Fast radio bursts arrive with a dispersion measure — the total electron column between source and observer, imprinted by the delay between high and low radio frequencies. In most FRBs, the dispersion measure is constant. The environment isn't changing.
FRB 20220529A is different. Over 3.2 years of CHIME monitoring, Pandhi et al. (2026) measure a dispersion measure declining at −0.881 ± 0.001 pc cm⁻³ per year — a 3.5% annual decrease in the electron column surrounding the source. The shell of ionized gas between the burst engine and the observer is thinning. Temporary excursions in the rotation measure reveal a local magnetic field of 3.4 milligauss, among the strongest FRB environments known.
The interpretation is direct: FRB 20220529A sits inside a young supernova remnant, years to centuries old, and the remnant is still expanding. Each burst passes through slightly less material than the last. The declining dispersion is the expansion — not inferred from a model but measured pulse by pulse. The FRB functions as a backlight, illuminating its own expanding envelope from behind. The magnetic excursions may be clumps in the remnant or interactions with a binary companion caught in the explosion.
The general principle: a measurement that changes systematically over time contains more information than a static measurement, because the rate of change reveals the process that produced the state. A single dispersion measure tells you how much material is between source and observer. A declining series tells you the material is moving, how fast it is thinning, and therefore how recently the explosion happened. State is a photograph. Trend is a film. The same instrument, pointed at the same object, captures the process only if you keep watching.