Five parameters are all you need

In ΛCDM, we demote the optical depth τ_reio to a derived parameter with analytic modeling and symbolic regression of 21cmFAST reionization timelines.

• arXiv URL: https://arxiv.org/abs/2405.13680
• journal URL: DOI URL

The key points are:

• We discover that reionization history has a universal shape, independent of cosmology or astrophysics, in both modeling and parameters;
• The Gompertz function, introduced 200 years ago to model human mortality, also well describes the mortality of neutral hydrogen in the Universe;
• Our reionization history is asymmetric in time, as preferred by simulation and observational data, unlike the conventional tanh model;
• We demote the reionization optical depth, the 6th cosmological parameter and an empirical one, to a derived parameter, as function of the other 5 cosmological parameters and astrophysics;
• We tighten the constraint on the optical depth τ_reio by 5 times, and that on the primordial scalar amplitude A_s by more than 2 times;
• For the first time, we “detect” the ionizing efficiency ζ_UV at high significance, the dominating parameter for reionization, a leap from the previous result consistent with zero.