Fiducial-Cosmology-dependent systematics for the DESI 2024 BAO Analysis

TL;DR

This paper assesses how the choice of fiducial cosmology affects BAO measurements in the DESI survey, finding a small systematic impact of about 0.1% on key parameters, through analysis of mock catalogues and real data.

Contribution

It introduces a comprehensive evaluation of fiducial cosmology dependence in BAO analysis, quantifying its systematic effect on DESI measurements using mock catalogues and real data.

Findings

Fiducial cosmology choice impacts BAO measurements by approximately 0.1%.

Systematic errors are consistent across different galaxy samples and redshift ranges.

The analysis confirms robustness of DESI BAO results against fiducial cosmology assumptions.

Abstract

When measuring the Baryon Acoustic Oscillations (BAO) scale from galaxy surveys, one typically assumes a fiducial cosmology when converting redshift measurements into comoving distances and also when defining input parameters for the reconstruction algorithm. A parameterised template for the model to be fitted is also created based on a (possibly different) fiducial cosmology. This model reliance can be considered a form of data compression, and the data is then analysed allowing that the true answer is different from the fiducial cosmology assumed. In this study, we evaluate the impact of the fiducial cosmology assumed in the BAO analysis of the Dark Energy Spectroscopic Instrument (DESI) survey Data Release 1 (DR1) on the final measurements in DESI 2024 III. We utilise a suite of mock galaxy catalogues with survey realism that mirrors the DESI DR1 tracers: the bright galaxy sample (BGS), the luminous red galaxies (LRG), the emission line galaxies (ELG) and the quasars (QSO), spanning a redshift range from 0.1 to 2.1. We compare the four secondary AbacusSummit cosmologies against DESI's fiducial cosmology (Planck 2018). The secondary cosmologies explored include a lower cold dark matter density, a thawing dark energy universe, a higher number of effective species, and a lower amplitude of matter clustering. The mocks are processed through the BAO pipeline by consistently iterating the grid, template, and reconstruction reference cosmologies. We determine a conservative systematic contribution to the error of $0.1\%$ for both the isotropic and anisotropic dilation parameters $\alpha_{\rm iso}$ and $\alpha_{\rm AP}$. We then directly test the impact of the fiducial cosmology on DESI DR1 data.