Testing Gravity and Cosmology with DESI Plus Weak Lensing

The combination of spectroscopic galaxies and weak lensing is a powerful cosmological probe, allowing for measurement of the power spectrum amplitude, matter growth rate, and constraints on modifications to General Relativity. With 20 million spectra over 14000 deg², the ongoing Dark Energy Spectroscopic Instrument (DESI) survey is ideally situated for these measurements in tandem with overlapping weak lensing surveys from KiDS, DES, HSC and CMB lensing data from Planck. To demonstrate the power of these measurements, we have made the highest-significance detection of galaxy-CMB lensing cross correlations (80-sigma), using Planck CMB lensing and unWISE infrared galaxies produced as part of DESI imaging. DESI will improve further on this measurement by providing precise spectroscopic redshifts, eliminating a major source of systematic error while still maintaining high statistical significance. I will additionally discuss ongoing simulations and preparatory work for DESI cross-correlations with cosmic shear. Finally I will discuss our methodology to ensure precise and accurate cosmological constraints from DESI-lensing cross-correlations, in tandem with the core DESI cosmology results from Baryon Acoustic Oscillations and Redshift Space Distortions.