Earliest Structures in the Universe can be explained by a Relativistic Cosmological Perturbation Theory

TL;DR

This paper develops a relativistic cosmological perturbation theory that explains the formation and timing of the universe's earliest structures, incorporating pressure effects and entropy perturbations.

Contribution

It introduces a coordinate-independent way to define energy and particle density perturbations and formulates a comprehensive relativistic perturbation theory including pressure and entropy effects.

Findings

Negative nonadiabatic pressure perturbations enabled rapid early growth of structures.

The theory explains the timing of structure formation shortly after decoupling.

Perturbations reached nonlinear phases early in the universe's evolution.

Abstract

A relativistic cosmological perturbation theory for the Friedmann-Lema\^itre-Robertson-Walker universe is presented that explains the masses and formation times of the first structures in our universe. First, it is shown that, without a coordinate system being used, quantities intended to represent energy density and particle number density perturbations can be defined in only one way. The Newtonian limit, where the pressure becomes zero, proves that these quantities are indeed the perturbations of the energy density and the particle number density. Then, after selecting a reference frame, a perturbation theory will be formulated based on these quantities. This formulation considers the local perturbation to the spatial curvature resulting from a density perturbation, the local fluid velocity due to pressure gradients caused by the self-gravity of the density perturbation, and entropy perturbations, all of which are necessary for structure formation. Pressure perturbations consist of two components: an adiabatic component caused by the density perturbation itself, and a random, nonadiabatic component resulting from the rapid, chaotic transition to the era when matter and radiation were decoupled. Immediately after decoupling, negative nonadiabatic pressure perturbations in various density perturbations enabled their rapid growth over a short period of time. This brief period ended when the total pressure perturbation became positive. Subsequently, the density perturbations gradually grew toward their nonlinear phase, which was reached early on.