  Cosmic acceleration is the most surprising cosmological discovery in many
decades. Testing and distinguishing among possible explanations requires
cosmological measurements of extremely high precision probing 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) in 2010.
The WFIRST mission 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
describe in this annual report the activities of the Science Investigation Team
(SIT) "Cosmology with the High Latitude Survey (HLS)" during the year 2017.
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 jointly address Galaxy
Redshift Survey, Weak Lensing and Cluster Growth and thus fully embrace the
fact that the imaging and spectroscopic elements of the HLS will be realized as
an integrated observing program, and they jointly impose requirements on
performance and operations. WFIRST is designed to be able to deliver a
definitive result on the origin of cosmic acceleration. It is not optimized for
Figure of Merit sensitivity but for control of systematic uncertainties and for
having multiple techniques each with multiple cross-checks. Our SIT work
focuses on understanding the potential systematics in the WFIRST DE
measurements.
