Emergence of cosmic structure from Planckian discreteness

TL;DR

This paper explores an alternative to inflationary models by proposing that cosmic inhomogeneities originate from Planck-scale spacetime discreteness, naturally producing scale-invariant perturbations without trans-Planckian issues or quantum-to-classical transition assumptions.

Contribution

It extends a quantum gravity-inspired model to quasi-de Sitter expansion, deriving the scalar perturbation spectrum with slow-roll parameters.

Findings

Scalar perturbation spectrum depends on slow-roll parameters

Inhomogeneities originate from Planck-scale discreteness

Model naturally produces scale-invariant perturbations

Abstract

In the standard inflationary paradigm the inhomogeneities observed in the CMB arise from quantum fluctuations of an initially homogeneous and isotropic vacuum state. This picture suffers from two well-known weaknesses. First, it assumes that quantum field theory remains valid at trans-Planckian scales, without modifications from quantum gravity. Second, it necessitates a quantum-to-classical transition in which fluctuations of a homogeneous quantum state become the classical inhomogeneities seen in the CMB. Recently, an alternative paradigm has been proposed in which such inhomogeneities are present from the very beginning, emerging from the assumed discreteness of spacetime at the Planck scale predicted by certain approaches to quantum gravity. Within this framework, scale-invariant scalar perturbations are generated naturally, without relying on trans-Planckian assumptions or invoking a quantum-to-classical transition. Specifically, inhomogeneities in the quantum state at the Planck scale propagate into semiclassical inhomogeneities on CMB scales. Here, we extend the aforementioned proposal to the most realistic case of a quasi-de Sitter expansion; in particular, we compute the scalar perturbation spectrum as a function of the slow-roll parameters, systematically encoded through the Hubble flow functions.