Probing dark energy with large-scale galaxy clustering: from instrumentation to simulation
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In the standard paradigm of cosmology, everything we observe now originated from initial quantum fluctuations in a small smooth region, which were frozen in during inflation and became primordial density perturbations on large classical scales. Under gravitational collapse, the overdensities seeded the formation of stars and galaxies. Mapping the large-scale structure of the universe at the Cosmic Frontier is a promising experimental avenue which will address in the next decade several pressing open questions in cosmology and particle physics, most notably the accelerating cosmic expansion. The observed distribution of galaxies and quasars traces the underlying matter density field and contains a wealth of information from signatures of primordial conditions to the background evolution rate. The Dark Energy Spectroscopic Instrument (DESI) is a next-generation, Stage IV dark energy experiment that will measure the expansion history of the universe through baryon acoustic oscillations and the growth of structure through redshift-space distortions with unprecedented precision. Ground-based at the Kitt Peak National Observatory, DESI features a new 8 deg² field-of-view corrector, 5000 robotically-actuated fibre positioners, and ten fibre-fed spectrographs. The 5-year survey beginning in 2020 will measure the spectra of 35 million galaxies and quasars up to redshift z ~ 3.5 in the 360 nm to 980 nm wavelength range, covering 14000 deg² of the sky. With an order of magnitude improvement over previous redshift surveys, DESI will place tight constraints on the dark energy equation of state, modified gravity, the existence of extra light species, neutrino masses, and models of inflation. ProtoDESI was a proof of concept commissioned in 2016 to mitigate the risks associated with DESI's challenging instrument design and precision requirements. Its simplified focal plane instrument housed 3 fibre positioners and a fibre photometry camera in place of spectrographs. ProtoDESI was successful as the first on-sky technology demonstration for DESI. For the official DESI focal plane instrument, the fibre positioning accuracy and, ultimately, the success of DESI, are grounded upon the stringent specifications of the focal plate structure (FPS) which directly holds the positioners. The FPS parts, consisting of ten focal plate petals (FPPs) and a focal plate ring, were fabricated with the required tolerances, comprehensively inspected, and aligned with appropriate shims and gauge blocks to ensure minimal loss of photons at the fibre tips. Adopting a coordinate measurement machine-based approach, we projected the fibre injection efficiency by measuring hardware features and modelling geometric transformations and fibre optics. The as-aligned, total root-mean-square optical throughput for 6168 positioner holes of 12 production FPPs (including two spares) is 99.88% ± 0.12%, well above the 99.5% project requirement. Finally, observations of galaxy clustering cannot be properly understood alone without accompanying theoretical motivations and numerical simulations in parallel. Cosmological N-body simulations have become indispensable for designing survey strategies, developing analysis methods, and making theoretical predictions. We quantify the shifts of the acoustic scale potentially resulting from galaxy clustering bias, which constitutes an increasingly significant source of theoretical systematics in distance measurements with the standard ruler. Utilising mock catalogues based on generalised halo occupation population of high-accuracy Abacus simulations in the largest volume to date for such tests, 48h⁻¹Gpc³, we find a 0.3% shift in the line-of-sight acoustic scale for one variation in the satellite galaxy population and a 0.7% shift for an extreme level of velocity bias of the central galaxies, while other models tested are consistent with zero shift at the 0.2% level after reconstruction. We note that these bias models produce sizeable and likely distinguishable changes at small scales that correlate with the shifts.
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