Five clues to capture dark matter

 In popular science works(2021 hermes bags), dark matter and dark energy are like twin brothers, and they are almost always mentioned at the same time, but in fact, their historical backgrounds are quite different. People perceive dark energy only in the last 20 years, while the understanding of dark matter has been over 135 years, and the understanding of the characteristics and functions of dark matter has undergone several important refreshes during this period.

　　 Clue 1: Extra gravity source

　　 As early as 1884, when researchers used the motion speed of stars in the Milky Way to estimate the total mass of the galaxy, they found that many stars were moving much faster than expected. It seems that a star moving at such a high speed should leave its orbit long ago, unless there is still a large amount of unobservable matter in the galaxy to provide additional gravitational constraints. According to calculations, the total amount of non-luminous stealth material far exceeds the total amount of all observable celestial bodies. No one can answer what these substances are, and dark matter is named after it.

This anomaly did not arouse too much attention from researchers at first. It should be known that at that time, due to the constraints of observation technology and theoretical models, the incomprehensible phenomena in astronomical observations can be said to be everywhere. In contrast, the matter of several stars that move too fast, despite being repeatedly discovered by researchers, is really not a remarkable phenomenon.

Until the 1970s, with the advancement of technology and the perfection of theories, a large amount of observational data became clearer and clearer, not only did the Milky Way have this mysterious additional source of gravitation, but other galaxies also existed universally, and in every galaxy, The total amount of this stealth material is exactly 6 times the observable mass. This was the first clear clue that dark matter left us. Since then, dark matter has been formally filed for investigation.

The most likely suspects are naturally the late products of stellar evolution, that is, the icy remains of the stellar nuclear fusion reaction. Our Milky Way has existed for more than 10 billion years, almost as old as the life span of the universe. Since this fire has been burning for more than 10 billion years, there is a lot of dense fog and black smoke around it, which seems to be a pretty intuitive picture.

　　 However, researchers soon discovered that there is an important difference between dark matter and stellar remains: dark matter is transparent. Although our surroundings are filled with dark matter that is 6 times the ordinary matter, there is no feeling of cloudy sky. The light from outside the galaxy can reach us unimpeded. What's more interesting is that, on the one hand, dark matter is completely transparent to light, and on the other hand, the gravitational field generated by its mass is like ordinary matter, which can break the light passing by.

This deflection of light produced by mass can produce a "gravitational lens" effect. The light emitted by a light source hidden behind a massive celestial body will be bent by the massive celestial body and then enter our astronomical telescope, so we will see some interesting images in the sky, just like passing through the bottom of a glass bottle The light seen.

Although ordinary celestial bodies and dark matter clusters both produce gravitational lensing effect, the way to distinguish between the two is very simple. Because dark matter itself is transparent to light, the gravitational lensing effect produced by its mass is like an invisible person with water on the body in a movie. It is very easy. distinguish.

It is based on the second clue of gravitational lensing that researchers can very accurately determine the position, shape and even movement of dark matter. In addition, according to the degree of light deflection, the quality of dark matter can also be estimated. The originally mysterious dark matter appeared obediently like this.

　　 With the help of the powerful method of gravitational lensing, the researchers not only quickly mapped the distribution of dark matter in various directions, but also unexpectedly found strong evidence of the existence of dark matter at the scene of a "car accident".

　　 Clue 3: Bullet Nebula

　　 At a distance of 3.7 billion light-years from us, there was a tragic galaxy collision event. To this day, the two galaxies can still be seen in the night sky after passing each other 100 million years later.

It can be seen from the photo that the luminous matter of the two galaxies is mainly concentrated in two white areas, followed by the red area, and the blue area the least. However, through the analysis of the gravitational lensing effect, the researchers were surprised to find that the area where the actual mass is concentrated is not in the place with the highest brightness, but runs in front of the luminescent material.

This shows that when the two galaxies collided and passed through each other, a large amount of non-luminous matter smoothly passed by, and only those luminous matter viscously rubbed against each other, slowing down the pace. This phenomenon is very consistent with the previous understanding of the nature of dark matter. The reason why dark matter is transparent to light is because it does not participate in electromagnetic interaction. This feature also makes dark matter like a ghost, which can smoothly pass through almost any object. We, made of ordinary materials, cannot pass through walls like the Taoist priests in Laoshan, and meteorites that hit the earth cannot pass through quietly, all due to the fetters of electromagnetic effects.

