Dispatches from Planet 3

by Marcia Bartusiak

  Only at the turn of this century, with the help of a space telescope aptly named Hubble, did Freedman and others confidently peg the universe’s current rate of expansion, as well as its age. A birthday cake for the universe would require around 14 billion candles.

  Astronomers have found some strange objects in this expanding universe—and these too are Einstein’s children. In the 1930s a young Indian physicist, Subrahmanyan Chandrasekhar, applied special relativity and the new theory of quantum mechanics to a star. He warned that if it surpassed a certain mass, it would not settle down as a white dwarf at the end of its life (as our Sun will). Instead, gravity would squeeze it down much further, perhaps even to a singular point.

  Chandrasekhar had opened the door for others to contemplate the existence of the most bizarre stars imaginable. First there was a naked sphere of neutrons just a dozen miles wide born in the throes of a supernova, the explosion of a massive star. Then there was the peculiar object formed from the collapse of an even bigger star or a cluster of stars—enough mass to dig a pit in space-time so deep nothing can ever climb out.

  Einstein himself tried to prove that such an object—a black hole, it was later christened—could not exist. He loathed what would be found at a black hole’s center: a point of zero volume and infinite density, where the laws of physics break down. The discoveries that might have forced him to acknowledge his theory’s strange offspring came after his death in 1955.

  Astronomers identified the first quasar, a remote young galaxy disgorging the energy of a trillion suns from its center, in 1963. Four years later, much closer to home, observers stumbled on the first pulsar, a rapidly spinning beacon emitting staccato radio beeps. Meanwhile, spaceborne sensors spotted powerful X-rays and gamma rays streaming from points around the sky. All these new, bewildering signals are believed to pinpoint collapsed objects—neutron stars and black holes—whose crushing gravity and dizzying spin turn them into dynamos. With their discovery, the once sedate universe took on an edge; it metamorphosed into an Einstein-ian cosmos, filled with sources of titanic energies that can be understood only in the light of relativity.

  Even Einstein’s less celebrated ideas have had remarkable staying power. As early as 1912 he realized that a faraway star can act like a giant spyglass, its gravity deflecting passing light rays and magnifying objects behind it. (See chapter 17.) But he eventually concluded this effect had “little value.” With today’s telescopes, though, astronomers are seeing galaxies and galaxy clusters act as gravitational lenses, offering a peek at galaxies farther out. Since the light-bending depends on the mass of the lens, the effect also lets observers weigh the lensing galaxies. They turn out to have far more mass than can be seen. It’s part of the universe’s mysterious dark matter, the roughly 90 percent of its mass that can’t be found in stars, gas, planets, or any other known form of matter.

  A cosmic web of dark matter is now thought to have governed where galaxies formed. Dark matter is the universe’s hidden architecture, and gravitational lensing is one of the few practical ways to “see” it. An effect Einstein thought insignificant has become a key astronomical tool.

  Theorists have also dusted off his discarded cosmological constant to explain a startling new discovery. Einstein’s “biggest blunder” is now starting to look like one of his greatest successes. Astronomers had assumed that gravity is gradually slowing the expansion of the universe. But in the late 1990s, two teams, measuring the distances to faraway exploding stars, found just the opposite. Like buoy markers spreading apart on ocean currents, these supernovae revealed that space-time is ballooning outward at an accelerating pace.

  For Einstein, the cosmological constant was a way to steady the universe. But if its repulsive effect—now called dark energy—is big enough, it could also drive the acceleration. “The need came back, and the cosmological constant was waiting,” says Adam Riess of the Space Telescope Science Institute, one of the discoverers of the acceleration. “It’s totally an Einsteinian concept.” So was that other prediction of general relativity recently confirmed, the gravitational waves emitted by the collision of such astronomical heavyweights as black holes and neutron stars (see chapter 20).

