Primordial black holes would have formed in the first fractions of a second
after the Big Bang, when dense pockets of energy in the primordial plasma
collapsed directly into black holes. Spanning an enormous range of masses,
from specks lighter than a mountain to objects rivaling the Sun, they have
long been considered a dark matter candidate because they formed before
nucleosynthesis and are non-baryonic.
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: accretion and Hawking evaporation. A new paper by
Jitumani Kalita and Debaprasad Maity at IIT Guwahati, published in the
Journal of Cosmology and Astroparticle Physics (JCAP 07, 2026), revisits
this competition with a level of rigor missing from previous work. 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. Smaller black
holes evaporate faster. A PBH with an initial mass around 10^9 grams would
evaporate completely before 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 has relied on the Bondi-Hoyle-Lyttleton
formalism, a non-relativistic approximation that ignores curved spacetime
near a rotating black hole. Kalita and Maity argue that this approximation is
insufficient for conditions in the radiation-dominated era, 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. 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 within the paper’s framework.
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 in the model. Under the assumptions of this study, PBHs
converge to the same low-spin evolutionary track regardless of their initial
spin.
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 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. Accretion makes PBHs
heavier, and heavier black holes evaporate more slowly. A PBH that would
have barely survived past BBN under the old 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 exist today, it must have survived 13.8 billion
years. 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 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 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.
Washed-out gravitational wave signatures
Rotating black holes produce a distinctive high-frequency feature in the
stochastic gravitational wave background (SGWB). 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.
The result changes predictions for future searches for high-frequency
stochastic gravitational-wave backgrounds.
A more complete picture
The Kalita and Maity paper represents a methodological step forward in PBH
physics. Their relativistic accretion framework produces qualitatively
different evolution for both mass and spin, and it tightens or shifts key
observational constraints in the field.
PBHs are uniquely sensitive probes of conditions during the radiation-
dominated era. Understanding how they grow, spin, and die 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, these
refined calculations will help interpret what the data reveal.
Clark – 1ban.news