Since the universe was born at a singularity

I believe we are all familiar with the "big bang theory of the universe", after all, this is the mainstream theory to explain the origin of the universe, we usually understand the theory that the universe was born in a very small, very dense, extremely high-temperature singularity, about 13.8 billion years ago, this singularity exploded, and then gradually evolved into what we now The universe as we see it now. But the question is, since the universe was born in a singularity, what is outside the singularity?

To answer this question, we need to first briefly understand what this so-called "singularity" is all about. The "big bang cosmology" is closely related to a landmark discovery that the universe we observe is in a state of continuous expansion.

This continuous expansion of the universe has led to the universe's matter being increasingly thin, and the overall temperature of the universe is also getting lower, according to which we can assume that at a certain point in time in the past, the density and temperature of the universe will be higher than now, if we rewind the evolution of the universe like a film in time, then the universe's matter will become denser and denser, the overall temperature will also be higher and higher.

Scientists are using this kind of time rewind to speculate on the evolution of the universe in the past. However, when the time rewind reaches a certain point in time about 13.8 billion years ago, the matter density and temperature of the universe become so high that they cannot be explained by modern physics, in other words, at this point, all known physical theories fail, and the state that the universe is in at this time is called "singularity".

Since singularity can no longer be explained by modern physics, it is meaningless to go back in time.

Through the above introduction, we can see that the "Big Bang cosmology" in the singularity, does not refer to a point located in space, but refers to a state of the universe.

Due to the speed of light, we can only observe a part of the universe, which is also known as the "observable universe", according to the "Big Bang cosmology", when the universe is in the singularity state, the size of the observable universe is less than the Planck length. Indeed, it can be considered a "very small point", and for the sake of discussion, we may refer to it as the "initial singularity".

The observable universe is only a part of the universe, and when the universe is in the singularity state, the "initial singularity" is also only a part of the universe, so if we want to describe the size of the entire universe in the singularity state, we have to include the unobservable universe, i.e., the "unobservable universe ".

The cosmological principle tells us that on the macroscopic scale of the universe, any point in space is physically the same, and the physical laws and physical quantities observed at any point in the universe are the same. On the other hand, the known observations show that the curvature of the universe is infinitely close to zero, and this means that the volume of the universe is probably infinite.

Accordingly, we can conjecture that the "unobservable universe" is composed of countless "observable universes" like ours, and if we go back in time to the time when the universe was in the singularity state, these "observable universes" become In other words, the universe at this time is composed of countless "initial singularities".

It should be noted that theoretically speaking, the volume, expansion rate, and the time we have to go back to any "observable universe" is finite, so the volume of these "initial singularities" is not infinitely small, but finite. So how large is a universe composed of an infinite number of "initial singularities" of finite size?

The answer is, of course, infinity. Therefore, even if we go back in time to the beginning of the universe as described by the "Big Bang cosmology", the volume of the universe in the singularity state is still infinite, and it is obvious that there is no "outside " concept.

In summary, the singularity described by the "Big Bang cosmology" is not a real point in space, but a state of the universe in which every part of the universe can be described as "extremely dense and extremely hot" What we usually understand as the "tiny point" is the state of our "observable universe" at this time, and it is only a part of the universe in this state.

So if we ask what is outside the singularity, it is equivalent to asking "what is outside the universe in the singularity state", because the universe is infinite, even in the singularity state, so there is no outside.