　　 This extremely rare phenomenon of separation of light sources and gravitational sources is not only the most powerful evidence for the existence of dark matter, but also has the important significance of screening and testing theoretical models. The collision site of the Bullet Nebula has also become a natural experimental field for various dark matter theoretical models.

　　 Clue 4: The structure of the universe

　　 In addition to the intuitive experimental field of the Bullet Nebula, there is another more important test scenario for the theoretical model, which is the early universe at the beginning of the Big Bang. In that cramped and hot environment, the entire universe is a pot of plasma soup, and the influence of electromagnetic interaction is far greater than the effect of gravity. In the process of rapid inflation, the particles move away from each other, and the electromagnetic interaction is rapidly weakened, and a pot of thick soup becomes a dish of loose sand.

　　 Then how did these scattered particles converge into galaxies? The answer can only be to rely on gravity. But if there is no dark matter in the universe, and only relying on the gravitational force generated by ordinary matter, a disc of scattered sand after the explosion will not have enough gravitational force. In such a universe, let alone regrouping into galaxies, it is difficult to find even a few particles larger than protons.

　　 Fortunately, the existence of dark matter that is insensitive to electromagnetic effects makes our universe not too fragmented. In that pot of thick soup, even if there is a high-energy plasma moving like a mad rabbit, dark matter can still be a virgin, supporting the overall structure of the universe. In the process of cooling down, ordinary matter will be attached to the dark matter skeleton structure due to the gravitational action of dark matter, which not only avoids an excessively uniform distribution form, but also inadvertently depicts the shape of the dark matter skeleton.

　　 It can also be seen from the facts listed in the previous clue that the real protagonist of the galaxy is actually dark matter. The ordinary matter celestial bodies we are familiar with every day are just small dots embedded in a large group of dark matter clumps. With the help of gravitational lensing effect, researchers can already directly outline the large-scale filamentous structure of the universe, including dark matter.

Researchers have repeatedly simulated the creation and structure formation process of the universe in the computer, and adjusted the initial parameters to investigate the conditions corresponding to the formation of the current universe. Almost all simulations clearly show that dark matter is an essential component of our universe. Without dark matter, the universe would not be what it is today.

Although photons appeared within a few seconds after the Big Bang, for a long period of time, the universe was filled with plasma and photons could not travel freely. It wasn't until 377,000 years later that the temperature cooled to the point where the nucleus and electrons combined to form an electrically neutral atom, the universe ended the "dark period" and the first batch of photons wandering freely appeared. That batch of photons can still be seen by us. This is the cosmic microwave background radiation. When we look around the microwave background radiation in all directions, what we actually see is what the universe looked like when it was 377,000 years old.

In the eyes of professionals, this seemingly chaotic picture contains a lot of extremely precious information, the most important of which is "baryon acoustic oscillations" (BAO, baryon acoustic oscillations). The so-called BAO, to put it simply, is the ripple of the density of baryons such as protons and neutrons.

　　 As mentioned earlier, the baby universe is a plasma soup. Dark matter is like water in a pot, in which baryons are evenly dissolved. Don't forget that this is a pot full of energy boiling and tumbling thick soup, it is destined to be unable to maintain a steady and even calm at all times. Those "noises" reverberating in the soup will cause corresponding changes in the density distribution of baryons. The tiny ups and downs left in the cosmic microwave background radiation today are the ripples caused by the acoustic oscillations in the pot of thick soup.

By carefully analyzing the fluctuation spectrum in the cosmic microwave background radiation, we can not only "listen" the various tones of the baby universe, and then judge many of the physical properties of the universe at that time, so the BAO traces are like fingerprints left by dark matter. An important comparison tool for verifying various dark matter hypotheses.

　　 In addition, the initial distribution state described by BAO is also a "standard ruler", which can measure the spatial changes in the subsequent expansion of the universe. The principle behind this "standard ruler" is easy to understand. Just as we use the reduced map and scale information to locate in an unfamiliar city, BAO will also tell us at what distance the probability of finding a galaxy is greater. It’s just that when we look up at the starry sky, what we see is the appearance of those celestial bodies at different points in time, so that we can just substitute the historical time factor into it, so as to help us in the present moment by comparing the standard ruler to overview the life of the universe All the moments.

