Berlin/Garching. Black holes are among the most amazing objects in the universe: their gravity is so strong that even light traveling at about 300,000 kilometers per second cannot escape them. That is why these objects could not be seen and why they were so difficult to detect. Black holes can explain the formation and movement of galaxies, and maybe even more. Recently, there has been a growing body of research suggesting that dark matter, which physicists around the world have been looking for for decades, may be partly or entirely composed of black holes.
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For now, this is still speculation. But newer instruments, such as the James Webb Space Telescope launched in late 2021, may shed light on whether there could be so-called primordial black holes — black holes that formed just after the Big Bang, before the first stars even appeared. This would contradict the modern standard cosmological model: according to it, a black hole is formed when a massive star explodes in a supernova at the end of its existence, and its core collapses due to huge gravity.
The existence of black holes has long been a subject of controversy.
The theoretical existence of black holes followed from the field equations that Albert Einstein (1879–1955) developed for his general theory of relativity. Shortly after the theory was published, the German physicist Karl Schwarzschild (1873-1916) discovered the possibility of the existence of such massive objects. The Schwarzschild radius, the event horizon of a black hole, is named after him, because you can’t see beyond it because light no longer escapes behind it.
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However, the existence of black holes has long been disputed in astronomy. It is now clear that they inhabit the universe. Astrophysicist Reinhard Gensel received the 2020 Nobel Prize in Physics for proving the existence of a 4.3-million-solar-mass supermassive black hole at the center of our Milky Way galaxy — along with Andrea Ghez and Roger Penrose.
The Nobel laureate is skeptical.
Hansel is skeptical about the existence of primordial black holes: “Most people I’ve talked to consider them very unlikely.” .
It was the British physicist Stephen Hawking who put forward the idea of primordial black holes in 1971 and 1974. Even then, it was about explaining the motion of galaxies. Because, according to the proven laws of gravity, many galaxies spin too fast in their outer regions. Therefore, back in 1933, the Swiss physicist Fritz Zwicky suggested dark matter, which is located in the outer regions of galaxies and reacts to the gravitational force of the center of the galaxy.
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Dark matter has not yet been discovered
Hawking was looking for evidence of the type of radiation he invented, emitted by black holes. Because of this so-called Hawking radiation – according to the theory – very small black holes could dissolve and emit certain gamma rays. But since this radiation was not detected, astrophysicists did not develop its approach and instead developed a theory of dark matter.
This could explain the formation of star clusters and galaxies hundreds of millions of years after the Big Bang. But, despite extensive experiments, even the slightest traces of dark matter have not yet been found. According to the current cosmological standard model, it is about five times more common than the ordinary matter known to us, which consists of stars, planets and people.
Where do giant black holes come from?
With improved astrophysical instruments, observations of the sky have been added that have raised questions. In 2015, physicists in the US were able to detect gravitational waves for the first time. They, too, arose from Einstein’s general theory of relativity: gravitational waves — ripples in spacetime — are produced when large masses are greatly accelerated. In the case of the gravitational waves detected in 2015, it was the merger of two black holes, each with about 30 solar masses.
However, according to the standard cosmological model, black holes in this mass range are not expected at all. In the meantime, gravitational waves have also been measured, which can come from black holes with a mass of up to 100 solar masses. Hansel says it’s not clear where black holes in this mass range come from. Experts are also surprised that black hole associations are registered quite often.
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Measurements of X-ray emission typical of the matter surrounding black holes, whose redshift indicates that the sources are very old, also confirm the formation of black holes in a very young universe: if there were no measurement error, the associated black holes would be older than the earliest stars. Another mystery is the discovery of the first quasars, which weigh billions of solar masses, just a few hundred million years after the Big Bang. Quasars – quasi-stellar radio sources – are radio wave signals emanating from a rotating disk of luminous matter surrounding a supermassive black hole.
These and other observations forced Gunther Hasinger to turn again to the theory of primordial black holes. Hasinger’s main job is scientific director of the European Space Agency Esa. During the first quarantine, he had time to review his own and third-party observational data, as well as review specialized articles on primordial black holes. “Theoretical considerations by Bernard Carr, Juan Garcia-Bellido and others convinced me,” says Hasinger.
What role does the Big Bang play?
In their analyses, these physicists link the formation of black holes with the formation of elementary particles immediately after the Big Bang. Simply put, their theory states that black holes of varying sizes formed as the extremely dense and hot universe expanded and cooled. Black holes with a planetary mass of about 100 million solar masses could be formed during the formation of various elementary particles.
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These black holes of different sizes may have contributed to the structure of the young universe in different ways. “When arguments are made against primordial black holes, usually only one type of black hole is considered,” Hasinger emphasizes. In addition, primordial black holes form clusters due to gravity, i.e. they are unevenly distributed in the universe. According to the new theory, black holes should be much more common than previously thought, including in our Milky Way.
This is consistent with various observations over the past few years:
• More and more black holes are being discovered in the Milky Way, the last of which is only 1000 light years away.
• Small gravitational lenses, so-called microlenses, are heavy objects that amplify the light of stars behind them. Some of these objects in the Milky Way are no larger than a planet or a small black hole in mass.
• The long-awaited Planet X, which is believed to influence the orbits of dwarf planets and asteroids at the edge of our solar system, has been calculated as a black hole the size of a tennis ball.
James Webb Space Telescope should provide answers
Using a simple model, Hasinger calculated that primordial black holes could match the estimated mass of dark matter. However, all of this evidence is not proof. But the ESA director is confident that new instruments, such as the James Webb Space Telescope, will provide data to answer this question.
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Because the telescope must be able to detect infrared radiation from a distance of 13 billion light years. Because light has traveled so long, these images are a glimpse of the world 13 billion years ago, 800 million years after the Big Bang.
“If we see galaxies that should have formed earlier than the standard cosmological model predicts, then this will be an important confirmation of the theory of primordial black holes,” Hasinger emphasizes. So the coming years will be exciting for astrophysicists.
Even if Nobel laureate Hansel thinks primordial black holes are unlikely, he is convinced that there are still many secrets to be discovered about black holes: “More than 100 years ago, Einstein presented his theory, but experts have only about 20 years of certainty about the existence of black holes – and this definitely not the end.”
RND/dpa
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