Why is the universe black? You think it's normal, but there's a cruel truth hidden

As a child, many people may have asked adults a question: why is the sky blue? And the answers given by adults are often bizarre.

In fact, blue is not the inherent color of the sky itself, it is just the color that occurs when sunlight hits the Earth's atmosphere and the atmosphere scatters sunlight.

When the sun sets, night covers the earth, and the quiet starry sky quietly emerges overhead, a new question arises: why is the universe black?

In fact, you are definitely not the first person to ask this question, because the ancients have always had an alternative curiosity and fear of the dark night, and this complex feeling is vividly reflected in the myths.

In Greek mythology, there is a "night goddess" named Nix, born in chaos, she has infinite divine power, and even makes Zeus, the king of the gods, tremble.

Of course, this kind of fear has not stopped people from exploring the dark night, but has stimulated human curiosity and desire for knowledge.

In the 18th century, when astronomer Edmund Halley was studying his own model of the universe, he found that if the universe is uniform and infinite, then the night should be bright, so he thought that the universe must be "dark" for a reason. .

Unfortunately, Halley did not find the answer until his death, because in Europe at the time, it was widely believed that the universe was static and infinite.

In 1823, the German astronomer Heinrich Obers summarized and summarized the theories and conjectures of predecessors, and formally proposed the famous "Obers Paradox", that is, if the universe is static and infinite, then the night should be It is the same as the day and not dark, because there are countless bright stars in the universe, and the light from these stars will always reach the earth to be seen by us.

Perhaps many people will think that this is a paradox? It is normal for the universe to be dark, because although there are many stars in the universe, under the huge distance scale, these luminous stars are like faint candles floating on the sea. How can they illuminate them all?

What's more, the universe is not empty, and there are large pieces of cosmic dust scattered around, most of the visible light should be blocked and absorbed by them, even if it is not fully absorbed, most of it is absorbed, which leads to the appearance of a dark night sky.

Is this really the case?

This conjecture may sound reasonable, but in fact it is not at all consistent with the actual observation. According to the first law of thermodynamics, after light is absorbed by matter, it will be released in the form of energy, and there is no absorption in the absolute sense.

So if the light that was supposed to illuminate the Earth's night sky is really absorbed by the dust, then the energy radiated by the dust can be observed with instruments. .

Since it is not because the light is blocked, why is the night sky not as bright as day? How to explain the "Obers Paradox"?

In the 19th and 20th centuries, explanations for the "Obers Paradox" were based on two major research findings.

The first finding was that the age of the universe is finite, which makes it impossible for light to travel through infinite space in finite time.

The second research finding is the Doppler effect, that is, the wavelength of the radiation emitted by an object changes due to the relative motion of the observer and the wave source.

The discovery of the Doppler effect proves that the light of distant planets is constantly moving away from us, and it also breaks the inherent idea that the universe is static.

Although these two findings can perfectly explain the "Obers paradox", but also some puzzling questions have arisen: Where did time begin? Why do celestial bodies keep moving away from the earth?

In the 1920s, the astronomer Edwin Hubble came to a conclusion after observing and studying dozens of extragalactic galaxies: the farther the galaxy is from the Milky Way, the faster it moves away from the Milky Way, which is also known as "Hubble's Law".

The discovery of "Hubble's Law" confirms the "Big Bang Theory" proposed by astrophysicist Georges Lemaitre, that is, the universe was born at a "point", and space and time all originated at that moment.

At this point, the explanation of "Obers' paradox" is relatively complete:

After the big bang, the light emitted by the stars has been propagating into almost infinite space for a limited time, resulting in longer and longer wavelengths and lower frequencies of the light reaching the earth, so the night sky we see is dark. one slice.

It should be noted that light is still there, and humans cannot see it mainly because they are redshifted into invisible light that cannot be directly observed by the naked eye. If you observe the night sky with special instruments, you will find that the night sky is still "bright".

In addition, the constant expansion of the universe is also the main reason why the night sky is dark. For example, there is a balloon that can expand infinitely. On the surface of the balloon, there are two ants that are one meter apart. They are both approaching each other at a speed of 1 centimeter per second. In theory, the two can meet in about 50 seconds.

But at this time, someone started to blow the balloon frantically. As the surface of the balloon became larger, the relative distance between the two ants was also increasing, until the balloon's inflation speed was much greater than the speed at which they approached each other, and they would never meet.

In the same way, the cosmic space is constantly expanding like a balloon, and the expansion speed exceeds the speed of light. Some stars have regressed to such a distance from the earth that the light they emit cannot run through the expanding space at all, so there is no will reach the earth to be seen by our eyes.

It is worth mentioning that the expansion of the universe may be the most desperate fact for human beings, because it means that more and more galaxies will retreat into regions that humans will never be able to observe, thus shrinking human beings in disguise The scope of activities and observations is unacceptable for a civilization with infinite development prospects and infinite possibilities.