Category Dreams of Other Worlds

Black Holes: From Speculation to Observation

In 1964, Riccardo Giacconi and his team discovered Cygnus X-1, a source in the constellation known as the Swan that is generally accepted as the first black hole detected. Cygnus X-1 has a mass nine times that of the Sun, with an implied event horizon size of 16 miles, smaller than a city. But in the early 1960s, X-ray astronomy was an emerging science,39 and a singularity was then considered purely theoretical.

Science writer Dennis Overbye recounts how physicist John Wheeler, based on his students’ prompting, eventually came to ac­cept the probabilities of the cataclysmic collapse of a star:

In 1939, J. Robert Oppenheimer, who would later be a leader in the Manhattan Project, and a student, Hartland Snyder, suggested that Einstein’s equations had made an apocalyptic prediction. A dead star of sufficient mass could collapse into a heap so dense that light could not even escape from it. The star would collapse forever while space­time wrapped around it like a dark cloak. At the center, space would be infinitely curved and matter infinitely dense, an apparent absurdity known as a singularity.

Dr. Wheeler at first resisted this conclusion, leading to a confron­tation with Dr. Oppenheimer at a conference in Belgium in 1958, in which Dr. Wheeler said that the collapse theory “does not give an ac­ceptable answer” to the fate of matter in such a star.

However, by 1967, during a presentation in New York, Wheeler had reconsidered and, “seizing on a suggestion shouted from the audience, hit on the name ‘black hole’ to dramatize this dire pos­sibility for a star and for physics.”40 Mitchell Begelman and Martin Rees likewise note that it was Wheeler who first officially used the term “black hole” in reference to a collapsed star and comment that “the name immediately caught on.” They write, “Here was something infinitely more mysterious, and perhaps much more sin­ister as well—a place where any form of matter or energy could enter, lose its identity, and be lost forever to the Universe.”41 In Black Holes and Time Warps, Caltech astrophysicist Kip Thorne similarly observes: “Of all the conceptions of the human mind, from unicorns to gargoyles to the hydrogen bomb, the most fan­tastic, perhaps, is the black hole. . . a hole with a gravitational force so strong that even light is caught and held in its grip; a hole that curves space and warps time.”42

In 1970, Stephen Hawking developed theorems that demon­strated black holes would be a standard phenomenon of Einstein’s general relativity and of the universe. By the mid-1970s, not sur­prisingly, black holes were resonating across the realms of fiction, popular music, art, and film. The Canadian rock group Rush re­leased “Cygnus X-1, Book 1: The Voyage” on their album A Fare­well to Kings (1977). It’s an extended rock composition written in homage to the X-ray emission source that Riccardo Giacconi discovered in Cygnus. The lyrics read in part:

In the constellation of Cygnus There lurks a mysterious, invisible force The Black Hole Of Cygnus X-1[. . . . ]


To telescopic eye Infinity

The star that would not die All who dare To cross her course Are swallowed by A fearsome force. . . .

I set a course just east of Lyra And northwest of Pegasus Flew into the light of Deneb Sailed across the Milky Way[. . . .]

The X-ray is her siren song My ship cannot resist her long Nearer to my deadly goal Until the Black Hole—

Gains control. . . .

Sound and fury Drowns my heart Every nerve Is torn apart. . . .43

Located approximately 6,000 light-years from Earth, Cygnus X-1 is now thought to be a high-mass X-ray binary system in which gaseous material from the blue supergiant star HDE 226868 is accreting onto a nearby black hole created by the collapse of a star nine times the mass of our Sun.44 The system emits powerful X-ray radiation as gas from the blue supergiant siphons onto, and is heated by, the black hole. Interest in Cygnus X-1 was further sparked by a public and congenial bet between Stephen Hawking and Kip Thorne regarding the physics of the object. By 1990, hav­ing argued that Cygnus X-1 was not a black hole, Hawking admit­ted to losing the wager.45

Cosmic Voyages

The characterization of Edwin Hubble maneuvering the 100-inch reflector like a mariner upon a vast sea evokes another celebrated captain who navigated the depths of space in the science fiction television series Star Trek launched in September 1966. As noted in the chapter on the Voyager mission, Captain James T. Kirk of the starship Enterprise, portrayed by William Shatner, commanded a fictional five-year mission to explore the far reaches of our galaxy.

When Gene Roddenberry began producing Star Trek, the world’s attention was turned toward space as the final frontier and the dream of traveling vast distances across our galaxy was at a fever pitch. The Apollo missions of the late 1960s and early 1970s cap­tivated audiences around the world, who watched with trepidation as Neil Armstrong, and through him humankind, stepped onto the surface of another world for the first time. We delighted in watch­ing the astronauts skipping and hopping across the lunar surface, and bounding lazily over cratered terrain in the lunar rover. The final episode of the original Star Trek series was broadcast in June 1969, just a month before Neil Armstrong and Buzz Aldrin stood on the lunar surface. Many in that generation came away from the truncated Apollo program wistful that the limits of money and politics prevented NASA from further manned lunar exploration.

