Cosmology with the WFIRST high latitude survey annual report, May 2017 WFIRST science investigation team galaxy redshift survey (Topic A), and weak lensing and cluster growth (Topic C)

Cosmic acceleration is the most surprising cosmological discovery in many decades. Even the least exotic explanation of this phenomenon requires an energetically dominant component of the universe with properties never previously seen in nature, pervading otherwise empty space, with an energy density that is many orders of magnitude lower than naive expectations. More broadly, the origin could derive from a novel, dynamically-evolving type of matter or, instead, signal deviations from General Relativity on the large scales and low densities probed by cosmological tracers. Testing and distinguishing among possible explanations requires cosmological measurements of extremely high precision that probe the full history of cosmic expansion and structure growth and, ideally, compare and contrast matter and relativistic tracers of the gravity potential. This program is one of the defining objectives of the Wide-Field Infrared Survey Telescope (WFIRST), as set forth in the New Worlds, New Horizons report (NWNH) Council [2010]. The WFIRST mission, as described in the Science Definition Team (SDT) reports [Spergel et al. 2013, 2015, hereafter SDT13 and SDT15 respectively], has the ability to improve these measurements by 1 - 2 orders of magnitude compared to the current state of the art, while simultaneously extending their redshift grasp, greatly improving control of systematic effects, and taking a unified approach to multiple probes that provide complementary physical information and cross-checks of cosmological results. We described in this document the activities of the Science Investigation Team (SIT) Cosmology with the High Latitude Survey. This team was selected by NASA in December 2015 in order to address the stringent challenges of the WFIRST dark energy (DE) program through the Project's formulation phase. This SIT has elected to address Galaxy Redshift Survey (GRS), Weak Lensing (WL) and Cluster Growth (CL) of the WFIRST Science Investigation Team (SIT) NASA Research Announcement (NRA) with a unified team, because the two investigations are tightly linked at both the technical level and the theoretical modeling level. Our team thus fully embrace the fact that the imaging and spectroscopic elements of the High Latitude Survey (HLS) will be realized as an integrated observing program, and they jointly impose requirements on instrument and telescope performance, operations, and data transfer. We also naturally acknowledge that the methods for simulating and interpreting weak lensing and galaxy clustering observations largely overlap. Many members of our team have expertise in both areas. WFIRST is designed to be able to deliver a definitive result on the origin of cosmic acceleration. If the growth rate of structure is inconsistent with the evolution of the Hubble constant, this would be the signature of the breakdown of General Relativity on cosmological scales. If the evolution of the Hubble constant is consistent with the growth rate of structure but inconsistent with vacuum energy, then this would imply that dark energy is dynamical. Either result would have a profound impact on our understanding of physics. WFIRST is not optimized for Figure of Merit sensitivity but for control of systematic uncertainties in the astronomical measurements and for having multiple techniques each with multiple cross-checks. Our SIT work focuses on understanding the potential systematics in the WFIRST dark energy measurements. In our proposal, we structured our planning around the series of deliverables described in §2. We will present in this detailed report our progress on these deliverables and illustrate that we either reached or exceeded our proposed expected milestones. Because the development of the science requirements is at the core of our proposed investigation, we present some broad aspects of our strategy in §3 before giving a summary of the High Latitude Imaging Survey (HLIS) and of the HLS Spectroscopic Survey (HLSS) science requirements as we formulated them to support the WFIRST Project Office in §4 and §5. We present our revised cosmological forecasts and associated trade studies in §6. We also address questions of survey operations and optimization in §7, our actions towards broad community engagement in §8 and discuss in §9 the other ways in which our SIT supported the WFIRST mission.