“Now entertain conjecture of a time
When creeping murmur and the poring dark
Fills the wide vessel of the Universe.”
William Shakespeare (1599)1
“God made darkness his secret place.”2
Night Falls
As the Earth rotates, the daytime side of our planet turns away from the Sun’s light and becomes night. The darkened continents lose their borrowed sunlight; a quiet hush passes over the land; and time seems to hesitate as the Sun drops below the horizon. Cool breezes refresh the air, fish come up to feed, swallows swoop through the air, and moonflowers open to the nightly glow.
Most familiar things slide away when evening falls. Daytime boundaries are lost, our familiar surroundings lose their form, houses become wrapped in a blanket of darkness, and entire cities seem to drain themselves of life. They dissolve into the shadows, fuse into the enfolding night, and vanish from view.
The night can bring delight.3 Fireflies, tiny specks of living light, can rise to twinkle in the dark, and the Moon and stars come into view. But even the stars seem insignificant and lost, as they try to light up the immense blackness that separates them. They resemble tiny fires in a cold, silent world.
Many astronomers nevertheless revel in the dark quiet of the night and the splendor it brings into view. There are luminous stars that heat and illuminate adjacent regions (Fig. 12.1), and immense, mysterious black places that are far bigger than the glowing regions (Fig. 12.2).4 At first sight, they look empty and without substance, characterized by an absence of anything we know, but closer scrutiny reveals that all that darkness is not an emptiness. It contains numerous fine, solid particles of dust that absorb and scatter starlight.5
Figure 12.1 Rosette Nebula Hot O and B stars in the core of this nebula exert pressure on the nearby interstellar material, trigger star formation, and heat the surrounding gas to a temperature of about 6 million degrees kelvin. (Courtesy of KPNO/CTIA.)
The ancient Chinese sage Lao-tzu wrote: “Darkness within darkness, the gate to all understanding,”6 and he was right. Astronomers know there is always something in the dark. It is often the kernel and substance of things to come. In the Earth, subterranean seeds lie slumbering below the cold winter surface. They are waiting to rise, bloom, and unfurl their beauty in the warmth of the spring. Humans begin their lives in the darkness of their mother’s womb, and stars form within immense dark places.
Figure 12.2 Dark clouds New stars may be born in the dense molecular clouds of the Carina Nebula. Energetic stellar winds and intense radiation from nearby massive stars are sculpting its outer edges. (A composite Hubble Space Telescope image courtesy of NASA/ESA/Hubble Heritage Project/STScI/AURA.)
Flowers, humans, and stars; they all keep appearing out of the dark and disappearing back into it. It may be the same darkness from which all of us and everything else have come and to where we are all going.
There has to be something more than dust out there if entire stars are being spawned from it. Just as the stars are themselves mainly composed of the lightest element hydrogen, it is hydrogen atoms that provide the main substance of interstellar space. This material has been additionally enriched with lesser amounts of heavier elements manufactured inside former stars.
When it ceases to shine, an entire star can be returned to the darkness from which it came. The celebrated American poet T. S. Eliot wrote some appropriate lines, when he was probably thinking of humans rather than stars:
“O dark dark dark. They all go into the dark,
The vacant interstellar spaces, the vacant into the vacant.”7
Figure 12.3 Light and dark Intense ultraviolet radiation from young, massive stars illuminates and shapes interstellar gas within the Omega Nebula, which is also designated as M 17. (A Hubble Space Telescope image, courtesy of NASA/ESA/J. Hester, ASU.)
To glimpse the full beauty of the world, we want to focus on both the light and the dark. It is the combination of brightness and shadow that make sunrise, sunset, or a stormy day so captivating. You see the contrast in the shadows cast on the ground by a drifting cloud, or in the Milky Way, where: “The unseen dark plays on his flute, and the rhythm of light eddies into stars and Sun, into thoughts and dreams.”8 Light and dark are always there, coexisting and depending on one another (Fig. 12.3).
Detecting the Unseen
For centuries astronomers viewed the Universe through the visible light rays emitted by stars. Their scrutiny began with unaided eyes, and was then extended using telescopes whose lenses and mirrors gathered in more light. This enabled the resolution of details that could not be seen before, and the detection of faint, otherwise invisible objects, such as the galaxies.
