Is there another coincidence problem at the reionization epoch?

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.

On the cosmological constant problem

“…This also implies, for instance, a current proximity to the maximal ratio of the density of critical mass shells to the critical background density [24]. Cosmological coincidences have furthermore been pointed out at recombination, reionization, and the star-formation peak [9]. It may be interesting to elaborate whether the framework presented here could establish a link between these observations or provide new approaches for other cosmological problems, leaving a range of further explorations to future work.…”

“…A curiosity of cosmic acceleration can furthermore be found in the comparable magnitude of its associated current energy density with that of matter, provoking the Why Now? conundrum [4,9].…”

Late-time acceleration by a residual cosmological constant from sequestering vacuum energy in ultimate collapsed structures

“…While these models have been invoked to address the enigma of cosmic acceleration, and sometimes the Why Now? conundrum of the presently coincident energy densities of the cosmological constant and the matter content [4,14], both attributed to the new cosmological constant problem, they usually do not provide a mechanism to circumvent the old cosmological constant problem of the non-gravitating vacuum energy (however, see, e.g., Ref. [15]).…”

Screening and degenerate kinetic self-acceleration from the nonlinear freedom of reconstructed Horndeski theories

“…Either the Universe turns out to be consistent with ΛCDM, which will motivate a more directed effort in tackling the cosmological constant problem (see, e.g., Refs. [70][71][72][73][74][75][76][77][78][79][80][81][82][83]). On the other hand, if recent observational tensions [3,84,85] persist then that will be strong evidence that the theory describing the Universe on cosmological scales requires revision and potentially will go beyond a cosmological constant.…”