Dark matter consists of black holes. Is it true?

Black holes are the most mysterious inhabitants of space. After all, no matter how hard we try, we will never know what is happening beyond the event horizon. This frightening darkness from which nothing can escape, not even photons of light. For many decades, these space monsters were considered hypothetical objects, and proof of their existence seemed unthinkable. The same can be said for dark matter, a hypothetical form of matter that cannot be observed and is estimated to make up about 85% of the Universe. Its existence, like black holes once upon a time, can be judged by the powerful gravitational effect that this substance has on cosmic objects, holding them together. Is it any wonder that the mysterious nature of both dark matter and black holes leads scientists to believe that these inhabitants of the Universe are connected and that dark matter can be detected by observing space monsters?

Dark matter consists of black holes. Is this true? One of the main mysteries of modern physics is the nature of dark matter. But how is it related to black holes? Image: d2r55xnwy6nx47.cloudfront.net. Photo.

One of the main mysteries of modern physics is the nature of dark matter. But how is it related to black holes? Image: d2r55xnwy6nx47.cloudfront.net

Contents

  • 1 Structure of black holes
    • 1.1 Jets, rotation and gravity
  • 2 The nature of dark matter
  • 3 How to detect dark matter?
  • 4 Holes in the theory

The structure of black holes

Being one of the most mysterious and exotic objects in the Universe, black holes are formed as a result of the collapse of massive starsand have such powerful gravity that not even light can escape their confines. The main elements of black hole structure include the event horizon, singularity, and accretion disk. But first things first.

Event Horizon– this is the boundary around a black hole, once beyond which everything – from matter to photons of light – disappears forever. In fact, the event horizon is a kind of «point of no return» – everything that crosses it becomes part of the black hole and can no longer interact with the outside world. The radius of the event horizon is called the Schwarzschild radius and depends on the mass of the black hole.

The structure of black holes. The event horizon of a black hole is the point of no return. Image: cdn.mos.cms.futurecdn.net. Photo.

The event horizon of a black hole is the point of no return. Image: cdn.mos.cms.futurecdn.net

As for the center of the black hole, there is a singularity in this place- the point at which the density of matter and the curvature of space-time become infinite. Here the laws of physics, as we understand them, cease to apply. Matter absorbed by a black hole is compressed to an infinitesimal size, creating a huge gravitational force.

More on the topic: What do black holes feed on and does this affect their appearance

Interestingly, what makes black holes observable is theaccretion disk, which consists of gas and dust and rotates around it. The material in the accretion disk is heated to extremely high temperatures by friction and gravitational forces, emitting radiation in various bands of the electromagnetic spectrum, including X-rays.

Jets, rotation and gravity

Some black holes , especially supermassive ones (those located in the centers of galaxies) can eject narrow jets of matter, called jets. These jets are ejected along the black hole's spin axes and can extend for thousands of light years. Jets form from material that was not consumed by the black hole and instead accelerates along its magnetic fields.

Jets, rotation and gravity. A jet - a plasma jet escaping from a black hole at its center - periodically changes its direction. Image: d182hggomw8pjd.cloudfront.net. Photo.

The jet, a plasma jet escaping from the black hole at its center, periodically changes its direction. Image: d182hggomw8pjd.cloudfront.net

There are also rotating or Kerr black holes, whose event horizons and structures differ significantly from their non-rotating counterparts. Such black holes create a “dragging” effect. space-time around you.

Black holes have a powerful impact on the surrounding space-time. They bend it so much that it causes effects predicted by Einstein's general theory of relativity, such as gravitational lensing (the bending of light around a black hole) and time dilation near the event horizon.

More Read more interesting articles about black holes and other inhabitants of the Universe on our channel in Yandex.Zen – articles that are not on the site are regularly published there!

And although black holes have moved from hypothetical objects to real objects in 2019, they are still the subject of intense study, as they provide unique opportunities to test our knowledge of gravity, quantum mechanics and the structure of the Universe.

