Category Dreams of Other Worlds


When we dream of other worlds, our dreams may be vivid and real and colorful, but they’re subject to the limitations of our senses. To visualize something, even if in our mind’s eye, we use the visual sense. For most of the history of astronomy we learned about the universe exclusively through visible light. Tens of thousands of years of naked-eye observations were followed by the first night time use of the telescope by Galileo in 1610, followed by a steady march of successively larger telescopes, culminating with the 8-11 meter behemoths of the present day, with dreams of even larger ones to come.1

The historical focus on visible light was natural, since the most powerful human sense evolved to match the peak energy output of our life-giving star. The cool outer envelopes of stars emit most of their radiation in the visible range, and galaxies are just assem­blages of stars. The night sky is visually rich. About 6,000 stars are visible to the naked eye at a dark site, but they are just an infini­tesimal sliver of the stellar plentitude of the universe. We live in a system of 400 billion stars, and the observable universe contains roughly 100 billion stellar systems, for a staggering total of 1023 stars.2 Also, the spectral transitions that are the fingerprints of dif­ferent types of atoms fall mostly in the visible wavelength range. Cosmic chemistry is practiced with telescopes that gather light and disperse it into its constituent wavelengths.

Nevertheless, just over two hundred years ago experiments showed there are wavelengths longer than the reddest waves of light and bluer than the bluest waves of light. A hundred years later, Wilhelm Rontgen performed the first systematic experiments to understand the nature of mysterious radiation that was gener­ated by high-energy particles moving in the vacuum of a sealed glass tube. In 1895, he announced the discovery of “X rays.” The details of the discovery are poorly documented because Rontgen had his lab notebooks burned when he died. X-rays electrified the scientific community and within a year their potential for medical imaging was realized. It was a lot longer before anyone discovered that they could be used for astronomy. Rontgen received the first Nobel Prize ever awarded for Physics in 1901.3

Just as X-rays provided a window into the human body, they now provide a window in space onto worlds barely imagined. X – ray telescopes have revealed the nature of black holes and neutron stars for the first time, they’ve discovered sizzling hot plasma in the centers of galaxy clusters and in the diffuse space between galax­ies, and they’ve let us make great strides in probing extraordinary black holes that lurk in the centers of all galaxies, ranging from a few million up to a few billion times the mass of the Sun. Their high energy relative to visible light means they’re often created by events of unimaginable violence, phenomena unknown to us fifty years ago (figure 10.1).

The Cosmic Distance Scale

What is the science yield from this premier facility, and how has it changed our view of the universe? In the early years of a space telescope, there’s often a concerted effort to put a lot of time into a few critical projects, so that major results will be obtained even if the telescope fails. There’s understandable tension involved in this decision, since the Hubble has always been oversubscribed by a large factor, and it keeps more astronomers happy if the time is thinly spread among many projects. Riccardo Giacconi, the first director of the Space Telescope Science Institute, was persuaded to set aside substantial time for peer-reviewed “Key Projects.” Early in the history of the facility, three of these were approved. Two of them were successful but are not well known—a medium – deep survey of galaxies,35 and a spectroscopic survey of quasars to probe the hot and diffuse gas in the intergalactic medium36—but the third is rightly considered one of the Hubble Space Telescope’s finest achievements.

As we saw in the chapter on Hipparcos, distance is fundamental to astronomy, yet it’s one of the hardest things to measure. In the everyday world we judge distance based on familiarity with the size of a tree or a person, for example, or the brightness of a lamp or a street light. But in the universe, objects like stars and galax­ies range over many orders of magnitude in size and brightness, so their apparent size or brightness is a very unreliable guide to distance. Is that a bright object far away or a dim object nearby? Is that a large object far away or a small object close by? This type of confusion is common. Considered in a different way, poorly deter­mined distances limit our knowledge of how the universe works. Without knowing how far away an object is, we don’t know its true size or its true brightness. That also means we don’t know its mass or luminosity, which means we can’t have a real physical un­derstanding of something that’s so remote that we’ll never be able to study it in the lab or gather a sample of it. This urge to measure distances accurately becomes acute in the study of the universe as one entity. In an expanding universe, precise distances are needed to answer fundamental questions: what is the size of the universe and how old is it?

Early in the life of the Hubble telescope, hundreds of orbits were devoted to measuring distances to three dozen galaxies. Fittingly, this Key Project used the method that Edwin Hubble had used in the early 1920s to show that the Andromeda nebula was a remote stellar system. Hubble identified examples of a well-known class of variable stars in M31, the Cepheid variables. Cepheids pulsate in a well-understood and well-regulated way, and this behavior mani­fests as a linear relationship between their luminosity and their period of variation.37 By finding Cepheids in a remote galaxy and measuring their periods with a series of images taken over hun­dreds of days, the star’s absolute and apparent brightness can be combined to derive the distance. Cepheids are very luminous and can be seen to large distances. But they’re buried in dense regions of overlapping star images so the sharp imaging of a telescope in space is needed to pick them out. Wendy Freedman, Rob Ken – nicutt, and Jeremy Mould led the project, accompanied by twenty – five other astronomers from around the world.