In the 20th century, unique telescopes, new technology, and novel detection equipment enabled astronomers to penetrate the darkness with radio waves and x-rays. They widened our cosmic vision by detecting invisible worlds that had been hidden in the dark for millennia. It was found that much of the Universe resides in the darkness, and remains out of direct sight to human eyes even with the aid of a visible-light telescope. Most of these unseen cosmic objects were totally unexpected, and no one predicted or even imagined many of them.9
What you observe anywhere in the Universe depends on how you look at it. The cold, dark interstellar spaces, for example, emit most strongly at long radio wavelengths, and very hot cosmic gases, with temperatures of millions of degrees kelvin, shine brightly at short x-ray wavelengths. It is only stars like the Sun, with disk temperatures of several thousand degrees kelvin that radiate intense visible light. It is all a matter of perspective, with invisible radiation disclosing some things, and visible light revealing others.
Cosmic Radio Broadcasts
It wasn’t until the closing years of the 19th century that anyone knew a thing about radio waves. The German physicist Heinrich Hertz first generated them in his laboratory in 1886,10 and the Italian entrepreneur Guglielmo Marconi pioneered global radio communications as a commercial venture shortly thereafter.11 But these developments did not lead directly to new windows on the Universe, and so had little bearing for astronomy.
Radio emission from the Milky Way was inadvertently discovered in the 1930s when the Bell Telephone Laboratories assigned Karl Jansky the task of tracking down and identifying natural sources of radio noise that were interfering with transatlantic radio transmissions at a wavelength of 14.6 meters. He constructed a rotating antenna, which pointed sideways at the horizon and permitted the identification of the interference, including the intermittent radio static produced by lightning discharges from distant thunderstorms.
Fortunately, the antenna’s wide field of view also pointed part way up into the sky, and thereby detected an extraterrestrial hiss of unknown origin, which was comparable in intensity to terrestrial lightning. By observing the variation of its intensity as a function of direction and time of arrival, Jansky established that the radio emission had to originate from outside the Solar System, and that the most intense radiation was coming from the direction of the constellation Sagittarius and the center of our Galaxy.12
This serendipitous discovery of cosmic radio broadcasts was reported by newspapers throughout the world, including the New York Times whose front-page headline for May 5, 1933 read: “New Radio Waves Traced to the Center of the Milky Way.” Jansky even appeared on a radio program, which rebroadcast his “star noise” so listeners could hear “the hiss of the Universe” by a direct long-line connection from the Bell Labs field station in Holmdel, New Jersey to a New York Broadcasting studio.
Although Jansky wanted to follow up his incredible discovery, the Bell Labs assigned him to other engineering problems more directly related to their objectives. The country was in the throes of the Great Depression, when jobs were scarce and Jansky was most likely glad to be employed. He never returned to research in astronomy after 1935, and died in 1950 at the age of 44 without ever receiving any scientific award for his profound discovery.
Moreover, astronomers almost completely ignored the result. Jansky did not publish in an astronomical journal, and his radio techniques were so much outside the conventional methods of astronomy that no traditional observatory contributed to new knowledge about it. So astronomers did not become aware of the different way of looking at the Cosmos until the 1940s, when an electrical engineer, amateur astronomer, and avid ham radio operator, Grote Reber, confirmed and extended Jansky’s findings and published them in the Astrophysical Journal.13 With the occasional help of a local blacksmith, he built with his own hands a 9.6-meter (31-foot) metal, dish-shaped radio telescope in the backyard of his home in Wheaton, Illinois, and after a few years of trying he used it to detect radio noise coming from the Milky Way.
Most stars other than the Sun do not emit detectable radio waves, and intense cosmic radio radiation originates in the space between them. This emission is attributed to cosmic-ray electrons traveling within interstellar space at a speed close to that of light. They are similar to the cosmic-ray protons and electrons impinging on the Earth’s atmosphere,14 but only the electrons emit radio waves.
Although long radio waves reveal the presence of invisible high-speed electrons in interstellar space, one particular radio wave, just 21 centimeters long, discloses cold hydrogen atoms in the space between the stars.
The Fullness of Space
Since most stars are mainly composed of hydrogen, it was supposed that the space they arose from ought to contain large amounts of the substance. And because new stars are still forming today, there ought to be plenty of hydrogen atoms out there right now. These atoms move slowly at the freezing temperatures of interstellar space and gently knock against each other, which stimulates invisible radio emission. The prediction and discovery of this radiation follows a paper trail of international scope, remarkable coincidences, and a professional courtesy of bygone times. It is also a tale of two graduate students and their ever so smart advisors.
