Hawking was asked: Which do you think is more important, your black hole radiation or the idea of no boundaries? Hawking sat in a wheelchair, controlling the mouse to type a line leisurely on the screen: others think that black hole radiation is more important because it has been universally accepted, but I think that the borderless idea is more important. In the book, Hawking even confidently says that the boundaryless idea is like the original heliocentrism and relativity, although many people can't understand it at first, but it will eventually become a theory that everyone believes. Knowing the borderless idea and the imaginary time will become a major leap forward in human intelligence, just as it is to know that the earth is round.
The borderless hypothesis is also called the borderless universe model, which means that in all the history of the universe, the universe, like the surface of the earth, is a finite closed surface without boundaries, but this surface is not two-dimensional, but four-dimensional. The author has understood this model as a five-dimensional space-time of three-dimensional space plus time and virtual time, but it is still very difficult to understand, because Hawking uses virtual time and Euclid space-time, which are difficult for ordinary people to understand. Today, the author describes it from another perspective, I don't know if it is more understandable and closer to the basic intention of this idea?
There are two key words in the borderless hypothesis, one is to sum the history of the universe, and the other is virtual time, that is to say, the boundaryless model contains all the possible histories of the universe in virtual time, and some people understand it as a parallel universe, although there is some truth, but it should not be completely consistent with Hawking's assumption.
We can think of the universe as a three-dimensional sphere expanding in real time, which may expand to a certain extent and contract, or it may remain stationary, or continue to expand. If we take time alone, this time is the historical path of the universe, which can be seen as a line segment or ray, and at every point on this line is attached a three-dimensional cosmic historical sphere. Now Hawking believes that in the virtual time perpendicular to real time, the universe has countless possible histories, and to depict the entire universe, it is necessary to sum all the cosmic historical paths in virtual time, that is, to sum the countless virtual time and the innumerable three-dimensional cosmic historical spheres attached to it. We do not consider the three-dimensional cosmic history sphere first, only sum the virtual time, so that from the singularity of the universe, diverging out of countless paths, returning to the possible end, the paths in these virtual time form a two-dimensional closed sphere, just like the earth, countless cosmic history from the South Pole, along the historical path (meridian) divergence, and finally reach the North Pole. In this way, the singularity of the universe is erased, and we no longer have to investigate what really happened before the singularity, because this model of the universe did not have a singularity, just as the concept was not before the Earth had no south pole. The South Pole is no different from any other point on Earth, and the singularity in the Model of the Boundless Universe is no different from other points.
If you superimpose the three-dimensional historical sphere into this model of the boundless universe, it is actually a five-dimensional cosmic historical sphere containing real time and virtual time, that is, the wave function of the entire universe. According to quantum mechanics, once intelligent beings observe the universe, this wave function collapses in virtual time, and the universe chooses a most likely history to present in real time, which is the four-dimensional space-time we are in, the real universe. Hawking further said that our universe is not a perfect sphere, the perfect sphere corresponds to the universe that has been expanding in a bursting manner in virtual time, and our universe is a flatter sphere at the south pole, so that after experiencing the inflationary stage, the expansion rate will slow down, and matter will fall together to form galaxies and stars, and there will be intelligent life like us. In virtual time, the history of this universe is characterized by a slightly flat and slight undulation of the South Pole, and scientists have found such ripples in the cosmic microwave background radiation, ranging from about one-ten-thousandth of a degree.
This is probably by far the closest and most understandable explanation of Hawking's original intent in the model of the borderless universe. But now this model is really not a mainstream cosmology, maybe too advanced, maybe it still needs to be perfected, maybe it will simply be falsified, who knows? There are so many inexplicable things in quantum mechanics, and if you want to expand it to such a large scale as the universe, you may not be able to do something more bizarre.
![Hawking was paralyzed at the age of 21 and confined to a wheelchair for half a century, how did his three children come?[fig]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)