Papers › Constraining exotic high-z reionization histories with Gaussian processes and the...
Constraining exotic high-z reionization histories with Gaussian processes and the cosmic microwave background
Hanyu Cheng, Ziwen Yin, Eleonora Di Valentino, David J. E. Marsh, Luca Visinelli
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The large-angle polarization anisotropies in the Cosmic Microwave Background (CMB) arise from 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 increases at redshifts z ≲10 as the first stars form, reionizing the intergalactic medium. We use \emph{Gaussian processes} to perform a model-independent reconstruction of the cosmic reionization history constrained by \textit{Planck} CMB data. Our approach recovers the standard reionization at z ≲10 and places stringent limits on any additional high-z reionization. From this reconstruction, we define a new derived parameter, the high-redshift contribution to the CMB optical depth, τ_(highz), whose posterior distribution provides robust constraints on exotic energy injection scenarios. We demonstrate this for decaying dark matter with particle masses in the range 𝒪(1 MeV). A companion paper applies this framework to multi-axion models. All data and code are publicly available at: \href{https://github.com/Cheng-Hanyu/CLASS_reio_gpr}{github.com/Cheng-Hanyu/CLASS\_reio\_gpr}.
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