This story began near the end of World War II (1939–1945), when astronomers at the Leiden Observatory in the Netherlands obtained smuggled copies of the Astrophysical Journal from America. When the Dutch astronomer Jan Oort read about Grote Reber’s unexpected discovery of cosmic radio static, he asked his graduate student Hendrik C. “Henk” Van de Hulst to find out if there were any spectral lines in the radio spectrum. He investigated the matter, and predicted a radio wavelength spectral line that might be detected from interstellar regions of electrically neutral, or unionized, hydrogen atoms.
Van de Hulst realized that these sources, now designated H I regions and pronounced H one regions, would be very cold and that most of the atoms would be in their lowest energy state. In this condition, the lone electron of the hydrogen atom has two possibilities in the direction of its spin, or rotation, and a rare collision between two of the atoms could cause one of them to flip over and reverse its spin direction. The atom is then in an unstable configuration, so its electron will soon flip back to its original state. This releases a small amount of energy and produces radiation at a wavelength of 21 centimeters. As Van de Hulst pointed out, these spin transitions will occur rarely in the cold tenuous interstellar gas; a given atom only undergoes the spin flip once every 11 million years. But an observer might well detect them when looking through the vast extent of interstellar space.
The prediction was published in 1945 within an obscure Dutch journal Nederlands tijdschrift voor natuurkunde, with an obtuse article title of “Radio Waves from Space: Origin of Radio Waves,”15 which did not tell the reader very much about what Van de Hulst had found. The Soviet theorist Iosif S. Shklovskii then confirmed the prediction seven years later, with greater detail, but in the Russian language.16
At about this time, Harold I. “Doc” Ewen, a graduate student at Harvard University, became interested in radio astronomy. Ewen’s advisor, Edward M. Purcell, asked his wife, Beth, to translate Shklovskii’s paper about the 21-centimeter radiation, and after reading it Purcell encouraged Ewen to build a radio receiver to search for it. They also had a copy of Van de Hulst’s original work, translated from the Dutch.
Ewen’s previous experience made him especially suited for building the radio detector. After completing undergraduate study at Amherst College, he joined the Navy, where he was trained in radar, first at Princeton University and then at the Radiation Laboratory of the Massachusetts Institute of Technology. As a radar officer in a naval airborne squadron during World War II (1939–1945) he learned all about repairing radio equipment, sometimes without any spare parts.
After the war, Ewen entered Harvard University on the GI bill, and to make ends meet he worked at the cyclotron particle accelerator of the University’s nuclear laboratory. His interest in detecting hydrogen became “a weekend thing, strictly a hobby on the side” of his real job. He put together a “detecting machine” using electronic components that were scavenged from other places, or brought with a $500 grant and $300 from Purcell’s pocket. Much of the equipment was borrowed every Friday, and returned each Monday, from the nuclear laboratory using a wheelbarrow.
Accurate measurements of the wavelength of the expected hydrogen emission, using terrestrial atomic hydrogen in a laboratory at Columbia University, enabled Ewen to tune his receiver to precisely 21.106 centimeters. And at Purcell’s suggestion, the receiver was switched between the wavelength of the expected signal and an adjacent one, with a difference that might contain the anticipated hydrogen radiation. [Such a wavelengthswitched, or frequency-switched, receiver has now been widely adapted by radio astronomers to remove unwanted noise from an observed signal containing a spectral feature.]
The receiver was connected to a simple horn antenna that was built of plywood, lined with copper sheeting, and mounted on a ledge just outside a high window. It pointed in a fixed direction up into the sky, which swept by as the Earth rotated, and the 21-centimeter transition was detected when the Milky Way entered the antenna beam.
Hendrik Van de Hulst happened to be serving as a Visiting Professor at the Harvard College Observatory at the time, and the Australian astronomer Frank Kerr was visiting Harvard on a Fulbright grant. So Ewen and Purcell had them over to describe their discovery, and urged them to have their people confirm the result.
At the meeting, the Harvard team learned for the first time that the Dutch group at Leiden had been actively trying to detect the radio transition for several years. So a description of the wavelength-switched receiver was provided to Van de Hulst, leading to the conversion of the Dutch system, and in a gracious move, which would be unheard of in today’s competitive scientific world, Purcell insisted that publication of their discovery be held up until the Dutch group confirmed it.
