Sky & Telescope - June 2022 - 21

GW190814: R. ABBOTT ET AL. / ASTROPHYSICAL JOURNAL LETTERS 2020, CC
BY 3.0; GW191103: R. ABBOTT ET AL. / ARXIV 2111.03606; GROUNDSHAKING:
VARUNYUUU / SHUTTERSTOCK.COM
GW190814
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GW191103
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SPINS Each black hole involved in a merger might be spinning, but
scientists can usually only calculate a range of possibilities in terms of
how fast and in what direction. Spins are measured from 0 to 1, where 1
is the maximum possible for a given black hole. The coordinates around
the circumference indicate the tilt of a black hole's spin axis relative to its
orbit: Zero degrees means the black hole was pointing straight up, 90°
that it was rolling on its side, and 180° that it was upside-down. Darker
colors indicate more likely spin values, and the larger object is the lefthand
hemisphere of each pair. For GW190814 (far left), the 23-solar-mass
black hole overwhelmed the signal, giving clear signs that it essentially
wasn't spinning while also masking any information about its companion's
spin. For GW191103, which involved black holes of about 12 and
8 solar masses, it's hard to tell how much the black holes spun or leaned.
lar systems. For example, stars with lower levels of heavy
elements grow larger than ones with higher levels, and they
make bigger back holes as a result. Galaxies' star-forming gas
was more pristine earlier in cosmic history, so bigger black
holes might thus be commoner at early cosmic times.
But even if they're made early, that doesn't mean they'll
merge early. Recent calculations by Lieke van Son (Center for
Astrophysics, Harvard & Smithsonian) suggest that stars able
to form black holes above 30 solar masses will also end up in
wider binaries than other stellar pairs. Wider binaries take
longer to merge, so we should see more high-mass black holes
merging later in cosmic history, she predicts.
Open the Floodgates
" If, for some reason, we never turned the detectors on again
- we're like, 'We're done, and we don't want to detect any
gravitational waves anymore - this would be a very unfulfilling
end to the story, " says Fishbach. " It really feels like we're
just at the beginning. "
Thanks to upgrades and the addition of a fourth detector
(Japan's mine-dwelling KAGRA), the next observing run will
have heightened sensitivity. Alerts will come every few days.
The rapid-response team will need a new strategy, perhaps
limiting wake-up calls to events that the automated system
flags as oddballs or involving neutron stars. " It's just a matter
of survival, " Hanna says. " My graduate students are unwilling
to not sleep. I've asked them, they've said no. "
But even with the brilliant upgrades that instrumentalists
GROUNDSHAKING
If the first gravitational-wave event had occurred 1 a.u.
away, it would have stretched and squeezed Earth on
a meter-level scale, causing planet-wide earthquakes.
(Fortunately, it was 90 trillion times farther away than that.)
devise, LIGO, Virgo, and KAGRA will ultimately be limited
by every astronomer's problem: size. " As scientists, we always
think about what comes next, " Vitale says. " We never live in
the moment. "
That next thing could be Cosmic Explorer, a proposed pair
of gigantic, ground-based detectors that would be 10 times
more sensitive than LIGO. " That simple factor of 10 brings
you from seeing one black hole every few days to seeing all of
the black holes in the universe, no matter where they are, "
he says. First light could come in the mid-2030s, around the
same time that astronomers will launch the Laser Interferometer
Space Antenna (LISA). LISA's sensitivity to lowerfrequency
gravitational waves will enable it to sense not only
smashing supermassive black holes but also stellar-mass
black holes years before they merge, giving ground-based
detectors a head's up.
In 15 years, astronomers may have seen more than
100,000 black hole collisions. The flood of discoveries will
answer questions about the objects' spins, formation, and
behavior across cosmic time, as well as what's happening at
the smallest and largest masses.
And hopefully, something unexpected will come, too. " It's
why I love what we are doing, " says Vitale. " Very often, when
we see something, it's the first time that humans have seen it. "
¢ Science Editor CAMILLE CARLISLE adores black holes of
all sizes. Follow discoveries in her blog, The Black Hole Files:
https://is.gd/bhfiles.
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Sky & Telescope - June 2022

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