“Before their eyes in sudden view appear
The secrets of the hoary Deep — a dark
Illimitable ocean, without bound,
Without dimension; where length, breadth, and heighth,
And time, and place, are lost.”
John Milton (1667)1
Spiral Nebulae, a Tale of Larger Telescopes
In the 17th and 18th centuries, telescopes revealed two types of cosmic objects that could not be distinguished with the unaided eye. They were the bright star clusters and the pale, cloudy nebulae. Charles Messier listed the 103 most prominent in his catalogue of 1781,2 and today they remain designated by the letter “M” followed by the number in his catalogue.
When telescopes were constructed with larger mirrors than ever before, they were naturally used to scrutinize the nebulae in Messier’s list in detail, and to see if they could be resolved into component stars or remained of misty disposition under close scrutiny. As an example, the English astronomer, William Herschel, used his most productive telescope, with a metal mirror of 18.7-inches (0.41 meters) across, to show in 1784 that many, but not all, of Messier’s round nebulae could be resolved into globular star clusters. His mirror was about five times bigger than that of his contemporary Messier.
Sixty years later, William Parsons, third Earl of Rosse, constructed an even bigger telescope at the family’s Birr Castle in Ireland (Fig. 6.1). The metallic mirror of the Leviathan of Parsonstown, as it was known, had a diameter of 6 feet, or 1.8 meters, and four times the diameter and sixteen times the light collecting area of Herschel’s instrument.
Figure 6.1 Leviathan of Parsonstown William Parsons, the third Earl of Rosse, surveyed the Heavens with this telescope from the grounds of Birr Castle, his family’s ancestral estate in Northern Ireland. The four-ton, 1.8-meter (72-inch) diameter speculum mirror, made of an alloy of copper and tin, was the largest in the world from 1845 to 1977, when it was surpassed by the 2.5-meter (100-inch) Hooker telescope at the Mount Wilson Observatory near Pasadena, California. (Courtesy of the Trustees of the Science Museum, London.)
With the added magnification of his gargantuan reflector, Rosse discovered, in the spring of 1845, an entirely new kind of cosmic object, the spiral nebulae. He showed, for example, that the fifty-first nebula in Messier’s catalogue, denoted M 51, has the spiral shape of an immense whirlpool, which Rosse attributed to its rotation. Since photography had not yet been introduced to astronomy, Rosse used fine drawings to display the structure of M 51 (Fig. 6.2), and fourteen other spirals.3
Figure 6.2 Spiral nebula The curved shape of the Whirlpool Galaxy, M 51, is illustrated in this drawing made by Lord Rosse when using his 1.8-meter (72-inch) diameter telescope in the spring of 1845; he subsequently found at least a dozen other nebulas with a spiral shape. [Reproduced from The Earl of Rosse, “Observations of the Nebulae,” Philosophical Transactions of the Royal Society, pages 110–124, plate 35 (1850).]
In the closing years of the 19th century, long exposures of photographic plates transformed the way the faint spiral nebulae were discovered and studied (Figs. 6.3, 6.4, 6.5). By 1900, the American astronomer James E. Keeler had conservatively estimated that 120,000 spiral nebulae were within the reach of his photographic telescope at the Lick Observatory in California, even though it was half the size of Lord Rosse’s mirror.4 At about the same time, other astronomers had begun to speculate that the Milky Way also has a spiral structure.5
Most of the thousands of newly discovered spiral nebulae were very faint and narrow in angular extent, which suggested that they are far away. But no one knew for certain whether the spirals are enormously large and exceedingly distant collections of innumerable stars, or small, nearby objects, little spinning wheels of gas that swirl in the interstellar spaces of the Milky Way. Their composition and structure was a matter of speculation until direct measurements of their exceptionally high speeds and enormous distances were obtained. These findings indicated that the spiral nebulae are huge stellar systems separated by vast stretches of empty space and that they are all participating in the expansion of the Universe.
Figure 6.3 Edge-on spiral The galaxy NGC 4565 is portrayed in this image taken from the 2.1-meter (82.7-inch) diameter telescope at the Kitt Peak National Observatory. (Courtesy of Bruce Hugo and Leslie Gaul/Adam Block/NOAO/AURA/NSF.)
A Quiet, Modest Man and a Vain, Ambitious One
Two Americans, V. M. Slipher and Edwin P. Hubble, jointly contributed to the discovery of the expanding Universe, although neither one of them ever claimed credit for it. One of them, Slipher, was a modest and private man working at an observatory that was mainly dedicated to searching for signs of life on Mars. The other astronomer, the pipe smoking, fly-fishing Hubble, was vain and egocentric, with access to the most powerful telescopes of his time. As far as the general public was concerned, Slipher remained largely unknown throughout his life, and became one of the unsung heroes of American astronomy. In contrast, Hubble was one of the most renowned astronomers of the 20th century.
Figure 6.4 Face-on spiral The Pinwheel galaxy M 101 has spiral arms that are about 170,000 light-years across. This portrayal is a composite of Hubble Space Telescope images superimposed on ground-based images taken at the 142-inch (3.6-meter) diameter Canada-France-Hawaii Telescope in Hawaii and at the 35-inch (0.9-meter) diameter telescope at the Kitt Peak National Observatory. (Courtesy of NASA/ESA/STSci/NOAO/AURA/NSF, K. Kuntz (JHU), F. Bresolin (U. Hawaii), J. Trauger (JPL), J. Mould (NOAO), Y.-H. Chu (U. Illinois, Urbana), J.-C. Cuillandre (CFHT), and G. Jacoby, B. Bohannan, and M. Hanna (NOAO).
Vesto Melvin Slipher, who was almost always referred to as “V. M.,” was born on a farm in Mulberry, Indiana on November 11, 1875, the second of nine children that grew to maturity. He graduated from high school in Frankfort, Indiana, taught briefly at a nearby country school, and at the age of 21 entered the Indiana University in Bloomington, where he studied mechanics and astronomy.
In 1901 Slipher began a fifty-three year career as an astronomer at the Lowell Observatory in Flagstaff, Arizona. He also acquired extensive property in and around Flagstaff, eventually bought a number of ranches, owned and operated a retail furniture store, and managed many rental properties.6 V. M.’s younger brother, Earl Carl, became a staff astronomer at the same observatory in 1905, and eventually served as Arizona State Representative, Mayor of Flagstaff, and Arizona State Senator.
