Savvas Koushiappas of Brown University and Abraham Loeb of Harvard University suggested a way to look for black holes that arose in the first minutes after the Big Bang. Their existence was predicted theoretically by Zeldovich and Hawking forty years ago, but they have not been able to observe them yet.
The proposed method relies on the observation of gravitational waves. The authors studied gravitational waves emanating from “ordinary” black holes formed as a result of the explosion of massive stars. Such waves, which came from small distances (by cosmic standards), should be observed more often than overcame enormous distances. Scientists have constructed models and determined the dependence of the frequency of such events on distance. It was found that the distance from which such signals should not come at all is within the reach of several detectors currently being designed.
The merging of black holes has its clear “signature” in the structure of gravitational waves. If such a signal comes from a distance that is greater than the “critical” one, it can be said with certainty that primary black holes are observed. If such black holes do not appear, you will have to revise the theory, which predicts their occurrence.
The calculations of Yakov Zeldovich in 1967 and Stephen Hawking in 1971 showed that in the early Universe, even before the formation of the first stars, black holes would appear. They are called primary black holes.
These objects, as calculated by scientists, should arise because of fluctuations (uneven distribution) of density in space, which must exist according to the laws of quantum mechanics. The mass of such black holes is different and depends on the time of occurrence of the object. For example, in the era of the formation of the first atomic nuclei (it ended only a few tens of minutes after the Big Bang) black holes could have appeared, exceeding by mass the Sun tens of millions of times. Earlier stages of the life of the universe should leave behind a much more modest black holes, including dozens of solar masses. It is such black holes that are found on the active detectors of gravitational waves LIGO and VIRGO.
And this is very serious. We still do not know if there are primary black holes in general. Observers did not find them. And how to do it? Black holes are therefore called black, that they do not emit electromagnetic waves, except Hawking radiation, too weak for observers.
Usually astronomers see black holes due to the radiation of the substance falling on them. So, for example, there is a black hole in the center of our Galaxy. But if there is a shortage around such an object, you can only wait for it to tear the star apart.
In 2015, gravitational waves were recorded for the first time. They discovered the long-awaited opportunity to observe the black holes of the stellar mass, which are not affected by a powerful flow of matter. When two such objects collide and merge into one, gravitational perturbations arise, which the detector registers.
But how do you know that the primary black holes have merged? After all, a black hole can be formed from a star, if its mass is not less than 30-40 solar. After burning out the thermonuclear “fuel”, the outer layers of such a star are dumped in a grandiose supernova explosion, and the nucleus turns into a black hole.
The answer to this question is contained in the new study. Scientists have calculated how often detectors should register gravitational waves from various distances, if it is a question of merging “stellar” black holes.
The measure of distance in the scale of the universe for specialists is a red shift. The authors found that even according to the most optimistic estimates at redshift 40 such events should be observed not more often than once a year, but at even greater distances should not be observed at all.
The thing is that the objects having a redshift of 40, we would see as they were only 65 million years after the Big Bang (which happened almost 14 billion years ago). The authors obtained their result based on the models of the early Universe.
At the same time, such distances are within reach of several gravitational wave detectors currently being designed, the sensitivity of which will be at least ten times higher than that of existing instruments. So, the authors conclude, if these detectors detect gravitational waves coming from such distances, it will be possible to conclude with confidence that they are generated by primary black holes.
And if they do not find it? Well, then we have to revise the theory, which predicts that such objects should be. This would be a serious challenge to modern cosmology.
Is it possible that black holes at such a distance from the Earth were formed as a result of stellar evolution? As explained in the research release, yes. But only if the standard theory is mistaken, assuming that the density fluctuations in the early universe were Gaussian. Such a discovery would also be a serious challenge to cosmology.
Moreover, these two options can be distinguished by observational data. The dependence of the frequency of events on the red shift will be different for primary black holes and in the case of a “non-Gaussian universe”. They coincide in one thing: both would become a very, very serious discovery.
A scientific article describing all these conclusions was published in the journal Physical Review Letters on November 30, 2017.
By the way, primary black holes are considered as one of the possible candidates for the role of mysterious dark matter, about which “Vesti.Nauka” (nauka.vesti.ru) repeatedly wrote.
We will explain. There are two basic evidences of the existence of this mysterious substance. The first is the observed picture of the irregularities in the relic radiation. This fact can not explain the standard cosmological model without invoking the hypothesis of particles unknown to science. But some scientists believe that it is necessary not to search for unknown particles, but to revise the theory.
The second evidence is the effect of gravity of dark matter on visible matter. Astronomers witness this gravity, for example, watching the movement of stars in galaxies and galaxies in clusters. Everything indicates that next to the “glowing” matter is one that does not radiate brightly in any ranges, so that observers notice it.
It is usually believed that this is exactly the same mysterious particles, since they still have to be introduced into the standard theory to explain the behavior of the CMB. But at least a partial contribution to this gravitation can be made by the most common substance, which simply has no reason to glow brightly. For example, it can be cold rarefied gas, brown dwarfs and so on. All this is a non-luminous, but ordinary matter in a crowd of experts called baryon dark matter. It is possible that in its motley structure the primary black holes occupy a place of honor.