Dispatches from Planet 3

by Marcia Bartusiak

  Cutaway of a massive star, building up heavier

  elements in its interior as it ages.

  (Illustration courtesy of Barbara Schoeberl, Animated Earth, LLC)

  If a star is massive enough, the equivalent of eight or more Suns, its core will continue fusing elements until iron is formed, which is the end of the line. The fusion of elements heavier than iron requires more energy than it releases. That’s when either a neutron star or black hole is born (as discussed in Chapter 7).

  Burbidge first became aware of the stars in 1923 at the age of four. The young Eleanor Margaret Peachey beheld them with vivid clarity during a nighttime crossing of the English Channel. Upon starting her studies at University College, London, in 1936, she discovered to her delight that astronomy could be a career as well as a hobby. Wartime blackouts, which kept the urban sky dark, allowed her to carry out research on the outskirts of London, using a telescope so antiquated that it was moved by a hanging weight.

  After the war, Peachey got married. She was eager to seek out the best telescopes and the best skies for observing, so she and her physics-trained husband, her closest collaborator over the years, moved to the United States. They eventually settled in California, where she began her lifelong commitment to opening up opportunities for women in science. At a time when women weren’t allowed to use the telescopes (or even stay overnight in the observatory’s dormitory), she fought for—and won—access to those atop Mount Wilson near Pasadena. “Thanks to her influence,” Caltech astronomer Anneila Sargent has said, “women can observe at any American observatory.” Burbidge puts it simply: “If you meet with a blockage, find a way around it.”

  Verifying that we are composed of stardust was the first in a long list of achievements. Burbidge made pioneering measurements of the masses of galaxies and specialized in quasars. These celestial entities are believed to be luminous objects at the centers of galaxies, where a spinning, supermassive black hole generates tremendous radiation. For many years she held the record for finding the most distant quasar, a feat listed in the Guinness Book of Records.

  Burbidge returned briefly to Great Britain in 1972 to serve as the first female director of the famed Royal Greenwich Observatory. But, happier at a telescope than a desk, she soon returned to the University of California at San Diego, where she studied quasars that emit large amounts of X-rays.

  She has not always embraced the majority opinion on celestial matters. Both Burbidges suspect that the true nature of quasars has not been fully revealed and that quasars are closer to us than most astronomers assume. In a 1994 memoir, she says that she is “continually surprised by the almost religious fervor with which most astronomers demand a single ‘Big Bang’ act of creation for the Universe.” Burbidge is more attracted to the notion, introduced by Hoyle, that matter was created in successive epochs, not just by a single event. Her unconventional views have often spurred the astronomical community to new lines of research.

  Burbidge has devoted more than eighty years to keeping watch on the universe. Unlike today’s astronomers, most of whom sit in control rooms watching data displayed on monitors, she has had the pleasure of sitting directly at a telescope. “To ride with the telescope,” she once recalled, “was an experience I wish I could share with today’s generation. . . . One could look out at the spectacular vision of the heavens.” She has held a front-row seat on a golden age of astronomy.

  CHAPTER SIXTEEN

  Dark Matters

  Searching for the universe’s main ingredient

  NEARLY half a mile beneath the surface of the Earth, within a cavern of an old iron-ore mine in northeastern Minnesota, special detectors cooled almost to absolute zero (–459.67 degrees Fahrenheit) are on the lookout. They serve the Cryogenic Dark Matter Search (CDMS), one of several projects around the world attempting to find a novel type of matter that has been long hypothesized but never seen. New particle physics theories, beyond the so-called standard model, suggest that all around us could be ghostly particles that blithely whiz through us with nary a nudge. The hope is that deep underground, far from disruptive cosmic rays, one of these exotic particles will occasionally bump into a detector and release an indisputable signal.

  If and when that happens, astronomers will be jumping for joy. Along with opening up new physics, the discovery of such weakly interacting massive particles (or WIMPs) might solve a cosmic mystery that has endured for more than eighty years. Those particles—distinct from those in the standard model, including the recently headlined Higgs boson—could be the long-sought “dark matter” thought to permeate the universe.

