Category Dreams of Other Worlds

Heart of Darkness

Black holes are black. That seems like a self-evident statement. Nothing can escape the event horizon of a black hole; the event horizon isn’t a physical barrier or a boundary but an information membrane, defining the region from which no particle or radiation can escape. Black holes are the ultimate expressions of general rel­ativity, where mass curves space so much that a region is pinched off from view.28

Black holes are the final states of massive stars. Every star is in a life-long battle between the forces of light and darkness. The light comes from fusion reactions creating pressure that pushes outward, while the dark is the implacable force of gravity pulling inward. In the end gravity always wins. When a star twenty or more times the Sun’s mass exhausts its nuclear fuel, the core collapses into a state so dense that nothing can escape, not even light. An isolated black hole would be black and undetectable. However, more than half of all stars are in binary or multiple systems, and that’s also true of the most massive stars that collapse catastrophically when their nuclear fuel is exhausted, forming a black hole. The rotation of the star is amplified in its newly compact state, so the black hole spins very fast. As material from a companion star is pulled onto the black hole, it forms a disk of gas, like water swirling into the drain of a bathtub.29 The disk is very hot, tens of thousands of degrees, and it glows in ultraviolet radiation and X-rays. Some hot plasma is accelerated along the pole of the spinning black hole, where it emits X-rays and gamma rays. So while a black hole is black, gas from a companion can be heated into pyrotechnic activity when it’s falling into the black hole (figure 10.2). The accretion process is well enough understood theoretically that X-ray signatures can be used to identify black holes. Some of the radiation comes from no more than 100 kilometers from the event horizon. The Chandra Observatory has played a vital role in this work.30

Chandra has the sensitivity to detect stellar black holes hun­dreds of light-years away. Only about twenty binary systems have well-enough measured masses to be sure the dark companion is a black hole, but X-ray observations can be used to identify black

Heart of Darkness

Figure 10.2. Black holes do not emit any energy or particles, but when a black hole is part of a binary system, gas is drawn from the companion onto an accretion disk that glows in X-rays. The binary orbit gives the mass of the black hole. The image is an artist’s impression, while the inset shows an X-ray spectrum which diagno­ses the temperature of the plasma near the black hole and gives clues to the black hole’s properties (NASA/CXC/M. Weiss/J. Miller).

holes with fairly high reliability. The examples studied with X – ray telescopes are the brightest representatives of a population of about 100 million black holes in the Milky Way.31

X-ray observations have also pushed the limit of our under­standing of black holes. In 2007, a research team used Chandra to discover a black hole in M33, a nearby spiral galaxy. The black hole was sixteen times the mass of the Sun, making it the most massive stellar black hole known.32 Moreover, it was in a binary orbit with a huge star seventy times the Sun’s mass. The formation mechanism of the black hole that placed it in such a tight embrace with its companion is unknown. This is the first black hole in a binary system that shows eclipses, which provides unusually ac­curate measurements of mass and other properties. The massive companion will also die as a black hole, so future astronomers will be able to gaze on a binary black hole where energy is lost as gravi­tational radiation and the two black holes dance a death spiral as they coalesce into a single beast.33

Characterizing Distant Worlds

Closer to home, astronomers have grown confident that there are habitable worlds in our galactic neighborhood as they harvest exoplanets and begin to detect objects close to the Earth in mass. The Hubble Space Telescope wasn’t in on the ground floor of the discovery of planets beyond the Solar System; that was the work of ground-based observers patiently working on small telescopes for decades. However, it has played a central role in going beyond detection to begin to characterize the planets.

With more than 850 exoplanets confirmed, and dozens more being discovered every week, the thrill of discovery is not what it was in the late 1990s. Almost all of these planets have been dis­covered by an indirect method, where the influence of an orbiting planet is seen in a periodic Doppler shift of the parent star. No one doubts that these planets are real, but the evidence of see­ing the planet is more direct. So it was very exiting when Hubble provided the first optical image of an exoplanet in 2008. A planet three times Jupiter’s mass is orbiting the bright star Fomalhaut, twenty-five light-years away.48 The planet is embedded in a dust disk of the young star and was spotted in archival images taken for a different purpose at two different epochs. Another research team has developed special imaging processing techniques that may be able to dig as many as a hundred new exoplanets out of the vast trove of Hubble archival images.

