There is also a saying that the universe is a twisted cycle with no edges.
The universe is finite and infinite, and it is a hypercircle.
Is the volume of the universe infinite? If the volume of space is infinite, we can imagine that there are infinite galaxies scattered in it, and their density is roughly the same. So many people can't understand, how can an infinite thing expand? Where will it expand? This question is not difficult to answer: infinite volume can expand, but the volume remains the same. Please remember what the tortoise taught us. However, when we compare this model with the "cosmic egg", there is a visualization problem. If there are galaxies everywhere, there will never be a cosmic egg with a limited volume, a shell, and there is no matter outside the shell. Therefore, the egg metaphor doesn't work.
Imagine that there is such an infinite universe, which is a sphere and contains a huge volume of space, including many galaxies. Now imagine that space is shrinking rapidly everywhere, just like Alice eating a magic cake in wonderland. In this way, the radius of the sphere will shrink smaller and smaller, but no matter how it shrinks, there is always endless space, and there are endless galaxies outside. If the sphere really shrinks to infinity, then, from a mathematical point of view, a thorny problem arises, that is, an infinitely shrinking infinite universe. This spherical universe still has no center and no boundaries. However, any such sphere, no matter how big it is at first, will be compressed into a point after infinite contraction. Astronomers believe that before the explosion, the universe was in this state of infinite contraction but unbounded.
In fact, having an available model of the universe can avoid many infinite disturbances. This model was put forward by Einstein himself in 19 17. After confirming the fact that space can bend, Einstein said that space can connect itself in many unexpected ways. You can make an analogy with the surface of the earth. The surface area of the earth is limited, but it is unbounded. No matter where a traveler goes, he will never touch the edge or boundary of the earth. Similarly, space may have a finite volume but no boundaries. Even if few people can really understand this strange picture, mathematics will outline the details for us. This object is called a hypersphere. If the universe is a super circle, in principle, astronauts can sail around the world like Magellan. He can fly in the same direction with a rocket until he reaches the starting point.
Although Einstein's hypercircular universe is limited, it has no center and no boundary (just like the earth's surface has no center and no boundary), so when it contracts, it is not like a cosmic egg. We can imagine that the hypercircle shrinks to infinity and its volume disappears, just like the surface of a circle shrinks to zero radius (see Figure 2).
The study of space that can expand and contract makes cosmologists put forward a theory of universe creation that is completely different from the Bible in details. The most striking thing in their scientific theory is that they believe that space itself, not just matter, was also created in the Big Bang. If you look at the inflated balloon in Figure 2 upside down, that is, the inflated balloon from scratch, treat it as a deflated balloon and use it as a model of the universe, then you will know roughly how modern physics tells about the creation of the universe. An important argument here is that no matter whether the universe we are talking about here is Einstein's hypercircle (that is, balloon model) or an infinite volume universe, it is impossible to imagine what space will look like after infinite contraction. At the initial moment of the Big Bang, space is infinitely shrinking, and that moment represents the time boundary when space no longer exists. Physicists call this boundary a singularity.