On the 23rd of November 2023, scientists
at LIGO detected a tiny subatomic
signal. The final echo of a cataclysmic
event. Somewhere halfway across the
cosmos, two black holes had collided
with incredible force. This epic crash
sent ripples through the universe,
powerful enough to be picked up here on
Earth, more than 7 billion light years
away. This isn't an unusual type of
event for LIGO to detect. But what was
unusual is that neither of the two black
holes involved in the collision should
even exist. According to our current
models, they fall into the so-called
mass gap, a range of black hole masses
that, as far as we know, are impossible
to create. Now, scientists are trying to
unpick this faint and unbelievably
intricate signal to learn how and why
these epic space giants met and solve
one of the biggest mysteries in
astronomy. Where did these impossible
black holes come from? I'm Alex Mccoan
and you're watching Astramm. Join me
today as we study the invisible to
reveal the impossible. We'll explore the
unbelievable technology that's letting
us penetrate black hole science and
discover how black holes, some of the
most extreme phenomena in space, are
once again pushing our understanding to
the limits.
All across the cosmos, pairs of black
holes are locked in a gravitational
dance. Over billions of years, their
orbits gradually tighten, bringing them
closer and closer until finally they
collide. On impact, the two black holes
merge. But in the process, something new
is born. One brand new, even more
massive black hole.
These epic collisions are intensely
powerful, releasing many times the mass
of our sun in bursts of energy. But they
are also some of the hardest events to
detect. Because black holes play by
their own rules, they have a habit of
trapping light and matter inside
themselves, meaning they don't emit
anything we can see or measure. Not yet,
anyway. So I guess the first thing we
need to ask is how do you see the
unseeable?
If we want to hunt black hole
collisions, we need to look at one of
Einstein's last untested predictions for
the theory of relativity. Einstein
predicted that when something massive
accelerates through the universe, it
should generate ripples in spaceime.
That is the very fabric of the universe
itself. We call these ripples
gravitational waves. Most of the time,
these fluctuations would be far too
small to measure. But theoretically,
something unbelievably massive
accelerating extraordinarily fast could
produce gravitational waves large enough
to detect on Earth. Just something like
two black holes colliding.
In the final death throws of their
spiral, binary pairs of black holes
approach the speed of light. And just as
Einstein predicted, in the last few
orbits before they merge for just a
fraction of a second, they create a
blast of gravitational waves that can be
incredibly powerful, rolling through
spaceime in all directions. With each
wave, spaceime itself contracts and
expands. But even for a black hole
merger, the amount we detect is tiny,
thousands of the size of a proton.
Einstein never saw his theory proved
because the technology did not exist to
make detections precise enough to
measure these changes. In fact, he
believed his theory was untestable. And
until 2015, it was. Almost exactly 60
years after Einstein's death, scientists
made their first direct observation of
gravitational waves with a truly
revolutionary detector. This is the
laser interpherometer gravitational wave
observatory or LIGO. You might remember
it from my previous video which you can
watch here, but essentially its job is
to detect gravitational waves.
LIGO consists of two detectors spaced
3,000 km apart. That's about a 3 and 1/2
hour flight on a plane, but a journey of
less than 10,000 of a second for a
gravitational wave. At each base, a pair
of 3 and 1/2 m tall concrete pipes
stretch at right angles into the
distance. The pipes are so long, more
than 4 km each, that they have to be
raised from the ground by a meter at
each end to keep them lying flat as
Earth curves beneath them. At the end of
each pipe sits a mirror. Each is 0.3 m
wide, weighing in at 40 kg, and polished
to a perfect sphere with sub nanometer
precision. The mirrors are suspended by
a system of pendulums made of fused
silica fibers, insulating them from the
slightest seismic tremor. This really is
crazily refined tech. But what's even
crazier is that it works. By firing
lasers down LIGO's detector arms,
scientists can measure the exact
distance between the two sets of
mirrors. When a gravitational wave rolls
through our planet, the rippling of
spaceime it causes, which by the way is
about 1,000th the size of a proton over
this scale, creates a pattern in the
reflected laser light that can be
measured by LIGO software. And with
that, we can detect gravitational waves.
Not only can we use this to spot
cataclysmic events happening billions of
light years away, but by analyzing the
signals, scientists can calculate the
scale of the event that has produced
them and paint a fairly detailed picture
of what was involved.
For 10 years now, LIGO and its
collaborators, mainly the Virgo detector
in Italy and the Kagura in Japan, have
been detecting black hole collisions
across the universe. The collaboration
has seen over 300 mergers and
revolutionized the field of black hole
studies. In fact, in 2017, its founders
won the Nobel Prize for physics for
proving the existence of Einstein's
gravitational waves. But this wasn't the
end of LIGO's astonishing discoveries.
