Does dark matter consist of very light particles? Estimation from the gravitational lensing effect | sorae portal site to space

There are countless galaxies, which are large clusters of stars in the universe. Many of them rotate, but as long as galaxies exist like galaxies in this universe, the speed of rotation of galaxies must be less than the speed at which a fixed star can be held back by gravity.

However, when we actually examined the rotational speed of the galaxy, we found that it was spinning so fast that the fixed stars could not be stopped by the gravity estimated from the galaxy’s estimated mass based on the fixed number. stars. I’m here. This result cannot be observed by electromagnetic waves such as light, and its existence can only be known indirectly through gravity.dark matter (Which indicates the existence of “dark matter”. Theoretically, the amount is more than four times that of ordinary matter that can be observed by electromagnetic waves.

Although the identity of dark matter remains unknown to this day, unknown elementary particles and composite particles made by combining these elementary particles have been proposed as one of the main candidates for many years. In this case, dark matter interacts weakly with gravity. (※ 1) It is considered a particle that can only be detected by the strength of weak interactions. It has a very short range and is difficult to detect. Dark matter may be right next to us, but it is thought to be elusive due to its ghostly properties.

* 1 … weak interaction: one of the four fundamental forces, and the only one that changes the type of quarks, the fundamental elementary particles that make up matter. Its range is extremely short, less than 1/10th of a meter, and so small that it fits into a nucleus.

If dark matter is made up of unknown particles, what kind of properties does it have?

In the 1970s, when the existence of dark matter was proven beyond reasonable doubt, it wasWIMP(Massive particles interact weakly.) WIMPs are fairly heavy particles, estimated to be at least ten times the mass of a proton.

Heavy particles are more difficult to move than light particles, so they hardly move when given energy such as heat. Because of this, WIMPs tend to clump together to form large clumps. This property is consistent with observations that dark matter exists in clumps in the present-day universe. Multiple theories predict the identity of the unknown particles that will make up WIMPs. For this reason, WIMPs were strong candidates for dark matter.

However, the prediction that the dark matter’s true identity is the WIMP is “gravitational lens effectThere was a difficult problem of not being able to explain well. According to general relativity, gravity is a distortion of space-time. Light has the property of traveling directly through space, but if space-time is distorted, distortion will follow. For example, the image of a distant galaxy can be distorted by the direction of light being bent by the gravitational source in front of it. This phenomenon is called gravitational lensing.

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By calculating from the degree of distortion of the image of the galaxy under the influence of gravitational lensing, we can know the strength of the gravitational source and the distribution of matter. The effect of gravitational lensing is an important clue to understanding the abundance, distribution and properties of dark matter, which is not easily observed.

However, if we assume that dark matter is composed of WIMPs, we find that there is a discrepancy between the expected gravitational lensing effect and the actual observational results. Because dark matter clumps formed by WIMPs produce relatively beautiful distortions of space-time, distorted images of galaxies should also have relatively beautiful shapes. However, it turns out that the already observed picture has a rather complex shape. Such images are difficult to explain unless the distortion of space-time is quite bumpy, which means that the density of dark matter is very uneven. The nature of WIMPs makes such distributions difficult to predict.

For heavy particles like WIMP, the predictions of gravitational lensing don’t match reality, so the dark matter identity is “axion」 (※ 2) Likes”light particlesThere is also an expectation that it will be. Because ultralight particles are much lighter than electrons, there is the problem that they are difficult to put together like a WIMP, but their properties as waves (※ 3) They are strongly expressed, so they tend to interfere with each other. The overlapping of ultralight particles means that the mass density of dark matter tends to be uneven, which is consistent with the observational result that space-time warping is quite bumpy. Ultralight particles with such properties are strong dark matter candidates along with WIMPs, but which one is more likely to be the right one has been an open question. In addition, there is literally an order of magnitude difference in weight between WIMPs and ultralight particles, and there are also significant differences in properties other than dark matter, which affects the construction of the underlying theory. Also in this regard, it’s an interesting question whether dark matter is actually WIMPs or lightweight particles.

* 2… Axion: One of the elementary particles not predicted in the Standard Model, the fundamental theory of elementary particle physics. Although it is said to have mass while being extremely light, less than 1/100 millionth of an electron, it has been searched for for many years as a prime candidate for dark matter, but has not yet been discovered.

* 3 … Objects and forces in this universe always have properties such as particles and properties of waves. However, roughly speaking, the heavier the particle, the more likely it is to appear as a grain, and the lighter the particle, the more likely it is to appear as a wave. The dark matter candidate, the light particles, is much lighter than the WIMPs, so it tends to exhibit wave-like properties.

A research team led by Alfred Amroth at the University of Hong Kong modeled the distortion of the galaxy’s image due to the effect of gravitational lensing and compared it to actual observational results. In recent years, technological innovations have made it possible to take high-resolution images of galaxies, making possible detailed comparisons of image distortions expected from the detailed distribution structure of dark matter and actual photographs. By building models according to WIMP theory and very light particles, it became possible to compare which is closer to the actual picture.

[▲ الشكل 2: صورة Quasar HS 0810 + 2554. مقسم إلى صور متعددة بسبب عدسة الجاذبية (Credit: NASA، ESA، A. Nierenberg (JPL)، T. Treu (UCLA))]

As a result of the research team’s comparison of WIMP models and lightweight particles,The model that ‘dark matter is made of very light particles’ matches the actual observational results betterhas been shown.

In this study, the result of applying the model to quasar 2001 HS 0810+2554 was particularly significant. The image of HS 0810+2554 splits into four due to the effect of gravitational lensing, but when we used a model to predict the position and brightness after splitting, we found that all images were reproduced in the ultralight particle model, while the WIMP model failed most of the time. This raises the possibility that dark matter may be made of very light particles.

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The prediction that dark matter is made of lightweight particles is consistent with other studies. For example, WIMP remains undiscovered until now, almost half a century after the search began. Particles in unexplored regions have such a large mass that even if WIMPs exist in that region, they do not match the properties of dark matter.

In addition, the properties of ultralight particles such as waves are consistent with observational results for “satellite galaxies”. About 50 small satellite galaxies have been discovered around the Milky Way, a much smaller number than predicted by standard galactic formation theories. If the dark matter is composed of ultralight particles, then relatively large satellite galaxies form only because the wave properties of the ultralight particles prevent the formation of galaxies lighter than a certain mass. That could explain the small number of galaxies

Furthermore, ultralight particles are expected to be elementary particles not included in the Standard Model (the basic theory of particle physics), so the discovery itself has extremely important physics implications. In this way, research that explores the identity of dark matter may also help solve mysteries in other fields.

However, the very light particles that might make up dark matter have yet to be discovered, making the search more difficult than searching with WIMP. If it is found, it will be one of the biggest discoveries in physics in recent decades, but so far there is no hint of it. Figuring out the identity of dark matter will take a long time.

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Text: Rare Aya