THE OVERARCHING OBJECTIVE FOR THE PROPOSED RESEARCH IS TO ENABLE AS MUCH INFORMATION AS POSSIBLE TO BE EXTRACTED FROM THE EUCLID AND WFIRST MISSION DATA TO REVEAL TANTALIZING SIGNATURES OF GRAVITY AND DARK SECTOR PHYSICS. THIS WILL BE ACHIEVED THROUGH THE DEVELOPMENT REFINEMENT AND APPLICATION OF TOOLS AND TECHNIQUES THAT EXTEND BEYOND THE CORE POWER SPECTRUM STATISTICS AND PRIMARY OBSERVATIONAL TRACERS AND ARE TUNED TO REVEAL THE COMPLEX PHENOMENOLOGY EXPECTED IN DARK SECTOR THEORIES INCLUDING WELL-MOTIVATED MODIFIED GRAVITY MODELS AND MASSIVE NEUTRINOS. THE PROPOSED WORK FOCUSES ON TOPICS UNDER TWO THEMES. SOME PROPOSED TOPICS FOCUS DIRECTLY ON THE STAND-ALONE SCIENCE RETURN FOR EUCLID AND WFIRST WHILE OTHERS ARE CENTERED ON LEVERAGING THE ADDITIONAL SCIENCE INFORMATION FROM COMBINING THE MISSION DATA WITH THAT FROM PLANCK AND CURRENT AND UPCOMING CMB EXPERIMENTS. THE FIRST THEME "REALIZING THE POTENTIAL OF THE KINETIC SUNYAEV ZEL'DOVICH (KSZ) EFFECT AS A DARK SECTOR PROBE" CENTERS ON REFINING KSZ EXTRACTION METHODS TO MAXIMIZE ITS USE WITH EUCLID AND WFIRST SPECTROSCOPIC AND PHOTOMETRIC GALAXY SAMPLES IN COMBINATION WITH UPCOMING CMB SURVEYS. TOPICS INCLUDE: QUANTIFYING AND MITIGATING SYSTEMATIC UNCERTAINTIES INTRODUCED BY CLUSTER MIS-CENTERING AND OPTICAL DEPTH MEASUREMENTS; EXTRACTING THE KSZ FROM LARGE SCALE STRUCTURE MORE GENERALLY NOT JUST FROM CLUSTERS AND; DEVELOPING REALISTIC PREDICTIONS FOR MODIFIED GRAVITY CONSTRAINTS WITH SYSTEMATICS ACCURATELY INCORPORATED. TOPICS IN THE SECOND THEME "UNSCREENING MODIFIED GRAVITY WITH DIVERSE STATISTICS AND TRACERS" EXPAND AND HONE THE SUITE OF DENSITY-SENSITIVE STATISTICS AND ASTROPHYSICAL TRACERS TO SAMPLE THE PROPERTIES OF GRAVITY IN A VARIETY OF ENVIRONMENTS. GRADUATE STUDENTS INVOLVED IN THE RESEARCH WILL DEVELOP A VARIETY OF THEORETICAL ANALYTICAL AND NUMERICAL SKILLS: CHARACTERIZING AND DISTINGUISHING MODIFIED GRAVITY THEORIES; BUILDING ADAPTING AND USING N-BODY AND HYDRO-DYNAMICAL SIMULATIONS; AND USING ANALYTICAL AND STATISTICAL SKILLS TO COMPARE THEORETICAL PREDICTIONS TO SIMULATED AND REAL OBSERVATIONS INCLUDING BEING ABLE TO MODEL AND INCORPORATE PRAGMATIC EXPERIMENTAL ERROR ANALYSES. THE PROPOSED RESEARCH DIRECTLY SUPPORTS THE PRIORITIES LAID OUT IN THE NASA ASTROPHYSICS ROADMAP "ENDURING QUESTS DARING VISIONS" IN ANSWERING ONE OF THE THREE DEFINING QUESTIONS "HOW DOES THE UNIVERSE WORK?" AND ENABLE NASA MISSIONS TO BROADEN THE WAYS IN WHICH THEY "PROBE THE ORIGIN AND ULTIMATE FATE OF THE UNIVERSE AND DETERMINE THE FORMS OF MATTER AND ENERGY THAT GOVERN IT BY MAPPING THE GROWTH OF COSMIC STRUCTURE THROUGH ITS HISTORY." THE GOALS OF THE PROPOSED RESEARCH ARE ESPECIALLY TIMELY GIVEN THAT DURING THE PROPOSAL TIME-FRAME THE EUCLID SATELLITE WILL LAUNCH AND WFIRST WILL BE MAKING CRITICAL DESIGN AND SURVEY CHOICES INFORMED BY THEIR IMPACT ON THE SCIENCE POTENTIAL. THE OUTCOMES OF THIS PROPOSED WORK COULD DIRECTLY FEED INTO EUCLID AND WFIRST COSMOLOGICAL PIPELINE DEVELOPMENT TO ENSURE THE MISSION DATA IS FULLY UTILIZED TO UNCOVER DISTINCTIVE FACETS OF THE DARK SECTOR.