When Clusters Collide: Constraints On Antimatter On The Largest Scales

TL;DR

This paper reviews observational constraints on antimatter in the universe across various scales, emphasizing that current data strongly limit the presence of antimatter on large cosmic scales, especially through galaxy cluster observations.

Contribution

It compiles and analyzes observational limits from X-ray and gamma-ray data, extending constraints on antimatter to larger scales using colliding galaxy clusters like the Bullet Cluster.

Findings

Antimatter is strongly constrained in galaxy clusters and larger scales.

X-ray and gamma-ray observations limit antimatter fractions to below 10^-6 in clusters.

Colliding clusters provide a method to extend antimatter constraints to scales of about 20 Mpc.

Abstract

Observations have ruled out the presence of significant amounts of antimatter in the Universe on scales ranging from the solar system, to the Galaxy, to groups and clusters of galaxies, and even to distances comparable to the scale of the present horizon. Except for the model-dependent constraints on the largest scales, the most significant upper limits to diffuse antimatter in the Universe are those on the Mpc scale of clusters of galaxies provided by the EGRET upper bounds to annihilation gamma-rays from galaxy clusters whose intra-cluster gas is revealed through its x-ray emission. On the scale of individual clusters of galaxies the upper bounds to the fraction of mixed matter and antimatter for the 55 clusters from a flux-limited x-ray survey range from < 5 x 10^(-9) to < 1 x 10^(-6), strongly suggesting that individual clusters of galaxies are made entirely of matter or, of antimatter. X-ray and gamma-ray observations of colliding clusters of galaxies, such as the Bullet Cluster, permit these constraints to be extended to even larger scales. If the observations of the Bullet Cluster, where the upper bound to the antimatter fraction is found to be < 3 x 10^-6, can be generalized to other colliding clusters of galaxies, cosmologically significant amounts of antimatter will be excluded on scales of order 20 Mpc (5 x 10^(15)M_Sun).