Theory 09/Circumgalactic medium/Radio arrays
Halos Are Denser Than Assumed
The claim
The density of the circumgalactic medium is set by the ratio of dispersion measure to path length through the halo, so fast radio bursts passing through halos measure gas that emission surveys cannot see.
01Mechanism
A radio pulse is delayed in proportion to the free electrons it passes; the delay divided by the path gives the density.
02Prediction
Dispersion should track halo mass and impact parameter.
03Refutation
Dispersion uncorrelated with halo geometry would remove the density inference.
04What makes it checkable
Fast radio bursts provide bright, brief, background sources whose dispersion measures can be decomposed by host and intervening halo.
05Why it was not testable before
The gas is too diffuse to emit detectably; it only becomes visible as a dispersion of something behind it.