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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.

The nature of forming planets is also influenced by the eventual size of the star itself. Current research suggests that bigger stars form with bigger disks around them, increasing the potential efficiency of making planets. Astronomers are also finding hints that the chemistry that transpires inside a protoplanetary disk is under the thrall of the parent star. These disk environments are great big cauldrons for all manner of atomic and molecular chemistry. Complex carbon molecules are forming, breaking apart, and being transported around in the disk. Our astronomical observations of young stellar systems reveal lots of chemical mayhem, but in among this are clear hints that systems producing smaller stars may have different chemistry taking place than those growing massive stars. The culprit may simply be the electromagnetic radiation streaming off the still-forming star itself. A big baby star makes more electromagnetic noise than a small one. Photons can both destroy fragile molecules and create pathways for other molecules to form. So the chemical makeup of planets may well be related to the size of their stellar parent, among other things.

Earth happens to be located in an orbit about its parent star that allows for a temperate surface environment. Liquid water can flow freely. Cocktails can be drunk on the beach. We don’t understand exactly how critical this really is, but the potential for liquid water on a planet seems to be a reasonable signpost for life. Water is both an essential biochemical solvent and a planet-wide contributor to geophysics and climate. Here too, the size and age of a star is a major factor in determining the orbital regimes that can harbor such a planet.

All this means that getting a world that has the right chemical and energetic richness to produce and sustain organisms hinges on many factors. This doesn’t prove that worlds like these are necessarily unlikely or very rare—just that they depend on a whole chain of interlinked steps, some of which we’ve now taken a quick look at. Our next step is to find the connection between these more local phenomena and those on a truly cosmic scale.

The first place to look is up, straight up, to the galaxies. Each one of these great stellar gathering places in today’s universe is a result of billions and billions of years of evolution. Dark matter, gas, dust, and stars coalesce, orbit, bump, explode, waft, and circulate in these systems. But as we’ve seen, galaxies are not all alike, and their global properties can affect the smaller details significantly. For example, the overall elemental mix available in a galaxy today can have a domino effect on the production of stars and planets. Less elemental richness can mean less-efficient cooling of nebular gas, which means fewer stars will form in the galaxy. That elemental ingredients list can also influence the comparative numbers of big stars and small stars. Fewer heavy elements forged in these stars mean fewer planets form around later generations of stars. And then, to add insult to injury, a dearth of these heavy elements directly impacts the raw chemistry that takes place around forming planets. That space chemistry makes a lot of carbon-based, organic molecules. We don’t yet fully understand how complex those molecules get at this stage, or how many of them could end up on the surfaces of new planets—especially the small, rocky Earth-like ones. But they may represent a “prebiotic” mixture for life. Instead of life having to wait millions of years for a young world to build complex molecules in some puddle, such worlds could receive a rich starter mix from space. This is admittedly speculative, but not unreasonable.

You can pick any one of those steps as a potentially critical hurdle for a galaxy to be able to generate the kind of environment that we’ve evolved out of. We can add several other factors into the mix. An environment subjected to blasts of intense cosmic radiation, whether as photons or particles, may be poorly suited for the growth of complex chemistry and molecules. For example, I’d bet that no Earth-like planet exists inside the jets of feeding supermassive black holes. That would surely be a horrible place for delicate biochemistry. Even being on the sidelines of such an intensely disruptive phenomenon might be detrimental to worlds otherwise suited to harboring life. In more general ways, we’ve discovered also how black holes can mold the universe around them. The key question now is to find out how this affects the chain of events leading to the formation of stars and planets that have the potential to generate, incubate, and sustain life. To tackle that we have to travel back to the very origins of supermassive black holes themselves.

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The most distant quasars exist in a very young universe, barely a billion years old. As we’ve seen, quasars are products of the appetite of the biggest and best-provisioned black holes. Surrounded by accreting matter, they pump out a prodigious amount of energy. But the age of these systems raises a fundamental question. These supermassive black holes must have formed almost contemporaneously with the first generations of stars in the universe. This is a great puzzle, because the way we think black holes form in today’s universe is from the catastrophic collapse of massive stellar remains. Once the mass of a spent stellar core or an object like a neutron star exceeds a certain threshold, there is only one way for it to go: down and in. There is no known pressure force that can resist the shrinking of such an object to inside its event horizon. But this produces a baby black hole only a few times the mass of our Sun. Even if it eats matter at the rate required to power something like a quasar, that amounts to only a few Suns’ worth of material a year. With a continual food supply, it would still take hundreds of millions of years to reach supermassive scales. So where could those first giant chasms have possibly come from?

