What Is the Cosmological Principle?

cosmological principle Any concrete substance that exists objectively has its own structure, which is in motion and change. Similarly, the objective existence of the "universe" should also have a structure and must constantly evolve. The composition of structure and form, in popular terms, refers to life, death, illness and death. The science that studies the structure and evolution of the universe is called cosmology.

The object of cosmological research is large-scale features of the entire observable space-time range. The distance scale that has been detected so far is 15 billion light years and the time scale is 10 billion years, including 100 million galaxies. According to measured data such as galaxy counts, radio source counts, and microwave background radiation, it is known that the spatial distribution of matter is uniform and isotropic in the space of more than 100 million light years. As a prerequisite for studying cosmology, cosmologists have established a working hypothesis. This assumption is called the principle of cosmology, which means that at any moment in the cosmological scale, the three-dimensional space is uniform and isotropic. Its meaning is ^[1]

Geocentric theory is a simple view of the universe that is most in line with human intuitive feelings. The sun, moon, and stars rise and fall, all revolving around the earth. The earth is intuitively considered to be at the center of the universe, and humans and their invented gods have gained a special place in the universe. The idea of geocentric theory has dominated human knowledge for more than a thousand years until the emergence of Copernicus's revolutionary heliocentric theory ^[2]

In addition to the great value of cosmological principles in epistemology, there are also many important applications in practical scientific research. Its most important application is that it can use the isotropic characteristics to establish the Robertson Walker metric, which can simplify Einstein's field equation and establish a standard cosmological model. In addition, the principles of cosmology are also reflected in the statistical research of astronomy. Let's take some simple examples to illustrate it.

There are a large class of flat galaxies similar to the Milky Way in the universe. These galaxies are often called disk galaxies. A distinctive feature of this type of galaxy is that it can see obvious spiral-like structures, so it is often called a spiral galaxy. The famous pinwheel galaxy M101 shown on the left in the picture, you can see its spectacular clockwise spiral arms.

The physical characteristics of spiral galaxies and their formation and evolution are one of the important research topics in modern astronomy. But because the galaxy has a disk-like structure, we can only see the characteristics of one direction from different angles. The two most extreme cases are a disk galaxy facing us completely, such as the picture

Windmill galaxy in China; the other case is completely sideways to us, such as the famous Straw Hat galaxy (right picture in the picture). However, the galaxy we are observing is just a projection. How do we know that the straw hat galaxy and the windmill galaxy are the projections of the same type of galaxies at different angles? The answer lies in the principles of cosmology. Because the universe is isotropic on a large scale, we have reason to believe that on a large scale, the spiral galaxy's disk is pointing randomly. If it is not random, we will get a special direction, and the existence of this direction will obviously violate the principles of cosmology. If a galaxy is a disk-like structure physically, then the projected image axis ratio (the long axis and the short axis ratio in the image) corresponds to different directions in space. And if the disk's spatial orientation is random, then we can statistically predict that the number of disk galaxies with different axial ratios is almost evenly distributed. In fact, observations show that the distribution of spiral galaxies with different axial ratios is indeed uniform. In addition to the axial ratio, there is a simpler feature that can verify the principle of cosmology, which is the direction of rotation of the spiral arm.

For example, the spiral arms of the pinwheel galaxy in the picture are clockwise. Since the direction of rotation of the arm depends entirely on the angle of observation, based on cosmological principles, we have reason to believe that the direction of rotation of the arm (clockwise or counterclockwise) should also be a random quantity. That is, within a certain range of scales, we should observe the same number of spiral galaxies clockwise and counterclockwise. Strangely, some researchers recently counted the number of galaxies with clockwise and counterclockwise spiral arms in different celestial areas, but got a shocking result: the number of clockwise spiral arms is more than counterclockwise! Such a result has caused fierce controversy. Some researchers have repeatedly studied this phenomenon, claiming that no significant difference was found between clockwise and counterclockwise. Some researchers believe that this amount of asymmetry may be related to people's psychology. When human's cognition is not obvious in the "rotation" direction of the arm, there will be a selection bias, and it is more inclined to clockwise rather than counterclockwise. However, a large-scale spiral arm identification using a machine learning method recently found that there are more galaxies clockwise. What the conclusion is or what it says, scientists will continue to explore in depth.