Why Is Space Dark And There Is Light On Earth.

Why is space dark and there is light on Earth? There's a question that often comes up during those evenings when friends gather and the conversation drifts to some incomprehensible physical principle related to astronomy. Usually under the starry sky, preferably in summer, it's not uncommon for someone to eventually look up and ask, why is space dark? The unfortunate person who asks this question often receives a variety of explanations, where complex concepts like relativity and cosmology are mixed with simpler ideas like light reflection, incorrectly of course. And when we say incorrectly, it is because the person asking the question is almost always confusing two very different phenomena, even though both involve space and darkness. One thing is to ask, why is the sky seen from space always completely dark even in the presence of the sun? Another is to wonder, if there are indeed billions and billions of stars in the universe, enough to cover the entire celestial sphere, why doesn't their light illuminate our nights like the sun does during the day? Folks, if you're up to it, keep watching, because the topic is really complicated. It was 9.07 a.m. Moscow time, on April 12, 1961, when the rocket engines ignited and lifted off, carrying a human into space for the first time. The cosmonaut, as you know, was Yuri Gagarin, and his first words after reaching orbit were, the sky is black, totally black, and along the edge of the Earth near the horizon there's a beautiful blue halo, from here I can also see the stars. At that time, this was already known in the scientific community, albeit only theoretically, but Gagarin's first-hand account helped bring this counterintuitive oddity to the attention of the general public. Indeed, the initial question might seem very simple at first glance. If you asked an elementary school child, the most predictable answer you might get is that at night there is no sun, and the light from the stars is too weak to illuminate the night. This answer isn't wrong, but it's at least incomplete and simplistic. People tend to think that during the day we can't see the stars because of the blinding sunlight. Nothing could be further from the truth. The reason for the invisibility of fixed stars during the day is due to the presence of Earth's atmosphere. The gases that make up Earth's atmosphere have the ability to uniformly scatter the light coming from the sun so that it seems to come from all directions, not just the sun. It's a bit like when you're driving at night and encounter the headlights of another car. If the night is dry and clear, the headlights only illuminate their respective stretch of road. But if the night is foggy, you'll see a whitish wall that prevents you from seeing even the edge of the road. On the Moon, which has no atmosphere, the sky is black even when the sun is high on the horizon. The same thing happens on any other rocky planet without an atmosphere. In short, the only reason space seems black is because it's empty. It's not black because it's dark, but because there's no matter there. There's nothing there to illuminate. Light travels in a straight line and doesn't hit anything that can scatter it in all directions. Even here on Earth, if we went up in a hot air balloon to about 32 kilometers in altitude or 20 miles, we would see the color of the sky gradually become darker blue, then transition to violet, and finally to the absolute black of interplanetary space. We would even be able to see the stars. And think about it, at that altitude, we still couldn't say we're in space. There isn't a precise point where space begins, considering that the atmosphere gradually thins out. However, there is the so-called Kármán line, located at 100 kilometers in altitude or 62 miles, which is conveniently where space begins according to international treaties. At 32 kilometers altitude, we would still be well within our atmosphere. And it might surprise you to know that, technically, even the International Space Station, moving at 400 kilometers altitude or 250 miles, is still considered to be within this gaseous layer, which probably extends up to 2000 kilometers or 1250 miles. And with this, we've reached the other side of the initial question, which despite the similarity in terms of darkness and space, is very different from what we've discussed so far. That is, if the universe is so vast and there are countless stars, why is the night sky dark? Shouldn't it be uniformly illuminated by all the stars in the universe? After all, there are stars and galaxies in the sky, which, assuming an infinite universe, would be so numerous that they would light up the night sky as brightly as the sun does during the day. The answer to this question, unlike the one we've just discussed, is actually closely tied to fundamental cosmological issues related to the evolutionary history of our universe. The paradox is now associated with the name of Oblers, a 19th-century German astronomer, who also discovered asteroids, Pallas and Pesta, and some comets. The idea of the universe at that time was based on the so-called perfect cosmological principle, which states the universe is infinite, has no privileged positions or directions, and is unchanging over time. Under these assumptions, and for simplicity, assuming all stars have the same brightness, the radiant energy reaching Earth is simply the product of the brightness of a single source multiplied by the number of sources. And since the volume is infinite, the number of sources and the total radiant energy must also be infinite. It was the Englishman Thomas Diggs who broke the Ptolemaic dogma of stars fixed to a crystalline sphere. In 1576, revisiting the concept of infinite space, which was already Democritus' idea two millennia earlier, he vigorously supported the vision of countless stars uniformly scattered in the infinite universe, leading to the question we're dealing with. If the universe is infinite, and thus the stars filling it are infinite in number, why isn't the night as bright as the day? Hey guys, just a moment before we continue! Be sure to join the Insane Curiosity Channel! Click on the bell, you'll help us to make products of even higher quality! His answer was that the most distant stars were too faint for their light to reach us, but as we will see, he was wrong. The paradox was also addressed by Kepler in the 17th century, and later revisited by both Halley and the Swiss astronomer Jean-Philippe Chézot. Chézot, although providing no better solution, that of light absorption, as we will see later, nonetheless managed to refute Diggs' answer, demonstrating that every star, even the most distant, contributes to the total brightness that should reach our night skies. Once Diggs was disproven, the problem