Dark Energy Survey Year 3 results: Marginalisation over redshift distribution uncertainties using ranking of discrete realisations

TL;DR

This paper introduces hyperrank, a novel method for marginalising redshift distribution uncertainties in weak lensing surveys, improving accuracy and computational efficiency over previous parametric models, demonstrated with DES Year 3 data.

Contribution

The paper presents hyperrank, a new non-parametric approach for marginalising over redshift uncertainties using discrete samples, enhancing flexibility and efficiency in cosmological analyses.

Findings

Hyperrank accurately marginalises over redshift uncertainties in simulations.

The method improves computational efficiency compared to traditional models.

Validation shows hyperrank's results are consistent with simpler marginalisation techniques.

Abstract

Cosmological information from weak lensing surveys is maximised by dividing source galaxies into tomographic sub-samples for which the redshift distributions are estimated. Uncertainties on these redshift distributions must be correctly propagated into the cosmological results. We present hyperrank, a new method for marginalising over redshift distribution uncertainties in cosmological analyses, using discrete samples from the space of all possible redshift distributions. This is demonstrated in contrast to previous highly simplified parametric models of the redshift distribution uncertainty. In hyperrank the set of proposed redshift distributions is ranked according to a small (in this work between one and four) number of summary values, which are then sampled along with other nuisance parameters and cosmological parameters in the Monte Carlo chain used for inference. This can be regarded as a general method for marginalising over discrete realisations of data vector variation with nuisance parameters, which can consequently be sampled separately to the main parameters of interest, allowing for increased computational efficiency. We focus on the case of weak lensing cosmic shear analyses and demonstrate our method using simulations made for the Dark Energy Survey (DES). We show the method can correctly and efficiently marginalise over a range of models for the redshift distribution uncertainty. Finally, we compare hyperrank to the common mean-shifting method of marginalising over redshift uncertainty, validating that this simpler model is sufficient for use in the DES Year 3 cosmology results presented in companion papers.