Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos

by Scharf, Caleb

  The original ideas of Zel’dovich and Salpeter would lead to the next conceptual leap, one that took this physics to a whole different scale. In 1969, a paper appeared in the scientific journal Nature that provided a single clean and elegant solution to the nature of the distant quasars and radio sources. It also laid out a new vision of the intimate relationship between black holes and galaxies. Its author was Donald Lynden-Bell, then in residence at the Royal Greenwich Observatory in Herstmonceux Castle in Sussex, some fifty miles south of London. Lynden-Bell was born in 1935 in Dover, southern England and had studied astronomy at Cambridge and worked at Caltech. Now he sat in the beautiful Tudor castle that some twenty years earlier had been purchased to house the Royal Observatory and its staff.

  An incredibly productive and fluent worker with mathematics and physics, Lynden-Bell has made contributions that span much of astronomy. His influential paper from 1969 is full of provocative and exuberant language. Even the first sentence describes the great radio lobe structure in some galaxies as a “dead or dying quasar.” He describes the Schwarzschild radius as the Schwarzschild “throat.” The slightly greater distance beyond which no stable orbit can exist around a black hole becomes the Schwarzschild “mouth.” But although his language was vivid and colorful, Lynden-Bell wasted no time getting to the point.

  By the end of the first paragraph he had laid out exactly how the giant radio-emitting structures seen across the universe seem related to “dead” quasars, how colossal the energy output had been from these compact regions, how gravitational energy is more than competitive in this case with stellar nuclear fusion as the energy source, and how there must be dead quasars in our area of the universe. Behind all this: supermassive black holes ranging from 10 million to a billion times the mass of the Sun. It’s a heady mix. By this time in 1969, there had already been discussion that the very distant quasars somehow represented a particular stage in the evolution of structure in the universe. Perhaps over the cosmic ages they morphed into the great radio-emitting lobes and clouds that were now being charted. Because we could only find the brighter and nearer quasars, there was a good chance we saw merely one in every thousand of the original objects. Lynden-Bell realized this implied that the true number of power sources for quasars in the universe was comparable to the number of galaxies. Matter accreting onto black holes a billion times the mass of the Sun could provide enough energy to be this power source. The logical conclusion was that the best place for these supermassive black holes was inside virtually every galaxy. At every galactic center, the great density of stars could provide ample fuel for the enormous appetites of black holes.

  It’s a work of great physical intuition, both outrageous and flawless. It goes on to present a mathematical description of how these giant black holes could eat matter and liberate energy. We’ll encounter this physics again in more detail. Most extraordinary of all, though, is not how this idea was presented, but what its implications were.

  If this hypothesis were true, then the universe would not only be full of small black holes that came directly from the deaths of large stars, but it would also be full of supermassive giant black holes. At the center of each galaxy—more than 100 billion of them, spread across almost 14 billion years of cosmic history—there must be black holes with event horizons that are tens of millions of miles across. This is a tall order to fill. While Lynden-Bell’s elegant hypothesis was consistent with many facts, it wasn’t immediately embraced by everyone. Arguments and disagreements swirled. Some astronomers felt that admitting giant black holes into our picture of the universe was too far-fetched. How was it possible for galaxies to contain these? How could you even grow such objects? Those dissenters suggested instead that vast agglomerations of stars dying as explosive supernovae could be responsible for the flood of energy from galactic centers. Others questioned whether the great velocities and apparent distances of quasars were real or an illusion of the gravitational redshifting of light. However, as time went by, more and more evidence mounted for precisely the kind of compact and incredibly dense regions at the hearts of galaxies across the universe that could only be matched by colossal black holes.

  Like the gurgling of water rushing down a drain, energy was pouring out of these places just as matter was pouring in. Astronomers realized—just as Lynden-Bell had suspected—that this was not a sustainable situation. Objects like quasars would shut down due to starvation. But it also became clear that far from simply “dying” in obscurity, supermassive black holes could assume a variety of states. Sometimes they would still be eating, gently grazing and slowly growing in mass and releasing energy. At other times they would be quiet, sleeping off the previous few millions of years of food. Exactly how they ate could vary. Exactly how energy was released could vary. What we glimpsed of the process was also highly subjective, depending acutely on the chance arrangement of structures around these holes and the direction of our viewpoint.

  To understand all this we need to take a very ambitious field trip. Our quest? To find out what really happens around a massive spinning black hole as our universe drains into it.

  4

  THE FEEDING HABITS OF NONILLION-POUND GORILLAS

  Once upon a time there lived a great monster. It made its home deep inside a castle that was deep inside a huge forest. No one had ever seen the monster, but over the centuries and millennia there had been clear signs of it stirring. Legend told that it trapped all things that came near. In its lair even time itself became sticky and slow, and its hot blue breath would burn through the strongest shield. Few dared to venture into its realm. Those who did either returned empty-handed with wide-eyed tales too strange to believe, or never came back at all. If you stood on the highest mountains in the land you could peer across the treetops and just see the haziest of outlines of the monster’s castle. Sometimes you might see a few strange clouds hovering over it, as if they were caught in a great swirl of atmosphere, and at night there might be an eerie glow reflected off the cool air. For years you’ve wondered about this enigmatic place and the monster within. Finally you decide that there is nothing else to be done but to go on your own quest, your own search for a glimpse of the beast. In this particular tale your starting point, and home, is our solar system, and the monster’s castle is deep in the galactic heart.

