Black holes with a nonconstant kinetic term in degenerate higher-order scalar tensor theories
Masato Minamitsuji, James Edholm
Abstract
We investigate static and spherically symmetric black hole (BH) solutions in shift-symmetric quadratic-order degenerate higher-order scalar-tensor (DHOST) theories. We allow a nonconstant kinetic term $X={g}^{\ensuremath{\mu}\ensuremath{\nu}}{\ensuremath{\partial}}_{\ensuremath{\mu}}\ensuremath{\phi}{\ensuremath{\partial}}_{\ensuremath{\nu}}\ensuremath{\phi}$ for the scalar field $\ensuremath{\phi}$ and assume that $\ensuremath{\phi}$ is, like the spacetime, a pure function of the radial coordinate $r$, namely $\ensuremath{\phi}=\ensuremath{\phi}(r)$. First, we find analytic static and spherically symmetric vacuum solutions in the so-called Class Ia DHOST theories, which include the quartic Horndeski theories as a subclass. We consider several explicit models in this class and apply our scheme to find the exact vacuum BH solutions. BH solutions obtained in our analysis are neither Schwarzschild or Schwarzschild (anti-) de Sitter. We show that a part of the BH solutions obtained in our analysis are free of ghost and Laplacian instabilities and are also mode stable against the odd-parity perturbations. Finally, we argue the case that the scalar field has a linear time dependence $\ensuremath{\phi}=qt+\ensuremath{\psi}(r)$ and show several simple examples of nontrivial BH solutions with a nonconstant kinetic term obtained analytically and numerically.