Sky & Telescope - June 2022 - 20

Black Holes Galore
for the merging objects except in special cases. (The remnant's
spin is pretty clear.) The 23-solar-mass black hole in
GW190814, for example, was so much larger than its companion
that its properties dominated the encounter
and the resulting waves, like a booming voice
overwhelming the other object's whisper. The
waves show that the big black hole essentially
wasn't spinning, indicating it's
likely a stellar skeleton.
But most of the spin information
we have comes from the population
analysis, because hints in the
cohort add up to reveal trends that
a single system can't. The results are
intriguing. Statistically speaking, the
black holes that collided show a small
preference for aligned spins, but roughly
30% were likely askew, telling us that
we're indeed seeing binaries made multiple
ways. The merging objects also spun slowly, if
at all, which sets them apart from the fast-spinning
black holes astronomers have seen paired up with stars in the
Milky Way. (No one knows why.) Notably, the range of spins
widens above 30 solar masses, which might also confirm that
the biggest black holes have diverse origins.
One of the biggest surprises is a connection between spins
and mass ratio, says LIGO astrophysicist Salvatore Vitale
Orbit axis + combined spins
Spin 1
Orbit axis
S = S1+S2
" None of the traditional
formation channels can
easily explain this, which
is why it's so interesting
and beautiful. "
-SALVATORE VITALE
(MIT). The black holes in binaries fall into two camps: either
equal mass and no spin, or mismatched in size with notable
aligned spins. " None of the traditional formation channels
can easily explain this, which is why it's so interesting
and beautiful, " he says.
There's a chance that AGN disks could
explain it, he adds. Mismatched systems
can form there, and the gas could
override the usual pell-mell nature of
dynamical hookups and force the black
holes to spin the same way as the disk
does. " The usual joke in Bayesian
analysis is, 'Would you bet a coffee,
a dinner, or your house on this?' "
he says, referring to the statistical
method used in this work. " I would bet
between a coffee and a dinner. "
Scientists likely won't have an answer
until we've found closer to 1,000 events, says
Fishbach. Most collisions so far involve pairs
of roughly 30-solar-mass black holes, the detectors'
sweet spot. " The things we're really confused about are, what
is happening at 60 solar masses and what's happening at
3 solar masses? We only have a handful of detections there
now. So even going from a sample size of one or two to, like,
five or 10 will be huge. "
Rising Tide
During the latest run, LIGO and Virgo saw gravitational
waves that had traveled up to 8 billion years to reach us. That
covers the last half of cosmic history, revealing how the rate
of mergers has changed with time. " Black holes were merging
more often in the past than they are now, " Doctor says.
" Now is the first time when we can confidently say that. "
The rate changes in step with star formation across the
Spin 2
universe, which peaked about 10 billion years ago. That suggests
that the vast majority of the black holes detected either
came from stars or were second-generation remnants instead
of being primordial black holes, hypothetical objects formed in
special conditions in the early universe. If such objects exist,
then they should merge at a roughly constant rate over time.
Primordial black holes are one of many suggestions to
SPIN Two black holes that orbit each other sometimes
also spin. Their spin axes can point parallel to the axis of
their orbital motion (as shown for object 2, a setup called
aligned), or they can be tilted (as with object 1) or even
upside-down (in which case the spin axis would be parallel
to the orbit's axis but the object would spin, say, clockwise
while orbiting counterclockwise). If the spins are misaligned,
the orbital plane will wobble (gray arrow).
explain dark matter. " This is one of these things that comes
in and out of fashion, " Hanna says. " It's a polarizing thing.
People are like, 'Oh, yeah, of course,' or 'No, you're nuts.
Why don't you go join the crackpots?' "
It's unlikely that observations will reach back to the peak
of cosmic starbirth in the next run, scheduled to begin at the
end of 2022. But the detections could still enable researchers
to determine how long a delay there is between star formation
and black hole mergers and if the merger rate changes
for different mass ranges, Kalogera says. " That is definitely
where we want to go next. "
This information could help scientists tease apart how the
binaries formed, down to the details of the original stel20
JUNE 2022 * SKY & TELESCOPE
BLACK HOLE BACKGROUND: SUDAKARN VIVATVANICHKUL; SPIN: GREGG
DINDERMAN / S&T, SOURCE: BHDYNAMICS.COM
http://www.BHDYNAMICS.COM
Sky & Telescope - June 2022

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