Dark matter theory? What is the dark matter theory?

Decades ago, dark matter was only a product of theory when it was first proposed, but now we know that dark matter has become an important part of the universe. The total mass of dark matter is six times that of ordinary matter, accounting for 1/4 of the energy density of the universe. More importantly, dark matter dominates the formation of the structure of the universe. The nature of dark matter is still a mystery, but if it is assumed to be a weakly interacting subatomic particle, then the large-scale structure of the universe formed from it is consistent with the observation. However, the recent analysis of the structure of galaxies and sub-galaxies shows that there is a difference between this hypothesis and the observed results, which also provides a useful place for many possible dark matter theories. These potential dark matter models can be distinguished by studying the density, distribution, evolution and environment of small-scale structures, which brings new dawn to the study of dark matter properties.

About 65 years ago, evidence of the existence of dark matter was first discovered. At that time, Fritz Zwicky discovered that the galaxies in the large cluster of galaxies had extremely high moving speed. Unless the mass of the cluster is more than 100 times the value calculated according to the number of stars in it, the cluster cannot bind these galaxies at all. After decades of observation and analysis, this has been confirmed. Although we still know nothing about the nature of dark matter, by the 1980s, dark matter, which accounts for about 20% of the energy density of the universe, was widely accepted.

[Caption]: Ordinary luminescent substances account for 0.4% of the total energy of the universe, other ordinary substances account for 3.7%, dark matter accounts for nearly 23%, and the other 73% is the dominant dark energy.

After introducing the theory of cosmic inflation, many cosmologists believe that our universe is flat, and the total energy density of the universe must be equal to the critical value (this critical value is used to distinguish whether the universe is closed or open). At the same time, cosmologists also tend to a simple universe in which the energy density appears in the form of matter, including 4% ordinary matter and 96% dark matter. But in fact, observation has never been consistent with this. Although there is a big error in the estimation of the total material density, this error is not enough to make the total material reach the critical value, and the inconsistency between this observation and the theoretical model becomes more and more acute with the passage of time.

When people realize that there is not enough matter to explain the structure and characteristics of the universe, dark energy appears. The only similarity between dark energy and dark matter is that they neither emit light nor absorb light. Microscopically, their compositions are completely different. More importantly, like ordinary matter, dark matter has gravitational self-attraction, and it gathers with ordinary matter to form galaxies. Dark energy is repelled by gravity and is almost evenly distributed in the universe. Therefore, dark energy will be missed when counting galaxy energy. Therefore, dark energy can explain the 70-80% difference between the observed material density and the critical density predicted by inflation theory. After that, two independent teams of astronomers observed the supernova and found that the universe is accelerating its expansion. As a result, the universe model dominated by dark energy becomes a harmonious universe model. Recently, the observation of wilkinson microwave anisotropy probe (WMAP) independently confirmed the existence of dark energy and made it a part of the standard model.

Dark energy also changes our understanding of the role of dark matter in the universe. According to Einstein's general theory of relativity, in a universe containing only matter, the density of matter determines the geometry of the universe, as well as its past and future. If you add dark energy, the situation is completely different. First of all, the total energy density (the sum of matter energy density and dark energy density) determines the geometric characteristics of the universe. Secondly, the universe has transitioned from a period dominated by matter to a period dominated by dark energy. About billions of years after the Big Bang, dark matter dominated the total energy density, but this has become a thing of the past. Now the future of our universe will be determined by the characteristics of dark energy, which is accelerating the expansion of the universe. Unless dark energy decays or changes its state over time, this accelerated expansion will continue.

However, we ignore the extremely important point that it is dark matter that contributes to the formation of the structure of the universe. If there were no dark matter, galaxies, stars and planets would not have formed, let alone human beings today. Although the universe is homogeneous and isotropic on a large scale, there are stars, galaxies, clusters of galaxies, giant holes and the Great Wall of galaxies on a smaller scale. On a large scale, the only force that can make matter move is gravity. However, the evenly distributed matter will not produce gravity, so all the cosmic structures today must come from the tiny fluctuations in the very early distribution of matter in the universe, and these fluctuations will leave traces in the cosmic microwave background radiation (CMB). It is impossible for ordinary matter to form a substantial structure in the cosmic microwave background radiation through its own fluctuation, because ordinary matter was not decoupled from radiation at that time.

