Why is the sky dark at night? How would you answer a seemingly simple question?
Presumably, most people will give a very obvious answer without thinking: the sky is dark at night, is it not because of the rotation of the earth, the sun goes down?! Even if you feel that your IQ has been insulted, you may be angry enough to add: This is also worth discussing?
Is that really the case? In fact, astronomers do not draw conclusions based on "experience" so easily.
And why the sky is dark at night is even a question in the scientific community that has no definite answer. From the time I really thought about it, puzzles and confusions haunt cosmologists and astronomers to this day.
Why is that said?
Because the stars in the sky are the stars hanging in the night sky!
Roughly speaking, our Milky Way contains between 10 billion and 400 billion stars, and the observable universe (part of which is no longer observable because of the expansion of the universe) contains at least 10 billion to 200 billion galaxies.
Even if you multiply by the smallest number, that's 10 billion multiplied by 10 billion! See clearly, oh, the order of magnitude is hundreds of millions.
As a result, most of us may not find the unit of quantity taught in the textbook to describe this total amount for a while. All in all, this is a very large, very large, even more than the human brain can imagine, and they all hang in our night sky!
Now, let's add another astronomical background: our universe may actually be infinite.
Therefore, when you look up at any small area of the sky, it should contain countless stars, so the intuitive corollary in line with "common sense" should be: we are in a vast universe, there are countless luminaries like the sun in the universe, and the night sky should be extremely dazzling and bright, like the ring lights in the operating room!
However, you and I both know that this is not the case.
This great contrast between the theoretical and factual results of this night sky has long been known in astronomy as olbers paradox.
What is the Obersian paradox
The Obers paradox is also known as the night black paradox or the luminosity paradox. Proposed in 1823 by the German astronomer Heinrich Albers and revised in 1826, the central idea was that if the universe were steady and infinite, the sky should be bright rather than dark at night. However, Olbers was not the first to pay attention to this problem, and similar ideas had been proposed by the physicist Kepler in 1610 and the Swiss astronomer Chasciakos in the eighteenth century. Even horror writer Edgar Allan Poe contributed to the problem.
Here's what the Obersians paradox is:
If the universe is infinite, with countless stars in it, then the entire sky should be completely covered by stars. And in the night sky in any direction we look at, we should be able to see the stars, so the night sky must be bright.
For your better understanding, imagine yourself in a dense forest. At this time, no matter where you look, you will inevitably only see layers of "tree walls", and you can only see the trunks of the trees connected into one piece, without any gaps. Maybe only when you're in a sparse grove of trees can you see out through the gap. And it's just a forest, it's big, but it's not infinite.
Now try to expand your imagination to an infinite universe full of countless stars! So many "starlight" gathered together that the night is as bright as day,
But in reality, the sky is dark. The problem has become complicated at this point, and there have been people in history who have struggled to find out why.
Explanation of the cause of the Obersian paradox
Over the centuries, there have been many attempts to explain the inexplicable phenomenon of the dark night sky. Here are some common explanations, but they are all ruled out.
- Some say that stars in the universe may be unevenly distributed. Makes sense, but this hypothesis has been denied by modern astronomical observations that the universe is completely isotropic.
- Some say that the number of stars in the universe is limited. But it still doesn't stand up, because even though the number of stars in the universe is limited, it's still huge enough to illuminate the entire night sky.
- Others say we can't see these stars because they're too far away and therefore too dark. Unfortunately, it is still wrong. Why? Suppose we slice the infinite universe into uniform spherical layers, centered on the Earth, and if one layer contains a certain number of stars, then due to the homogeneity of the universe, the layer twice as far from the Earth will contain four times more stars. However, according to the law of inverse squares, when viewed from Earth, stars farther away are four times darker, which means that the total brightness of each layer is still the same. Therefore, no matter how far away, each layer of stars produces the same amount of light. As a result, the sky will still be evenly illuminated.
- It is also said that space is full of interstellar dust, which blocks the light of distant stars. This explanation feels very correct, but it is still rejected by scientists. Because according to the law of conservation of energy, starlight acts as energy, so much energy will inevitably heat up the cosmic dust. Dust will also begin to re-radiate absorbed light and shine like stars.
