Sky & Telescope - April 2025 - 40

BLACK HOLE BACKGROUND: SUDAKARN VIVATVANICHKUL / SHUTTERSTOCK.COM
Primordial Black Holes
Finding " too many " galaxies at early times won't close
the case on primordial black holes, Natarajan warns. Early
galaxies might simply have been better at making stars than
we expected. Nor does every supermassive black hole have to
begin as a PBH. Some seeds might start as the cores of overmassive
stars, or as big gas clouds that collapse directly into
black holes. Natarajan and her colleagues have found at least
one object that shows all the signs of being a direct-collapse
black hole (S&T: Jan. 2024, p. 8).
But when there's more than one way for something to
happen, then chances improve that it does, she says - and
there's more than one way to make a primordial black hole.
" I'm completely open to the idea that the PBH is formed in
the very early universe, " she says, " and that some fraction of
the supermassive black holes we see were seeded by them. "
Game Changer
Proving or disproving that primordial black holes exist will be
difficult. Astronomers might spot the gamma-ray flash from
an evaporating black hole, for example. Or we might find
signs in the universe's chemical makeup. The existence and
evaporation of tiny PBHs in the early cosmic soup could have
impacted the distribution of protons and neutrons, and even
the production of atomic nuclei such as helium.
Some scientists are convinced that gravitational-wave
detectors are going to solve the conundrum. As astronomers
push out to greater distances, they're learning how the
rate of black hole mergers changes with time. If the black
holes come from stars, the rate should track the cosmic
star-formation rate, which peaked 10 to 11 billion years ago
(at a redshift of 2 to 3) and has dwindled since. Mergers of
primordial black holes, on the other hand, should become
steadily more common as we look further back in time -
and they should happen long before stars formed in the first
few hundred million years.
Data from the third LVK observing run show that the rate
of black hole mergers increases with look-back time, but that
run only reached to about a redshift of 1, or some 8 billion
years into the past. The detectors might reach far enough in
the ongoing fourth observing run to see the turnover. If a
subset of black holes of certain masses merge more or less
often than those of other masses, scientists might be able to
tease the two populations apart. One of those populations
could be primordial.
LVK won't be able to detect individual sources much earlier
than the peak of star formation, but the fifth observing run,
tentatively expected to start in 2027, should turn up something
else: the collective hum of countless black holes merging
at early times. This hum is like the music of an orchestra,
created by a specific combination of instruments, Biscoveanu
explains. Scientists could analyze this gravitational-wave
background to determine which combination of merging
objects would best match the hum they hear. One combination
could include primordial black holes.
It's more likely, however, that answers will have
CAN REGULAR
BLACK HOLES EXPLAIN
DARK MATTER?
No. Based on studies of the
cosmic microwave background,
we know that only about 5%
of the universe's contents are
baryonic, whereas dark matter
makes up about 25%. Normal
black holes are an itsy-bitsy
fraction of all baryonic matter. So
there aren't enough of them to
explain dark matter.
to wait until the next generation of gravitationalwave
detectors, coming in the mid- to late 2030s.
These projects may be able to detect mergers as
early as a few tens of millions of years after the
Big Bang. If scientists find merging black holes
that early, the objects won't be from stars.
Ten years ago, says Carr, very few people
believed in primordial black holes. Even
he didn't really believe in them when he
started working on them in the 1970s.
(He's 60% to 70% convinced now.) He
stresses that it ultimately doesn't matter
whether primordial black holes pan out as
dark matter or as a large fraction of today's
gravitational-wave events - those aren't the
only reasons PBHs are interesting. " As long
as they're somewhere, I don't care where it is,
or even how much it is, " he says. " Because if
they exist, that's going to be really exciting. "
¢ Science Editor CAMILLE CARLISLE remains
perpetually caught by the gravitational pull of all
things related to black holes.
40 APRIL 2025 * SKY & TELESCOPE
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Sky & Telescope - April 2025

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