The second piece to the problem of where all structures and objects come from is a little trickier: it’s all about resistance to building, or even destruction. So is there a yin to the yang of cosmic evolution? Indeed there is. Matter in the universe, rather perversely, also creates many obstacles to its own assembly. The most fundamental hurdle for matter comes from the basic phenomenon of its own pressure, which is in turn related to its temperature. The component atoms or molecules of a material such as a gas are buzzing and bustling around, and we gauge this by talking about how hot or cold the gas is. The hotter the gas and the faster the typical motion of these particles, the greater the particles’ thermal energy. The colder it is, the more sluggish the particles get. Eventually, close to absolute zero, they should all stand still, but for their inherent quantum wiggling and jostling.
What we experience as gas pressure here on Earth is the combination of this thermal motion and the number of atoms or molecules in a particular region. With a ping, ping, ping, the gas particles of our atmosphere bounce against our skin, inside our lungs, and against one another. When you blow up a balloon you are filling it with trillions of air molecules that beat against the rubbery material, causing it to stretch and expand. The thermal motion of the gas creates this property of matter called pressure, which resists efforts to contain it. This is precisely how pressure and temperature work against gravity. Matter will try to fall into, pour into, the deep wells and bowl-like distortions in spacetime caused by mass. But the incessant motion of that matter is like having an infestation of springy fleas that you’re trying to trap inside smaller and smaller boxes. The moving molecules just don’t want to be confined. It’s further complicated because matter tends to get even hotter as it becomes compressed. The same thing happens when you try to pump air into a bicycle tire. The forces of gas pressure resist compression, and some of the energy from your arms is converted into heat, causing the pump to get warm. That heat comes from the speeded-up thermal motion of the gas particles. The hotter the gas gets, the greater the pressure. It is a major obstacle to building objects in the cosmos, yet evidently not an insurmountable one, or you and I would not be here discussing it.
Objects can also explode. Massive stars have a bothersome tendency to end millions of years of nuclear fusion in great cataclysms we call supernovae. Similarly, white dwarfs may be fed just a little too much matter and exceed the critical Chandrasekhar mass, the largest-size object that can be supported by the quantum electron pressure that we encountered before. They implode disastrously. Radiation and particles burst forth to disperse all that carefully gathered material, like an impatient child getting frustrated with a house of cards. And we’ve seen that black holes, small and large, can generate huge outputs of energy with incredible efficiency. All these phenomena act against the gravitational gathering of matter, but despite this our universe clearly reaches a balance. It has to, or else there would either be nothing much present except tenuous gas and dark matter—or all matter would be locked up in black holes. The success of constant construction is clearly tempered by the success of ongoing obstruction; we are surrounded by a shifting and dynamic impasse. And a key discovery that will help us understand this impasse, this point of equilibrium, begins with an extremely famous and rather intimidating family.
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First there was old grandpa Erasmus Darwin, a physician and a highly regarded and historically important natural philosopher in the 1700s. Then, two generations later, along comes Charles. He sails off from England in his twenties for a five-year journey to the exotic southern oceans of Earth. He returns and later helps revolutionize our view of the nature of life. One of his sons, George, makes profound contributions to physics and celestial mechanics. And later one of George’s sons, named Charles in cyclical fashion, becomes a highly respected physicist who helps apply quantum mechanics to the problems of atomic physics in the early twentieth century. Pity the Darwins’ neighbors—personally, I’d focus on making my lawn look better than theirs.
In this brilliant family, it is Charles’s son George Darwin who plays a small but pivotal role in advancing our understanding of how matter in the universe ends up making planets, stars, and even the great clusters of galaxies. It begins with an almost offhand, but deeply insightful, comment by George in the late 1800s. At the time, scientists were working to understand the origins of stellar systems in our own galaxy. A popular theory was the “nebular hypothesis,” which posited that stars and planets formed out of interstellar gas and dust, somehow coalescing and condensing out of this material— although how and why an interstellar cloud, or nebula, would want to do that was a point of uncertainty. While George Darwin’s main scientific efforts went into the complex subject of gravitational tides on planets and moons, he was also very aware that the nebular hypothesis really needed someone to tackle the physics behind it. In a paper published in 1888, he succinctly described what was missing from the theories of the time: the mathematical description of how a rotating cloud of gas could give way to its own gravitational forces to condense into stars and planets. It was a clearly phrased challenge, waiting for someone bold enough to pick it up.
