"We can now easily say that the universe is innocent and that people's thinking is boundless, because we can't see the edges, we can't think through many things, and people's lives are limited."

The origin and evolution of the universe is one of the most fundamental issues in the study of physics. The Big Bang universe model built on the principle of general relativity and cosmology (that is, the distribution of matter on a large scale in the universe is highly uniform and isotropic). This model tells us that about 13.7 billion years ago, was when the Big Bang happened. The universe was in an extremely dense and extremely hot state, forming space and time. At that point, the universe was born, and it has evolved through expansion and cooling. In this process, the universe experienced several important periods such as the synthesis of primitive light elements, photon decoupling, and the formation of neutral atoms, and the formation of first-generation stars. The galaxies, earth, air, water, and life are in this ever-expanding space-time.

In the 1920s, astronomer Hubble observed from the galaxy spectrum red shift (a spectrum of the celestial body to the long-wave (red) end) observation that all the galaxies in the universe are moving away from each other; the distance is farther. The greater the speed of retrogression, the more proportional the two are. This proportional coefficient is called Hubble's constant. This law is called Hubble's law. Based on this, the concept of "expanded universe" and the "Big Bang universe model" are created. In the 20th century, a large number of astronomical observations and astrophysical studies confirmed this model.

This model is further strongly supported by the Wilkinson Microwave Background Anisotropy Detector (WMAP) and Sloan Digital Sky Survey (SDSS) astronomical observations and the accurate measurement of cosmological parameters. This is undoubtedly a brilliant achievement on the path of human exploration of the mysteries of the universe and the basic structure of matter. The results of WMAP tell us that the common matter in the universe is only 4%, 23% of the matter is dark matter, and 73% is dark energy. SDSS also gives similar results. From the point of view of the basic structure of matter, common materials such as trees, tables, and our own human beings are made up of molecules and atoms. However, molecules and atoms are not fundamental. The most basic particles known so far are the quarks and leptons described by the standard model of particle physics and the interacting particles (photons, gluons, etc.). The Beijing Electronic Negative Collider is a systematic study of the quarks of quarks and taus.

So what is dark matter? Dark matter is a non-luminous substance inferred from astronomical observations in the universe. It consists of nonluminous objects, halos, and non-baryonous neutral particles, but it has a significant gravitational effect. For example, for a galaxy considering the rotational speed of a celestial body far from its center, if the area where the material exists is the same as the area where the light exists, the Newton's law of gravitation can tell that the farther away from the center, the smaller the speed should be. However, this is not the case with astronomical observations. This shows that there are invisible dark matter. Current astronomical observations and structural formation theories strongly suggest that about one-third of the universe is dark matter. The neutrino is a kind of dark matter particle, but the results of WMAP and SDSS show that its quality should be very small, and can only occupy a tiny proportion in dark matter. The vast majority should be the so-called neutral weakly acting heavy particles. It is still not clear what they are. Theoretical physicists speculate that they may be the lightest supersymmetric particles in the theory of supersymmetry, are stable, and are left behind like a microwave background photon during the evolution of the universe. At present, scientists from all over the world, such as the DAMA experiment of the Sino-Italian Scientists Cooperation Group, and the AMS experiment led by Mr. Ding Yizhong, are currently conducting various accelerator and non-accelerator experiments to try to find such dark matter particles.

(Figure 1: Expanded universe, this picture shows the change in the expansion rate that was born since the universe 1.4 billion years ago. The shallower the curve, the faster the rate of expansion. The apparent curve changes about 750 million years ago, the flight in the universe. The objects begin to separate at a faster rate.)

(Figure 4: Blue in the picture is dark matter, red luminescent substance, the two have been separated.)

So what is dark energy? In cosmology, dark energy is the conjecture of some people, referring to a space filled with negative pressure energy. It is an invisible energy that can drive the movement of the universe. All the stars and planets in the universe are driven by dark energy. According to the theory of relativity, this negative pressure is similar to an anti-gravity over long distances. Today, this conjecture is one of the most popular solutions to the problems of the expansive expansion of the universe and the loss of matter in the universe. There are mainly two models of dark energy: cosmological constants (ie, a constant energy density that is uniformly filled with space), and quintessence (a dynamic field where the energy density changes over time and space). Distinguishing between the two may require more precise measurements of the expansion of the universe and a deeper understanding of the expansion velocity over time. Since the speed of the universe's expansion is described by the cosmological equation of state, the measurement of the matter-state equation of dark matter is one of the most important problems in observing cosmology today.