Figure 6.5 Endless galaxies A remote cluster of galaxies, designated CL 0939 + 4713 for its coordinates on the sky, as it looked about 10 billion years ago when the light we see was emitted and the Universe was two-thirds of its present age. (A Hubble Space Telescope image courtesy NASA, STScI, and Alan Dressler, Carnegie Institution.)
Percival Lowell, a wealthy Bostonian, funded the construction of the Lowell Observatory in order to observe Mars in the clear, cool air of the San Francisco Mountains near Flagstaff. He believed that intelligent Martians had constructed canals on the planet to transport water from its melting polar caps to parched equatorial deserts, and wrote popular books about them. At the time, the spiral nebulae were thought to resemble our Solar System in its early formative stages, and Lowell instructed V. M. to examine the spiral nebulae to find out more about the beginning of our own planetary system.
When he observed the spectral lines of the brightest spiral nebulae in 1914, Slipher unexpectedly discovered that most of them are moving away from us at velocities well in excess of those of any other known cosmic object. Although these results did not become widely known, they did have a noticeable impact on several astronomers who corresponded with V. M. and traveled to Lowell Observatory to find out more about his velocity measurements.
After Slipher finished his observations of bright spiral nebulae, the more flamboyant Edwin Powell Hubble determined their remote distances and received most of the public attention.
Hubble was born on November 20, 1889 in Marshfield, Missouri and grew up there and in Wheaton, Illinois, a suburb of Chicago, with a comfortable life as the son of a successful insurance executive. As a teenager, he earned spending money delivering morning papers. He also had a stern father who gave him “good lickings,” which he later said “did him a lot of good.”7
Edwin was a strong, gifted, and confident high-school athlete, and a smart, charismatic, handsome, polished and self-assured young man. His biographer, Gale E. Christianson, has also portrayed him as ambitious, insensitive and shallow, a social climber and a liar.8
Hubble attended the University of Chicago, receiving his undergraduate degree in mathematics and astronomy in 1910. In the same year, he was elected as a Rhodes scholar despite a B– undergraduate grade average, at the recommendation of the eminent physicist Robert A. Millikan, who Edwin had worked for as a laboratory assistant. [Hubble attended Queens College at Oxford University in England, where he was trained in English law and also studied literature and Spanish.]
In 1913 Edwin joined his family in Louisville, Kentucky, where he became a high school teacher for a year, and at the age of 24 entered graduate school at the University of Chicago to study astronomy at its nearby Yerkes Observatory; his subsequent doctoral thesis in 1917 concerned photographic investigations of faint nebulae.9 Edwin then joined the American Army, and with the rank of Major was sent to France in September 1918 a few months before the end of the First World War (1914–1918). Hubble enjoyed the plain hard living, the simple food, the discipline, and the adventure of army life.10
When the war was over, he returned to the United States, and in 1919, at the age of 30, began a position as staff astronomer at the Mount Wilson Observatory near Pasadena, California. He was one of the first to use the unprecedented light-collecting power of the observatory’s 100-inch (2.5-meter) reflector to explore the realm of the faint spiral nebulae, and pioneered determinations of the distances to the brightest ones.
Hubble met Grace Burke, the daughter of a wealthy Los Angeles banker, when she was staying at the Kapteyn cottage on Mount Wilson with the wife of another astronomer. At the time, Grace was married to the geologist Earl Lieb, also from an established California family — his father later became president of Stanford University. When Grace’s husband died, from inhaling carbon monoxide while exploring a coal mine, Grace and Edwin began a romance that culminated in their marriage in 1924.
By the following year, Hubble had found that the bright spiral nebulae are too distant to be located in the Milky Way, which indicated that the observable Universe is much larger than many astronomers thought it was at the time. Five years later, he showed that the greater the distance of a spiral nebula, the faster it is moving away from us. This meant that the known Universe is getting bigger as time goes on, and also established its great but finite age.
Edwin’s observations made him famous, and he and his wife Grace relished the notable friendships that resulted from his notoriety. They socialized with the likes of Charlie Chaplin, Frank Capra, Paulette Goddard, Harpo Marx, and William Randolph Hearst, and were constant companions of Aldous and Maria Huxley who shared their disdain for the lower class. Hubble appeared on the cover of the February 9, 1948 issue of Time magazine, was photographed with Hollywood stars, and well after his death NASA named its Hubble Space Telescope after him.
Despite his fame, Hubble held whatever beliefs he had very close to his chest. When asked about them, he replied that: “The whole thing is so much bigger than I am, and I can’t understand it.”11 On another occasion, Hubble wondered about the relationship of his observations to an understanding of God, and stated that: “We do not know why we are born into the world, but we can try to find out what sort of a world it is — at least in its physical aspects.”12 To his surprise, Hubble’s name and observations were even mentioned by Pope Pius XII, in a 1951 address entitled “Proofs for the Existence of God in the light of Modern Natural Science.”13
In his later years, Hubble noted that the scientific realm is restricted to just one aspect of the Universe, which deals with probable knowledge of the observed world of Nature. This knowledge comes from observation and experiment, and can be easily communicated and tested. There is another world, he supposed, that science cannot enter, one of eternal, ultimate truth. “Sometimes, through the strangely compelling experience of mystical insight,” Hubble wrote, “a man knows without the shadow of doubt, that he has been in touch with a reality that lies beyond mere phenomena.”14 This ecstasy, this sense of wonder, he declared, is a private revelation that can only be understood by those who experience it.
Hubble’s assistant Allan Sandage related this awe and wonder to a religious sentiment, commentating that: “There has to be some organizing principle. God to me is a mystery but is the explanation for the miracle of existence — why there is something rather than nothing.”15
Not much was said, as far as this author can discover, about Slipher’s beliefs. Like Hubble, he was mainly concerned with observations of the natural world, so lets get on with his unexpected discovery of the high velocities of spiral nebulae.
The Spiral Nebulae are Moving Very Fast
Vesto M. Slipher, known as V. M., made the unanticipated discovery that spiral nebulae are moving at exceptionally high speeds. Many astronomers then thought that the bright centers of spiral nebulae were newborn stars, and that the surrounding spiral arms were nascent planetary systems, which whirled and rotated around the central star just as the Earth revolves about the Sun. So Slipher set out to observe their rotation, but that is not what happened! He discovered that they were nearly all moving away from the Earth and each other at velocities of up to 1,100 kilometers per second, and much faster than any star.