The Key Project had a goal of measuring the distance scale to an accuracy of 10 percent. This might seem unduly modest, but it’s a reflection of the difficulty of the experiment. In the decade leading up to the launch of Hubble, astronomers had disagreed by 50 percent or more on the distance scale. In particular, the goal was to measure a quantity called the Hubble constant to within 10 percent. The Hubble constant is the current expansion rate of the universe, measured in units of kilometers per second per Megapar­sec. Hubble discovered a linear relation between the distance and the recession velocity of a galaxy. The Hubble constant sets that scaling. So if the Hubble constant is 70, then for every Megaparsec (3.3 million light-years) increase in distance, the recession velocity increases by 70 kilometers per second (157,000 mph). On average, this scaling says that a galaxy a million light-years away is reced­ing from the Milky Way at 47,000 mph, a galaxy 10 million light – years away is receding at 470,000 mph, and so on. Inverting the Hubble constant allows us to project the expansion back to zero separation of all galaxies, and so gives us a good estimate of the age of the universe. By measuring distances out to 20 Megaparsecs (or 66 million light-years), the Key Project would determine the size, expansion rate, and age of the universe.

The formal result of the Key Project was the measurement of a current expansion rate of 73 kilometers per second per Megapar­sec, with a random error of 6 and a systematic error of 8, published in the twenty-eighth paper in the series from team members, which indicates just how productive and prolific one Hubble project can be.38 Synching the distance scale in this way has been a foundation stone in the emergence of “precision cosmology,” where overall attributes of the universe are becoming very well-determined. Al­though the Key Project worked in our cosmic “back yard,” or the local 0.01 percent of the visible universe, it leveraged and helped to improve distance measurements both inward and outward. Hub­ble also was able to measure parallaxes for Cepheids in the Milky Way (along with Hipparcos, which we’ve previously discussed) and tether the distance scale in direct geometric measures. Know­ing Cepheid distances for several dozen galaxies calibrated the use of other types of distance indicators, such as supernovae and the well-defined rotation properties of galaxies, which can be used out into the remote universe.

Grooves in the Cosmic Pond

As early as 1937, James Jeans observed that “the tendency of mod­ern physics is to resolve the whole material universe into waves, and nothing but waves. These waves are of two kinds: bottled-up waves, which we call matter, and unbottled waves, which we call radiation or light.”48 WMAP’s image of the early universe would seem to confirm Jeans’s observation that the universe might be un­derstood in terms of waves. Even as sound waves traveling through the Sun’s plasma reveal information about its structural composi­tion, fluctuations in the plasma that comprised the very early uni­verse reveal critical data about its matter density and structure. Amedeo Balbi, a member of ESA’s Planck mission, the successor to WMAP, writes, “The primordial plasma resonated like an enor­mous bell, and the mechanism which started the vibrations could plausibly only be one: a period of inflation that occurred a tiny fraction of second after the big bang.”49

Balbi points out that galaxies are clustered throughout space and they have a very slight preference for spacings that can be thought of as corresponding to sound waves. These galaxy sepa­rations designate grooves or “acoustic peaks,” as in a wave, and represent gravity and temperature variations on the cosmic micro­wave background, which evolved into a subtle imprint on the dis­tribution of galaxies. One wave that’s clearly detected in the galaxy distribution has a scale of 300 million light-years.50 Astrophysicist Jean-Pierre Luminet likewise claims that the early universe “‘rang’ like a musical instrument.” He explains that the variations or peaks and troughs we see in the cosmic microwave background reveal details of the universe’s mass and density in much the same way acoustic waves reverberating through a drum reveal its struc­tural properties. “If you sprinkle sand on the surface of a drum,” writes Luminet, “and then gently vibrate the drum skin, the grains of sand will assemble into characteristic patterns” that reveal data regarding “the size and shape of the drum” or “the physical nature of the drum skin.”51 Similarly, in analyzing the variation in clusters and superclusters of galaxies, we are gathering details about the matter density of the primeval universe.

University of Virginia astronomer Mark Whittle likewise char­acterizes the variations in the microwave background as similar to the crest and trough of sound waves: “The waves are actually very long; they’re many thousands of light years, and so they cor­respond to frequencies, pitch which is very, very low by human standards, roughly 50 octaves below human ears.”52 Whittle has produced a sound file simulating what the big bang would sound like if modulated for the human ear. Describing his recreation of the acoustic peaks in the cosmic microwave background as a “sort of a raw, deep roaring sound,” Whittle is quick to note that there’s “actually musicality present.”53 He describes WMAP’s image of the microwave background as “a microscope. . . a telescope. . . a time machine, all rolled into one, and stored in it is enough infor­mation to kind of diagnose what the character of the universe is today, what its future will be and what its birth was.” Of this “ex­traordinary document,” Whittle reminds us that embedded in the image is the primeval narrative of the universe “written by nature in nature’s own language.”54 COBE and WMAP (and now Planck) have given nature a voice.

Amedeo Balbi observes in The Music of the Big Bang, “Ripples in the matter density of the early Universe had to leave a perma­nent imprint in the ancient cosmic light—as a seal impressed into wax.”55 Cosmologists theorize that etched into the primordial plasma at the beginning of time were density and heat variations that became the galaxy clusters we observe today. Balbi’s meta­phor of matter density fluctuations imprinted on the cosmic mi­crowave background as indentations in wax brings to mind an early method of creating phonograph records by etching grooves in wax. Early sound recordings involved “engraving” data into a wax overlay of a zinc record. Music or voice data were recorded by tracing with a sharp stylus a spiral onto beeswax coating the zinc record. The disc was then treated with chemicals that pre­served the grooves where the stylus had removed the wax, after which the phonograph record could be played.