Although the universe exhibits uniformity and isotropy on the largest scales, stars, galaxies, galaxy clusters, and the galaxy Great Wall exist on smaller scales. The force that can promote the movement of matter on a large scale is only gravity. However, evenly-distributed materials do not generate gravity. Therefore, all the cosmic structures of today must have originated from the tiny fluctuations in the distribution of matter in the very early universe, and these fluctuations will leave marks in the cosmic microwave background (CMB). However, ordinary substances cannot form a physical structure through their own fluctuations and they do not leave traces of the cosmic microwave background radiation, because ordinary substances have not been decoupled from radiation at that time.

Dark energy is a landmark achievement of cosmological research in recent years. There are two main proofs of dark energy support. One is that a large number of observations on distant supernovae indicate that the universe is accelerating, and that the speed of galaxy expansion is not constant as Hubble's law describes, but it is accelerating. According to Einstein's gravitational field equation, the phenomenon of accelerating expansion deduces that there is "dark energy" with negative pressure in the universe. Another piece of evidence comes from recent studies of microwave background radiation that accurately measure the total density of matter in the universe. However, we know that all common substances and dark matter add up to about 1/3 of them, so there is still a shortage of about 2/3. This shortage of material is called dark energy, and its basic feature is that it has negative pressure. It is almost evenly distributed in the space of the universe or does not cluster at all. Recent WMAP data shows that dark energy accounts for 73% of total matter in the universe. It is worth noting that for normal energy (radiation), baryons and cold dark matter, the pressure is non-negative, so there must be an unknown negative pressure substance that dominates today's universe.

However, the basic theory of physics cannot explain the observed dark energy. Dark energy is the biggest challenge faced by physics in the 21st century. Dark matter and dark energy are called “two dark clouds” in this century's astronomy. Physics has just begun the exploration of this new type of substance, dark energy. There are many opinions, but only some speculations and assumptions are far from forming a basic and reasonable explanation. Scientists are planning to launch new exploration satellites to conduct more accurate and systematic observations of large-scale space in the universe, further study the law of accelerated expansion of the universe, determine the form and physical characteristics of dark energy, and different dark energy forms will lead to very different The law of the expansion of the universe. Solving this problem requires new theories. Once such a theory is found, it is likely to be a quantum theory of interaction and unity that people have long sought, including gravitation.

Dark matter evidence

In the 1930s, the Nobel Prize winner, Milligan, was committed to building the California Institute of Technology in Pasadena, southern California, into a world-class research institute. The first scholar he employed for astrophysical research was Swiss scientist Fritz Zwicky. In 1934, Zwicky studied the movement of galaxies within the galaxy cluster and for the first time raised the possibility of the existence of dark matter. The galaxies have a tendency to clump in the universe and have a gravitational connection between them. The system composed of them is called a galaxy group; if there are more than tens of galaxy groups bound together by gravity, they are hundreds, thousands, and so on. Thousands, then called galaxy clusters. There are hundreds and thousands of galaxies in the galaxy cluster. Because they are bound by gravity, the speed and gravity of the galaxy must be balanced to prevent it from derailing. The stronger the gravitational force, the faster the movement. However, Zwicky discovered that the galaxy within the galaxy cluster is far from enough to produce such large gravity, and there must be other substances that humans cannot see. He called it dark matter.

At present, people can only know through the effect of gravitation that a large amount of dark matter exists in the universe. The earliest evidence of the existence of dark matter originated from the observation of the rotational velocity of globular galaxies. Modern astronomy through gravitational lens, large-scale structure formation in the universe, microwave background radiation and other research shows that: we currently understand only about 4% of the universe, dark matter accounts for 23% of the universe, and 73% is a Dark energy that causes the universe to accelerate expansion. Li Zhengdao, winner of the Nobel Prize in 1957, believes that dark matter and dark energy account for 99% of the universe's mass. Dark matter cannot be directly observed, but it can interfere with the light waves or gravitational force emitted by the star, and its existence can be clearly felt. Scientists have made many assumptions about the properties of dark matter, but it has not been fully proved until now.