  At first the going is easy on your journey. The stars are familiar and friendly. Out here in the Orion spur of the great spiral disk of the Milky Way, stellar systems are spaced with an average of about five to ten light-years between them. Finding a comfortable path through is not difficult. Even the rivers of dusty darkness between the galactic arms are easy to cross, and traveling the first twenty thousand or so light-years is a breeze. After a while, though, things begin to change. This is the beginning of the galactic axial hub. Like the distorted yolk of a huge fried egg, the central region of the galaxy inside about four thousand light-years is a gently bulbous but elongated structure. It contains a far higher density of old red and yellow stars than out in our suburbs. The woodlands begin to thicken up in here as we ease our way toward the inner sanctum. More and more stars begin to block the way, and we are constantly shifting our path in order to slide through.

  Pressing on, we finally enter the true galactic core. Some six hundred light-years across, this interior forest is densely packed with stars buzzing around in their orbits. Compared to home, the skies are coated with star after star after star. At the edge of this core, where we first enter, stars are packed together a hundred times more densely than around our solar neighborhood. At the very middle, there are hundreds of thousands more than we are used to. The going is extremely tough and slow, and it gets worse and worse as we descend inwards. This is the oldest undergrowth, part of the ancient barrier to the center. Something else exists in here, too. A rather piecemeal and shabby disk of material encircles the entire core, made of hydrogen gas clouds. It blocks the view from some directions, and as we move farther down, another structure now begins to reveal
itself. There is a flattened ring of gas rotating about the very center of the galaxy. It’s composed of atoms and molecules, and it is unlike anything else in the Milky Way. It is a rich and substantial formation, a hundred times denser than a typical nebula. Its outer edge is still some twenty light-years out from the galactic center, but its inner lip descends to within only about six light-years. Tilted at a rakish angle to the plane of the entire galaxy, it spins at about sixty miles a second. Most of it is hydrogen gas, but nestling in among this pure stuff are other compounds: oxygen and hydrogen in simple combination, molecules of carbon monoxide, and even cyanide. Every hundred thousand years or so, the inner part of this molecular ring makes one complete circuit around the center of the galaxy. This impressive structure at first looks serene, but closer inspection reveals the scars of terrible violence. Some great cataclysm has recently blasted the ring, pushing some of the gas into clumps and lumps and scorching other parts. It is a strange and ominous gateway.

  Moving cautiously inside the ring, we take stock of what is happening around us. We are within an incredibly dense and constantly moving swarm of stars. It seems like chaos, yet through this noisy buzz we can see something distinctly peculiar happening up ahead. We pause in flight to watch as several of these innermost stars move along their orbits. Remarkably, these orbits are not only around something unseen ahead of us at the center, but they are extraordinarily fast as the stars swing by that invisible focal point. One star whizzes through its closest approach at velocities approaching 7,500 miles a second. That’s astonishing, considering that our homeworld, Earth, orbits the Sun at less than twenty miles a second, and even the planet Mercury moves at barely thirty miles a second. For the star to achieve an orbital velocity of that magnitude, it must be moving around a huge mass. We perform the calculation. Deep within a tiny volume at the galactic center is an unseen something that is 4 million times more massive than the Sun. There is nothing else this dark body can be except a colossal black hole.

  How we have come to build this detailed picture of the environment at the center of our galaxy is a tale of technological prowess and skilled insight. One of the greatest achievements of astronomy in the late twentieth century and early twenty-first century has been the discovery that our own galaxy, the Milky Way, harbors a supermassive black hole at its center. It provides a vital context for the rest of our story, and a key reference point. But there are still limits to how much detail we can see when we peer this deep into the inner galactic sanctum. At present we have to rely on a number of indirect astronomical phenomena to tell us more. For example, tenuous hot gas is being measurably expelled from this tiny region. X-ray photons are also streaming out, and roughly once a day they flare up and brighten by a hundredfold. It’s tempting to imagine that somewhere inside this central core are moths flying too close to an open flame, and sometimes we see their unfortunate demise. Altogether these characteristics represent clear signs that matter is sporadically entering the maw of a brooding monster.

  Figure 9. The innermost region of our own galaxy mapped at microwave frequencies. This image, spanning approximately twelve light-years, reveals an extraordinary structure of irradiated gas centered on a bright object that astronomers associate with the central massive black hole. As the image suggests, this gaseous structure is in motion around and toward a central point.

  We see another signature in the great loops of magnetized gas that surround this whole region, aglow in radio waves that flood out into the galaxy. They are part of the very same extraterrestrial radio signal that Karl Jansky first saw in the 1930s with his simple radio telescope in a field in New Jersey. Yet despite all this activity, the black hole at the center of the Milky Way is operating on a slow simmer compared to the brilliant distant quasars that can shine as brightly as a hundred galaxies. It’s a brooding, hulking beast, not a blazing pyre. But to really place it in context, we should size things up and compare this local environment to the rest of the cosmos.

