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

by Scharf, Caleb

  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 explo
ding 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.