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Critical gravity in the Chern-Simons modified gravity

We perform the perturbation analysis of the Chern-Simons modified gravity around the AdS$_4$ spacetimes (its curvature radius $\ell$) to obtain the critical gravity. In general, we could not obtain an explicit form of perturbed Einstein equation which shows a massive graviton propagation clearly, but for the Kerr-Schild perturbation and Chern-Simons coupling $θ=kx/y$, we find the AdS wave as a single massive solution to the perturbed Einstein equation. Its mass squared is given by $M^2=[-9+(2\ell^2/k-1)^2]/4\ell^2$. At the critical point of $M^2=0(k=\ell^2/2)$, the solution takes the log-form and the linearized excitation energies vanish.