In 1990, the U.S. Space Shuttle sent the Hubble Space Telescope to space. According to Hubble telescope observations, people have calculated that the age of the universe is about 12 billion to 14 billion years; however, astronomers are known to have some ancient globular clusters, which are about 14 billion to 16 billion years old. This shows a contradiction that the age of the universe is actually younger than some globular clusters. Later, an unexpected discovery shocked the scientific community. Astronomers call it a type Ia supernova, which can be used as a standard candlelight because the brightness of the light emitted by the explosion is fixed. Knowing the standard candlelight and then observing its brightness with a telescope, you can determine its distance. Ten years ago, two independent astronomical research teams composed of Liss et al. and Palmer et al. almost simultaneously announced that using type Ia supernovas as standard candles, they discovered that the universe is accelerating. Previously, almost everyone believed that the expansion of the universe must be decelerating, because gravitational force only slows down the expansion. The discovery of the exploding universe indicates that the age of the universe is longer than people originally thought, and that either there is repulsion or there is negative pressure that scientists call dark energy.

In 2006, American astronomers used the Chandra X-ray telescope to observe the galaxy cluster 1E 0657-56. They inadvertently observed the process of galaxy collisions. The galaxy clusters collided with power and made dark matter separate from normal matter. Direct evidence of the existence of dark matter. Marusa Bradac is from the Kavli Institute for Particle Astrophysics and Cosmology at the US Department of Energy's Stanford Linear Accelerator Center (SLAC), and he and his colleagues are one and a half billion light years away from us. The Galaxy Cluster carried out this epoch-making observation. This study is based on observations of the Bullet Cluster. The bullet galaxy cluster is an unusual cosmic structure. It is actually formed by two groups of galaxies meeting each other and crossing each other. As the two galaxy clusters collide at a speed of 100 million miles per hour, the luminous substances contained within them squeezing and decelerating due to interaction forces other than gravitational force. However, because the dark matter in the two galaxy clusters does not have such repulsive forces, they do not slow down and pass directly through. This causes the dark matter to go in front of the luminescent material, so that each galaxy cluster is divided into two parts: the dark matter first, and the luminescent matter second. The researchers compared the X-ray image of the luminescent material and the measurement of the total mass of the galaxy cluster, thus detecting the separation of dark matter and luminescent material. They observed the gravitational lensing effect caused by galaxy clusters. In this phenomenon, the light emitted from the rear galaxy cluster was deflected due to the gravitational attraction of galaxy clusters. The quality of the galaxy clusters was different. The greater the deflection, the greater the declination of the light indicates the greater the mass of the galaxy cluster—the result is the total mass of the galaxy cluster. By using the Hubble Space Telescope, the Magellan Telescope, and the Very Large Telescope to measure light deflection, the team was able to restore the distribution of mass in the bullet galaxy cluster. Then they compared it to the X-ray image of the luminescent material photographed by the Chandra X-ray telescope and found four separate masses of matter: the two of them, the larger ones, consisted of dark matter, which accelerated away from the point of impact; The other two smaller ones consist of luminescent substances, which move slowly around the collision point. The separation of galaxy clusters into two clusters in space demonstrates the existence of two substances, and the huge differences they exhibit show the strange nature of dark matter.

(Figure 2: Computer simulation of dark matter.)

(Figure 3: Dark matter in the bullet galaxy cluster has been separated from the luminescent material.)

The cryogenic dark matter search project (CDMS) aims to use a detector to detect the interaction between particles and find the motion caused by dark matter particles. In 2009, American scientists detected two signals that could come from dark matter particles in a laboratory located on the campus of the University of California. However, they also stated that the similarity between these signals and dark matter particles is not high. They installed more advanced laboratory equipment about 714 meters underground from the Souden Coal Mine in Minnesota for the Phase II Low Temperature Dark Matter Search Project (CDMSII). The dark matter phenomenon will be disturbed by the cosmic rays that enter the earth. To reduce the influence of cosmic rays on the background signal of the muon particles, the only way is to move to the depths of the ground, so that we can surely confirm the composition of dark matter.

