Little Red Dot Cosmology: A Matter-Era Baryon Acoustic Oscillations Probe of CDM

The discovery of Little Red Dots (LRDs) with JWST provides a new source population for cosmology in a largely unmapped period of cosmic history. LRDs are most numerous at 4 z 9, thereby bridging existing low-redshift large-scale-structure measurements and the cosmic microwave background. In this epoch, the Universe is deep in the matter-dominated era, where dark energy is dynamically negligible and the expansion history is tightly predicted in Λ CDM, making these redshifts a clean test of the standard cosmological model. Recent measurements indicate that LRDs have number densities of n about10-4 h cubed Mpc-3, a bias rising from b about 3 to about8 over this interval, and distinctive spectrophotometric signatures -- a remarkably favorable combination for baryon acoustic oscillation (BAO) measurements. Using a Fisher forecast with these fiducial inputs, we find that a wide-field spectroscopic survey of LRDs over a DESI-like (14,000 deg squared) footprint delivers a percent-level isotropic distance scale measurement of D V/r d across four bins spanning 4 z 9. LRDs are therefore a compelling target for future wide-field near- to mid-infrared spectroscopy aimed at mapping large-scale structure deep into the matter-dominated era.