Is a dark matter of the black hole
Are the black holes the solution to the riddle?
Günther Hasinger, the Scientific Director of the European Space Agency (ESA), was supposed to speak in the traditional Magnus House in Berlin. But in Corona times it turned into a "webinar" - his lecture for the around 200 registered participants was broadcast from Madrid, and the audience sat in front of their computer screens. The great interest was understandable, as it was once again about the mysterious dark matter. Science has been dealing with this phenomenon for 90 years without being able to say what it is.
There must be more mass in space than can be seen
From observations of stars in galaxies and of galaxies in galaxy clusters, we know that their movements can only be explained if there is much more mass than can be seen. This dark matter works through its gravitation, but otherwise remains hidden. This is by no means a subtle little thing, because according to recent studies, around 80 percent of the universe consists of that ominous "substance," that is, much more than the proportion of visible objects in the universe.
Understandably, the researchers want to know what is behind it. There were no limits to the imagination. In the last few decades numerous ideas about the possible nature of dark matter have developed. Speculative elements in the research process are necessary, but they must be verifiable. Elementary particle physicists in particular "constructed" numerous hypothetical particles with certain properties as candidates for dark matter. But despite the most modern technology, none of them has yet been found. That way you explained something you couldn't understand with something else you couldn't prove.
But we have known objects in the universe for a long time, which - like dark matter - cannot be seen and which nonetheless verifiably exist: the black holes. Because of the enormous density of matter in them, their gravity is so great that no electromagnetic waves can escape them, so they remain invisible. The mechanism for the formation of black holes, which has long been known, results from the evolution of massive stars that end in a supernova explosion at the end of their "life cycle" and collapse to form a black hole. The typical masses of such black holes are between one and 100 solar masses. But in the meantime we also know other "types" of black holes with very different masses, which must also have very different origins behind them. For example, the gigantic black hole in the main galaxy of the Virgo galaxy cluster weighs around three billion solar masses. The black hole in the center of our Milky Way system, on the other hand, comprises "only" around four million solar masses.
Recent discoveries have shown several "intermediate" black holes with masses in the range of 10,000 to 100,000 solar masses, which represented something like a missing link in the chain. In addition, there are the "primordial" black holes postulated by Stephen Hawking and Bernard J. Carr in the early 1970s. These downright tiny things, about the size of an atomic nucleus, but with a mass of one billion tons, are said to have originated immediately after the Big Bang. Whether these hypothetical structures have a sufficiently high life expectancy to still exist today - after 13.8 billion years - is hotly debated in the scientific community. It is extremely difficult to prove it. At best, they can be detected using »microlensing«. They then act like an optical lens, bend space-time in their environment, which can be seen from the stars directly behind them. Above all, the "Optical Lensing Gravitational Experiment" (OGLE) at the Warsaw University Observatory is dedicated to such investigations. Several dozen microlens effects have just been found there; in 20 of them the distances could be determined by the GAIA mission. It is assumed that these objects are primordial black holes in the mass range of one to ten solar masses, because normal stars should be optically observable at this distance. If this interpretation turns out to be true, we now have a gigantic mass spectrum of very different black holes.
X-ray satellites provide observation data
Hasinger is known internationally as a proven specialist in X-ray and gamma-ray astronomy. Since black holes cannot be seen, he and numerous cooperation partners from all over the world investigated the question of what proportion the various black holes have in the background radiation that reaches us from all directions in the cosmos. The main part is formed by the well-known cosmological background radiation (3-K radiation), which comes as the "echo of the big bang" from around 380,000 years after the Big Bang. The wavelengths of this radiation are in the microwave range.
However, the diffuse X-ray background remained mysterious for a long time, as it could not initially be assigned to any individual sources. It took around three decades until this was finally achieved thanks to increasingly better X-ray satellites such as ROSAT, Chandra and others. It turned out that the majority of this radiation is caused by the growth of supermassive black holes in the nuclei of active galaxies. The black holes suck up matter from their environment, which leads to violent gamma emissions in the corresponding accretion disks.
The remainder that is still missing - as shown by correlations between measurements in the X-ray range and in the infrared range - can be easily explained if one ascribes it to the primordial black holes. On the basis of a mass distribution of the primordial black holes assumed by his Spanish colleague Juan Garcia-Bellido, Hasinger estimated their contribution to the X-ray background radiation and found a good agreement.
In the discussion, Günther Hasinger made no secret of the fact that he particularly liked his “speculation” about dark matter as the effect of primordial black holes of different masses because it does not require any additional assumptions. As with "Ockham's razor", named after the English philosopher Wilhelm von Ockham (1288-1346), all other explanations can simply be shaved off if this one is correct. Ockham once said that the best of several theories is always the simplest. However, this is also an unproven proposition. One can now look forward to the verdict of Hasinger's colleagues, but above all to even better measurement data from the eROSITA X-ray telescope, which was launched in 2019, and the planned “Advanced Telescope for HighEnergy Astrophysics” (ATHENA), which should not become active until 2031 at the earliest.
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