More than a decade passed, and finally, in 1902, a young physicist employed at the University of Cambridge named James Jeans took George’s comment to heart. Consulting with the now-senior Darwin, he dutifully produced a fifty-three-page treatise, “The Stability of a Spherical Nebula.” In this work, Jeans provided the mathematical and physical basis for understanding the fundamentals of what we now call “gravitational collapse.” The essence of it is simple, the practice rather more complex. Jeans established that in a structure like a nebula there are two opposing forces at play. One is gravity. The matter in a nebula has mass, and so it will tend to fall together, shrinking itself. The other force is the natural pressure of the gas. This is the springy force that resists the inward fall of material.
A blob of dense nebula represents more mass than a blob of less-dense nebula, so the greater the density, the greater the gravitational forces at work. But density is also related to gas pressure, temperature, and composition. Jeans saw that high temperature and low density would make it harder for a nebula to condense to make objects like stars. Conversely, low temperatures and high densities would make it easier for gravity to pull material together. Jeans also realized that if you measured just the temperature and density in a nebula, you could immediately calculate the size of a region that would be hovering in balance, just poised to collapse. A smaller region would have insufficient gravity to overcome its gas pressure. A bigger region would have insufficient gas pressure to resist gravity’s embrace. This critical point later became known as the Jeans Mass.
In other words, if you find a nebula that is bigger than its Jeans Mass, then it is almost inevitable that it will be in the process of collapsing and condensing to make stars. Similarly, any cloud of gas that is actively cooling down by emitting radiation stands a good chance of cooling enough that it begins to collapse under its own gravity—especially if its mass is only a little less than the value of the Jeans Mass.
But surely one can just see whether or not a nebula is collapsing, right? Why bother with all this calculation? The problem concerns the human timescale versus the timescales of the cosmos. We’d have to wait around for hundreds of thousands of years to really notice a nebula collapsing to make stars. We’re just too puny and short-lived. Instead we must rely on clues like those we get from Jeans’s equation. He found a way for us to deduce actions of matter that are happening at a snail’s pace from our terrestrial perspective.
We now know that there are many hideous complications to this simple picture. These include the elastic-like effects of interstellar magnetic fields, flowing motions in the nebula, and the endless lumpiness and complexity of material spread around in our galaxy. However, Jeans’s insight is still critical. In a general form it applies across the universe, from the very first generations of stars to those forming in the spectacular
Orion nebula in our night sky. Gravity must always overwhelm pressure in order to make objects in the cosmos. It also provides the basis for the next piece of our story, which is all about places not behaving the way you might expect them to.
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Clusters of galaxies might not at first seem to be the likeliest candidates for unveiling the mysteries surrounding the life cycle of black holes in the universe. While a supermassive black hole can occupy a volume similar to that encompassed by the orbit of Neptune, a big cluster of galaxies can occupy a region some 30 million light-years across. The black hole is only 0.00000000001 times the size of the cluster. That’s the size of the period at the end of this sentence compared to one-third of the distance to the Moon. Nonetheless, there is indeed a very special relationship between these two vastly different structures, and it’s one that is connected to the constructive and destructive elements of the cosmos.
I’ve said it before: galaxy clusters are the cathedrals of the universe. These vast systems can contain hundreds, even thousands of galaxies. In this way clusters are the largest “objects” in the cosmos, the great big conglomerations of material at the intersections of the cosmic webbing of matter. As such, they also represent a nearly closed environment, an astrophysical biosphere in which physical phenomena are captured and contained. They are gravitationally bound together within the spacetime distortion of a quadrillion Suns’ worth of mass, composed of dark matter, gas, and stars. As a result, the escape of material is seldom an option.