On the other hand, the tiny fluctuations of dark matter that are not coupled with radiation are magnified many times before the decoupling of ordinary matter. After the decoupling of ordinary matter, the dark matter that had gathered together began to attract ordinary matter, and then the structure we have observed now was formed. So this requires an initial fluctuation, but its amplitude is very, very small. The substance needed here is cold dark matter, hence its name, because it is a non-relativistic particle without thermal motion.

Before we begin to explain the validity of this model, we must explain one last important thing. For the aforementioned small disturbance (fluctuation), in order to predict its gravitational effect at different wavelengths, the small disturbance spectrum must have a special shape. Therefore, the initial density fluctuation should be independent of the scale. That is to say, if we decompose the energy distribution into the sum of a series of sine waves with different wavelengths, then the amplitudes of all sine waves should be the same. The success of inflation theory lies in that it provides a good dynamic starting mechanism and forms such a small disturbance spectrum independent of scale (its spectral index n= 1). The observation result of WMAP confirmed this prediction, and its observation result was n = 0.99±0.04.

But if we don't know the nature of dark matter, we can't say that we already know the universe. Now we know two kinds of dark matter-neutrinos and black holes. However, their contribution to the total amount of dark matter is very small, and most of the dark matter is still unclear. Here we will discuss the possible candidates of dark matter, the structural formation caused by it, and how we can combine particle detectors and astronomical observations to reveal the properties of dark matter.

The most promising dark matter candidate

For a long time, the most promising dark matter is only the basic particle in the hypothesis, which has the characteristics of long life, low temperature and no collision. Longevity means that it must be as old as the present universe or even longer. Low temperature means that they are non-relativistic particles when decoupled, and only in this way can they cluster quickly under the action of gravity. Because the clustering process takes place in a smaller range than the Hubble horizon (the product of the age of the universe and the speed of light), and this horizon is very small compared with the present universe, the dark matter cluster or dark matter halo initially formed is much smaller than the scale and mass of the Milky Way. With the expansion of the universe and the increase of Hubble's horizon, these initial small dark matter halos will merge to form larger-scale structures, which will later merge to form larger-scale structures. The result is the formation of structural systems with different volumes and masses, which is consistent with the qualitative observation. On the contrary, relativistic particles, such as neutrinos, can't form the structure we observed because they move too fast during the gravitational clustering period. Therefore, the contribution of neutrinos to the mass density of dark matter can be ignored. The measurement results of neutrino mass in solar neutrino experiment also support this point. Collisionless means that the interaction cross section of dark matter particles (with dark matter and ordinary matter) can be ignored in dark matter halo. These particles are bound to each other only by gravity, and in the dark matter halo, they run unimpeded with a broad orbital eccentric rhythm spectrum.

Low temperature collision-free dark matter (CCDM) is promising for several reasons. Firstly, the numerical simulation results of CCDM structure formation are consistent with the observation results. Secondly, weakly interacting massive particles (WIMP), as a special subclass, can well explain its abundance in the universe. If the interaction between particles is weak, they are in thermal equilibrium in the first trillionth of a second of the universe. Then he began to lose his balance because of annihilation. According to the estimation of their interaction cross section, the energy density of these substances accounts for about 20-30% of the total energy density of the universe. This is consistent with observation. The third reason for CCDM's optimism is that some attractive candidate particles are predicted in some theoretical models.

