DES Y1 results: Splitting growth and geometry to test $\Lambda$CDM
  We analyze Dark Energy Survey (DES) data to constrain a cosmological model
where a subset of parameters -- focusing on $\Omega_m$ -- are split into
versions associated with structure growth (e.g. $\Omega_m^{\rm grow}$) and
expansion history (e.g. $\Omega_m^{\rm geo}$). Once the parameters have been
specified for the $\Lambda$CDM cosmological model, which includes general
relativity as a theory of gravity, it uniquely predicts the evolution of both
geometry (distances) and the growth of structure over cosmic time. Any
inconsistency between measurements of geometry and growth could therefore
indicate a breakdown of that model. Our growth-geometry split approach
therefore serves as both a (largely) model-independent test for
beyond-$\Lambda$CDM physics, and as a means to characterize how DES observables
provide cosmological information. We analyze the same multi-probe DES data as
arXiv:1811.02375 : DES Year 1 (Y1) galaxy clustering and weak lensing, which
are sensitive to both growth and geometry, as well as Y1 BAO and Y3 supernovae,
which probe geometry. We additionally include external geometric information
from BOSS DR12 BAO and a compressed Planck 2015 likelihood, and external growth
information from BOSS DR12 RSD. We find no significant disagreement with
$\Omega_m^{\rm grow}=\Omega_m^{\rm geo}$. When DES and external data are
analyzed separately, degeneracies with neutrino mass and intrinsic alignments
limit our ability to measure $\Omega_m^{\rm grow}$, but combining DES with
external data allows us to constrain both growth and geometric quantities. We
also consider a parameterization where we split both $\Omega_m$ and $w$, but
find that even our most constraining data combination is unable to separately
constrain $\Omega_m^{\rm grow}$ and $w^{\rm grow}$. Relative to $\Lambda$CDM,
splitting growth and geometry weakens bounds on $\sigma_8$ but does not alter
constraints on $h$.