In 2023, this observatory made yet
another find that once again shocked
astronomers.
It was a signal scientists called
GW231123.
And what it showed was seemingly
impossible.
GW231123
lasted about a tenth of a second, but
stood out clearly. A blip in the
readings about 20 times louder than the
typical detector noise. The signal was
so strong that the team of scientists
who were involved in the discovery might
have written it off as an anomaly until
the same measurement came in from the
second detector on the other side of the
states. The team still had to check this
wasn't a random blip by simulating
thousands of years worth of fake data.
They found that the probability of
random noise mimicking GW231123
was less than once in 10,000 years. In
other words, they were pretty sure it
was real. So, they set about working out
exactly what it could be. The models
that best explained the patterns they
saw in the waveform of the signal were
those of highly spinning black holes
colliding. But even then, the signal was
quite unusual. It appeared to be just
the second half of the black hole
collision, what's known as the ring down
phase. As the new black hole formed, it
rang like a dampening bell before
settling down. And it was the
gravitational waves released during this
end portion of the merger that LIGO had
detected.
Taking into account all these
irregularities, the team concluded that
around 7 billion lighty years away, two
black holes had merged in a powerful
collision to form one single black hole
with a gargantuan mass somewhere between
182 and 251 times that of our sun. This
was a stunning discovery. Before this
event, the most massive black hole ever
seen after a merger had been about 140
solar masses. At the moment of the
collision, it was easily the brightest
event in the entire cosmos, emitting 15
solar masses of energy. That's nearly
three quintessillion or three followed
by 48 zeros jewels. It would take the
sun about 200 trillion years to emit
that much energy. It's more than what
was produced by every single other star
in the universe combined at that moment.
And what's more, all that energy was
released as gravitational waves, which
spread out through the fabric of
spaceime, traveling billions of light
years until eventually they reached our
planet. With further complex analysis of
the signal, the scientists were also
able to tease out information about the
two individual black holes involved in
the crash. And this is where things got
strange. Based on Einstein's theory of
relativity, theoretical physicists have
built a huge library of theoretical
waveform templates. Each one simulates
the signal a different black hole binary
would produce if heard by LIGO. For
example, heavier black holes merge
faster, creating a shorter, higher pitch
signal. Scientists compare each new
signal to their vast catalog of
theoretical collisions to see which is
the best fit. It turned out that the two
black holes that produced this titanic
space event were 103 and 137 times the
mass of the sun and spinning about
400,000 times faster than Earth. Close
to the speed of light. That's almost as
fast as it's physically possible for
them to go. And this revelation
presented a huge mystery because until
now physicists did not believe that
either of these black holes should be
able to exist.
In some ways black holes are very
simple. They are such pure and extreme
phenomena that they can be defined by
only three characteristics. Spin or
angular momentum, mass and electric
charge. For many years, scientists have
predicted a specific subset of black
holes that should not exist.
Specifically, black holes with masses
roughly between 50 and 130 times that of
our sun. These are bigger than your
average black hole, but not the super
massive monsters that live at the center
of galaxies. This is called the mass
gap. And to understand it, we have to
look at how black holes typically form.
When really large stars reach the end of
their lives and start forming iron in
their core, they eventually collapse
because there is not enough radiation
pressure to prevent the outer layers
from falling in under gravitational
pressure. These stars then explode as
type 2 supernova. If the original star
is very massive, typically more than 20
to 30 times the sun's mass, and the
remnant core left after the supernova
explosion exceeds the neutron star
limit, which is around 2.2 to three
solar masses, then it will keep
collapsing, forming a black hole. But
there are even more massive stars,
giants, which formed in the early
universe with between 130 and 250 solar
masses. These stars do one of two
things. Some end in violent supernova
that tear the star apart completely
without leaving the super dense core
behind that is required to form a black
hole. Others, more specifically
pulsating red giants, throw off layer
after layer, reducing its mass. Then the
final much smaller star would collapse
into a neutron star or a smaller black
hole again preventing the formation of
an intermediate mass black hole. For
almost 10 years it seemed like our
predictions were right as gravitational
wave detectors like the LIGO Virgo Kagra
collaboration did not find any black
holes of this intermediate size. Then
GW231123
turned up and revealed two black holes
merging both within the mass gap. Our
theory at least in some cases was wrong
and scientists have had to come up with
a new hypothesis.
One possibility is that they grew
through a series of smaller black hole
mergers. That the collision LIGO
detected was just the latest in a long
chain of black holes eating each other
and getting bigger as they join forces.