One effect of Star Trek first airing in the runup to the moon landing was that the series inspired the first space-faring genera­tion, including many at NASA. William Shatner writes of the origi­nal series that “every time [NASA] launched a manned rocket our ratings went up, meaning people were very interested in space: and when our ratings went up Congress voted more money for the space program.” Even if we don’t assign causation, this is interest­ing. The perceived interconnection between Star Trek and NASA extended to NASA itself. Shatner recalls:

NASA officials often invited us to launches, and finally I decided to go to one of them. They treated me as space royalty, eventually allowing me to sit in the LEM, the moon landing module, with an astronaut. I was lying in the hammock like seats. . . looking out of the small win­dows at the universe displayed as the astronauts would see it. The astronaut, who was teaching me how to fly this craft, told me to look at a certain section of the star system—and as I did, flying beautifully across the entire horizon came the Starship Enterprise.59

That model of the Enterprise was assembled, Shatner points out, by some of the same engineers who built the Apollo spacecraft that landed on the Moon.

So compelling was Roddenberry’s vision that in the late 1970s, the space agency recruited Nichelle Nichols, who played commu­nications officer Lieutenant Uhura in the original series, to help in attracting young people to NASA’s program. In 1977, NASA had received roughly 1,600 applications for new hires, but fewer than one hundred from women, and far fewer from minorities. Within four months of Nichols’s recruiting, applications rocketed up to 8,400, with approximately 1,650 from women and 1,000 from minority candidates.60 Years later, in 1992, during the STS 47 mission, Space Shuttle astronaut Mae Jemison, the first Afri­can American woman in space, made it her practice, in honor of Nichelle Nichols, to begin each of her shifts with “Hailing frequen­cies open,” an often scripted line for Nichols.61 That same year, in 1992, in recognition of Star Trek’s contribution to the popular fas­cination with space exploration, a portion of Roddenberry’s ashes were flown aboard Space Shuttle Columbia.

Fred Hoyle predicted over sixty years ago that when “the sheer isolation of the Earth [had] become plain to every man whatever his nationality or creed. . . a new idea as powerful as any in his­tory will be let loose.” Hoyle optimistically expected “this not so distant development may well be for the good, as it must in­creasingly have the effect of exposing the futility of nationalistic strife” and would inevitably reconfigure “the whole organization of society.”62 Hoyle’s point about a new understanding of Earth in the context of the vastness of space is in part why Star Trek and, later, the Hubble Space Telescope, garnered such global interest. Both NASA and Star Trek contributed to a deep cultural narrative about the possibilities of exploring the universe through interna­tional collaboration. Only twenty-four men traveled to the Moon, but unimaginable surveys of deep space have been realized via the Hubble Space Telescope. The telescope continues to capture and convey stunning images of galaxies and nebula and inspire a new generation who dreams of a time when not just a handful of care­fully selected astronauts, but they themselves can explore worlds within our galaxy and beyond. That powerful cultural dream has been realized as Hubble’s cameras extend our vision to the very limits of the observable universe.

When the Apollo 8 astronauts toured the world upon return from the Moon, command module pilot Michael Collins recalls that regardless of which nations they visited, people congratulated them on the fact that “we,” humankind, made it to the Moon.63

Collins noted that people around the world embraced the Moon landing as a collective human achievement. The Hubble Space Telescope, an international effort between NASA and the Euro­pean Space Agency (ESA), is similarly embraced and loved globally as a shared human achievement. Hubble’s remarkable images have forever altered our understanding of the cosmos and demonstrate how far humankind has reached across the universe in our as yet most palpable survey of the intergalactic abyss.

Science in the Matrix of Culture

No longer the stuff of science fiction, black holes pervade all facets of culture. Wallace and Karen Tucker, in recounting the history of the Chandra X-ray Observatory, note that the telescope has of­fered “substantial observational evidence for the existence of black holes,” which they claim have since “become part of the popu­lar literature as a metaphor for an irretrievable loss of material, time, money, and so on.”46 In the hands of English novelist Mar­tin Amis, in Night Train (1988), black holes become a metaphor for death.47

But black holes are understood in the public arena as far more substantial than a mere metaphor. In 2008, it was reported that physicists at CERN’s Large Hadron Collider could produce min­iature black holes that in a runaway scenario might swallow the Earth. In fact, public concern over the possibility of scientists man­ufacturing microscopic black holes led to the facility posting an information and safety web page that reassures general audiences of the following:

Speculations about microscopic black holes at the LHC refer to par­ticles produced in the collisions of pairs of protons, each of which has energy comparable to that of a mosquito in flight. Astronomical black holes are much heavier than anything that could be produced at the LHC. According to the well-established properties of gravity, described by Einstein’s relativity, it is impossible for microscopic black holes to be produced at the LHC. There are, however, some specula­tive theories that predict the production of such particles at the LHC. All these theories predict that these particles would disintegrate im­mediately. Black holes, therefore, would have no time to start accret­ing matter and to cause macroscopic effects.48

