Is there another coincidence problem at the reionization epoch?

TL;DR

This paper investigates a coincidental alignment between the reionization epoch and the intersection of radiation and cosmological constant densities, suggesting a potential physical connection supported by Bayesian analysis.

Contribution

It introduces a novel statistical approach to assess the significance of the coincidence between reionization and density intersections, proposing a possible physical link in cosmology.

Findings

Strong Bayesian evidence for the coincidence from current data.

Future observations could clarify the physical significance of this alignment.

The connection may relate to star-formation history and cosmic background dynamics.

Abstract

The cosmological coincidences between the matter and radiation energy densities at recombination as well as between the densities of matter and the cosmological constant at present time are well known. We point out that moreover the third intersection between the energy densities of radiation and the cosmological constant coincides with the reionization epoch. To quantify the statistical relevance of this concurrence, we compute the Bayes factor between the concordance cosmology with free Thomson scattering optical depth and a model for which this parameter is inferred from imposing a match between the time of density equality and the epoch of reionization. This is to characterize the potential explanatory gain if one were to find a parameter-free physical connection. We find a very strong preference for such a concurrence on the Jeffreys scale from current cosmological observations. We furthermore discuss the effect of choice of priors, changes in reionization history, and free sum of neutrino masses. We also estimate the impact of adding intermediate polarization data from the Planck High Frequency Instrument and prospects for future 21 cm surveys. In the first case, preference for the correlation remains substantial, whereas future data may give results more decisive in pro or substantial in contra. Finally, we provide a discussion on different interpretations of these findings. In particular, we show how a connection between the star-formation history and the cosmological background dynamics can give rise to this concurrence.