Dark matter is a kind of invisible matter proposed theoretically that may exist in the universematerialIt may be the main component of cosmic matter, but it does not belong to any known matter constituting visible celestial bodies.The phenomenon suspected to violate Newton's universal gravitation found in a large number of astronomical observations can be well explained under the premise of the existence of dark matter.Modern astronomy, through the movement of celestial bodiesNewtonUniversal gravitationPhenomenonGravitational lensing effectThe formation of the large-scale structure of the universeMicrowave background radiationThe observation results show that dark matter may exist in a large number of galaxies, star clusters and the universe, and its mass is far greater than the total mass of all visible objects in the universe.Combined with the observation and standard of microwave background radiation anisotropy in the universecosmological models (Λ CDM model) can determine that dark matter accounts for 85% of the total mass of all matter and 26.8% of the total mass and energy of the universe.
A widely accepted theory holds that dark matter is composed of "weakly interacting massive particles" (WIMP), whose mass and interaction intensity are near the electric weak scale, and the observed residual abundance is obtained through thermal decoupling during the expansion of the universe.In addition, there are also hypotheses that dark matter is composed of other types of particles, such as axions, inert neutrinos, etc.
It was the astronomer Jacobus Kapteyn who first proposed the possible existence of "dark matter". In 1922, he proposed that the possible existence of invisible matter around stars could be inferred indirectly from the motion of star systems[1]。In 1932, astronomer Jan Oort conducted a dark matter study on the motion of stars near the solar system.However, no conclusive conclusion has been reached about the existence of dark matter.[2]In 1933, astrophysicist Fritz Zwicky measuredBackhair ClusterThe velocity of each galaxy in relative to the cluster.[3]Using the potential force theorem, he found thatVelocity dispersionToo high. The gravity generated by the mass of visible galaxies in the cluster cannot bind them in the cluster. Therefore, there should be a lot of dark matter in the cluster, whose mass is at least 100 times that of visible galaxies.In 1936, S. SmithVirgo Galaxy ClusterOur observation also supports this conclusion.[4]However, the breakthrough conclusion of this concept failed to attract the attention of the academic community at that time.In 1939, astronomer Horace W. BabcockAndromeda Galaxy Spectral study of,[5]It shows that the rotation speed of stars in the outer region of the galaxy is much faster than that throughKepler's lawIt is expected to be larger, corresponding to a larger mass to light ratio.This suggests that there may be a large amount of dark matter in the galaxy.In 1940, Oldt studied the velocity of stars in the outer region of NGC3115, and pointed out that its total mass light ratio could reach about 250[6]。In 1959, F. D. Kahn and L. Woltjer studied the relative motion between the Andromeda Nebula and the Milky Way[7]Through the speed at which they approach each other and the distance between them, it can be inferred that the mass of dark matter in our local galaxy cluster is about ten times greater than that of visible matter.An important evidence for the existence of dark matter comes from Vera Rubin and Kent Ford's research on the rotation speed of stars in the Andromeda Nebula in 1970[8]。Using high-precision spectral measurement technology, they can detect the relationship between the rotation speed and distance of the peripheral stars far away from the galactic core region around the galaxy.according toNewton's law of universal gravitationIf the mass of the galaxy is mainly concentrated on the visible stars in the galactic core, the speed of the stars outside the galaxy will decrease with the distance.However, the observation results show that the speed of stars outside the galaxy is constant in a large range.This means that there may be a large amount of invisible matter in the galaxy not only distributed in the core of the galaxy, but also its mass is far greater than the total mass of light-emitting stars.In 1973, M. S. Roberts and A. H. Rots used the 21 cm characteristic spectral line observation technology to detect the velocity distribution of the gas outside the Andromeda Nebula, which also confirmed this conclusion from another angle[9]。In the 1980s, a large number of new observational data supporting the existence of dark matter appeared, including the gravitational lens effect when observing background galaxy clusters, the temperature distribution of hot gas in galaxies and star clusters, and the anisotropy of cosmic microwave background radiation.The theory of the existence of dark matter has gradually been widely recognized by the astronomical and cosmological circles.According to the comprehensive analysis of existing observation data, the main component of dark matter should not be any known microscopic elementary particles.Today's particle physics is trying to explore the properties of dark matter particles through various means.