Slipher reported his extraordinary findings for 15 spiral nebulae in August 1914 at the 17th annual meeting of the American Astronomical Society. With characteristic modesty, he stated that: “In the great majority of cases the [spiral] nebula is receding … the striking preponderances of the positive sign [outward velocity] indicates a general fleeing from us or the Milky Way.”16 His astonishing discovery received a standing ovation from the audience.
For the next decade, Slipher worked almost alone in his pioneering measurements of the high-speed motions of spiral nebulae, and it wasn’t easy. Heroic exposure times and precise hand-guiding of his telescope were required for periods of 20, 40 and even 80 hours to obtain the elusive spectra of the faint spirals, and to infer radial velocities from them. Few such observations had therefore even been attempted, and it explains why almost no one else tried the measurements for many years.
Although the brilliant Andromeda nebula, denoted M 31, is moving toward us, most of the brightest spiral nebuae that were within the range of Slipher’s small 24-inch (0.6-meter) refractor are moving away. By 1917 he had discovered that at least 20 of them are in outward motion.17
After six more years he had discovered that 41 out of 45 spiral nebulae are fleeing from us and each other. By this time, he had largely completed observations of the brighter spiral nebulae whose spectra could be measured using his small telescope; so Slipher moved on to other things, including spectroscopic observations of the aurora, comets, planets, the night sky, and stars, as well as the search for a distant Planet X beyond Neptune.
It has been suggested that Slipher’s discoveries did not become well known because he worked in the remote, scientific backwaters of the Lowell Observatory whose primary activity involved searching for life on Mars, rather than the mainstream astronomy that was being carried out at other places such as the Mount Wilson Observatory in California. But that is not the case. V. M. traveled to Mount Wilson, in 1918 and 1921, and many prominent astronomers visited him to discuss his measurements of spiral nebulae between 1922 and 1925, including Knut Lundmark of the Observatorium at Uppsala, Sweden, A. S. (Arthur Stanley) Eddington from the University of Cambridge, Georges Lemaître of the University of Louvain, Belgium, and Edwin Hubble from Mount Wilson. Slipher also mailed his radial velocity measurements to these visitors, and others such as Gustaf Strömberg in Sweden and Harlow Shapley in the United States.
Moreover, Henry Norris Russell, one of the most influential American astronomers of the first half of the 20th century, was exceptionally familiar with Slipher’s discovery. Russell returned frequently to the Lowell Observatory for refreshing summer visits with his family in the 1920s and 1930s. They would take camping trips with V. M. and his wife to places like the Canyon de Chelley, the Grand Canyon, and the Painted Desert. Young astronomers who lived on the Observatory’s Mars Hill dated Russell’s two daughters, Lucy and Margaret, and Margaret, the youngest, married one of them — Frank Edmondson in 1934.
Slipher’s unexpected discovery of the high-speeds of spiral nebulae was therefore well known by contemporary astronomers, but their implications remained controversial. Russell could not believe that almost all of them were moving away from us, and neither could A. S. Eddington, who proposed that there must be a non-moving solution related to the curvature of space-time.18
The spiral nebulae might or might not be fleeing rapidly away from the Milky Way, but at the time of Slipher’s discovery no one yet knew how big they were or how far away.
The Spiral Nebulae are Very Distant
If the spiral nebulae are moving at high velocity, they could not be gravitationally confined within the Milky Way, at least for very long. Already in 1914, for example, Ejnar Hertzsprung of the Astrophysikalisches Observatorium in Potsdam, Germany had written to Slipher to congratulate him on his “beautiful discovery of the great radial velocity of some spiral nebulae. It seems to me,” he wrote, “that with this discovery the great question, if the spirals belong to the system of the Milky Way or not, is answered with great certainty to the end, that they do not.”19 Nevertheless, some astronomers thought that the spiral nebulae ought to be confined within the Milky Way.
The issue was presented during the now-famous Shapley-Curtis debate over “The Scale of the Universe” during a meeting of the National Academy of Sciences held in April 1920,20 but no one could settle the controversy for certain because the distances to the spirals had not been directly measured. Subsequently, Edwin Hubble was able to use the new 100-inch (2.5-meter) reflector on Mount Wilson to detect Cepheid variable stars in bright nearby spiral nebulae, which permitted measurements of their enormous distances.
In late 1923 he decided to look for novae in the Andromeda nebula, M 31, and accidently discovered that a suspected nova varied in brightness like a Cepheid variable star with a long period of about a month. But this important discovery was not immediately accepted. On receiving Hubble’s letter reporting the unexpected finding, in February 1924, Harlow Shapley replied that it was “the most entertaining piece of literature I have seen for a long time.”21 To Shapley, then the Director of the Harvard College Observatory, Hubble’s finding was just a big joke, apparently because Shapley was convinced that the spiral nebulae did not contain stars.
Figure 6.6 Andromeda The nearest spiral galaxy, the Andromeda Nebula, M 31, as photographed using the 100-inch (2.54-meter) diameter reflector of the Mount Wilson Observatory in California. Two smaller galaxies are also shown in this image. (Courtesy of the Mount Wilson Observatory.)
Undaunted, Hubble began a detailed hunt for Cepheids in M 31 (Fig. 6.6), as well as in the great spiral in Triangulum, M 33. These two had the largest angular extents of the known spiral nebulae, and were presumably the closest of the many thousands of spiral nebulae that had by then been photographed. By exploiting the light gathering power of the great 100-inch reflector, Hubble could detect the light variation of individual Cepheid stars in these two close spiral nebulae, which had never been done before.
Night after night, he used the enormous reflector to photograph the two spiral nebulae. By comparing hundreds of photographs, he found Cepheids that periodically brighten and dim like clockwork. Within each spiral, the stars of longer variation period were the brighter ones, which was consistent with the increase of luminosity with period that Harlow Shapley had found for Cepheids in globular star clusters over a range of shorter periods of days.
By the end of 1924, Hubble was able to measure the variation periods of enough Cepheids to infer the distances of M 31 and M 33 using Shapley’s period-luminosity relations for Cepheids in globular star clusters. Once the Cepheid luminosity was established from its variation period, that luminosity could be combined with the observed brightness to determine the distance. Hubble found that both M 31 and M 33 were at a distance of nearly a million, or 1,000,000, light-years. Both spirals had to reside outside the Milky Way!