A year later J. J. Abrams’s film Star Trek (2009) depicted Romu – lans generating, at will, miniature black holes that could devour starships and planets. Actually Star Trek and black holes have a history that goes back to the same year Wheeler started using the term. The idea of a collapsed or black star was floated in the first season of the original Star Trek series in an episode titled “Tomor­row is Yesterday,” first aired in January 1967, in which the starship Enterprise encounters a “black star of high gravitational attrac­tion.”49 As Captain Kirk and the crew attempt to escape its gravity well, their ship is slung into a time warp that sends them plunging back through time. The Enterprise ends up adrift in Earth orbit in 1968, just days prior to the first manned Moon mission. Though not referred to as a black hole, the black star that triggers their adventure was the product of Einstein’s singularity and Oppen – heimer and Snyder’s collapsed star.

From space artists like David A. Hardy to amateur astronomers like Stephen Cullen, many have speculated what Cygnus X-1 might actually look like.50 Hardy’s rendering depicts powerful X-ray jets shooting from the poles of the binary system’s black hole. Cullen actually photographed those jets in May 2009. “Luckily, Cygnus X-1 inhabits a region of space thick with gas that emits specific kinds of radiation when hit by shock waves, energetic particles, and relativistic jets from black holes,” writes Cullen, who notes that astronomer David Russell, and a team working with him, had already obtained “visual evidence of a jet-powered nebula caused by the northern jet slamming into the interstellar medium.” But Cullen thought there might be a powerful jet emerging from the southern pole, and there was. “Unlike the northern region, the im­ages did not show a well-defined shell of shocked gas,” he recalls. “What I did see, however, was a diffuse, fan-shaped glow that traced a path directly back to Cygnus X-1.”51 Having obtained clear images of the previously undetected jet, Cullen subsequently invited Russell’s comments and together the astronomers collabo­rated to publish Cullen’s results. The discovery demonstrates the extent to which the Chandra Observatory has prepared astrono­mers to anticipate what might be explored in wavelengths beyond visible light.

Not only have Chandra’s X-ray observations demonstrated pro­found aspects of the high-energy universe otherwise unavailable to telescopes, but its images have opened the universe for visually impaired readers as well. In 2007, astronomy educator Noreen Grice and astronomers Doris Daou and Simon Steele published Touch the Invisible Sky in which Braille text and especially pre­pared Braille images detail discoveries by Chandra, Spitzer, and other telescopes that reveal the universe in electromagnetic wave­lengths not visible to the human eye.52 In that text and other NASA

Braille books, readers can experience the infrared, ultraviolet, and X-ray universe, through their fingertips.53

The popular fascination with black holes seems hinged on the fact that a singularity and its event horizon contort our wildest imaginings about the physical universe. We think we can wrap our minds around just about any natural phenomena, but black holes defy all logic at the point of the singularity and leave unanswered questions regarding the physics at its core. Kip Thorne asserts that black holes have much to tell us about the birth and the future of the universe: “Gravitational-wave detectors will soon bring us observational maps of black holes, and the symphonic sounds of black holes colliding—symphonies filled with rich, new informa­tion about how warped space-time behaves when wildly vibrating. Supercomputer simulations will attempt to replicate the sympho­nies and tell us what they mean, and black holes thereby will be­come objects of detailed experimental scrutiny.”54 Like the music of the universe discussed in the chapter on WMAP, the universe has more to tell us in the narratives or symphonies we are yet to learn from black holes. One thing at least seems certain. The new understandings of massive black holes that Chandra is unfolding will inevitably be woven into the matrices of human culture and language, poetry and music, and even our dreams.


Awareness of the size and age of the universe is hard-won knowledge that has taxed scientists for the past 2,500 years. To ancient cul­tures, the sky was a proximate canopy that circled overhead, and there was no sense of the vast distance to the stars, let alone the idea that something might lie beyond those pinpoints of light. The ancient Greeks were the first civilization to spawn a class of philosopher-scientists, who applied logic and mathematics to their observations of the sky.

Cosmology has its root in the Greek idea of “cosmos,” or an orderly and harmonious system. In the Greek view, the antitheti­cal concept of “chaos” referred to the initial state of the universe, which was darkness or an abyss.1 Thus, order emerged from disor­der when the universe was born. Pythagoras is believed to be the first to use the term cosmos, and the first to say that the universe was based on mathematics and numbers, although in truth, so lit­tle is known about Pythagoras and his followers that direct attri­bution of these ideas is impossible. Pythagoras is also credited with “harmony of the spheres,” a semi-mystical, semi-mathematical idea that simple numerical relationship or harmonics were mani­fested by celestial bodies, with the overall result having commonal­ity with music. Pythagoreans didn’t think the music of the spheres was literally audible.2

Aristotle’s geocentric cosmology dominated Western thought for nearly two millennia, but Aristarchus developed a heliocentric

cosmology that implied large distances to the stars, so as not to observe a parallax shift from one season to another. Mapping the stars in the third dimension didn’t become possible until parallax was measured in the nineteenth century, giving William Herschel an inkling of the extent of the system of stars that we inhabit. Twin foundational discoveries by Edwin Hubble early in the twentieth century—the distances to the nebulae, or galaxies, and the uni­versal recession velocities of galaxies—set the stage for modern cosmology. By the mid-twentieth century, the universe was known to be billions of years old and billions of light-years in extent.

