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 are 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.

Figure 1

Late-time observations measure correlations of the adiabatic density fluctuation $\zeta (x)$ produced from nonlinear time evolution in the early Universe. The particle’s propagation is illustrated by the solid lines, while the dashed line represents the absence of the corresponding mode at late times. Left: Quantum-vacuum fluctuations arise as the correlated production of three particles due to nonlinear effects. This process would violate energy conservation in flat space, and thus produces no poles at physical momenta [31]. Right: Classical fluctuations only arise in a state containing physical particles, as local variations in the particle density; see, e.g., Refs. [25, 26, 27, 28, 29, 30]. The nonlinear evolution that leads to net particle creation also allows for decays (or annihilation). These processes appear as poles at physical momenta.