  In this paper we analyze a suite of cosmological simulations of modified
gravitational action f(R) models, where cosmic acceleration is induced by a
scalar field that acts as a fifth force on all forms of matter. In particular,
we focus on the bispectrum of the dark matter density field on mildly
non-linear scales. For models with the same initial power spectrum, the dark
matter bispectrum shows significant differences for cases where the final dark
matter power spectrum also differs. Given the different dependence on bias of
the galaxy power spectrum and bispectrum, bispectrum measurements can close the
loophole of galaxy bias hiding differences in the power spectrum.
Alternatively, changes in the initial power spectrum can also hide differences.
By constructing LCDM models with very similar final non-linear power spectra,
we show that the differences in the bispectrum are reduced (<4%) and are
comparable with differences in the imperfectly matched power spectra. These
results indicate that the bispectrum depends mainly on the power spectrum and
less sensitively on the gravitational signatures of the f(R) model. This weak
dependence of the matter bispectrum on gravity makes it useful for breaking
degeneracies associated with galaxy bias, even for models beyond general
relativity.
