Can small-scale baryon inhomogeneities resolve the Hubble tension? An investigation with ACT DR4

Small-scale inhomogeneities in the baryon density around recombination have been proposed as a solution to the tension between local and global determinations of the Hubble constant. These baryon clumping models make distinct predictions for the cosmic microwave background anisotropy power spectra on small angular scales. We use recent data from the Atacama Cosmology Telescope to test these predictions. No evidence for baryon clumping is found, assuming a range of parametrizations for time-independent baryon density probability distribution functions. The inferred Hubble constant remains in significant tension with the SHOES measurement.

Automated summary

Small-scale inhomogeneities in baryon density around recombination have been suggested as a solution to discrepancies in Hubble constant determinations. Predictions from baryon clumping models were tested using data from the Atacama Cosmology Telescope, but no evidence of baryon clumping was found. The inferred Hubble constant remains significantly different from measurements by SHOES.