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ORIGINS: PART II
Our existence in this place, this microscopic corner of the cosmos, is fleeting. With utter disregard for our wants and needs, nature plays out its grand acts on scales of space and time that are truly hard to grasp. Perhaps all that we can look to for real solace is our endless capacity to ask questions and seek answers about the place we find ourselves in. That is not such a bad thing. Ignorance is far scarier than knowledge. One of the questions we are now asking is how deeply our specific circumstances are connected to this majestic universal scheme of stars, galaxies, and, of course, black holes. Given that we now see how the origins of black holes and galaxies are intimately linked, and how the subsequent evolution of both is tied together, it’s not unreasonable to ask what we might owe to these pinhole punctures in spacetime. Fortunately, we live in an era where we can begin to answer that question.
Our gloriously vast universe contains at least 100 billion galaxies. Generations of careful observation, mapping, and extrapolation have gone into producing this estimate. All but a small handful of these great stellar systems are invisible to the naked human eye. Indeed, the tiniest and dimmest systems are in the majority. Dwarf galaxies, miniature versions of the fuzzy stellar swarms that we call ellipticals, are the most numerous systems in the cosmos. They’re incredibly hard to spot, though, since they can consist of as few as a couple of million stars and be only a few hundred light-years across. They’re so faint that they vanish out of sight for all but the most persistent and well-equipped observers. The big, more easily seen galaxies are broadly divided. The great disks of spirals are almost always large. They can span hundreds of thousands of light-years, and they can contain a trillion stars. Away from the intense environment of galaxy clusters, they represent more than 70 percent of all large systems. Ellipticals can also be huge, but in number they amount to only about 15 percent of all large galaxies.
We are part of this great intergalactic jungle, and to finish my argument about the relationship of black holes to life in the cosmos, I’m first going to dig deeper into the story of our own very particular watering hole.
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The Milky Way itself is a big system, even by the standards of spirals. Its 200 billion stars amount to a mass approximately 100 billion times that of our Sun, and its disk stretches across a diameter of 100,000 light-years. Our parent star and our home planet are positioned toward the outer edge of this vast plate, although by no means at the edge of the matter it contains. The visible stars represent just one aspect of a slowly rotating, center-orbiting wheel of dust, gas, and dark matter. Every 210 million years, we complete another circumnavigation of the Milky Way. Since the Sun formed more than 4.5 billion years ago in a long-dissipated clutch of other new stars, we have made this galactic round trip just over twenty times.
Our biggest neighbor is the Andromeda galaxy, separated from the Milky Way by a gaping void of 2.5 million light-years of intergalactic space. Our eyes see only the barest hint of a hazy patch at its location. In truth, its light is spread out across the sky in a great band some six times the size of the full Moon. It is a giant spiral, but it is quite different from the Milky Way. While our galaxy is still actively producing a few new stars every year, Andromeda has descended into late middle age. It’s not without baby stars, but they are forming at one-third to one-fifth the rate that they do in the Milky Way. Andromeda’s central cloud of very old stars is also far more prominent than that of the Milky Way. Nestled inside this central stellar hive in Andromeda is a black hole 100 million times the mass of our Sun. Just as in most other galaxies, this hole is one-thousandth the mass of the old stars surrounding it.
Figure 16. The spiral galaxy known as NGC 6744, widely considered to be a close match for the structure of our own Milky Way galaxy. It is 30 million light-years from our location.
In 4 billion to 5 billion years, the curved spacetime containing the masses of Andromeda and the Milky Way will cause them to merge. In fact, they’ve already started falling toward each other. Although this encounter will happen at a velocity of more than a hundred miles a second, it will not be a collision in the traditional sense of the term. There is so much space between the tiny points of condensed matter in stars that the galaxies will simply drift and flow into each other with little violence. Exactly how intimate this vast embrace will be is unclear, and it will play out over hundreds of millions of years. But eventually the combined content of these two great systems may settle into something resembling an elliptical galaxy, and Andromeda and the Milky Way will be no more.
Regardless of the outcome, by the time this slow collision begins our Sun will have used up the hydrogen fuel in its core, which will contract inward as gravity acts against the diminishing pressure in its center. The shrinking interior will get hotter and will flood the upper layers of the solar atmosphere with radiation, inflating them outward. The Sun will grow to a bloated and gouty red-giant star, engulfing what remains of the inner planets, including Earth. Whether or not our distant descendants are still around to witness these events, they will no doubt mark the end of our birthplace. This tiny scrap of rock and water that took life from microscopic single-celled organisms to beings like us in just a few billion years will be erased. But until then, we have a chance to understand what makes the Milky Way tick, and how it compares to all other galaxies.