As early as 1931, Georges Lemaitre apparently wondered whether information about the universe had been recorded in the primordial quantum, even as information is preserved in the grooves of a phonograph record. Lemaitre observed that “the whole story of the [universe] need not have been written down in the first quantum like the song on the disc of a phonograph.”56 However, the WMAP results suggest that, in fact, it was. From Lemaitre to Balbi, the technology of the grooved record and the shape of an acoustic wave served as apt metaphors for peaks in the cosmic microwave background.57 Had we some other technology for recording sound in the time of Lemaitre, perhaps astronomers would have characterized these acoustic peaks in other terms. On the other hand, scientists find that acoustic and electromagnetic waves are means by which the universe, the stars, and planets tell their story. Astronomers and cosmologists, since Pythagoras, have never given up on finding music in the cosmos. And now they have.

X-rays in Popular Culture

News of Rontgen’s discovery of X-rays rippled around the world. “When Rontgen discovered X-rays in November of 1895 by plac­ing his hand between a Crookes Tube and a florescent screen, the medical applications of the technology were immediately appar – ent.”4 By January 1896, Rontgen had issued a report regarding his experiments to media in Europe and the United States. The public was immediately fascinated by the possibility of seeing through seemingly solid objects or inside the human body, and the topic made headlines in newspapers and weeklies. Apparently, X-r ay

X-rays in Popular Culture

Figure 10.1. At the end of the nineteenth century, X-rays were harnessed for medical imaging, where dense material like bones cast a shadow in X-rays. In the astronomical realm, a very hot gas or plasma emits X-rays, as do regions near compact objects like black holes. The X-rays may also be absorbed by intervening material, and they can only be detected by a satellite above the Earth’s atmosphere (NASA/CXC/M. Weiss).

“mania” swept through the public sphere but at a level far sur­passing a simple cultural fad, explains Nancy Knight, who notes that “X-rays appeared in advertising, songs, and cartoons.”5 John Lienhard similarly reports, “Seldom has anything taken hold of the public imagination so powerfully and completely. . . . Right away, magazine cartoons celebrated the idea. A typical one showed a man using an X-ray viewer to see through a lady’s hat at the theatre.”6

Edwin Gerson has investigated the speed with which X-rays were incorporated into the branding of products in the United States, and he explores why so many marketers used the term X – ray to appeal to consumers. Being a medical doctor, Gerson began collecting household products dating back to the 1890s, none of which have anything whatsoever to do with X-rays. One could purchase Sniteman’s X-Ray Liniment for horses and cattle, and Patt’s X-Ray Liniment for people. X-Ray Golf Balls, ostensibly to improve the accuracy of one’s game, were sold by John Wana – maker of New York. There was the D-cell X-Ray flashlight battery, X-Ray Stove Polish, X-Ray Cream Furniture Polish manufactured in New York, and X-Ray Soap made in Port Huron, Michigan. X-Ray Blue Double Edge razor blades were touted as “the fin­est blade known to science,” while drinking fountains for chicken houses were marketed by the X-Ray Incubator Company in Wayne, Nebraska. Housewives could purchase an X-ray Coffee Grinder or the X-Ray lemon squeezer, and the X-Ray Raisin Seeder was available as early as November 1896. A company from Baltimore, Maryland, sold boxes of X-Ray headache tablets, with 8 pills for ten cents promising relief in just fifteen minutes.7 Clearly, adver­tising by means of non sequitur is not just a phenomenon of the modern age.

The popular fascination with X-rays continued for decades. By 1924, an X-ray shoe fitter was deployed by shoe vendors to image both feet inside a new pair of shoes. David Lapp reports that in the United States “there were approximately 10,000 of these fluo – roscopes in use, being made by companies such as Adrian X-Ray Shoe Fitter,” and that by the early 1950s most shoe stores were using the devices.8 As Gerson points out, “The public was simply astonished with X rays, and advertisers played off this spellbound attention by adding the name to almost any type of product. . . . Not only did the image of the X ray convey a sense of cutting-edge technology, it also functioned as a metaphor for ‘powerful unseen truth and strength.’”9

Researchers and medical personnel working with X-rays were frequently exposed to dangerous or fatal levels of radiation, in­cluding Elizabeth Fleischmann, who at age 28 read a news article on how to build an X-ray device and by 1900 became the best medical radiologist in the United States. But in ten years’ time, Fleischmann had been so badly overexposed to radiation by im­aging her patients and developing the films that she did not sur – vive.10 At the time, the effect of exposure to radiation was poorly understood and this ignorance played out in dangerous exposure for both medical practitioners and those working with the new technology for purposes of entertainment. Nancy Knight, for in­stance, indicates that “papers reported daily” on Thomas Edison’s efforts to X-ray a person’s brain in order to figure out how the brain functions.11

Allen Grove reports that some speculated at the time that the new technology might afford a kind of X-ray vision that might make it possible to capture on film the images of ghosts, appar­ently a popular notion in England. He also comments that within months of Rontgen’s announcement, theaters were headlining performances inspired by the popularity of X-rays.12 Piano sheet music for American composer Harry L. Tyler’s “X Ray Waltzes” from 1896 depicted on its cover a man holding his hand under an X-ray tube. Beneath the man’s hand appears a skeletal X-ray image. In fact, in 2010, Tyler’s waltzes were dusted off and fea­tured in a BBC broadcast titled “Images that Changed the World.” And, any discussion of X-rays in popular culture must recall that in the 1930s, when writer Jerry Siegel and artist Joe Shuster in­vented the prototypical superhero, it was inevitable that Superman would be gifted with X-ray vision. Though Superman didn’t have such ability at the outset, in later adventures he is able to see in multiple wavelengths of the electromagnetic spectrum, including radio waves, infrared, ultraviolet, and eventually X-rays.13