Yet again, the devil is in the details. One theory is that the very first generations of stars in the universe are responsible for producing giant holes. Compared to today’s stellar objects, some of these firstborn could be unusually massive, hundreds of times the size of the Sun. The pristine hydrogen and helium gas of the young universe cools less efficiently than today’s polluted interstellar gas, so a nebular cloud maintains its pressure and doesn’t give way as gravity gathers more and more mass together. This can result in the formation of stellar giants. Once nuclear fusion is triggered, these stars burn quickly and produce black holes. By merging with one another and gulping down surrounding gas, they might grow quickly to supermassive sizes. But we don’t know for sure; there may not be enough feedstuff around these holes for them to grow so fast.

Alternatively, under the right conditions the growing mass of a young galaxy could conceivably produce a giant black hole directly in its center. This is a possibility that a number of scientists have studied in detail. Matter pours into the swelling gravity well of an infant galaxy. A sufficiently huge blob of gas may form, collapse under its own weight, and simply speed past all the stages that would otherwise turn it into billions of individual stars. The end result is a directly formed, factory-fresh supermassive black hole. This is awfully tricky, though—absolutely pristine hydrogen and helium and perfect conditions would be required to allow such a lot of extraordinarily dense gas to gracefully condense to a single point.

A third, and arguably more plausible, route has to do with the natural messiness of structure formation in the universe. We are pretty confident that the largest galaxies in the cosmos begin as close-knit groupings of smaller component baby galaxies. These fall together within their mutual gravity well, colliding and merging to eventually settle as a giant galaxy. The colossus 12 billion light-years away with which I began this book represents a stage not so long after that kind of agglomeration.

Supercomputer simulations of these primordial environments indicate that the process of collision and merger of these baby galaxies can generate enormous whirlpools of turbulence. It’s not unlike when you pull an oar through water. Behind the paddle the water rushes back into the trough, swirling and churning. These turbulent regions should draw in the material from the colliding galaxies, gathering it in a giant and unstable disk within which spiraling waves direct the gas to the center. Here it is concentrated to a level that pushes it right through the barrier of instability, the critical balance point that James Jeans first determined. Gravity takes over, and a weird star forms that’s more than ten thousand times the mass of the Sun. In the blink of a cosmic eye the core of this object gives way, and the matter has fallen inside its event horizon to form the giant seed of a supermassive black hole. It all happens so fast that the rest of the galactic gas has no time to disperse or to condense into skittering stars. This gives the new black hole an opportunity to gobble it up and grow very quickly.

We don’t yet know with certainty whether any of these three routes operate in the young universe. Youthful galaxies definitely collide and coalesce. Maybe this helps bring more and more fresh matter into the hungry beaks of the nesting baby black holes, allowing them to bulk up. Perhaps, too, the vast turbulent vortices of colliding galactic pieces can generate the overweight clouds of gas that will swiftly collapse into massive holes. Like the hogging bulk of a cuckoo chick sneakily planted in another’s home, they might snatch all the food. Something, for sure, is producing supermassive black holes within the first billion years of existence of the universe. Since this is when the first generations of stars are also produced, there should to be ample opportunity for the properties of the black holes to become linked with the properties of those stars. In some cases, perhaps multiple holes, cosmic neighbors, catch each other in their mutual gravitational pull and merge. We’ve certainly glimpsed more than one giant hole in systems like the distant bubble-blowing maelstrom 4C41.17 that my colleagues and I got to know so well. Such sticky embraces could both boost the growth of the most massive black holes and leave a calling card on the surrounding stellar swarms.