  The first person to wonder about this unseen cosmic ingredient was an irascible physicist named Fritz Zwicky. A Bulgarian-born Swiss national, Zwicky arrived at Caltech in 1925 to study the properties of liquids and crystals. But that was just for starters. An aggressive and stubbornly opinionated man, he regularly annoyed his physics and astronomy colleagues by studying anything he pleased. Along the way he championed some pretty wild ideas, some of which proved their worth decades later. In 1933, as noted in an earlier chapter, he was the first to propose that a supernova—the total destruction of a star—left behind an extremely small and dense object that he called a “neutron star.” The first such object wasn’t detected until 1967.

  Given his eclectic scientific style, it’s not surprising that Zwicky also spied one of the first signs that the universe’s ledger books were not quite balancing. He had decided to examine all the velocity information then available in the literature on the galaxies situated within the famous Coma cluster, a rich group of hundreds of galaxies some 330 million light-years distant. His statistical analysis revealed that the galaxies were moving around in the cluster at a fairly rapid clip. But adding up all the visible light being emitted by these galaxies, he realized that there was not enough luminous matter to bind the speeding objects to one another through the force of gravitation.

  “It is difficult to understand why under these circumstances there are any great clusters of nebulae remaining in existence at all,” he eventually concluded. The situation seemed paradoxical. With the Coma galaxies buzzing around so nimbly, the cluster should have broken apart long ago, but it was still very much intact. Zwicky reasoned that some kind of unseen matter must pervade the Coma cluster to provide additional gravitational glue. In his report to the Swiss journal Helvetica Physica Acta in 1933, Zwicky referred to this invisible substance as dunkle Materie, or dark matter.

  Dark matter was traced in these six different galaxy clusters and depicted as mistlike clouds that surround each cluster.

  (NASA, ESA, D. Harvey [École Polytechnique Fédérale de Lausanne, Switzerland], R. Massey [Durham University, United Kingdom], the Hubble SM4 ERO Team, ST-ECF, ESO, D. Coe [STScI], J. Merten [Heidelberg/Bologna], HST Frontier Fields, Harald Ebeling [University of Hawaii at Manoa], Jean-Paul Kneib [LAM], and Johan Richard [Caltech])

  Zwicky’s suggestion was largely ignored for several decades. Astronomers at the time figured the dilemma would disappear once they could analyze the motions of galaxies in more detail. They presumed that “weighing” a cluster of galaxies would prove more complicated than Zwicky had supposed.

  The issue wasn’t revived until the 1970s, largely owing to Vera Rubin, an astronomer with the Carnegie Institution of Washington. Early in her career she had dabbled in quasar research, the study of the universe’s most energetic galaxies. Having been recently discovered, quasars were then the hottest topic in astronomy, but Rubin came to dislike the field’s cutthroat pace and so decided to seek a less stressful topic. She eventually turned her attention to a problem far less controversial, even boring: the rotation of spiral galaxies. In doing this, she teamed up with W. Kent Ford, who had recently perfected a new electronic instrument that made it easier to record the spectrum of a galaxy, the data needed to measure the rotation.

  At this point, astronomers just assumed that a galaxy rotated much like our solar system, following the laws of gra
vity set down by Isaac Newton. The stars closest to the galaxy’s massive center would travel faster than those farther out in the disk, where the gravitational influence is diminished—just as the inner planets in our solar system practically race around the Sun, while the outer planets move at a far slower pace. But in spiral galaxy after spiral galaxy, Rubin, Ford, and a team of Carnegie postdocs found a far different pattern. To their surprise, they revealed that the stars and gas at a disk’s edge traveled just as fast as matter closer to the galaxy’s center.

