2. GALACTIC EPOCH

Hierarchy of Structures

DESCENDANTS OF OUR CIVILIZATION MAY never become advanced enough to journey far enough from our Milky Way Galaxy to look back and witness the full grandeur of our extended home in space; the finite speed of light is too limiting, the Galaxy too vast. A literal picture of our resident swarm of a hundred billion stars floating proudly and silently in the void of space may forever elude us. Yet, from our Earth-based vantage point in the suburbs of our Galaxy—nearly thirty thousand light-years from its hub—astronomers study the variety and spread of other colossal star systems well beyond our own Milky Way. Many of these distant galaxies launched their light—some of the very light in tonight’s nighttime sky—long before Earth and the Sun even emerged in the firmament.

Deep space harbors myriad objects looking strangely unlike stars. Photographic exposures, taken with even small, backyard telescopes, often reveal fuzzy, lens-shaped images resembling disks more than the bright, round points typical of stars. The eighteenth-century German philosopher Immanuel Kant regarded these blurry blobs of light, like so many flattened yet luminous puffs of cotton, as individual “island universes” far outside the confines of our Milky Way. We now know that labeling each of them a universe—“the totality of all things”—presents a clear semantics problem, but he was correct in thinking that these peculiar patches of light reside way beyond the well-known stars that constitute the familiar constellations.

Large, modern telescopes have since revealed these remote beacons to be entire galaxies, each a huge collection of matter comparable to our Milky Way, measuring some hundred thousand light-years across, or a billion billion kilometers. Replete with hundreds of billions of stars bound loosely by gravity, each galaxy harbors more stars than all the people who have ever lived on Earth. Silently and majestically, galaxies twirl in the deep reaches of the Universe—vast hordes of radiation, matter, and perhaps life—simultaneously granting us a feeling both for the immensity of the Universe and for the minuteness of our position in it.

That position, when internalized, often resembles a boat adrift at sea. For there are as many galaxies in the Universe as there are stars in our Galaxy—all told, probably as many stars in the observable cosmos as grains of sand on all the beaches of the world—some 10²², to be numerical about it. Yet organized patterns abound—grand dynamical processions like the pinwheeling of individual galaxies and the recessional of many more galaxies—provided we are willing to ponder the big and the broad.

Objects identified as galaxies in photographs often display spiral shapes much like our Milky Way or the neighboring Andromeda Galaxy. (Andromeda is the one distant galaxy that our naked eyes can see, perhaps best with averted vision, as a faintly glowing oval amid the constellation of the same name.) Each has a central bulge from which sport thin spiral regions, or “arms,” chock-full of stars. The apparent prevalence of spiral galaxies is just that—apparent—largely the result of whorled patterns easily noticed among the many other patches of light in the nighttime sky. In reality, galaxies have an array of morphologies, and spirals are not the most common type of galaxy in the Universe.

After decades of effort in the mid-twentieth century led by the American astronomer Edwin Hubble, inventories of extragalactic matter beyond our Milky Way are now nearly complete for that relatively local part of the Universe in which we live. At least for normal, baryonic matter, that is, for we are still unsure what to make of the issue of “dark matter” raised in the Particle Epoch as strongly inferred but not yet seen. The most abundant galaxies are shaped like footballs and officially called elliptical galaxies. Some are less elongated, akin more to very large beach balls. Others resemble fat cigars and even thin cigars, but that’s probably due to their tilted perspective. None of the ellipticals exhibits internal structure of any kind, and they are notably devoid of any spiral arms. Regardless of their shape, most elliptical galaxies harbor the usual hundred billion or so stars, typically spread across a hundred-thousand-light-year domain. A minority of them measure up to ten times that size and contain trillions of stars, but otherwise elliptical galaxies are undistinguished, albeit monumental, throngs of stars.

Spiral arms are not the only trait that elliptical galaxies lack. Hardly any cool gas drifts among the stars of these galaxies; they have little or no interstellar matter. This implies that all the elliptical galaxies are old. Stars, which originate from interstellar matter, apparently did so in these galaxies long ago, leaving hardly any loose gas for the continued formation of future generations of stars. Analysis of the radiation emitted by individual stars within the ellipticals further proves that those stars are also old. Evidently, nearly all their interstellar matter was used up eons ago, thus quenching the star-formation process.