Even though the Australians were not actively pursuing a search for the hydrogen radio emission, and did not have a detection system in operation at the relevant wavelength, they assembled the necessary components soon after receiving word from Kerr, and repeated the detection about a month after the Dutch had. A coordinated report from all three centers was then published in the journal Nature in July 1951.17,18
The detected line was seen in emission, and the line profiles could be used to estimate the temperature of the dark interstellar hydrogen atoms. Using this technique, the Leiden research group found that they are very cold indeed, with a temperature of about 100 degrees kelvin, well below the freezing temperature of water at 273 kelvin.
After the pioneering detection, the Navy called Ewen to return to active duty because of the Korean conflict, and the Harvard astronomy department did not immediately follow up his discovery. It was the Dutch and Australian groups that took advantage of the fact that the 21-centimeter radiation propagates right through the curtains of interstellar dust that hide most of our Galaxy from view at the optical wavelengths of visible light.
By laboriously turning an old German radar antenna in various directions by two small hand cranks every few minutes for much of a two-year period, the Leiden group collected observations to show that the interstellar gas seen in the northern hemisphere describes four extended arm-like features. In the meantime, the Sydney group used a radio telescope that looked straight up, and could not be moved, to discern similar features in the southern Milky Way as the Earth’s rotation turned these regions past the telescope’s field of view. The two surveys were combined to give the Leiden-Sydney map of our Galaxy in which neutral hydrogen atoms are concentrated in several elongated, nearly circular features that resemble spiral arms. (Fig. 12.4).19
Figure 12.4 Arms of the Milky Way The distribution of the 21-cm radiation from hydrogen atoms in the plane of our Galaxy, the Milky Way, displays arm-like features. Large circles are spaced 6,500 light-years apart and the small circle and the letter S in the top middle denote the Sun. [Adapted from Jan H. Oort, Frank J. Kerr, and Gart Westerhout: “The Galactic System as a Spiral Nebula,” Monthly Notices of the Royal Astronomical Society 118, 379–389 (1958).]
Soon after the discovery of interstellar atomic hydrogen at radio wavelengths, astronomers began to speculate about the possibility of detecting molecules with radio waves, which are sensitive to the coldest clouds of interstellar matter. But observations of any molecule first required accurate measurements of the frequencies or wavelengths of its spectral features in the terrestrial laboratory. Charles Townes played an important role in these pioneering investigations.
Masers, Lasers, and Interstellar Molecules
Charles H. Townes grew up in a devoted Southern Baptist family. He spent his childhood on a 20-acre cotton farm in Greeenville, South Carolina, where he attended public schools and then graduated from the local Furman University, a Baptist Institution, in 1935 at the age of 19. He completed work for a Masters degree in physics at Duke Univeristy a year later, and then entered the graduate school at the California Institue of Technology where he attended lectures by Albert Einstein, Willie Fowler, Robert Millikan, J. Robert Oppenheimer, Richard Tolman, and Fritz Zwicky. In Townes’ account of these and later times, he notes how chance conversations and encounters of life lead, in totally unpredictable ways, to the events that shape a career.20
In 1939, with a fresh Ph.D. degree in hand, Townes went to work for the Bell Telephone Laboratories in New York, where he helped design radar bombing systems for the U.S. Air Force throughout World War II (1939–1945). The radar was supposed to operate at a short wavelength of 1.25 centimeters, which would provide high angular resolution, but it was eventually scrapped because radiation at this wavelength cannot travel very far in the Earth’s atmosphere before it is absorbed by water molecules there.
This got Townes interested in how molecules absorb and emit energy, and he began a program of molecular spectroscopy at microwave wavelengths, from 0.1 centimeters to 10 centimeters, that he continued for two decades as a faculty member at Columbia University. During this time, he and graduate students built a maser filled with ammonia gas, NH3, in which the molecule turns itself inside out, like an umbrella in a high wind. [The nitrogen atom, N, vibrates back and forth in the plane of the three hydrogen atoms, H, at the same 1.25-centimeter wavelength he had investigated for radar purposes.]