  To do that, let’s return briefly to our map of forever, still contained in the sack that was delivered to the doorstep two chapters ago. In our neighborhood of the universe, encompassing a mere 6 billion years or so of light travel time, the intensely bright quasars occur in only about one out of every hundred thousand galaxies. In other words, they are extremely rare creatures. For that reason, we should not be too surprised that the Milky Way isn’t one of the galaxies that contain a quasar. Those other galaxies with great radio lobes and ray-like jets extending outward are even more rare; the most prominent examples are over 10 million light-years from us. But at greater distances, further back in cosmic time, the situation is very different. In fact, between 2 billion and 4 billion years after the Big Bang, fiercely energetic quasars were a thousand times more common. We think that roughly one in a hundred galaxies held a quasar in its core at any moment. This was a golden age for these objects, powered by the voracious appetites of supermassive black holes.

  No single quasar lasts for very long, however. With monumental effort, astronomers over the past several decades have surveyed and studied these enigmatic objects, and piece by piece they’ve reconstructed their history. Like paleontologists building the skeletons of long-gone creatures and covering them with reconstructed flesh, so too have astronomers rebuilt the lifestyle of the supermassive black holes that drive quasars. We find that a typical quasar will only light up for periods that last between 10 million and 100 million years, a tiny fraction of cosmic history. Because of this, we know that more than 10 percent of all galaxies in the universe have actually hosted a brilliant quasar during their lifetimes. It just means that wherever or whenever we look, we never get to see them all switched on at once.

  But why do quasars die out with cosmic time? It is a question that remains unresolved. Even this basic description of the cosmic distribution of quasars is the result of decades of intense research. (The history of that effort is a fascinating one, but a story for another day.) We can, however, make some reasonable speculations about the life cycles of quasars. First, they are powered by supermassive black holes that, as they devour matter, produce an output of energy far greater than in other environments. The electromagnetic shrieks of material falling into a black hole are what we see during this process. This suggests that the enormous energy of quasars is deeply connected to the availability of consumable matter and the rate at which it is being consumed. The more matter falls in, the bigger the hole can become, and the bigger the hole, the more energy it can extract from that matter. Eventually, though, this material seems to run out. Quasars live fast and big and die after a blaze of glory that must depend acutely on the detailed nature of matter consumption by supermassive black holes.

  The most distant quasars we know of (going back to within a billion years of the Big Bang) are typically also the most luminous. In other words, as the cosmic clock ticks, and new quasars come and go, they gradually become dimmer. The astronomical jargon used for this is “downsizing.” (Who says scientists don’t have a sense of humor?) All quasars, however, from the brightest to the faintest, are powered by the most massive of the supermassive black holes. They are the elite—the big guys. They also occur in the bigger galaxies in the universe. This is an important connection to make, because it begins to tie the evolution of supermassive black holes to the evolution of their host galaxies, their great domains.

  Indeed, astronomers have found something else peculiar and critically important going on in galaxies. The mass of their huge black holes is generally fixed at one-thousandth of the mass of the central “bulge” of stars surrounding the galactic cores. These are typically the old stars that form a great buzzing cloud around galactic centers. Sometimes that central cloud can even dominate the whole galaxy. Careful astronomical measurements have revealed that a galaxy with a big bulge of central stars will also have a big central supermassive black hole, and a galaxy with a small bulge will have a smaller black hole—according to the 1,000:1 mass ratio. But while this relationship i
s strikingly clear in many galaxies, it is not entirely universal. For example, the Milky Way is pretty much “bulgeless.” Its central stars are in more of an elongated block or bar, not a swarm thousands of light-years across. And, as we’ve seen, our own supermassive black hole is a comparatively petite monster of 4 million times the mass of the Sun. By contrast, the nearby spiral galaxy of Andromeda has a great big bulge of central stars and contains a supermassive black hole that we think is 100 million times the mass of the Sun, neatly fitting the expected size. Why there should be this relationship between central stars and black holes is a mystery at the forefront of current investigations. We will find it to be of the utmost importance as we dig deeper into the relationship between black holes and the universe around them. But the next step in following this story is to get our hands dirty again with the business of feeding black holes.

  *

  We can make a number of broad arguments to describe how energy is produced from the distorted spacetime surrounding dense concentrations of mass in the cosmos. I made some of those in the previous chapter, and emphasized the power involved. The idea certainly sounds feasible: there’s plentiful energy to spare, but specific physical mechanisms are needed to convert the energy of moving matter into forms we can detect. Otherwise, it’s like stating that burning gasoline releases a lot of energy and therefore an engine could be driven by gasoline. That might be true, but it doesn’t demonstrate how an internal combustion engine works. In our case, the processes of energy generation and conversion are particularly complicated because of the exotic nature of black holes. Unlike an object such as a white dwarf or a neutron star, a black hole has no true surface. Matter that gets close to the event horizon will essentially vanish from sight for an external observer. There is no final impact onto a solid body, no final release of energy from that collision. So whatever is going on just outside the event horizon is absolutely critical to understand.