ON POSSIBLE VARIATION IN THE COSMOLOGICAL BARYON FRACTION

The fraction of matter that is in the form of baryons or dark matter could have spatial fluctuations in the form of baryon-dark matter isocurvature fluctuations. We use big bang nucleosynthesis calculations compared with observed light-element abundances as well as galaxy cluster gas fractions to constrain cosmological variations in the baryon fraction. Light-element abundances constrain spatial variations to be less than 26%-27%, while a sample of relaxed galaxy clusters shows spatial variations in gas fractions less than 8%. Larger spatial variations could cause differential screening of the primary cosmic microwave background (CMB) anisotropies, leading to asymmetries in the fluctuations, and ease some tension with the halo-star Li-7 abundance. We also show that fluctuations within our allowed bounds can lead to B-mode CMB polarization anisotropies at a non-negligible level.