Townes has recalled that the former and current chairmen of his department in Columbia University, who were both Nobel laureates for their work with atomic and molecular beams, asked him to discontinue this work, since they didn’t think it would work and their research depended on the same source of support as his. But Townes kept on with what interested him, and was thankful that he had come to Columbia with tenure, so they couldn’t fire him for doing what he wanted. He was also grateful that his funding agencies in the military did not have any specific expectations or instructions. Townes was instead thrilled, intrigued and stimulated by the beauty of nature, from a calm sea to a stormy one, from an atom to a field of wild flowers, or an insect, bird, fish, star or galaxy.21
The term maser is an acronym for microwave amplification by stimulated emission of radiation; the reader may be more familiar with the laser, where the letter “l” denotes visible light.22 When focused to a small point, laser beams can produce intensities of light billions of times that at the Sun’s surface.
The basic idea had been anticipated by Einstein in 1917 when he explained how radiation could induce, or stimulate, still more radiation when it hits an atom or a molecule provided the energy of the incoming photon equals the energy that can be lost by an atom or a molecule in making a transition from a higher to a lower energy state.
Charles Townes led a somewhat nomadic life, moving from place to place and entering other fields, to turn over new stones and see what is under them. While at Columbia, he took an interest in astronomy and at the request of Hendrik van de Hulst attended an international symposium on radio astronomy, in 1955, where he specified what molecules might be found in interstellar space. This included his precise laboratory measurements of the vibrational and rotational transitions of molecules that might be detected at microwave or radio wavelengths, including the carbon monoxide, CO, water, H2O, and hydroxyl, OH, molecules.23 But radio astronomers could not observe these molecules in interstellar space until new methods of spectral analysis and radio telescopes with surfaces accurate to a few centimeters were constructed.
Townes served as Vice President and Director of Research at the Institute for Defense Analysis in Washington, D.C. from 1959 to 1961, and during the next six years, from 1961 to 1967, he was Provost at the Massachusetts Institute of Technology, or MIT for short. In 1963 Alan Barrett used a new digital receiver designed by Sander Weinreb at MIT to obtain the first observations of intersellar OH at a wavelength of 18.005 centimters, or a frequency of 1,667 MHz, that Townes had specified.24 The discovery was confirmed within days by other radio astronomers, with an unexpected consequence.
In some places, the OH molecules were emitting energy of high brightness temperature at the spectral line wavelength, rather than absorbing it. They were acting like cosmic masers that were being pumped into high energy levels by radiation from stars and by collisions. It was subsequently found that interstellar water molecules are even more powerful masers, which can emit much more power at a single wavelength than the Sun does at all wavelengths. It is an amazing coincidence that the masers invented in the terrestrial laboratory were also operating in interstellar space.
In 1964 Townes received the Nobel Prize in Physics, which he shared with Nicolai G. Basov and Alexander “Sasha” Prokhorov of the Lebedev Physical Institute in Moscow, for “fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle.” In simpler terms, they received the prize for their invention of the maser and laser.
Throughout his career, Townes has reflected on the similar goals, faith and insights of science and religion. Scientists have faith that there is an unchanging order in the Universe, from the Earth to the larger Cosmos, and that this order is understandable by human beings who seek to discover it. Religious persons have faith that they can understand the purpose and meaning of the Universe and how we fit into it; their discoveries often come about by great revelations. Townes states that discovery in science does not usually come from the “scientific method” of logical deduction from observed facts, and that these discoveries are often accidental, intuitive, and sudden. He believes that science and religion will ultimately converge and join together in a common pursuit of the truth.25
Townes was appointed University Professor at the University of California in 1967, with a location at the Berkeley campus. In this position, he and his graduate students discovered ammonia and water in interstellar space.26 This was soon followed by the detection of the embalming fluid formaldehyde, and carbon monoxide by other groups.27
These discoveries triggered an avalanche of molecular searches in which groups of young radio astronomers armed with the latest laboratory measurements engaged in an extraordinarily competitive fight to be the first to detect the next interstellar molecule. The net result has been the discovery of a pharmaceutical array of hundreds of interstellar molecules, including complex organic molecules such as ethyl alcohol, or ethanol, the substance that gives beer, wine and liquor their intoxicating power.
The molecules reside within dense, dark and dusty, interstellar clouds that typically span up to 120 light-years and harbor a total mass of up to a million times the mass of the Sun, mainly in the form of molecular hydrogen. These hydrogen molecules do not emit radio spectral lines, but their presence can be inferred by their collisional excitation of carbon monoxide.