On December 21, 2009, scientists discovered dark matter in the Souden Mine, which is by far the most powerful evidence of dark matter. Other experiments are also exploring signals from dark matter, such as Lux experiments. The American Fermi Space Telescope is trying to locate dark matter, looking for evidence of its annihilation in space (when dark matter collides, two particles will generate gamma rays that can be received by the detector), but there is currently no discovery.

In October 2010, cosmologists said that they had found the most convincing evidence of dark matter particles in the depths of the galactic core. The mysterious matter of this land collided together to generate gamma rays more frequently than other adjacent areas in the sky. Like ordinary matter, dark matter has gravity, and it is with their help that billions of stars have accumulated into the galaxy. However, it is difficult for this substance to interact with ordinary substances and people cannot see it. Microneutrons are the only dark matter particles that have been found in the laboratory, but they are almost zero mass, and they account for only a small percentage of the cosmic energy part of dark matter. Astrophysicists believe that the large part of the rest is made up of weakly interacting massive particles (WIMPs), which are about 10 to 1000 times more energetic than protons. If two dark matter particles collide, they will destroy each other and produce gamma rays. The team found these signals in the data gathered in a 100 light-year-old area at the core of the Milky Way. Hope explained that they are concerned about this area because it is the favorite gathering place for dark matter. The density of dark matter in this area of the Milky Way is 100,000 times that of the Galaxy. In short, the core of the galaxy is a place where dark matter gathers together and often collides.

(Figure 5: In January 2007, scientists announced for the first time that dark matter in the universe was a drawn Three-dimensional digital map.)

(Figure 6: The gamma rays that collided with mysteries of the galactic core)

In November 2010, thanks to the Hubble Space Telescope and the cosmic gravitational lens effect, scientists have successfully acquired the most accurate map of dark matter in a giant galaxy cluster so far. When a large amount of dark matter accumulates, just as a galaxy consisting of conventional matter will contain hundreds or even thousands of galaxies, the collected dark matter will generate a huge gravitational effect, causing the rays emitted by the far distant galaxy to bend when passing near it. , thus forming a lens-like effect. Earth astronomers have already photographed multiple images and artifacts of the same galaxy forming around the center of gravity. This is called the 'gravitational lens effect.' This is a visual evidence of the existence of dark matter. When light from a distant galaxy passes near a galaxy cluster, the light will deflect due to the galaxy's gravitational pull. At this time, the galaxy cluster looks like a lens. Looking in this direction, you will see it. The huge light arc is even a few different images of the same galaxy.

(Figure 7: The blue area in the figure is the distribution of dark matter that scientists superimposed on the images of the Hubble Telescope.)

(Figure 8: China Dark Matter Research Base.)

Chinese scientists challenge modern physics "two dark clouds"

Dark matter and dark energy are called 'two dark clouds' in modern physics and astronomical clear skies in the 21st century. Uncovering the mysteries of dark matter and dark energy will be another major leap for human beings to understand the universe, and may lead to a new Physics revolution. On March 18, 2010, the National '973' project "Theoretical research on dark matter and dark energy and experimental pre-research" was launched in Beijing. This signifies that Chinese scientists will challenge the mystery of the 'two dark clouds.'

"As more and more accurate cosmological data are available, the evidence for the existence of dark matter and dark energy becomes more and more clear. With the implementation of a series of higher-accuracy astronomical observation experiments, it indicates that The golden age of cosmological research has begun," said Wu Yueliang, chief scientist of the project, academician of the Chinese Academy of Sciences and director of the Institute of Theoretical Physics of the Chinese Academy of Sciences.

In terms of experimentation, the Zijinshan Observatory of the Chinese Academy of Sciences used the advanced thin-ionization ionization energy analyzer (ATIC) detector to discover the "super" energy spectrum of high-energy electrons, which may be related to dark matter annihilation. The results were published in the British "Nature" on November 20, 2008. In the magazine, it was selected as the major research progress in the field of world physics in 2008 selected by the American Physical Society and the European Physical Society; a number of experimental physicists at the Institute of High Energy Physics, Chinese Academy of Sciences participated in the long-term detection of dark matter by the Italian DAMA experimental group and reported relevant Possible signals of dark matter particles; Shanghai Jiaotong University has a basis for XENON detection technology for direct detection of dark matter; Tsinghua University has conducted long-term research on low background, high energy threshold high purity germanium detectors.

