A tiny black hole the size of an atomic nucleus but with the mass of a mountain emerged in the first moments after the Big Bang, when random clumps of energy collapsed into such objects. If enough of these holes existed, they could have temporarily become the universe's main content, then evaporated via Hawking radiation — the slow 'melting' of a black hole as it releases energy back into space.
But a black hole doesn't hang in a vacuum. It pulls in dust and gas like a whirlpool dragging in leaves. This is accretion — matter falling onto the hole. The new analysis shows that such 'feeding' not only inflates the hole but also stretches its reign. The universe stays under the sway of these invisible giants-in-miniature for longer.
When the holes finally vanish, their departure is not silent. The abrupt change in the gravitational field generates gravitational waves — ripples in spacetime itself, like the rings from a stone tossed into a pond. These waves, arising from density inhomogeneities, are called isocurvature. Their energy is limited by what we know about primordial nucleosynthesis — the epoch when the first atomic nuclei formed. Waves that are too strong would have disrupted this process.
Taking accretion into account significantly shifts the allowed bounds for primordial black holes: they must be lighter and rarer than previously thought. Even if black holes merge with each other to form larger ones, the constraints remain strict. Isocurvature gravitational waves turn out to be a more sensitive probe than the mergers themselves.
So tiny black holes, barely born, manage to gain weight and delay their finale. And we, observing the ripple of space, can peer into that epoch when the universe was younger than a second.
as written by the authors arXiv:2605.15772
In this work, we study the effects of accretion on the primordial black hole (PBH) reheating scenario. PBHs could form from primordial fluctuations. If they have the right mass and abundance, they could dominate the Universe and complete the reheating entirely through Hawking radiation. We find accretion effects on the BH can not only increase the BH mass, but also prolong such early matter domination. The consequence of the accretion is further investigated using isocurvature induced gravitational waves (GWs), which are generated right after the sudden evaporation of the BHs from the oscillation of the gravitational potential. Big Bang nucleosynthesis limits on the energy density of the GWs put important constraints on the PBH domination scenario. Inclusion of accretion shifts such constraints significantly towards smaller formation mass and smaller initial abundance. Furthermore, the PBH could undergo mergers leading to extended mass functions. We find similar shifts in the allowed parameters with the inclusion of accretion for the merger constraint. We find the constraints from isocurvature GWs typically stronger than the constraints from mergers.
A primordial black hole with a mass of 10¹⁵ grams (like a small asteroid) has the size of a proton, and its temperature is billions of times hotter than the Sun's core: it would shine brighter than a star, but only in gamma rays.
In Terry Pratchett's novel 'The Pillars of Creation', tiny black holes were used as an energy source — much like the scenario of heating the early universe.