It is the tiny, solid dust particles in the giant molecular clouds that help block out the harsh radiation in space, and enable chemical reactions to form complex, delicate molecules from the atomic constituents of the interstellar gas. These clouds are black and exceedingly cold, with temperatures of only about 10 kelvin, radiating almost exclusively in the microwave region of the electromagnetic spectrum.
Under the right circumstances, a giant, massive molecular cloud collapses under its own weight, eventually forming up to a million stars. In fact, some stars are now forming in these clouds. They are the present-day incubators of newborn stars.
In the 1980s, it was additionally discovered that atoms and molecules are not the only substance out there in the black cosmic places. The distant parts of our Galaxy are rotating so fast that they ought to fly apart unless the gravitational force of some other, unseen material is holding it all together. The existence of this dark matter was also anticipated by observations of motions within clusters of galaxies.
Dark Matter
The Universe is permeated with an unusual substance that is not of the ordinary kind we can see with our eyes. It is known as dark matter, which sounds like the title of a mystery thriller or an old-fashioned film noir. Astronomers use the name for something that emits no visible light or any other kind of radiation, and hence is dark, but it interacts gravitationally like ordinary matter. It is something we cannot directly see and yet know must be there.
The astronomer Agnes Clerke noticed the possible cosmic implications of this dark matter more than a century ago, when the entire Universe was thought to consist only of stars. She wrote of dark stars, whose presence might be inferred from their gravitational effects on the motions of visible stars, and stated that: “Unseen bodies may, for ought we can tell, predominate in mass over the sum total of those that shine. They supply possibly the chief part of the motive power of the Universe.”28
How do we know that dark matter exists when we can’t see it? Astronomers infer its presence by observing the way visible stars, interstellar gas, or galaxies move. The gravitational forces of the dark matter hold swirling stars and gas in at the edges of galaxies, and keeps clusters of galaxies from flying apart. It is something like noticing a powerful wind from the way it shakes a tree.
Back in 1937, the eccentric Fritz Zwicky, a Swiss astronomer working at the California Institute of Technology, showed that the Coma cluster of galaxies ought to be breaking apart unless large amounts of unseen matter were keeping it intact.29 Zwicky estimated the mass of individual galaxies in the cluster from their luminosities and from their observed rotational motions, and he used the random motions of the galaxies in the cluster to estimate the mass of the entire cluster required to hold it together. He found that the sum of the masses of the galaxies was not enough to keep the cluster dynamically stable. That is, there was not enough luminous matter in the visible galaxies to gravitationally hold them together and keep the cluster intact. Their motions had to be balanced by the gravitational pull of some hidden substance that was much more massive than the sum of the masses of the visible galaxies the cluster contains.
A few years earlier Zwicky had even introduced the term dunkle materie, or “dark matter” for the invisible stuff, and concluded: “If this is confirmed, we would arrive at the astonishing conclusion that dark matter is present with a greater density than luminous matter.”30
As Zwicky proposed, the dark matter also acts like a zoom lens, and magnifies remote galaxies too faint to be otherwise seen.31 When the light from a distant galaxy passes through an intervening cluster of galaxies, the light rays are bent, diverted, focused and magnified by its unseen dark matter. Observations of this gravitational lensing have been used to detect the presence of dark matter in a cluster of galaxies, and to determine the concentration and distribution of the unseen matter within the cluster (Fig. 12.5).32,33
Figure 12.5 Cluster of galaxies and gravitational lens A Hubble Space Telescope image of a rich cluster of galaxies, designated Abell 2218, whose invisible dark matter deflects light rays and magnifies, brightens and distorts images of objects that lie far beyond the cluster. (Courtesy of NASA.)
Dark matter is not solely confined to clusters of galaxies. Substantial imperceptible matter envelops individual galaxies as well. This totally unexpected result was discovered by measuring the rotational motions of gas and stars in the outermost reaches of nearby spiral galaxies. Such movements are inferred from the Doppler shifts of spectral lines of bright stars and emission nebulae, seen at optical wavelengths, or interstellar hydrogen gas detected at radio wavelengths near 21 centimeters.
