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We know that today's universe is extremely empty. if all the matter in the universe is scattered and evenly distributed, there are only six hydrogen atoms per cubic meter! This is equivalent to only one grain of sand in such a large space as the earth! Although today's universe is extremely sparse, according to the Big Bang theory, the early universe should be very dense. If the density is high enough to form a black hole, why didn't the universe collapse directly into a large black hole?

According to general relativity, when matter is compressed to a small enough scale, the density of matter is so dense that the surrounding space-time is so distorted that even light cannot escape, so it is called a black hole. This range of inescapable matter varies with the mass of matter, showing different black hole sizes.

Black holes in nature are usually formed by the death and collapse of massive stars, that is, constant star black holes. In addition, there are larger black holes, such as supermassive black holes at the center of galaxies, which can be as massive as tens of billions of times the mass of the sun. This is just a naturally occurring black hole that has been found in nature, and in theory, it can become a black hole as long as the density is high enough, even if the mass is not so large. For example, if you are compressed to the size of a nucleus, you become a miniature black hole at the particle level. It can be seen that whether it is a large black hole or a small black hole, there must be matter in order to become a black hole, which is exactly what the universe lacked at the beginning of the Big Bang.

Excluding the singularity of the Big Bang, there was no matter in the Planck period before 1 Planck time (10 ^-43 seconds), although the radius of the universe was only 1 Planck length. Not only matter, but even the four basic forces are not independent at this time, but they are still unified "super forces" at this time. After that, gravity was the first to separate from the super force, and although there was gravity, there was still nothing in the universe that could be called matter.

At 10 ^-36 seconds, an important phase transition has taken place in the universe, which not only separates the force, but also gives birth to a kind of large mass strange particle-dark matter, which only weakly interacts with normal matter. Although it is not clear what dark matter is, it is certain that it has a gravitational pull. Now that we have both gravity and matter, we should be able to produce black holes this time, right? Coincidentally, before dark matter began to gather, the universe immediately entered a period of frantic expansion.

Inflation is an extremely short but exponential expansion experienced by the universe in its very early days. In only 10 ^-33 seconds, the whole universe expanded by 10 ^ 26 times, equivalent to the instant expansion from an atom to the size of an entire galaxy! This speed is much faster than the speed of light, and the dark matter has been scattered evenly throughout the universe before it can gather. Some people may wonder: isn't the movement of this matter faster than the speed of light? Because the expansion of the universe is a kind of expansion of space itself, it is not the real motion of matter, it can be said that it is only a kind of visual superluminal speed, so it does not violate the theory of relativity.

To sum up, even if the universe was high enough in its early days, it would not collapse into a black hole. Another key point is that this kind of "cosmic black hole" and "black hole in the universe" are not the same thing. The black holes we are talking about usually appear in space, where the originally flat space is bent by mass. In other words, the black hole is also in space, it is just a special place in space. But there is no space outside the universe, at least not what we call it, so strictly speaking, this "cosmic black hole" is not the kind of ordinary black hole described by general relativity.

Although the universe itself cannot become a black hole, will matter or some kind of energy in the universe collapse into a black hole?

Some density fluctuations may indeed cause some regions to be dense enough to produce small black holes, which can be called "primary black holes". But it is still impossible for all matter in the universe to collapse into a black hole, because the density of matter in the universe is too uniform.

Imagine a point at the center of a uniform sphere where the resultant forces in all directions cancel each other out, and the resultant force is exactly zero. Just like if you safely reach the center of the earth, then you will be basically in a state of weightlessness.

For places outside the center, can't the matter still gather there? Don't forget that there is no so-called center in the universe, or there is a center everywhere in the universe, which is about the shape of the universe.

As we have said before, according to the observations at this stage, our universe as a whole is flat. This flatness does not mean that the universe is a two-dimensional plane, but that its overall curvature is 0, which can be understood as a three-dimensional Euclidean space. What does a flat universe mean? It means that our universe may be infinite and endless. For a space that extends infinitely in all directions and where matter is evenly distributed, the resultant force is zero no matter where you are.

You may wonder again: didn't the universe go through the expansion phase of the Big Bang, then it should have changed from small to big? if the universe is infinite, what is the process from small to big?

It has to be said that this is indeed an interesting and profound question, and it can only be said that the current observations support both cases. So how do we understand it?

In cosmology, when we say that the universe changes from small to large, it does not mean the total size of the universe, but the scale factor of the universe. The scale factor can be understood as a parameter describing the expansion of the universe. In the early days of the Big Bang, the scale factor of the universe was very small, that is, the points in the universe were very close together. With the passage of time, these points begin to move away from each other, corresponding to the increase of the scale factor. In other words, it gets bigger just because the distance between the internal points becomes larger, and it may already be infinite for the universe as a whole. In short, the expansion of the universe does not require the universe to be infinite or finite. An infinite universe can still expand from a very dense state. From this point of view, the two do not conflict.

I know, it's really hard to understand, it's kind of like the concept of eternal inflation that we've introduced before. Our universe is only a part of the whole universe, just a region where inflation has stopped. Beyond that, it is either a region that is still inflationary or another parallel universe that has stopped inflationary.

Of course, whether it's the Big Bang, the flat universe, or the theory of eternal inflation, these are just theories and guesses based on existing observational evidence. So, we don't have to care too much about the answers to the questions such as whether the Big Bang exists or whether the universe has a boundary. These theories, or even all scientific theories, only represent the correctness of the present stage. If there is new subversive evidence in the future, then the theory will be revised accordingly, and the worst thing is to re-establish a better theory. But this does not mean that the previous theories are useless, and their existence is still of great significance as a connecting link between the preceding and the following for the whole history of scientific development.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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