In the 2010s, physicists working at the Large Hadron Collider learned to create in the lab a substance that doesn't exist in the ordinary world. When two lead nuclei, accelerated to nearly the speed of light, collide, a quark-gluon plasma is born for an instant — a 'soup' of the smallest known particles of matter that filled the Universe during the first microsecond after the Big Bang. This soup is incredibly hot — hundreds of thousands of times hotter than the center of the Sun.

When a jet of particles (a narrow stream flying with enormous energy) is born in such a soup, it behaves like a motorboat on water. The boat pushes water aside, leaving behind diverging waves and a zone of frothy, churned-up water. In the same way, a jet plowing through the plasma pushes matter sideways and forward, and behind it forms a region where there are fewer particles than before the jet passed. Physicists call this zone the diffusion wake.

For a long time, this wake was sought in the data, but the signal was weak and drowned in noise. Now the CMS collaboration at the Large Hadron Collider has collected data from thousands of lead-lead collisions and compared them with proton-proton collisions, where there is no plasma. It turned out that behind the jet in the plasma, a noticeable dip in particle number indeed forms — the statistical significance exceeded five standard deviations. That's the threshold beyond which physicists speak of a discovery, not a random fluctuation.

The new measurement is important not only as confirmation of a beautiful picture. It provides a way to 'probe' the properties of the quark-gluon plasma: how viscous it is, how quickly it transfers energy, how it reacts to a particle passing through it. Comparison with theoretical models showed that without accounting for the diffusion wake, it's impossible to describe the data. This means the plasma behaves not like a dilute gas, but like a liquid with very low viscosity — an almost perfect liquid, similar to what might have existed in the early Universe.

Now that the wake has been reliably detected, physicists can use it as a tool: vary the jet energy, see how the wake changes, and from that reconstruct a detailed picture of what happened in the first instants after the Big Bang. The boat leaves behind not just waves: it leaves a map of the depths.