Recombination induced thermodynamic Gaussian cosmological baryonic fluctuations

TL;DR

This paper proposes that large, scale-invariant Gaussian fluctuations in baryonic matter arise from thermodynamic effects during recombination, potentially explaining discrepancies in cosmic microwave background data and Hubble constant measurements.

Contribution

It introduces a novel mechanism where thermodynamic fluctuations during recombination generate scale-invariant density fluctuations, impacting cosmological observations within standard and modified gravity models.

Findings

Large thermodynamic fluctuations occur near recombination due to sound speed drops.

These fluctuations can decouple the primordial spectrum from observed CMB features.

The mechanism may explain the Hubble constant tension and effects in modified gravity scenarios.

Abstract

In some instances, e.g. near phase transitions, thermodynamic fluctuations become macroscopically relevant, and relative amplitudes grow far above the standard $N^{-1/2}$ scale, with $N$ the number of particles. Such large fluctuations are characterised by a scale invariant Gaussian power spectrum. In this letter I show that the abrupt drop in the baryonic sound speed across recombination leads to conditions resulting in such large thermodynamic Gaussian fluctuations in the ionisation fraction of the baryons. Under pressure equilibrium, this will result in a mechanism for generating scale invariant density and temperature fluctuations in the baryonic component, inherent to the thermodynamics of the baryons themselves. Within a $\Lambda$CDM framework, this extra random fluctuation source leads to a decoupling of the inflationary relic small wave number spectrum and the amplitude of the Gaussian random fluctuations at frequencies higher than the first acoustic peak, an effect which could explain the mismatch between cosmic microwave background (CMB) inferences and local kinetic determinations of the Hubble constant. Within modified gravity theories in absence of dark matter, the mechanism proposed serves as a source for random Gaussian density fluctuations in the acoustic peak region.