What dark matter hides and why scientists still could not prove its existence

In matters relating to the nature and properties of dark matter, astronomers are still at an early stage.

study, first of all, because the reality of its existence has not yet been proven.

The theory of the existence of this substance wasIt was launched over 40 years ago as an explanation of the discrepancy between the mass of all visible objects in the galaxy and the mass of the galaxy itself. Astronomer Vera Rubin, who first discovered the discrepancy, determined that this invisible substance is extremely common, and it consists of a large part of the universe. Today we know this substance as dark matter.

Vera Rubin. Photo: Carnegie Institution for Science / carnegiescience.edu

Although astronomers have at least threeevidence that dark matter exists, none of the attempts to detect direct evidence of its existence and to determine its properties was not successful.

However, the work of scientists from Yale University inled by Peter van Dokkum, published in the journal Nature in March 2018, scientists more than ever brought scientists closer to finding yet another proof of the existence of this substance.

What do astronomers know about dark matter?

Dark matter is a substance that is notinteracts with other matters by means of electromagnetic (EM) or strong nuclear forces. The absence of electromagnetic interactions means that it cannot emit, absorb, reflect, refract or diffuse light. This, of course, makes it a rather complicated subject for observation. However, about 85% of all matter in the universe is dark matter.

So far, scientists have no practical evidence that dark matter really exists, but there is theoretical. Here are the three main ones.

Galactic rotation curves

When one object rotates around another,an object in orbit must be constantly accelerated to the central (or, more precisely, they both accelerate to their combined center of mass). Without this acceleration, the orbital body will simply fly away.

The faster the orbital body moves, themore acceleration is required to keep it in orbit. Since in this case the acceleration is due to gravity, this means that the central mass must be larger.

This knowledge allows scientists to "weigh" differentparts of the galaxy, as well as measure rotational speeds, comparing redshifts on the approaching and receding sides of the galaxy. When weighting, astronomers see a discrepancy between the mass of all objects in the galaxy and its total mass.

Redshift - shift of the spectral lines of chemical elements inred (long wavelength) side. This phenomenon may be an expression of weak diffuse scattering, the Doppler effect or gravitational redshift, or a combination of them. For the first time, the shift of the spectral lines in the spectra of celestial bodies was described by the French physicist Hippolyte Fizeau in 1848 and suggested the Doppler effect caused by the radial velocity of a star to explain the shift.

Gravity lensing

According to the general theory of relativity, anythe time it passes through the gravitational field is slightly distorted. It acts as a gravitational lens and can produce, for example, “Einstein rings”, as in the image below.

Einstein’s General Theory of Relativity statesthat the gravity of such large space objects as galaxies bends the space around it and deflects the rays of light. When this occurs, a distorted image of another galaxy - the source of light.

The “Einstein Ring” in the image above isdistorted image of one galaxy (it is highlighted in blue), located behind the other (red) galaxy in the center. Light from blue propagates in all directions, but is bent by the gravity of a red galaxy. This means that light, which, for example, was originally directed directly to the Earth, will never reach our planet - unlike light, which had a different direction, but was distorted by a lens and proceeds as if from all directions at once. This process explains the appearance of the ring.

In weak gravitational lenses statisticalanalysis of the distortions in the light that we receive allows us to “notice” the gravitational field between the Earth and distant galaxies. Often in this field there is more mass - respectively, and more matter - than scientists can explain.

An example of gravitational lensing, which from the point of view of the existing theory proves the presence of dark matter, is a photograph of the Bullet cluster of galaxies located in the constellation of Carina.

The picture shows the consequences of the collision of two galaxies. Red in the image shows areas of visible matter, blue - dark matter, the presence of which is determined by gravitational lensing.

This distinction is due to the fact thatMost of the luminous matter in a cluster of galaxies is in an intracluster medium — in a hot, dense plasma. When parts of the plasma collide with each other, a significant amount of the substance slows down and remains in the center. But dark matter weakly interacts with matter, so its components from the two clusters can freely pass through each other - this leads to the separation shown in the photo.

