Chapter 158: Dark Matter

Lost in thought, Hua Feng heard a voice speaking by his ear: "Kid, I must say you have some comprehension. To be honest, I am Sun Wukong, the Great Sage Equaling Heaven who once created havoc in the Heavenly Palace. As for now, my identity in Dawn Academy is Sun Xing. I can see that your physique is different from ordinary people, bearing quite a resemblance to my own spiritual stone monkey nature born of heaven and earth."

"You are... Sun Wukong?" Hua Feng asked in somewhat incredulous astonishment.

Because no matter what, Sun Wukong existed as a super-heroic figure in the heart of any Chinese person; he had never imagined that one day he would truly encounter a character from mythology in reality.

"All right, don't be so surprised. There are still many things in this world you do not understand. Now I need you to know about something called dark matter," Sun Wukong continued immediately.

Hua Feng felt a vast amount of information rush into his mind: Dark matter is a hypothesized invisible form of matter proposed to exist in the universe. It may constitute the primary component of the universe's matter, yet it does not belong to any currently known type of matter that forms visible celestial bodies. Phenomena observed in numerous astronomical studies that appear to violate Newton's law of universal gravitation can be well explained under the assumption that dark matter exists.

Modern astronomy, through observations of celestial body motion, gravitational lensing effects, the formation of large-scale structures in the universe, and the cosmic microwave background radiation, indicates that dark matter likely exists in abundance within galaxies, galaxy clusters, and throughout the universe, with its mass far exceeding the total mass of all visible celestial bodies combined. Combined with observations of the anisotropies in the cosmic microwave background radiation and the standard cosmological model (the ΛCDM model), it can be determined that dark matter accounts for 85% of the total mass of all matter in the universe.

Currently, a widely accepted theory suggests that dark matter is composed of weakly interacting massive particles (WIMPs), whose mass and interaction strength are near the electroweak scale, acquiring their currently observed relic abundance through a thermal freeze-out process during the universe's expansion. In addition, there are hypotheses suggesting that dark matter is composed of other types of particles, such as axions and sterile neutrinos.

The possibility of the existence of "dark matter" was first proposed by the astronomer Jacobus Kapteyn, who suggested in 1922 that the invisible matter potentially existing around stellar systems could be inferred indirectly through their motions. In 1932, the astronomer Jan Oort conducted research on dark matter regarding the motion of stars near the solar system, though he failed to reach a definitive conclusion on its existence. In 1933, the astrophysicist Fritz Zwicky used spectroscopic redshifts to measure the relative velocities of individual galaxies within the Coma cluster.

Using the virial theorem, he discovered that the velocity dispersion of the galaxies in the cluster was too high; the gravity generated solely by the mass of the visible galaxies within the cluster was insufficient to bind them together. Therefore, a vast amount of dark matter should exist within the cluster, with a mass at least a hundred times greater than that of the visible galaxies.

S. Smith's observations of the Virgo cluster in 1936 also supported this conclusion. However, this groundbreaking conceptual conclusion failed to attract much attention from the academic community at the time. In 1939, the astronomer Horace W. Babcock, through spectroscopic studies of the Andromeda Nebula, showed that the rotational velocities of stars in the outer regions of the galaxy were much higher than expected according to Kepler's laws, corresponding to a large mass-to-light ratio.

This hinted at the potential existence of a large amount of dark matter within the galaxy. In 1940, Oort's study of the rotational velocities of stars in the outer regions of the galaxy NGC 3115 indicated that its total mass-to-light ratio could reach approximately 250. In 1959, F.D. Kahn and L. Woltjer studied the relative motion between the mutually attracting Andromeda Nebula and the Milky Way; through their approach velocity and mutual distance, they deduced that the dark matter in the Local Group inhabited by humans is about ten times more massive than the visible matter.

Important evidence for the existence of dark matter came from the 1970 studies of stellar rotational velocities in the Andromeda Nebula by Vera Rubin and Kent Ford. Using high-precision spectroscopic measurement techniques, they were able to detect the relationship between the rotational velocity of outer stars—far from the galactic nucleus—around the galaxy and their distance from the center.

According to Newton's law of universal gravitation, if the mass of a galaxy were primarily concentrated in the visible stars of the galactic nucleus, the velocities of stars in the outer regions of the galaxy would decrease with distance.

However, observational results showed that over a considerable range, the velocities of stars in the outer parts of the galaxy remained constant. This implied that a vast amount of invisible matter in the galaxy was not merely distributed in the core region, and its mass far exceeded the total mass of the luminous stars. In 1973, M.S. Roberts and A.H. Rots used 21-centimeter neutral hydrogen line observational techniques to probe the velocity distribution of gas in the outer parts of the Andromeda Nebula, confirming this conclusion from another angle.

