Cosmological Constraints from Cross-Correlation of Planck CMB lensing and DESI-like Emission-Line Galaxies in Legacy Surveys

Understanding the cause of the cosmic acceleration is one of the outstanding questions in physics. While dark energy is the leading explanation, its exact nature is unknown, and theories such as modified gravity also offer alternative explanations. A complementary probe to studying dark energy and modified gravity theories is via measuring the growth of structure. The high redshift emission-line galaxies (ELGs) in particular trace the growth of structure in an era when the dark energy density was not significant and consequently can serve as an ideal testing ground for modified gravity theories. In this project, we select DESI-like ELGs from the Legacy Surveys DR9 imaging data and cross-correlate their positions with the Planck 2018 CMB lensing map to measure the growth of structure, linear bias, and matter density in tomographic bins 0.6 < z < 1.1 and 1.1 < z < 1.6. We conduct a full 3 X 2 analysis (C_gg, C_κκ, C_κg) and account for imaging systematics in the final cosmological inferences. We further calibrate the ELG photometric redshifts using the DESI Survey Validation dataset. In the talk, I will specifically discuss our final measurements and how the same framework can be used by the upcoming galaxy and CMB lensing surveys to provide robust cosmological constraints.