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Can we distinguish black holes?



One of the most fundamental predictions of Einstein's Theory of Relativity is the existence of black holes. Despite the recent discovery of gravitational waves from binary black holes by LIGO, the direct detection of electromagnetic waves remains elusive, and astronomers are looking for it with radio telescopes.

Astrophysicists at Goethe University Frankfurt and staff members of the ERC-funded project BlackHoleCam in Bonn and Nijmegen have self-consistent and realistic images of the shadow of an accreting supermassive black hole – such as the black hole candidate Sagittarius A * (Sgr A *) in the heart our galaxy – created and compared relativity and in another theory of gravity. The goal was to test whether Einstein's black holes can be distinguished from those in alternative theories of gravity.

Not all of the light rays (or photons) produced by matter falling into a black hole become trapped in the event horizon, a region of space-time from which nothing escapes. Some of these photons reach distant observers, so when a black hole is observed, a "shadow" is expected directly against the background sky. The size and shape of this shadow depends on the properties of the black hole, but also on the theory of gravity.

Because the greatest deviations from Einstein's theory of relativity are expected very close to the event horizon and generate alternative theories of gravity Various predictions on the properties of the shadow, direct observations of Sgr A * represent a promising approach to test gravity in the strongest regime , Making such images of the black hole is the main goal of the International Event Horizon Telescope Collaboration (EHTC), which combines radio data from telescopes around the world.

Scientists from the BlackHoleCam team in Europe, who are part of the EHTC, ​​have now gone a step further and investigated whether it is possible between a "Kerr" black hole from Einstein's gravity and a "Dilaton" black Exploring the hole that is a possible solution to an alternative theory of gravitation Researchers studied the evolution of matter that falls into the two very different types of black holes and calculated the radiation that was emitted to construct the images. In addition, real physical conditions were used in the telescopes and in the interstellar medium to produce physically realistic images.

"To capture the effects of different black holes, we used realistic simulations of accretion disks with nearly identical initial settings. Simulations used cutting-edge codes and lasted several months on the supercomputer of the institute, LOEWE," Dr. Yosuke Mizuno, lead author of the study.

In addition, expected radio images obviously have limited resolution and image fidelity. Using realistic image resolutions, scientists were surprised to find that even highly non-einstein black holes could disguise themselves as normal black holes.

"Our results show that there are theories of gravitation in which black holes can mask Einsteinian, so new techniques for analyzing EHT data are needed to distinguish them," notes Luciano Rezzolla, professor Goethe University and head of the Frankfurt team. "While we believe the General Theory of Relativity is correct, we must be open-minded as scientists, fortunately future observations and advanced techniques will solve these doubts," concludes Rezzolla.

"Independent information from a circulating pulsar, We are actively looking for these ambiguities," says Michael Kramer, Director at the MPI for Radio Astronomy in Bonn. Heino Falcke (Professor at Radboud University), who proposed using radio telescopes 20 years ago to model the shadow of black holes, is optimistic.

"There is little doubt that the EHT will eventually receive strong evidence for a black hole shadow. These results encourage us to refine our techniques beyond the current state of the art and to make even sharper images in the future." [1

9659002] Research work

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Goethe University Frankfurt

Understanding space and time



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