By the 1980s, a large batch of new observational data supporting the existence of dark matter emerged, including gravitational lensing effects when observing background galaxy clusters, the temperature distribution of hot gas within galaxies and clusters, and anisotropies in the cosmic microwave background radiation. The theory of the existence of dark matter has gradually gained widespread acceptance in the astronomical and cosmological communities.

According to comprehensive analyses of existing observational data, the primary component of dark matter should not be any currently known microscopic elementary particle. Contemporary particle physics is striving through various means to explore the particle properties of dark matter.

Although dark matter has not yet been directly detected, abundant evidence already indicates its massive presence in the universe, for example:

Galactic Rotation Curves and Dispersion Velocity Distributions:
Galactic rotation curves describe the relationship between the orbital velocities of visible celestial bodies in spiral galaxies and their distances from the galactic center. Based on observations of the mass distribution of visible celestial bodies in spiral galaxies and calculations using the law of universal gravitation, bodies further out should orbit the galactic center more slowly than those closer to the center.

However, measurements of the rotation curves of numerous spiral galaxies show that the speeds of outer bodies are nearly identical to those of inner bodies, remaining far higher than expected.

This suggests the presence of massive, invisible matter within these galaxies. Combined with the virial theorem, the matter distribution within a galaxy can be calculated from the dispersion velocity distribution of its visible celestial bodies.

This method is similarly applicable to measuring the matter distribution of elliptical galaxies and globular clusters. The results show that, with few exceptions, the matter distributions of most galaxies and clusters do not match the observed distributions of visible matter, with visible matter accounting for only a minor fraction of the total mass of galaxies and clusters.

Galaxy Cluster Observations:
The mass distribution of galaxy clusters can primarily be derived through three different methods:
1. Observing the motions of galaxies within a cluster and calculating them via gravitational theory.
2. Observing X-rays emitted by galaxy clusters. Hot gas capable of emitting X-rays is ubiquitous in galaxy clusters; when the gas achieves hydrodynamic equilibrium within the cluster's gravitational field, the mass distribution of the cluster can be inferred from its temperature.
3. Gravitational lensing effects. According to general relativity, light originating from behind a galaxy cluster is bent when passing through a massive galaxy cluster, analogous to an optical lens. The distribution of matter within the galaxy cluster can be calculated based on the degree of bending of the background light. These three methods are independent and mutually supportive, making galaxy cluster observations an important tool for studying dark matter. Currently, these observations consistently indicate that the total mass of matter in galaxy clusters vastly exceeds the total mass of visible matter within them.

Cosmic Microwave Background Radiation:
On a cosmological scale, through precise observations of the anisotropies in the cosmic microwave background radiation, the total amount of dark matter in the universe can be determined. Current observations indicate that dark matter contributes 26.8% of the total energy of the universe, conventional matter constituting celestial bodies and interstellar gas accounts for only 4.9%, and the remaining 68.3% consists of dark energy driving the accelerated expansion of the universe.

N-body gravitational simulations of cosmic evolution by supercomputers show that collisionless, low-velocity dark matter particles gradually clump together under gravity, a process capable of forming the large-scale structures we see today. The dark matter distribution of these structures possesses a universal mass distribution. Low-velocity dark matter is favorable for the formation of large-scale structures, whereas high-velocity particles tend to smooth out structures. Therefore, neutrinos are not supported as primary dark matter particle candidates.

The existence of dark matter has been widely accepted; however, very little is currently known about its properties. The currently known properties of dark matter encompass only a limited number of aspects:
1. Dark matter participates in gravitational interactions and therefore must possess mass, though the mass of individual dark matter particles has not yet been determined;
2. Dark matter must be highly stable; since evidence for dark matter exists at different stages of cosmic structure formation, dark matter must be stable on the timescale of the age of the universe (tens of billions of years);
3. Dark matter basically does not participate in electromagnetic interactions; its interaction with photons must be so weak that dark matter is essentially non-luminous. Dark matter also basically does not participate in strong interactions, otherwise the process of primordial nucleosynthesis would be perturbed, altering the abundances of light elements and leading to inconsistencies with current observational results.
4. Computer simulations of the formation of large-scale cosmic structures reveal that the velocity of dark matter must be far lower than the speed of light—hence "cold dark matter"—otherwise our universe would be unable to form the currently observed large-scale structures under gravity;

Synthesizing these fundamental properties, it can be concluded that dark matter particles do not belong to any known elementary particle. This poses a challenge to the currently highly successful Standard Model of particle physics.