  The baryonic acoustic signature in the large-scale clustering pattern of
galaxies has been detected in the two-point correlation function. Its precise
spatial scale has been forwarded as a rigid-rod ruler test for the space-time
geometry, and hence as a probe for tracking the evolution of Dark Energy.
Percent-level shifts in the measured position can bias such a test and erode
its power to constrain cosmology. This paper addresses some of the systematic
effects that might induce shifts: namely non-linear corrections from matter
evolution, redshift space distortions and biasing. We tackle these questions
through analytic methods and through a large battery of numerical simulations,
with total volume of the order ~100[Gpc\h]^3. A toy-model calculation shows
that if the non-linear corrections simply smooth the acoustic peak, then this
gives rise to an `apparent' shifting to smaller scales. However if tilts in the
broad band power spectrum are induced then this gives rise to more pernicious
`physical' shifts. Our numerical simulations show evidence of both: in real
space and at z=0, for the dark matter we find percent level shifts; for haloes
the shifts depend on halo mass, with larger shifts being found for the most
biased samples, up to 3%. From our analysis we find that physical shifts are
greater than ~0.4% at z=0. In redshift space these effects are exacerbated, but
at higher redshifts are alleviated. We develop an analytical model to
understand this, based on solutions to the pair conservation equation using
characteristic curves. When combined with modeling of pairwise velocities the
model reproduces the main trends found in the data. The model may also help to
unbias the acoustic peak.