Rontgen’s discovery had a deep shaping effect on the arts. This was because X-rays were an entree into a hidden world, the “world within the world” represented by the interior structure of atoms and molecules. In 1912, less than twenty years after the initial dis­covery, physicist Max von Laue showed that X-rays were a form of electromagnetic radiation because they diffracted when inter­acting with matter. A year later, the father and son team of William Henry and William Lawrence Bragg described mathematically how X-rays scattered within a crystal, winning the Nobel Prize in Physics for their work in 1915. This opened the door to X-ray crystallography, a powerful tool for measuring the spacing and arrangement of atoms within a solid. The impact of this discovery was particularly strong in the visual arts, which were undergoing their own revolution with the onset of Fauvism, Cubism, and Fu­turism. The X-ray was liberating because to artists it “proved that the external is not valid, it’s just a false layer.”14

Art historian Linda Dalrymple Henderson contends that X-rays suggested the possibility of stepping into the fourth dimension. She writes, “From the 1880’s to the 1920’s, popular fascination with an invisible, higher dimension of space—of which our famil­iar world might be only a section or shadow—is readily apparent in the vast number of articles and the books. . . published on the topic.” Henderson has demonstrated that modern artists read­ily embraced X-rays, and their ability to expose realities and per­spectives not perceptible to the human eye, as suggestive of four­dimensional space.15 Cubist paintings, with their simultaneous depiction of all sides of a seemingly transparent object, and their suggestion of interior planes, explains Henderson, “are testaments to the new paradigm of reality ushered in by the discovery of X – rays and interest in the fourth dimension. Such paintings are new kinds of ‘windows’—in this case, into the complex, invisible reality or higher dimensional world as imagined by the artist.”16 The in­spiration was sometimes very direct. According to historian Arthur Miller, “Picasso’s Standing Female Nude” (1910) was inspired by the power of X-rays to glimpse beyond the visible: what you see is not what you get. In this case, the inspiration was X-ray photo­graphs taken to diagnose the illness of Picasso’s mistress, Fernande Olivier. Superposed on a background of planes, her body lies open to reveal pelvic hip bones made up of geometrical shapes: forms reduced to geometry—the aesthetic of Cubism, inspired by mod­ern science.”17 From the arts, to medicine, and particularly through astronomy and space exploration, X-rays have profoundly altered human culture.

Dark Force

The Hubble Space Telescope has touched every area of astronomy. But some of its key contributions have profoundly shaped our view of the universe. One of the original hopes for the Hubble was to extend the Key Project, which measured the local or current ex­pansion rate, and measure the entire expansion history of the uni­verse over cosmic time. Hubble’s sensitivity and resolution allow it to observe supernovae in distant galaxies. A Type 1a supernova is a white dwarf star that detonates as a result of mass steadily siphoned onto it from a massive companion star. When the white dwarf exceeds the Chandrasekhar limit, which we encountered in the last chapter, it collapses and then explodes as a supernova. This well-regulated process means it’s a “standard bomb” with an in­trinsic brightness that doesn’t vary from one supernova to another by more than 15 percent. At peak brightness, the supernova rivals its parent galaxy in brightness (figure 11.4), so these explosions can be seen to distances of 10 billion light-years or more.39

In the mid-1990s, two groups studying supernovae saw some­thing utterly unexpected.40 In an expanding universe, the effect of normal matter and dark matter is to slow down the expansion rate. By looking back in time with more and more distant super­novae, these researchers had expected to see supernovae appear­ing slightly brighter than expected for a constant expansion rate. The reasoning is that deceleration reduces the distance between

Dark Force

Figure 11.4. Although the Hubble Space Telescope is relatively small by modern standards, its sharp imaging and sensitive instruments allow it to see supernovas at distances of billions of light-years. When they die, these stars rival the bright­ness of their surrounding galaxies, enabling the distances to those galaxies to be measured (NASA/STScI/P. Garnavich).

us and a supernova relative to constant expansion, making it ap­pear brighter. Instead, they saw the opposite effect: the distant su­pernovae were dimmer than expected. The interpretation was the distance to the dying star was larger than expected. Rather than decelerating, the universe has been accelerating! Cosmic accelera­tion is a very puzzling effect, because it implies a force acting in op­position to gravity. No such force is known in physics, so the cause of the phenomenon was called “dark energy,” where that phrase is really just a placeholder for ignorance. Dark energy seems to have the character of the cosmological constant, the term that Ein­stein added to the solutions of his equations of general relativity to suppress natural expansion (and which he later called the greatest blunder of his life). It’s new and fundamental physics.

The discovery of cosmic acceleration was based on images and spectra taken with ground-based telescopes. The Hubble Space Telescope didn’t make the initial discovery. But confirming the re­sult, extending the measurements to higher redshift, and putting constraints on the nature of dark energy—all of those have been essential contributions. Hubble’s depth has been used to trace the expansion history back over two thirds of the age of the universe. The data show that acceleration reverts to deceleration more than 5 billion years ago.41 Astronomers can now apportion the two major components of the universe: dark matter and dark energy. Dark energy accounts for 68 percent, dark matter accounts for 27 percent, diffuse and hot gas in intergalactic space is about 4.5 percent, and all the stars in all of the galaxies in the observable uni­verse amount to only 0.4 percent of the cosmic “pie.” Dark forces govern the universe.