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We live in a time of unprecedented cosmic exploration. The tools of modern astronomy are unlike anything else in the history of our species. We have the technological prowess to construct exquisite instruments like the Chandra X-ray Observatory and the Hubble Space Telescope. We can also co-opt the powers of computer automation and global communications to examine previously unimaginable volumes of the universe. Indeed, much of the astronomy we’ll practice in the future will involve a degree of rich mapmaking and information gathering the likes of which we have never seen before. Giant new telescopes will sweep the skies, and every few nights they will produce a record of hundreds of millions of cosmic objects, from stars to galaxies. And they will repeat this again and again. They will do for the universe what something as mundane as a security camera does for a city street: constantly monitoring, constantly growing our library of data, and producing a map of the cosmos of ever-increasing levels of detail in both space and time.
Our early steps toward this new type of astronomy have already begun. One such effort has been the project known as the Sloan Digital Sky Survey. This extraordinary enterprise has surveyed over 35 percent of Earth’s night sky since the year 2000, detecting 500 million astrophysical objects. Sloan’s modest telescope is just over eight feet in diameter, but it scans across swath after swath of the heavens above New Mexico, with its digital camera recording untold cosmic photons. More than a million of the objects it has captured are galaxies, an incredible sampling of the local universe that penetrates 2 billion years deep into cosmic time. But how to pick through such a wealth of data?
In 2007, a consortium of astronomers launched a project called Galaxy Zoo. The idea was simple, but challenging to implement. The galaxies within the Sloan survey needed to be classified—somehow we needed to assign the correct physical label to all the detected systems in order to extract robust statistical facts about them. The characterizations are familiar: spirals and ellipticals, and subdivisions within these kingdoms. This kind of classification might sound like a straightforward task—surely a computer could be used to “recognize” galaxies. However, nature is tricky, and mistakes made at the level of just a few percent can mess everything up. There is an enormous range of natural variation in structures, as well as confusing quirks—and just plain hiccups in the data. Even very clever computer algorithms can be fooled, especially when you have a million systems to work on.
Since the advent of telescopic astronomy four hundred years ago, the human eye and brain have proved themselves to be remarkable image analyzers. With just a small amount of training and practice, a human being can distinguish galaxy types with incredible efficiency. It’s like looking at dried flowers or squashed bugs—after a while you can race through samples with little hesitation. There’s a daisy, a lily, a rose, another daisy. There’s a beetle, a fly, another fly, a mosquito—the human mind is a brilliant pattern-recognition machine. The real problem facing the Galaxy Zoo project was the sheer scale of its goals. The scientists wanted to classify a million galaxies; they also required at least twenty duplicate identifications for each possible galaxy, in order to weed out mistakes. Not even a dedicated group of scientists could find the time or perseverance to accomplish this.
The solution was to harness the power of the human hive, to “crowdsource” astronomy like never before. Soon after it launched, Galaxy Zoo put out a request for volunteers on the Internet. Within a month of its call for human eyes, eighty thousand people had carved out time to look at the million galaxies ten times over. They were scientists, students, bus drivers, retirees, amateur astronomers, kids, athletes, writers, artists, doctors—individuals from all walks of life. It was an amazing example of the joyful and satisfying spirit of cooperation. Just a year later, a staggering 150,000 people had made more than 50 million classifications. The project continues to this day, expanding into the details of galaxies and into new data from the huge archives of the Hubble Space Telescope’s two decades in orbit.
Huge sets of data like the information gathered through Galaxy Zoo have allowed astronomers to tackle questions that used to be near-impossible challenges. It is like having census data from an entire continent instead of from a few quirky and obscure neighborhoods. Exactly how we will find ourselves interpreting the results will likely play out over the coming decades, but for now we can ruminate on some of the most compelling discoveries. For us, a key one is the link between galaxy properties and the supermassive black holes that they host. At last we have a way to avoid the confusing peculiarities of individual galaxies and to look instead at how they match up against a million other systems.
