There are two well-known problems in theories of the origin of galaxies that scientists have been trying to solve for a long time. New research is helping to find an approach to both: Scientists have suggested that supermassive black holes could form directly from dark matter. Physicists comment on this theory for Forskning.
This may explain how black holes formed so early in the history of the universe, according to new research.
It looks like most galaxies have a black hole at the center.
The mass of these monsters is hundreds of thousands, and sometimes billions of times more than the mass of an ordinary star. They consume gas and stars that get too close.
Black holes can occur when a large star dies and collapses into itself. But scientists aren't sure where the supermassive giants came from at the beginning of the universe's history.
Carlos R. Argüelles of the Argentine National University of La Plata, along with three scientific colleagues, carefully considered the following assumption: Could supermassive black holes form directly from dark matter?
Dark matter is a kind of substance in the universe about which we know very little and the volumes of which we have not yet been able to calculate. Probably, the mass of this invisible substance is six times the mass of the visible substance of the universe.
It has been suggested that dark matter is simply ordinary atoms that are difficult to detect. As well as brown dwarfs and gas. But most physicists believe that this is a special, not yet discovered type of particles.
Scientists believe that dark matter is located, like a scattered bubble or halo, around galaxies, and also extends much further than the outermost stars.
“In the galaxy we see stars, but most of the galaxy - namely, dark matter - we do not see. We call this the dark matter halo,”said Torsten Bringmann, a professor at the University of Oslo who studies dark matter, to Forskning.
Dark matter creates additional gravity that keeps the stars in their current locations. Also, a dark matter halo, for example, could explain why the Milky Way is spinning faster than it should.
Researchers who have published a new theoretical study suggest that supermassive black holes formed directly from dark matter at the beginning of the universe.
Scientists have found that clouds or halos of dark matter sometimes have a dense core. And this core can become so dense that at some point the matter collapses to the state of a supermassive black hole.
Scientists speculate that in small, dwarf galaxies, this critical density may not have been reached. In them, the core of dark matter can only resemble a black hole, and the outer halo explains the observed rotation curves of such galaxies.
“This model shows that dark matter halos can have dense cores. It is possible that they will play a decisive role in our efforts to understand how supermassive black holes form,”Carlos R. Argüelles wrote in a press release.
According to scientists, this hypothesis may explain how supermassive black holes were able to form in the universe so early.
Some huge black holes were created long before the universe was a billion years old. In 2017, NASA reported that it had discovered the oldest black hole discovered.Just 690 million years after the Big Bang, this giant was already 800 million times more massive than the Sun.
According to the Big Norwegian Dictionary, there are several assumptions as to what they could have formed from. For example, from ordinary black holes, a kind of "seeds" that attracted more and more matter and grew for a long time.
Another hypothesis is that several black holes from clusters of dead stars simply merged together. Also, supermassive black holes could have formed directly from collapsing gas clouds at the beginning of the universe.
"This new scenario for their emergence could probably provide a natural explanation of how supermassive black holes appeared at the beginning of the universe, without the need to assume that stars or some 'seeds' appeared first in the form of ordinary black holes, which then grew unrealistically quickly.", explains Arguelles.
It was assumed before
Thorsten Bringmann of the University of Oslo examined the new study.
This is not the first time scientists have suggested that dark matter is the source of supermassive black holes, he said.
“It claims to have done the most realistic analysis of all of them to date,” says Bringmann. "Scientists describe the distribution of dark matter particles and their movement at the beginning, that is, when halos or these structures are formed."
As a result, they got halo models with a large black hole in the middle.
Forskning: Is there a difference between a supermassive black hole made of dark matter and ordinary matter?
Thorsten Bringmann: No. This is one of the great findings of general relativity. A black hole is described by only three numbers. Mass, rotation speed and charge.
In fact, the charge is not particularly important either, because if the black hole turns out to be negatively charged, it will immediately attract a positive charge to itself.
“So in fact only two numbers completely describe a black hole. You can throw a whole library into it, and all the information will disappear, and only two numbers will remain."
Bringmann explains that scientists have chosen a fairly common dark matter candidate called sterile neutrinos.
Neutrinos are elusive "ghostly particles" that barely interact with ordinary matter. Scientists have suggested that neutrinos may have heavier "brothers", which are called sterile neutrinos. These hypothetical particles are only influenced by gravity - they are no longer affected by any natural forces.
Just below the border
The researchers suggested that the mass of such a particle should be just below the limit beyond which its existence was ruled out by observation, says Bringmann.
What is called the standard model in cosmology, ΛCDM (Lambda-CDM Model), describes the evolution of the universe with the help of dark energy and "cold" dark matter. Many people agree with this model.
“The fact that dark matter is cold means only that at the beginning of the formation of the structure of the universe it does not have a noteworthy speed. It picks up speed only when there is a gravitational potential, and gravity accelerates it,”says Bringmann.
This sets a limit on how fast particles can move at the very beginning and how much mass they can have.
“If they move too fast, then the modeling does not form a structure, and this is not consistent with what we see on a larger scale,” says Bringmann.
The particles in the study are as fast or "hot" as possible within the model.
According to Bringmann, if we determined the properties of dark matter completely at random, we would hardly come to such a result.
"But it is quite possible."
One of many possible solutions
Bringmann recalls that there are two well-known problems in theories of galaxy formation that scientists are trying to solve.
“We know that, for example, in the center of our galaxy - and, by and large, in the center of all galaxies - there are huge black holes. They are needed to understand how, at an early stage in the development of the universe, galaxies in general were formed.
But one problem is that no one really knows where these black holes came from. When scientists model the formation of galaxies, most of the time they just say, "Okay, let's just assume that giant black holes existed from the beginning."
And when the simulation is started after that, everything is very similar to what we observe in reality thanks to powerful telescopes.
The second problem is called the “cusp problem,” and it concerns the distribution of dark matter in dwarf galaxies, Bringmann explains. Here, when modeling, it turns out not quite what is expected. Too much dark matter is produced in the center of dwarf galaxies.
“Remarkably, this new study addresses both of these problems with the same description. It's incredibly interesting,”says Bringmann.
But there are other models that can help solve these problems. Probably, there will be no consensus on which of them is the best.