Walsh and his colleagues reported their suspicion that a gravitational lens was at work, and further observations by other astronomers at the Palomar Observatory in California confirmed that conjecture. The lens turned out to be a giant elliptical galaxy, a member of a rich cluster of galaxies located halfway between the quasar and Earth.
It wasn’t long before astronomers uncovered many other cases of gravitational lensing throughout the celestial sky—and not just multiple images of pointlike objects. When entire galaxies are lensed by intervening galaxies or even clusters of galaxies, their broader shapes are often smeared into long arcs and rings. That’s how the Cheshire Cat got its grin.
Gravitational lensing, however, has turned out to be far more than an amusing or pretty optical effect. Today it is one of astronomy’s most valuable tools. The amount a light beam is deflected depends on the total mass of the gravitational lens. So, by carefully measuring the deflections, astronomers can “weigh” entire clusters of galaxies. Their results have confirmed that around 90 percent of the mass in these clusters is indeed composed of an unknown dark matter. Moreover, both the position and intensity of the arcs formed around the cluster of galaxies allow astronomers to map how this matter is distributed through and around the cluster. Such information is offering clues as to the true nature of dark matter.
And just as Zwicky forecast eight decades ago, gravitational lenses are magnifying the images of galaxies residing in the most distant regions of the universe, galaxies that would have been too small or faint to be seen with a telescope alone. All of these applications are helping astronomers trace the growth of galaxies and clusters of galaxies through time, to examine how cosmic structures have evolved and changed over the eons. “The vistas we uncover with this new gravitational telescope,” writes astronomer Evalyn Gates in her book Einstein’s Telescope, “will take us further than ever . . ., providing answers that may unlock the door into a deeper understanding of the fundamental nature of space, time, matter, and energy.”
With improved technology, astronomers have also come to see individual celestial objects act as gravitational lenses, the enterprise that both Eddington and Einstein had deemed hopeless. Background stars in our Milky Way and in the Magellanic Clouds are seen to briefly magnify—microlens—due to dark objects passing in front of them. This is one way that astronomers have revealed the presence of both brown dwarf stars and extrasolar planets, objects too dim to be seen directly. Such an amazing accomplishment brings a smile to astronomers’ faces—or even a broad, mischievous grin.
CHAPTER EIGHTEEN
Rivers of Galaxies
Once thought to be illusory, superclusters of
galaxies are now being well mapped
THE Milky Way has a new address. For more than six decades it’s been known that our galactic home is perched at the edge of a long and vast collection of galaxies called the Virgo Supercluster. But an international team of astronomers announced in recent years that we belong to an even larger assembly in this sector of the universe. Led by R. Brent Tully of the University of Hawaii at Manoa, the team dubbed this gargantuan structure “Laniakea,” which means “immense heaven” in Hawaiian.
This finding proves, once again, that galaxies are very sociable creatures. Even though space-time is continually stretching, moving most galaxies away from one another as the universe expands, gravity keeps adjacent neighbors together, even drawing them closer, forming arrangements across a range of sizes.
The Milky Way, for example, is part of a small collection right here in our galactic neighborhood. Edwin Hubble named it (rather uninspiredly) the “Local Group.” One end is anchored by our home galaxy, surrounded by a bevy of dwarf galaxies; the Andromeda and Triangulum galaxies dominate the other end, with their own small companions. But the Local Group pales in comparison to the richest clusters. The Coma cluster, located some 300 million light-years away in the direction of the Coma Berenices constellation, contains thousands of galaxies hovering together like a dense cosmic flash mob.
The Hubble Space Telescope imaged a large portion (several
million light-years across) of the Coma cluster. The spherical
cluster is more than 20 million light-years in diameter and
contains thousands of elliptical and disklike galaxies.
