Origins: Fourteen Billion Years of Cosmic Evolution (11 page)

What facts can we cite to support this bold assertion? Since astrophysicists have no way to see back to the universe’s first 0.000000000000000000000000000000000001 of a second, they do the next best thing, and use scientific logic to connect this early epoch to times they can observe. If the inflationary theory is correct, the initial fluctuations produced during that era, the inevitable result of quantum mechanics—which tells us that small variations from place to place will always arise within an otherwise homogeneous and isotropic fluid—would have had the opportunity to become regions of high and low concentrations of matter and energy. We can hope to find evidence for these variations from place to place in the cosmic background radiation, which serves as a proscenium that separates the current epoch from, and also connects it with, the first moments of the neonate universe.

As we have already seen, the cosmic background radiation consists of the photons generated during the first minutes after the big bang. Early in the universe’s history, these photons interacted with matter, slamming into any atoms that happened to form so energetically that no atoms could exist for long. But the ongoing expansion of the universe in effect robbed the photons of energy, so that eventually, at the time of decoupling, none of the photons had energies sufficient to prevent electrons from orbiting around protons and helium nuclei. Since that time, 380,000 years after the big bang, atoms have persisted—unless some local disturbance, such as the radiation from a nearby star, disrupts them—while the photons, each with an ever-diminishing amount of energy, continue to roam the universe, collectively forming the cosmic background radiation or CBR.

The CBR thus carries the imprint of history, a snapshot of what the universe was like at the time of decoupling. Astrophysicists have learned how to examine this snapshot with ever-increasing accuracy. First, the simple fact that the CBR exists, that their basic understanding of the history of the universe is correct. And then, after years of improving their abilities to measure the cosmic background radiation, their sophisticated balloon-borne and satellite instruments gave them a map of the CBR’s tiny deviations from homogeneity. This map provides the record of the once minuscule fluctuations whose size increased as the universe expanded during the few hundred thousand years after the era of inflation, and which then grew, during the next billion years or so, into the large-scale distribution of matter in the cosmos.

Remarkable though it may seem, the CBR provides us with the means for mapping the imprint of the long-vanished early universe, and for locating—14 billion light-years away in all directions—the regions of slightly greater density that would become galaxy clusters and superclusters. Regions with greater-than-average density left behind slightly more photons than regions with lower densities. As the cosmos became transparent, thanks to the loss of energy that left the photons unable to interact with the newly formed atoms, each photon embarked on a journey that would carry it far from its point of origin. Photons from our vicinity have traveled 14 billion light-years in all directions, providing part of the CBR that far-distant civilizations at the end of the visible universe may even now be examining, and “their” photons, having reached our instruments, tell us about what things were like long ago and far away, in the times when structures had barely begun to form.

Through more than a quarter of a century following the first detection of the cosmic background radiation in 1965, astro-physicists searched for anisotropies in the CBR. From a theoretical viewpoint, they desperately needed to find them, because without the existence of CBR anisotropies at the level of a few parts in a hundred thousand, their basic model of how structure appeared would lose all claim to validity. Without the seeds of matter they betray, we would have no explanation for why we exist. As happy fate would have it, the anisotropies appeared precisely on schedule. Just as soon as cosmologists created instruments capable of detecting anisotropies at the appropriate level, they found them, first with the COBE satellite in 1992, and later with far more precise instruments mounted on balloons and on the WMAP satellite described in Chapter 3. The teeny fluctuations from place to place in the amounts of microwave photons that form the CBR, now delineated with impressive precision by WMAP, embody the record of cosmic fluctuations at a time 380,000 years after the big bang. The typical fluctuation sits only a few hundred thousandths of a degree above or below the average temperature of the cosmic background radiation, so detecting them is like finding faint spots of oil on a mile-wide pond that make the water plus oil a tiny bit denser than average. Small though these anisotropies were, they sufficed to get things started.

