Light from quasar J0744+2052 traveled to us for almost 12 billion years. Along the way, it bumped into hydrogen clouds that snatched individual lines from the spectrum — like a comb sifting through delicate frequencies. Astronomers call this pattern the Lyman-alpha forest in spectroscopy. But within this apparent chaos, order unexpectedly emerged. Like an echo of a long-silenced explosion, a characteristic scale — about 30 kiloparsecs, the diameter of the Milky Way — froze into the distribution of lines.

The event itself resembled a thunderclap: the rapid expansion of ionized bubbles — at a speed of about 10 km/s — pushed the plasma, compressing it to scales of 40 kiloparsecs. After the front passed, pressure gradients evened out the density, leaving a “memory” of the thermal shock. This pressure smoothing scale is the acoustic imprint of reionization. In the numerical simulation Sherwood-Relics, where the end time of reionization and the amount of injected energy were varied, researchers saw a distinct peak in the power spectrum of peculiar gas velocities. That is exactly the peak they looked for in real data.

The key to the discovery was the analysis of the quasar flux spectrum from Boera et al. (2019) at redshift 4.2–5.0. Applying an empirical model, they extracted the peak position and obtained direct estimates of the smoothing scale: at z=4.2 it exceeds 33.78 kiloparsecs, at z=4.6 it is 32.36 ± 8.35 kiloparsecs, and at z=5.0 — 31.27 ± 18.28 kiloparsecs. The numbers matched indirect predictions from theory and simulations strikingly well.

Previously, the thermal history of reionization was inferred indirectly — from the general suppression of small-scale fluctuations in the spectrum. Now we have a concrete acoustic feature in our hands, an independent tuning fork that will allow us to calibrate the entire analysis of the Lyman-alpha forest. This sharply reduces uncertainties in the search for light particles of dark matter: their own influence on small scales was hard to separate from thermal effects.

The early universe rang out — and its reverberation, imprinted in absorption lines, is only beginning to tell us about the first billion years. Hearing this echo directly, measuring its loudness and pitch, means holding the key to one of the darkest epochs. New spectroscopic surveys like DESI and WEAVE promise to boost precision by factors of tens, and then the cacophony of the first bubbles will turn into a clear signal.