(NASA/ESA/Hubble Heritage Team [STSci/AURA])
Even before astronomers knew that many of the nebulae they were observing all over the heavens were distant galaxies, they noticed how some of these cosmic clouds crowded together. In the eighteenth century, astronomer William Herschel wrote about Coma’s “remarkable collection.” Having built the largest telescopes in his time, he was able to spot this prominent swarm more than two centuries ago.
But how far did this tendency go? Were there also, astronomers asked, clusters of galaxy clusters? That question took quite a while to answer. In the 1930s, both Harvard astronomer Harlow Shapley and the Swedish astronomer Erik Holmberg spoke of “metagalactic systems” or “metagalactic clouds,” what we today call superclusters. To these observers’ eyes, some of the clusters appeared to form even larger assemblies.
But, around the same time, Hubble photographed selected regions of the sky and concluded the opposite: that clusters were distributed fairly uniformly across the heavens. Hubble at the time was embracing the cosmological principle, the idea that on the very largest scales the universe must be “isotropic”—looking about the same no matter in which direction you looked. To him, galactic groupings stopped at clusters. This view was so strong that few dared to question it, and Hubble’s opinion prevailed for many years . . . until a feisty French astronomer began to alter that widely held belief.
During World War II in France, astronomer Gérard de Vaucouleurs had been an expert observer of Mars, but by the early 1950s he had traveled to Australia to work at the Mount Stromlo Observatory. There he performed a tedious yet very important chore: a revision of one of astronomy’s bibles, the Shapley-Ames catalog of bright galaxies. It changed his professional life. While updating the catalog’s listings to include Southern Hemisphere galaxies, he couldn’t help but notice (with the aid of his telescope) that the Milky Way, along with its Local Group neighbors, is caught on the outskirts of a much larger system of galaxies. Altogether this system is generally arranged as a flat disk, made up of multiple clusters of galaxies. On a celestial map, it appears as a long band that stretches across both the northern and southern skies. The Virgo cluster, a huge collection of hundreds of galaxies located some 65 million light-years away, serves as the disk’s centerpiece.
De Vaucouleurs was seeing what Holmberg and Shapley had already noticed, but he was more tenacious. In a 1953 scientific paper, he gave this grouping a distinct name. He called it the “Local Supergalaxy,” what later became known as either the Local or Virgo Supercluster. In the 1980s de Vaucouleurs recalled that his suggestion was largely received with resounding silence. “It was considered as sheer speculation, even nonsense,” he told me. “Some prominent astronomers even told their students that it was an insane topic to work on. The concept that the universe was isotropic was too strong. It was dogma.”
But a few listened and gradually examined the idea further. More and more evidence piled up as other astronomers began to carry out their own surveys of galaxies across the heavens, with new instrumentation that enabled them to find both nearby and distant clusters that were once too faint to be counted. “All of a sudden,” wrote Italian astronomer Andrea Biviano in a review of this history, “researchers had a catalogue of clusters, and they could start to look at them as a population, rather than as individual objects.”
By 1961 the Virgo Supercluster was not alone. That year UCLA astronomer George Abell, the most noted cluster hunter of his era, examined all the data gathered so far by both him and others and pointed out other potential superclusters, each “large cloud” stretching up to 160 million light-years from end to end. Abell counted seventeen more nearby in our universe. As for
the Virgo Supercluster, Abell declared that an independent survey found “striking confirmation of de Vaucouleurs’ hypothesis.”
But acceptance did not come readily. No one could yet explain how such large structures could remain stable over the eons. More than that, some astronomers wondered if they were being deceived. Our eyes are very sensitive to patterns, a trait that enabled our ancestors to spot a predator amid the jungle foliage. For a long time, many were wary that superclus-ter proponents were merely tracing out shapes in a random distribution of clusters, much the way early planetary astronomers found “canals” on Mars.
Starting in the 1980s, however, as astronomers were able to determine the distances to more and more galaxies and clusters, they produced three-dimensional maps of the heavens. They discovered they weren’t being fooled at all. In fact, the distribution of galaxies was more astounding than they had ever imagined. Galaxies appear to congregate as if they are on the surfaces of huge, nested bubbles, with the bubble interiors nearly devoid of galaxies. Evidence suggests that this cosmic foam originated in the Big Bang, owing to perturbations surging through the primordial soup.
