2.1 × 10⁻⁹ — roughly the temperature variation in the cosmic microwave background, the ancient light that reached us from the earliest times. If compared to an ocean, it's ripples of fractions of a millimeter high over many kilometers. This tiny unevenness holds the history of the Universe's birth. A fraction of a second after the Big Bang, space began to expand rapidly — scientists call this inflation. The idea was proposed by Alan Guth and Andrei Linde. In those moments, the Universe acted like the most powerful accelerator in history: quantum fields were boosted to incredible energies. Juan Maldacena was the first to propose using this as a 'cosmological collider' — a tool to search for invisible particles.

The main role in inflation was played by a special field — the inflaton. But other fields could have existed alongside it. Their quantum fluctuations under the influence of gravity during expansion should have left traces in the cosmic microwave background. These traces would appear as patterns of slightly warmer and slightly cooler spots. They would disrupt the usual randomness. Such order in disorder — like a fingerprint on glass. If helper particles indeed appeared at the dawn of time, their imprint would show up in how triplets of spots are grouped on the sky.

The analysis for the first time considered strong, not weak, field interactions, going beyond the simple assumptions of the Standard Model (the basic set of particle physics laws). Up to seven different particle exchange channels were taken into account. The key parameter — mixing — indicates how easily the helper particles 'mixed' with the inflaton. Using Bayesian inference (a method comparing data with different theories), predictions were compared with actual Planck satellite data. The improvement in fit was tiny — less than a random fluctuation. Simply put, there is no signal. The upper limit on mixing was about 170 in arbitrary units.

This result strikes down many models that invoked additional fields to explain inflation. It shows that if such fields exist, they are either very light or barely interact. One could say that the universal collider is empty for this range of particles. But the absence of a signal is not a failure. It indicates that nature at the earliest stages is simpler than assumed.

Future telescopes, such as CMB-S4 and the Simons Observatory, will improve sensitivity by a factor of 2–5. But this may not be enough to capture the faint whisper of helper particles. Much more hope lies in maps of radiation from the dark ages, which carry information about millions of independent oscillations. For now, that very 2.1 × 10⁻⁹ recorded in the sky map remains silent about the first trillionth of a second. The universal collider awaits new observers with more sensitive instruments.