If a central black hole and the cloud or bulge of stars in a galactic center form contemporaneously, then each may imprint its properties on the other. A big cloud of condensing gas, perhaps swirled into a focusing vortex, could produce both a large black hole and a large set of new stars. A smaller amount of material would produce a lesser black hole and fewer stars. Once a concentration of matter collapses within its event horizon, the whole region quickly shuts down. The outflow of energy from this black hole acts to sweep out any leftover gas and prevent much further growth of anything. In effect, it puts a date stamp on the process. This may be reflected in the relationship we see now between black hole mass and the stars of galactic cores. A similar process might also take place during later episodes of black hole growth: when material from intergalactic space falls into a galaxy, it could trigger star formation while firing up the gravity engine at the center. In this way, the growth of the black hole and the formation of stars could be pushed along in tandem.

If we go even further back in cosmic time, there is something else, an effect that might implicate smaller black holes in the conjoined history of stars and galaxies. As I’ve noted before, about 380,000 years after the Big Bang the universe cooled enough to become transparent in appearance. Until this time it was opaque, as hot hydrogen and helium nuclei and loose electrons whizzed around and a thick soup of photons scattered back and forth between these particles. At this early time, dark matter was smeared out and diffuse, a shadowy component waiting for gravity to take hold. But as the cosmos cooled to a few thousand degrees, the typical energy of the photons fell below an important threshold. They were no longer prone to being absorbed and rerouted in the clouds of electrons and nuclei that were trying to couple with one another. Bona fide atoms could form without interference and the photons could fly freely across the universe, becoming the cosmic background radiation, the remnants of this hot stage. It was a critical moment in the history of the universe.

For a hypothetical observer, though, it marked the beginning of what was possibly the most monumentally dull episode the cosmos has ever gone through. For roughly the next 100 million years, the universe was dark and increasingly chilly. It was like a particularly bad winter in northern Europe. Astronomers refer to this period as the “dark ages” of the cosmos—with good reason, since there was nothing interesting there: no stars, no galaxies, nothing to light it up. Of course, matter was at work slowly gathering itself down into all its self-imposed hollows and valleys in spacetime, but otherwise, all was quiet.

Eventually gravity got its way. The first stars formed, and their radiation poured out into the pristine void. After a hundred million years of solitude, the primordial gas of the universe was buffeted by energetic photons again. Ultraviolet light stripped electrons from their atoms, and now the cosmos became a great Swiss cheese of cold dark gas full of heated, ionized holes surrounding hotly burning stars. This immediately and irrevocably altered the environment for the formation of the next generations of stars. For astronomers this has been and still is a vital subject of investigation, because what happens next is critical in establishing the entire history of stars and galaxies that leads all the way up to the present day.

And this is where, just possibly, the phenomenon of black holes steps in to dramatically subvert and alter the very building blocks of this newly awakened young universe. In 2011, an intriguing study appeared from a group of astrophysicists led by the Uruguayan-born astronomer Felix Mirabel. Their idea is deceptively simple. Increasingly, as scientists attempt to mimic the physical conditions of this teenage cosmos using sophisticated computer simulations, there is evidence that the first stars may have formed not as individuals, but with brothers and sisters. In today’s universe most stars are actually part of a pair or a bigger group. It seems that when fertile conditions exist for the formation of stars, nature finds it easier to form them together, often orbiting around each other. It is a good bet that conditions 13 billion years ago would have produced lots of paired-up, binary stars.

However, no two stars are identical, and it is likely that of two massive stars born as a pair, one will live faster than the other. Once it depletes its nuclear fuel, a big star really has only one way to go, and that is to implode and form a black hole a few times the mass of our Sun. In the right configuration, the black hole can then begin to consume its companion. Stellar matter will be torn off and swept into a disk that accretes around and into the hole. In this familiar process, the frictional heating of the disk releases energy as photons, reaching up to the X-ray regime. It is precisely this scenario that powers our own local black hole prototype system of Cygnus X-1, detected in 1964 by rudimentary X-ray telescopes. In Cygnus X-1 a blue supergiant star is feeding matter into a disk around a black hole some ten times the mass of our Sun.