We find that there is a significant difference between the supermassive black holes inside elliptical galaxies and those inside spiral galaxies. In today’s least-massive elliptical galaxies, the least-massive black holes are also the most active black holes—they are still eating and producing energy. The opposite is true in spiral galaxies: in these systems it is the most-massive black holes that are producing the most energy. This sounds like a stunning reversal in behavior until we realize that the most-massive black holes in spiral galaxies are in fact about the same size as the least-massive black holes in elliptical galaxies.
We can interpret this to mean the following. In today’s universe the most-massive black holes are effectively has-beens. It doesn’t matter where they are—most of them have eaten their fill and are certainly not going to light up like quasars again. They are starving. Whatever activity they exhibit is typically modest: enough, for example, to regulate the flow and cooling of matter deep inside a galaxy cluster. The lower-mass black holes, from a few million to a few tens of millions of times the mass of the Sun, are the main players in our surrounding universe. They are still growing, albeit rather gently and sporadically. Thus, the quasars and the great elliptical galaxies have exhausted themselves by leaping out of the starting gate, while the spirals and their more modest black holes have been biding their time. It is the ultimate race between the tortoises and the hares. In fact, some observations suggest that the level of black hole growth we see in these tortoises today is larger than it was a few billion years ago. Only after nearly 14 billion years are they finally hitting their stride.
Where, then, does our galaxy, the Milky Way, sit among these grand tortoises? The answer reveals something quite profound, but first we have to understand how to get there. When astronomers talk about matter being fed into supermassive black holes, they talk about “duty cycles,” just like the episodic sloshing of clothes inside a washing machine. The speed of a black hole duty cycle describes how rapidly it changes back and forth from feeding on matter to sitting quietly. The periodic distribution of the great bubbles floating up through clusters of galaxies is an excellent example of a duty cycle made visible. Detecting the presence of black holes is far easier when they are “switched on,” and the faster this cyclical behavior, the more black holes you will detect at any instant in a region of the cosmos. It’s like being in a completely darkened room full of very hungry mice. If you toss out some pieces of cheese, the fastest runners will quickly scurry from crumb to crumb, and you will count many of them simply by listening. The slow ones take big pauses between snacks, and you will count far fewer at any given moment.
The results of surveys like the Sloan and the Galaxy Zoo indicate that this duty cycle is related to the overall stellar contents of a galaxy. These contents are a critically important clue to the nature of a galactic system. The stars in a galaxy can be reddish, yellowish, or bluish; blue stars are typically the most massive. They are therefore also the shortest-lived, burning through their nuclear fuel in as little as a few million years. This means that if you detect blue stars in the night sky, you’re catching sight of youthful stellar systems and the indications of ongoing stellar birth and death. Astronomers find that if you add together all the light coming from a galaxy, the overall color will tend to fall into either a reddish or a bluish category. Red galaxies tend to be ellipticals, and blue galaxies tend to be spirals. In between these two color groups is a place considered to be transitional, where systems are perhaps en route to becoming redder as their young blue stars die off and are no longer replaced. With nary a sense of irony, or indeed color-mixing logic, astronomers call this intermediate zone the “green valley.”
Surprisingly, over the past billion years it is the largest green valley spiral galaxies that have the highest black hole duty cycles. They are home to the most regularly growing and squawking giant black holes in the modern universe. These galaxies contain 100 billion times the mass of the Sun in stars, and if you glance at any one of them, you are far more likely to see the signs of an eating black hole than in any other variety of spiral. One in every ten of these galaxies contains a black hole actively consuming matter—in cosmic terms they are switching on and off constantly.
The physical connection between a galaxy being in the green valley and the actions of the central black holes is a puzzle. This is a zone of transition, and most galaxies in the universe are either redder or bluer than this. A system in the valley is in the process of changing; it may even be shutting down its star formation. We know that supermassive black holes can have this effect in other environments, such as galaxy clusters and youthful large galaxies. It might be that these actions are “greening” the galaxies. It might also be that the circumstances causing the transformation of a galaxy are feeding matter to the black hole.
