Five parameters are all you need (in $\Lambda$CDM)
Abstract
The standard cosmological model, with its six independent parameters, successfully describes our observable Universe. One of these parameters, the optical depth to reionization , represents the scatterings that Cosmic Microwave Background (CMB) photons will experience after decoupling from the primordial plasma as the intergalactic medium transitions from neutral to ionized. depends on the neutral hydrogen fraction , which, in turn, should theoretically depend on cosmology. We present a novel method to establish the missing link between cosmology and reionization timeline using symbolic regression. We discover the timeline has a universal shape well described by the Gompertz mortality law, applicable to any cosmology within our simulated data. Unlike the conventional tanh prescription, our model is asymmetric in time and a good fit to astrophysical constraints on . By combining CMB with astrophysical data and marginalizing over astrophysics, we treat as a derived parameter, tightening its constraint to . This approach reduces the error on the amplitude of the primordial fluctuations by a factor of 2.3 compared to Planck's PR3 constraint and provides a commanding constraint on the ionization efficiency . We expect further improvements in the near term as reionization constraints increase and our understanding of reionization advances.
Cite
@article{arxiv.2405.13680,
title = {Five parameters are all you need (in $\Lambda$CDM)},
author = {Paulo Montero-Camacho and Yin Li and Miles Cranmer},
journal= {arXiv preprint arXiv:2405.13680},
year = {2024}
}
Comments
Correctly accounting for ionization efficiency enables us to place leading constraints by combining our universal asymmetric reionization model with CMB and damping wing constraints on $x_\mathrm{HI}$. By marginalizing over $\zeta_\mathrm{UV}$, we achieve powerful constraints on $A_\mathrm{s}$ and $\tau_\mathrm{reio}$, with uncertainties around 3% for the former