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Standard rulers such as the baryon acoustic oscillation (BAO) scale serve asworkhorses for precision tests of cosmology, enabling distance measurementsthat probe the geometry and expansion history of our Universe. Aside from BAOmeasurements from the cosmic microwave background (CMB), most standard rulertechniques operate at relatively low redshifts and depend on biased tracers ofthe matter density field. In a companion paper, we explored the scientificreach of nulling estimators, where CMB lensing convergence maps arecross-correlated with linear combinations of similar maps from line intensitymapping (LIM) to precisely null out the low-redshift contributions to CMBlensing. We showed that nulling estimators can be used to constrain the highredshift matter power spectrum and showed that this spectrum exhibitsdiscernible BAO features. Here we propose using these features as a standardruler at high redshifts that does not rely on biased tracers. Forecasting sucha measurement at $z \sim 5$, we find that next-generation instruments will beable to constrain the BAO scale to percent-level precision at $7.2 \%$, whileour futuristic observing scenario can constrain the BAO scale to $4\%$precision. This constitutes a fundamentally new kind of BAO measurement duringearly epochs in our cosmic history.

A New Probe of the High-z BAO scale: BAO tomography With CMB $\times$ LIM-Nulling Convergence