This was an astounding discovery, and Hubble knew it. So he had these findings published in an article in The New York Times, on November 23, 1924, and then read in absentia by the Princeton astrophysicist Henry Norris Russell on New Year’s Day, 1925, at the thirty-third annual meeting of the American Astronomical Society in Washington, D.C.22 The historic paper, entitled “Cepheids in Spiral Nebulae,” caused an overwhelming sensation.
Hubble had broken through the ancient stellar Heavens and moved the outer boundary of the observable Universe far beyond it. The Cosmos had to be much larger than anyone had previously demonstrated, and it transformed astronomers’ view of the Universe. The Milky Way was relegated to just one of a multitude of spiral nebulae separated from each other by immense regions of apparently empty space.
Although Hubble failed to acknowledge it, the Estonian astronomer Ernst Öpik had inferred a large distance for M 31 three years before Hubble. While at the Armagh Observatory in Northern Ireland, Öpik used the rotation velocity of M 31 to estimate its mass at about 2 billion Suns. By assuming that its mass to luminosity ratio is comparable to that of the stars in the Milky Way, he obtained the luminosity of M 31 and combined that with its observed brightness to determine in 1922 a distance of about 1.5 million light-years.23
Hubble did not even mention or reference Öpik’s previous, related work, but Hubble would soon make an even more dramatic discovery. The outward velocities of spiral nebulae increase with their distances, which suggested that the Universe is blowing itself apart.
Discovery of Cosmic Expansion
By 1929 Hubble had used the unparalleled 100-inch reflector of the Mount Wilson Observatory to measure the distances of 24 spirals, which were all incredibly far away and even more distant than Andromeda. When he compared these distances to their radial velocities, mainly provided by V. M. Slipher, he found that the more distant a spiral nebula is, the faster it is rushing away from us, at least to a velocity of about 1,000 kilometers per second (Fig. 6.7).24 Not only was the observable Universe far bigger than had previously been thought, it was also expanding and carrying the spiral nebulae outward in all directions, with the fastest ones having moved the greatest distance.
A velocity-distance relation had been anticipated in 1922 by the German astronomer Carl Wirtz,25 and in 1925 the Swedish astronomer Knut Lundmark confirmed that the more distant the spiral nebula, the faster it was receding.26 Hubble’s unique contribution was the measurement of accurate distances using Cepheid variable stars, rather than just estimating the distances from the observed brightness of the spiral nebulae.
Figure 6.7 Hubble’s diagram A plot of the radial velocity of nearby extragalactic nebulae, or galaxies, as a function of their distance, published by Edwin Hubble in 1929. Here the velocity is in units of kilometers per second, abbreviated km s–1, and the distance is in units of millions of parsecs, or Mpc, where 1 Mpc is equivalent to 3.26 million light-years. [Adapted from Edwin P. Hubble, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae”, Proceedings of the National Academy of Sciences 15, 168–173 (1929).]
With outlandish arrogance, Hubble failed to acknowledge, in his renowned 1929 paper, any previous observational evidence for a velocity-distance relation. He did not mention the published results of either Wirtz or Lundmark. Incredibly, he did not even acknowledge Slipher’s velocity measurements or cite them with a reference! Hubble also used Harlow Shapley’s methods of inferring cosmic distances from Cepheid variable stars.
Both Slipher’s and Shapley’s results had been well known by the astronomical community for more than a decade, and the failure to mention their work confirmed that Hubble was not an overly modest or generous person. His oversight was eventually corrected with acknowledgment of Slipher’s pioneer work,27,28 but Shapley subsequently noted that Hubble never did acknowledge his priority.29
The previous investigation that Hubble did cite was a theoretical speculation by the Dutch astronomer Willem de Sitter back in 1917. He had found that one consequence of Einstein’s General Theory of Relativity could be an imitation motion in which more distant clocks would run more slowly; hence distant atoms, vibrating like slow clocks, would show a redshift increasing with the square of the distance.30 In his pioneering 1929 publication, Hubble supposed that the velocity-distance relation, when corrected for solar motion, might represent this de Sitter effect, and in a letter to de Sitter, written in 1930, Hubble stated: “The velocity-distance relation among nebulae has been in the air for years — you, I believe, were the first to mention it.”31
In 1929 Hubble was probably unaware of another relativistic derivation of the velocity-distance relation by the Belgian astronomer and diocesan priest, Abbé Georges Lemaître. It was published two years before in an obscure Belgian journal using observational results given to Lemaître when he visited both Slipher and Hubble. It is a little harder to excuse Hubble’s neglect of the California Institute of Technology physicist H. D. Robertson’s 1928 theoretical prediction of the relation using the available data.32 Their offices were within walking distance of each other.
Nevertheless, most astronomers of the time were not inspired by relativity theory and theoreticians would argue for decades about which relativistic model might correspond with reality.33 Moreover, de Sitter’s interpretation without an expansion was simply wrong, and Hubble’s mention of it was the last time he left the observable world to entertain theoretical speculations about it.
Astronomers now explain the velocity-distance relation in terms of an expanding Universe, in which the galaxies are all rushing away from us, dispersing and moving apart and occupying an ever-increasing volume. Yet, Hubble never did interpret his observations this way, and other eminent astronomers also wondered about such an explanation. In 1929, the renowned American astronomer Henry Norris Russell found that: “The notion that all the galaxies were originally close together is philosophically rather unsatisfactory.”34
But Hubble knew he was onto something. With great foresight he initiated a program with Milton L. Humason using the 100-inch reflector to extend the velocity-distance relation to velocities as large as 20,000 kilometers per second and to distances as large as 100 million light-years.35
Milton Humason has a colorful life story. He dropped out of school at age 14, and never received a formal education after the eighth grade. He became a tobacco-chewing gambler and reputed “ladies man,” and began his career at Mount Wilson as a mule driver taking construction material and equipment up the mountain. In 1917, he became janitor at the observatory, and volunteered to be a night observing assistant. In succeeding years he progressed from assistant to observer to a self-trained staff astronomer more skilled at observing than Hubble when it came to obtaining the spectra of faint nebulae.