One of the many surprises in exoplanet research was the exis­tence of hot Jupiters, planets like one of the giant planets in our Solar System but orbiting closer to their stars than Mercury does to the Sun. These planets present an opportunity because the chances of an alignment that lead to an eclipse as seen from the Earth greatly increase. Over a hundred exoplanets have been seen to eclipse their parent star, dimming it slightly for a few hours. These events are repeatable and Hubble has observed several exoplanet eclipses with its spectrographs.49 The spectrum of the star shows absorption from a trace amount of heavy elements, but when the transit occurs, some starlight filters through the atmosphere of the gas giant and an extra imprint of absorption is added to the spec­trum from ingredients in the planet atmosphere. So far, sodium, oxygen, carbon, and hydrogen were detected in the atmosphere of one planet, and carbon dioxide, methane, and water (or rather, steam!) in the atmosphere of another.50 Both are Jupiter-sized and far too hot to be habitable, but these observations are showing the path for the detection of biomarkers: the chemical imprints of biology in the atmosphere of an exoplanet. This may well be the way we first discover life beyond Earth.

Finally, Hubble has contributed to the statistical understanding of exoplanets. Looking toward the crowded stellar bulge of the Milky Way 26,000 light-years away, the telescope found sixteen exoplanet candidates orbiting a variety of stars.51 Five of the plan­ets are in an extreme category not yet found by any other search method: super-rapid orbiters that whirl around their stars in less than a day. They were discovered in 2006 using the transit method. Extrapolating to the entire galaxy, the survey projects to 6 billion Jupiter-sized planets in the Milky Way. Theory predicts terrestrial planets in roughly equal or greater numbers than giant planets, so the distant worlds are out there, just waiting for us to reel them in.

Stellar Cataclysm

The formation of a black hole is just one half of the story when a massive star dies. As the star reaches the end of its life and all fusion energy sources are exhausted, it suffers a dramatic gravita­tional collapse. The crushing force of in-falling gas squeezes the core into a black hole, but much of the mass rebounds outward and the outflowing gas meets more in-falling gas and heats it mo­mentarily to billions of degrees. Heavy elements like gold and sil­ver and platinum are created in that thermonuclear blast and they surf into space to become part of future generations of stars.34 This is a supernova.

For centuries and millennia after the supernova event, outrush – ing gas interacts with the interstellar medium to form a delicate filigree of nebulosity and filaments of glowing gas. Supernova remnants are most clearly seen in X-rays, and with Chandra their structure can be studied in great detail. The spectrometers on Chandra have mapped the ejection of iron and silicon and oxygen and nitrogen, elements essential for planet-building and for life.35 Cosmic alchemy is part of our story since every carbon and oxy­gen atom in every person was once part of a previous generation of stars. The imagers on Chandra have observed the shock-heated filaments in enough detail to model the process that accelerates particles to within a fraction of a percent of the speed of light (plate 18).

Chandra has observed supernova remnants at different stages of their evolution and used them to piece together the expansion history of a typical dying star. This research has an intriguing con­nection to human history. Tycho’s supernova of 1572 in the con­stellation of Cassiopeia and the Crab Nebula supernova of 1054 in the constellation of Taurus were two of the brightest “guest” stars of the last millennium, visible to the naked eye and in the historical record of many cultures.36 Recently, Chandra data led to a revised age for the supernova remnant RCW 86, and the detonation date of AD 185 perfectly matches the notation of a guest star in records of the Chinese court astronomers, making this the first supernova in recorded human history.37

At the center of a supernova remnant is either a black hole or a neutron star. Neutron stars aren’t as exotic as black holes, but they are quite bizarre. The pressure of gravity forces protons to merge with electrons and create pure neutron material; without the electrical repulsion to keep the particles separate they are as close together as marbles in a jar. A neutron star is like a giant atomic nucleus with an atomic number of 1057. Chandra can see neutron stars via the high-energy phenomena spawned by their in­tense magnetic fields. A neutron star has a magnetic field of about 1012 Gauss, equivalent to 10 billion refrigerator magnets. They are spinning at typical speeds of 60 rpm and sometimes as quickly as 45,000 rpm. Imagine something a million times the mass of the Earth that’s the size of a city and spinning nearly a thousand times a second! Chandra images of dozens of neutron stars have revealed glowing clouds of magnetized particles blowing away from the neutron star like a wind.38

