Are redshift-space distortions actually a probe of growth of structure?
  We present an impact of coupling between dark matter and a scalar field,
which might be responsible for dark energy, on measurements of redshift-space
distortions. We point out that, in the presence of conformal and/or disformal
coupling, linearized continuity and Euler equations for total matter fluid
significantly deviate from the standard ones even in the sub-horizon scales. In
such a case, a peculiar velocity of total matter field is determined not only
by a logarithmic time derivative of its density perturbation but also by
density perturbations for both dark matter and baryon, leading to a large
modification of the physical interpretation of observed data obtained by
measurements of redshift-space distortions. We reformulate galaxy two-point
correlation function in the redshift space based on the modified continuity and
Euler equations. We conclude from the resultant formula that the true value of
the linear growth rate of large-scale structure cannot be necessarily
constrained by single-redshift measurements of the redshift-space distortions,
unless one observes the actual time-evolution of structure.