As we study other spiral galaxies in the nearby universe, we do find evidence that the black holes pumping out the most energy have influenced their host systems across thousands of light-years. In some cases, the fierce ultraviolet and X-ray radiation from matter feeding into the holes can propel wind-like regions of heated gas outward. These wash across a galaxy’s star-forming regions like hot-weather fronts spreading across a country. Exactly how this impacts the production of stars and elements is unclear, but it’s a potent force. Equally, the trigger for such violent output from the central hole can influence the broader sweep of these systems. For example, the inward fall of a dwarf galaxy captured by the gravity well of a larger galaxy stirs up material to funnel it toward the black hole. It is like fanning the embers of a spent fire to relight it. The gravitational and pressure effects of that incoming dwarf galaxy can also dampen or encourage the formation of stars elsewhere in the larger system. Some or all of these phenomena could help link a supermassive black hole to the age (and hence color) of the stars around it.
Remarkably, astronomers have recently realized that our Milky Way itself is one of these very large green valley galaxies. What this means is that our supermassive black hole should be on a fast duty cycle, which is quite a surprise. I’ve talked about the black hole lurking at the center of our galaxy; it didn’t seem so active—in fact, it betrays itself most convincingly by its effect on the orbits of galactic core stars. By this measure, it is only 4 million times the mass of the Sun, a relative whippersnapper. Yet according to our canvassing of the universe, it should also be one of the very busiest.
To paraphrase Humphrey Bogart, of all places in all the galaxies in all the universe, we had to go and find ourselves in this one. It is of course tempting to be skeptical: we haven’t thought of our galaxy as playing host to a particularly hungry supermassive black hole. But perhaps this is just a question of timing, of our short lives compared to the lifetime of the cosmos. We need to find out what’s going on—do we really live in a quiet or a busy intergalactic neighborhood? Intriguingly, some dramatic evidence now suggests that our received wisdom is due for an overhaul. That evidence comes from viewing the Milky Way through some very special glasses.
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The most energetic form of electromagnetic radiation is the gamma-ray photon. Gamma rays have wavelengths less than the size of an atom; they are highly penetrating, much more so than X-rays, and will travel through anything but the thickest sheets of metal or rock. On Earth they originate from the processes occurring within unstable atomic nuclei as part of natural radioactivity. For example, gamma rays produced from the isotope Cobalt-60 are used by the food-processing industry to irradiate and sterilize products like meat and vegetables. Out in the universe, they come from some of the most violent and energy-rich events: stellar implosions, hypersonic shock waves, and the effects of ultra-relativistic particles streaking across space.
For decades a particularly mysterious and persistent set of gamma-ray photons have been finding their way into astrophysical detectors. Although the signal proved hard to pin down, it was clear that these ever-present gammas were coming from a very particular direction—from the inner regions of our own galaxy. It was an ominous sign of fierce processes occurring somewhere deep within the Milky Way.
Eventually, X-ray telescopes, like the Roentgen Satellite we’ve already encountered, began to pick up tentative signs of immense structures jutting out from our galactic core. These zones of X-ray light were difficult to spot because of their extreme faintness, but astronomers were able to see that they resembled conical funnels opening out toward intergalactic space, spanning thousands of light-years. Their presence suggested that a release of energy, some kind of outflow or vast galactic wind blowing from the inner galactic sanctum, was propelling tenuous hot gas outward.
During the early twenty-first century, astronomers were also charting out the mottled tapestry of the cosmic background radiation through their microwave receivers. These stretched remnants of the photons from the dawn of the universe contained something unusual, too—another tantalizing glimpse of a huge structure. As the scientists analyzed the great microwave sky maps, they saw hints of a subtle tinge, a haze covering that same inner zone of our galaxy. It suggested that the cosmic photons might be passing through some kind of structure composed of fast-moving particles. The photons were being altered on their way to us, their energies shifted by something lurking in this region.
In 2010, a small team from Harvard University led by astronomer Doug Finkbeiner announced a remarkable discovery. Two years earlier NASA had launched a new observatory into orbit. Named Fermi after the famous physicist Enrico Fermi, this instrument represented a huge advance in the way we study gamma rays from space. It could produce high-fidelity gamma-ray images, opening up new cosmic vistas to astronomers. As Fermi orbited the Earth, it constructed a map of the entire sky, scooping up gamma-ray photons from every corner of the universe. Finkbeiner and his team analyzed this map in meticulous detail. They painstakingly combed through it, plucking out all the bright and noisy objects that were blocking the view from our cosmic vantage point. It’s like trying to chart the underlying forms of a large and moonlit city. You have to remove the glare of the office windows, car headlights, and streetlights before you can see the outlines of the buildings.