In theory, researchers from the Institute of Theoretical Physics of the Chinese Academy of Sciences, the Institute of High Energy Physics of the Chinese Academy of Sciences, the National Astronomical Observatory of the Chinese Academy of Sciences, Peking University, Tsinghua University, the University of Science and Technology of China, and Fudan University proposed theoretical models and mechanisms for explaining dark matter and dark energy. Work with international influence.

According to Wu Yueliang, the project of the '973' project is set up as five projects, which are the theoretical research of dark matter and related new physical phenomenology, space exploration experimental research of dark matter, advanced research of dark matter underground exploration, and dark matter tonnage The pre-study and dark energy theoretical research and ground-surveillance scheme of the helium detector were studied.

The overall academic idea of the project is to play the role of theoretical advancement and experimental pre-research, emphasize the advantages of combining theoretical research with experimental detection design and multi-disciplinary integration, from the underground, the ground to the space a variety of means complement each other, constitute an organic In order to carry out indirect and direct detection of dark matter, to provide reliable physical basis and feasible experimental design and effective detection program, to promote China's space satellite celestial particle physics experimental platform, deep underground laboratory in Sichuan Jinping National, Antarctic hail National ground astronomical telescope observation experiment base construction.

"The project has focused on the main advantages in the field of dark matter and dark energy, and formed an interdisciplinary research team." Wu Yueliang said, "the research team has a solid theoretical basis for the theoretical study of dark matter and dark energy, including particle physics theory. , gravitation theory, grand unification theory (such as superstring theory) and other basic theories; has extensive experience in accelerators, detector construction and related technologies, and international cooperation; has long-term research in experimental groups and has achieved important results with accumulated rich experience .

China Dark Matter Research Base

China’s first extremely underground laboratory—“China Jinping Underground Laboratory” was unveiled and put into use on the Jinping Hydropower Station in Yalong River, Sichuan on December 12, 2010. Jinping underground laboratory has a vertical rock coverage of 2,400 meters. The world’s most rock-covered laboratory. Its completion marks that China already has a world-class, clean, low-radiation research platform and can independently carry out the most advanced basic research topics such as dark matter detection. At present, the dark matter detectors of the experimental group of Tsinghua University have taken the lead in the laboratory and started the detection work. In 2011, the Shanghai Jiaotong University and other research teams will also enter here to carry out the dark matter detection research.

During the construction of the Ertan hydropower station, at the foot of the Jinping Mountain in Sichuan, 18 kilometers of tunnels that can pass through cars were built. The above is more than 2,500 meters thick mountain rocks. These ordinary tunnels, in the eyes of cosmological researchers who are struggling to find an experimental environment. Shanghai Jiaotong University's Research Institute for Particle Physics, which was just established in February 2010, used the Jinping Mountain Tunnel as a construction site for underground laboratories. This will be the site of the first experiment after the establishment of the Institute, specifically 'acquisition of' dark matter. At present, it is the most superior dark matter detection environment in the world.

The reason why it is called optimal, according to Ji Xiangdong, director of the Physics Department of the University of Jiaotong University and director of the Institute of Particle Physics Cosmology, uses the underground tunnel built during the local construction of the hydropower station. The excavation is 40 meters long and wide at the side. , 6 meters high space for each. Therefore, it is more convenient to use than some underground laboratories in foreign countries that are 'born out' of mines. They do not have to go up and down the elevator, and they can drive on the ground by car. The 2500-meter deep tunnel is even rarer because the deeper it is buried, the less cosmic ray interference will be.

Nikai Xuan, a research fellow at the Jiaotong University Institute of Particle Physics, is the head of Jiaotong University’s Jiaotong University’s international cooperation project XENON, and is also the head of the experimental data analysis team. In the last year, he worked in the famous Gran Sasso laboratory in Italy. Gran Sasso's laboratory is built 1400 meters below the ground. It is also built on underground tunnels and is the largest underground laboratory in the world. There are more than a dozen large and small experiments at the same time. There are dark matter detection and neutrino detection.