The unimaginably high velocities indicated that the spiral nebulae, which are now known as spiral galaxies, participate in a uniform flow that gathers speed with distance. Although individual galaxies might dart here and there, even collide if near enough to each other, these localized motions are limited in velocity by gravitational interactions, and they are relatively slow when compared to the recession velocities of remote galaxies. In other words, the galaxies, or extra-galactic nebulae, have to take part in an expanding Universe, for any other plausible explanation only applies to speeds much lower than those of the most distant galaxies. The Milky Way became just one Galaxy among many, with a capital G to show it is ours.
Both Hubble and Humason were nevertheless cautious about interpreting their results and hesitant to attach any significance to them beyond the observations themselves. “The interpretation of the redshifts as velocities of recession is controversial,” wrote Humason in 1931, “for the present we prefer to speak of these velocities as apparent.”36 And in their collaborative paper of the same year, these two astronomers described “the ‘apparent velocity-displacements’ without venturing on the interpretation and its cosmologic significance.”37 Hubble declined to interpret the velocity-distance relation in terms of an expanding Universe throughout his life.
He thought that cosmological models were a forced interpretation of the observational results, and as far as he was concerned theoretical cosmology consisted to a large extent of irrelevant, unverified speculations. Even as late as May 1953, shortly before his death by heart attack, Hubble was reluctant to accept the redshift as a literal expansion and referred to his famous relation as the law of the red-shifts, which should be formulated as an empirical relationship between observed facts.38
[By this time the term redshift, denoted by the lower case letter z, had entered the description; for low redshifts it is the ratio of the velocity, V, of the object to the velocity of light, c. The renowned Hubble’s law states that the velocity V of a galaxy at distance D is given by V = H0 × D, where the symbol H0 is known as the Hubble constant. It is a fundamental measure of the Universe with the same value for any galaxy.]
Hundreds of billions of galaxies have now been located with telescopic eyes, and there isn’t any end in sight. They are as numerous as snowflakes in a storm or grains of sand at the seashore, and each galaxy is composed of billions of stars. And as far as we know, the Universe has no perceptible outer boundary. Moreover, the entire Universe looks practically empty, for immense regions of apparently vacuous space separate most of the galaxies from each other.
The overwhelming immensity of space had been imagined in the 17th century, by the French philosopher Blaise Pascal who wrote: “The whole visible world is only an imperceptible dot in nature’s ample bosom…. Nature is an infinite sphere whose center is everywhere and circumference is nowhere…The eternal silence of these infinite spaces frightens me.”39
Pascal was right about the dark immensity of space, its overwhelming largeness, which has been confirmed by modern astronomical surveillance of the enormously distant horizons of the observable Universe.
Children and adults can still find the darkness scary. That could be why they tell ghost stories around campfires at night. Many astronomers nevertheless revel in the dark quiet of the night and the splendor it brings into view. Their understanding of the fullness of the dark places, and the formation of stars within it, has made the spacious Universe more inclusive and less threatening.
When did the Expansion Begin?
By reversing the expansion of the observable Universe, you can find out when the galaxies started on their outward journey. Assuming a constant speed, that expansion age is obtained by dividing the distance of any remote extragalactic nebula, or galaxy, by its velocity. It is known as the Hubble time, the reciprocal of the Hubble constant.
If the expansion has continued at a steady pace with the rate determined by Hubble’s measurements, the expansion began about 1.8 billion, or 1.8 × 109, years ago. But that is less than the age of the crust of the Earth, accurately dated in 1956 at 4.6 billion years from the radioactive decay of its oldest rocks.40 How could the Earth be older than the expanding Universe?
Well, it isn’t! In 1952, a quarter of a century after Edwin Hubble had set up the extragalactic distance scale, Walter Baade announced that he had corrected the distance scale, owing to two different kinds of pulsating stars, and this meant that a steady expansion began about 3.6 billion years ago.41,42 That still wasn’t long enough ago, but Hubble’s assistant and protégé, Allan Sandage, eventually corrected the identification of the brightest stars in Andromeda, and in 1958 announced that the Hubble time is close to 13 billion years, and significantly older than the Earth.43 [When Harvard’s Department of Astronomy invited Sandage to talk about his measurements of the Hubble constant, which was being estimated in other ways by Harvard graduate students and faculty, Sandage wrote back saying that his mother had taught him not to talk to the village idiot.44]
By the turn of the 20th century, the dust had settled and most astronomers now accept a Hubble time of 13.7 billion years. Wendy Freedman and her colleagues at the Carnegie Observatories in Pasadena, for example, used the Hubble Space Telescope to scrutinize Cepheid variable stars in nearby galaxies and to obtain in 2001 an expansion age of about 15 billion years (Fig. 6.8).45
The Universe is therefore considerably older than the oldest terrestrial rocks, and we can all be reassured that the expanding Universe had a definite beginning. Moreover, astronomers have gazed out to the remote edges of the observable Universe, where cosmic objects can move nearly as fast as light.
Figure 6.8 Recent Hubble diagram The Hubble Space Telescope has been used to obtain these measurements of the radial velocities and distances of Cepheid variable stars in galaxies. The radial velocity is given in units of kilometers per second, denoted as km s–1, and the distance is in units of a million parsecs, or Mpc, where 1 Mpc is equivalent to 3.26 million light-years. These data have been used to measure the expansion rate of the Universe, also known as the Hubble constant and designated H0, obtaining H0 = 75 ± 10 km s–1 Mpc–1. [Adapted from Wendy L. Freedman et al., “Final Results From the Hubble Space Telescope Key Project to Measure the Hubble Constant,” Astrophysical Journal 553, 47–72 (2001).]
Radio Stars, Radio Galaxies, Quasars and Supermassive Black Holes
When astronomers first looked out at the radio sky, they found that the brightest emission coincided with the Milky Way, so it was natural to suppose that they were stars. But because the nearest and visibly brightest stars, other than the Sun, did not exhibit detectable radio emission, and because some discrete radio sources were a hundred million times brighter than the Sun at radio wavelengths, the English radio astronomer Martin Ryle argued for the existence of a new kind of radio star located between the optically visible stars.46
Ryle’s research group at the University of Cambridge constructed arrays of modest-sized radio telescopes and connected them together to simulate a single large radio telescope. This improved the angular resolution of discrete radio sources and the sensitivity needed to detect the weaker ones. They opened up the sky to a host of previously unknown sources that are uniformly distributed in space, and not just within the Milky Way. At a debate over their origin, Ryle stuck to his radio-star proposal, but Thomas Gold, who was then lecturing at Cambridge, noted that the source distribution favored an extragalactic origin far outside our own Milky Way Galaxy.47 With remarkable foresight, he also reasoned that if stars were to radiate intense radio emission they must be collapsed stars, which would have strong magnetic fields magnified by gravitational collapse.
