On Possible Variation in the Cosmological Baryon Fraction

TL;DR

This paper investigates potential spatial variations in the cosmological baryon fraction using nucleosynthesis and galaxy cluster data, exploring implications for CMB anisotropies and primordial element abundances.

Contribution

It constrains the extent of baryon fraction fluctuations and discusses their effects on cosmic microwave background and primordial element observations.

Findings

Baryon fraction variations are constrained to less than 27% by nucleosynthesis.

Galaxy cluster data limit spatial variations to under 8%.

Possible fluctuations could generate detectable B-mode polarization in the CMB.

Abstract

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 anisotropies, leading to asymmetries in the fluctuations and ease some tension with the halo-star 7Li abundance. Fluctuations within our allowed bounds can lead to "B-mode" CMB polarization anisotropies at a non-negligible level.