What is the shape of the initial state?

TL;DR

This paper proposes a way to characterize the initial quantum state of the Universe's curvature perturbations during inflation, linking observable correlators to the initial state's structure and pre-inflation physics.

Contribution

It introduces a parameterization of the initial quantum state and extends the in-in formalism to include this structure in correlator calculations.

Findings

Different correlator shapes probe various initial state structures.

CMB and large scale structure measurements provide complementary information.

Shapes of correlators reveal clues about pre-inflation physics.

Abstract

We argue that a plausible operational definition for an initial state of the Universe is the initial quantum state of the curvature perturbations generated during inflation. We provide a parameterization of this state and generalize the standard in-in formalism to incorporate the structures in this state into the computation of correlators of the perturbations. Measurements of these correlators using both the CMB as well as large scale structure probe different structures in the initial state, as they give rise to bi- and tri-spectra peaked on different shapes of triangles and quadrilaterals in momentum space. In essence, the shapes implied by the correlators feed directly into information about the shape of the initial state and what physics could have preceded inflation to set this state up.