Gradually they peeled away the layers of the chart … and there, beneath everything else, they found something quite extraordinary. There was a faint structure in the gamma-ray light coming from the inner galaxy. It was spread across the sky, and it looked exactly like a pair of bubbles. One emerged on either side of the galaxy, to the “north” and to the “south,” a vast pair of globe-like wings reaching twenty-five thousand light-years up and away into intergalactic space. Glowing with gamma-ray photons, these bubbles are anchored at their bases to the very core of the Milky Way.
We think that the gamma-ray photons from these structures come from lower-energy photons that are boosted by fast-moving particles such as electrons. This is exactly the mechanism we’ve seen in the larger structures surrounding the host galaxies of jet-spewing supermassive black holes. It is the process that we found lighting up the colossal bubbles rising from a black hole in the youthful universe. It originates with particles moving close to the speed of light, accelerated from the regions close to an event horizon.
It’s still possible that these galactic bubbles are the result of an enormous flurry of stellar birth and death taking place in the galactic core millions of years ago. Such a “burst” of thousands of stars can produce great outflows of radiation and matter that could conceivably produce similar structures. But there is additional evidence indicating that these gamma-ray bubbles really are the signposts of an episode of black hole growth and activity that occurred within the last hundred thousand years.
When we took our journey toward the galactic center, we found a variety of large and intriguing structures, from giant rings of dense gas to other clumps and clouds of material. We’ve known these to be cold forms, made of frigid molecules sitting in the chill of interstellar space, or tepid and dull clouds of gas. Yet astronomers have found that some of these otherwise dark structures are glowing with X-ray light. This glow has a very particular flavor. It comes from cold atoms of iron that have been agitated until they release X-ray photons. The best explanation is that this agitation is really a form of reflection. X-ray light washes across the cool nebula, where it is absorbed and re-emitted toward us. In this case the gas acts as a giant hazy mirror, and scientists have concluded that the only plausible original source for this reflected radiation is an intensely energy-rich environment at the very core of the galaxy. But because the X-rays we see are echoes off clouds that are three hundred light-years from the galactic center, it means that we are watching a time-delayed playback. From our perspective, something big and powerful in the very core of the galaxy was throwing out a million times more X-ray light three hundred years ago than it is today.
The pieces of evidence are adding up to a compelling picture of our home environment. If the Milky Way obeys the rules that we see in tens of thousands of other galaxies, then it must contain a black hole that is getting fed very regularly. From the gamma-ray bubbles and the ravaged molecular rings of the inner galaxy to the ghostly echoes of X-ray light produced three hundred years ago, there is every reason to believe that we harbor a black hole that is indeed very active. The hole may not be the largest or the most prolific at producing energy when it eats, but it’s a busy object, a stormy chasm in our midst. Centuries ago, it burned bright to create the ethereal reflections from the galactic core. Perhaps twenty-five thousand years ago it erupted on an even greater scale to blow the vast bubbles that glow bright in the gamma-ray sky. We should expect the re-ignition of this gravitational engine at any time. If only John Michell or Pierre-Simon Laplace had had a space-borne telescope at their disposal when they looked up to the stars in their scientific quests—the sky in the 1700s would have been rather spectacular!
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Clearly, our Milky Way and its central black hole belong to a special club. They hold a distinctive status within today’s universe, one that points to a possible connection between the cosmic environment and the phenomenon of life here on Earth. Scientists and philosophers sometimes discuss what are called “anthropic principles.” The word anthropic is derived from ancient Greek and means that something pertains to humans, or to the period of human existence. Anthropic principles usually tackle the awkward question of whether or not our universe is somehow just right for life to occur. The argument goes that if just a few fundamental physical laws, or physical constants, in the universe were just a bit different, it would have failed to produce life. But we don’t currently have good explanations for why the physical parameters of the universe are what they are. So the question stands out: Why did our universe turn out so suitable for life at all? Isn’t that incredibly unlikely?
Like many scientists, I grow uncomfortable when faced with these questions. We’re determined to try to overcome any prejudice that we are “special” in any way. Just as Copernicus proposed that the Earth is not at the center of the solar system, we are not central to the universe. Indeed, the universe described by Einstein’s field equation has no meaningful center. But some of the anthropic arguments are trickier to respond to. One possible solution to the discomfort of assigning ourselves a special status hinges on a conceptual and physical picture of nature that allows for multiple realities, or multiple universes. For example, if our universe is merely one of many that exist within a higher-dimensional version of spacetime, then there’s no surprise that we exist here. We simply exist in a universe that has the conditions that allow for the phenomenon of life—there is nothing special about it. It’s just an island that has the right climate.