  If the planets in our solar system acted like this, Jupiter, Uranus, and Neptune would have careered off into interstellar space long ago. Rubin recognized that a huge reservoir of extra matter, imperceptible to her instruments, had to be tucked away somewhere to keep the stars from flying out of the galaxy. It was the Coma cluster problem all over again, but this time within an individual galaxy. Modeling this effect, theorists figured that each spiraling disk must be embedded in a large sphere of invisible matter to keep the luminous galaxy intact. They also knew that it couldn’t be just ordinary matter that wasn’t glowing, as the Big Bang didn’t make enough regular particles to account for the dark matter required.

  Fritz Zwicky coined the term

  dunkle Materie (dark matter).

  (Photograph by Fred Stein, courtesy of

  the American Institute of Physics Emilio

  Segrè Visual Archives.)

  Some radio astronomers had measured a few of these fast galactic spins earlier, but by 1978 Rubin and her team had measured more than two hundred. This arsenal of data at last took the dark-matter problem off the back burner and turned it into one of the most active concerns in astronomy—an effort that continues to this day. While some astronomers initially questioned Rubin’s findings, recent and more varied measurements have removed nearly all doubt.

  Vera Rubin measuring her spectra in the 1970s.

  (AIP Emilio Segrè Visual Archives, Rubin Collection)

  Some of the best evidence to date is based on an effect known as “gravitational lensing.” Astronomers, for example, have aimed the Hubble Space Telescope at massive galaxy clusters to map their dark matter. While astronomers can’t directly see the dark matter, they can view its gravitational effects, especially in the way it bends light arriving from the distant galaxies behind it, much like a lens. What results is an arty view of the cluster, filled with myriad arcs, bands, and rings of light (see chapter 17). The amount of light bending, using Einstein’s rules of general relativity, provides the means to weigh the dark matter in the cluster and map its distribution.

  On top of that, the exquisite measurements now made of the cosmic microwave background, the remnant radiation left over from the Big Bang, tell us that there is five times as much dark matter in the universe as there is of the ordinary elements that make up the stars, nebulae, and us. We’re merely the icing on the cosmic cake. What this invisible stuff is remains one of astronomy’s greatest mysteries, and yet the answer to dark matter’s composition may not come from out there—the farthest recesses of space-time—but possibly from instruments that stand watch deep down in the Earth.

  CHAPTER SEVENTEEN

  Cosmic Funhouse

  An amusing relativistic effect turns into

  an important astronomical tool

  THERE’s always something delightful that catches my eye after the Sun has set: on one evening the artistic swoosh of a crescent moon, on another the striking pattern of stars that forms the Orion constellation, whose appearance in the Northern Hemisphere heralds the coming of winter. So, when looking up at the nighttime sky I often smile.

  And the cosmos, I have learned, is smiling back . . . literally.

  While searching through images collected by the Sloan Digital Sky Survey, astronomers from Great Britain, Russia, and Spain announced in 2009 that they had come across a familiar face in the direction of the constellation Ursa Major—that of the disappearing Cheshire Cat in Lewis Carroll’s Alice’s Adventures in Wonderland. The two “eyes” of the cat are giant elliptical galaxies, each the brightest member of a small group of galaxies. Both groups are situated some 4.6 billion light years away. More recently, NASA’s Chandra X-ray Observatory discovered that these two sparse clusters are, in fact, racing toward one another at around 300,000 miles (480,000 kilometers) per hour and will eventually merge about one billion years from now.

  This group of galaxies has been nicknamed the “Cheshire Cat” because of its resemblance to the smiling feline in Alice’s Adventures in Wonderland. The two “eyes” are elliptical galaxies, while the “grin” and “face” are formed by galaxies farther out, whose images are stretched out by gravitational lensing. This picture is a composite, blending an optical image with an X-ray image.