One of the candidate particles is the neutral companion, a particle proposed in the supersymmetric model. Supersymmetry theory is the basis of supergravity and superstring theory, which requires that every known fermion should have an accompanying boson (not yet observed), and every boson should also have an accompanying fermion. If supersymmetry still exists today, the accompanying particles will have the same mass. However, due to the spontaneous breaking of supersymmetry in the early universe, the mass of accompanying particles has also changed today. Moreover, most supersymmetric partners are unstable and decay soon after supersymmetry is broken. However, one of the lightest partners (the mass is in the order of 100GeV) avoids decay due to its symmetry. In the simplest model, these particles are electrically neutral and have weak interaction-they are ideal candidates for WIMP. If dark matter is composed of neutrons, underground detectors can detect these particles when the earth passes through dark matter near the sun. In addition, it must be pointed out that this detection does not mean that dark matter is mainly composed of WIMP. Current experiments can't determine whether WIMP accounts for most or only a small part of dark matter.

Another candidate is axion, a very light neutral particle (its mass is in the order of 1μeV), which plays an important role in the grand unified theory. Axons interact with each other through tiny forces, so it can't be in thermal equilibrium, so it can't explain its abundance in the universe well. In the universe, the axion is in the state of low-temperature boson condensation, and now the axion detector has been built and the detection work is in progress.

Problems existing in CCDM

Because CCDM is integral, the standard model is special in mathematics. Although some of its parameters have not been accurately measured so far, we can still test this theory on different scales. The largest scale that can be observed at present is CMB (Thousand Mpc). The observation of CMB shows the original distribution of energy and matter, and it also shows that this distribution is almost uniform and has no structure. The next scale is the distribution of galaxies, from several Mpc to near 1000 Mpc. At these scales, the theory and observation are in good agreement, which also gives astronomers the confidence to extend this model to all scales.

But on a smaller scale, from 1Mpc to the galactic scale (Kpc), there is inconsistency. This inconsistency appeared a few years ago, and its appearance directly led to the crucial question of whether the current theory is correct. To a large extent, theorists believe that the inconsistency is more likely to be caused by our improper assumptions about the characteristics of dark matter, and it is unlikely to be an inherent problem of the standard model itself. First of all, for large structures, gravity is dominant, so all calculations are based on Newton and Einstein's laws of gravity. On a smaller scale, the hydrodynamic action of high temperature and high density substances must be included. Secondly, the fluctuation on a large scale is small, so we have accurate methods to quantify and calculate. But on the scale of galaxies, the interaction between ordinary matter and radiation is extremely complicated. The following are the main problems on a small scale. Substructures may not be as common as predicted by CCDM numerical simulation. The number of dark matter halos is basically inversely proportional to its mass, so many gravitational lens effects caused by dwarf galaxies and small dark matter halos should be observed, but the current observation results have not confirmed this. Moreover, dark matter around the Milky Way or other galaxies, when merged into galaxies, will make the originally thin galactic disk thicker than what is observed now.

The density distribution of dark matter halo should increase sharply in the nuclear region, that is to say, with the decrease of the distance from the center, its density should increase sharply, but this is obviously inconsistent with the central region of many self-gravity systems we have observed. As observed in the study of gravitational lenses, the core density of galaxy clusters is lower than that calculated by the halo model of massive dark matter. The dark matter in the core region of ordinary spiral galaxies is less than expected, and the same situation also occurs in some galaxies with low surface brightness. Dwarf galaxies, such as companion galaxies of the Milky Way, Yufu galaxies and Tianlong galaxies, have uniform density centers, which is in sharp contrast with the theory. The silver disk scale and angular momentum of hydrodynamic simulation are smaller than those observed. In many galaxies with high surface brightness, there are rotating rod structures. If this structure is stable, the density of its core is required to be less than the expected value.

It is conceivable that solving these more and more problems will depend on some complex but common astrophysical processes. Some conventional explanations have been put forward to explain the aforementioned structural loss phenomenon. On the whole, however, the current observational evidence shows that there is a contradiction between the predicted high density and the observed low density from the giant galaxy cluster (with a mass greater than 10 15 solar mass) to the smallest dwarf galaxy (with a mass less than 109 solar mass).