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

by Scharf, Caleb

  This evidence, together with the incredible mechanical effects of black hole–inflated bubbles, indicates that black holes play a crucial role in the evolution of cosmic structures across time. They regulate the production of new stars in the giant galaxies of today’s universe, and have likely done so throughout cosmic time. And in the very distant past, they could limit how big those galaxies could ever hope to grow in the first place, acting like frustrated farmers trying to keep their weeds under control. What we saw in 4C41.17 was the turbulent youth of a giant galaxy in the beginnings of a great cluster, and despite all the young stars pouring out radiation, and the extraordinary shroud of stellar dust from their brief lives, at the core of it all was still a point of infinite density and relentless force. Because of it this galaxy could never grow beyond a certain size. The more matter fed into its core, the more food there was for the black hole, and the more the black hole would pump out disruptive energy. The X-ray glow of cosmic photons that had been boosted by this energy was as luminous as a trillion suns, and could easily be the tipping point for stemming the growth of the galaxy. It could explain the difference between the number of stars we thought the universe should make in these places and the number that it actually does.

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  After the rush to analyze and then report our findings, there was enough of a lull to meditate on what we had discovered. While our interpretation seems reasonable, there is always room for future investigation. Regardless, the most remarkable aspect for me was my own shifting sense of what a massive galaxy in the young universe might be like. The mental picture that I’d held up to this point was rather static. Surely something like a galaxy would form serenely and majestically from the gentle condensation of material out of the pristine sea of hydrogen, helium, and dark matter? Of course, we knew that there were quasars out there. They were chewing up matter voraciously and spitting out enough energy to change the balance of atoms, ions, and molecules in the young universe. But they were still quite sparse, and half a century after we’d begun to understand their physical nature, they continued to seem a little disconnected from the galaxies that hosted and fed them. Yes, we also knew that there were generations upon generations of new stars in those early eons. A vigorous amount of stellar birth, life, and death was pumping raw elements into the cosmos. These new components were condensing out as the massive clouds of dust that shrouded so much of the first stellar light, hiding it away from us. But despite all this, the great galaxies still felt like noble and vast castles, steady and reliable.

  By stark contrast, in the dull name of 4C41.17 we’d looked into a maelstrom. In a billion or two years this would indeed be another giant galaxy, almost certainly an elliptical fuzzball of hundreds of billions of stars. But we were witnessing a stage where it was a pit of seething radiation and particles, much of that driven by the thrashing forces of a growing supermassive black hole. As much as gravity was trying to draw the galaxy together, it was being resisted by this outflow of energy. Clearly, that resistance was ultimately going to be futile, or else the local cosmos of today would be a very different place. This is part of the very same problem facing our theories and simulations of the growth and evolution of cosmic structures, the simplest of which predict far more stars than we actually find in the universe. What we see in 4C41.17 is a direct clue to the way in which nature limits and restrains the growth of the most massive galaxies. The energy generated by the comparatively microbial specks of the black holes in their cores helps hold them in check.

  There was one more thing that this extraordinary colossus on the other side of the universe had to reveal to us, and it came from the environment it occupied. When Smail and his colleagues had taken their original deep images in the lurking submillimeter part of the electromagnetic spectrum, they had found something else. Around the distant radio-emitting galaxies like our 4C41.17, there would often be other glowing mounds that were visible in the pictures. In keeping with the grand scheme of a cosmic web of matter in the universe, these young agglomerations of mass tended to cluster together. See one and you were a little more likely to see another nearby, and another near that, and so on.

