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

by Scharf, Caleb

  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 vibrati
on. 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.