These kinds of mergers typically impart
large gravitational wave recoil kicks
that can unbind the remnant from its
host system. This could be overcome in a
dense enough environment like a globular
star cluster where there could be lots
of black holes which could interact
often with high escape velocities. It's
a tempting idea and it's a process that
we do believe can happen. But in this
case there is a problem. You see I told
you that black holes are simple. They
have mass, spin and charge. Well, every
time they merge, they add their two
masses together, but they also combine
their spins. Since the spins are mostly
in random directions, they tend to
average out and merges rarely yield
daughter black holes with high spin
magnitudes. The two black holes involved
in this collision, however, were
spinning at near the speed of light,
making GW231123
not just the most massive, but also the
fastest spinning black hole binary
confidently detected with gravitational
waves yet. So, some researchers
concluded that something other than
prior mergers must account for the
tremendous masses of the progenitor
black holes. The mystery continued. That
is until scientists at the Flat Iron
Institute Center for Computational
Astrophysics came up with a new theory.
The team led by or God argued that the
answer might lie in one of the most
overlooked features of black hole
formations, their parent stars magnetic
fields. They created complex simulations
for a 250 solar mass star and modeled
its life from the moment it started
burning hydrogen to when it runs out and
collapses in a supernova. The simulation
showed that in fact the star would have
burned through much of its mass as fuel
by the time it collapsed, ending its
life sitting at just 150 times the sun's
mass. Now sitting just above that pesky
mass gap. But the team still needed to
work out what happened after that 150
solar mass object went bang. Did it
match the high mass, fast spinning black
holes LIGO saw? The team came up with a
second set of much more complex
simulations to study the aftermath of
the supernova. They imagined the
supernova's remnants as a black hole
sitting at the center of a cloud of
leftover stellar material laced with
magnetic fields. First, they asked their
simulation to imagine what would happen
if the parent star had been
non-rotating. As they expected, in this
instance, the cloud of detritus fell
into the black hole. Next, they asked
the simulation to imagine that the
parent star was spinning and spinning
fast. In this scenario, the cloud of
dust formed a disc around a spinning
black hole. As the disc accretes into
the black hole, it spins faster. This is
where the magnetic fields come in. The
team told the simulation to predict the
outcome if the stellar dust cloud
surrounding the newborn black hole was
full of magnetic fields. In a similar
way to what we see happening in active
galactic nuclei forming quazars and
blazars, the magnetic fields in the
spinning disc were twisted up, exerting
pressure on the debris and eventually in
some cases forming relativistic jets
that blasted material away from the
accretion disc, reducing the material
available to feed the black hole. The
stronger the magnetic fields, the
greater this effect. In extreme cases
with very strong magnetic fields, up to
half of the stars original mass can be
blasted away from the black hole. There
was also another cool discovery to be
found in the simulations. The team found
that magnetically driven explosions
could also impart what is called
stochastic momentum to the black hole,
likely resulting in a potentially
significant natal kick compared to the
unmagnetized direct collapse black hole.
In other words, each time the magnetic
fields generate an explosion of matter
away from the region around the black
hole, it sped up. If that kick was too
powerful, it could unbind the binary
pair. But just the right amount could
spin the black hole faster without
breaking the system. With the right
spinning star and the right magnetic
field, the team could produce a fast
spinning black hole that sits in the
mass gap, just like those detected in
signal GW231123.
One of the exciting results of this work
is that it suggests a connection between
the mass of a black hole and how fast it
spins. Strong magnetic fields can slow
down a black hole and carry away some of
the stellar mass, creating lighter and
more slowly spinning black holes. Weaker
fields allow heavier and faster spinning
black holes.
Unfortunately, for now, astronomers know
of no other black hole systems on which
this connection can be observationally
tested. However, future observations may
find more such systems they could study
to confirm this connection. And the
theory also predicts something we might
actually be able to see. The magnetic
field triggered explosions could
generate short-lived high luminosity
jets similar to very energetic gammaray
bursts. And these might be observable.
Looking for these gammaray signatures
would help confirm or godlibs and his
team's formation process and reveal how
common these massive black holes might
be in the universe. If the connection of
mass and spin is confirmed as a
fundamental pattern in black holes, it
would help astronomers gain a deeper
understanding of the fundamental physics
of black holes. Just as exciting, this
result would prove that the mass gap is
fillable, meaning there may be plenty
more intermediate mass black holes
hiding out there in the darkness,
waiting for us and our next generation
of instruments to find them. It will
take years for the black hole community
to fully unravel all the implications of
this signal, and I for one cannot wait
to see what more it has to teach us.