Imagine the expanding universe with a brake and an accelerator. The “driver” isn’t very competent so they press both pedals at the same time. Dark matter is the brake because gravity slows the ex­pansion. Dark energy is the accelerator. In the first two thirds of the expansion history, the dark matter dominates and the expansion slows with time. But the effect of dark matter weakens, because the density and the gravity force go down, while dark energy has a constant strength in both time and space. So the pressure on the brake eases while the accelerator is pressed the same amount, and about 5 billion years ago all galaxies started to separate at ever – increasing rates. Some cosmologists consider it an unexplained coincidence that dark energy and dark matter—two mysterious entities with fundamentally different behavior—happen to have roughly the same strength and crossed paths relatively recently in cosmic time. This is the only time in cosmic history they are close to equal strength; for most of the early history of the universe, dark matter was utterly dominant and forever into the future dark energy will dominate. This coincidence only sharpens the enigma.

Precision Cosmology

WMAP has not only put us “in tune” with the cosmos; it has re­fined and sharpened our view of the extraordinary event that cre­ated all matter and energy 13.8 billion years ago.

WMAP has taken quantities that were poorly known or only hinted at and turned them into well-measured cosmological param – eters.58 The temperature is measured to a precision of a thousandth of a degree. Since space can be curved according to general relativ­ity, the universe can act as a gigantic lens. To do this vast optics experiment we look at the microwaves that have been traveling across space for billions of years. The fundamental harmonic of the microwave radiation sets the size of the “spot.” Radiation from that typical spot size travels through space and the angular size can be magnified or de-magnified depending on whether space has positive or negative curvature, which is like the universe acting as either a convex or a concave lens. WMAP has shown that the spot size doesn’t change so the universe is behaving like a smooth sheet of glass. The inference is that space is not curved; the universe is flat to a precision of 1 percent.59 This is just as expected from inflation.

WMAP has measured the mass and energy of the universe with unprecedented precision. The ratio of fundamental to first harmonic powers depends on the baryonic or normal matter con­tent of the universe. Ordinary atoms only make up 4.6 percent of the universe, to within 0.1 percent. The strength of the second harmonic is sensitive to dark matter, and shows that dark matter is 23 percent of the universe, to within 1 percent.60 Knowing the mass and energy content of the universe, General Relativity can be used to calculate the current age. It’s 13.73 billion years with a precision of 1 percent or 120 million years, though that precision depends on assuming that space is exactly flat (which is a good assumption). These numbers have been refined and slightly altered by Planck. The cosmic “pie” has dark slices and just a sliver of vis­ible stuff (figure 12.4).

We’ve also been able to learn about the epoch when the first stars and galaxies formed using the WMAP data. If stars formed soon enough after recombination, they would have ionized the still-diffusing gas. That would have recreated the conditions where photons bounce off electrons, as was routinely the case before re­combination. This late scattering imprints polarization on the mi­crowave radiation, analogous to sunlight being polarized when it bounces off a water surface. Polarization requires a special direc­tion, and that can only be seen if photons travel relatively freely before interacting. Before recombination no polarization is ex­pected. WMAP saw a polarization indicating that 20 percent of the photons were scattered by a sparse fog of ionized gas a couple of hundred million years after the big bang. This is surprising because astronomers didn’t expect the first stars to form that quickly.61

The sum of all WMAP’s improvements leaves little doubt that the universe began with a hot big bang. The model is described quite precisely and any competing idea would have to clear some very high bars of evidence to be viable. It’s extraordinary that we can know some attributes of the overall universe better than we know attributes of the Earth.

Precision Cosmology

Figure 12.4. Observations of the microwave background radiation combined with ground-based observations determine the contents of the universe. Most of the universe is in the form of enigmatic dark matter and dark energy, with just a small component of the normal matter that comprises all stars, planets, and people (NASA/WMAP Science Team).

A Gentle Astrophysical Giant

In 1923, Wilhelm Rontgen died as one of the most celebrated physicists of his time. A modest man, he had declined to take out a patent on his discovery, wanting X-rays to be used for the benefit of humankind. He also refused to name them after himself and donated the money he won from his Nobel Prize to his university.

That same year Subrahmanyan Chandrasekhar started high school in a small town in southern India. Chandra, as he was universally known, would grow into another great but modest scientific fig­ure. Chandra means Moon or luminous in Sanskrit. He was one of nine children and although his family had modest means, they valued education; therefore he was home schooled since local pub­lic schools were inferior and his parents couldn’t afford a private school. His family hoped Chandra would follow his father into government service but he was inspired by science and his mother supported his goal. In addition, he had a notable role model in his uncle, C. U. Raman, who went on to win the Nobel Prize in Physics in 1930 for the discovery of resonant scattering of light by molecules.18

Chandra’s family moved to Madras and he started at the univer­sity there, but was offered a scholarship to study at the University of Cambridge in England, which he accepted. He would never live in India again. At Cambridge he studied, and held his own, with some of the great scholars of the day. As a young man he fell into a controversy with Arthur Eddington, at the time the foremost stellar theorist in the world, which upset him greatly. Eddington refused to accept Chandra’s calculations on how a star might con­tinue to collapse when it had no nuclear reactions to keep it puffed up. Chandra was a gentle man, unfailingly polite and courteous. In part due to this conflict, he chose to accept a position at the Uni­versity of Chicago, where he spent the bulk of his career.