Filamentary superclusters stand out where the bubble-like surfaces intersect. At the time of de Vaucouleurs’s death in 1995, many of these superclusters were well mapped, with astronomers naming them after the constellations in which they can be found, such as Coma, Leo, Hercules, Perseus-Pisces, and Centaurus.
The enclosed line circles the Laniakea structure. The Milky Way galaxy (located at black dot on right) is on the edge of the Virgo Supercluster streaming inward.
(Copyright © 2014, Nature Publishing Group)
And these superclusters are not static. That’s how Tully and his colleagues found Laniakea. They saw that the Virgo Supercluster is being gravitationally drawn, like a river flowing downhill into a larger sea, toward a dense collection of galaxies known as the “Great Attractor.” By tracing the movement of galaxies directed toward the Great Attractor, they could define the borders of the new Laniakea Supercluster.
Home to some 100,000 galaxies, Laniakea stretches more than 500 million light-years across, nearly five times larger than our original Virgo abode, which is now a mere branch. Formerly caught in a supercluster suburb, the Milky Way finds itself in Laniakea’s hinterlands.
CHAPTER NINETEEN
The Big Dipper Is Crying
The well-known constellation looks as if it is
leaking cosmic rays
ONE year after it opened in 2007, the Northern Hemisphere’s largest cosmic-ray detector, the Telescope Array situated in western Utah, began observing a relatively large number of ultra-high-energy cosmic rays emanating from just below the handle of the Big Dipper. What might be the exact cause of this “hotspot” of rays? Even after years of observation and study, no one knows for sure. “All we see is a blob in the sky,” says University of Utah astrophysicist Gordon B. Thomson, “and inside this blob there is all sorts of stuff—various types of objects—that could be the source.”
An international team of astronomers found the hotspot by tracking a cascade of secondary particles that showered down upon the Earth and were captured by the Telescope Array when particularly powerful cosmic rays—those above 57 billion billion electron volts (14 million times the energy of the particles accelerated recently in the Large Hadron Collider)—hit the atmosphere. The array, a high-tech wonder, consists of more than five hundred scintillation detectors, each about the size of a Ping-Pong table, spread out over three hundred square miles of desert like myriad chess pieces. These measure the secondary particles that rain down upon the surface when a cosmic-ray shower hits the Earth’s atmosphere. Positioned around these detectors are three stations, each with a set of mirrors watching for blue flashes also created by the incoming cosmic rays. It’s a modern-day method that’s a far cry from the cruder instruments used by the discoverers of cosmic rays a century ago.
In this time-lapse photo, stars appear to rotate above a section of Utah’s Telescope Array, which is aimed at detecting highly energetic cosmic rays from space.
(Ben Stokes, University of Utah)
At the start of the twentieth century, researchers were only just discovering that charged ions reside in the air. Did this ionization originate from the Earth’s crust, they wondered, or from radioactivity within the atmosphere itself? Or perhaps from even farther out, the atoms getting ionized by some type of radiation journeying from the Sun?
Fascinated by this mystery, Theodor Wulf, a German priest and physicist, built a sensitive electroscope (the era’s standard charge detector consisting of wires or metal leaves suspended in a vessel) and, while on a trip to Paris in 1910, took his new instrument to the top of the Eiffel Tower, then the world’s tallest structure. Figuring the radiation emanated from the ground, he expected to measure a far weaker signal nearly one thousand feet (300 meters) above the cityscape. But instead, his signal was surprisingly strong. Perhaps, Wulf mused, it was coming from radioactive iron within the tower’s ornate lattice.
Victor Hess (center) in 1911 about to depart on
an air-balloon flight from Vienna.