The Enigmatic Center of Our Galaxy

Twenty years ago, evidence began to accumulate that the center of the Milky Way galaxy contained a dark mass that could not be accounted for as normal stellar remnants.55 Star motions in the center were so rapid they indicated a huge black hole, about four million times the mass of the Sun. The evidence for this supermas­sive black hole is now better than for the more conventional black holes that result from the death of a massive star. The mass is not just based on stellar velocities but on tracking the entire stellar orbits as they loop around the central dark object.56

Chandra did not contribute the evidence that cemented the case for a large black hole in the galactic center, but it showed that the black hole was unusually anemic, emitting far less high-energy ra-

The Enigmatic Center of Our Galaxy

Figure 10.3. At the center of the Milky Way galaxy, 27,000 light-years away, in the direction of the Sagittarius constellation, is a very dense star cluster. In this X-ray image, spanning 3 light-years, the bright regions represent hot gas from overlapping supernova remnants and to the lower left of the bright region, dynamical evidence indicates a black hole about 4 million times the mass of the Sun. The actual center of the galaxy is marked and is an ultra-compact radio source called Sag A* (NASA/CXC/MIT/F. Baganoff).

diation than other massive black holes. Even as it devours 1019 kg of material each second, it radiates energy a million times less ef­ficiently than a set of stars of equivalent mass. Yet, while the black hole in the galactic center is very feeble, it’s not boring. Ten years ago, an X-ray flare was seen coming from the vicinity of the black hole and since then, hundreds of flares have been seen, occurring almost daily.57 They raise the level of X-ray emission tens or hun­dreds of times above the quiescent state (figure 10.3). Such rapid flares must be created within ten times the event horizon scale, close to the point of no return. The galactic center also harbors a source of energy intense enough to create anti-matter in the form of positrons.58

The real mystery is the inactivity of the galactic center. The cen­ter of our galaxy harbors a massive star cluster with star densities thousands of times higher than those seen in the solar neighbor­hood, and there is plenty of gas available within 10-20 light-years. So why is the black hole so quiet? The best guess is that it’s cur­rently starved because explosive events have cleared away much of the gas from around it. Chandra has provided evidence for this explanation. There are lobes of X-ray emission that indicate the black hole was more active 5,000 or so years ago, and blobs of plasma emerging from the center argue for quasi-periodic activity. Remarkable observations a few years ago saw a cloud of gas near the black hole brighten and fade in only a few years, responding to an X-ray pulse that had traveled for three hundred years to get there. We can infer that the black hole was a million times brighter three hundred years ago, when Queen Anne had just ascended to the throne in England and the largest town in her American colo­nies was Boston, with a population of seven thousand.59 However, the galactic center is about 27,000 light-years from Earth, so all of this action really happened in the late Stone Age before humans settled down into civilizations and the information is just reaching us now.

A Beginning for the Universe

It was only in the most recent seconds of human existence, com­paratively speaking, that the universe began to take shape in the human imagination. For the vast majority of the 200,000 years since the emergence of Homo sapiens, the reaches of the depths of space were unfathomable. In just the last hundred years humans began to discover the universe and develop a basic understanding of its mass and age. One key to unfolding our current view was detection and mapping of the cosmic microwave background, the remnant light and heat of the big bang. The temperature of the vacuum of space is 2.725 K, a trace above absolute cold, exactly what the universe should have cooled to if it had expanded to its current size from a hot and dense initial state 13.8 billion years ago. NASA’s COBE and WMAP spacecraft have mapped this sig­nature of the moment when the abyss of space and everything in it came into being.

Prior to and throughout the first third of the twentieth century, most people, even most astronomers, simply assumed that the uni­verse always existed. Science historian Helge Kragh comments, “The notion of a universe of finite age was rarely considered and never seriously advocated.”3 Astronomers knew very little about the depths of space before the 100-inch telescope at Mount Wil­son Observatory near Pasadena became operational. At that time, they intensely debated whether the Milky Way comprised the en­tire universe, or whether the nebulae might lie far beyond our gal-

axy. On an October night in 1923, American astronomer Edwin Hubble, working with the 100-inch, then the largest telescope in the world, observed a variable star in M31, the Andromeda Neb­ula, which ultimately confirmed that it was millions of light-years beyond the Milky Way. The announcement of Hubble’s result in 1925 radically altered the scientific and public understanding of our place in space. Just a few years later, in 1929, Hubble and his assistant Milton Humason again rocked the world by reporting that remote galaxies were racing away from the Milky Way at 700 miles per second or faster. Their observations indicated that the universe was expanding, a seemingly preposterous idea that Albert Einstein himself initially refused to believe.