Silently and majestically, galaxies twirl in the vast reaches of the Universe . . .

The infrequence of star formation in elliptical galaxies contrasts sharply with the abundance and activity of interstellar matter within spiral galaxies. Here, there are also a variety of shapes, though all the spirals are basically flattened disks resembling double sombreros clapped brim to brim. Some spiral galaxies have a large central bulge, mostly made of intact stars and diffuse gas, around which the spiral arms are tightly wrapped. Others have a more open pattern of arms emanating from an intermediate-sized central region. Still others have a rather small center from which long, stringy arms protrude, often making it hard to recognize these as spirals.

Spiral galaxies are known to contain lots of gas, and dust, too, mixed throughout the vast spaces among their stars. The oldest stars extend into the galaxies’ spherical halos, but the youngest stars are found exclusively in the thin disks. Furthermore, observations during the past few decades have shown that stars are still forming, most of them in the arms. Unlike the old elliptical galaxies, spiral galaxies have a good deal of vitality—which doesn’t necessarily mean that the spirals are young. Rather, they are simply still rich enough in interstellar gas to provide for ongoing stellar birth.

Some spiral galaxies sport a peculiar feature that has astronomers puzzled—namely, a linear “bar” of stellar and interstellar matter passing through their midsections. For these so-called barred spirals, the arms stem from near the ends of bar rather than from the central bulge itself. The puzzle concerns the way the bars form, evolve, or maintain themselves. Even our own Milky Way is now judged to perhaps have a small one passing through its galactic nucleus.

A final, catchall class of galaxies groups all those termed irregular galaxies, the type most widely seen in the Universe. These are oddly shaped structures of stars, gas, and dust whose visual appearance prevents their placement in any of the other categories. The irregulars clearly have much interstellar matter yet no organized structure, spiral arms, or central bulge. By and large, irregulars tend to be a bit smaller than other types of galaxies, so some astronomers call them dwarf galaxies. They often do seem to be dominated by the larger spiral or elliptical galaxies near which they are usually found. In fact, irregular galaxies seldom exist alone in space; they are mostly allied with larger “parent” galaxies of the spiral or elliptical variety.

Their proximity to the big galaxies is probably telling us something: The irregulars might be severely distorted regular galaxies that have experienced close encounters, and thus great tidal disruptions, with their parent galaxies. Or, they might be leftover building blocks of the larger galaxies into which they have not yet fallen. Some observations do hint at possible bridges of hydrogen gas connecting parent and irregular galaxies, suggestive of interactions between them. We shall return to these issues when later discussing mergers and acquisitions among galaxies rife with evolutionary change.

Our Milky Way Galaxy has a few small, companion irregular galaxies, most notably the Large and Small Magellanic Clouds, so named for the sixteenth-century Portuguese voyager Ferdinand Magellan whose round-the-world expedition first brought word of these great fuzzy patches of light to Europeans living in the Northern Hemisphere. Resembling dimly luminous atmospheric clouds and seen easily with the naked eye, they can be viewed only from locations south of Earth’s equator, making them spectacular targets for first-time northerners traveling south, though they have undoubtedly served as celestial wonders to residents of the Southern Hemisphere since the dawn of civilization. Though one is slightly larger than the other, each is roughly a hundred times smaller than our own Milky Way system—meaning that they house “only” a billion or so stars—and both reside not quite two hundred thousand light-years away. The Magellanic Clouds probably orbit our Galaxy, just as Earth orbits the Sun or the Moon orbits Earth. The periods of these irregular galaxies are long by human standards, however, and their orbital paths have not yet been firmly established.

Actually, these famous celestial objects (at least to residents “down under”) might not deserve the term galaxy at all, not even irregular galaxy. Though they do contain about a billion solar masses and do measure some ten thousand light-years across, they reside at a distance from our Galaxy that is only fifty percent again of its disk’s typical extent. Thus, if our Milky Way system does harbor matter (dark or otherwise) in an extended halo—as many astronomers now suspect—then the Magellanic Clouds may well be nothing more than rich regions of star formation in the halo of our own Galaxy. Perhaps all such dwarf structures now classified as irregular galaxies will turn out to be residents of the outer realms of larger, well-categorized galaxies—and therefore not genuine galaxies at all.

