Ghost Garden: How Neutrino Showers Sprout Dark Matter Seeds ✛ ✦

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For a long time, cosmology was dominated by the belief, originating from Hawking: during the radiation era, primordial black holes do not grow. The issue lies in the scales: their gravitational radius (Schwarzschild radius) is minuscule — like a pinhead in an ocean — compared to the mean free path of radiation in the hot plasma. Born from quantum ripples predicted by Alan Guth during inflation, these seeds of darkness seemed doomed bystanders — like seeds thrown onto scorching concrete, with no chance to sprout. But fresh analysis uncovers an unexpected garden: the ubiquitous and nearly elusive neutrinos become life-giving moisture for them.

The key shift occurs when the universe cools enough that the neutrino mean free path λν begins to exceed the Schwarzschild radius $r_s$ of the black hole. At this moment, neutrinos stop piercing straight through the hole — they get captured by its gravity. The authors introduce a smooth weighting function W = exp(-κ $r_s$/λν) to describe this transition and compute the mass evolution. For holes with initial masses from 10³ to 10⁵ solar masses, a real feast begins: they increase their mass hundreds of times, deforming the original spectrum.

This growth has far-reaching consequences. First, it boosts the fraction of dark matter locked in PBHs by about 13%, which is significant for future surveys. Second, an additional peak emerges in the mass spectrum in the 'sweet spot' region, while the peak linked to electron-positron annihilation shifts toward larger masses. The distorted profile can now serve as a probe of early-universe physics: it's sensitive to the equation of state during the QCD transition and even to subtle lepton asymmetry effects.

Finally, the work connects the micro- and macrocosmos: the weak interaction of neutrinos, described by the Standard Model, directly influences the formation of large-scale structure. Perhaps the enigmatic 'little red dots' that James Webb sees at the dawn of expansion of the universe are precisely those black holes grown on a neutrino diet. And future gravitational wave detectors, like LIGO and LISA, may catch the mergers of these ancient monsters, testing the hypothesis. Thus the ghostly shower transforms cosmic seeds into giants, rewriting the story of the dark sector.