In the WMAP map of the cosmic background radiation, the larger hot spots tell us where gravity would overcome the expanding universe’s dissipative tendencies and gather together enough matter to manufacture superclusters. These regions today have grown to contain about 1,000 galaxies, each with 100 billion stars. If we add the dark matter in such a supercluster, its total mass reaches the equivalent of 10
16
Suns. Conversely, the larger cool spots, with no head start against the expanding universe, evolved to become nearly devoid of massive structures. Astrophysicists just call these regions “voids,” a term that gains meaning from being surrounded by something that is not a void. So the giant sheets and filaments of galaxies that we can trace on the sky not only form clusters at their intersections but also trace walls and other geometric forms that give shape to the empty regions of the cosmos.

Of course, the galaxies themselves did not simply appear, fully formed, from concentrations of matter a tiny bit denser than average. From 380,000 years after the big bang until about 200 million years later, matter continued to gather itself together, but nothing shone in the universe, whose first stars were yet to be born. During this cosmic dark age, the universe contained only what it had made during its first few minutes—hydrogen and helium, with traces of lithium. With no elements heavier than these—no carbon, nitrogen, oxygen, sodium, calcium, or heavier elements—the cosmos contained none of the now common atoms or molecules that can absorb light as a star begins to shine. Today, in the presence of these atoms and molecules, the light from a newly formed star will exert pressure upon them that pushes away massive quantities of gas that would otherwise fall into the star. This expulsion limits the maximum mass of newborn stars to less than one hundred times the Sun’s mass. But when the first stars formed, in the absence of atoms and molecules that would absorb starlight, infalling gas consisted almost entirely of hydrogen and helium, providing only token resistance to stars’ output. This allowed stars to form with much larger masses, up to many hundred, perhaps even a few thousand, times the mass of the Sun.

High-mass stars live life in the fast lane, and the most massive live the most rapidly of all. They convert their matter into energy at astonishing rates, as they manufacture heavy elements and die explosive youthful deaths. Their life expectancies amount to no more than a few million years, less than a thousandth of the Sun’s. We expect to find none of the most massive stars from that era alive today, because the early ones burnt themselves out long ago, and today, with heavier elements common throughout the universe, the highest-mass stars of old cannot form at all. Indeed, none of the high-mass giants has ever been observed. But we assign them the responsibility for having first introduced into the universe almost all of the familiar elements we now take for granted, including carbon, oxygen, nitrogen, silicon, and iron. Call it enrichment. Call it pollution. But the seeds of life began with the long-vanished first generation of high-mass stars.

During the first
few billion years after the time of decoupling, gravitationally induced collapse proceeded with abandon, as gravity drew matter together on nearly all scales. One of the natural results of gravity at work was the formation of supermassive black holes, each with a mass millions or billions of times the mass of the Sun. Black holes with that amount of mass are about the size of Neptune’s orbit and wreak havoc on their nascent environment. Gas clouds drawn toward these black holes want to gain speed, but they can’t, because there’s too much stuff in the way. Instead, they slam into and rub against whatever came in just before them, descending toward their master in a swirling maelstrom. Just before these clouds disappear forever, collisions within their superheated matter radiate titanic quantities of energy, billions of times the Sun’s luminosity, all within the volume of a solar system. Monstrous jets of matter and radiation spew forth, extending hundreds of thousands of light-years above and below the swirling gas, as the energy punches through and escapes the funnel in all ways it can. As one cloud falls, and another orbits-in-waiting, the luminosity of the system fluctuates, getting brighter and dimmer over a matter of hours, days, or weeks. If the jets happen to be aimed straight at you, the system will look even more luminous, and more variable in its output, than those cases in which the jets point to the side. Viewed from any appreciable distance, all of these black hole–plus–infalling-matter combinations will appear amazingly small and luminous in comparison to the galaxies we see today. What the universe has created—the objects whose birth we have just witnessed in words—are quasars.