Planet Earth is finite; beyond it stretches the scant flimsiness of interplanetary space. Our Solar System is also finite; beyond it lies the near vacuum of interstellar space. And our Galaxy, in turn, is finite; beyond it exists the absolute material void of intergalactic space. Perhaps beyond even that, then, the arrangement of galaxies in space is also finite. Which brings to mind the obvious question: How are the galaxies spread throughout the expansive tracts far from the Milky Way? Is there some boundary, or terminus, beyond which galaxies are no longer seen? Or do they reside everywhere, all the way out to the limits of the observable Universe?

Within the “neighboring” realm of a few million light-years, astronomers know of a few dozen galaxies. Giant spirals, such as our own Milky Way and Andromeda, are found among small ellipticals and many irregulars, such as the Magellanic Clouds. Surprisingly, some nearby galaxies have been discovered even as recently as the past decade, such as the Sagittarius Spheroid, a newly found dwarf galaxy only eighty thousand light-years distant yet mostly obscured by our Galaxy’s central bulge. Evidently, these two-score galaxies are bound together by their own mutual gravitational attraction—a mammoth version of the same natural phenomenon that holds stars in galaxies, planets around stars, and people on Earth. In all, these “local” galaxies are clustered within a volume whose diameter is some five million light-years. Including our Milky Way, the whole bunch is known as the Local Group. It constitutes our extended neighborhood in space.

Several million light-years comprise a significant chunk of cosmic real estate. Do note two important things about it. First, we have suddenly made a large jump in spatial dimensions, from the hundred-thousand-light-year size of our Milky Way to this five-million-light-year size of our Local Group. Galaxy clusters represent a distinctly higher level of hierarchical structure in the Universe—structure well beyond that of individual galaxies. Second, the Milky Way doesn’t lie at the center of this cluster of galaxies. Not only is Earth not the center of our Solar System and the Sun not the center of our Galaxy, but our Galaxy is also not the center of the much larger Local Group. Though we might like to think so, humankind is not at any special, unique, or privileged location in the gargantuan, perhaps infinite, Universe.

Many more than a few dozen galaxies reside in the Universe. Time exposures made with large telescopes reveal thousands of galaxies within virtually any small field of view. In all, astronomers estimate that some forty billion other galaxies inhabit the observable Universe. And virtually all of them are much farther away than even the distant members of our local galaxy cluster. For millions of light-years beyond the edge of the Local Group, there appears to be nothing—no galaxies, no stars, no gas or dust—just empty intergalactic space.

Strive to appreciate the far recesses of deep space outside the Local Group. Searching a seemingly interminable void, we occasionally sight a “field” galaxy scattered lonely here and there. Not until we reach a distance of some sixty million light-years away do we find another galaxy cluster, an unmistakable volume of space brimming with galaxies. This cluster is especially rich, containing not just forty galaxies as does our own Local Group; the so-called Virgo Cluster harbors nearly three thousand galaxies. Try to visualize in mind’s eye thousands of individual galaxies all clustered in a swarm, each one housing about a hundred billion stars. No wonder most people have trouble appreciating the immensity of matter, space, and time in the Universe. Astrophysicists are no different; we, too, share the burden of trying to fathom such humongous sizes and scales, including astronomical numbers of astronomical objects.

Galaxy clusters like these populate the Universe throughout. They are not figments of our imagination. Their existence is fact, as hundreds have now been mapped and cataloged through numerous observations. In much the same way that galaxies are collections of stars, galaxy clusters are collections of galaxies. And beyond them, in turn, galaxy super-clusters (or clusters of clusters) apparently also exist on colossal scales of typically hundreds of millions of light-years across. Both the Local Group and the Virgo Cluster are mere members of such a larger system—perhaps. These truly titanic structures occupy a most lofty rung—the greatest established to date—in the hierarchy of material assemblies within the Universe: particles, atoms, molecules, dust, planets, stars, galaxies, galaxy clusters, and now galaxy superclusters.

