New probe of the high-redshift Universe: Nulling CMB lensing with interloper-free line intensity mapping pair lensing

Intermediate redshifts between galaxy surveys and the cosmic microwave background (CMB) remain unexplored territory. Line intensity mapping (LIM) offers a way to probe $z\gtrsim 1$ Universe, including epoch of reionization dark ages. Via exact nulling lensing kernel, we show that LIM lensing, in combination with (resp., CMB) can uniquely pre-reionization) Universe. However, foregrounds are key hurdle this futuristic technique. While continuum be controlled by discarding modes perpendicular line sight (low $k_\parallel$ modes), interloper haven't been addressed context lensing. In paper, quantify bias for first time, derive ''LIM-pair'' estimator which avoids it exactly after cross-correlating CMB This new quadratic works combining two maps different lines, from same redshift, whose interlopers uncorrelated. As result, foreground avoidance method is robust even large changes amplitude power non-Gaussianity. The cross-spectrum LIM-pair thus currently theoretical uncertainties modeling at high redshift.