How to "make" dark matter? Scientists also think of many ways. The first method was the astronomical observation method, but it was impossible to answer "What is dark matter?" Later, people used indirect detection and direct detection. The former is the detection of ordinary matter particle signals generated by collisions of dark matter, and is usually detected by ground or space telescopes; the latter, by collisions of nuclei and dark matter, detects the signals generated by collisions. On the ground, because of the large number of cosmic rays, these signals can interfere with direct detection and affect their ability to discriminate. Therefore, underground laboratories can help the detectors 'block' interference and allow them to focus on their work.

As far as scientific researchers are concerned, it is still unknown as to what can be used to detect dark matter. Ji Xiangdong said: “Particle physics seeks the deepest mysteries of matter, and what impact it will have on our future lives. We are still not known. Just as electric inventions were invented, people could not imagine later television or computers. In any case, every scientific discovery has made people more aware of the material world."

Dark Matter Candidates

For a long time, the most promising dark matter is only the basic dark particles in the hypothesis. It has the special characteristics of long life, low temperature, and no collision. Low temperatures mean that they are non-relativistic particles when they are decoupled, and only then can they be quickly grouped under gravity. Long life means that its life must be equal to or even longer than the current age of the universe. Since the clustering process takes place in a smaller range than Hubble's horizon (the product of the age of the universe and the speed of light), and this horizon is very small relative to the current universe, the first dark matter mass or dark matter halo formed is better than the Milky Way. The scale is much smaller and the quality is much smaller. With the expansion of the universe and the increase in the horizon of Hubble, the first halo of small dark matter will merge to form larger-scale structures, and these larger-scale structures will later merge to form larger-scale structures. The result is the formation of structural systems of different volumes and qualities, qualitatively consistent with observations. On the contrary, for relativistic particles, such as neutrinos, the structures we have observed cannot be formed due to the speed of the mass gravitation. Therefore, the contribution of neutrinos to the mass density of dark matter is negligible. The measurement of neutrino mass in the solar neutrino experiment also supports this. Collisionless refers to the negligible interaction of dark matter particles (dark matter and common matter) in small and medium dark matter halos. These particles rely solely on gravitational force to bind each other, and in a dark matter halo they orbit with an unobstructed, eccentric orbit spectrum.

There are several reasons why low temperature collision-free dark matter (CCDM) is viewed. First, the numerical simulation results of CCDM structure are consistent with observations. Second, as a special sub-class, weakly interacting large mass particles (WIMP) can explain its abundance in the universe. If the interaction between the particles is weak, they are in thermal equilibrium in the first trillionths of a second of the universe. Later, due to the annihilation of them, they began to get out of balance. According to their interaction cross-section estimation, the energy density of these substances accounts for about 20-30% of the total energy density of the universe. This is consistent with the observations. The third reason CCDM is favored is that some theoretical models predict some very attractive candidate particles.

One of the candidates is the neutralino, a particle proposed in a supersymmetric model. Supersymmetry theory is the basis of supergravity and superstring theory. It requires that each known fermion has an accompanying boson (not yet observed), and that each boson must also have an accompanying fee. Yonago. If supersymmetry continues to this day, the companion particles will all have the same mass. However, due to spontaneous symmetry in the early super-symmetry of the universe, the quality of particles that accompany it today has also changed. Moreover, most of the super-symmetric companion particles are unstable, and decay soon after the super-symmetry breaks out. However, there is a lightest companion particle (mass in the order of 100 GeV) that avoids the occurrence of decay due to its own symmetry. In the simplest model, these particles are electrically neutral and weakly interacting - ideal candidates for WIMP. If the dark matter is composed of neutrals, then when the earth passes through the dark matter near the sun, the underground detector can detect these particles. It is also important to note that this detection does not mean that dark matter is mainly composed of WIMP. The current experiment cannot determine whether WIMP accounts for most of dark matter or just a small part.

Another candidate is the axion, a very light neutral particle whose mass is on the order of 1 μeV, which plays an important role in the grand unification theory. The axons interact with each other with very little force, so they cannot be in thermal equilibrium and therefore cannot explain its abundance in the universe. In the universe, the axicon is in the condensed state of cryogenic bosons, and now an axicon has been built and the detection work is ongoing.

In the universe, the stars interact with each other and perform a variety of regular orbital motions. However, in some motions, we cannot find the substances that correspond to their effects. Therefore, it is assumed that there may be invisible substances in the universe.