Observational evidence
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Although dark matter has not been directly detected, there is a large amount of evidence that it exists in the universe, such as:
Galaxy rotation curve and dispersion velocity distribution
The galaxy rotation curve describesspiral galaxy The relationship between the orbit speed of the visible objects and their distance from the center of the galaxy.According to the observation of the mass distribution of visible objects in the spiral galaxyLaw of universal gravitationAccording to the calculation, the peripheral objects should rotate around the center of the galaxy more slowly than the central objects.However, the measurement of the rotation curve of a large number of spiral galaxies shows that the speed of the outer objects is almost the same as that of the inner objects, which is much higher than expected.This implies the existence of massive invisible matter in these galaxies.Combined with the potential force theorem, the material distribution in the galaxy can be calculated from the dispersion velocity distribution of visible objects in the galaxy.This method is also applicable to measurementElliptical galaxyandGlobular clusterMaterial distribution.The results show that the material distribution of most galaxies and star clusters is not consistent with the observed distribution of visible matter except for a few, and the mass of visible matter is only a small part of the total mass of galaxies and star clusters.
Galaxy cluster observation
The mass distribution of galaxy clusters can be obtained mainly by three different means: (1) Observe the movement of galaxies in the cluster and calculate it by gravity theory.(2) Observe the X-ray produced by galaxy clusters.The hot gas that can emit X-rays is ubiquitous in the galaxy cluster. When the gas reaches the hydrodynamic balance in the gravitational field of the galaxy cluster, the mass distribution of the galaxy cluster can be inferred from its temperature.(3) Gravitational lensing effect.According to the general theory of relativity, the light from behind the galaxy cluster will bend when passing through the massive galaxy cluster, which is similar to the lens in optics.The distribution of matter in the cluster can be calculated according to the bending degree of background light.These three methods do not affect each other and support each other, making galaxy cluster observation an important means of studying dark matter.These observations consistently show that the total mass of matter in star clusters is far more than the total mass of visible matter.
Cosmic microwave background radiation
On the cosmic scale, the total amount of dark matter in the universe can be determined by fine observation of the anisotropy of the microwave background radiation in the universe.Observations show that 26.8% of the total energy of the universe is contributed by dark matter, while only 4.9% of the conventional matter constitutes celestial bodies and interstellar gases, and the remaining 68.3% is the dark energy that promotes the accelerated expansion of the universe.[10]
The formation of large-scale structure of the universe
Large Computer Simulation of the Evolution of the Universe with N-body Gravity[11-13]The collisionless low-speed dark matter particles gradually gather into clusters under the action of gravity, which can form the large-scale structure we see today.The dark matter distribution of these structures has a universal mass distribution.Low velocity dark matter is conducive to the formation of large-scale structure.The high-speed particles tend to smooth the structure.Therefore, neutrinos are not supported as the main candidate for dark matter particles.
Known Attributes
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The existence of dark matter has been widely recognized, but the properties of dark matter are poorly understood.The known properties of dark matter only include a limited number of aspects:
(1) Dark matter participationGravitational interaction, so it should have mass, but the mass size of a single dark matter particle cannot be determined.
(2) Dark matter should be highly stable. Because there is evidence of dark matter at different stages of the formation of the cosmic structure, dark matter should be stable on the time scale of the cosmic age (10 billion years).
(3) Dark matter basically does not participate in electromagnetic interaction, and the interaction between dark matter and photons must be very weak, so that dark matter basically does not emit light;Dark matter also does not participate in strong interaction, otherwisePrimary nucleosynthesisWill be disturbed,Abundance of light elementsIt will change, which will lead to inconsistency with the current observation results.
(4) According to the computer simulation of the formation of the large-scale structure of the universe, the speed of dark matter movement should be much lower than the speed of light, that is“Cold dark matter”Otherwise, our universe cannot form the observed large-scale structure under the action of gravity.
Synthesize these basic attributes.It can be concluded that dark matter particles do not belong to any of the basic particles we know.This is very successful for the currentStandard Model of Particle PhysicsPose a challenge.
Dark matter candidate
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Weak interactionThe massive particle (WIMP) is one of the most widely discussed dark matter candidates. It refers to a certain stable particle whose mass and interaction intensity are near the electric weak scale, and the known residual abundance is obtained through the thermal decoupling mechanism.WIMP should be basically electric neutral and color neutral, so it is not directly involved in electromagnetic and strong interaction.Neutrinos also do not participate in strong interaction and electromagnetic interaction, but because they move at a speed close to the speed of light in the universe, they belong to "hot dark matter" and are not enough to be the main component of dark matter.In the standard model of particle physics known to mankind, there are no particles that meet these properties at the same time, which means that WIMP must be a new physical particle beyond the standard model.The WIMP predicted by existing theories includes the lightest supersymmetric companion particle in the supersymmetric model[14], such as ultrameso;The smallest Kaluza Klein excited state particle in the extra dimension theory;T-odd particles in Little Higgs model.
Another dark matter candidate is axion, a very light neutral particle, which is associated with the joint symmetry breaking of charge conjugation parity inversion in strong interactions.The axions interact with each other by a very small force, so they cannot be in thermal equilibrium with the background radiation. Therefore, the residual abundance will not be obtained by thermal decoupling, but can become cold dark matter by breaking the vacuum state.