Given that relativity theory recognized space and time as insep­arable, the Belgian astronomer and Jesuit priest Georges Lemai- tre interpreted Hubble and Humason’s findings of the “runaway” galaxies as meaning only one thing. The universe itself was ex­panding, which in turn suggested that the universe must have been smaller, denser, and hotter in the distant past. Among Lemaitre’s many contributions to cosmology, three of his most simple and yet profound ideas were that the universe had a beginning, that both relativity and quantum theory were needed to explain this origin in terms of expanding space-time, and that Edwin Hubble’s reced­ing galaxies were evidence of this cosmic expansion.

Lemaitre was the first to suggest that Hubble and Humason’s redshifted nebulae indicated the expansion of space-time itself.4 In 1931, in the journal Nature, Lemaitre offered a short proposal for what English astronomer Fred Hoyle later derogatorily dubbed the big bang. In that article, Lemaitre postulated “the beginning of the universe in the form of a unique atom, the atomic weight of which is the total mass of the universe.”5 He theorized inflation of the cosmos from a “primeval nebula” or “primeval atom” of dense matter. Working from Einstein’s theory of general relativity as well as emerging theory in quantum mechanics of elementary atomic particles, Lemaitre proposed that the universe inflated from a dense, highly compacted soup of subatomic particles that at a moment of quantum instability resulted in the unfolding of space. “What’s remarkable about his Nature letter,” writes John Farrell, “is that—apart from discussing the idea of a temporal beginning of the cosmos—it marks the first time that a physicist directly tied the notion of the origin of the cosmos to quantum processes.”6 Even before the neutron had been discovered, Lemaitre understood that the beginnings of the universe could be explained in part via quan­tum theory and argued that “all the energy of the universe [was] packed in a few or even in a unique quantum.” By Lemaitre’s es­timation space and time or space-time could “only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the [universe] happened a little before the begin­ning of space and time.”7 Lemaitre depicted the early universe as analogous to a “conic cup,” the bottom of which represents “the first instant at the bottom of space-time, the now which has no yesterday because, yesterday, there was no space.”8

In 1934, in “The Evolution of the Universe,” Lemaitre outlined his “fireworks theory of evolution” in which the stars and galax­ies, having evolved over billions of years, were merely “ashes and smoke of bright but very rapid fireworks.”9 He described our situ­ated view from Earth, scanning the night skies, as we look back toward the primordial past: “Standing on a well-chilled cinder, we see the slow fading of the suns, and try to recall the vanished bril­liance of the origin of the worlds.”10 Lemaitre additionally intuited that a fossil light would be the signature of the universe’s begin­ning. As James Peebles, Lyman Page, and Bruce Partridge point out: “One learns from fossils what the world used to be like. The fossil microwave background radiation is no exception.”11 Lemai – tre expected evidence of a fossil radiation from the early stages of the universe could be detected. Thinking in 1945 that cosmic rays were the signatures of this fossil light, Lemaitre supposed that these “ultra-penetrating rays” would reveal the “primeval activity of the cosmos” and were “evidence of the super-radioactive age, indeed they are a sort of fossil rays which tell us what happened when the stars first appeared.”12 Just weeks before his death in 1966, Lemaitre celebrated learning of the discovery of the cos­mic microwave background, the fossil light he had anticipated. Throughout his career, he debated with Einstein whether or not his cosmological constant was a repulsive force that “could be un­derstood as a vacuum energy density.”13 Cosmologists now regard Einstein’s cosmological constant as indicative of the effects of dark energy contributing to the universe’s expansion (figure 12.1).

A Beginning for the Universe

Billions of years from now

Figure 12.1. The recession of galaxies implies the universe is expanding; using general relativity, the expansion history can be calculated. The curves show the past and future expansion of the universe in terms of matter content (Dm) and dark energy content (D). Observations agree with the upwards curving dashed line. The expansion history of the universe was dominated initially by dark mat­ter and more recently by dark energy (Wikimedia Commons/BebRG).

Supermassive Black Holes

In the 1960s, long before the Milky Way was known to have a supermassive black hole, violent activity was seen in the centers of some distant galaxies. The manifestations were gas motions a hun­dred times faster than would be seen in a normal stellar system, intense radio emission concentrated in a tiny region of the galactic nucleus, and radiation across the electromagnetic spectrum from radio waves to gamma rays. Some of the strongest X-ray sources in the sky are galaxies hundreds of millions of light-years away, which means they have an X-r ay emission thousands of times stronger than a galaxy like the Milky Way. Rapid variability of the radio, optical, and X-ray emission localized the intense activity to regions light-days across, only ten times bigger than the Solar Sys­tem. No star cluster can pack so much radiation into such a small volume; the only plausible explanation was a gravitational engine: a black hole.60

Imagine a city like Los Angeles viewed from high above in a helicopter, where every streetlight, every house light, and every car light is standing in for a star. There are probably 100 million lights in the greater Los Angeles area, so in this analogy each light rep­resents a thousand stars. Los Angeles is like the disk of a spiral galaxy. Now imagine a light one inch across in the city center that emits a hundred times more radiation than all the city lights put together. That’s the intensity of an active galactic nucleus. It would be clearly visible from an immense height, long after the individual lights in the city had faded from view. We can see an incredibly bright point of light but the surrounding galaxy is too faint to see. That’s a quasar.

