FRB-QSO Pairs Could Trace the Missing Baryons: A 2030s HST Window Before HWO

This arXiv preprint (v1, June 2026) is a forward-looking proposal rather than new observations, projecting that ~10^5 arcsecond-localized FRBs at z<1 by 2035 will enable thousands of close pairs with UV-bright QSOs over 20,000 deg². The methodology relies on Monte Carlo forecasts combining current FRB localization rates from next-generation interferometers with existing QSO catalogs, yielding ~100 pairs at θ<1' for joint DM/RM and HST/COS absorption analysis. Sample limitations include dependence on unproven detection yields and the finite HST lifetime. The paper correctly identifies that FRBs supply phase-independent electron columns while QSO spectra resolve ionization states, yet understates synergies with existing CGM datasets such as the COS-Halos survey (Tumlinson et al. 2013, ApJ, 777, 59) and recent CHIME/FRB DM catalogs (CHIME/FRB Collaboration 2021, ApJS, 257, 59). These connections would allow direct partitioning of the cosmic DM budget into CGM, IGM, and halo components on a sightline-by-sightline basis. A decade-long program could constrain feedback energetics and non-thermal pressure support, topics only sparsely addressed in current hydrodynamical simulations. The 2030s timing is critical before the Habitable Worlds Observatory shifts UV priorities.