(New York Times)
Continuing this quest, a number of scientists started taking measurements aboard balloons, which could reach higher altitudes, but their results were contradictory. It was Austria’s Victor Francis Hess who gathered the first convincing evidence that the radiation was arriving from outer space. Hess, an ardent amateur balloonist, was a physicist at the newly opened Institute for Radium Research in Vienna. His moment of discovery came on August 7, 1912, after he and two companions took off in a hydrogen-filled balloon from the Bohemian town of Aussig for the seventh in a series of flights he had been conducting that year. Using three electrometers of improved accuracy, he detected a noticeable increase in his ionization readings as his balloon rose to an altitude of 3.3 miles (5.3 kilometers). In fact, the ionization was three times higher than on the ground. Hess knew he was too far up for this radiation to be arriving from below. That meant it must be Höhenstrahlung, as he called it, “radiation coming from above.”
This was not a eureka moment for the scientific community, however. Many were still skeptical, including the world-renowned Caltech physicist Robert A. Millikan, who in the 1920s used unmanned balloons to take his instruments to even greater heights, up to nine miles (fourteen-and-a-half kilometers). As late as 1924 he reported that “the whole of the penetrating radiation is of local origin.” But after continuing his measurements atop mountains and aboard airplanes, he was at last convinced of their extraterrestrial nature. Millikan, a bit of a showboater, didn’t mind that America’s newspapers gave him all the credit for the find, with no mention of Hess.
Millikan has “found wild rays more powerful and penetrating than any that have been domesticated or terrestrialized . . . probably completing [an] alphabet for the language by which the stars communicate with man,” reported the New York Times on November 12, 1925. “The mere discovery of these rays is a triumph of the human Mind that should be acclaimed among the capital events of these days.”
Millikan, like many others at the time, believed the radiation was electromagnetic in nature. Because the radiation was so penetrating, he figured the wavelengths had to be shorter than gamma rays. At a meeting of the National Academy of Sciences, he called them “cosmic rays.” With great imagination, he declared that the highly energetic photons were released when particles in interstellar space somehow condensed into higher elements. To Millikan, cosmic rays were the “signals broadcasted throughout the heavens of the births of the common elements . . . the birth-cries of the infant atoms.”
This led to a raging battle between Millikan and University of Chicago physicist Arthur H. Compton, who was sure that the interstellar “rays” were actually particles. The debate between the two was so fierce that the national press regularly covered this scientific tussle. The particle model finally won in 1932, once Compton sent teams of researchers around the globe, fr
om Alaska to New Zealand, and fully demonstrated that the rays varied in intensity with latitude. The cosmic rays increased in number as the researchers traveled from the equator to the poles. That meant they were particles getting deflected by the Earth’s magnetic field: the field lines point toward the poles, and particles swoop more easily toward the polar regions than equatorial latitudes. (Photons are not diverted by magnetic fields.) With the controversy settled over the rays’ true nature, full credit was also restored. It was Hess who was awarded the 1936 Nobel Prize for his original discovery more than two decades earlier. The fact that everyone continued to call the alien particles cosmic rays was Millikan’s consolation prize.
As the use of Geiger counters and cloud chambers grew more sophisticated, physicists came to see that cosmic rays were mostly protons, but could also be atomic nuclei or electrons. They enter the Earth’s atmosphere, in a range of energies, from all directions of the celestial sky. Some five quintillion (5 1018) strike the Earth’s atmosphere each second. Upon colliding with air molecules, the primary rays generate a cascade of secondary particles that plummet to the ground (and get detected by such instruments as Utah’s Telescope Array).
Cosmic rays gave birth to the field of particle physics. By carefully studying cosmic-ray interactions, physicists came to discover new and bizarre elementary particles, beyond the plain-vanilla proton, electron, and neutron. In 1932 the positron (the electron’s antimatter mate) was discovered in a cosmic-ray cloud chamber; by 1937 the track of a speeding muon (a heavy electron) was similarly spotted in a chamber photograph.
Dispatches from Planet 3 Page 10
