  The observed galaxy power spectrum acquires relativistic corrections from
lightcone effects, and these corrections grow on very large scales. Future
galaxy surveys in optical, infrared and radio bands will probe increasingly
large wavelength modes and reach higher redshifts. In order to exploit the new
data on large scales, an accurate analysis requires inclusion of the
relativistic effects. This is especially the case for primordial
non-Gaussianity and for extending tests of dark energy models to horizon
scales. Here we investigate the latter, focusing on models where the dark
energy interacts non-gravitationally with dark matter. Interaction in the dark
sector can also lead to large-scale deviations in the power spectrum. If the
relativistic effects are ignored, the imprint of interacting dark energy will
be incorrectly identified and thus lead to a bias in constraints on interacting
dark energy on very large scales.