  What this meant was that in any good X-ray image targeting one of the distant dusty places, there would likely be others. If they harbored supermassive black holes, then these too might show up as bright points of X-ray light as those energetic photons burrowed their way out through the thick, dusty shrouds. Again, Smail and I picked through the data we had for 4C41.17, as well as examining images of two other structures that were also snapshots of young systems, just a couple of billion years old. To our surprise, not only did we find points of intense X-ray emission apparently inside the dusty shrouds of the young galaxies, but in some cases they came in pairs: twin pinpoints of light next to each other. In one, at the hairy edge of uncertainty in among noisy data, there was evidence of even a triple arrangement—three points of X-ray light peeking out. They were tiny clusters of photons on the screen, just edging into statistical believability, but what might elicit a fatigued shrug from other scientists can excite an astronomer to sleepless nights. Luck and hunches play a huge role in the exploration of the universe.

  We realized that we were seeing supermassive black holes that might have originally belonged to separate youthful galaxy structures, stellar teenagers adrift in the young universe. These wayward creatures were now merging and coalescing, setting off frenetic waves of star formation and pumping out dust. Through all this, our X-ray images were penetrating down into their hearts and cores. We know that pairs of supermassive black holes are not something we see much of in today’s universe. Only some 4 percent of galaxies are thought to harbor multiple giant black holes. This meant that the ones we had found 12 billion years in the past were likely en route to merging with each other, eventually hiding their origins within a single event horizon. Such arrangements had been posited for some time as a way to smooth out and reorganize the orbits of stars in galactic cores to better match what astronomers were seeing in the nearby universe. Two orbiting black holes are a dynamical blender, reshaping the paths of stars around them.

  Given a little more time, perhaps a few million years, these distant giants would find one another through their gravitational pulls. Swirling around as an ever faster orbiting pair, they would eventually combine in a crescendo of gravitational radiation, sending ripples in spacetime ringing out across the universe. It looked like this was clear evidence for one route to growing supermassive black holes in the universe: simply have them eat each other. In doing so, they should also leave their fingerprints on the galaxies that host them, disrupting and rearranging the motions of stars around them, leaving an extremely important set of telltale crumbs around the cookie jar.

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  Our distant colossus and its brethren represent the extreme end of the galactic flora and fauna. They are the giant trees in the richest parts of the cosmic tropical rain forests. In these environments there is now little doubt that supermassive black holes have played a major, likely dominant, role in sculpting the forms that we see. Twelve billion years ago, and even earlier, they served as regulators and law-keepers to stem the flood of new stars as matter cooled and condensed. Since then they have continued to hold matter at bay. The great bubbles inside clusters have mixed together elements and steadied the transformation of raw hot gas into new stars and planets. But this happens in synchronization with the inflow of that matter, an astonishing symphony of feedback and balance. These great systems breathe in and breathe out.

  Elsewhere, off in other galaxies, supermassive black holes are also making their presence felt. But in these other copses and clumps of galactic trees and shrubs, the interplay between construction and destruction is more complex. We happen to find ourselves living in a very large spiral galaxy, the Milky Way. It’s an interesting terrain, neither a backwater nor one of the universe’s greatest cathedral-like structures—the giant elliptical galaxies within clusters. It’s natural to wonder what inf
luence black holes have had on this place, and what role they might continue to play. This brings us to the final part of our story, the search for the origins and nature of our own galactic environment and perhaps even of life itself.

  7

  ORIGINS: PART I

  We know that black holes are end states for matter, but not all of them hide away out of sight. Spacetime itself curves to ridiculous extremes to create the ultimate entrapment at these locations. Yet as cosmic gas, dust, and stars approach the event horizon, their dissolution generates vast amounts of energy that pours noisily back out into the universe. In all of nature, black holes are the most efficient engines for converting matter into energy. This energy plays a vital role across cosmic time: it helps control the production of stars, limits the size of the greatest galaxies, and trims rivers of cooling material into mere rivulets.