After reviewing the five important clues of dark matter, let us return to the most basic question: what exactly is dark matter? This question is obviously still under exploration, and there is still a long way to go before the answer is revealed. However, from the clues listed above, we already have a lot of basis for judgment, and when faced with various models proposed by theoretical researchers, we can make choices with confidence.

　　 There are so many different theoretical models of dark matter, which can be roughly divided into the categories included in the following table.

A type of theory represented by MOND (modified Newtonian dynamics), although the sword goes slant, it also has a place in academia. This theory believes that there is no dark matter at all. The real reason for the excessively fast stars observed in the galaxy is not an additional source of gravity, but our existing gravity theory needs to be revised. This theory is extended to the context of the theory of relativity, which is the TeVeS (Tensor-vector-scalar gravity) theory.

At first, the way this theory corrects gravity is not natural, and the way to introduce additional parameters seems a bit awkward. However, a few years ago, the Dutch physicist Professor Verlinde proposed the entropy-gravity theory to explain the possible sources of the correction term in MOND. MOND theory makes up for this weak link [1].

　　 Of course, the flaws of the MOND theory are still very obvious. For example, when faced with the dislocation and separation of the dark matter and the light source like the bullet nebula, it is difficult for MOND to provide a strong theoretical explanation. Although the asymmetric gravitational field effect can barely be achieved through hard construction, the deliberate taste is too heavy and lacks physical convincing, so it has not become the mainstream.

　　 The theories that are also denied by the Bullet Nebula also include macroscopic dark matter such as MACHO (massive compact halo objects). If the dark matter is the ashes left after burning in the galaxy, then these debris should obviously be behind the movement of the light source, rather than running in front of the light source as shown by the Bullet Nebula. Even though MACHO does not contain burning debris, but primitive matter that has existed since the birth of the universe, as long as this matter is affected by electromagnetic effects during the collision, it cannot reasonably explain the image displayed by the bullet nebula.

　　 In addition, it is difficult for theories like MACHO to obtain observational support from the large-scale structure of the universe or the information carried by the microwave background radiation BAO. In fact, the researchers who support MACHO essentially denied the existence of dark matter that played a special role in the bisque of the early universe. Many observational facts tend to believe that dark matter must intervene in the evolution of the universe early enough to be able to produce the current appearance, otherwise the Big Bang will not be able to form a galaxy so far, only a piece of scattered sand.

　　According to the clues that have been discovered, it is almost impossible to play the role of dark matter in any existing known matter, only some special states of known particles, or simply some new particles that have not yet been discovered.

　　 Looking for the invisible guardian

　　 Some theoretical researchers are looking for various possible combinations like splicing Lego within the framework of the existing standard model, and one by one, they are testing whether their physical properties are consistent with dark matter. This type of work occasionally makes some progress. For example, some researchers have discovered that a particle composed of 6 quarks can better meet the expected requirements. However, this type of work generally still belongs to a niche branch, and more researchers are focusing on how to modify and expand the existing standard model to propose a theoretical model of new particles.

　　 In fact, in the minds of most theoretical researchers, the study of dark matter has long been integrated with the extension of the Standard Model-dark matter is also protecting our buildings from collapsing. Since the discovery of phenomena such as neutrino oscillation and strong CP, the existing standard model has been determined to have deficiencies and must be revised. Some research advances promoted by this, such as the seesaw mechanism and PQ symmetry, have become well-known hot topics in related research fields.

　　 In addition, the supersymmetry theory has been proposed for many years. Although it has not been confirmed by experiments, its elegant form has made many theoretical researchers unwilling to give up, and they are still working hard to turn it into one of the many possible options for extending the standard model.

　　In the new models proposed by these theoretical extensions, some new particles will appear, the characteristics of which are very consistent with the characteristics of dark matter. One of the most compelling options at the moment is the axe. If experiments can confirm the discovery of axons, not only can it capture a dark matter particle, but it can also solve the strong CP problem that has plagued physicists for a long time, thereby raising the theoretical understanding of symmetry and antimatter to a new height.