Equation of state of dark energy in f(R) gravity
  f(R) gravity is one of the simplest generalizations of general relativity,
which may explain the accelerated cosmic expansion without introducing a
cosmological constant. Transformed into the Einstein frame, a new scalar degree
of freedom appears and it couples with matter fields. In order for f(R)
theories to pass the local tests of general relativity, it has been known that
the chameleon mechanism with a so-called thin-shell solution must operate. If
the thin-shell constraint is applied to a cosmological situation, it has been
claimed that the equation-of-state parameter of dark energy w must be extremely
close to -1. We argue this is due to the incorrect use of the Poisson equation
which is valid only in the static case. By solving the correct Klein-Gordon
equation perturbatively, we show that a thin-shell solution exists even if w
deviates appreciably from -1.