  The mighty jolt of cosmic birth probably also generated gravity waves, which would still be resonating through the cosmos. These remnant ripples could hold direct evidence of the fleeting moment when physicists believe all of nature’s forces were united. If so, Einstein’s gravity waves could at last offer clues to something he tried and failed to develop: a “theory of everything.” Physicists are still seeking such a theory—a single explanation for both the large-scale force of gravity and the short-range forces inside the atom.

  Catching these faint echoes of the Big Bang is a major goal of NASA’s next generation of space astronomy missions, a plan the agency has tagged “Beyond Einstein.”

  Beyond Einstein? Not by a long shot. Einstein might be startled by the universe as we understand it today. But it is unmistakably his.

  CHAPTER TWENTY-NINE

  The Big Burp

  The universe began in warp drive

  NEARLY four decades ago Alan Guth, now an MIT physicist, introduced the astounding idea that our universe began not with a bang, but with a sort of cosmic burp—a brief moment of superaccelerated expansion that transformed a subatomic smudge of energy into a cosmos capable of generating galaxies, stars, and planets. Ever since, this idea has been avidly investigated and challenged.

  Guth trained in particle physics and had no plans to pursue cosmology until the late 1970s. Then, in 1978, he and a fellow postdoc at Cornell University, Henry Tye, were analyzing theories on the unification of the forces of nature. Guth and Tye wondered whether unification in the very early universe might have given rise to magnetic monopoles: hypothetical particles that have only one magnetic pole, either north or south. Continuing to work together after Guth moved to another postdoc position at Stanford University, the two concluded that, indeed, so many monopoles would have been created in the standard conception of the Big Bang that, as Guth said, “we began to wonder why the universe was here at all. [The monopoles’] tremendous weight would have closed the universe back up eons ago.” The monopoles would be so gravitationally attracted to one another, an expansion could never get started. Space-time would have collapsed.

  To explain why that didn’t happen, the two young researchers surmised that the early universe “supercooled” as it expanded, keeping the forces unified a bit longer as temperatures plunged, just as water can sometimes supercool and remain liquid below its freezing point under certain conditions. According to their calculations, such supercooling would have curbed huge numbers of monopoles from being produced.

  MIT physicist Alan Guth.

  (Betsy Devine/Wikimedia Commons)

  Things really got interesting when Guth decided one night to quickly check how such supercooling might have affected the expansion of the newly born universe. On December 6, 1979, around 11:00 p.m., the young physicist sat down in his makeshift home office and began to work on a series of calculations that within a couple of hours covered four pages. The title at the top of the first page, recorded in small, precise black letters, proclaimed his ambitious intention: he was tackling nothing less than the evolution of the universe.

  Guth was dealing with the arcane tools of his trade—concepts called “Higgs fields” and “false vacuum states.” But, as Guth put down his pen around 1:00 a.m., the bottom line was undeniable. If his equations were valid, the universe did not just expand at the moment of its birth, it tore outward like a fanciful science-fiction spaceship in warp drive. Perhaps inspired by the double-digit rises in the cost of living at the time, Guth came up with an appropriate name for this brief period of hyperacceleration: he called it inflation.

  Inflation began around 10–35 second into our birth, when the universe was less than a trillionth the size of a proton. Guth saw that the proposed supercooling endowed the universe with a tremendous potential
energy, not unlike a rock precariously perched on the edge of a precipice. In this state, gravity, normally a force that draws things together, did a turnabout and became repulsive, causing space-time to balloon outward at a superaccelerated rate for an infinitesimal fraction of a second. But that was enough of a window for our subatomic speck of a cosmos to double in size sixty to a hundred times over. Once inflation ended (when the universe was about the size of a marble or larger), its latent energy was converted into all the particles and radiation that surround us today. It was inflation’s demise that actually put the bang into the Big Bang, providing our cosmos with all its necessary building materials. As Guth likes to put it, “The universe is the ultimate free lunch.” A lot came out of nearly nothing.

  A diagram showing the universe’s evolution. The far left depicts the earliest moment, when a period of “inflation” produced a burst of growth. For the next several billion years, space-time continued to expand, more recently speeding up as dark energy came to dominate the expansion.

