Maximal freedom at minimum cost: linear large-scale structure in general
  modifications of gravity
  We present a turnkey solution, ready for implementation in numerical codes,
for the study of linear structure formation in general scalar-tensor models
involving a single universally coupled scalar field. We show that the totality
of cosmological information on the gravitational sector can be compressed -
without any redundancy - into five independent and arbitrary functions of time
only and one constant. These describe physical properties of the universe: the
observable background expansion history, fractional matter density today, and
four functions of time describing the properties of the dark energy. We show
that two of those dark-energy property functions control the existence of
anisotropic stress, the other two - dark-energy clustering, both of which are
can be scale-dependent. All these properties can in principle be measured, but
no information on the underlying theory of acceleration beyond this can be
obtained. We present a translation between popular models of late-time
acceleration (e.g. perfect fluids, f (R), kinetic gravity braiding, galileons),
as well as the effective field theory framework, and our formulation. In this
way, implementing this formulation numerically would give a single tool which
could consistently test the majority of models of late-time acceleration
heretofore proposed.
