Chapter 159 Sagittarius

Weakly Interacting Massive Particles (WIMPs) are among the most widely discussed dark matter candidates. They refer to stable particles whose mass and interaction strength lie near the electroweak scale, having obtained their currently known relic abundance through the thermal freeze-out mechanism. WIMPs are expected to be fundamentally electrically and color neutral, and thus do not directly participate in electromagnetic or strong interactions.

Neutrinos also do not participate in strong and electromagnetic interactions, but because they travel through the universe at speeds close to the speed of light, they are classified as "hot dark matter," which is insufficient to serve as a primary component of dark matter.

In the currently established Standard Model of particle physics, there are no particles that simultaneously satisfy these properties, implying that WIMPs must represent new physics particles beyond the Standard Model.

WIMPs predicted by existing theories include: the lightest supersymmetric partners in supersymmetric models, such as neutralinos; the lightest Kaluza-Klein excitation states in extra-dimensional theories; and T-odd particles in the Little Higgs model.

Another dark matter candidate is the axion, an extremely light neutral particle associated with the breaking of charge conjugation-parity (CP) joint symmetry in strong interactions.

Axions interact via extremely feeble forces, preventing them from achieving thermal equilibrium with background radiation; thus, they do not acquire their relic abundance through thermal freeze-out, but can become cold dark matter via the breaking of the vacuum state.

Although numerous astronomical observations of dark matter have been conducted, its composition remains incompletely understood. Early theories of dark matter focused on obscured conventional stellar bodies, such as black holes, neutron stars, aging white dwarfs, and brown dwarfs.

These objects are generally classified as Massive Compact Halo Objects (MACHOs). However, years of astronomical observations have failed to locate a sufficient quantity of MACHOs.

It is generally believed that elusive baryonic matter (such as MACHOs and certain gases) does contribute partially to dark matter effects, but evidence indicates that such matter constitutes only a minor fraction.

The remainder is termed "non-baryonic dark matter." Furthermore, observational data—including galactic rotation curves, gravitational lensing, cosmic structure formation, the baryon fraction in galaxy clusters, and cluster abundances (combined with independently derived evidence of baryon density)—indicate that 85% to 90% of the mass in the universe does not participate in electromagnetic interactions.

This "non-baryonic dark matter" is generally surmised to consist of one or more fundamental particles distinct from conventional matter (such as electrons, protons, neutrons, and neutrinos).

Due to the absence of direct detection evidence for the existence of dark matter, several theories attempt to explain existing astronomical observations without introducing dark matter. A prominent example is Modified Newtonian Dynamics (MOND), which asserts that Newtonian or Einsteinian gravitational theories are incomplete and that gravity behaves differently across various scales. However, evidence for dark matter stems from numerous independent observational phenomena, making it exceptionally challenging to explain all of these phenomena simultaneously through gravitational theories alone without invoking dark matter.

In particular, the observed pronounced spatial separation between visible matter and its mass center in colliding galaxy clusters, as demonstrated in the "Bullet Cluster" event, serves as compelling observational evidence supporting the existence of dark matter rather than the need for modified gravitational theories.

Even though dark matter particles interact only weakly with conventional matter, they may still be detectable by precision experimental instruments. The detection methods currently employed by scientists can be divided into three categories: first, detecting dark matter particles as they directly interact with matter within a detector, known as "direct detection"; second, searching for signals of ordinary matter produced by the self-decay or annihilation of dark matter in the universe, known as "indirect detection"; and third, searching for artificially produced dark matter particles in particle colliders, known as "collider detection."

1. Direct Detection. If dark matter is composed of microscopic particles, a vast number of dark matter particles must be passing through the Earth at every moment.

If one of these particles collides with an atomic nucleus within the detector material, the detector can register the change in the nucleus's energy and, by analyzing the nature of the collision, deduce the properties of the dark matter. However, for WIMPs, because their interactions with ordinary matter are exceptionally weak, the probability of them being captured by a detector is likewise exceedingly low.

To minimize interference from other types of cosmic rays, direct dark matter detection experiments are frequently conducted deep underground. Currently, dozens of underground dark matter detection experiments are underway worldwide.

No direct detection experiment has yet found conclusive evidence for the existence of dark matter particles. The results of these experiments have nevertheless placed stringent constraints on the mass and interaction strength of dark matter particles.

2. Indirect Detection. Given that a vast quantity of dark matter particles exists within the Milky Way, it should be possible to detect the conventional fundamental particles produced by their annihilation or decay. Indirect detection involves searching astronomical observations for such annihilation or decay signatures, which include high-energy gamma rays, electrons and positrons, protons and antiprotons, neutrons, neutrinos, and various cosmic-ray nuclei.

