The space we are in is actually the interior of a black hole, which is the view of the black hole cosmology. The current mainstream cosmological model is the Big Bang theory, and there are many non-mainstream cosmological models. The black hole universe model is one of them.
1. Why is the universe inside a black hole?
The question begins with the expansion of the universe.
We know that the universe is expanding at an accelerating rate. The implication here is that new space arises from every space. As a result, objects in the universe are all moving away from each other. However, closer objects will be pulled together by gravity or electromagnetic force, and the distance will not increase. The effect of space expansion can only be seen between galaxies that are very far away.
In the example above, both the distance between cat-mouse and cat-dog increased due to space expansion, however, the increase in cat-dog distance was significantly greater than the increase in cat-mouse distance. Due to the superimposed effect of space expansion, the farther away the object, the greater the increase in distance between them. That is, the farther away an object is from us, the faster it will move away from us.
From this, it is not difficult to imagine that galaxies that are far enough away from us, objects far away from us, will reach the speed of light. This distance is called hubble radius. Galaxies outside the Hubble radius are moving away from us faster than the speed of light.
In the black hole universe model, the Hubble radius is also the Schwarzschild radius of the black hole where our universe is located. If we compress an object below the Schwarzschild radius, the escape velocity on the surface of the sphere reaches the speed of light. In other words, the sphere becomes a black hole.
The formula for calculating schwarzschild radius is very simple:
where G is the gravitational constant, M is the mass of the object, and c is the speed of light.
So, if we compress everything within hubble radius into a black hole, how big will its Schwarzschild radius be? If the known mass of the universe (calculated from the observed density of the universe) is substituted into the above formula, the Resulting Schwarzschild Radius and Hubble Radius are very close. Considering the accuracy and error of the observation, it can be considered that the two values are equal.
In other words, the space we are in is already a black hole.
The density of the universe is very low, equivalent to the size of the Earth, with an average mass of only one grain of sand (. The fact that such a sparse space can form a black hole may be incredible to many people. Black holes often give the impression of being dense — if the Sun is compressed into a black hole, its Schwarzschild radius is only 3 kilometers, while the Earth's black hole has a Schwarzschild radius of less than 1 centimeter.
As you can see from the formula above, the Schwarzschild radius is proportional to the mass, while the radius of the object is proportional to the cube root of the mass (if the density is constant).
So, when the mass of the object increases, the Schwarzschild radius grows faster than the radius of the object itself. For an object of great mass, its Schwarzschild radius will reach or even exceed the object's own radius.
However, most cosmologists disagree with this evidence. They believe that the Very Close Schwarzschild radius and hubble radius are just a coincidence.
2. The universe inside a black hole
The traditional understanding of black holes is that when an object falls into a black hole, it will continue to fall toward the center of the black hole, and finally fall into the singularity of the center of the black hole— a point of infinite density and infinitesimal volume.
However, this is not the only black hole structure that may exist. The black hole's strong gravitational pull makes space-time extremely curved, creating a closed space-time isolated from the outside. In such an enclosed space, if matter is roughly evenly distributed, general relativity allows it to maintain a stable internal structure without collapsing into a singularity.
Well, in the absence of external matter and energy influences, the black hole universe is a static universe. When the black hole universe inhales external matter, its volume also increases due to the increase in mass, and from the inside of the black hole universe, it manifests itself as the expansion of cosmic space.
So, in the black hole cosmic model, the explanation of the expansion of the universe does not require the concept of dark energy. A black hole universe usually begins with a stellar-level black hole. Whether it's inside as a singularity or a baby universe is the same for external observers. When it absorbs the matter and radiation of the outer universe (the parent universe), it will slowly grow.
In a galaxy, when a star reaches the end of its life, it becomes a white dwarf, a neutron star, or a black hole. Eventually, they converge at the center of the galaxy, forming a super black hole with more than 1 billion solar masses. And inside this black hole is another universe like ours.
Stellar-grade black holes were born out of supernova explosions, so their internal temperatures are extremely high (higher than those of neutron stars), but they do not radiate energy outward. As the black hole universe expands, the temperature gradually decreases due to the dispersion of energy. When a black hole universe expands to the size of our universe, the temperature has dropped to 3K. This is the microwave background radiation we observe.
The rate at which the black hole universe expands depends on the amount of matter inhaled. Our cosmic boundary expands at the speed of light, which means it needs to suck in 10,000 solar-mass matter per second.
For some black holes with relatively small masses (such as stellar-level black holes left by supernova explosions), the curvature of space-time around them varies greatly, and matter is torn into pieces when it enters the black hole. Massive black holes (such as the super black hole at the center of a galaxy) are called "gentle black holes," and matter can pass through the event horizon intact and into the black hole — to another universe.
3. Model of the black hole universe
The black hole cosmic model is a multi-layered hierarchical structure. Our universe is in a larger universe (the parent universe); the other universes in the parent universe are the sister universes of our universe; the black holes in our universe (stellar-level black holes and super black holes) all contain subcosmums;
Similarly, our sister universes have their own subcosms; outside the mother universe is the larger grandmother universe. The total number of layers in this model depends on the size of the entire space.
If the whole of space is infinite, then the number of cosmic levels is also infinite; conversely, it is finite. In the process of the expansion of the black hole universe, it may meet the sister universe and merge into a larger universe.
The above is a brief introduction to the black hole universe model. It is another cosmological model that differs from the Big Bang theory, and has different interpretations of many phenomena in astronomical observations, such as space expansion and microwave background radiation. This theory is currently considered an "alternative cosmology" that is not shared by most cosmologists. However, from the 1970s to the present, there are still some cosmologists who are working on this theory.