In these intergalactic biospheres, the majority of normal matter exists in the form of extremely hot and tenuous gas—gas that’s so hot that electrons are stripped from atoms to leave them as ions, the positively charged nuclei and negatively charged electrons coexisting to create plasma. This plasma outweighs all the stars in the galaxy clusters. Most of it is primordial hydrogen and helium, slurped down into the gravity well of the cluster by the same circumstances of imbalance discovered by James Jeans. This deep well is in turn dominated by the unseen dark matter that outweighs all the normal matter in gas and galaxies by about ten times. The captured gas falls ever inward within the gravity well, but as it accelerates it crashes into itself, and converts the energy of this waterfall-like pouring motion into the thermal motion of individual atoms. This is how the gas heats up, and a temperature of 50 million degrees is not unusual inside the biggest galaxy clusters. And the more massive the cluster, the higher the temperature can go.
Hotter gas means higher pressure, and that can stop gravity from compressing the gas any more. Instead it just sits and seethes in the gravitational bucket of the cluster. But over time this gas can also cool off. It can do this by rearranging any electrons that have managed to reattach themselves to ions, squirting out photons of light and releasing that energy. It can also cool as the electrons are decelerated by the electric fields between themselves and the oppositely charged ions of the gas.
This is much like the shrieks of rubber hitting rubber at the bumper-car rink in an amusement park. That noise is energy being lost as the cars whack into their surroundings. Similarly, electrons buffeted about in a plasma emit photons of light to bleed off energy. It’s like the processes of radiation emission seen in particle accelerators that we talked about in the last chapter. The scientific name for the phenomenon is a wonderful mouthful: bremsstrahlung (brems-stra-lung) comes from the German bremsen (to brake) and Strahlung (radiation), and literally means “braking radiation.” Among its many fascinating characteristics, bremsstrahlung from the cooling gas inside galaxy clusters is not visible to the human eye because it’s in the form of X-rays. And, like the bumper cars, the more tightly packed together the electrons are, the more energy they can get rid of—the more rubbery shrieks of X-ray photons—and the faster things cool down.
The first evidence for the existence of this superhot gas in clusters emerged in the late 1960s during the dawn of X-ray astronomy. Unlike the sharp, pinpoint-like X-ray emission of neutron stars and black holes, galaxy clusters are big and cloudy. When you see the X-ray image of gas in a galaxy cluster, you are a direct witness to the amazing dent made in spacetime, filled up with matter like water poured into a bowl. Yet this gas is remarkably tenuous. A cubic meter may contain a total of only a thousand ions and electrons. We only notice it because we see the cumulative light from a depth of millions of light-years into this thin fog. This gas also squeezes in toward the core of a cluster, because of the typical shape of the spacetime bowl. It’s a bit like a soufflé that’s partially successful. All is light and fluffy, except for the embarrassingly thick gummy patch at the bottom. This creates an intriguing conundrum: We know that this bowl of gas cools off by emitting X-ray photons. The primary route by which it does that (the bremsstrahlung) is related directly to how dense the gas is—how many of the electrically charged electrons and ions there are in any given region. If particles are closely packed, the cooling happens faster. So the cluster should be cooling most rapidly at its center, where the gas is densest. But cooler gas means lower pressure, which leads to gravity squashing matter further together, making it even denser and allowing it to cool even faster.
This can act as a runaway process, not unlike a car perched on a hilltop without its emergency brake on. At the top of the hill the slope is gentle, and if I inadvertently lean on the car, it moves just a bit at first. But if I fail to jump in and apply the brakes, then it picks up even more speed, until eventually all I can do is watch in horror as it whizzes down the hill and off an inconveniently located cliff.
