Possible Detection of a Black Hole So Big It ‘Should Not Exist’
Black
hole physicists have been excitedly discussing reports that the LIGO and Virgo
gravitational-wave detectors recently picked up the signal of an unexpectedly
enormous black hole, one with a mass that was thought to be physically
impossible.
“The
prediction is no black holes, not even a few” in this mass
range, wrote Stan Woosley, an astrophysicist at the University
of California, Santa Cruz, in an email. “But of course we know nature often
finds a way.”
Seven
experts contacted by Quanta said they’d heard that among the
22 flurries of gravitational waves detected by LIGO and Virgo since April, one
of the signals came from a collision involving a black hole of unanticipated
heft — purportedly as heavy as 100 suns. LIGO/Virgo team members would neither
confirm nor deny the rumored detection.
Chris
Belczynski, an astrophysicist at Warsaw University, previously felt so sure
that such a large specimen wouldn’t be seen that in 2017 he placed a bet with
colleagues. “I think we are about to lose the bet,” Belczynski said, “and for
the good of science!”
Belczynski’s
former confidence came from the fact that such a big black hole can’t form in
the usual way.
Black
holes — dense, paradox-ridden spheres whose gravity traps everything, even
light — form from the contracting cores of fuel-spent stars. But in 1967, three
physicists at the Hebrew University in Jerusalem realized that when the core of a dying star
is very heavy, it won’t gravitationally collapse into a black hole. Instead,
the star will undergo a “pair-instability supernova,” an explosion that totally
annihilates it in a matter of seconds, leaving nothing behind. “The star is
completely dispersed into space,” the three physicists wrote.
A
pair-instability supernova happens when the core grows so hot that light begins
to spontaneously convert into electron-positron pairs. The light’s radiation
pressure had kept the star’s core intact; when the light transforms into
matter, the resulting pressure drop causes the core to rapidly shrink and
become even hotter, further accelerating pair production and causing a runaway
effect. Eventually the core gets so hot that oxygen ignites. This fully
reverses the core’s implosion, so that it explodes instead. For cores with a
mass between about 65 and 130 times that of our sun (according to current estimates), the star is completely obliterated. Cores
between about 50 and 65 solar masses pulsate, shedding mass in a series of
explosions until they drop below the range where pair instability occurs. Thus
there should be no black holes with masses in the 50-to-130-solar-mass range.
“The
prediction comes from straightforward calculations,” said Woosley, whose 2002 study of this “pair-instability mass
gap” is considered definitive.
Black
holes can exist on the other side of the mass gap, weighing in at more than 130
solar masses, because the runaway implosion of such heavy stellar cores can’t
be stopped, even by oxygen fusion; instead, they continue to collapse and form
black holes. But because stars shed mass throughout their lives, a star would
need to be born weighing at least 300 suns in order to end up as a 130-solar-mass
core, and such behemoths are rare. For this reason, most experts assumed black
holes detected by LIGO and Virgo should top out at around 50 solar masses, the
lower end of the mass gap. (The million- and billion-solar-mass supermassive
black holes that anchor galaxies’ centers formed differently, and rather
mysteriously, in the early universe. LIGO and Virgo are not mechanically
capable of detecting the collisions of supermassive black holes.)
That
said, a few experts did boldly predict that black holes in the mass gap would
be seen — hence the 2017 bet.
At a
meeting that February at the Aspen Center for Physics, Belczynski and Daniel Holz of the University of Chicago
wagered that “black holes should not exist in the mass range between 55 and 130
solar masses because of pair instability,” and thus that none would be detected
among LIGO/Virgo’s first 100 signals. Woosley later co-signed with Belczynski
and Holz.
But Carl Rodriguez of the Massachusetts
Institute of Technology and Sourav Chatterjee of
the Tata Institute for Fundamental Research in Mumbai, India, later joined
by Fred Rasio of Northwestern University, bet
against them, wagering that a black hole would indeed be detected in the mass
gap, because there’s a roundabout way for these plus-size black holes to form.
Whereas
most of the colliding black holes that wiggle LIGO and Virgo’s instruments
probably originated as pairs of isolated stars (binary star systems being
common in the cosmos), Rodriguez and his co-signers argue that a fraction of
the detected collisions occur in dense stellar environments such as globular
clusters. The black holes swing around in one another’s gravity, and sometimes
they catch each other and merge, like big fish swallowing smaller ones in a
pond.
Inside
a globular cluster, a 50-solar-mass black hole could merge with a 30-solar-mass
one, for instance, and then the resulting giant could merge again. This
second-generation merger is what LIGO/Virgo might have detected — a lucky catch
of the big fish in the pond. “This can really only happen in clusters,”
Rodriguez said. If the rumor is true, he, Chatterjee and Rasio will each
receive a $100 bottle of wine from Belczynski, Holz and Woosley.
But
there are other possible origin stories for the putative big black hole.
Perhaps it started out in an isolated binary star system. After the first star
collapsed into a black hole, it might have grown by stripping matter from its
companion star. Later, the second star would have collapsed as well, then
eventually the two would have collided and merged, sending gravitational waves
cascading through the fabric of space-time.
The
LIGO/Virgo team quickly announces every potential gravitational-wave event and
the region of sky from which it originated, so that other telescopes can swivel
in that direction. But the tight-lipped team has yet to publish detailed
information about any event from the current observing run that began in April,
such as the inferred sizes of the colliding objects. The team plans to reveal
all by the spring of 2020 at the latest. If the oversize black hole is among
the results, the analysis should also reveal how fast the hole and its
companion were spinning when they collided; this information will help favor
one origin story or the other, or neither.
The
rumor is “pushing us to alternative formation mechanisms,” said Chris Fryer, an
astrophysicist at Los Alamos National Laboratory who has studied binary black
hole formation and the mass gap. “In any event it will be an exciting event —
if it’s true.”
As for
Woosley, he still feels certain the mass gap exists, despite possible
exceptions. “A likely outcome will be that when we have hundreds of black
holes, we will indeed see a cliff at around 50,” he said, “but with a few
events in the gap because nature abhors a vacuum.”
This
article was reprinted on TheAtlantic.com.
