Gradient expansion formalism for nonlinear superhorizon perturbations
  We develop a theory of nonlinear cosmological perturbations on superhorizon
scales where a characteristic length scale of perturbations is longer than the
Hubble radius, in general theoretical frameworks. Our formalism is based on the
spatial gradient expansion approach by adopting the ADM decomposition.
Nonlinear superhorizon perturbation including both scalar (curvature
perturbation) and tensor (gravitational waves) modes can be dealt with valid up
to a second-order in the expansion. First we will review the formalism for a
standard general relativity (GR) gravity plus a general kinetic single scalar
(k-inflation) with a general form of the potential in the context of
inflationary cosmology. That is the basic overview of our procedure. Then it
can be extended to more general framework, that is (1) beyond k-inflation
(Galileon inflation), (2) a multi-component scalar field with a general kinetic
term and a general form of the potential and also (3) beyond Einstein gravity
(general scalar-tensor theory), which can lead to several kinds of modified
gravity. These theories are motivated not only inflation, but also the topic of
dark energy. We provide a formalism to obtain the solution and construct
nonlinear curvature perturbation in such general theoretical situation and it
can be applied to the calculation of the superhorizon evolution of a primordial
non-Gaussianity beyond the so-called $\delta N$ formalism, showing fully
nonlinear interaction of both scalar and tensor modes.