  Supermassive black holes are also closely related to the sizes of the ancient clouds of stars that surround them. This is true whether the singularity contains a million times the mass of the sun or 10 billion times that amount. It is like judging the size of a pot of honey sitting in a garden by the size of the swarm of bees surrounding it. This relationship exists wherever we find clouds of ancient stars buzzing around the centers of galaxies. It is true for the ellipticals as well as for the majestic spirals, in which clouds or bulges of old stars sit at the center of great disklike wheels of slowly rotating matter. But some galaxies lack this central, puffed-up cloud of old stars; our Milky Way is one such system. In situations like these there are still central black holes, and they can be a million to a hundred million times the mass of the Sun, but somehow the processes at play in other galaxies didn’t arise in the same way to link them to central stellar swarms.

  In all cases, though, an intimate relationship clearly exists between giant black holes and their host galaxies; they have “coevolved.” That’s extraordinary, given that they are such disparate structures: one is tens of thousands of light-years across, the other a billion times smaller. From place to place, galaxy to galaxy, this coevolution is also quite varied, suggesting that the particulars of history and circumstance must play vital roles. We can see and smell the signs of fundamental mechanisms at work, but we’ve yet to join all the dots.

  As I asserted at the opening of this book, the presence and behavior of black holes in the universe could very well be connected to the origins of life. It’s an outrageous-sounding proposition that the extreme and seemingly remote behavior of black holes has anything to do with the capacity of this universe for life. In order for me to make good on my promise to illuminate that connection, we need to take a careful look at the chain of phenomena that we think go into making stars, planets, and living things before coming back to complete our story about black holes. This inevitably leads us to questions about our own particular cosmic circumstances, and I think the answers are quite surprising.

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  A terrified spider scuttles across the wall while a flower unfurls its petals in a vase. Off in the street a dog idly barks at something real or imagined, and deep in the ocean a school of fish darts and swoops around a cloud of frantically paddling krill. Something slimy grows on the underside of a muddy rock while, together with the 100 trillion bacteria in our guts, we sit in our chairs as electrical pulses zip around our brains. This is life.

  Here on Earth it is at once a collection of extraordinarily complex and simple phenomena, involving molecular structures and microscopic machines that organize and reorganize matter in a network of self-sustaining processes. The timescale over which these processes operate stretches from nanoseconds to billions of years. Yet for all this multilayered complexity, the fundamental actions are basic. Energy and matter are exchanged with the environment, and the organization of shape and form, at first on very small scales, is offset by an increase in environmental disorder. A single-celled microscopic organism maintains its cell membranes and internal structure at the expense of plucking and inserting material out of and into its surroundings. Quadrillions of these tiny life-forms can change a planet. They alter its atmosphere and modify its surface chemistry. In effect, they geo-engineer it into something new while building their own ordered cells. Eventually, they may even produce a busy multicellular chicken that leaves behind its own merry trail of disorder in the search for food and energy.

  Right now we have only one example of life to study: that which exists on a small rocky planet orbiting a modest star in the 14 billionth year of this universe. There is nothing about the nature of life on Earth, however, that suggests it is anything but a fair sample of the mechanisms that could arise anywhere. For example, terrestrial life consists of carbon, hydrogen, oxygen, and nitrogen, plus some other elements. The characteristics of the chemical bonding among these compounds are such that an extraordinary array of complex and energy-efficient molecular structures can form—from amino acids to DNA. There is no obvious example of an alternative chemical set in the cosmos that can do this.

  We don’t really know the when, how, or why of life’s origins, but it’s clear that there are some fundamental prerequisites. The first is the elemental mix necessary to produce biologically important molecules. The second is a location, or sequence of locations, for that chemistry to be incubated in and to ultimately occupy. A third requirement emerges as the wheels of life are set in motion, and that is a supply of energy, whether in the form of raw atomic or molecular materials, or thermal energy, or electromagnetic radiation that can drive chemical reactions. In short, the recipe for life calls for ingredients, pots and pans, and a continually hot oven.

