Halo modelling in chameleon theories
  We analyse modelling techniques for the large-scale structure formed in
scalar-tensor theories of constant Brans-Dicke parameter which match the
concordance model background expansion history and produce a chameleon
suppression of the gravitational modification in high-density regions. Thereby,
we use a mass and environment dependent chameleon spherical collapse model, the
Sheth-Tormen halo mass function and linear halo bias, the Navarro-Frenk-White
halo density profile, and the halo model. Furthermore, using the spherical
collapse model, we extrapolate a chameleon mass-concentration scaling relation
from a LCDM prescription calibrated to N-body simulations. We also provide
constraints on the model parameters to ensure viability on local scales. We
test our description of the halo mass function and nonlinear matter power
spectrum against the respective observables extracted from large-volume and
high-resolution N-body simulations in the limiting case of f(R) gravity,
corresponding to a vanishing Brans-Dicke parameter. We find good agreement
between the two; the halo model provides a good qualitative description of the
shape of the relative enhancement of the f(R) matter power spectrum with
respect to LCDM caused by the extra attractive gravitational force but fails to
recover the correct amplitude. Introducing an effective linear power spectrum
in the computation of the two-halo term to account for an underestimation of
the chameleon suppression at intermediate scales in our approach, we accurately
reproduce the measurements from the N-body simulations.
