Constraining exotic high-z reionization histories with Gaussian processes and the cosmic microwave background

The large-angle polarization anisotropies observed in the cosmic microwave background (CMB) are generated by Thomson scattering of CMB photons off free electrons in the post-recombination Universe. In the standard $Λ$ cold dark matter cosmological model, the free electron density becomes large at redshifts z 10 due to the formation of the first stars, leading to reionization of the intergalactic medium. In this work, we use Gaussian processes to give a model-independent reconstruction of the reionization history of the Universe constrained by the Planck CMB measurements. This method gives a consistent reconstruction of the standard reionization at z 10, and strongly constrains any additional high-z reionization. We use our results to construct a new derived parameter expressing the high-z contribution to the CMB optical depth, τhighz. The posterior distribution on τhighz can be used to set accurate constraints on models for exotic energy injection, which we demonstrate for the case of decaying dark matter with particle mass in the range O(1\,MeV). A companion paper demonstrates the use of this methodology to test ensembles of multi-axion models. Our results can be recast to different models using the provided data and code: {https://github.com/Cheng-Hanyu/CLASS_reio_gpr}{github.com/Cheng-Hanyu/CLASS_reio_gpr}.