It is known that the large structure of the universe is foamy. The galaxies are clustered into the “Great Wall of Galaxies,” the connecting fibers of the foam, and the fibers are huge "cosmic voids," that is, large bubbles with a diameter of 1 to 300 million light. year. If there is not an additional gravitational attraction of dark matter that cannot be seen, it will be impossible to maintain such a large void, just as the roof and the bridge are too large to support.

Although our universe is expanding, the galaxies in high-speed motion do not spread apart. If there is only visible matter, their gravitational force is not enough to hold the galaxies together.

We know that 99.86% of the mass of the solar system is concentrated in the center of the solar system, the sun. Therefore, the planets close to the sun are attracted by the sun's gravity and are larger than the planets far from the sun. Therefore, the speed of the planets near the sun running around the sun is high. It is faster than a planet farther from the sun in order to produce greater centrifugal acceleration (centrifugal force) to balance larger solar gravitational forces. But in the center of the galaxy, although there are more stars and black holes, the stars near the center of the galaxy are not moving faster than the distant stars. This shows that the quality of the galaxy is not concentrated in the center of the galaxy. There must be a lot of dark matter in the outer region of the galaxy.

The brightness of the celestial bodies reflects the quality of the objects. So astronomers often use the galaxy's brightness to calculate the quality of galaxies, but also to calculate the quality of galaxies through gravity. However, the quality of the Milky Way derived from the gravitational force is more than ten times the mass of the Milky Way estimated from the brightness and even five thousand times in the peripheral area. Therefore, there must be a lot of dark matter there.

Dark Matter Distribution Shapes

Dr. Nobuhiro Okabe, a postdoctoral researcher at the Academia Sinica and the Institute of Astronomy and Astrophysics of the Academia Sinica, recently participated in a group of transnational teams that for the first time confirmed the current prediction model for dark matter in the astronomical world. This model describes the dark matter in large quantities. Clusters of galaxies show a flat distribution that is approximately elliptical. The paper was published on the April 23, 2010, website of the Monthly Notices of the Royal Astronomical Society.

The research team used the Subaru Telescope’s Prime Focus Camera (also known as Suprime-Cam) to observe 25 massive galaxy clusters and to measure the dark matter spatial distribution of these galaxy clusters in detail with a gravitational lens. Clusters of galaxies are ideal sites for studying the distribution of dark matter because they contain thousands of galaxies and contain a large amount of dark matter. The researchers used Suprime-Cam to obtain a wide-angle image of the massive galaxy clusters (the galaxy clusters are located at a distance of 3 billion light-years away from Earth), and used these images to measure and analyze the distribution of dark matter.

After detailed analysis of the gravitational lens effect in the image, the team obtained clear evidence of the distribution of dark matter in these galaxy clusters. On average, the distribution of dark matter presents a very flat shape rather than a simple spherical outline. The degree of flattening is quite large, and the ratio of the major axis to the minor axis of the elliptical shape is equivalent to 2:1. This discovery set a feat, and for the first time successfully demonstrated that astronomers used gravitational lenses to directly detect the flattening of dark matter distribution. The results of the study also show that the degree of flattening observed is consistent with the mainstream theoretical results of dark matter.

(Figure 9: Subaru Suprime-Cam captures an ultra-wide-angle image of the A2390 galaxy cluster (2.7 billion light-years from the Earth). The shades of purple are dark matter with many distant galaxies (usually about 80 miles from Earth). The estimated distribution of the gravitational lens effect is about a hundred million light years. The deeper purple region represents a higher density of dark matter, and it can be seen that the distribution of dark matter extends along the northwest-southeast direction.)

(Figure 10: This figure shows the use of a gravitational lens to measure the distribution of dark matter. Different colors represent differences in density of dark matter, with reddish colors representing higher densities. Black ellipses show distorted patterns of background galaxies; locations of distant galaxies. It is systematically distorted into a black elliptical shape, and the distortion is caused by the gravitational lens effect (in fact, all background galaxies have their own shape and direction, so the shapes of multiple galaxies are averaged, and are captured by the gravitational lens effect. The degree of distortion.) The left and right graphs show the dark matter distributions of "spherical" and "elliptical." The different distorted patterns show that the dark matter distribution can be measured by two-dimensional lens distortion.)