The controversy was settled when improved radio interferometric techniques were used to refine the position of the intense radio source Cygnus A, setting the stage for the crucial optical identification that would transform our knowledge of the radio Universe. Armed with the accurate radio position, two German-born American astronomers, Walter Baade and Rudolph Minkowski, used the giant 5.0-meter (200-inch) optical telescope on Mount Palomar, California to find the visible-light counterpart of Cygnus A in 1954.48 It is an elliptical galaxy of redshift 0.057, which meant that it is receding from us at 5.7 percent of the velocity of light. And its distance, of about 750 million light-years, was inferred from the linear relationship between redshift and distance, known as Hubble’s law.
Once this distance had been established, the enormous absolute radio luminosity of Cygnus A was realized. It wasn’t just emitting a faint crackle and hiss, but instead a colossal, shattering roar, like a lion instead of a household cat. Every second, Cygnus A emits as much power in radio waves as a million million, or 1012, Sun-like stars radiate in visible starlight. It turned out to be a new type of radio galaxy, whose radio luminosity is comparable to its optical one.
The news that Cygnus A is a galaxy, and not a star, embarrassed and humiliated Ryle. When he saw the identifying photographs, at a scientific conference, he threw himself on a nearby couch — buried his face in his hands — and wept.49 But he soon recovered his composure, realizing that the radio galaxies could be used to probe the distant Universe and provide tests of different cosmological models. And Ryle’s group did just that, abandoning the radio star hypothesis and using their comprehensive and definitive Cambridge radio surveys to demonstrate strong evolutionary effects over cosmic times.50 And the world forgave Ryle’s earlier mistake, for he was awarded the Nobel Prize in Physics in 1974, primarily for his development of the aperture synthesis of radio telescopes.
When interferometeric measurements showed that the angular extents of some other intense radio sources were much narrower than radio galaxies, it was for a time believed that the first true radio stars of small physical size had been discovered. Allan Sandage used the 200-inch Mount Palomar telescope to look in the direction of one of them, 3C 48 — the 48th source in the third Cambridge radio-source catalogue, and there was no galaxy to be found. Instead, Sandage discovered a bright blue object, no bigger in angular size than a star, and with a totally confusing line spectrum that was unlike anything ever seen before.
The key to the mystery was provided when the Moon happened to pass in front of another bright radio source, 3C 273, in 1962. The radio astronomer Cyril Hazard, then at the University of Sydney, realized that a careful timing of the disappearance and reappearance of the occulted radio source would establish a precise position, since the location of the Moon’s edge is known accurately for any time.
So Hazard and his colleagues used the occultation method to show that 3C 273 is a double radio source, one component of which apparently coincides with a blue stellar object.51 This coincidence prompted Maarten Schmidt to confirm the identification and obtain an optical spectrum using the 200-inch Mount Palomar telescope, which indicated a completely unexpected and exceptionally high recession velocity of 0.16 percent of the velocity of light.52 When he told his colleague Jesse Greenstein about the discovery, Greenstein produced a list of emission line wavelengths for 3C 48, and within minutes they had found that it is rushing away with an even faster motion at 37 percent of the velocity of light.
When these velocities are used to infer distances using Hubble’s law, it is found that 3C 48 and 3C 273 are located at distances of billions of light-years. And when their observed brightness is combined with these distances, the intrinsic visible-light power is comparable to that of 10 million million, or 1013, Sun-like stars.
Since the bright objects appeared star-like in visible light, they became known as quasi-stellar radio sources, a term that was soon shortened to quasars. The quasars had, in fact, been ignored as stars on optical photographs for years. But a casual inspection of the optical sky would never have led to the discovery of quasars. About 3 million stars look brighter than the brightest quasar, 3C 273.
What accounts for the extraordinary power of the remote radio galaxies and quasars? They are likely energized by a compact, supermassive black hole. Its powerful gravity pulls in surrounding stars and gas, forming a flat, orbiting accretion disk that spirals into the black hole. As proposed by Martin Rees, who was later elevated to England’s House of Lords as Baron Rees of Ludlow, powerful magnetic fields generated by the rotating black hole turn the whirling accretion disk into an enormous dynamo.53 It uses the hole’s rotational energy to accelerate charged particles and squirt them out in diametrically opposite directions along the rotation axis at about the speed of light. They continuously feed the two radio lobes commonly found symmetrically placed from the center of radio galaxies and quasars.
The classic example is M 87, a giant elliptical galaxy whose central spinning disk of hot gas indicates that a super-massive black hole resides at its center. A one-sided jet of gas emerges nearly perpendicular to the disk, and stretches out into one of the two lobes of the radio galaxy Virgo A, numbered 3C 274 in the Cambridge survey (Fig. 6.9). The motions of bright knots in the jet indicate that they are traveling outwards at about half the speed of light. And Very Long Baseline Interferometry observations with widely separated radio telescopes reveal that M 87’s jet emerges from a region at most six light-years across, most likely harboring the super-massive black hole that produces the jet.
To power the youthful activity of a quasar by accretion, there has to be about one solar mass per year of gas flowing into the black hole. So billions of stars or the equivalent amount of gas must be consumed as a quasar or radio galaxy evolves over the course of billions of years. The supply dwindles away over time and the activity dies down, but the black hole does not disappear. Most galaxies probably contain supermassive black holes at their center. The ones in the older, nearby galaxies are the starving remains of former quasars, with a dwindling supply of material that once fed a higher rate of activity. They are found in ordinary nearby galaxies, like Andromeda and the Milky Way, whose cores are old surviving fossils of former quasars.
Astronomers decipher the history of our expanding Universe by watching massive stars explode into oblivion.
Figure 6.9 Radio jet The bright radio source Virgo A, also designated 3C 274, coincides with M 87, a giant elliptical galaxy located in the Virgo cluster of galaxies at a distance of about 50 million light-years. This radio map, made with the Very Large Array, shows two elongated lobes, one on either side of the center of M 87. The most intense radio emission comes from a jet that emerges from the core of the galaxy and stretches some 8,000 light-years into one of the two lobes. The observed high-speed motion of bright knots in the jet implies that its radio-emitting electrons are traveling at nearly the speed of light. Observations of the rates at which stars and gas clouds revolve within the central core of M 87 indicate that it contains a compact massive object, most probably a supermassive black hole of about 3 billion solar masses. (Courtesy of NRAO/AUI/NSF.)
