Stationary black holes and stars in the Brans-Dicke theory with $Λ >0$ revisited (2209.11011v3)

Abstract: It was shown a few years back that for a stationary regular black hole or star solution in the Brans-Dicke theory with a positive cosmological constant $\Lambda$, endowed with a de Sitter or cosmological event horizon in the asymptotic region, not only there exists no non-trivial field configurations, but also the inverse Brans-Dicke parameter $\omega^{-1}$ must be vanishing. This essentially reduces the theory to Einstein's General Relativity. The assumption of the existence of the cosmological horizon was crucial for this proof. However, since the Brans-Dicke field $\phi$, couples directly to the $\Lambda$-term in the energy-momentum tensor as well as $\Lambda$ acts as a source in $\phi$'s equation of motion, it seems reasonable to ask : can $\phi$ become strong instead and screen the effect of $\Lambda$, at very large scales, so that the asymptotic de Sitter structure is replaced by some alternative, yet still acceptable boundary condition? In this work we analytically argue that no such alternative exists, as long as the spacetime is assumed to be free of any naked curvature singularity. We further support this result by providing explicit numerical computations. Thus we conclude that in the presence of a positive $\Lambda$, irrespective of whether the asymptotic de Sitter boundary condition is imposed or not, a regular stationary black hole or even a star solution in the Brans-Dicke theory always necessitates $\omega^{-1}=0$, and thereby reducing the theory to General Relativity. The qualitative differences of this result with that of the standard no hair theorems are also pointed out.