Mirabel and his colleagues realized that if this pairing of stars and holes indeed occurred at the end of the universal dark ages, the cosmic environment could have been radically altered. X-rays are far more penetrating than ultraviolet photons of light, reaching much farther out into the universe before getting ensnared and absorbed. But they have a similar effect on atoms, ripping off electrons and creating electrostatic carnage. In this scenario, the energy from baby black holes eating up their companion stars floods across vast distances. It fundamentally alters the shape and form of structure in the young universe. Instead of a Swiss cheese topology of cold atoms and molecules filled with hot ionized holes, the cosmos would be cooked more uniformly. This both helps and hinders the formation of new stars. The extra heating of gas by these X-rays could slow down the production of the next batches of stellar objects. But in counterintuitive fashion, X-rays that penetrate deep into the densest cores of baby galaxies can actually encourage the rudimentary chemistry of hydrogen gas, which provides a route for new objects to condense.

This is because pure atomic hydrogen has a hard time cooling down. Atoms may bump and scatter against one another, but there are few ways for that energy of motion to be transferred into electromagnetic radiation that can fly away. It’s a different story for hydrogen molecules, though, where two hydrogen atoms are joined together. This molecule can rotate like a bandleader’s little baton. It can also wiggle and vibrate like a spring with weights on both ends. So when hydrogen molecules bump and bash into each other, some of that energy of motion is transferred into their rotation and vibration. It can then escape as low-energy infrared photons. This provides a new and unique route for the gas to cool off. The energy of motion, the thermal energy, of the gas gets transferred into the wiggling molecules, which in turn spit it out as photons that carry the energy away. For this reason, molecular hydrogen cools much faster than simple, single atoms of hydrogen.

But making hydrogen molecules in situ is a horribly inefficient process. Remarkably, the disruptive influence of X-ray photons is incredibly beneficial to this simple chemistry. X-rays can strip electrons from atoms, and in doing so they provide a jump start for the atomic nuclei to bind together, like an electrostatic lighter fluid. The speedily cooling hydrogen molecules make it far easier for gravity to pull material together, since the gas pressure is reduced. While less-energetic photons can’t penetrate into dense clouds of gas, X-rays can. And by making hydrogen into molecules in these dense spots, they put the gas on the fast track to making new stars. This theoretical scenario is certainly plausible. In this case, not only do supermassive black holes play a unique and critical role in sculpting the structures of the universe, but small black holes could also be of fundamental importance at the dawn of stellar astrophysics.

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Our conclusion is that the very first black holes—large and small—can leave an imprint on all subsequent stellar generations and galactic environments. The production of new elements and the opportunities for planetary systems all hinge on these early effects, as well as on the long-term behavior of galaxies and the black holes they contain. But not all places are created equal, and most stars that are in the cores of places like galaxy clusters are quite old now. Whatever elements their dying siblings managed to spit out into the void are dispersed in the hot intergalactic gas of these vast gravitational crucibles. Very little of it is recycled back into a state from which it can become new stars or planets. Black holes bear great responsibility for this situation. Ten billion years ago, they restricted and limited what was an explosive growth of stars and elements. Since then, they have continued to hold matter at bay. The multiplicity of black holes that we found in the great dusty mountains of submillimeter-emitting material from 10 billion years ago tallies with their early formation inside the merged splatters of baby galaxies. It also tallies with a picture in which massive black holes often merge with one another, leaving telltale signs in the way that stars are spread across galactic centers. On a much smaller scale, we see aspects of the same behavior in individual galaxies. Those with great swarms of old central stars harbor the most massive black holes. These galaxies are also limited in how many new stars they have been able to make over the past few billion years, and where they could make them.

Some of these places must present a much less fertile terrain, given what we know about the cosmic requirements for life. They may be poorer in condensing elements, and are probably poorer in fresh stars with pristine new worlds. But is this true? Are these locations really unfavorable places for life? The challenge we face is that we only have a single example to serve as context: for now, we know only one planet like Earth. But I would argue that this information still lets us learn something fundamental. We exist in a specific place at a specific cosmic time, in a particular part of a particular galaxy in a particular type of region in the universe. Since that environment is part of the conjoined evolution of black holes and their host galaxies, we can ask what special things link us directly to that history.