Completing Hubbles Work

The Hubble Deep Field and the Ultra Deep Field images would have fascinated Edwin Hubble perhaps most of all, since they plumb the depths of space near the edge of the observable universe and reveal our universe as it was 400 million years after the big bang. In current cosmological models based on the big bang, the universe has an edge in time rather than space. Looking out, we look back in time due to the finite speed of light. At the limits of Hubble’s vision, no edge in space has been detected. Rather, we’ve looked so far back in cosmic time that we’re seeing the epoch where the first small galaxies were created and began merging into the larger and mature galaxies we see around us now. The edge in time corresponds to “first light,” but beyond that was an early period called the “dark ages” when there were no stars and galax­ies. Hubble has been so successful that it has almost completed the fundamental task of seeing the entire population of galaxies in the visible universe over all cosmic time. The Ultra Deep Field images provide new information regarding how highly structured spi­ral galaxies are formed. Astronomers now understand that large spiral galaxies represent mergers, over eons, of fragments of the young, chaotic galaxies seen at the faintest levels of brightness.52 “To some degree, we can follow the stages of galaxy formation right into the fog bank of the Dark Ages and beyond, even toward the big bang itself,” writes Jeff Kanipe, who describes the Hubble Space Telescope “as both a pathfinder and a plumb line into the vast depths of our cosmic origins.”53 The Deep Field images repre­sent humankind’s first glimpse of those origins.

The Hubble Space Telescope is named in recognition of Edwin Hubble, an observational astronomer who in the 1920s and 1930s worked at Mount Wilson Observatory with the Hooker 100-inch telescope, then the largest reflecting telescope in the world. Dur­ing his lifetime Hubble was accepted as “the world’s preeminent observational astronomer,”54 in part because the 100-inch reflector allowed Hubble to seek answers to some of the then fundamen­tal questions of astronomy—whether there were galaxies beyond the Milky Way, how to create an accurate distance ladder to such galaxies, and what could be known at the limits of the observ­able universe. In 1923, Hubble determined a distance to the An­dromeda galaxy of roughly 800,000 light-years. Though today we know it lies about 2.2-2.3 million light-years from our gal­axy, Hubble’s calculation settled once and for all the debate over whether anything existed beyond the Milky Way galaxy. It was a monumental find. Hubble accomplished this through Henrietta Leavitt’s research into the regular periodicities of Cepheid variable stars. Upon discovering and charting a Cepheid variable in An­dromeda, Edwin Hubble could determine its intrinsic brightness to calculate a distance to the galaxy.55 Later, in 1929, he used the 100-inch reflector, and the inestimable skills of Milton Humason as an observer and photographer, to determine that galaxies are racing away from each other at remarkable speeds. Hubble didn’t use the phrase when he published his result, but this marked the discovery of the expanding universe.56

Perhaps it was Grace, Edwin Hubble’s wife, who dubbed him “the mariner of the nebulae.” She was an eloquent writer whose memoirs of Hubble are archived at the Huntington Library in San Marino, California. Gale Christianson’s biography, which draws upon Grace’s unpublished memoirs of Hubble, vividly recreates the scene of the astronomer at work at his telescope: “Like a captain on the bridge of a great ship, the master mariner barked out his or­ders—so many hours and so many degrees. Then came the metallic whining of the traverse, a series of loud clicks, a final heavy clang of the Victorian machinery as the 100-inch was clamped” into place for viewing far-flung galaxies, or nebulae as Hubble referred to them. Christianson extends the metaphor, characterizing Hubble as a sentinel mariner “[s]lipping, night after night, silent and alone, past the distant shoals, of the nebulae.”57 Hubble himself wrote of astronomical exploration in the terms of oceanic exploration. He spoke of the 100-inch reflecting telescope as performing reconnais­sance at the “dim boundary—the utmost limits of our telescopes,” as Hubble in the cold early morning hours would “search among ghostly errors of measurement for landmarks that are scarcely more substantial.”58

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).