Boiling milk develops a trembling skin. By watching its movements, you can guess the strength of the fire beneath the pot and the fat content of the milk. In a similar way, astrophysicists peer into the ancient microwave radiation — the oldest light that has reached us from an era when the universe was only 380,000 years old. This radiation is not perfectly smooth: it has ripples, tiny temperature variations. And these ripples preserve a memory of what was happening in the very first moments.
In those times, space was filled with a swarm of light, nearly elusive particles — neutrinos and possibly others not yet discovered. They zipped around at tremendous speeds, collided, scattered, and through their combined energy influenced how the universe cooled and expanded. The imprint of this influence is etched into the ripples of the microwave background. The main task is to measure exactly how many such particles there were. Physicists call this parameter Neff. If it turns out to be slightly more than three, it means there is something beyond the three known types of neutrinos.
The new work focuses on the future experiment CMB-S4 — a network of telescopes that will scan the sky for years in search of these ripples. The authors calculated how accurately different telescope configurations could measure Neff. The best of the configurations considered would achieve an uncertainty of less than 0.03 over seven years of observations. That is enough to reliably detect even one new type of invisible particle. For comparison, they also calculated an ideal, practically unattainable limit — what could be seen if the telescopes captured all possible signal without interference. This shows where there is still room to grow.
Such a precise measurement of Neff is not just a quest for a record. It ties together several eras: the first fractions of a second when particles were born; the first minutes when protons and neutrons began to assemble into the nuclei of light elements; and those 380,000 years when light finally could travel freely through the cooled space. Returning to our pot: by the trembling of the milk skin, you will now be able to tell not only the strength of the fire, but also whether there was a secret ingredient added to the milk at the very beginning.
Neutrinos are so ubiquitous that every second, about 100 billion of these particles from the Sun pass through the nail of your thumb, and you don't even notice.