Early Modified Gravity: Implications for Cosmology
  We study the effects of modifications of gravity after Big Bang
Nucleosynthesis (BBN) which would manifest themselves mainly before
recombination. We consider their effects on the Cosmic Microwave Background
(CMB) radiation and on the formation of large scale structure. The models that
we introduce here represent all screened modifications of General Relativity
(GR) which evade the local tests of gravity such as the violation of the strong
equivalence principle as constrained by the Lunar Ranging experiment. We use
the tomographic description of modified gravity which defines models with
screening mechanisms of the chameleon or Damour-Polyakov types and allows one
to relate the temporal evolution of the mass and the coupling to matter of a
scalar field to its Lagrangian and also to cosmological perturbations. The
models with early modifications of gravity all involve a coupling to matter
which is stronger in the past leading to effects on perturbations before
recombination while minimising deviations from Lambda-CDM structure formation
at late times. We find that a new family of early transition models lead to
discrepancies in the CMB spectrum which could reach a few percent and appear as
both enhancements and reductions of power for different scales.
