Astrophysicists are testing Einstein’s General Theory of Relativity near black holes’ surfaces.
Astrophysicists at ICTS-TIFR are testing Einstein’s Equations very close to the event horizon of black holes, the surface of black holes from within which even light cannot escape. To do so, they are using gravitational waves emitted by two black holes while colliding with each other and merging to form a single, more massive black hole.
The General Theory of Relativity describes gravitation as a warping of spacetime, the arena on which the universe displays its cosmic dance. Einstein’s Equations, the cornerstone of the theory, have been tested repeatedly through various experiments. They have come out with flying colours, but scientists are still sceptical.
Black Holes Are the Testbed of the General Theory of Relativity
The theory gives similar results to Isaac Newton’s gravitational force formula when physicists deploy it on most objects around us: the motion of objects on the Earth, the motion of the Moon around the Earth, or the motion of the planets around the Sun. However, Newton’s formula gives inaccurate results when the effect of gravitation becomes strong, like near the Sun or stars or their denser counterparts: neutron stars or black holes. Astrophysicists rigorously tested the accuracy of the General Relativity in such scenarios, such as Mercury’s orbit around the Sun, in the early twentieth century.
Physicists have long been testing General Relativity using merging black holes. According to the theory, when black holes collide, they emit gravitational waves. These undulations of the spacetime around the black holes reach detectors on the Earth, and help astrophysicists study the black holes’ properties. As the dancing black holes merge to form a bigger black hole, physicists have difficulty deciphering exactly when this happens.
The difficulty stems from Einstein’s Equations of General Relativity being a set of ten related equations. Solving them is tricky because each equation informs the other. Usually, researchers use approximations to solve these equations. However, these approximations become invalid for merging black holes where gravity becomes too strong.
Astrophysicists at ICTS have recently used large supercomputers to solve Einstein’s Equations. Prayush Kumar, one of the ICTS researchers, explained that the solutions tell them how black holes behave, zooming around at close to light speed under the effects of strong gravitation.
The team has found that when the black holes approach close to each other, they deform each other from their original shapes. The researchers study the deformations of surfaces inside the event horizons, a region of spacetime that no one can observe directly. The researchers call these surfaces ‘dynamical horizons’.
This simple animation, by Vaishak Prasad, another ICTS researcher, show the deformations of the dynamical horizons.
Merging, Spinning Black Holes Testify General Relativity
Vaishak said that their calculations consider all the complications required by General Relativity. He has proposed a new approach to directly calculate the evolution of the black holes’ dynamical horizons. The calculations take three to four weeks for each merger, even using the most powerful supercomputers at ICTS-TIFR. Vaishak added this is the first time someone has done such full-scale simulations in India.
Prayush explained how, in his previous studies with his collaborators, he found that these deformations are related to the gravitational waves detected on Earth much before and much after the black holes merge. He said that after the black holes have merged, a spinning black hole appears that “rings like a bell.”
Vaishak’s study deciphered how the black holes’ dynamical horizons deform before and after the merger. “We found that even the non-spinning black holes’ dynamical horizons deform,” said Vaishak. The horizons’ evolution affects the gravitational waves detected on the Earth, contrary to what earlier studies have found, he added.
The results are interesting because this could be a direct way of studying what happens inside the deformed black holes’ event horizons. In its full complexity, the General Theory of Relativity can describe nature even here.
Presently, the ICTS research group is investigating various questions about these extreme events to test the General Theory of Relativity. Prayush said the picture is not entirely rosy. For example, they are not sure why the final black holes ring down like a bell. Alternative explanations that do not fit into the theoretical framework of the General Theory of Relativity could exist. And, until scientists actively look for such inconsistencies, they will never find them.
The search for the perfect theory of gravitation is still on.
To know more about the exciting prospects, read the following papers:
- Vaishak Prasad: Tidal deformation of dynamical horizons in binary black hole mergers and its imprint on gravitational radiation
- Yitian Chen, Prayush Kumar, Neev Khera, Nils Deppe, Arnab Dhani, Michael Boyle, Matthew Giesler, Lawrence E. Kidder, Harald P. Pfeiffer, Mark A. Scheel, and Saul A. Teukolsky: Multipole moments on the common horizon in a binary-black-hole simulation
- Vaishak Prasad, Anshu Gupta, Sukanta Bose, Badri Krishnan: Tidal deformation of dynamical horizons in binary black hole mergers
- Vaishak Prasad, Anshu Gupta, Sukanta Bose, Badri Krishnan, and Erik Schnetter: News from Horizons in Binary Black Hole Mergers
The author thanks Prayush Kumar and Vaishak Prasad for discussions.