That’s all quite entertaining stuff, but it also makes us think a little more about exactly what the laundry list of conditions is for life in a universe. It really is striking that the Milky Way, containing us, lands smack-dab in the sweet spot of supermassive black hole activity. It is possible that this is not mere coincidence, and the first question that springs to mind is whether our solar system experiences direct physical ramifications of the activity of a 4-million-solar-mass black hole some twenty-five thousand light-years away. Could it affect the suitability of our suburban galactic neighborhood for life-bearing planets? When our central black hole switches on, eating and pumping out energy, the evidence doesn’t suggest that it’s enormously bright from our viewpoint. The huge gamma-ray glowing bubbles extending out from the galactic disk definitely indicate some pretty hefty energy production, but not directed toward us. If larger events ever occurred, they must have been in the distant past, perhaps even prior to the formation of our solar system 4.5 billion years ago. Since then, our central monster has likely had only modest physical impact on distant galactic suburbs like those of our solar system.
From the point of view of life, this may be a good thing. A planet like the Earth could be sideswiped by a large increase in ambient interstellar radiation in the form of high-energy photons and fast-moving particles. Radiation can have a deleterious effect on the molecules inside organisms, and it can even affect the structure and chemistry of our atmosphere and oceans. We may be relatively well shielded at 25,000 light-years from the galactic center, but if we lived closer to the galactic core it might be a different story. So the fact that we don’t live on a planet closer to the core may not be coincidental. Similarly, perhaps we shouldn’t be surprised to find ourselves here at this time, rather than billions of years in the past or in the future.
Our galaxy has, like so many others, coevolved with its central supermassive black hole. Indeed, the clues we seek may be less about the question of how our central black hole can directly influence life on Earth, and more about the role it plays as an indicator of the present state of our galaxy in general. The connection between supermassive black holes and their galaxies provides us with a real tool for gauging galactic history. The ferocious quasars of the younger universe are linked to the biggest elliptical galaxies, mostly sitting in the cores of galaxy clusters. These galaxies formed hard and fast and early, the excitable hares in the race. By now their stars are almost all old, and their raw gas is mostly far too hot to form new stars or planets. Other ellipticals, those great dandelion heads of stars, seem to have formed later as galaxies merged. Something along the way has “quenched” their formation of stars. We think that less-violent, but still incredibly powerful, output from supermassive black holes is an excellent candidate for this regulatory role. The spirals with bulges of central stars jutting high above and below the galactic disks also show the signs of an intimate history with their central black holes. They follow some of the same patterns as the ellipticals. In both, the central black hole mass is one-thousandth of the mass of the surrounding stars. Our neighbor Andromeda is one of these systems, its generous stellar bulge covering a black hole more than twenty times the size of ours.
Lower down the pecking order are bulgeless galaxies, like many spirals. Although the Milky Way is a huge galaxy, one of the biggest in the known universe, it harbors a relative pipsqueak of a black hole. The lack of a stellar bulge is a mystery: either the galaxy had less raw material to form from in the first place, or the regulating black hole never really kicked in, or fewer small galaxies and clumps of matter have fallen into the system across time. The incredibly numerous dwarf galaxies also come up short in the black hole department. The true dwarfs of the galactic zoo are quite pitiful things, often with just a few tens of millions of stars or so, and little sign of the gas or dust that will make new ones. Those that are rich in interstellar soup are often so dark, so devoid of stars, that it is as if someone forgot to light the fuse.
Our galaxy still makes stars, at a rate of approximately three solar masses a year. This isn’t much on an individual human timescale, but it means that at least 10 million new stars have been born in the Milky Way since our ancestors started walking upright somewhere in the Olduvai Gorge. This is not bad for a place within a universe that is almost 14 billion years old. The giant galaxies of the young universe, blazing with the quasar light from their cores, are in some senses long burnt out. The annoyed belches of their central black holes quench the formation of any new stars; the rippling waves from their flatulent bubbles of relativistic matter prevent material from cooling down and condensing into stellar systems. A tortoise among these hares, the Milky Way still trudges along.
