mohammad atazadeh

The Schwarzschild solution was obtained in 1916 as the first solution to Einstein’s field equations based on spherical symmetry, which was considered a new idea in the use of symmetry at the time. In this study, it is tried to reformulate the solutions of Einstein’s field equations with spherical symmetry in both the presence and absence of the cosmological constant based on the invariants (or first integrals) of the symmetry groups. The method used in this article is a combination of four well-known symmetries: Lie point symmetry, λ-symmetry, Darboux polynomials, and the extended Prelle-Singer method. In this method, to resolve the Schwarzschild problem, a combination of the mentioned symmetries is used in such a way that the independent invariants are obtained in a systematic and algorithmic way. Based on this, a symmetry theory is provided for us. With the help of this theory, one can use symmetries in the best possible way, far from any confusion, and achieve the desired solutions with a specific solution, which is to find independent invariants.

In solving the Brans-Dicke (BD) equations in the BD theory of gravity, their linear independence is important. This is due to fact that in solving these equations in cosmology, if the number of unknown quantities is equal to the number of independent equations, then the unknowns can be uniquely determined. In the BD theory, the tensor field $g_{\mu \nu}$ and the BD scalar field $\varphi$ are not two separate fields, but they are coupled together. The reason behind this is a corollary that proposed by V. B. Johri and D. Kalyani in cosmology, which states that the cosmic scale factor of the universe, $a$, and the BD scalar field $\varphi$ are related by a power law. Therefore, when the principle of least action is used to derive the BD equations, the variations $\delta g^{\mu \nu}$ and $\delta \varphi$ should not be considered as two independent dynamical variables. So, there is a constraint on $\delta g^{\mu \nu}$ and $\delta \varphi$ that causes the number of independent BD equations to decrease by one unit, in such a way that in the equations that have been known as BD equations, one of them is redundant. In this paper, we prove this issue, that is, we show that one of these equations, which we choose as the modified Klein-Gordon equation, is not an independent equation, but a result establishing other BD equations, the law of conservation of energy-momentum of matter and Bianchi's identity. Therefore, we should not look at the modified Klein-Gordon equation as an independent field equation in the BD theory, but rather it is included in the other BD equations and should not be mentioned separately as one of the BD equations once again.