So many reasons can not be explained, resulting in a long time this problem has become a puzzle, for a long time lingering in the minds of scientists.
If none of this is the cause, there should be some credible theory to explain, then
What is the most credible answer to the Obers paradox?
After entering the 21st century, the current scientific community advocates the following two theories to explain the night black paradox.
1. The universe has ages
At present, human calculations through astronomical data show that our universe is about 13.8 billion years old. The Big Bang, the creation of the universe, occurred about 13.7 billion years ago. About 400 million years after the Big Bang, the first stars were born, followed by the evolution of galaxies and various celestial bodies.
That is to say, at this time, the universe appeared the light of the first star, and then shuttled between the universes. And light takes time to propagate, so we can only observe stars that are 13.8 billion light-years away from us, and the light of stars beyond this distance, we can't see.
Since the size of the universe is infinite, many stars and galaxies are invisible to us because their light has not reached our earth until now.
2. The universe has been expanding rapidly
The expansion of the universe was discovered by Edwin Hubble's continuous observations of the universe in 1929. He saw light from distant stars and galaxies, stretched spectrally as these stars were rapidly moving away from us. This phenomenon is known in astronomy as "redshift": it occurs when the wavelength of light becomes longer and moves toward the red end of the electromagnetic spectrum. The light of the very distant stars of the universe, because the universe itself expands, its wavelengths are stretched so large that they become infrared. The human eye can't see infrared light, so we naturally don't see very distant stars and galaxies at night.
Due to the existence of redshift, the early cosmic starlight has actually been shifting toward microwaves that are longer and less energetic than infrared wavelengths, which are more perceptible to the human eye. The huge amount of radiation of the Big Bang itself has long since moved to the wavelength of microwaves, which is what astronomers often call the cosmic microwave background radiation. This kind of microwave actually covers our entire universe and the sky, and the rustling sound on the radio and the snowflake spots on the TV are caused by these microwave interferences.
In other words, our night sky and universe are not "dark", they are bright in themselves, but they are filled with microwaves that the human eye cannot see.
The following figure shows the cosmic microwave background radiation, and you can clearly see that the microwave light is evenly distributed throughout the universe. This confirms Orbes's speculation, except that he would not have thought at that time that the universe had an age, that the universe was expanding, and that light would change.
The humble phenomenon that the night sky is "dark" actually confirms that the universe is unstable and has become one of the evidences of the Big Bang theory.
At the same time, it also foreshadows a very bad future for the universe, and even the ultimate fate of the universe that we do not like very much.
Now, according to the latest observational data, the universe has expanded at a rate of more than 700,000 kilometers per second, which is almost 2.3 times the speed of light (many people will say, didn't Einstein's theory of relativity say that matter cannot exceed the speed of light?). But scientists believe that the expansion of the universe itself is not the motion of matter, so faster than the speed of light is reasonable.) While our observable universe is therefore up to 93 billion light-years in diameter, almost all distant galaxies are moving faster than light away from Earth. The light emitted by these distant galaxies and stars will never be able to see.
Civilizations there will never be able to see or contact us, because electromagnetic waves and light are the same thing.
Like other civilizations in the universe, in the distant future, we will all become cosmic islands, breeding civilizations in the few galaxies near the planet.
If we were to assume the most extreme situation, if the time is stretched for a very long time, the sky visible to the naked eye will be completely dark. If there had been life then, he would have even thought that the universe had always been dark.
At this time, it is almost the ultimate fate of the universe: heat death of the universe, the entropy of the universe will reach the maximum, the effective energy in the universe will be converted into heat energy, and the temperature of all matter will reach the thermal equilibrium, so that there is no energy in the universe that can maintain motion or life.
Such a cosmic fate is inevitably desperate.
Going back to the question of why the night sky is dark, you now finally understand the simplicity of this problem.
Summary: Wherever you look at the sky, there is at least one star or galaxy. You don't see that much because their light hasn't reached our planet yet, or has moved red beyond visible light.
This is why the "bright" night sky is black in the eyes of the human eye .
Written by: The Secrets of the Universe