that the universe is not infinitely old tells us, logically, that it had a beginning , which we know today, thanks to other complementary evidence, such as the Big Bang . However, the Big Bang theory introduces new problems to the Olbers paradox. Since the universe started out infinitely dense, hot, and small, and then expanded and cooled , there was a time when the entire universe was as hot as the surface of a star and emitted as much light as that. We should observe that same light reaching us from the most distant regions of the universe and the sky should not be dark, even living in a finite universe and in which we do not observe infinite stars. This light is known as the cosmic microwave background . The only reason it doesn’t result in a bright sky is because the expansion of the universe since that first light was emitted has caused its wavelength to expand as well, reducing its energy out of the visible part of the electromagnetic spectrum .
So the night sky is uniformly bright and not dark, if we look at it at the correct wavelength.
