Adapted from Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos, by Caleb Scharf, by arrangement with Scientific American/Farrar, Straus and Giroux, LLC (US), Penguin Press (UK), Hayakawa Shobo (Japan) and Prószy´nski (Poland). Copyright © 2012 by Caleb Scharf.
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. 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 black holes.
Lots of cosmic phenomena can potentially influence the existence of life, but some are a little more important than others. Black holes are on that list 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 gigantic spinning electrical battery capable of expelling matter at nearly light speed across tens of thousands of light-years. Black holes also ensnare nearby matter like nothing else can—they are the universe's ultimate competitive eaters. And like a competitive eater, they often ingest matter in great gulps rather than steadily snacking.
Matter falling into a black hole does not go down quietly. It moves at a tremendous speed as it approaches the event horizon, spiraling around in hypervelocity loops if the black hole is spinning. Should that material intersect and collide with anything else on the way, the potential exists for an enormous release of kinetic energy, converted into the motion of atomic and subatomic particles and electromagnetic radiation. Produced well before reaching the event horizon, these particles and photons can escape, surging back out into the universe. A crude analogy is to liken this to water draining noisily from a bathtub. As the liquid falls into the drainpipe, some of its swirling kinetic energy is converted into sound waves, water bashing against molecules of air. The sound waves move faster than the water, and they escape. In the case of a giant black hole, the energy expelled during such digestive episodes can have wide-ranging effects on the surrounding galaxy.
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 supermassive black hole at the center of our own Milky Way galaxy is quiet now, but it, too, switches on from time to time. The duty cycle astronomers have inferred for our central black hole turns out to share a connection with the overall flavor of the galaxy. It also offers intriguing hints about how the solar system manages to support life.
The results of astronomical surveys indicate that the duty cycle of a giant black hole relates, surprisingly, to the host galaxy's stellar medley. The same dynamical processes that send matter hurtling into a black hole—and therefore set its duty cycle—likely influence the kinds of stars that populate a galaxy, and the energy pouring out of a flaring black hole at the peak of the duty cycle can spice up the galaxy's stellar contents. 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 are 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.”
Over the past billion years it has been the largest so-called green valley spiral galaxies that have had 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 10 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 hole is a puzzle. This is a zone of transition, and most galaxies 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 their actions are “greening” the galaxies. It might also be that the same circumstances causing the transformation of a galaxy are feeding matter to the black hole.
As we study other nearby spiral galaxies, 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 windlike 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 is a potent force. Equally, the trigger for such violent output 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 explain why a supermassive black hole's activity roughly correlates 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. The black hole lurking at the center of our galaxy does not seem so active—in fact, it betrays itself most convincingly by its insidious effect on the orbits of galactic core stars. By this measure, it is only four million times the mass of the sun, a relative whippersnapper. Yet according to our canvassing of the universe, it should be one of the very busiest.
To paraphrase Humphrey Bogart, of all the 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 have not 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.
Indeed, it appears things were quite different not so long ago. We see x-rays echoing off interstellar clouds of gas that are 300 light-years from the galactic center. From our perspective, then, something big and powerful in the very core of the galaxy was throwing out a million times more x-ray light 300 years ago than it is today. And in 2010 a small team from Harvard University announced a remarkable discovery: a faint but enormous structure in the gamma-ray light coming from the inner galaxy. It was spread across the sky and looked exactly like a pair of bubbles, each reaching 25,000 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; they may be the signposts of an episode of black hole growth and activity that occurred within the past 100,000 years.
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. The hole may not be the largest or the most prolific at producing energy when it eats, but it is a busy object, a stormy chasm in our midst. We should expect the reignition of this gravitational engine at any time.
Fast, Not Furious
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 only 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 do not 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 to be so suitable for life at all? Isn't that incredibly unlikely?
Like many scientists, I grow uncomfortable when faced with these questions. We are determined to try to overcome any prejudice that we are “special” in any way. Just as Copernicus proposed that Earth is not at the center of the solar system, we are not central to the universe. Moreover, the universe described by modern cosmology has no meaningful center. Yet 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 it is 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 is just an island that has the right climate.
That is 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 black hole some 25,000 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 does not suggest that it is 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 before the formation of our solar system 4.5 billion years ago. Since then, our central monster most likely has had only a 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 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, affecting even 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, it might be a different story. The fact that we do not live on a planet closer to the core may not be coincidental. Similarly, perhaps we should not 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 speak both to the question of how our central black hole can directly influence life on Earth and to its role 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, black hole–powered quasars of the younger universe generally occur in the biggest elliptical galaxies, mostly sitting in the cores of galaxy clusters. These galaxies formed hard and fast and early; by now their stars are almost all old, and their raw gas is mostly too hot to form new stars or planets. Other ellipticals, those huge 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 1/1,000th the mass of the surrounding stars. Our neighbor Andromeda is one of these systems, its generous stellar bulge covering a black hole more than 20 times the size of ours.
Lower down the pecking order are bulgeless galaxies, like many spirals. Although the Milky Way is a vast 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, evincing 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 is not much on an individual human timescale, but it means that there have been at least 10 million new stars born in the Milky Way since our ancestors started walking upright somewhere in Olduvai Gorge in Tanzania. 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 burned out. The annoyed belches of their central black holes quench the formation of any new stars; the rippling pressure waves from their flatulent bubbles of nearly light-speed matter prevent material from cooling down and condensing into stellar systems. Meanwhile the Milky Way keeps trudging along.
Perfect for Life
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 pasts 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 great circulating pressure waves disturb the disk of gas and dust. The stars 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 colossal 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. Too close to a supernova, and there is 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 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 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.
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 to occur. 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.
The entire chain of events 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 numbers 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.
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.