When contemplating the congested confines of rich galaxy clusters—such as Virgo, with its thousands of members, or the appropriately named Hercules Cluster, with its estimated hundred thousand galaxies—it’s hard to avoid the impression that galactic traffic jams must be common. Just as atoms collide when confined in a closed container or hockey players in a enclosed rink, the random motions of galaxies within a galaxy cluster could conceivably induce phenomenal collisions among these huge material constructs.

Galaxies do indeed collide. A good deal of observational evidence proves that they do so, and quite often. Numerous celestial images show two or more galaxies interacting, some of them tearing each other apart. While, in many photographs, galaxies lie along the same line of sight yet are really far separated in space, others are physically near one another, especially those within the galaxy clusters. Whether galaxies are colliding head-on or only experiencing close encounters cannot often be easily determined, for detectable motions among the distant galaxies typically take millions of years—which is why no human has ever witnessed the full panoply of a galaxy collision, as much as we note its effects virtually frozen in time.

. . . humankind is not at any special or unique location in the gargantuan, perhaps infinite, Universe.

At first thought, collisions among giant galaxies might be expected to create a mind-boggling crunching of matter, complete with spectacular explosions and superlative fireworks. Surprisingly, that doesn’t happen much at all. Such collisions, in fact, are rather quiescent. The stars in each galaxy more or less just glide past one another as the two galaxies slide through each other. That’s because stars themselves hardly ever collide; they are, after all, small objects by cosmic standards. While astronomers have plenty of direct photographic evidence for galaxy collisions, no one has ever witnessed or imaged a collision between two stars—not even in our own Milky Way, which we can see more closely and clearly.

This oddity occurs because galaxies within most clusters are bunched fairly tightly. The distance between adjacent galaxies in a given cluster averages a million light-years, which is only about ten times greater than the size of a typical galaxy. This doesn’t really give them much room to roam around without crashing. By contrast, stars within a galaxy are spread out much more thinly. The average distance between stars in a galaxy is roughly five light-years, millions of times greater than the size of a typical star. Said another way, if our Sun were the size of an apple, its closest neighbors would be some two thousand kilometers away. Hence, stellar collisions are extremely rare within any one galaxy, with the possible exception of their central regions. When two galaxies collide, the population density of stars merely doubles, leaving ample space for the stars to meander without sustaining much damage. To be sure, the interstellar contents, and perhaps the stars as well, of each galaxy are likely rearranged by the tidal forces induced by gravitational interactions, but no spectacular explosions result, even if the collision is head-on.

Evidence of galaxies colliding.

Like majestic ships passing in the night, these two spiral galaxies (called NGC 2207 and IC 2163) are experiencing a close encounter. They might eventually even suffer a head-on collision. Representative of many such galactic collisions seen in deep space, scenes like this one are common in the Universe. In roughly a billion years, after several more interactions, these spiral galaxies will probably merge into a single, colossal, elliptical galaxy. Source: Space Telescope Science Institute.

That’s not to say that the pushing, shoving, and shocking of the interstellar gas doesn’t cause any change. In fact, among the loose gas they wreak relative havoc! Bursts of star formation erupt in interacting galaxies like hurricanes in a pas de deux. In recent years, astronomers have imaged numerous “starburst galaxies,” where the internal gas of colliding galaxies has been disturbed and rearranged enough to trigger sudden episodes of new stars in the disks of both. Additionally, already formed stars appear agitated, oddly orbiting like frenzied moths around a lamp, while other stars seem to be ejected along streamers stretching as much as a hundred thousand light-years from the site of the collision. So, although dramatic fireworks following direct hits among stars are most unlikely, computer simulations do show that the ensuing commotion causes the galaxies to glow about fifty percent brighter for a hundred million years or so owing to their many newborn stars. Later in this Galactic Epoch, we shall address the evolutionary implications of mergers and acquisitions as galaxies collide, mutually attract, and agglomerate.

Completing our inventory of the large-scale spread of matter in space, we naturally pose the next obvious question: Are there even greater assemblies of matter in the Universe, or do galaxy superclusters top the cosmic hierarchy? At present, astronomers are unsure. Some data imply clusterings of galaxy superclusters—or at least nonrandom arrays of galaxies on the largest scales yet observed—but this evidence is shaky and subject to debate. If correct, though, this would mean that our Local Group along with several other galaxy clusters embody a galaxy supercluster centered near the rich Virgo Cluster and that, in turn, these tens of thousands of galaxies form part of an even larger structure on the order of several hundred million light-years in diameter—which is more than a thousand times larger than the size of the Milky Way itself.

