Origin of Primordial Inhomogeneities and Anisotropies from a Symmetric Quantum State

Determine a physically and mathematically consistent mechanism by which the homogeneous and isotropic Bunch–Davies vacuum state in inflationary cosmology evolves to yield primordial inhomogeneities and anisotropies together with a classical phase-space distribution that can seed the observed large-scale structure of the universe.

Background

The paper reviews the standard cosmological lore in which the universe begins in the homogeneous and isotropic Bunch–Davies vacuum during inflation and is later described in terms of classical evolution in phase space. It argues that the transition from an intrinsically quantum state to a classical distribution ("classicalization") is not satisfactorily derived and that unitary evolution preserves the initial symmetries.

The author emphasizes that a concrete, satisfactory derivation is missing for how a symmetric quantum vacuum leads to classical primordial inhomogeneities and anisotropies needed to form stars, galaxies, and clusters. This gap is identified as a central open question concerning the origin of structure and the arrow of time tied to initial conditions.

References

Therefore, the appearance of primordial inhomogeneities and anisotropies required for the formation of the observed large-scale structure in the universe by no means appears to be settled and is an open question (see, e.g. [Berjon]).

— Demystifying Arrow of Time  (2405.15796 - Kakushadze, 2024) in Section 5 (Special Initial Conditions: Why, How, …)

It is therefore interesting to study how the higher-order statistics of $\mathcal R$ are affected when the stochastic noise is treated as classical at the homogeneous matching times compared with the classicality bound $\sigma{\rm cl}(k)$.

— Classical Noise in Transient USR Inflation  (2609.19478 - N. et al., 16 Sep 2026) in Section 4, subsection “Perturbations at homogeneous matching times”