Signals of a Quantum Universe
  Structure in the Universe is widely believed to have originated from quantum
fluctuations during an early epoch of accelerated expansion. Yet, the patterns
we observe today do not distinguish between quantum or classical primordial
fluctuations; current cosmological data is consistent with either possibility.
We argue here that a detection of primordial non-Gaussianity can resolve the
present situation, and provide a litmus-test for the quantum origin of cosmic
structure. Unlike in quantum mechanics, vacuum fluctuations cannot arise in
classical theories and therefore long-range classical correlations must result
from (real) particles in the initial state. Similarly to flat-space scattering
processes, we show how basic principles require these particles to manifest
themselves as poles in the $n$-point functions, in the so-called folded
configurations. Following this observation, and assuming fluctuations are (i)
correlated over large scales, and (ii) generated by local evolution during an
inflationary phase, we demonstrate that: the absence of a pole in the folded
limit of non-Gaussian correlators uniquely identifies the quantum vacuum as the
initial state. In the same spirit as Bell's inequalities, we discuss how this
can be circumvented if locality is abandoned. We also briefly discuss the
implications for simulations of a non-Gaussian universe.
