  Any hint of non-gaussianity in the cosmological initial conditions will
provide us with a unique window into the physics of early universe. We show
that the impact of a small local primordial non-gaussianity (generated on
super-horizon scales) on the statistics of collapsed objects (such as galaxies
or clusters) can be approximated by using slightly modified, but gaussian,
initial conditions, which we describe through simple analytic expressions.
Given that numerical simulations with gaussian initial conditions are
relatively well-studied, this equivalence provides us with a simple tool to
predict signatures of primordial non-gaussianity in the statistics of collapsed
objects. In particular, we describe the predictions for non-gaussian mass
function, and also confirm the recent discovery of a non-local bias on large
scales (arXiv:0710.4560, arXiv:0801.4826) as a signature of primordial
non-gaussianity. We then study the potential of galaxy surveys to constrain
non-gaussianity using their auto-correlation and cross-correlation with the CMB
(due to the Integrated Sachs-Wolfe effect), as a function of survey
characteristics, and predict that they will eventually yield an accuracy of
Delta f_{NL} ~ 0.1 and 3 respectively, which will be better than or competitive
with (but independent of) the best predicted constraints from the CMB.
Interestingly, the cross-correlation of CMB and NVSS galaxy survey already
shows a hint of a large local primordial non-gaussianity: f_{NL} = 236 +/- 127.