  (NASA/WMAP Science Team)

  Inflation explained a longtime mystery: the uniformity of the universe from end to end. Caught in an unusual state of expansion, the growing cosmic seed was able to maintain a uniform density as space-time hyperaccelerated outward, so that our universe ended up looking pretty much the same in all directions. Guth was initially elated by this finding, until he discovered a fatal flaw in his scenario: at the end of his rip-roaring burst, he ended up with a chaotic collection of tiny “bubble universes,” none looking like ours. But in the following years, other theorists, such as the Russian physicist Andrei D. Linde, now at Stanford, figured out ways to get one of Guth’s many bubbles to balloon into a suitable cosmos.

  Yet how do you obtain proof of such a fantastic event, one that occurred at the birth of time itself? If astronomers could peer back with their telescopes to the initial fireball, they wouldn’t see anything at all. Much as the Sun’s hot outer layers prevent us from gazing to its core, the universe at this time was a blurry soup of plasma, impossible for any optical, radio, or X-ray telescope to probe. The universe didn’t become truly transparent until it was about 400,000 years old—when electrons settled down with protons and neutrons to form atoms, and the primordial photons were at last able to travel through the universe unimpeded. Stretched out by the universe’s expansion, remnant radiation from the Big Bang now exists as a wash of microwaves bathing the entire universe. Detecting that “cosmic microwave background” tells us how the universe was doing several hundreds of thousands of years after the Big Bang—but no earlier.

  Clever theorists, however, found a way around this obstacle. They predicted that quantum fluctuations, tiny jitters in the universe’s initial seed, would have blown up to astronomical scales as the universe whizzed outward. And it was those perturbations that helped organize primordial matter into the clusters and galaxies we see today. Valuable support for that idea came when balloons and satellites—sent into space to measure the microwave background with exquisite precision—captured a signal related to temperature with just the pattern of fluctuations predicted by the inflationary models. But competing models for the early universe’s behavior, which didn’t involve inflation, offered similar predictions.

  By the 1990s, theorists offered a more powerful test for inflation—they suggested that primordial gravity waves, generated during inflation, would engrave a unique signature upon the cosmic microwave background. A consequence of Einstein’s general theory of relativity, gravity waves are actual ripples in the fabric of space-time, jiggles that alternately stretch and squeeze anything in their path. Searches for this gravity-wave signature were initiated by a number of groups, including teams that have set up special radio telescopes on the icy terrain of the South Pole, notable for its thin, dry air, the best conditions for gathering celestial microwaves (other than in space).

  The effect being sought is very subtle. They are looking for a slight swirling pattern on the remnant Big Bang radiation, which indicates it has become “polarized” (the electric fields oscillating back and forth in one preferred orientation). Theory suggests that the gravity waves, as they rippled space-time in the infant universe, had given the light a little kick that caused its orientation to curl, a pattern that only inflationary gravity waves could imprint. Seeing such a pattern would make the case for inflation far stronger. If verified, it’s the sort of scientific finding that might prompt its discoverers to think about a Scandinavian vacation in order to pick up their Nobel Prizes.

  CHAPTER THIRTY

  The Great Escape

  “Black holes ain’t so black”

  GAMMA rays from deep space were discovered by accident in the early 1970s. A group of United States satellites called Vela (“watch” or “vigil” in Spanish) had been put into orbit to make sure nations around the world were complying with the 1963 nuclear test ban treaty. Sifting through the satellites’ vast archive of recordings, researchers from the Los Alamos National Laboratory found one event, a burst of gamma rays recorded on July 2, 1967, that didn’t look at all like a covert nuclear-bomb test, either in space or on Earth. They soon found similar bursts in the records, and all appeared to come from outside the solar system.