The nature of the dark matter

Physicists have been talking about dark matter for a long time and a lot, because according to observations and available data, it must exist. Moreover, in order to explain gravitational anomalies in space, dark matter must be approximately 5 times more common in the Universe than ordinary visible matter – for example, in our galaxy there is 15 times more dark matter than ordinary matter.

The nature of dark matter. Researchers judge the existence of dark matter by its gravitational effect on space objects. Image: static.scientificamerican.com. Photo.

Researchers judge the existence of dark matter by its gravitational effect on space objects. Image: static.scientificamerican.com

The problem, however, is that dark matter, by its very nature, is also virtually undetectable because it interacts very weakly with ordinary matter, except by gravity. And although the nature of dark matter remains a mystery, most researchers believe that this hypothetical substance consists of unknown elementary particles.

Unfortunately, despite decades of effort, no experiments have detected new particles that could be responsible for dark matter, says Przemek Mroz of the Warsaw University Astronomical Observatory in Poland.

Mroz is the lead author of two papers recently published in Nature and the Astrophysical Journal Supplement that test whether dark matter can be explained by another mysterious class of objects in the universe: black holes.

The nature of dark matter. Physicists want to understand how dark matter and black holes are related? Image: www.universetoday.com. Photo.

Physicists want to understand how dark matter and black holes are connected? Image: www.universetoday.com

Recall that since the discovery of black hole mergers in 2015, about 100 cases have been identified. These black holes are typically 20 to 100 times heavier than our Sun, and black holes previously discovered in the Milky Way are typically only 5 to 10 solar masses. «Explaining why these two populations of black holes are so different from each other is one of the biggest mysteries of modern astronomy,«, the article says.

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How to detect dark matter?

In an attempt to answer these and other questions, researchers have proposed that the larger black holes discovered by the LIGO and Virgo experiments are primordial black holes formed in the early Universe. As gravitational wave detectors are used to detect new black holes, physicists speculate that such primordial black holes could account for a significant portion, if not all, of dark matter. Moreover, although black holes do not emit light, this theory can be tested.

A corollary of Einstein's theory of general relativity is that massive objects can bend light around them. This effect is calledgravitational lensing. When a massive object, such as a black hole, comes between Earth and other objects, such as galaxies, those galaxies grow in size and their brightness increases.

How to detect dark matter? The greater the mass of the object causing gravitational lensing, the longer the brightness of the bodies behind it increases. Image: spacegid.com. Photo.

The greater the mass of the object causing gravitational lensing, the longer the brightness of the bodies behind it increases. Image: spacegid.com

Note that lensing by objects the size of the Sun lasts several weeks. But the gravitational lensing of black holes with a mass of more than 100 solar masses would last for several years.

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The idea that gravitational lensing could help study dark matter was first put forward in the 1980s by Polish astrophysicist Bogdan Paczynski. Experiments have shown that black holes smaller than the Sun may make up less than 10% of dark matter. But these early experiments were not sensitive to lensing on longer time scales.

A hole in the theory

Now, astronomers from the Optical Gravitational Lensing Experiment (OGLE) have presented new results from 20 years of observation of 80 million stars in the nearby Large Magellanic Cloud. If dark matter in the Milky Way consisted only of black holes, researchers would expect to see 258 microlensing events. Instead, their results showed only 13.

A hole in the theory. The diagram shows galaxies with yellow dots. Expected and observed microlensing events by massive objects toward the Large Magellanic Cloud, seen through the Milky Way halo. Image: phys.org. Photo.

The diagram shows galaxies with yellow dots. Expected and observed phenomena of microlensing by massive objects towards the Large Magellanic Cloud, visible through the halo of the Milky Way. Image: phys.org

This indicates that massive black holes may make up no more than a few percent of dark matter. To be precise, black holes with a mass of 10 solar masses can contain no more than 1.2% dark matter. Black holes with a mass of 100 solar masses account for 3.0% of dark matter, and black holes with a mass of 1000 solar masses account for 11% of dark matter, explains Mroz.

Obtained in the study the data show that primordial black holes cannot account for a significant fraction of dark matter, and simultaneously explain the observed black hole merger rates measured by LIGO and Virgo. Either way, the mystery surrounding what makes up most of dark matter remains unsolved.


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