On a reconstruction procedure for special spherically symmetric metrics in the scalar-Einstein-Gauss-Bonnet model: the Schwarzschild metric test

The 4D gravitational model with a real scalar field $\varphi$, Einstein and Gauss-Bonnet terms is considered. The action contains the potential $U(\varphi)$ and the Gauss-Bonnet coupling function $f(\varphi)$. For a special static spherically symmetric metric $ds^2 = (A(u))^{-1} du^2 - A(u) dt^2 + u^2 d\Omega^2$, with $A(u) > 0$ ($u > 0$ is a radial coordinate), we verify the so-called reconstruction procedure suggested by Nojiri and Nashed. This procedure presents certain implicit relations for $U(\varphi)$ and $f(\varphi)$ which lead to exact solutions to the equations of motion for a given metric governed by $A(u)$. We confirm that all relations in the approach of Nojiri and Nashed for $f(\varphi(u))$ and $\varphi(u)$ are correct, but the relation for $U(\varphi(u))$ contains a typo which is eliminated in this paper. Here we apply the procedure to the (external) Schwarzschild metric with the gravitational radius $2 \mu $ and $u > 2 \mu$. Using the ``no-ghost'' restriction (i.e., reality of $\varphi(u)$), we find two families of $(U(\varphi), f(\varphi))$. The first one gives us the Schwarzschild metric defined for $u > 3 \mu$, while the second one describes the Schwarzschild metric defined for $2 \mu < u < 3 \mu$ ($3 \mu$ is the radius of the photon sphere). In both cases the potential $U(\varphi)$ is negative.

K. K. Ernazarov, V. D. Ivashchuk · 2024