Relic radiation

During the first few hundred thousand years afterThe Big Bang The universe was hot enough to ionize strongly. This temporarily rendered it almost opaque to light - photons rotated like any other particle. However, when everything was cool enough, significant amounts of protons and electrons merged into neutral hydrogen, which became transparent enough for most of the surrounding light. This process happened quite quickly (from the point of view of cosmological time) - as a result, all the light contained in the Universe, relatively speaking, was suddenly released outside, taking a picture at that stage of its evolution. So you can simply describe the relic radiation.

To fix this light, scientists cansend radio telescopes in any direction - and depending on the field of observation, the temperature will vary slightly. The difference in temperature is due to the presence or absence of dark matter in this area.

What is unusual found in the first galaxy?

DF2 - a galaxy that belongs to a large groupled by the massive elliptical galaxy NGC 1052. The galaxy attracted the attention of scientists by the fact that it looked different in the photographs taken by the Dragonfly and Sloan Digital Sky Survey (SDSS). On the first, the galaxy was a spot of weak light, while on the second, a group of point objects.

Based on these observations, scientists led byPeter van Dokkum identified ten globular clusters (large groups of old stars) inside the galaxy and found that they move three times slower than in the presence of a large amount of dark matter. The fact is that if the mass of the galaxy were greater than the mass of visible objects, the clusters would rotate faster.

The scientific community rated the publication critically- as an error the researchers called what they observed only for ten clusters and only for two nights. Skeptics felt that scientists might have overlooked key details of the motion of star clusters, and this as a result distorted their estimate of the mass of the galaxy and its apparent matter.

And in the second?

The only way to prove the correctness of theirObservation was the search for a second galaxy, which would contain the minimum amount of dark matter - and in March 2019 such a galaxy was discovered.

The researchers published two scientific articles - infirst, they re-measured the mass of the DF2 using the improved Hubble Camera and the Keck Observatory's ten-meter telescope in Hawaii. This time, astronomers observed not only the speed of movement of clusters, but also the speed of rotation of the stars inside them. As a result, scientists have found that DF2 is a transparent ultradiffuse galaxy, the size of which roughly corresponds to the Milky Way. Only stars in it turned out to be about 200 times smaller.

The second article was devoted to the discovery of suchDF2 galaxies - DF4, which is located in the same cluster next to the galaxy NGC 1052. The researchers believe that, firstly, galaxies with a minimal amount of dark matter are not uncommon, and, secondly, that a large galaxy could “steal” the dark matter from their smaller neighbors.

How can the absence of dark matter be evidence of its existence?

To understand the statement that the absence of a darkmatter in two galaxies confirms its presence in the Universe in accordance with the General Theory of Relativity, it is worth considering criticizing the idea of ​​the presence of dark matter.

Some scientists do not agree that in the universethere is dark matter, and theoretical evidence of its presence is attributed to the so-called modified Newtonian dynamics (MOND). This alternative theory says that gravity on a cosmic scale does not work the way Isaac Newton or Albert Einstein predicted. This means that the General Theory of Relativity, on which theories about the existence of dark matter are built, does not work in the case of galaxies.

For example, theoretical physicist Eric Verlinde fromThe University of Amsterdam in 2016 published a scientific article in which it considered gravity as a by-product of quantum interactions and suggested that the additional gravity attributed to dark matter is the effect of dark energy - the background energy interwoven into the Universe space-time fabric.

In other words, Verlinde believes that dark matter is not matter, but only the interaction between ordinary matter and dark energy.

Discovery of scientists from Yale Universitydemonstrates that dark matter can be separated from ordinary matter, provided that both of the detected galaxies behave in accordance with the standard theory of gravity. That is, the processes occurring in them can be explained using the equations discovered by Newton and Kepler.

What are the questions

The discovery of astronomers if it succeedsfinally confirm in the course of future observations, challenges the existing theory about the formation of galaxies. In particular, we are talking about the assumption that the larger NGC 1052 could “steal” the dark matter from DF2 and DF4. If this is really possible under the condition of maintaining the orderliness that is observed in both of the observed galaxies, then astronomers will have to completely reconsider the mechanism of their formation and existence.

"We hope to find out how commonthese galaxies and whether they exist in other areas of the universe. We want to find more evidence that will help us understand how their properties are consistent or not consistent with our current theories. We hope that this will allow us to take another step in understanding one of the greatest mysteries in our universe - the nature of dark matter, ”said Dokkum in a conversation with Astronomy.