Probing gravity with redshift-space distortions: effects of tracer bias
  and sample selection
  We investigate clustering properties of dark matter halos and galaxies to
search for optimal statistics and scales where possible departures from general
relativity (GR) could be found. We use large N-body cosmological simulations to
perform measurements based on the two-point correlation function (2PCF) in GR
and in selected modified gravity (MG) structure formation scenarios. As a
test-bed, we employ two popular beyond-GR models: $f(R)$ gravity and the normal
branch of the Dvali-Gabadadze-Porrati (nDGP) braneworld. We study a range of
simulated halo and galaxy populations and reveal a noticeable MG signal in the
monopole and quadrupole moments of the redshift-space 2PCF, and in the
so-called clustering wedges. However, once expressed in terms of the linear
distortion parameter, $\beta$, the statistical significance of these signals
largely diminishes due to a strong degeneracy between MG-enhanced clustering
and modified tracer bias. To circumvent this, we consider statistics less
dependent on the bias: relative clustering ratios. We generalize the monopole
ratio proposed in earlier work to multipole moments and clustering wedges, and
introduce a new estimator of the $\beta$ parameter. The clustering ratios we
extract foster noticeable differences between MG and GR models, reaching a
maximum deviation of 10\% at 2$\sigma$ significance for specific variants of
$f(R)$ and nDGP. We show that such departures could be measured for $\beta$ if
non-linear effects at intermediate scales are correctly modeled. Our study
indicates that the clustering ratios give great promise to search for
signatures of MG in the large-scale structure. We also find that the selection
of an optimal tracer sample depends on a particular statistics and gravity
model to be considered.