Astronomers naturally thought that the bright center of a spiral galaxy was the most massive region, which gravitationally controlled the motions of the stars outside it. In this case, the stars and interstellar material would revolve around the massive center at speeds that decrease with their distance from it, just as the more distant planets move with slower speeds around the central massive Sun. But the outer peripheries of some nearby spiral galaxies, which can be scrutinized in detail, are moving too fast to be constrained by anything that can be directly seen. They must be held in by the gravitational attraction of large amounts of invisible dark matter.
Around 1970, Vera C. Rubin and W. Kent Ford, at the Carnegie Institution of Washington, D.C., showed that at least one galaxy, the nearby Andromeda Nebula, or M 31, was not rotating as expected.34 When they measured the rotational velocities from the visible-light spectra of emission-line regions, they remained rotating at high speeds at large distances from the center of the galaxy.
Radio astronomers then showed that remote clouds of hydrogen atoms are spinning about the centers of other spiral galaxies unexpectedly fast. The orbital velocity of the gas remains constant with increasing distance from the center of spiral galaxies; well beyond the visible stars (Fig. 12.6).35 The radio astronomical results were not fully appreciated until Rubin, Ford and their colleagues turned their full attention to the spinning movements of a host of spiral galaxies in the following decade.36 Like Andromeda, their outermost regions circle the centers of these galaxies as quickly as the inner parts closer to the centers. Substantial amounts of unseen, non-luminous matter are required to keep the outer parts from moving out of their galaxy’s control.
This indicated that most of the material within galaxies is not concentrated near their center, where the visible-light luminosity is greatest. Their outer parts are filled with an invisible, non-luminous substance, a dark matter, which is far more massive than anything we can see. It also meant that the spiral galaxies don’t end where their light does, and that they extend further than the eye can see even with the aid of any telescope.
Well outside the boundary of its visible light, there are appreciable amounts of unseen matter that keeps the fast-spinning visible material connected to a galaxy and holds it together. Astronomers estimate that each nearby spiral galaxy is immersed within, and enveloped by, a halo of dark matter at least 10 times more massive than its visible component, extending as far as a million light-years.
There is more than meets the eye in our Galaxy as well. The mass of its visible inner regions, inferred from the orbital motion of the Sun about the galactic center, is about 100 billion Suns. But the rapid motions of dwarf satellite galaxies, which revolve about our Milky Way at distances of up to a million light-years, indicate that a great reservoir of unseen matter also envelops our Galaxy. In order to hold onto and retain these dwarf companions, the invisible parts of our Galaxy must outweigh the visible ones by a factor of about 10. Most of this mass does not lie within the plane of the Milky Way, but beyond it in a dark halo, which extends out to a million light-years from the galactic center.
The visible stars of nearby spiral galaxies must be sandwiched within a similar massive darkness to retain their shapes.37 About 90 percent of the mass of galaxies has to reside in this much larger unseen halo giving off neither visible light nor any other radiation to let us know it is there or what it might be composed of.
Figure 12.6 Dark halo envelops a spiral galaxy The rotation velocity of the spinning spiral galaxy NGC 3198 plotted as a function of distance from its center (bottom). The observed emission from the 21-cm line of hydrogen atoms indicates that a halo of dark matter contributes most of the mass at distant regions from the center (top). The rotation velocity is given in units of kilometers per second, or km s–1. [Adapted from T. S. Van Albada and colleagues, “Distribution of Dark Matter in the Spiral Galaxy NGC 3198,” Astrophysical Journal 295, 305 (1985)].
These discoveries were a consequence of the pioneering investigations of Fritz Zwicky and Vera Rubin, who both preferred to find out things by themselves.
Working Alone — Fritz Zwicky and Vera Rubin
After graduate study in mathematics and physics at the Swiss Federal Institute of Technology in Zurich, Fritz Zwicky travelled to the California Institute of Technology, abbreviated Caltech, where he began investigating cosmic rays and eventually became Professor of Astronomy.
Fritz preferred to work alone, and once planned to write an autobiography entitled Operation Lone Wolf. Eccentric, aggressive and independent, he liked to be right, and to show others were wrong. [Also see Exploding Stars in Chapter 6] Zwicky thought his distinguished colleagues stole his ideas and hid their own mistakes. He also rarely gave a grade in his courses better than an average C, since in his opinion the students never did very well.
Fritz was indeed quite a clever and accomplished fellow. He pioneered the study of extragalactic exploding stars, the supernovae, foretold the existence of city-sized neutron stars, inferred the presence of dark matter in clusters of galaxies, predicted the existence of cosmic gravitational lenses, and contributed to research on cosmic rays, jet propulsion, and the first manmade satellites.