Exploding Stars, the Supernovae
At the end of its bright, shining life, an entire star can explode and spew out its insides like phosphorescent javelins, seeding space with ingredients for the next generation of stars. As an example, the Sun, Earth and the rest of our Solar System formed 4.6 billion years ago from interstellar material that had been enriched by previous generations of massive stars that were born, lived and perished in explosions within the Milky Way. Such exploding stars also occur in other galaxies, where for a few weeks they can outshine all the rest of the galaxy. They have been given the name supernovae, because of their exceptional brightness.
In 1934 Walter Baade, an astronomer at the Mount Wilson Observatory, and the Swiss astronomer Fritz Zwicky, who had moved to the nearby California Institute of Technology in 1925, communicated to the United States National Academy of Sciences a remarkable pair of papers on supernovae.54 In one paper, they demonstrated that the enormous total energy emitted in the supernova process corresponds to the complete annihilation of an appreciable fraction of the star’s mass. The other publication discussed their prediction that a supernova explosion accelerates charged particles to high energies, most likely accounting for the energetic, cosmicray particles that rain down on the Earth from all directions. They were right on both counts.
Fritz was quite a character. Eccentric, aggressive and independent Professor Zwicky had the dangerous habit of telling everyone just what he thought of him or her. He was always out “to show those bastards,” often meaning his colleagues at the California Institute of Technology, or Caltech for short, and once called them “spherical bastards,” because, he said, “they were bastards any way you looked at them.”
Such abrasive and outspoken behavior didn’t endear him to his fellow human beings, but Zwicky didn’t care — he knew he was smarter than most of the Philistines he had to work with. And his quick intelligence and irascible personality made him a formidable and indefatigable interrogator of German scientists after World War II (1939–1945), including Wernher von Braun and others associated with the German V-2 rockets.
You might say that Fritz had some kind of personality disorder, but that’s what universities help protect — the exceptionally bright who don’t always get along with “normal” people. And in Zwicky’s case it paid off. He reasoned that supernovae ought to be exploding all the time in remote galaxies, given their immense number and the many billions of stars they each contain. So Fritz began a patrol of the night sky with a 3.5-inch camera that he mounted on the roof of a building at Caltech, accompanied by the laughter of his faculty colleagues. Much to his dismay, he did not find any supernovae for the first two years, but in 1937 he detected one.55
It took decades before astronomers found out what makes a star explode, and it turned out that there are two ways to detonate the explosions. Both types are symptoms of advancing stellar age, and brilliant one-way trips to complete destruction, but they originate in different kinds of stars and are detonated in separate ways. One type can occur in a close binary system, with a white dwarf, the shrunken dense remnant of a former low-mass star, circling another nearby ordinary star (Fig. 6.10).56 When the nearby companion star expands, as the result of its normal evolution, hydrogen from its outer atmosphere spills onto the white dwarf, compressing and heating the star and adding mass to it. As soon as the white dwarf reaches its maximum possible mass, it can’t take any more and is pushed over the edge of stability into explosion. It has become a type I supernova that shines with the light of billions of Suns, and no star is left behind.
Figure 6.10 Type I supernova The strong gravitational attraction of a white dwarf can pull a nearby main sequence star into an elongated shape. When sufficient amounts of hydrogen in the outer atmosphere of the companion star spills over onto the white dwarf, the entire star can explode into a type I supernova.
Since every one of these nuclear-powered type I explosions come from identical stars at their maximum mass, with explosions triggered under very similar conditions, they are expected to display a uniformity in their maximum light output and the shape of their light curve. This makes them very useful as a “standard candle” for measuring the distances of very remote galaxies, and to thereby determine the pace of cosmic expansion.
In the other kind of supernova explosion, the type II variety, a very old and massive star blows up all by itself. It follows the creation of an iron core within an evolving, massive supergiant star. Since nuclear reactions cannot continue in the iron core, there is nothing left to heat the star and to generate the motions that are needed to support it. The iron core collapses under its own weight into a neutron star or black hole in less than a second, and an explosion blows away all the rest of the in-falling matter. There is little uniformity amongst these gravity-powered type II explosions of single, massive stars without a close companion.
Breaking Free
Walter Baade realized that supernovae might be used to measure the distant parts of the expanding Universe.57 With this in mind he teamed up with Zwicky to search for supernova explosions using the wide-angle field of view and high speed of a Schmidt telescope, which could survey large areas of the sky in a reasonable time. Its inventor, Bernhard Schmidt, had worked at the Hamburg Observatory with Baade, and in 1935 Zwicky traveled to Hamburg to visit Schmidt. Upon his return, Zwicky and Robert Millikan convinced George Ellery Hale to allocate funds for an 18-inch Schmidt, in preparation for the 200-inch reflector that the Rockefeller Foundation had endowed.
The sky patrol with the new Schmidt telescope was a collaborative effort between Zwicky, Baade, and another German astronomer, Rudolph Minkowski, who had left a Professorship at the University of Hamburg in 1935 to begin a twenty-five year career at the Mount Wilson Observatory. Zwicky and Josef Johnson identified supernovae with the Schmidt telescope; Baade used the Mount Wilson reflectors to study the shape of their rising and declining light intensity, known as the light curve; and Minkowski used these telescopes to obtain their line spectra.58
By 1941 about 50 supernovae had been detected in the supernova sky patrol, and that year, Minkowski announced that there are basically two kinds of supernovae, designated as type I and type II, based on whether or not lines of the element hydrogen appear in their spectra.59
The sky patrol was interrupted by the Second World War (1939–1945), which also destroyed the supernova collaboration. Zwicky incorrectly accused Baade, who was an “alien” German citizen, of being a Nazi, and threatened to kill him if he showed up on the Caltech campus. From then on, Baade refused to be left alone in a room with Zwicky.
