Our human world is bathed in light. First, the illuminated side of the Earth receives approximately 10 trillion photons per square centimeter per second. These photons come from the outer layer of the distant natural giant thermonuclear reactor, which is what we call the sun. Then, there are photons wandering around in any cubic centimeter of open space. Some are microwaves left over from the cosmic thermal Big Bang 13 billion years ago, while others are photons produced by distant stars and various astrophysical phenomena in the universe.
We are also full of artificial electromagnetic radiation from the earth. We, warm and soft human beings, are powerful infrared beacons walking. Our magical chemical metabolism can turn energy into heat and emit photons to the environment. If you wear a pair of glasses that can sense infrared wavelengths in the electromagnetic spectrum, you will see a radiant world: here is your own light, your cats and dogs, pet parrots, and even flapping insects. The light from that tiny bit of muscle on the body.
However, when we set our sights on the vast universe, it becomes more and more obvious that our radiant world seems a bit unusual in the universe. One of the brightest hints is that every time the sun goes down, our world plunges into darkness. If we live in a truly infinite and eternally unchanging universe, then we may be able to see the sky full of stars as soon as we look up no matter where we go. All stars are stacked infinitely so that there should be light visible everywhere. .
It’s a bit contradictory, isn’t it? In cosmology, we call it the “Obers paradox”, which was proposed in 1823 by the German astronomer Obers. Although Oberst tried to solve this long-established problem mathematically, he still failed to provide a good answer to the question “Why is the universe mostly dark?” In the end, it was Edgar Allan Poe who first gave a plausible explanation for Oberst’s paradox in 1848. Poe’s qualitative analysis is that maybe the universe is too young to fill the sky with light.
There are some subtleties in the modern interpretation of Obers’ paradox, but in fact it can be boiled down to the fact that we do not live in an infinite and eternal universe . The universe is not only limited in age, but the history of making stars (with finite lifespans) is also very complicated, and the universe is also expanding, diluting the intensity of light reaching us from afar. Therefore, the sky above our heads looks unevenly lit, and compared with the daily environment in which we live, most universes especially like to “swallow” photons.
But the story does not end there, because there are some strange things in the history of stars. First, this is related to the actual formation of stars. Although we have a (still incomplete but) confident understanding of the basic physical causes and processes of the formation of individual stars or star clusters—starting with gravitational condensation and the collapse of the interstellar medium, things are trickier when it comes to entire galaxies.
Research over the past three decades has shown that about 10 billion to 11 billion years ago, the “star-making” activity in the entire universe reached a peak. Since then, although new stars still form from time to time in the universe, the frequency of star making has been much lower than before. Basically, it can be said that almost all the stars that the universe can make—about 95% of stars—have already been made. In the future, there will only be fewer and fewer newly born stars, and occasional galaxy mergers or other unexpected events will occur during this period.
But there is still a big question here. What limits the number of stars that the universe can make? This is also an enduring topic of astronomical discussion, especially the topic related to the composition of stars in a single galaxy, which is often mentioned. For example, our current cosmological paradigm (or a paradigm recognized by most scientists) is that we live in a universe dominated by dark matter, and in this universe dominated by dark matter, the largest galaxy should be Formed only recently, it is composed of low-level, gravitationally pushed and merged small galaxies. However, if you study those huge galaxies carefully, you will find that most of them are composed of earlier stars, indicating that they have existed for at least a long time.
To explain this, astronomers introduced the concept of “quenching”. In this process, some activities inhibit or prevent the formation of new stars in the galaxy. Unsurprisingly, you need a very powerful mechanism to suppress or prevent starmaking activities on such a large scale. Then the biggest suspect behind this is the supermassive black holes that exist in the centers of most galaxies. These black holes will swallow everything around them. When matter approaches the surface of a black hole, they release energy in the form of photons and particles. This outwardly radiating energy just blows away the interstellar gas that could have gradually cooled and condensed into new stars, thereby inhibiting the formation of new stars.
Of course, this specific process is not well understood. But there is a new and very tantalizing clue that the mass of supermassive black holes seems to be related to the total mass of stars in their galaxies. This clue is very surprising, because even a supermassive black hole with a mass exceeding one billion times the mass of the sun has the same size as a sun. So it is possible that a galaxy with a diameter of tens of thousands of light-years is actually closely related to the very small dot in its center.
One explanation for this is that there is an impulse feedback mechanism. If a black hole grows by swallowing the same interstellar gas that forms a new star, then this process will also cause the radiated energy to be swallowed by the black hole itself, and the radiated and absorbed energy will be blown away and blown away to make the black hole grow. The interstellar gas also prevents the starmaking activity of the entire galaxy. Under this mechanism, the mass of the black hole and the total number of stars in the galaxy are always in a state of simultaneous growth throughout their life cycle.
This also contributes to one of the strangest characteristics of black holes. Although the universe is “one-way motion”-space and time curve toward the surface of a black hole, and nothing can escape, but black holes can also provide explanations for some of the brightest and far-reaching phenomena observed in the universe. These phenomena, in turn, may greatly limit the number of stars in the universe; they also play a role in inhibiting the accumulation of light in the universe. There is an old saying that there is no light without darkness. For the universe, we can also say the same: without light, there is no darkness.
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