  It is in this shopping list that the connections between life and its broader cosmic environment come into sharp focus. Earlier, I described how stars build the heavier elements of the universe. They are nuclear pressure cookers, stuffing protons and neutrons together until they can squeeze in no more. While the primordial elements of hydrogen and helium will always be the most abundant cosmically, the next in line are oxygen and carbon. These are generally made deep in the cores of massive stars, although some of these elements are also produced for short periods in the outer shells of aging stars. The very heaviest elements are produced inside the most massive stars, more than eight times the mass of the Sun, and during their violently explosive deaths as supernovae. Very heavy elements are also produced when objects like white dwarfs are tipped over the edge by more material falling onto them. Pushed through Chandrasekhar’s magical limit of quantum pressure support, these dense stellar remnants briefly compress and forge additional elements before spewing them out into the universe in a supernova explosion.

  Over lots and lots of time, these heavier elements pollute the interstellar and intergalactic gases as great spills of atomic nuclei, diffusing farther and farther into the depths of space. New stars will condense out of this gas where gravity can overcome the resistance of pressure and energy, and the cycle of stellar formation begins again. As we’ve seen, this can be quite a battle in some locations, and black holes bear a great responsibility for regulating and limiting this process throughout the cosmos.

  The precise details of how this matter condenses into new stars and planets are at the forefront of modern scientific inquiry, not least because we are in the midst of searching for other worlds that might harbor life. We now have the technological means to detect and study planets around other stars, as well as the environments of young, protostellar, protoplanetary systems. This so-called “exoplanetary science” is nothing short of a revolution. Since the Epicurean philosophers of ancient Greece and probably well before, we’ve questioned whether ours is one of many such worlds in the cosmos. Finally, after centuries of trying, we’ve indeed begun to discover these other solar systems.

  Stars form at the center of thick disks of gas and dust that coalesce from nebulae, not unlike the disks of matter that form around some black holes. These fat platters of material, known as protoplanetary disks, can be a thousand times wider in radius than the distance between
the Earth and the Sun. Planets condense and grow out of these disks through a variety of possible routes, complicated by the effects of gravitational dynamics. Over a few million years, what was once a beautifully smooth and pristine wheel of matter becomes pocked and lumpy with these coagulating worlds. At the same time as the planets are forming, the disk of material is also being evaporated away. Flooded by radiation from the new and increasingly hot central star and from neighboring stars, it simply boils off. Astronomers can see this happening, and it fundamentally limits the formation of planets. It is much like the way our earthly seasons change, from fertile spring to slow-growing summer and eventually to winter. The stars and the planets that we are left with are in many senses mere fossils of this episode of intense activity. And intense it is. The major planets of a solar system like our own form in about 30 million years, an extremely short time in the grand scheme of things—a mere 0.3 percent of the lifetime of the parent star. We do not yet understand many details of this process, but our observations of alien systems are revealing vital clues that also have something to say about possible life in these places.

  One such signpost is the richness of the elements in a protoplanetary disk. In fully formed exoplanetary systems, we see this on vivid display. The heavy-element content of a star is a good indicator of the elemental mix in the original planet-forming material, and astronomers can measure this quantity through the spectrum of a star’s light. It goes hand in hand with the likelihood of finding planets. The more heavy elements we detect, the more planets are likely to exist around that star, and the more massive they typically are. This makes a lot of sense. Where there are greater quantities of substances like carbon and silicon, there is more raw material for efficiently forming embryonic planetary bodies.

  Water also plays a major role within nascent planetary systems. The relatively high abundance of oxygen across the universe, together with plentiful hydrogen, means that water molecules crop up all over the place. In the disk of material around young stars, water plays a key chemical role within the gases and the youthful chunks of condensing material. When water freezes, it also provides a major source of solid matter that helps drive the gravitational agglomeration of protoplanets. Just as the environment in our solar system gets warmer as we get closer to the Sun, so does the environment in the disk of material around a baby star. Conversely, farther away from these warm inner zones, water freezes into a solid and actually helps accelerate the growth of big chunky planet-like lumps. A major fraction of the solid interiors of planets such as Uranus and Neptune are composed of water ices for this very reason.