Through the 1990s astronomers realized that every galaxy con­tains a black hole, with a mass that scales with the mass of the old stars in the center of the galaxy.61 Moreover, these black holes are not active most of the time, so are not feeding and spewing out the energy that makes them easy to detect. The Milky Way is a medium-size galaxy with a 4-million-solar-mass black hole that’s currently inactive. The brightest quasars harbor black holes a thou­sand times larger, or a few billion times the mass of the Sun. These extraordinary gravitational engines form and grow quite naturally as galaxies form and grow but steadily consume matter, and by occasional mergers. It’s remarkable that nature makes black holes spanning a factor of a billion in mass, from a few times the mass of the Sun to the mass of a small galaxy. Their sizes range by the same factor of a billion, but they all share the properties of having an event horizon and (it is supposed) a central singularity. Strange as it might seem, massive black holes do not represent an extremely dense state of matter. The size of the event horizon is proportional to mass, but the volume increases by the cube of the size, so the more massive a black hole is the less dense it is within the event horizon. The 3-billion-solar-mass beast at the heart of the elliptical galaxy M87 is not much denser than water! Modest density does not, however, reduce their intrinsic strangeness (figure 10.4).

Supermassive Black Holes

Figure 10.4. M87 is a giant elliptical galaxy with a “super-volcano” of X-ray activity originating from a black hole about 3 billion times the mass of the Sun at its center. The black hole sends out jets of material along the poles of its spin axis that create shock waves and interact with the intergalactic medium in a way analogous to a large volcano interacting with Earth’s atmosphere (NASA/ CXC/KIPAC/N. Werner/E. Million).

Chandra has played a vital role in telling the story of supermas­sive black holes and their behavior. In nearby galaxies it has de­tected vast bubble-like cavities and wavelike ripples in hot gas near the center, signs of “blowback” from the central engine.62 X-rays provide evidence of repetitive explosive activity, where mass falls onto the black hole, sparking it into activity and triggering jets of high-energy particles, which then evacuate the nearby volume and starve the black hole until new material accumulates. In this way, a massive black hole can be inactive most of the time. Chandra has also seen binary black holes in the centers of merging galaxies.

In one case, two huge black holes are only 3,000 light-years apart and will merge in the next hundred million years, provoking a cataclysm of gravity waves and new activity and yielding a single galaxy with a larger black hole.63 This is direct evidence for the manner in which galaxies and their embedded black holes have grown by mergers and acquisitions over billions of years.

Black holes and galaxies evolve on cosmic timescales of billions of years, so to figure out how they change the strategy is to make deep surveys that capture the census of active and inactive black holes at a range of distances or redshifts. The Chandra Deep Field was stared at for 2 million seconds, over three weeks, to create the deepest X-ray image of the sky ever made. This deep field, covering a patch of sky smaller than a postage stamp held at arm’s length, is combined with a shallower but wider survey to tell the full story.64 One of us (CDI) has participated in this research, with a survey of active galaxies in a two square degree region that netted more than two thousand supermassive black holes, and showed their evo­lution in cosmic time. This survey was sensitive enough to catch feeble accretors and black holes similar in size to the one in the Milky Way.

The broad conclusion of these surveys is that star formation and black hole growth in a galaxy are tightly linked. The heavi­est black holes, a hundred million times the Sun’s mass or larger, ate voraciously in the first few billion years after the big bang, and have been grazing or fasting since then. Black holes ten to a hundred million times the Sun’s mass had a more well-regulated diet, and because they consume smaller proportions of their gas and dust early on, they continue to grow even today. X-rays are crucial to telling this story, since they can penetrate regions that would extinguish optical and ultraviolet radiation.65 The typical size of a galaxy forming a supermassive black hole of a particular mass reduces with cosmic time, so the biggest black holes formed first. Massive galaxies in the far-flung universe misspent their youths by building monstrous black holes. This result is appar­ently at odds with the hierarchical mode of formation of structure in the universe, where small objects form first and gradually grow into larger objects. This unexpected phenomenon has been called “downsizing.”66

A by-product of this research is the solution to an old mystery of the X-ray sky. When the first X-ray satellites took their data in the 1960s and 1970s they saw a faint diffuse glow that spanned the entire sky. This X-ray “background” was unexplained. Surveys with Chandra and XMM-Newton show that the background is ac­tually composed of a myriad of pinpoints of X-ray emission from active galaxies 3-8 billion light-years away. Also, a majority of the emission is from galaxies where the nuclear activity is shrouded by dust and would not have been visible to an optical telescope. These surveys have also shown that black holes in the most mas­sive galaxies are “green.” By comparing the available fuel with the energy required to evacuate cavities in the central regions, the ef­ficiency can be calculated. It’s high because the mass is accreted slowly and smoothly and the energy is extracted very close to the event horizon of the black hole. If a family car was as efficient as one of these supermassive black holes, it would get a billion miles to a gallon of gas.