His name is associated with the Chandrasekhar limit, the theo­retical upper bound on the mass of a white dwarf star, or about 1.4 times the Sun’s mass. Above this limit, the force of gravity over­comes all resistance due to inter-particle forces, and a stellar corpse will collapse into an extraordinary dark object, either a neutron star, or if there’s enough mass, a black hole. In the 1970s, he spent several years developing the detailed theory of black holes. Chan­dra was amazingly productive and diverse in his scientific interests. He wrote more than four hundred research papers, and ten books, each on a different topic in astrophysics. For nineteen years, he was the editor-in-chief of The Astrophysical Journal, guiding it from a modest local publication to the international flagship jour­nal in astronomy. He mentored more than fifty graduate students while at the University of Chicago, many of whom went on to be the leaders in their fields. In 1983, he was honored with the Nobel Prize in Physics for his work on the theory of stellar structure and evolution.

Through his long career, Chandra watched the infant field of X-ray astronomy mature. The telescopes flown in sounding rock­ets in the early 1960s were no bigger than Galileo’s spyglass. Just before its 1999 launch, NASA’s Advanced X-Ray Astrophysical Facility was renamed the Chandra X-ray Observatory (CXO) in honor of this giant of twentieth-century astrophysics.19 In a span of less than four decades, Chandra improved on the sensitivity of the early sounding rockets by a factor of 100 million.20 The same sensitivity gain for optical telescopes, from Galileo’s best device to the Hubble Space Telescope, took four hundred years—ten times longer!

Dark Force Redux

Hubble has weighed in on the other major ingredient of the uni­verse: dark matter. The existence of dark matter was first indicated in an observation of the Coma Cluster of galaxies by the maver­ick Caltech astronomer Fritz Zwicky. Zwicky measured redshifts or radial velocities for galaxies in the cluster and saw velocities that were much higher than anticipated. The galaxies were buzzing around like angry bees at over 500 kilometers per second, over a million miles per hour.42 If the cluster had the mass indicated by the stars in all the galaxies, it wasn’t enough to keep those galaxies bound in one region of space. In fact, visible mass was insufficient by a factor of ten. The Coma Cluster should be flying apart. But it’s not, so Zwicky hypothesized an invisible form of matter to hold it together. It had to be a form of matter that exerted gravity but didn’t radiate light or even interact with radiation—dark matter.

This observation was so odd and so unexpected that most as­tronomers simply set it aside. (It didn’t help that Zwicky was bril­liant but extremely cantankerous, and he made a lot of enemies in the profession with his blunt and often rude comments.) But in the 1970s the rotation of spiral galaxies also indicated unseen forms of matter extending far beyond the visible stars. Astronomers re­visited Zwicky’s observation and found that it was correct, and that other clusters of galaxies showed the same effect. Dark matter was a ubiquitous feature of the universe, on galactic scales and beyond galaxies in the space between them.

The Hubble Space Telescope has cemented the measurement of dark matter in a very elegant way. Einstein’s theory of general rela­tivity says that mass bends light, and this prediction was confirmed in 1919 with starlight bending around the limb of the Sun during a solar eclipse. We’ve seen that both Cassini and Hipparcos had a hand in showing that relativity was correct. Zwicky realized that a galaxy could bend light by a detectable amount and he urged astronomers to search for the effect. Lensing was finally seen for the first time five years after Zwicky died, when the twin-i mage mirage of a single quasar was observed in 1979. In the mid-1980s, the phenomenon of lensed arcs was discovered with 4-meter tele­scopes on the ground. Each lensed arc is an image of a galaxy be­hind a rich cluster, magnified and distorted by the cluster. Cluster lensing has a very particular signature because the arcs are frag­ments of concentric circles centered on the cluster core. The beau­tiful part of the effect is that the gravitational deflection is sensitive to all mass, light or dark, so it’s a reliable way to weigh a cluster.

Hubble’s exquisite imaging has been used to study lensed arcs in dozens of clusters.43 Each background galaxy that gets imaged is a little experiment in gravitational optics (and a confirmation

Dark Force Redux

Figure 11.5. The cluster of galaxies Abell 2218 acts as a gravitational lens, where its visible and dark matter distort and amplify the light of more distant, back­ground galaxies. They are seen as tiny arcs in this Hubble image, many of which are concentric with the center of the cluster. The lensing effect is an affirmation of general relativity, where mass causes space-time curvature and the universe acts like a gigantic optics experiment (NASA/ESA/SM4 ERO Team).

of general relativity). In some clusters there are hundreds of little arcs, so the mass measurement of the cluster is very reliable— it’s like having an optics experiment with hundreds of light rays (figure 11.5). The observations clinch the fact that dark matter exceeds normal matter by a factor of six, or the ratio of the 27 percent to 4.5 percent contributions mentioned earlier. They also allow the dark matter to be mapped in the cluster, and the spatial distribution is critical information in helping decide the physical nature of this universal but mysterious substance.

Hubble has also played a pivotal role in locating a large type of dark object: massive black holes. Since the discovery of quasars in the 1960s, it has been clear that only a gravitational “engine” could generate so much energy from such a small volume. The energy source for quasars is thought to be a supermassive black hole, billions of times more massive than the Sun. As we saw in the last chapter, black holes aren’t always black. Nothing can escape from the event horizon, but a black hole will gather hot gas into a rotating disk around it. The disk siphons material into the black hole while the poles of the spinning black hole act as giant particle accelerators. The accretion disk emits huge amounts of ultraviolet and X-ray radiation while the jets emit radio waves. For a long time, astronomers thought that the galaxies surrounding the qua­sars were special because they housed such a black hole. Over the last fifteen years, Hubble has used its spectrographs to study stellar velocities near the centers of apparently normal galaxies near the Milky Way. Often, the data showed a sharp rise in star velocities near the nucleus of the galaxy.44 Calculating the density of mat­ter that would generate such high velocities in such a small vol­ume, the only possible explanation was an efficient gravitational engine—a black hole.

