Revisiting Primordial Black Hole Accretion, Evaporation, and Cosmological Consequences
Primordial black holes occupy a peculiar place in cosmology. Unlike the supermassive black holes that anchor galaxies or the stellar-mass black holes born from collapsing stars, PBHs would have formed in the first chaotic fractions of a second after the Big Bang. Dense pockets of energy in the primordial plasma could have collapsed directly into black holes, producing objects spanning an enormous range of masses, from microbe-sized specks lighter than a mountain to beasts rivaling the Sun. Because they formed before nucleosynthesis and are non-baryonic by nature, they have long been considered a candidate for dark matter.
But proving that PBHs actually exist, let alone that they make up a significant fraction of the Universe’s missing mass, requires understanding two opposing forces that govern their evolution: accretion, which makes them grow, and Hawking evaporation, which makes them shrink. A new paper by Jitumani Kalita and Debaprasad Maity at the Indian Institute of Technology Guwahati, published in the Journal of Cosmology and Astroparticle Physics (JCAP 07, 2026), revisits this competition with a level of rigor that has been missing from the literature. Their key innovation is a fully general-relativistic treatment of accretion onto spinning (Kerr) black holes embedded in the radiation-dominated early universe.
The accretion-evaporation balance
Hawking radiation, first predicted by Stephen Hawking in 1974, causes black holes to emit particles from just outside their event horizons. The rate of emission is inversely proportional to the black hole’s mass squared, meaning smaller black holes evaporate faster and more violently. A PBH with an initial mass around 10^9 grams would evaporate completely before Big Bang Nucleosynthesis (BBN) began, while one with a mass of roughly 10^15 grams would survive to the present day.
Accretion works in the opposite direction. As a PBH moves through the dense radiation bath of the early universe, it pulls in surrounding matter, gaining mass. The standard treatment of this process has relied on the Bondi-Hoyle-Lyttleton formalism, a non-relativistic approximation that ignores the complexities of curved spacetime near a rotating black hole. Kalita and Maity argue that this approximation is insufficient, particularly for the first few hundred thousand years after PBH formation, when the universe was hot and dense enough for accretion to compete seriously with evaporation.
A relativistic framework for spinning black holes
The team derived a new spin-dependent accretion efficiency, labeled lambda Kerr, which governs how efficiently a Kerr black hole pulls in surrounding radiation. They then constructed coupled differential equations for the simultaneous evolution of PBH mass and spin, incorporating both relativistic accretion and spin-dependent Hawking emission.
The results are striking. For a slowly spinning PBH, relativistic accretion can increase its mass by a factor of roughly 4.5 over its initial value, substantially more than non-relativistic models had predicted. Even more surprisingly, the process rapidly suppresses the black hole’s rotation. Accretion drags in fluid with little net angular momentum, diluting the spin. A PBH that began life as a near-maximally rotating Kerr black hole, with spin parameter close to 0.99, ends up with a spin near zero long before it has a chance to evaporate significantly. By the time evaporation becomes important, essentially all PBHs have become effectively non-rotating Schwarzschild black holes.
This spin-down effect has major consequences. It means that the detailed spin distribution of PBHs at formation, something that depends on the specific mechanism by which they were created, gets erased very early. Whatever initial conditions produced them, PBHs all converge to the same low-spin evolutionary track.
Stronger bounds from Big Bang Nucleosynthesis
One of the paper’s most impactful findings concerns the BBN constraint. If a PBH evaporates after BBN begins, its decay products would inject high-energy particles into the primordial plasma, altering the light element abundances in ways that disagree with observations. This sets an upper limit on the initial mass of PBHs: heavier ones live too long.
The new relativistic treatment strengthens this bound by a factor of roughly 4 to 5. After accounting for accretion-boosted mass growth, the maximum initial mass a PBH can have before it violates BBN constraints drops from about 3.3 x 10^8 grams to about 7.3 x 10^7 grams. The physics is straightforward. Accretion makes PBHs heavier, and heavier black holes evaporate more slowly. A PBH that would have barely survived past BBN under the old non-relativistic calculations now clearly overstays its welcome.
Implications for dark matter
The study also reshapes the parameter space for PBHs as dark matter candidates. For a PBH to be present in the universe today, it must have survived the full 13.8 billion years since its formation. The minimum initial mass required for survival drops from roughly 1.2 x 10^15 grams in the evaporation-only scenario to about 2.7 x 10^14 grams when relativistic accretion is included, again because accretion adds mass, extending the lifetime.
This shifts the viable window for PBH dark matter toward lower initial masses, tightening constraints on the fraction of dark matter that PBHs could represent. The study also examines dark matter produced directly by Hawking emission. PBHs that evaporate can generate stable particles that persist as a dark matter relic, and the updated mass-evolution tracks alter the expected abundance of such particles. The constraints on the initial abundance of PBHs become stronger by orders of magnitude in many mass ranges.
Washed-out gravitational wave signatures
Rotating black holes produce a distinctive high-frequency feature in the stochastic gravitational wave background (SGWB), a random hum of gravitational radiation that fills the universe. This feature has been considered a potential observational signature of a primordial black hole population. Because the frequency depends on the black hole’s spin, it could in principle distinguish PBHs from other sources.
The newly identified accretion-driven spin-down effectively erases this signature. By the time PBHs begin emitting gravitational waves at detectable levels, their spins have already been suppressed to near zero. The high-frequency bump disappears from the predicted SGWB spectrum. Future gravitational wave observatories like LISA, the Einstein Telescope, and Cosmic Explorer will need to revise their search strategies if they hope to isolate PBH contributions from other cosmological sources.
A more complete picture
The Kalita and Maity paper represents a methodological step forward that touches nearly every corner of PBH physics. Their fully relativistic accretion framework is not a minor correction. It produces qualitatively different evolution for both mass and spin, and it tightens or shifts essentially every observational constraint in the field.
The work also highlights how much remains unknown about the earliest moments of cosmic history. PBHs are uniquely sensitive probes of conditions during the radiation-dominated era, when energies were high and physics was unexplored. Understanding how they grow, spin, and die is not just an exercise in black hole theory. It is a way of reading the imprints of processes that happened when the universe was less than a second old.
As gravitational wave detectors grow more sensitive and cosmological observations become more precise, these refined calculations will be essential for interpreting what the data actually say. For now, the message from IIT Guwahati is clear: the standard non-relativistic treatment of PBH accretion has been masking a richer, more constrained story about the role these ancient objects may have played in shaping the cosmos.
Images:
