The theory of gravity, popular among physicists, is poorly applicable to the real world. Astronomers came to this conclusion by looking at what happens in the immediate vicinity of black holes. The researchers also proposed a new way to build models of black holes. An employee of the Ural Federal University named after the first President of Russia B. N. Yeltsin (UrFU) and her colleague from the University of Tokyo were presented in the magazine Classical and Quantum Gravity.
Today, most scientists believe that black holes are real objects, and not just mathematical solutions to the equations of general relativity. However, modern physics has accumulated many prerequisites for revising this theory. All fundamental interactions known to science have already been described in "quantum language", except for gravity. These inconsistencies indicate that the theory of relativity is only one approximation to the ultimate theory of gravity.
One of the simplest versions of such an extended theory is the assumption that the gravitational constant entering the equations is not a constant, but a field that can change in time and space. Scientists at the modern level of accuracy cannot measure this slowly changing field and only therefore perceive it as a constant. If we accept this hypothesis, then gravity arises with a scalar (given at each point by only one number) field. This is how the first and simplest theory of gravity with a scalar field, the Brans - Dicke theory, was formulated. Today the class of theories of gravity with a scalar field is very wide; such theories are considered one of the most promising ways to expand general relativity.
In a new work, Daria Tretyakova from UrFU, together with a colleague from the University of Tokyo, investigated one of the theories of this class - the so-called Horndesky model. Horndesky is the most general possible class of theories of gravity with a scalar field, where there are no instabilities, that is, there are no unusual parameters of matter (for example, negative or imaginary mass).
At the cosmological level (the scale at which the Universe can be considered as a single object of study), models of this class, which have symmetry with respect to the shift in space and time of the scalar field, have proven themselves well, allowing one to describe the rapidly expanding Universe without invoking additional theories. The authors decided to test these models more rigorously and versatile. Astronomers studied Horndesky's models on the astrophysical scale of individual space objects and found that black holes in models that have successfully proven themselves in cosmology are unstable.
These models are poorly suited to describe the real Universe, because today it is believed that black holes exist and, on the contrary, are quite stable. The situation, however, is fixable: scientists have proposed a way to build Horndesky models, ensuring the stability of black holes in the framework of such theories. Now the authors plan to subject the newly proposed models to standard tests: to check the adequacy on the cosmological and astrophysical scales.