That we live in a large spiral galaxy with very little central stellar bulge and a modest central black hole may be a clue to the type of galaxies best suited to life: ones that did not spend their past building colossal black holes and fighting the demons unleashed in the process. New stars continue to form in a galaxy like ours, but with different vigor from other systems. Most new stars are forming on the edges of the spiral arms as these great circulating waves disturb the disk of gas and dust. They are also forming farther from the galactic center than they used to. Astronomers say that we live in a region of “modest” star formation. Very active star formation produces an awfully messy environment. It builds the massive stars that burn through their nuclear fuel the fastest, ending up as great supernova explosions. Planetary atmospheres can be blasted away or chemically altered by radiation. Fast-moving energetic particles and gamma rays can pummel the surface of a world. Even the flux of ghostly neutrinos released in stellar implosion is intense enough to damage delicate biology. And those are just the moderate effects. Live too close to a supernova and there’s a good chance your entire system will be vaporized.
Yet these are also the very mechanisms by which the rich elemental stew inside stars spreads out into the cosmos. This raw material creates new stars as well as planets. They are planets with complex chemical mixtures of hydrocarbons and water, layered and dynamic, stirred by the heat of heavy radioisotopes, with billions of years of geophysics ahead of them. So somewhere in between the zones of forming and exploding young stars and the nursing homes and graveyards of ancient ones is a place that is “just so,” and our solar system resides in just such an environment. It is far enough from the galactic center, but not too close to the busy and explosive realms of stars that are forming right now. Of course, all this will change in 5 billion years, when the Andromeda galaxy comes sailing into us.
The connection between the phenomenon of life and the size and activity of supermassive black holes is quite simple. A fertile and temperate galactic zone is far more likely to occur in the type of galaxy that contains a modestly large, regularly nibbling black hole rather than a voracious but long since spent monster. The fact that there are any galaxies like the Milky Way in the universe at this cosmic time is intimately linked with the opposing processes of gravitational agglomeration of matter and the disruptive energy blasting from matter-swallowing black holes. Too much black hole activity and there would be little new star formation, and the production of heavy elements would cease. Too little black hole activity, and environments might be overly full of young and exploding stars—or too little stirred up to produce anything. Indeed, change the balance at all and you change the whole pathway of star and galaxy formation. As we’ve seen, even the presence of small black holes, as the universe emerged from its cosmic dark ages, may have helped direct these chains of events.
The entire pathway leading to you and me would be different or even nonexistent without the coevolution of galaxies with supermassive black holes and the extraordinary regulation they perform. The total number of stars in the universe would be different. The number of low- and high-mass stars would be different. The forms of the galaxies would likely be different, and their organization of gas, dust, and elements would almost certainly be different. There would be places that had never been scorched by the intense synchrotron radiation of a supermassive black hole. There would be other places that had never received that jolt, that kick in the pants, that got star or planet formation up and running.
Lots of cosmic phenomena are connected to the existence of life, but some are a little more important than others. Black holes are on that list, and it’s because of their unique nature. No other object in the universe is as efficient at converting matter into energy. No other object can act as a great electrical battery capable of expelling ultra-relativistic matter across tens of thousands of light-years. No other object can grow so massive yet still be so comparatively uncomplicated. A black hole is a dent in spacetime described by just three fundamental quantities: mass, spin, and electrical charge.
Take a look at your hand. It contains atoms of carbon, oxygen, and nitrogen that were forged a million miles below the surface of another star billions of years ago. Your hand also contains hydrogen that was present at the very beginning of the universe. All these elements have felt the forces of black holes. And right now, a tiny fraction of the vast electromagnetic sea of photons racing through the universe is reaching down through our atmosphere, hitting those ancient atoms of your flesh. Some of these photons originated in the fearsome spinning of matter around black holes, or from the accelerated jets of particles rushing at near light speed out into the cosmos. We are awash in their radiation, but it is nothing new to the atoms in your hand. As the cosmic dark ages lifted 13 billion years ago, some of the primordial hydrogen in your body was likely buffeted and tickled by the radiation of feeding black holes. Billions of years later, the stars that built your heavy elements existed because of the history of gravity and energy in one zone of what was becoming the Milky Way galaxy.
This fertile corner of the cosmos has been governed by all that has gone on around it, including the behavior of the black hole at our galactic center. The very places that have sealed themselves away from the rest of the universe have served as one of the most influential forces shaping it. We owe so much to them.