Line Intensity Mapping Without Cosmic Variance

The current and future generations of line intensity mapping surveys promise dramatic improvements to our understanding of galaxy evolution and large-scale structure. A line intensity map provides a census of the cumulative emission from all galaxies in a given volume, including faint objects that are undetectable individually. Furthermore, cross-correlations between line intensity maps and galaxy redshift surveys allow measurement of galaxy luminosities and cosmic growth without the limitations of cosmic variance. Using the Fisher information matrix, we derive simple expressions describing the sensitivities obtainable by these cross-correlation surveys, focusing on cosmic variance evasion. Based on this analysis, we discuss the landscape of future potential cross-correlations between intensity maps and galaxy redshift surveys. We find the sensitivities of future cross-correlation measurements will be limited primarily by the galaxy number density in galaxy redshift surveys. This conclusion advocates for cross-correlation with extremely deep galaxy redshift surveys, such as those planned for the Nancy Grace Roman Space Telescope. Resulting detections would enable a highly sensitive census of the entire line-emitting galaxy population, resolving mysteries related to large-scale structure and galaxy evolution including the cosmic star formation rate.