What is most clear from the latest cosmic maps of matter on the largest scales probed thus far are the irregularities: galaxies seem to be arranged in networks of filaments, or sheets, surrounded by relatively empty regions of space known as voids. A colossal sponge might be a good visual image, or perhaps an immense bubble bath. The so-called Great Wall, a lengthy arc of several thousand galaxies extending some half-billion light-years across the sky nearly three hundred million light-years away, is the nearest and most prominent of these giant features. An even bigger “wall,” sporting nearly a hundred thousand galaxies about one-and-a-half billion light-years long and about a billion light-years away, is currently the largest known structure in the Universe. The bright galaxies’ locations resemble spider webs or the neural structure of the human brain, whereas the dark voids, often measuring hundreds of millions of light-years wide, are almost completely absent of any galaxies. The most likely interpretation of these maps—the largest ones ever made—is that individual galaxies, and even whole galaxy clusters, are spread across the surface of vast “bubbles” in space. Much like soapy water, the gigantic bubbles ostensibly fill the entire Universe, whereas the voids are the interiors of those bubbles. Furthermore, the galaxies seem distributed like beads on strings only because the observed two-dimensional maps are actually crossectional cuts through the real three-dimensional bubbles. The densest of the galaxy clusters and perhaps the superclusters apparently lie in regions where several bubbles meet—at intersections and nodes of vast cosmic filaments, that is, at some of the great crossroads of the Universe. The observed, “frothy” patterns of galaxies in deep space might be telling us something about our origins, for those patterns are probably traceable to the earliest parts of the Particle Epoch.

All told, individual galaxies contribute little to the large-scale architecture of the Universe as a grand cosmic system—but they are key to unraveling that architecture. Each galaxy is essentially a passenger on an expanding, foamy framework, much like humans who have little bearing on the overall tectonics of Earth yet ride along with the drifting continents. On the other hand, galaxies can be used to probe the framework of the Universe, in much the same way that geologists probe the structure of Earth. Metaphorically, galaxies resemble billiard balls whose motions can help determine the size and shape of a playing table, or, better yet, golf balls that can survey the curved topology of a putting green. Cosmologists thereby analyze the radiation emitted by distant galaxies to unravel the very fabric of the Universe, a vitally important endeavor for any full appreciation of the cosmos.

We have reached the limits of telescopic exploration—at least as pertains to the size and scale of organized structures. We have also broached the realm of conjecture at the upper end of those structures. Let’s pause for a moment to recapitulate the mental picture before us: We live on planet Earth, which orbits the Sun. The Sun, in turn, is just one of hundreds of billions of stars in the immense Milky Way. Our Galaxy is moreover only one of many residents of the Local Group, which, in turn, is merely an undistinguished galaxy cluster near the periphery of what might be an even larger galaxy supercluster. And so on, among the filaments, voids, and potentially greater structures in the Universe.

At every level in our inventory, nothing seems special about our Earth, our Sun, our Galaxy, our Local Group. Evidently, mediocrity reigns throughout.

Such is our niche in the Universe.

Astronomers have charted normal galaxies out to several billion light-years. Many galaxylike objects are also known to exist beyond this galaxy horizon, but their fuzziness makes it tough to place them into any of the normal galaxy categories. More importantly, the basic character of many of the most distant objects differs from those nearby. By and large, objects more than several billion light-years away are more “active,” to a certain extent more violent. Overall, the radiative powers of the active galaxies are much greater than those of the nearby spirals and ellipticals. Furthermore, the active galaxies emit copious amounts of different kinds of radiation—for example, X rays from the interior cores of those galaxies and radio waves from the environments well beyond their cores.

The adjective “normal,” used to describe the elliptical, spiral, and irregular galaxies, conveys that those objects radiate the accumulated light of large numbers of stars. Much of their emitted energy is of the visible type, supplemented by lesser amounts of radio, infrared, ultraviolet, or X-ray radiation. That’s because stars, too, emit mostly in the visible part of the spectrum. But this is not true for the active galaxies. Some of them are completely invisible to us, undetectable with even the world’s largest optical telescopes. Their presence is sensed and studied by radio and infrared telescopes on Earth and by orbiting satellites capable of capturing higher-energy photons. Radiation from the active galaxies, then, is largely inconsistent with the summed emission of myriad individual stars. To be blunt about it, astrophysicists are unsure if active galaxies really have many stars.

