The LSST Dark Energy Science Collaboration (DESC) Science Requirements Document

Abstract: The Large Synoptic Survey Telescope (LSST) Dark Energy Science Collaboration (DESC) will use five cosmological probes: galaxy clusters, large scale structure, supernovae, strong lensing, and weak lensing. This Science Requirements Document (SRD) quantifies the expected dark energy constraining power of these probes individually and together, with conservative assumptions about analysis methodology and follow-up observational resources based on our current understanding and the expected evolution within the field in the coming years. We then define requirements on analysis pipelines that will enable us to achieve our goal of carrying out a dark energy analysis consistent with the Dark Energy Task Force definition of a Stage IV dark energy experiment. This is achieved through a forecasting process that incorporates the flowdown to detailed requirements on multiple sources of systematic uncertainty. Future versions of this document will include evolution in our software capabilities and analysis plans along with updates to the LSST survey strategy.

Overview of the LSST Dark Energy Science Collaboration Science Requirements Document

The LSST Dark Energy Science Collaboration (DESC) aims to facilitate dark energy analysis through the LSST, emphasizing five primary probes: weak lensing (WL), cluster lensing (CL), large-scale structure (LSS), strong lensing (SL), and supernovae (SN). This document, serving as a Science Requirements Document (SRD), focuses on defining the methodological and performance prerequisites necessary to achieve high precision in measurements of dark energy parameters using LSST data.

Research Objectives and Methodology

The DESC seeks to achieve a comprehensive cosmological analysis that meets the standards of a Stage IV dark energy program independently from other experiments. The research is aligned with the Dark Energy Task Force (DETF) standards, which serve as benchmarks for improvements over previous-era analyses. The emphasis is on providing multiple levels of constraints on dark energy models with distinct dependencies on cosmic growth and geometric parameters. The SRD outlines two classes of systematics: "calibratable," which can be pre-determined and controlled, and "self-calibrated," which are addressed through joint analysis of cosmological and systematic uncertainties.

High-Level Outcomes

The document outlines several high-level requirements:
- A combined figure of merit (FoM) of 500 for the complete dataset, indicating a tenfold enhancement over Stage II analysis, inclusive of both self-calibrated and calibratable uncertainties.
- Substantial improvement over previous benchmarks for each probe individually (e.g., LSS, WL, SN), with a focus on ensuring that no single probe dominates in systematic uncertainty compared to statistical uncertainty.

The overarching analysis involves detailed forecasting of the dark energy equation of state parameters $(w_0, w_a)$ and involves setting specific requirements on systematic uncertainties to prevent them from being a superior noise source relative to statistical uncertainties.

Specific Analysis Techniques

For each cosmological probe:
- LSS and WL analyses involve galaxy clustering measurements and cross-power spectra. Improved linear and nonlinear bias models are vital in these forecasts to understand the breakdown of the power spectra at smaller scales.
- CL studies leverage tomographic approaches incorporating cluster counts and lensing efficiency. The mass-observable relations (MOR) and their uncertainties remain a focal point for these analyses.
- SL and SN analyses require stringent photometric calibration standards. They're currently formulated through Gaussian assumptions and involve significant improvements to light curve modeling and spectroscopic calibration between low-redshift anchors and high-redshift samples.

Computational Techniques

The computational element is handled through various simulation and modeling efforts:
1. CosmoLike is a pivotal tool offering non-Gaussian covariance support, essential for LSS and WL, evaluating nonlinear power effects and intrinsic alignments while accommodating systematic variances in the power spectra.
2. The integration of CosmoSIS for SN and SL forecasts allows for Monte Carlo methods to account for uncertainties in parameter spaces beyond the nominal two-dimensional space, preparing for more intricate marginalizations over diverse nuisance parameters.

Future Directions

Key recommendations include expanding this framework:
- Enhancing observational strategy impacts, with quantifiable cadence simulations, into forecasts.
- Developing consistent model verification for systematic effects across probes.
- Incorporating neutrinos and other possible cosmological extensions in forecast models.
- Cultivating DESC's methodological adaptability to potentially adjust models with new ancillary data and improved theoretical understanding in future document iterations.

In conclusion, the DESC SRD defines a strategic and comprehensive approach to addressing dark energy measurement challenges with LSST data. It outlines methodologies to ensure rigorous standards and continuous improvement in systematic controls and inference methodologies across all cosmological probes, aiming to safeguard and advance the scientific reach of LSST in cosmology.