Although many astronomical observations have been made on dark matter, its composition has not yet been fully understood.Early theories of dark matter focused on some hidden conventional matter stars, such as black holes, neutron stars, aging white dwarfs, brown dwarfs, etc.These stars are generally classified as massiveCompact celestial body(Massive Compact Halo Objects, MACHOs), however, astronomical observations over the years have failed to find enough MACHOs.It is generally believed that the hard to detect baryonic matter (such as MACHOs and some gases) does contribute part of the dark matter effect, but the evidence indicates that such matter only accounts for a small part of it[15-16]。The rest is called "non baryonic dark matter".In addition, observational data such as the rotational speed curve of galaxies, gravitational lens, the formation of cosmic structure, the proportion of baryons in galaxy clusters, and the abundance of galaxy clusters (combined with independent baryon density evidence) also indicate that 85% - 90% of the mass in the universe does not participate in electromagnetic effects.This kind of "non baryonic dark matter" is generally supposed to be composed of one or more basic particles different from conventional matter (electrons, protons, neutrinos, etc.).
Since there is no direct detection evidence for the existence of dark matter, there are also some theories that try to explain the existing astronomical phenomena without introducing dark matter.A typical type of theory is the modified Newton Dynamics (MOND), which claims that Newton's or Einstein's gravity theory is incomplete and gravity will behave differently at different scales.However, the evidence for the existence of dark matter comes from many unrelated observational phenomena. It is very challenging to explain all these phenomena at the same time only through gravitational theory without introducing dark matter.In particular, the apparent separation of visible matter from its mass center in the colliding cluster observed in the "bullet cluster" case is observational evidence supporting the existence of dark matter rather than the need to modify the gravitational theory.
Detection means
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Even if dark matter particles interact weakly with conventional matter, dark matter particles may be detected by sophisticated experimental instruments.The detection methods adopted by scientists can be divided into three categories: one is to detect the direct interaction between dark matter particles and the matter in the detector, which is called "direct detection";The second is to find the signal of dark matter's own decay or annihilation in the universe to produce ordinary matter, which is called "indirect detection", and the third is to exploreParticle colliderThe artificially generated dark matter particles are called "accelerator detection".
(1) Direct detection.If dark matter is composed of microscopic particles, there should be a large number of dark matter particles passing through the earth every moment.If one of the particles strikes the atomic nucleus in the detector material, the detector can detect the change of atomic nucleus energy and understand the properties of dark matter by analyzing the nature of the impact.However, for weakly interacting massed particles (WIMPs), because their interactions with ordinary matter are extremely weak, the probability of being caught by the detector is also very weak.In order to shield the interference of other kinds of cosmic rays to the maximum extent, the direct detection experiments of dark matter are often conducted deep underground.There are dozens of underground dark matter detection experiments in the world.There is no conclusive evidence that dark matter particles exist in direct detection tests.The results of these experiments strongly limit the mass and interaction strength of dark matter particles.
(2) Indirect detection.Since there are a large number of dark matter particles in the Milky Way, it should be possible to detect the conventional elementary particles generated by their annihilation or decay. Indirect detection is to look for such annihilation or decay signals in astronomical observations, including high-energy gamma rays, positrons and electrons, protons and antiprotons, neutrons, neutrinos and various cosmic ray nucleons in cosmic rays.Experiments using indirect detection methods can be carried by satellites or space stationsSpace probeCollect cosmic ray particles directly, or observe the shower or Cherenkov light effect when high-energy cosmic ray particles enter the earth's atmosphere on the ground.By analyzing the number and energy spectrum of various particles in cosmic rays, we can extract the information about the decay or annihilation of dark matter in the universe.The difficulty of indirect detection of dark matter is that there are many high-energy ray sources in the universe that are not generated by dark matter, and cosmic rays have to undergo a complex propagation process from generation to arrival near the Earth.At present, the understanding of the generation and propagation process of cosmic rays is not comprehensive, which brings challenges to the search for dark matter signals in cosmic rays.There are manyDark matter spaceThe detection experiment is in progress.
(3) Collider detection.Another way to find dark matter is to generate dark matter particles in the laboratory.stayHigh-energy particleIn collision experiments, undiscovered particles including dark matter particles may be generated.If dark matter particles are generated in the collision, because they are difficult to be directly detected by the detector, the total energy and momentum of the collision product particles detected by the detector will be lost.This is a characteristic of invisible particles.Combined with direct or indirect detection means, it can help determine whether the particles generated in the collider are dark matter particles.
scientific research
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In 2022, an analysis by American astrophysicists,It is confirmed that the universe is composed of about two-thirds of dark energy and one-third of matter, which has accelerated its expansion in the past billions of years, mainly in the form of dark matter.[17]
