Every fast radio burst is like a resounding strike of a bell in the void. And in its fading ring, the entire structure of space is encoded: over billions of light-years, the radio pulse is slightly delayed by every free electron it meets. This delay — the dispersion measure — grows with path length, but until now we didn't know just how many electrons lurk inside the distant galaxy itself. Now, thanks to painstaking analysis of 90 localized FRBs, the picture gains depth. Ionized gas doesn't just exist — it evolves, like a living organism, pulsing to the rhythm of stellar births and deaths.

At the heart of this story lies a simple, almost musical idea: a cosmic echo. The host dispersion measure DM_host is the cumulative reverberation of electron gas that we capture when looking through a galaxy. As redshift z carries us into the past, this echo should change. The parameter $n_z$, the exponent in the law DM_host ∝ (1+z)^$n_z$, works like a volume knob: the higher z, the louder the voice of ancient plasma. In the work based on data from DSA and CRAFT surveys, its value came out as 1.62 — a signal not yet freed from noise, but clearly rising. The echo metaphor helps us feel: the farther back in time, the denser and richer was the plasma of galaxies.

A first glance at the DM vs. z plot reveals the drama: without evolution, most points are mere random noise, but once we introduce a power-law growth, distant bursts at z > 1 obediently align with the predicted curve. These events become the key. If we assume that the bulk of FRB population is linked to star-forming regions — as indicated by their predominant association with young populations — then the index $n_z$ ≈ 1.6 corresponds to an increase in electron density as (1+z)^{2.6}. This rate almost exactly mirrors the cosmic history of star formation. In other words, we hear the fading echo of an era when galaxies seethed with stars. Diffuse ionized gas, not compact HII regions, sets the overall trend. Magnetars, born from short-lived massive stars, fit the picture perfectly: their evolution steps in lockstep with the rate of new star formation.

This approach is more than an elegant touch. It extends into questions that have challenged radio astronomy for decades. Even Karl Jansky, who first heard an extraterrestrial radio signal, and Jocelyn Bell Burnell, who discovered pulsars, laid the foundation on which FRB cosmology now stands. Today, building on Hubble's law as interpreted by Edwin Hubble, we use this evolution to cross-check with numerical simulations such as IllustrisTNG and FIRE-2. Ignoring $n_z$ leads to an overestimation of redshift by Δz≈0.3 at DM around 1000–2000 pc cm⁻³ — a serious error for the search for hidden baryons and the epoch of helium reionization.

The outlook is compelling. When projects like CHIME Outriggers and DSA-2000 begin delivering thousands of new events with precise localizations, and mass-spectroscopy from DESI and EUCLID determines redshift for most of them, we will be able not only to refine $n_z$ but also to break this parameter down into components: separate disk contribution from halo contribution, quiescent galaxies from starbursting ones. Thus FRBs evolve from fleeting cosmic screams into chroniclers of galactic evolution — silent witnesses to how, over billions of years, the matter from which we are made gathered and dispersed. And one day, combining these data with X-ray observations and emission lines, we will assemble a feature-length film about the life of baryons in the Universe. For now, we listen to the echo of millions of suns.