Sky & Telescope - April 2025 - 36

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PRIMORDIAL BLACK HOLE BOUNDS: BEATRIZ INGLESSIS / S&T, SOURCE: BRADLEY KAVANAGH / GITHUB.
COM/BRADKAV/PBHBOUNDS; NUMBER OF PUBLICATIONS BY YEAR: BEATRIZ INGLESSIS / S&T, SOURCE:
SAO ASTROPHYSICS DATA SYSTEM
Primordial Black Holes
Or collapse could have happened during a phase transition.
At certain moments in the primordial universe, conditions
changed dramatically, akin to when liquid water freezes. During
these transitions, the pressure dropped. A region that otherwise
wouldn't be dense enough to collapse suddenly might.
The primordial universe experienced several phase transiWHERE
THEY COULD HIDE Thanks to various observations, scientists
have placed limits on the fraction of dark matter that primordial
black holes of a given mass could account for. Here, gray regions are
the excluded possibilities, labeled by measurement type. Additional,
contended limits exist but aren't shown.
From the Cosmic Stew
Back when the universe was a fraction of a second old, it was
filled with matter and radiation. This plasma was not perfectly
smooth; it had knots of higher density in random places.
As the nascent universe expanded, the plasma soup diluted
and cooled. Dense knots naturally expanded more slowly
than the surrounding, diffuser stuff. Under the right conditions,
some knots could have packed enough mass into the
right amount of space to stop expanding, overcome their
internal pressure, and collapse, creating black holes.
To overcome the plasma's pressure takes special circumstances,
explains Bernard Carr (Queen Mary University of
London, UK), who has worked on primordial black holes for
some 50 years. Collapse could happen if the smallest density
fluctuations were more severe than the ones on larger size
scales that later gave rise to galaxies and cosmic structure -
fluctuations we see in the cosmic microwave background, the
afterglow from this early era.
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Year
RISING INTEREST For the first several decades, there were few research
papers with " primordial black hole " in the title. But after scientists
announced in 2016 the first detection of gravitational waves from merging
black holes, interest surged.
36 APRIL 2025 * SKY & TELESCOPE
tions. At each epoch, knots of a certain mass and size were
most likely to collapse. The smallest primordial black holes,
those squeaking in at 10−5 gram (about one-tenth the mass
of a hungry flea), would have formed at the Planck time,
10−43 second after cosmic history began. Larger black holes
would have formed later. A spurt should have occurred at
10−5 second during what's called the quantum chromodynamics
(QCD) transition, Carr says, when subatomic particles called
quarks combined to create things like protons. Black holes
formed at this time would have most commonly had a mass
roughly equivalent to that of the Sun.
Only a tiny fraction of the universe would have collapsed
to make these objects. Anne Green (University of Nottingham,
UK), who has spent her career exploring solutions to the
dark matter problem, is skeptical that conditions could have
been so precisely tuned to make exactly the right number of
primordial black holes at the right time such that they could
explain dark matter. Too many too early, and they would
upset the balance between radiation and matter.
" You've got to get the perturbations just right to make all
of the dark matter primordial black holes, and not to have
created a universe that just made the primordial black holes
and nothing else, " she says.
The smallest primordial black holes would have evaporated
away long ago - in fact, it was studying their hypothetical
quantum properties that helped lead Stephen Hawking to
discover that black holes evaporate. The temperature of an
evaporating black hole's radiation is inversely proportional
to its mass, so as a black hole shrinks, it heats up. Those still
evaporating today would have a mass of 1 quadrillion grams
(1015 g, or the mass of a small comet), packed into a space
a little larger than a proton. Such black holes would radiate
gamma rays, albeit slowly - they'd sit " sizzling away " over
billions of years, Carr says, before losing their remaining
mass in an explosion of gamma rays.
Gamma rays do suffuse the universe in a high-energy
haze, from a variety of sources. To stay within the limits set
by this haze, at most one-billionth of the universe's total contents
could be made up of primordial black holes of cometlike
masses.
An Endless Sea of . . . Nothing?
Primordial black hole (PBH) enthusiasts split on which mass
should be the target of ongoing searches. Some, including
Carr, favor a solar mass, while others focus on objects as
lightweight as planets and asteroids.
The first potential evidence seemed to support stellar-mass
objects. In 1997, astronomers reported their results looking
for dark matter in the form of massive compact halo objects
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