Not much happened with the supernova search for nearly half a century, when improvements in technology enabled astronomers to use some of the explosions to probe the depths of our Universe. The digital age had arrived and computers could be used with charge-coupled detectors, abbreviated CCDs. The electronic detectors collect almost 100 percent of the incident light, as compared with about 1 percent for photographic emulsions, which can make a small telescope with a CCD as powerful as a large telescope using photographic plates.
It was a time of big astronomy and big science, and the new surveys of the supernovae were big enterprises. It lasted for decades, and involved groups and teams of astronomers from all over the world using telescopes distributed around the globe in Australia, Chile, the United States and even the Hubble Space Telescope. Most importantly, the supernova search had a purpose beyond just watching stars explode. The inquisitive astronomers wanted to use them to find out more about what kind of expanding Universe we live in.
By collecting and recording supernova light from both nearby and distant galaxies, out to large redshifts and half-way across the observable Universe, the astronomers could measure expansion speeds out to a large fraction of the speed of light and see how the rate of cosmic expansion has changed since the light was emitted from distant supernovae. Just about everyone expected that the Universe would slow down as it continued to expand, due to the attractive gravitational force of matter within it. This meant that supernovae at great distances, which emitted their light long ago, should appear to be receding faster than those nearby.
Astronomers therefore attempted to compare the expansion velocities of very distant supernovae with those in nearby galaxies, expecting to determine the mass density of the Universe. Well, they didn’t see the expected, and to everyone’s surprise, observations of supernovae in remote galaxies showed that the Universe is speeding up, expanding at a quickening pace, accelerating outward and running away. The galaxies have severed the bonds of gravity, and will never return.
It took nearly 20 years of work by several teams to arrive at these conclusions, and it was a difficult road to follow.60 In the early 1980s, it was found that a particular subclass of type I supernovae, designated type Ia, exhibit amazingly uniform line spectra and light curves indicating a similar origin and common peak luminosity. These type Ia’s are identified by both the absence of hydrogen spectral features and the presence of a silicon absorption line — type Ib does not display the silicon feature but does show pronounced helium ones.
Then doubts arose over whether or not type Ia supernovae are all the same, for some of them were 10 times more luminous at peak intensity than others, so standardized light curves had to be developed.61,62 Two international teams were organized to observe the distant, high-redshift supernovae reliably and quickly. The first one, dubbed the Supernova Cosmology Project, began in 1988 at the Lawrence Berkeley National Laboratory. Under the direction of Saul Perlmutter, they created software that would allow computers to find supernovae automatically from digital images taken with a small, dedicated telescope. The equipment would automatically subtract images of the night sky taken about a month apart, in times of new Moon, and anything that remained after the subtraction was a new source of light, most likely a supernova.
By the mid 1990s Perlmutter and his colleagues from Europe, Chile and America were finding a lot of supernovae, further examining them in spectral detail using major ground-based telescopes, and, occasionally, the Hubble Space Telescope.
Another group, dubbed the High-z Supernova Search, joined the quest in 1994, employing similar techniques to the Supernova Cosmology Project. The “High-z” denotes large redshifts, z, and therefore supernovae in extremely distant galaxies. This group, led by Brian P. Schmidt of the Mount Stromlo Siding Springs Observatory in Australia and the Australian National University, included Adam G. Riess, then a graduate student in the astronomy department at Harvard University.
The motivation of the Supernova Cosmology Project was to measure the amount of invisible dark matter in the Universe by detecting how much it slows the cosmic expansion. At first the group reported, in 1997, that they had found just what they were looking for, but stressed that the results were not precisely established, and then came to the opposite conclusion with less uncertainty a year later.63
In the meantime, the High-z team was convinced that Perlmutter’s original conclusion was wrong. The breakthrough came in 1998 when Adam Riess, of the High-z team and by then a post-doctoral student at the University of California at Berkeley, declared that analysis of 16 distant supernovae indicated that the cosmic expansion has unexpectedly sped up during the past 5 billion years.64 Saul Perlmutter’s group nearly simultaneously arrived at a similar result using 42 high-redshift supernovae, with publication in the following year.65
The two programs had independently reached the same conclusion using different supernovae and different analytical techniques. By clocking how much the cosmic expansion has changed since the light was emitted from those distant, high-redshift exploding stars, when compared with nearby low-redshift ones, they found that the expansion is speeding up and accelerating as time goes on, rather than slowing down.
In 2011 the Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt and Adam G. Riess “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”
The unexpected runaway expansion suggested that some unknown repulsive force permeates the Universe and counteracts the combined, mutual gravitational attraction of all the matter in the Universe, pushing the galaxies faster and faster apart. It is called dark energy.
Cosmic Inflation
An inflation theory may describe what happened in the first fraction of a second of the expanding Universe. The novel idea was introduced by the MIT astronomer Alan H. Guth in 1981,66,67 and further developed at Stanford University by Andrei D. Linde within a year.68 During inflation the Universe was driven apart by a repulsive gravity, unlike the attracting kind we are used to, and operating on a very small scale in both space and time. Owing to its inherent instability, the burst of inflation soon decayed away and came to an end, in a time far less than one second, releasing its remaining energy into heat and material particles. This accelerated expansion in the first miniscule moments of the Big Bang, this inflation, most likely obliterated all evidence of previous events, in a day without a yesterday.
The cosmic inflation theory was challenged by physicists at Princeton and Harvard Universities for its current lack of definitive, observed predictions,69 which might be compared to stirring up a hornet’s nest. Particle physicists and cosmologists at MIT and the University of California, Berkeley responded to the challenge with specific quantitative predictions that match observations of the cosmic microwave background radiation.70,71
Regardless of the observational verification, the inflation idea has some imaginative consequences. As proposed by Alex Vilenkin at Tufts University,72 and independently by Linde,73 cosmic inflation indicates that multiple invisible Universes may have emerged from nothing, and other potentially unseen Universes could be waiting to arrive. If this is the case, our observable Universe might be but a small component within a vast assemblage of other Universes that we cannot see. They are together known as the Multiverse.
Each Universe could start with its own Big Bang, with different settings to Nature’s fundamental constants and natural laws. Although any individual Universe, including our own, may live and die, the Multiverse is supposed to be forever. The eternal, self-reproducing Universes could just keep on arising from the vacuous nothing, like bubbles in the foam of a river.
All of these imaginary Universes are highly speculative. They may always remain unobservable and unverifiable, eternally unknown and unknowable, so their possible existence might never be tested, even if the cosmic inflation theory is confirmed in our own Universe.