Quasars were discovered during the early 1960s, as astronomers began to use telescopes equipped with detectors sensitive to invisible domains of radiation, such as radio waves and X-rays. Their galaxy portraits could therefore include information about the galaxies’ appearance in those other bands of the electromagnetic spectrum. Combine this with further improvements in photographic emulsions, and a new zoo of galaxy species emerged from the depths of space. Most remarkable among them were objects that, in photographs, look like simple stars, but—quite unlike stars—produce extraordinary amounts of radio waves. The working description for those objects was “quasistellar radio source”—a term quickly shortened to quasar. Even more remarkable than the radio emission from these objects were their distances: as a class, they turned out to be the most distant objects known in the universe. For quasars to be that small and still visible at immense distances meant that they had to be an entirely new kind of object. How small? No bigger than a solar system. How luminous? Even the dim ones outshine your average galaxy in the universe.

By the early 1970s, astrophysicists had converged on supermassive black holes as the quasar engine, gravitationally devouring everything in its grasp. The black hole model can account for how small and bright quasars are, but says nothing of the black hole’s source of food. Not until the 1980s would astrophysicists begin to understand the quasar’s environment, because the tremendous luminosity of a quasar’s central regions prevents any sight of its much fainter surroundings. Eventually, however, with new techniques to mask the light from the center, astrophysicists could detect fuzz surrounding some of the dimmer quasars. As detection tactics and technologies improved further, every quasar revealed fuzz; some even revealed a spiral structure. Quasars, it turned out, are not a new kind of object but rather a new kind of galactic nucleus.

In April 1991,
the National Aeronautics and Space Administration (NASA) launched one of the most expensive astronomical instruments ever built: the Hubble Space Telescope. The size of a Greyhound bus, directed by commands sent from Earth, the Hubble Telescope could profit from orbiting outside our ever-blurring atmosphere. Once astronauts had installed lenses to correct for mistakes in the way its primary mirror had been made, the telescope could peer into previously uncharted regions of ordinary galaxies, including their centers. Upon gazing into those centers, it found the stars moving inexcusably fast, given the gravity inferred from the visible light of other stars in the vicinity. Hmmm, strong gravity, small area . . . must be a black hole. Galaxy after galaxy—dozens of them—had suspiciously speedy stars in their cores. Indeed, whenever the Hubble Space Telescope had a clear view of a galaxy’s center, they were there.

It now seems likely that every giant galaxy harbors a super-massive black hole, which could have served as a gravitational seed around which the other matter collected or may have been manufactured later by matter streaming down from outer regions of the galaxy. But not all galaxies were quasars in their youth.

The growing roster
of ordinary galaxies known to have a black hole at their center began to raise eyebrows among investigators: A supermassive black hole that was not a quasar? A quasar that’s surrounded by a galaxy? One can’t help but think of a new picture of how things work. In this picture, some galaxies begin their lives as quasars. To be a quasar, which is really just the blazing visible core of an otherwise run-of-the-mill galaxy, the system has to have not only a massive, hungry black hole but also an ample supply of infalling gas. Once the supermassive black hole has gulped down all the available food, leaving uneaten stars and gas in distant, safe orbits, the quasar simply shuts off. You’ve then got a docile galaxy with a dormant black hole snoozing at its center.

Astronomers have found other new types of objects, classified as intermediate between quasars and normal galaxies, whose properties also depend on the bad behavior of supermassive black holes. Sometimes the streams of material falling into a galaxy’s central black hole flow slowly and steadily. At other times episodically. Such systems populate the menagerie of galaxies whose nuclei are active but not ferocious. Over the years, names for the various types accumulated: LINERs (low-ionization nuclear emission-line regions), Seyfert galaxies, N galaxies, blazars. All of these objects are generically called AGNs, the astrophysicist’s abbreviation for galaxies with “active” nuclei. Unlike quasars, which appear only at immense distances, AGNs appear both at large distances and relatively nearby. This suggests that AGNs fill in the range of galaxies that misbehave. Quasars long ago consumed all their food, so we see them only when we look far back in time by observing far out in space. AGNs, in contrast, had more modest appetites, so some of them still have food to eat even after billions of years.