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

Astronomers are still at an early stage when it comes to questions concerning the nature and properties of dark matter.

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

The theory about the existence of this substance wasput forward more than 40 years ago as an explanation for the discrepancy between the mass of all visible objects in a 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 makes up most 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 spectral lines of chemical elementsto the red (long wavelength) side. This phenomenon may be an expression of weak diffuse scattering, the Doppler effect or gravitational redshift, or a combination of these. The shift of spectral lines in the spectra of celestial bodies was first described by the French physicist Hippolyte Fizeau in 1848 and proposed the Doppler effect caused by the radial velocity of the 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, statisticalanalyzing the distortions in the light we receive allows us to “notice” the gravitational field between the Earth and distant galaxies. There is often more mass—and therefore more matter—in this field than scientists can explain.

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

The image shows the aftermath of the collision of two galaxies. The red in the image shows areas of visible matter, the blue shows 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 afterAt the Big Bang, the universe was hot enough to become highly ionized. This temporarily made it almost opaque to light—the photons rotated like any other particle. However, when things cooled enough, significant amounts of protons and electrons combined to form neutral hydrogen, which became transparent enough to most of the light surrounding it. This process happened quite quickly (in terms of cosmological time) - as a result, all the light contained in the Universe, relatively speaking, was suddenly released out, taking a snapshot at that stage of its evolution. This is a simplified way to describe the cosmic microwave background radiation.

To detect this light, scientists canpoint radio telescopes in any direction, and depending on the observation area, the temperature will change slightly. The difference in temperature is explained by the presence or absence of dark matter in this region.

What is unusual found in the first galaxy?

DF2 is a galaxy that is part of a large groupled by the massive elliptical galaxy NGC 1052. The galaxy caught the attention of scientists because it looked different in photographs taken by the Dragonfly and Sloan Digital Sky Survey (SDSS). In the first, the galaxy appeared as a spot of faint light, while in the second, it was 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 if there was a lot of dark matter. The fact is that if the mass of the galaxy was greater than the mass of visible objects, the clusters would rotate faster.

The scientific community assessed the publication critically— the researchers’ mistake was that they observed only ten clusters and only for two nights. Skeptics believed that scientists may have overlooked key details about the motion of star clusters, resulting in their estimates of the galaxy's mass and visible matter being skewed.

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 - inThey were the first to remeasure DF2's mass using Hubble's Advanced Camera and the 10-meter telescope at Keck Observatory in Hawaii. This time, astronomers observed not only the speed of movement of the clusters, but also the speed of rotation of the stars inside them. As a result, scientists have established that DF2 is a transparent ultra-diffuse galaxy, the size of which is approximately the same as the Milky Way. Only there were about 200 times fewer stars in it.

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 Erik Verlinde fromUniversity of Amsterdam published a scientific paper in 2016 that examined gravity as a byproduct of quantum interactions and suggested that the additional gravity attributed to dark matter is an effect of dark energy - background energy woven into the fabric of space-time of the Universe.

In other words, Verlinde believes that dark matter is not matter, but only an 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

Discovery by astronomers, if successfulconclusively confirmed through future observations, challenges existing theory about galaxy formation. In particular, we are talking about the assumption that the larger NGC 1052 could “steal” dark matter from DF2 and DF4. If this is truly possible, provided that the order that is observed in both observed galaxies is preserved, 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.