Essentially the same thing should happen in a galaxy cluster. The thicker gas in the core cools faster by pumping out more X-ray photons. As it cools its pressure will decrease, and gravity will cause it to slump inward. If the temperature of this gas drops by just a factor of three, it will shrink down inside the gravity well to become twenty times denser. It’s James Jeans’s argument for the collapse of nebula gas all over again: when temperature and density drop below a magic level, gravity takes over. Inside a galaxy cluster, this rapidly cooling gas will begin to roll down the hill, toward the center. But unlike my unfortunate car trundling down a slope, this gas is also supporting lots more gas above it, the rest of the soufflé. Take away that support and the material above will also slump inward. So the outer gas will in turn pour down the bowl sides, increasing in density and cooling more rapidly. It’s as if I had tied my car to the front of a whole chain of other cars, all destined for the cliff edge.
In the outer realms of a galaxy cluster the gas cools at a very slow rate. In the center, though, the runaway process can cool down hundreds of times the mass of the Sun in gas every year. That may not sound like much, but a typical cluster has been around for billions of years, so that adds up to an awful lot of material turning into a thick cold nebula. And thick cold nebulae, as James Jeans saw, have a tendency to collapse further, condensing into stars.
This characteristic of galaxy clusters began to emerge into scientific discussions in the mid-1970s. By this time, early generations of orbiting telescopes had found intriguing signs of dense and bright X-ray emitting gas in the very centers of a couple of these huge systems. One of the scientists trying to understand these measurements was Andrew Fabian. The English-born Fabian was one of a new breed of astronomer, cutting his teeth as a doctoral student with rocket-borne X-ray detection experiments. Launched from Australia and Sardinia, the rockets gave him ten-minute-long peeks above Earth’s atmosphere and into the cosmos. Continuing his postdoctoral career at the University of Cambridge—still his scientific home today—Fabian, together with his student Paul Nulsen, joined a few other scientists around the world in eagerly studying the physics behind the intriguing new images of galaxy clusters. At the center of some, the density of the gas and its X-ray emission indicated it was cooling down in less than 10 million years, the blink of a cosmic eye. The investigators quickly realized that this could be precisely the signature of a runaway process, and this great cosmic downfall of gas soon earned t
he name “cooling flow.”
Most large galaxy clusters also have a big galaxy sitting in their centers. These central objects are of the elliptical class, each a dense dandelion-like cloud of hundreds of billions of stars. All that cooling cluster gas should end up in these central objects. It could even be responsible for building these galaxies in the first place, and they should still be rife with the formation of new stars. But there’s a catch: something is amiss. Nature is not playing ball. Gas cools in clusters, all right, but most of it never actually gets to the point of making stars—a huge problem for what scientists thought was an obvious process.
By the early 1990s, X-ray observations were sophisticated enough to allow for more precise estimates of just what was going on in the centers of the clusters. In most of these systems, the central concentration of X-ray light appeared to match up with the theoretical predictions for “cooling flows.” In some cases the X-ray data was good enough to allow astronomers to estimate the gas temperature itself, which indeed seemed to be dropping in cluster cores. Many scientists staunchly advocated that cooling flows were vital parts of galaxy clusters. They had good reason to. Everything seemed to fit together—or at least almost everything.
The biggest stumbling block was the sheer amount of material that was thought to be cooling down until it no longer emitted X-rays. If it was turning into cool nebula-like clouds these should in turn be condensing into new stars, lots and lots of them. But there wasn’t much evidence to support this. The giant galaxies at the centers of clusters simply didn’t contain a huge excess of young stars. Where cooler gas could be seen, there was very little compared to expectations. In 1994, Fabian tried to come up with a number of explanations for what was going on. It was conceivable that whatever the cooling gas was turning into was simply dark, and effectively invisible. It could be dust, or small and tepid stars. Some gas could be dropping into near invisibility as cold molecules of simple compounds like carbon monoxide. It was also possible that more complex physics was controlling the gas and hiding it. Magnetic fields might be channeling and constraining the gas, and perhaps hot and cold gases were coexisting in complicated structures that fooled our telescopes.
Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos Page 13