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For the first few hundred million years after the Big Bang, the universe was dark: there were no stars yet, and space was filled with cold hydrogen. This gas emits radio waves at a wavelength of 21 centimeters — like a quiet whisper that reaches us across 13 billion years. But nearby, other sources blare: galaxies, remnants of exploded stars, and instrument noise. They are thousands of times louder and drown out the hydrogen.

To make out the whisper, astronomers compare it with a map of galaxies. Galaxies and hydrogen gather in the same places — where there is more matter. Usually, the correlation is sought in pairs: you take two points and see how similar they are. But the hydrogen signal is more complex: its brightness varies in patches that cannot be described by simple pairs. The new method, called kNN CDF, looks not at pairs but at whole constellations of neighbors — how many nearest points surround each one. It's like comparing not two voices but an entire choir.

Tests on simulated data showed that the new approach detects the correlation even when interference is strong, while the old method fails. Moreover, it distinguishes between scenarios of how the first stars ignited and ionized the gas. At the same fraction of ionized hydrogen, two models can give identical pairwise correlations but different neighbor patterns. This means we can learn not only 'when' but also 'how' the universe emerged from darkness.

So the whisper of the first stars becomes more audible. By comparing it with a map of galaxies in a new way, we can make out details that were previously lost in the noise.