Probing the Physics of Dark Energy with LSST: Large Scale Structure and Supernovae

The Large Synoptic Survey Telescope (LSST) will make a huge leap forward in understanding the mysterious 'dark energy' that drives the accelerating Universe. Its unprecedented combination of sky area and depth allows for multiple, independent probes to address fundamental questions: Is dark energy consistent with Einstein's cosmological constant? Does it change with space or time? Does modified gravity provide a better explanation of cosmic acceleration? The LSST design is advanced but leaves significant freedom in the cadence of when a given field on the sky will be observed in each filter as well as the specific dithering pattern used to obtain more uniform sky coverage. Because the LSST Project plans to finalize this survey strategy in the next few years, there is a golden but short-lived opportunity to optimize it for dark energy studies. The P.I.s currently serve as co-convenors for the Large Scale Structure and Supernova Analysis Working Groups of the LSST Dark Energy Science Collaboration (DESC), respectively, and our activities will involve coordination of those working groups as well as research on specific Key Projects identified in the DESC 2015 Science Roadmap, as summarized below. We propose the following program of experimental high energy physics research at the Cosmic Frontier to be conducted as part of LSST DESC:

Overview of PI Gawiser's Research Plan: We will test dithering strategies (both the timescale and the pattern of telescope pointing shifts) recommended by our previous work on the next round of LSST Image Simulations to measure resulting systematics for galaxy colors, large-scale structure, weak lensing, and galaxy clusters. These systematics will then be imprinted in full-sky catalog-level simulations and used to improve the LSST DESC analysis pipeline with the dual goals of removing systematics and accounting for residual systematics in the error budget on cosmological parameters. Deep Drilling Fields will be simulated at the catalog level to explore their capability to ameliorate systematic errors at main survey depth. We will then recommend to the LSST Project observing strategies for both the main survey and the Deep Drilling Fields that are optimized for dark energy studies.

Overview of Co-PI Jha's Research Plan: We will study the LSST cadence and observing strategy for the main survey and deep-drilling fields to optimize the scientific return on dark energy physics from LSST observations of type-Ia supernovae (SN Ia). We also propose to develop the next generation of SN Ia light-curve fitters, software tools that turn observed photometry into cosmological distances necessary for understanding the nature of dark energy. We will make fundamental improvements in the methods used, incorporating new results regarding SN Ia intrinsic color variations, reddening/extinction by dust, and environmental dependencies to SN Ia luminosity, building upon a training set based on new, large SN Ia samples. The tools developed will be critical in reducing astrophysical systematic uncertainties that would otherwise limit the precision and accuracy of dark energy parameter constraints from LSST supernova cosmology.