Vera Rubin also liked to work mainly alone, but she preferred to avoid controversy and was never as outspoken as Zwicky. As a child Vera liked to watch the stars move through the sky every night, and by high school she knew she wanted to become an astronomer. Vera Cooper had graduated from Vassar College and acquired a husband, Robert Rubin, before she was 20. She joined him in graduate studies at Cornell University, where her 1951 masters degree speculated that galaxies might move around on their own with an extra sideways motion apart from their outward expansion. Such peculiar, streaming motions were subsequently observed.
When Vera’s husband started a job at the Johns Hopkins Applied Physics Laboratory, Vera entered a doctoral program in astronomy at the nearby Georgetown University, where George Gamow served as her advisor. Her doctoral thesis, completed in 1954, involved the clumped distribution of galaxies. By that time she had two children, and another two by 1960. Her astronomical career slowed down as a result of family life, and did not accelerate until she got a job in 1965 at the Department of Terrestrial Magnetism, abbreviated DTM, at the Carnegie Institution of Washington. It was known for the freedom it gave its researchers without overt publication pressures.
At the DTM, Vera Rubin joined W. Kent Ford who had built an image tube device that enabled them to record spectra of galaxies in two to three hours rather than the thirty or more hours that it then took with other methods. In the early 1970s Vera began using these spectra to measure the rotation velocities of spiral galaxies at various distances from their centers, partly because of plain old curiosity about how that motion might be related to the galaxy’s structure, and also because no one else was doing it. This led to the discovery that spiral nebulae, including our own Galaxy, are embedded within extended halos of dark matter.
Vera disliked mediocrity, and insisted on working on problems outside the main stream of astronomy. As she expressed it: “I liked doing things that other people were not doing, [tended] to work pretty much alone,” and “did not interact with theorists at all.”38 She saw no conflict between her Jewish religion and science, and viewed them as separate aspects of her life.
Zwicky’s critical attitude extended to religion, for he thought God was not needed to account for the already miraculous wonders of Nature.
The discoveries of extensive amounts of invisible dark matter in the Universe were subsequently confirmed and extended by spacecraft observations of the cosmic background radiation.
Elusive Dark Matter and Enigmatic Dark Energy
Two kinds of dark matter have been found to exist. The first kind is made of the familiar baryons that make up ordinary terrestrial material, as well as stars and galaxies. The name of these baryons is derived from the Greek barys for “heavy,” and the protons and neutrons that provide most of the weight of atoms are baryonic. For lack of a better name, the other kind of dark matter has been named non-baryonic matter. This substance had to be around to give the first galaxies shape and form.
As the expanding Universe cooled, some regions of matter must have gathered together by their mutual gravitation to become the first stars and galaxies. However, the expansion of the Universe would pull any primeval baryons apart as soon as they started to coalesce. The extra gravitational attraction of dark matter is required to clump primordial fluctuations of the early Universe into the clusters of luminous galaxies that are now found in the Universe.
The first few minutes of the Big Bang provide clues to the amount of baryons in visible and invisible form, both then and now. The hydrogen, deuterium, and most of the helium that are now observed in the Universe originated back at these early times. When the observed amounts of these light elements are compared with the calculated quantities produced in the Big Bang, it is found that there has to be roughly 10 times as many invisible baryons in the Universe as visible ones that make up the stars. So the bulk of the baryons now residing in the Cosmos are not luminous, at least in visible light.39
Even these unseen baryons cannot fully account for dark matter. Detailed analysis of the irregularities in the cosmic background radiation, by instruments aboard WMAP and Planck, indicate that 82% of the total mass of the Universe does not consist of ordinary atoms or their sub-atomic consitutents.40,41 In other words, most of the dark matter is unlike any matter we know about, and the amount of this unknown substance far transcends ordinary baryonic matter in visible or invisible form. So perhaps it might also be called dark gravity, since its gravitational attraction is the only thing we know for certain about it.
This unseen substance is not the only unsolved mystery. There is the dark energy inferred from the runaway expansion of the Universe. The Planck allsky survey indicates the Universe contains about 5% ordinary matter, 26% dark matter, and 69% dark energy. Astronomers are now actively seeking to know more about dark matter and dark energy.