The abnormal power and odd character of these mostly distant astronomical objects imply that the Universe was once more robust than it is today. They confirm the idea, described in the previous Particle Epoch, that the first few billion years of the Universe must have been a tumultuous period, quite unlike the more tranquil state surrounding us now in space and time. Since physical conditions were undoubtedly different in the earlier Universe—and recalling that probing great distances in space equals searching far back in time—it shouldn’t surprise us that remote objects seen in their youth differ from nearby, older ones. What is enigmatic—in fact, downright astounding—is the enormous amount of energy radiated by some of the most powerful active galaxies. Their total release of energy often stretches astrophysical understanding to its limits.

To gain some perspective, an average star such as our Sun emits in any one second the equivalent of about a billion-megaton nuclear bomb—an impressive feat in and of itself. Yet our Galaxy is a hundred billion times more powerful because, after all, it contains that many more stars. By contrast, an active galaxy is generally a hundred to a thousand times more energetic than that. Active galaxies can launch in one second as much radiation as the Sun emits in about a million years.

Now imagine the equivalent of a hundred normal galaxies all packed into the space usually occupied by one. This is the crux of the problem encountered while trying to fathom the monstrously active galaxies. Decades ago, it was fashionable to suggest that these objects were the sites of spectacular galaxy collisions. However, as noted above, computer simulations now show that even such collisions would not produce energy in the amount required nor much explosiveness at all.

The fact that active galaxies often emit more invisible than visible radiation implies that these objects differ fundamentally from normal galaxies. What’s more, some of the active galaxies’ cores are extremely luminous while others sport huge lobes that resemble wings, all of which further exacerbate their many oddities, making them among the hardest objects in the Universe to decipher. Perhaps we shouldn’t even be calling them galaxies.

The gross emission features of some active galaxies can be explained by invoking a distinctly nonstellar mechanism. Called the “synchrotron process” after the laboratory accelerators (sometimes called synchrotrons) used to study subatomic particles, this nonthermal action describes the emission of radiation when charged elementary particles interact with magnetic fields. No stars are involved, nor is any heat per se, hence the term “nonthermal.” The radiation arises simply from fastmoving particles, especially electrons, traveling through magnetized regions of space.

Magnetism presumably pervades all things, not just Earth, the Sun, and the Solar System but also entire galaxies. Although the magnetic forces in typically diffuse galaxies are some millions of times weaker than on Earth, magnetism can still play a significant role, especially when its effects mount across an entire galaxy. For many active galaxies, especially a subclass known as radio galaxies, the emitted radiation arises from a pair of oppositely aligned and hugely extended lobes that often span a million light-years; that’s a single object equal to some ten times the size of our Milky Way, in fact comparable to the Local Group of galaxies. Fortunately, images of a handful of these objects—most notably one of the closest (at three billion light-years!) of the active galaxies, known only by its catalog name of 3C273—also reveal a kind of Rosetta Stone: a jet of high-speed matter fired from the core of the galaxy out into the intergalactic medium, thus “feeding” the extended lobes farther away. The velocity of the outflowing matter in the jets typically measures fifty thousand kilometers per second, or nearly two-tenths of the speed of light, and some of the most energetic ones surpass half of light’s speed. The jets themselves not only point toward the huge lobes from which most of the invisible radiation arises, but, more tellingly, they also point back to the central nucleus where the energy is actually produced.

Laboratory experiments have proved that when charged particles, particularly electrons, are injected into a magnetic field, they spiral around much like the needle of a compass thrown spinning through the air. Magnetism slows the particles, causing some of their kinetic energy to be changed into radiant energy (which is why the process is technically termed “nonthermal bremsstrahlung,” or breaking radiation). The amount of radiation emitted from a single encounter of an electron and magnetism is not terribly large in the laboratory. But in the case of a huge galaxylike object, the radiation can mount fiercely because of vast numbers of electron encounters. Furthermore, the emitted radiation is theorized to be of the radio variety, in accord with what is observed.

