We’ve never seen them directly, yet we know
they are there, lurking within dense star clusters or wandering the dust lanes of the
galaxy, where they prey on stars or swallow planets whole. Our Milky Way may harbor millions of black
holes, the ultra dense remnants of dead stars. But now, in the universe far beyond our galaxy,
there's evidence of something far more ominous, a breed of black holes that has reached incomprehensible
size and destructive power. Just how large, and violent, and strange can
they get? A new era in astronomy has revealed a universe
long hidden to us. High-tech instruments sent into space have
been tuned to sense high-energy forms of light – x-rays and gamma rays – that are invisible
to our eyes and do not penetrate our atmosphere. On the ground, precision telescopes are equipped
with technologies that allow them to cancel out the blurring effects of the atmosphere. They are peering into the far reaches of the
universe, and into distant caldrons of light and energy. In some distant galaxies, astronomers are
now finding evidence that space and time are being shattered by eruptions so vast they
boggle the mind. We are just beginning to understand the impact
these outbursts have had on the universe: on the shapes of galaxies, the spread of elements
that make up stars and planets, and ultimately the very existence of Earth. The discovery of what causes these eruptions
has led to a new understanding of cosmic history. Back in 1995, the Hubble space telescope was
enlisted to begin filling in the details of that history. Astronomers selected tiny regions in the sky,
between the stars. For days at a time, they focused Hubble’s
gaze on remote regions of the universe. These Hubble Deep Field images offered incredibly
clear views of the cosmos in its infancy. What drew astronomers’ attention were the
tiniest galaxies, covering only a few pixels on Hubble’s detector. Most of them do not have the grand spiral
or elliptical shapes of large galaxies we see close to us today. Instead, they are irregular, scrappy collections
of stars. The Hubble Deep Field confirmed a long-standing
idea that the universe must have evolved in a series of building blocks, with small galaxies
gradually merging and assembling into larger ones. You can see evidence of this pattern by looking
out into the universe. Many galaxies are gyrating around one another. Some are crashing together, others ripping
each other apart. Gravity calls the tune as these galaxies draw
together, exchanging stars and gas, and, over time, merging to form larger composite galaxies. This came to be known as the hierarchical
picture of cosmic history, in which the universe evolved from the ground up, with its structures
growing larger and larger over time. A team operating at the Subaru Observatory
atop Hawaii’s Mauna Kea volcano examined one of the deepest galaxies known, whose light
has taken nearly 13 billion years to reach us. It was a messenger from a time not long after
the universe was born. This object is known as a quasar, short for
“quasi-stellar radio source.” It offered a stunning surprise. A small region in its center is so bright
that astronomers believe its light is coming not from a collection of stars, but from a
single object of at least a billion times the mass of our sun. This beacon is generated by gas falling onto
the object and heating up to extreme temperatures. The only thing known to generate this much
power is a swirling caldron, where space suddenly turns dark as it merges into a giant black
hole. For astronomers, the question was: how did
this black hole get so big so early in the history of the universe? It likely got its start in an early generation
of stars, often known as population 3 stars. Made up of hydrogen, they are thought to have
been hundreds of times the mass of the sun. These giant stars burned hot and fast, and
died young. A star is like a cosmic pressure-cooker. In its core, the crush of gravity produces
such intense heat that atoms are stripped and rearranged. Lighter elements like hydrogen and helium
fuse together to form heavier ones like calcium, oxygen, silicon, and finally iron. When enough iron accumulates in the core of
the star, it begins to collapse of its own weight. That can send a shock wave racing outward
that literally blows the star apart in a supernova. At the moment the star dies, if enough matter
falls into its core, it can collapse to a point, forming a black hole. The first generations of stars and black holes
burst onto the cosmic scene in a time of incredible turbulence. Within primordial gas clouds, stars were being
born in dense knots. They gave rise to black holes that began to
swallow more and more matter. A computer simulation of the early universe
shows just how quickly these voracious monsters were able to grow. The project, by scientists at Carnegie Mellon
University, was designed to recreate a region in the early universe that measured over a
hundred million light years on a side. It shows what took place in the first one
billion years of cosmic history. This virtual universe is set in motion by
equations describing the properties of gas, the energy released in star birth and the
outward motion of time and space. The result: an intricate cosmic web, with
gravity drawing matter into filaments and knots like a vast tangle of interconnected
spiders’ webs. Inside the densest regions is where the largest
galaxies, and black holes, grew. Here, circles indicate the appearance of black
holes deep in the data. As they gain weight, by eating up their surroundings,
the circles grow larger. A few, in the largest galaxies, reach ultra
massive proportions, billions of times the mass of the sun. These black holes were not just swallowing
gas. The orbiting Chandra X-Ray Observatory was
dispatched to look into distant galaxies for black holes on growth spurts. Scientists looked for pockets of gas and stars
glowing hotly in X-ray light. What Chandra found was that the core of some
distant galaxies countained hot pairs, twin supermassive black holes drawn together by
gravity. Black holes by nature resist this dark marriage. As the two approach each other, they go into
an orbit that could last virtually forever. To learn what allows them to merge, we go