The Hot Big Bang

Pivotal roles in science are often taken by outsiders. The “father” of the big bang model was a part-time lecturer at the Catholic University of Leuven when he published a paper in a journal that was little read outside Belgium. George Lemaitre was a Catho­lic priest who had moved from civil engineering into physics and astronomy. Einstein was initially skeptical, but at a seminar by Lemaitre in Princeton in 1935, he expressed his admiration for the beauty of the theory.

The other intellectual parent of the scientific theory of creation was a Russian emigre called George Gamow. This physicist was elected to the Academy of Sciences of the U. S.S. R. at the age of twenty-eight, the youngest person ever to gain that honor. With his student Ralph Alpher, he showed that the early hot universe could have produced the amount of helium we see, which is too much to have been produced by stars. Gamow jokingly had his colleague Hans Bethe listed on the paper so it would read Alpher-Bethe – Gamow, in a pun on the first three letters of the Greek alphabet; Bethe had no other role in the paper.14 In 1948, the same year, Gamow and Robert Herman predicted that the afterglow should have cooled down after billions of years, filling the universe with microwave radiation at a temperature of five degrees above abso­lute zero, or 5 K.15 However, nobody pursued the prediction, due in part to a lack of widespread awareness of the theory, and in part to the primitive state of microwave technology at the time.

The new theory was given its catchy name by Fred Hoyle in a BBC radio broadcast in 1949.16 Even though he advocated the rival “steady state” theory, which didn’t involve a hot and dense early phase for the universe, he claimed the label was descriptive and not pejorative. As Hoyle noted, with typically sardonic wit, the big bang was an audacious theory: the entire universe, holding enough matter to yield more than a trillion trillion stars in many billions of galaxies, somehow emerged instantaneously and without any precedent from an iota of space-time! Steady state theory called for the gradual creation of matter in the vacuum of space between the receding galaxies. Although spontaneous creation of matter was ad hoc physics, it seemed like a more modest proposition.

For fifteen years, the status of the theory remained tentative. The universal recession of galaxies certainly pointed to a time when the universe was smaller, denser, and hotter. The explanation of the fact that the universe is a quarter helium by mass (and 10 percent by number of atoms) was a success, but the difficulty of measuring cosmic abundances of other light elements meant no further progress could be made on testing this idea, called big bang nucleosynthesis. Counts of extragalactic radio sources indicated the population was evolving and argued against the steady state theory. The missing ingredient was a decisive observation that fa­vored the hot big bang. It came by accident in 1964 when two engineers working at Bell Labs in Holmdel, New Jersey, detected a microwave signal of equal intensity in every direction in the sky (figure 12.2). NASA’s Wilkinson Microwave Anisotropy Probe, or WMAP, is the illustrious descendant of this pioneering experiment.

The Hot Big Bang


Figure 12.2. Theorists working with the expanding universe model predicted that the universe should be filled with relic radiation from the big bang, diluted and cooled by the expansion to just under 3 K. NASA’s COBE satellite measured the spectrum and shows the radiation had exactly the predicted temperature; the data fit the model so well that the error bars are smaller than the thickness of the curve (NASA/COBE/FIRAS Science Team).

Hunting Dark Matter and Dark Energy

Chandra has also weighed in on two of the profound mysteries of cosmology: the nature of dark matter and dark energy. These two components of the universe account for 95 percent of its be­havior, yet the physical basis for them is not known and is not part of standard physics. Dark matter outweighs normal matter by a factor of six and binds galaxies and clusters and stops them from flying apart, as well as causing the expanding universe to decelerate for most of the first two thirds of its existence. Dark energy dominates dark matter by a factor of three and has caused the cosmic expansion to accelerate in the most recent third of the universe’s existence.67

Dark matter and dark energy don’t interact strongly with nor­mal matter like the atoms in our bodies, so stealth and cunning must be used to ensnare them and measure them. In both cases the laboratories used are rich clusters of galaxies, consisting of thousands of galaxies moving swiftly under the action of gravity and a huge cloud of superhot gas, so energetic that it emits X – rays. In 2006, an object called the Bullet Cluster was used for a convincing demonstration that dark matter actually exists. In the Bullet Cluster, a bullet-shaped cloud of hundred-million-degree gas is produced by a high-speed collision between a large cluster and a smaller one. The hot gas was slowed by the collision, due to a drag force analogous to air resistance. By contrast, the dark mat­ter hardly interacted at all and sailed through during the collision, ending up on either side of the hot gas.68 This result would not have occurred unless weakly interacting dark matter dominated the mass of both clusters. In particular, alternate theories of grav­ity, conjured up to avoid needing dark matter, fail to explain the observations. It seems we have to live with dark matter.