  The duration of the bursts ranged from less than a tenth of a second to some thirty seconds—they were popping off like a cosmic flashbulb, flickering for a moment, then fading away. Over the succeeding years various countries launched space detectors that were specifically designed to discern the origin of these powerful cosmic eruptions (gamma rays have the most energy of any electromagnetic radiation), and gradually an answer emerged. Today it’s generally accepted that the most common bursts emanate from the gravitational collapse of massive stars—located as far away as the most distant and ancient reaches of the universe—into black holes; others have their origin in collisions between pairs of neutron stars.

  Each successive generation of gamma-ray instrumentation offered better and better timing resolution. And that presented physicist David Cline at the University of California, Los Angeles, and several colleagues with a unique opportunity. Plowing through the data from seven gamma-ray detectors, they came to suspect that what they called Very Short Gamma Ray Bursts—those lasting less than a tenth of a second—might represent a class of phenomena with a distinct cause.

  How to explain these ultra-brief, super-high-energy bursts? Cline and his colleagues claim they could be evidence for tiny “primordial black holes,” perhaps with the mass of a small asteroid packed into the volume of an atomic nucleus, that formed within the extreme densities of the early universe—a phenomenon first predicted by Stephen Hawking in the 1970s. That would be big news in the physics community, if true, for such bursts would then offer the means to study what happens when general relativity (the rules that govern the universe at large) merges with quantum mechanics (the tenets of the atomic world). Such a union of the macrocosm with the microcosm has long been sought by physicists.

  Hawking’s musings were partly sparked during a visit to Moscow in the fall of 1973, where he talked with Soviet physicists Yakov Zel’dovich and Alexander Starbinsky. Those two men had suggested that under special circumstances—that is, when a black hole rotates—it should convert that rotational energy into radiation, thus creating particles. This emission would continue until the spinning black hole wound down and stopped turning.

  Devising his own mathematical attack on the problem, Hawking was surprised to discover that all black holes—spinning or not—would be radiating. As Hawking later put it, “Black holes ain’t so black.”

  Hawking announced his discovery in February 1974 at a quantum gravity conference held in England, and his report was soon published in the journal Nature. In this endeavor, Hawking looked at the black hole from the perspective of an atom and found that quantum mechanical effects caused black holes to create and emit particles as hot bodies would. As a consequence, the black hole slowly decreases in mass and eventually disappears in a final exp
losion. Such a finding turned black-hole physics upside-down; a black hole, by definition, holds on to everything it swallows. It’s supposed to emit nothing and never go away.

  Hawking estimated it would take more than 1060 years, far longer than the age of the universe, for a regular black hole, weighing a few stellar masses, to disappear. But what if extremely small holes were created in the turbulence of the Big Bang? They could be popping off right now. Hawking estimated that in its final breath—its last tenth of a second of life—that tiny object would release the energy of a million one-megaton hydrogen bombs.

  Needless to say, his fellow physicists were not enthralled by this idea. At that February conference, it was greeted with total disbelief. At the end of Hawking’s talk, the chairman of the session, John Taylor from Kings College, London, got up and responded, “Sorry, Stephen, but this is absolute rubbish.”

  But gradually, over the following two years, it came to be recognized that Hawking had made a startling breakthrough: his argument demonstrated that gravitation and quantum mechanics were somehow deeply connected. Even though these two laws of nature have yet to be fully joined, here was evidence that unity was achievable.

  Hawking saw that space-time gets so twisted near a black hole that it enables pairs of particles (a matter particle and its antimatter mate) to pop into existence just outside the black hole. You could think of it as energy being extracted from the black hole’s intense gravitational field and then converted into particles.

  Illustration of a black hole evaporating,

  releasing radiation over time.

  (APS/Alan Stonebraker)

  But because we’re talking about the submicroscopic scale, the exact line of the black hole’s boundary is quite fuzzy. So, at times, one of the newly created particles can disappear into the black hole, never to return, while the other remains outside and flies off. As a result, the hole’s total mass-energy is reduced a smidgen. This means the black hole is actually evaporating. Ever so slowly, particle by particle, the black hole is losing mass.