back to the ideas developed by Albert Einstein. He said that when massive bodies accelerate
or whip around each other, they literally disturb the fabric of space, causing it to
ripple like a disturbance on a pond. When these ripples move outward, they carry
with them the energy of the pair’s orbit, causing them to spiral closer. When this dance of death comes to an end,
that’s when the pair joins together to form a larger black hole. That moment may be approaching for a quasar
called OJ-287, at 3.5 billion light years away. Flareups in the surrounding region have led
astrophysicists to conclude that another black hole is flying around it. By measuring the giant's gravitational hold
on its companion, astronomers estimate its mass at 18 billion solar masses. For a time, OJ-287 was the largest black hole
ever detected. It no longer is. Deep in the heart of the Coma galaxy cluster,
a mere 321 million light years away, lies a giant eliptical galaxy known as NGC 4889. Astronomers used several large telescopes
to measure the speed at which stars are orbiting around the center. They used that data to calculate the mass
of the central object, a whopping 21 billion solar masses, give or
take a few billion. Theoretically there are no limits to how much
weight a black hole can gain. And yet even the largest black holes, and
their host galaxies, seem to obey limits. What holds them back has to do with the way
clusters of galaxies evolve, a pattern long noted by scientists. This computer simulation shows the evolution
of a galaxy cluster in the early universe. The gravity of the entire region draws small
galaxies by the thousands, along with great streams of gas, into the center. So why doesn’t the central galaxy, and the
black hole that resides within it, capture all this matter? Why don’t they swallow the entire cluster? You can see the answer in a region called
MS0735. At two and a half billion light years away,
it appears in visible light to be a typical galaxy cluster. In X-ray light, you can see that it’s enveloped
in a cloud of hot gas, measured at nearly 50 million degrees. Hollowed out of this cloud are two immense
cavities up to 600,000 light years across. That’s enough room in each to stuff 600
galaxies the size of our Milky Way. Now add in a radio image of the cluster. You can see two vast streams of matter pushing
out from the center. That’s a give-away that the cavities were
formed by an eruption in the core of the giant central galaxy. Two jets, shooting out of a central black
hole, have launched blast waves that plowed through the gas that makes up the inter-galactic
medium. The
energy it took to carve out these Xray cavities is remarkable, the equivalent of several billion
supernovae, according to one calculation. In fact, this has been referred to as the
largest single eruption recorded since the big bang. It was generated by a black hole that weighs
in at around 10 billion solar masses. Black hole jets like this have been seen all
around the universe, including in our own cosmic neighborhood. This is the famous M87 galaxy, at the center
of the Virgo galaxy cluster, around 50 million light years away Astronomers have been intensively studying
the black hole that lurks in its heart, and recently estimated its mass at 6.6 billion
solar masses. It powers a pair of high-powered jets that
are plowing through the galaxy. But how does a black hole, a creature famous
for hiding in the dark, emit this much energy? Think of a black hole as the eye of a cosmic
hurricane, kept rotating by all the stars, gas, and other black holes that fall into
it. As this matter flows in, it forms a spinning
donut-like feature called an accretion disk, which works like a dynamo. The spinning motion of the disk generates
magnetic fields that twist around and channel some of the inflowing matter out into a pair
of high-energy beams, or jets. How much energy depends on the black hole’s
gravity, and how much matter has already crashed through the event horizon. Is this just another frightening spectacle
of Nature? Or is it part of a more profound process at
work? It shows that a monster black hole will not
be forcefed. The largest black holes in the universe probably
rose between 10 and 12 billion years ago, the age of the quasars. By releasing energy in the form of jets, they
heated up their surrounding regions. This prevented gas from collapsing into the
central galaxy, and allowed smaller galaxies on the periphery to form and grow. But the impact the black holes did not stop
there. This Chandra image of the Hydra A galaxy cluster
shows the same immense hot cavities, glowing in X-ray light, as well as a jet blasting
out of its central galaxy. Gas along the edge of the jet was found to
contain high levels of iron and other metals probably generated by supernova explosions
in the center. By pushing these metals into regions beyond,
a black hole seeded more distant galaxies with the elements needed to form stars and
solar systems like ours. The black holes in these more remote galaxies
then seeded their own environments. This is what might be happening in Centaurus
A, also known as the “hamburger galaxy.” Peering through the dense dust lanes that
dominate our line of sight, astronomers have come to believe that it’s actually two galaxies
in the act of colliding. In X-rays, you can see a jet erupting from
the center. This computer simulation shows the effect
of such a merger on black holes. As the two galaxies pass by each other, the
pull of gravity disrupts their spiral shapes, forcing huge volumes of gas into their cores. As the black holes begin to feed, they emit
blast waves that push much of the loose gas out beyond the boundaries of the new galaxy. In the final steps of this cosmic dance, the
two black holes merge, and emit one final blast. To think that our Earth, our solar system,
ourselves are the beneficiaries of these far-away monsters. The largest black holes have played dual roles
in a great cosmic struggle. They are the product of gravity’s relentless
inward pull, the force that has drawn matter into galaxies, clusters, and the structures
they form. But with their incredible power, they emit
energy that pushes back on gravity. In so doing, these strange and powerful objects
have become the master architects of space and time. 1
