The response might be deemed obvious, but the question is not strictly related to our day and night cycle, this is a more complex question, deeply interconnected with the history and evolution of our cosmos.
Certainly, if we asked this question to any person, his answer would probably be something like “duh! because the sun is on the other side of the planet dummy.”; yet, even when the answer might be correct, (albeit a little rude). This only explains our perception of the day and night cycle on earth, but in essence does not answer the question of why in the night there is no more light, only the one we receive from the stars and temporarily from the moon, when reflecting the sunlight. In greater detail, why if the estimated number of stars is so large, with current estimates of one billion trillion stars, or numerically expressed: 1,000,000,000,000,000,000,000; which means that there are more stars in the universe than grains of sand in all the planet’s beaches (Yes, this is an interesting fact, since it is estimated that the earth has a total of 7.5 trillion grains of sand). So why, with that immense number of stars, we are not flooded with light coming from all of them?
Also called the “dark night sky paradox”, is the argument that the German Astronomer Heinrich Wilhelm Olbers proposed in the eighteenth century about this dilemma; and based on the current knowledge at that time on how the universe was constituted, which was considered static, infinite, homogeneous and with an infinite number of stars. His assumption was that the light of all these stars should cover every point in the sky, and the accumulated of this starlight would not leave any space without light in the sky; and we would be in a space totally embraced in light coming from the stars.
In this phenomenon, it was also considered the inverse square law (concept visited in this article); but in this case, to estimate the luminosity coming from stars at different distances. It can be considered that at a close distance from us on Earth, the number of stars would be relatively low, but they will have high luminosity, greater than from stars at bigger distances. On the contrary, at a greater distance, the number of stars multiplies, and even when their luminosity decrease, given their higher distance; as this light comes from a larger amount of stars, the accumulated light intensity would be the same as that of the accumulated of nearby stars. If we consider this, each “layer” of stars increases equally the total luminosity, and if the universe is infinite, as it was considered then; the we would have an infinite number of layers, and therefore, an immense amount of light. But it is obvious that this is not the case; and the sky is not covered with starlight, especially evident at night.
This question remained unanswered for centuries, being proposed by astronomers such as Johanes Kepler and Edmond Halley, and finally attributed to Olbert. It was until the early twentieth century, with the work of Einstein and his General Theory of Relativity, and the discoveries of Edwin Hubble, that an answer was finally considered.
Why is it dark at night?
Einstein theories complement the work of the mathematician James Clerk Maxwell, who established that the speed of light is finite and constant. In addition to this fact, Einstein with his special theory of relativity demonstrated how the speed of light travels at the same speed for any “observer”, regardless of their relative speed at which might move, and he concluded that the simultaneity of events is relative. That is, regardless of the relative object’s moving or resting state, the speed of light remains constant. And with the definition of his general theory of relativity, he concluded something much more intriguing. The equations of this theory showed that the universe was not static, but a universe that was expanding, growing steadily. This was opposed to the still prevalent conceptualization of a static universe, and this was the reason why Einstein added the “cosmological constant” in his equations to counteract this effect and maintain the result of a static universe.
It was between the years 1920 and 1930 when Hubble made his greatest discoveries, first, identifying that the so-called nebulae were in fact Galaxies at extreme distances, far beyond our own Milky Way, changing the understanding of how big the universe really is. And his even more important discovery of how the universe is expanding, growing continuously. Which confirmed the initial proposal from Einstein, making evident that the cosmological constant adjustment was not required; Einstein called this his “biggest blunder”.
So then, why is the sky dark at night?
Finally, the answer. If the universe is expanding, and light travels at a finite speed of 300 thousand kilometers per second (186,000 mi/s), this explains why the idea of a luminosity that increases with the accumulation of stars both near and far is incorrect. The light that can reach us is only the one that has had enough time to travel the distance that separates us from its origin star. And in many cases, for example, young stars located at a very great distance, the light haven’t had enough time to travel the distance to Earth. Added to this, with the expanding universe the light from more distant sources, must travel even more, and this distance is always increasing. This does not mean that the star where this light comes from is moving away; it’s just that, because of the universe expansion, more space is constantly being generated between the light source and us.
Another important result of Einstein and Hubble’s work is that, when analyzing the expansion of the universe, it can be concluded that, if “we reverse the time”, we’ll find out that there was a moment when the entire cosmos was contained in a single point of origin; this is what’s known as the Big Bang or the “origin of the universe”. And how long ago does is this event occurred? Current estimates place the Big Bang as an event that occurred 13.8 Billion years ago; certainly less than infinite, as in the static universe where light could have had the time to travel to us regardless of its distance. This means that the most distant light we can observe is that of the origin of the universe, this is the microwave background radiation. The photo of the Universe in its infancy.
Although this is an oversimplification of this phenomenon; these are the conditions that prove that light is limited in is motion both by distance and by time, for it to travel toward us; and therefore, not all the light of the stars can reach us; thus, giving us the opportunity to enjoy dark skies at night.
Saludos, Alex – ScienceKindle.