But these black holes were surprising in two ways. First, they weren’t as massive as the black holes that powered quasars. They ranged from 10 million to a few hundred million times the mass of the Sun. Second, the galaxies otherwise looked completely normal. Apparently, these black holes weren’t active even though there was plenty of “food,” or gaseous fuel in the center of these galaxies. As the data accumulated, it became clear that every galaxy has a black hole, with mass proportional to the mass of old stars in the galaxy, but those black holes must only be active a small fraction of the time and inert the rest of the time.45

This result has led to a paradigm shift in extragalactic astron­omy. There’s no division between “normal” and “active” galaxies— all galaxies are active at some level but not all the time. Black holes are a standard component of a galaxy. There are a few intermedi­ate mass black holes residing in globular clusters and in dwarf galaxies, filling in the mass range from a thousand to a million solar masses. Nature knows how to make black holes spanning a factor of a billion in mass! The low end of the range is collapsed stellar corpses the size of a city and the high end is behemoths in galaxies ten times larger than the Milky Way. Moreover, in their active phases black holes eject mass and quench star formation and generally alter the properties of the surrounding galaxy. The co-evolution of galaxies and black holes is now a major field of research; with its exquisite resolution and sensitivity Hubble is playing a big role. Some time in the first hundred million years or so after the big bang, the first stars and galaxies formed, and black holes began to grow at the same time.

Probing the Very Early Universe

The landscape in cosmology has shifted. Scientists no longer worry about the validity of the big bang model. It rests on a sturdy tri­pod of evidence. One leg is the expansion of the universe as traced by the recession of galaxies. Another leg is the cosmic abundance of light elements in the first three minutes, when the temperature was 10 million degrees. The third is the microwave background radiation. WMAP has advanced the level of diagnostic power of the third piece of evidence to a level that is unprecedented. The big bang model has so far passed all tests with flying colors.

The frontier of cosmology involves gaining better physical un­derstanding of dark matter and dark energy, and pushing tests of the big bang to earlier eras. WMAP has “weighed” dark matter with better accuracy than ever before, and it has shown that dark energy is an inherent property of space-time itself (as with Ein­stein’s “cosmological constant) rather than being a particle or field existing in space-time. The current frontier is the epoch of infla­tion, an incredible trillion trillion trillionth of a second after the big bang. Inflation was motivated by the unexpected flatness and smoothness of the universe, plus the lack of space – time glitches surviving to the present day, like monopoles and strings.

Inflation got early support from WMAP data showing that the strength of temperature variations was independent of scale. But in current theory, “inflation” is not one thing; it’s an umbrella for a bewildering array of ideas about the infant universe. Since it refers to a time when all forces of nature except gravity were unified, in­flation models involve speculation as to the fundamental nature of matter. Superstrings are one such concept for matter, but almost all the theories have to incorporate gravity in some way, so the theory of the very early universe is proximate to the search for a “theory of everything.”

In 2003, the WMAP team announced a new measurement re­sulting from the polarization measurements and increasingly re­fined temperature maps. The temperature variations are generally equal in strength on all scales, but as data improved, it became clear they were not exactly equal on all scales. The sense of this deviation was exactly as predicted by the favored inflation mod­els, and different from predictions by rival theories for the early expansion (exotic “cold big bang” models were also ruled out). In 2010, using WMAP’s seven-year data, the team even managed to use the data to confirm the helium abundance from the big bang, and they put constraints on the number of neutrino species and other exotic particles.62 Most gratifyingly, the data confirmed the fundamental correctness of the model of temperature variations as resulting from acoustic oscillations. The piper’s tune is better understood than ever before (plate 22).

Inflation is required by the data, and cosmologists are homing in on the correct model. Part of the inflation landscape is the fact that the physical universe—all that there is—is much larger than the visible universe—all we can see. Inflation also motivates the idea of the multiverse: parallel universes with wildly different proper­ties that emerge from the quantum substrate that preceded the big bang.63 Our dreams of other worlds should now be expanded to encompass other universes filled with worlds. The ambition and scope of these theories is extraordinary, and it undoubtedly would have amazed Pythagoras to know how far we’ve taken his ideas of a universe based on mathematics and harmony.

The Chandra X-ray Observatory

Until the Chandra X-ray Observatory was launched, the best X – ray telescopes were only as capable as Galileo’s best optical tele­scope, with limited collecting area and very poor angular resolu­tion. With Chandra, X-ray astronomers gained several orders of magnitude of sensitivity, and the ability to make images as sharp as a medium-size optical telescope. Chandra was the third of NASA’s four “Great Observatories.” The others are the Hubble Space Tele­scope, launched in 1990 and still doing frontier science, the Comp­ton Gamma Ray Observatory, launched in 1991 and deorbited in 2000 after a successful mission, and the Spitzer Space Telescope, launched in 2003 and currently in the final “warm” phase of its mission since its liquid helium coolant ran out in 2009 (plate 17).

It took a while to open the X-ray window on the universe be­cause the Earth’s atmosphere is completely opaque to X-rays. In the 1920s, scientists first proposed using versions of Robert God­dard’s rocket to explore the upper atmosphere and peer into space. However, this idea wasn’t realized until 1948, when a re-purposed V2 rocket was used to detect X-rays from the Sun.21 The next few decades saw the development of imaging capabilities for X-rays and new detector technologies, and X-ray astronomy tracked the maturation of the space program. The first X-ray source beyond the Solar System, Scorpius X-1, in the constellation of Scorpius, was detected by physicist Riccardo Giacconi in 1962.22 This in­tense source of high-energy radiation is a neutron star, the end result of the evolution of a massive star where gravity crushes the remnant to a state as dense as nuclear matter. The intense X-rays result from gas being drawn onto the neutron star from a com­panion and being heated violently enough to emit high-energy radiation.