Dark energy is even more ephemeral, announcing its presence (and ubiquity) only by the effect it has on the cosmic expansion. The long and hard search for independent evidence of its existence settled on clusters of galaxies. In a study that took nearly a de­cade to complete, researchers showed that rich clusters suffer from “arrested development.”69 It’s more difficult for clusters to grow when space is being stretched. By comparing the size and age of clusters with simulations of how they should grow under differ­ent conditions of cosmology, the results cement the interpretation of dark energy as a universal repulsive agent. They also rule out alterations to gravity theory and confirm that general relativity is a good description of the behavior of matter and radiation on large scales. The enigma of dark energy has not been solved, but its status as the biggest challenge in both physics and cosmology has been enhanced. Whether it is a black hole devouring a companion, massive black holes causing mayhem in the centers of galaxies, or clusters being pulled apart by the accelerating expansion of space, Chandra has provided data to illustrate the violence of the uni­verse we inhabit.

A result of these insights is a new sense of the power of gravity. Stars are powered by gravity, and gravity also governs the nature of the most compact and energetic objects we’ve ever discovered. Dark forces even govern the expansion of the universe. When we think of other worlds, we think of planets illuminated by stars, where any life that exists is beholden to the energy from the star. But in a fundamental sense, starlight is just the inefficient leakage of radiation from mass-energy conversion by fusion. The true source of the starlight is gravity—a star is a gravitational engine. Black holes and neutron stars have no light but they have intense gravity. While they seem alien and utterly different from our world, given suitable protection or adaptation living creatures might be able to live near these compact stellar husks, using gravity to power their dreams.

Cosmic Microwave Background Radiation

Scientific discovery rarely unfolds smoothly or predictably. Eight years before the theoretical prediction of relic radiation, Andrew McKellar measured the spectra of stars and discovered interstel­lar material that was excited to a temperature of 2.3 K.17 He had no explanation for the excitation, which is caused by the radia­tion from the big bang.18 While Gamow’s prediction of a universe bathed in cold, microwave radiation sat in the literature, several experimenters had the detection of the radiation within their grasp but either did not control systematic errors well enough or were not aware of the importance of the observation. Robert Dicke at Princeton was, and by 1964 he and his team were hot on the trail of the big bang signature. But as they were preparing a radiometer on the roof of the physics building they got a call from Bell Labs. “Boys, we’ve been scooped,” was Dicke’s memorable response.19 The Nobel Prize was awarded in 1978 to Arno Penzias and Robert Wilson of Bell Labs for their discovery.

What was the nature of the radiation that Penzias and Wilson detected? To understand this involves rewinding the history of the

universe to its very early epochs. The Hubble expansion is a lin­ear relationship between distance and recession velocity: more dis­tant galaxies are moving away from us quicker. Although at first glance this seems to imply that we have a privileged location in the universe, a hypothetical observer in another galaxy would see exactly the same relationship that Hubble saw. In a uniform three­dimensional expansion each observer thinks they are the center of the universe. Since all cannot be, none are. Nor can we see an edge to the universe, so we can’t place ourselves with respect to a boundary. The Copernican principle holds. There’s no discernable center to the universe.

Reversing the expansion projects to a time when all galaxies were on top of each other: the big bang. But a simple backward extrapolation overestimates the age of the universe because matter tugs on other matter, so the expansion rate has slowed since the earliest epochs. In an expanding universe model the major observ­able is redshift, a stretching of the radiation from distant galax­ies due to the expansion of space-time itself. Redshift is simply related to the factor by which the universe has expanded since the radiation was emitted. One plus the redshift is the expansion fac­tor. Ironically, the universe is easier to understand in early times. Before structure forms, the universe behaves just like a simple gas, where the temperature and the average density both increase going back toward the big bang. Once gas starts to collapse by grav­ity, the physics is very complex. Stars and galaxies started form­ing when the universe was about ten times smaller than it is now, about 13 billion years ago.20

Extrapolating further backward, there was a time when the uni­verse was much denser than it is now and hot enough that atoms were ionized. Electrons liberated from atomic nuclei interacted with radiation and stopped the photons from traveling freely. It was as if the universe was shrouded in an impenetrable fog. As the universe expanded and thinned out and cooled, it became trans­parent and radiation could travel without interruption. This spe­cial epoch is the earliest time we can “see” into the universe. In a big bang model, the background radiation comes from a time when the universe was a thousand times smaller than it is today, and a thousand times hotter. Infrared photons from 380,000 years after the big bang, when the temperature was about 3000 K, have been stretched a thousand-fold to become microwave photons in a vast and frigid universe with a temperature a little below 3 K.

The picture of the sky in microwaves is an extraordinary baby picture of the universe. Imagine as an adult that you were shown a picture of yourself a few hours old. Since those waves are from the universe as a whole, they permeate space and they travel in every direction through expanding space. There are trillions of relic photons from the big bang in any volume like that of one breath. However, their radiant intensity coming from any direction in space is only 0.00001 Watt or a ten-millionth of a light bulb.21 If you can find an old-fashioned image tube TV and tune it between stations so you see only static, about 1 percent of the white specks on the screen are interactions of the dots of phosphor with those microwaves.22 The big bang is all around us.