The Discovery of Relevant Various Scientists

In the 1930s, the Dutch astrophysicist Oort pointed out that in order to explain the movement of stars, it is necessary to assume that there are dark matter near the sun; in the same year, Ziewicz also considered the movement of galaxies from the galaxy galaxy. There is a large amount of dark matter in the galaxy cluster; the theoretical analysis of American astronomer Bacco also shows that near the sun, there are substances that are almost invisible to the luminous substances.

So, what is the dark matter near the sun and on the galactic plane? Astronomers believe that they may be extremely faint brown dwarfs not observed by general optical telescopes or planets 30 to 80 times the mass of wood planets. Photographs of sky photos taken by large-field telescopes have revealed M dwarf stars that are darker than 14 stars and less than half the mass of the sun. Since the Sun is located near the center plane of the Milky Way, it can be deduced from the number of detected M dwarfs that they probably provide the other half of the missing mass of the Milky Way. And every M-type star shines for tens of thousands of years. Therefore, it is believed that there must be a lot of these "small stars" in the galaxy after the "burning" of the "corpses", enough to provide all the dark matter required for theoretical calculations.

Observational results and theoretical analysis indicate that there are massive dark halos in the periphery of spiral galaxies. So, what invisible substances are contained in dark halo? The British Astronomy family thinks that there may be three candidates: the first is the low-mass stars or planets described above; the second is the massive mass of about 2 million times the mass of the sun that was collapsed by supermassive stars. Black holes; the third type is bizarre particles, such as neutrinos whose mass may be 20 to 49 electron volts and electrons, electrons of 105 electron volts, or the great unification theory currently supported by scientists. Demand particles.

According to Iris, a particle physicist at the European Nuclear Research Center, the galaxy halo and the best candidate for dark matter in the galaxy cluster are S-particles required by the theory of supersymmetry. This theory holds that the elementary particles (eg, photons) of each known particle must have a pair of particles (eg, a photon of a certain quality). Iris recommends four best dark matter candidates: photon, higgs, neutrino, and gravitational particles. Scientists also believe that these particles are also cold dark matter candidates in the vast universe of space between galaxy clusters.

Up to now, many astronomers believe that more than 90% of the substances in the universe are hidden by 'dark matter.' However, what is the dark matter in the end is still a mystery, yet to be further explored by people.

January 6, 2006 reported that for the first time in history, scientists from the University of Cambridge Institute of Astronomy have successfully identified some of the physical properties of dark matter that are widely distributed in the universe. At present, scientists engaged in this research are preparing to publicly publish the results of this research in recent weeks.

According to astronomers, according to current statistical data, it is likely that the dark matter that we can't normally see accounts for 95% of all the total mass of the universe.

In this study, scientists conducted a total of 23 nights of research on dwarf galaxies not far from the Milky Way using powerful high-power astronomical telescopes (including VLT, Very Large Telescope, which was set up in Chile). Scientists have since We also concluded from about 7,000 calculations that the dark matter content of the dwarf galaxies they observed was more than 400 times that of other common materials. In addition, the particles of these dwarf galaxies can move at a speed of up to 9 kilometers per second, and their temperature can reach 10,000 degrees Celsius.

At the same time, scientists have also observed that there is a huge difference between dark matter and other common substances, such as: Although the temperature of the observation target is so high, such high temperature does not produce any radiation. According to Professor Jerry Gilmore, who led the study, it is very likely that dark matter particles are not composed of protons and neutrons. However, scientists had previously believed that dark matter should be composed of some "cold" particles, and these particles would not move too fast.

Dark matter research experts also stated that the smallest continuous dark matter fragment in the universe is also about 1,000 light-years old, and the mass of such dark matter fragments is about 30 times that of the Sun. Scientists also determined the density of dark matter particles distribution in this study. For example, if there are 1023 particles per cubic centimeter of space on Earth, then such a large space for dark matter can only accommodate about three points. One of the particles.

As early as the 1930s, the Swiss scientist Fritz Zwikho assumed that there was some unknown dark matter in the universe. He also pointed out that if the luminescent materials in the galaxy cluster are linked to each other by their own gravitational force, their quantity must be increased by another 10 times. The invisible gravity material used to make up for this vacancy is the dark matter we are talking about today. Although the amount of dark matter stored in the universe is much higher than other common materials, the nature of dark matter cannot yet be fully expressed by scientists.