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

TL;DR

The paper presents the LSST DESC Science Requirements Document (SRD) v1, quantifying the dark energy constraining power of five probes (weak and strong lensing, large-scale structure, galaxy clusters, and supernovae) and outlining a forecast-driven framework to translate that power into concrete analysis-pipeline requirements. It introduces a two-class taxonomy of systematic uncertainties—self-calibrated and calibratable—and allocates an error budget to ensure calibratable systematics remain subdominant, while forecasted FoMs guide progress toward a stand-alone Stage IV program. The document specifies high-level FoM targets (e.g., >500 jointly, with robust standalone viability) and provides detailed, probe-specific requirements for redshift, photometry, and shear calibrations across Year 1 and Year 10 scenarios, including blinding mandates. It also discusses software validation against external tools, the role of Stage III priors, and anticipated future enhancements across all probes as software and ancillary datasets evolve. Overall, the SRD establishes a concrete path to achieving and validating LSST DESC’s ambitious dark energy science goals, with transparent budgeting and a clear plan for software and methodological improvements over time.

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.

Figures (15)

• Figure :
• Figure C1: $95\%$ confidence ellipses for the supernova science case as a function of prior on $\Omega_m$. For low-matter universes, the dark energy equation of state lies along the positive $w_0-w_a$ degeracy direction, however a tight prior around the fiducial matter density focuses and rotates the dark energy equation of state parameters along the degeneracy direction for acceleration.
• Figure D2: The positions of the first four BAO peaks as a function of redshift are shown in color, while the horizontal lines show the centers of our $\ell$ bins in this DESC SRD version. In order to Nyquist sample, we need two $\ell$ bins (i.e., horizontal lines) between each colored line. A finer binning, and possibly linear rather than logarithmic $\ell$ binning, would yield a more optimal measurement of the BAO feature from the galaxy angular power spectrum.
• Figure D3: Diagram indicating sources of systematic uncertainty for the LSS analysis on which we would like to place requirements in the DESC SRD. The direction of the arrows indicates the flow from overall systematic uncertainty to broad systematics categories to the specific physical effects on which we place requirements. As shown, there are several issues that contribute to both redshift and number density uncertainty. The green / lavender boxes indicate sources of uncertainty on which we do / do not place requirements in this DESC SRD version, respectively.
• Figure D4: Diagram indicating sources of systematic uncertainty for the WL analysis on which we would like to place requirements in the DESC SRD. The direction of the arrows indicates the flow from overall systematic uncertainty to broad systematics categories to the specific physical effects on which we place requirements. As shown, there are several low-level issues in the right-hand column that contribute to multiple categories of uncertainty in the middle column. The green / lavender boxes indicate sources of uncertainty on which we do / do not place requirements in this DESC SRD version, respectively. The cyan boxes indicate those for which more R&D beyond the DESC's DC2 may be needed in order to place requirements. For some of the green boxes, we currently only have software infrastructure to place requirements through their impact on one class of uncertainty; such connections are shown as green arrows.