Giacconi is another “giant” in astrophysics—as leading scientist for X-ray observatories from Uhuru in the 1970s to Chandra in the 1990s, first director of the Space Telescope Science Institute, and winner of the Nobel Prize in Physics in 2002. This last and ultimate accolade fittingly came almost exactly a century after the award of the first Nobel Prize in Physics for the discovery of X – rays to Wilhelm Rontgen. Born in Genoa, Italy, his early life was disrupted by the Second World War; as a high school student, he had to leave Milan during allied bombing raids. He returned to complete his degree and started his life as a scientist in the lab, working on nuclear reactions in cloud chambers. With a Fulbright Fellowship he moved to the United States and forged his career there. He had his hand in all the pivotal discoveries of X-ray as­tronomy: the identification of the first X-ray sources, character­ization of black holes and close binary systems, the high-energy emission that emerges from the heart of some galaxies, and the nature of the diffuse X-rays that seem to come from all directions in the sky.23

The growth in the number of celestial X-ray sources gives a sense of how each new mission has advanced the capabilities: 160 sources in the final catalog of the UHURU satellite in 1974, 840 in the HEAO A-1 catalog in 1984, nearly 8,000 from the com­bined Einstein and EXOSAT catalogs of 1990, and about 220,000 from the ROSAT catalog of 2000. Chandra has had a wider-field but lower sensitivity counterpart in the European XMM-Newton mission, also launched in 1999. These two X-ray satellites have detected a total of over a million X-ray sources.

Chandra was launched by the Space Shuttle Columbia into a highly elliptical orbit that takes it a third of the way to the Moon. At five tons, it was the most massive payload launched up to that time by the Space Shuttle. The elongated orbit gives it lots of “hang time” in the perfect vacuum of deep space and lets science be done for 55 hours of the 64-hour orbit. This comes at the expense of the spacecraft being unserviceable by the Shuttle when it was still flying, so the facility has to work perfectly. In fact, the only tech­nical problem was soon after launch when the imaging camera suffered radiation hits during passage through the Van Allen radia­tion belts; it is now stowed as the spacecraft passes through those regions. The spacecraft had a nominal five-year mission at time of launch but it’s producing good science well into its second decade and is expected to last at least fifteen years.24

One of two different imaging instruments can be the target of incoming X-rays at any given time. The High Resolution Camera uses a vacuum and a strong electric field to convert each X-ray into an electron and then amplify each one into a cloud of electrons. The camera can make measurements as quickly as 100,000 times per second, allowing it to detect flares or monitor rapid variations. Chandra’s workhorse instrument is the Advanced Camera for Im­aging Spectroscopy. With 10 CCDs, it has one hundred times bet­ter imaging capability than any previous X-ray instrument. Either of these cameras can have one of two gratings inserted in front of it, to enable high – and low-resolution spectroscopy. Spectroscopy at X-ray wavelengths is a bit different from optical spectroscopy. The spectral lines seen by Chandra are usually very high excitation lines of heavy elements like neon and iron, coming from gas that’s kept highly agitated by high-energy radiation or violent atomic collisions.25

There are three major differences between optical and X-ray detection of sources in the sky. The first is the way the radiation is gathered. X-rays falling directly on silvered glass have such high energy that they penetrate the surface and are absorbed, like tiny bullets. X-ray telescopes use a shallow angle of incidence, so that the photons bounce off the mirror like a stone skimming off water. Chandra uses a set of four concentric mirrors, six feet long and very slightly tapered so they almost look like nested cylinders. This method of gathering radiation makes it difficult to achieve a large collecting area. The second difference is the much higher energy of X-rays. Chandra measures photons in a range of energy from 0.1 to 10 keV, or 100 to 10,000 electron volts, which is a standard unit of measure for photons. On the electron volt energy scale, two numbers that bracket this range are 13.6 eV, the modest energy required to liberate an electron from a hydrogen atom and 511 keV, the rest mass energy of an electron. For reference, photons of visible light have wavelengths 10,000 times longer and energies 10,000 times lower.

With each photon packing such a punch, a typical astronomical source emits far fewer X-ray photons than visible photons, so each is very valuable. The goal in X-ray astronomy is to detect every photon individually. Very few photons are required to detect a source (this is helped by the fact that the “background rate” is low; X-rays are not created by miscellaneous or competing sources, so the X-ray sky is sparsely populated). In some of the deepest ob­servations made by Chandra, two or three photons collected over a two-week period is enough evidence to declare that a source has been detected. There are papers in the research literature with more authors than photons!

Chandra unlocks the violent universe because celestial X-rays have high energy and can only be produced by extreme physical processes.26 The Sun and all normal stars are very weak X-ray sources because their cool outer regions produce thermal radia­tion peaking at visible wavelengths. It would take a gas at hun­dreds of thousands or millions of degrees to emit copious X-rays; diffuse gas with this very high temperature is distributed between galaxies. Another way X-rays can be made is when particles are accelerated to extremely high energies; they release the energy in a smooth spectrum that extends to X-rays and even gamma rays.27 Despite the million plus X-ray sources that have been cataloged, there are thousands of times more optical sources, so the X-ray sky is relatively quiet. But many of those X-ray sources are extremely interesting because they’re situations where matter has been sub­ject to extreme violence.