remained unresolved, continuing to puzzle the best minds of the time and suggesting a plethora of explanations. One of the first considered by Chézot himself was the hypothesis that there was a large amount of interstellar dust that could prevent the observer from seeing the most distant stars. Indeed, it is well known that observing objects at low galactic latitude near the plane of our galaxy, represented in the sky by the so-called Milky Way, is very problematic due to the presence of dust. For example, in studies of the distribution of extragalactic sources, such regions are deliberately excluded to avoid systematic effects in the statistical analysis of the data. From the perspective of Olber's paradox, however, this justification is simply wrong. Any absorbing material, after a sufficient period, would reach thermodynamic equilibrium with the light hitting it and would start emitting as much energy as it receives, recreating the original problem. Another possible explanation is that our universe, despite its infinite size, has only a finite number of stars. However, observational estimates of the amount of luminous matter are so large that they do not solve the paradox. Even with a finite number of stars, the sky would still be bright at night. In the 19th century, Herschel was the first to propose the so-called hierarchical solution to the paradox, later taken up by many other astronomers until the beginning of this century. The model is based on the Kantian idea of hierarchy of clusters of increasing size and suggests that our observable universe is just one of a series of nested universes, like a giant Russian doll. It can be shown that if the larger clusters were sufficiently less dense than the smaller ones, this could solve the paradox. More modern versions of this model are based on a fractional distribution of sources, appearing identical to itself when viewed on different scales. For example, the stars could indeed be infinite but arranged in such a way that they hide behind each other like the trunks of a dense forest. In this way, only a limited area of the sky would be bright, and the paradox would be resolved. This solution may seem artificial, ad hoc, but it is theoretically correct. For example, if the stars were distributed in a fractal manner, there could be large regions of the sky completely devoid of luminous sources, and the sky would appear completely dark, except for small regions. It is worth noting that the most recent observational data on the large-scale spatial distribution of galaxies and galaxy clusters seem to agree, at least over a limited range of scales, with a hierarchical model. However, extending this model to larger scales contradicts the observation of the so-called cosmic background radiation, which is currently considered the most compelling evidence of the universe's homogeneity and isotropy on a grand scale. The modern view of the universe is very different from that of the last century. On one hand, the theory of general relativity, developed by Einstein, has connected space and time, thanks to the fact that the speed of light is finite. Since it is not possible to communicate at speeds faster than this, there is always a finite time to connect two different regions of the universe. On the other hand, in 1929, Hubble discovered, through the observation of redshift of some lines in the spectra of galaxies, that the universe is expanding, something already predicted by general relativity. As a consequence, the perfect cosmological principle fell. The universe is not immutable at all. There is an evolution of the universe, whose properties therefore change over time. The development of the theory led to the Big Bang model, which predicts an initial time, about 13.7 billion years ago, from which the expansion of the universe originated. This model predicts the existence of the so-called cosmological horizon, which encloses the region that can send light signals to an observer on Earth. The solution to the fundamental equations of the model, named after the Russian mathematician Friedmann, shows that this region is finite and not infinite. Based on these new interpretations, quite complicated, so I apologize if my explanation wasn't clear or worse if it was boring, there are two possible explanations for Olber's paradox. The first is that the universe is expanding and, as a consequence, there is a redshift of the light emitted by distant celestial objects. This is due to the so-called Doppler effect, the same effect that makes the whistle of an approaching train sound higher pitched than that of a departing train sound lower pitched. For the most distant objects, this redshift would be so great that it would prevent their observation. The second is that again due to the expansion, the energy emitted by distant stars and galaxies would be spread over larger volumes, appearing much weaker to an observer on Earth. The combination of these two factors means that the total brightness received is not infinite, but finite. In this view, the fact that the night sky is dark is simply proof of the universe's expansion. Another possible explanation for the paradox within the Big Bang model comes from the age of the universe. Since it is relatively young and not eternal, the time available for galaxies to produce radiation is also finite, and obviously less than the age of the universe since it takes some time for them to form. As a result, there wouldn't have been enough time to reach the thermodynamic equilibrium that would make the night sky as bright as the surface of a star. These last two solutions are distinct, but can be connected through the current view of the universe, the Big Bang model. As we've seen, it predicts both the finite age of the universe and its expansion. Therefore, it becomes important to understand which of the two effects has a greater weight in explaining the paradox. Recently, it has been possible to address the problem quantitatively, showing that both effects are important, but to different extents. The fact that galaxies have a finite age explains the reduction in the intensity of intergalactic radiation, also called the extragalactic light background, by a factor of 10, while the effects of the universe's expansion can justify a reduction by only a factor of 3. So, despite what some specialized cosmology books claim, the solution to Olber's paradox is not purely a relativistic effect. What is truly decisive is that there was a time in history of the universe when there were no galaxies like the ones we know today, and therefore no possibility of filling the universe with visible radiation. Ultimately, we are left with a great satisfaction. When we look out the window at night and gaze at the dark sky, we are actually conducting a simple but very important cosmological experiment.