Quasinormal modes of a Schwarzschild black hole surrounded by free   static spherically symmetric quintessence: Electromagnetic perturbations
  In this paper, we evaluated the quasinormal modes of electromagnetic perturbation in a Schwarzschild black hole surrounded by the static spherically symmetric quintessence by using the third-order WKB approximation when the quintessential state parameter $ w_{q}$ in the range of $-1/3<w_{q}<0$. Due to the presence of quintessence, Maxwell field damps more slowly. And when at $-1<w_{q}<-1/3$, it is similar to the black hole solution in the ds/Ads spacetime. The appropriate boundary conditions need to be modified. 
Yu Zhang, Yuanxing Gui, Fei Yu and Fenglin Li