Experiments employing indirect detection methods may utilize space-borne detectors mounted on satellites or space stations to directly collect cosmic-ray particles, or observe ground-based air showers or Cherenkov radiation effects produced when high-energy cosmic-ray particles enter the Earth's atmosphere.

By analyzing the quantity and energy spectra of various particles within cosmic rays, information regarding the decay or annihilation of dark matter in the universe can be extracted. The difficulty of indirect dark matter detection lies in the abundance of high-energy radiation sources in the universe that are not generated by dark matter, compounded by the complex propagation history cosmic rays undergo from their origin to their arrival near Earth.

Current understanding of the generation and propagation of cosmic rays remains incomplete, presenting a challenge to the identification of dark matter signatures within cosmic rays. Presently, multiple space-based dark matter detection experiments are ongoing globally.

3. Collider Detection. Another approach to searching for dark matter is to produce dark matter particles in the laboratory. In high-energy particle collision experiments, undiscovered particles, including dark matter particles, may potentially be generated. If dark matter particles are produced in a collision, their elusiveness to direct detection by the apparatus will manifest as missing total energy and momentum among the detected collision product particles. This serves as a characteristic signature of invisible particle production. When combined with direct or indirect detection methods, this can help verify whether the particles produced in the collider are indeed dark matter particles.

The Milky Way is the stellar system containing the solar system, comprising between 150 billion and 400 billion stars, a multitude of star clusters and nebulae, various types of interstellar gas and dust, and black holes. Its total visible mass is estimated at 210 billion times the mass of the sun.

As Hua Feng finished scanning this information roughly, a new stream of data followed closely behind. Hua Feng noticed this batch pertained to the Milky Way.

"Great Sage, what does this mean?" Hua Feng asked, feeling thoroughly perplexed by this unprompted infusion of information from Sun Wukong.

"Pay no mind to the details for now; just commit these facts carefully to memory. In time, you will understand why I have done this. This era is no longer the one I once knew; it took me a hundred years simply to readapt to this world. Today, this world faces an unprecedented threat, and I am compelled to act," Sun Wukong replied, his tone growing somewhat agitated.

Hearing this, Hua Feng pressed no further and continued to digest the massive influx of data.

Most of the stars in the Milky Way are concentrated within an oblate spheroid spatial region resembling the shape of a discus. The bulging central portion of this oblate body is called the "nuclear bulge," with a radius of approximately 7,000 light-years. The central region of the bulge is termed the "galactic nucleus," while the surrounding expanse is the "galactic disk." Beyond the galactic disk lies a larger spherical region containing fewer stars and a lower density, known as the "galactic halo," which spans 70,000 light-years in diameter.

In the past, the Milky Way was believed to be a spiral galaxy much like the Andromeda Galaxy, but recent research indicates that the Milky Way is actually a barred spiral galaxy.

Ninety percent of the matter in the Milky Way consists of stars. These stars vary greatly in type and can be categorized into five stellar populations based on their physical properties, chemical composition, spatial distribution, and kinematic characteristics. The youngest Extreme Population I stars are primarily distributed along the spiral arms within the galactic disk, whereas the oldest Extreme Population II stars reside mainly in the galactic halo. Stars frequently congregate into clusters. In addition to a vast number of binary systems, over a thousand star clusters have been identified within the Milky Way. The galaxy also contains gas and dust, which account for roughly 10% of its total mass. This gas and dust are unevenly distributed, forming nebulae in some regions while drifting sparsely through interstellar space.

Since the 1960s, a multitude of interstellar molecules, such as carbon monoxide and water, have been discovered.

Molecular clouds serve as the primary sites of star formation. The core region of the Milky Way, namely the galactic center or nucleus, is a highly unusual place. It emits intense radio, infrared, X-ray, and gamma-ray radiation whose exact nature remains unclear; a supermassive black hole likely resides there, with an estimated mass reaching 4 million times that of the sun.

In 1971, British astronomers Donald Lynden-Bell and Martin Rees analyzed infrared observations and other properties of the Milky Way's central region, proposing that the central engine of the galaxy should be a black hole. They predicted that if their hypothesis were correct, a compact radio-emitting source ought to be observable at the galactic center, with radiation characteristics matching those observed from synchrotrons in laboratories on Earth. Three years later, precisely such a radio source was discovered: Sagittarius A.