  (X-ray: NASA/CXC/UA/J. Irwin et al.; Optical: NASA/STScI)

  But what’s most captivating about this celestial formation is its “grin,” a lustrous smirk generated by the two elliptical galaxies and their surrounding matter. As the light waves from background galaxies farther out come upon the gravitational influence of all this matter in their journey through space, the distant light gets bent and stretched into long arcs. With a powerful enough telescope, you can see such smiles all over the celestial sky. The Cheshire Cat is only one of many examples of this funhouse effect that is fully explained by Einstein’s general theory of relativity.

  With his new gravitational theory, introduced in 1915, Einstein posited that space and time join up to form a palpable object, a sort of boundless rubber sheet (although in four dimensions). Masses, such as a star or planet, indent this flexible mat, curving space-time. With that image in mind, he predicted that a beam of starlight would noticeably shift as it passed by a massive celestial body, following the curved pathway. It was a prediction that thrust Einstein into the public eye: when astronomers, who were monitoring a 1919 solar eclipse, saw starlight graze the darkened Sun and get deflected by exactly the calculated amount, Einstein became world-famous overnight. “Lights All Askew in the Heavens,” blared the headline in the New York Times, “but Nobody Need Worry.”

  In this situation, the Sun had become the gravitational equivalent of an optical lens. Instead of glass deflecting the light rays, gravity was doing the job. It wasn’t long before others wondered whether such “gravitational lensing” might be sighted farther out. In 1920 the British astronomer Arthur Eddington considered the possibility of seeing multiple images of a star, if that star were properly situated behind another stellar body. Although the physical principle is not the same, you might think of the starlight as a stream of water that comes upon a rock and gets diverted into several streams on either side of the stone. Thus our eyes detect multiple images of the star, rather than just one. But in the end, Eddington figured that the effect would be so weak “as to make it impossible to detect it.”

  Four years later, the Russian physicist Orest Chwolson noted that if the distant star were aligned just right—precisely behind a star that acts as a gravitational lens—its light would spread out to form a ring that completely surrounds the lens. Einstein was already aware of these possibilities. As early as the spring of 1912, three years before he published his general theory of relativity, he carried out some calculations of gravitational lensing in his notebook and jotted down the possibility that a lens might not only create a double image of a star, but might also magnify the intensity of the star’s light. However, he then dropped the subject.

  Einstein didn’t return to the problem until 1936, and then only after he was prodded by a young Czech electrical engineer and amateur scientist, who asked him to once again consider cosmic lensing. “Some time ago, [Rudi] W. Mandl paid me a visit and asked me to publish the results of a little calculation, which I had made at his request,” wrote Einstein in his paper for the journal Science titled “Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field.” He went on to say it was “a most curious effect” but also concluded (like Eddington) that there was “no h
ope of observing this phenomenon directly,” since it defied “the resolving power of our instruments.” Privately, Einstein wrote the editor of Science that his findings had “little value, but it makes the poor guy [Mandl] happy.”

  But at the California Institute of Technology, Fritz Zwicky, both physicist and astronomer, thought otherwise. The following year in Physical Review he pointed out that “extragalactic nebulae [galaxies] offer a much better chance than stars for the observation of gravitational lens effects.” Acting like a giant magnifying glass, the galactic lens would enable astronomers to “see [other] nebulae at distances greater than those ordinarily reached by even the greatest telescopes,” wrote Zwicky. It was a prescient vision, but one that was not confirmed for another forty-two years.

  In 1979, British astronomer Dennis Walsh was closely examining a photographic plate to locate the visible counterpart of a newly discovered radio source, 0957+561, when he noticed that the radio object’s position coincided with two star-like bodies, not just one. Additional telescopic observations from the Kitt Peak National Observatory in Arizona confirmed that the cozy pair were quasars. The spectra of these quasars were nearly identical, which hinted that they were not simply the chance alignment of two separate objects (which often happens). A celestial object’s spectrum is as distinctive and exclusive as a fingerprint or personal sample of DNA. The spectral matchup strongly suggested that Walsh was seeing the same quasar—the brilliant core of a young galaxy some nine billion light years distant—but in duplicate.