Theory 02/Physics, Cosmos & Matter /Cosmology
Dispersion Measures Count Filaments
Dispersion is attributed to the host. Some of it may be the web between.
Excess dispersion in fast radio bursts measures the filling factor of the cosmic web's filaments rather than the host galaxy's contribution, so bursts at the same redshift should show a spread set by sightline filament crossing.
01The evidence
Dispersion measures exceed what host galaxies can plausibly supply, and the excess varies widely between bursts at similar redshift.
02The tension
Models assign the excess to host galaxy halos, with a wide prior. The prior absorbs the spread rather than explaining it.
03The mechanism
A sightline crosses a variable number of filaments, each containing warm plasma. The free electron column therefore varies with the geometry of the crossing rather than with the endpoint.
That makes the spread a map of the web, and predicts bursts whose sightlines cross more filaments should show larger excess at fixed redshift.
04What this is not
Those models put the excess at the source. This puts it along the path, which explains the spread at fixed redshift.
05Predictions
- P1Excess dispersion should correlate with sightline filament count.
- P2Bursts at equal redshift should spread.
- P3The spread should be anisotropic with the web.
06Refutation
Excess dispersion uncorrelated with sightline structure would restore host attribution.
07What it changes
It would make bursts a cosmic web probe.