That said, the details of the emission mechanism within many active galaxies remain enigmatic, even assuming repeated injections of fast and numerous electrons into the galaxies’ lobes. Although the synchrotron process gives us an inkling of the type of abnormal event responsible for the emission of such intense radio power, active galaxies also display a kind of explosiveness that requires continual acceleration of electrons to speeds close to that of light itself. Moreover, large clumps of plasma are observed and occasionally tracked moving outward, forming the extended lobes so characteristic of many of these active galaxies. The implication is that fast-moving matter is violently ejected in opposite directions by extraordinarily energetic events at the cores of these galaxies.

What might be the source of such great energy? Can any known means explain such outbursts on truly galactic scales? Somewhat ironically, black holes can—or so astronomers think. But before encountering these denizens of Nature, do note that the active galaxies are still not the most energetic objects in the Universe. An additional, extraordinarily luminous class of active astronomical objects has been monitored for several decades now—objects so puzzling that they sometimes seem to defy the currently known laws of physics. These are the innocuous-looking, though inordinately powerful, quasi-stellar sources—quasars for short. Resembling common stars, the quasars’ very great distances mean that they not only rival the energy emission problems of active galaxies; quasars actually exacerbate those problems. Here’s why:

Not only are the quasars the most energetic objects in the known Universe, but their radio and optical radiation is highly variable, often displaying variations from week to week, occasionally from day to day. The implication is straightforward: Galaxy-sized objects could never synchronize their front-to-back emission to produce such rapid and coherent time variations; otherwise, the intensity of those variations would be blurred and not as sharp as observed. Expressed another way, cause-and-effect arguments demand that no object can flicker more quickly than radiation can cross it. Thus, daily variations imply that quasars cannot be much larger than a light-day across, or roughly the diameter of our Solar System. The enormous power of the quasars, ranging from a hundred on up to a million times that of our Milky Way Galaxy, must then arise from a region much smaller than our Milky Way, in fact tiny by cosmic standards. All of which drives us further toward the idea of compact black holes as candidates for the quasars’ central engines.

Quasar emission mechanisms—whatever they really are—must operate, again by comparative cosmic standards, within an almost unbelievably small realm of space, conceivably well less than a single light-year. Try to imagine the equivalent of a hundred or more normal galaxies all packed into a region comparable to the Solar System. That’s an indication of the anomalous state of affairs needed to appreciate the Herculean quasars, certainly among the most baffling objects in all the Universe.

Black holes. Although perhaps best treated in the context of stars in the next Stellar Epoch, the most massive black holes likely arose during the Galactic Epoch, roughly a billion years after the Matter Era began. Observations made during the 1990s imply that black holes reside in the hearts of most galaxies—relatively dormant holes at the cores of normal galaxies and extremely energetic ones powering the active galaxies. So, rather than sidestepping this important issue—a central topic in much of astrophysics today—let’s consider the phenomenon of black holes now.

A black hole is a region containing a huge amount of mass occupying a relatively small volume. It’s not an object per se so much as a hole, and one that’s dark to boot. Such a hole still exerts gravity, to be sure exceptionally strong gravity, great enough to warp spacetime severely in its vicinity. Its two main features—large mass and small size—guarantee an enormously strong gravitational force. Why? Because one-half of the law of gravity states that its force directly relates to the mass in question. The other half dictates that gravity is inversely proportional to the square of the distance over which the mass is spread—the inverse-square law again, as noted in the Particle Epoch. And because the distance term is squared, the gravitational force grows spectacularly when distances separating any two parts of an object decrease, which is exactly what happens for a compressed object like a black hole.

Gravitational theory—either Newton’s or Einstein’s, they both make this prediction—stipulates that when any object having a mass of about three times that of the Sun is no longer countered by an countervailing force (such as heat in a star, or rotation in a cloud), that object will collapse indefinitely, crushing matter to the dimensions of a point. It implodes catastrophically without limit; apparently nothing can stop it.

Can anyone possibly grasp such a seemingly ridiculous phenomenon? How can an entire star (or larger) shrink to the size of an atom (or smaller), while presumably on its way to infinitely small dimensions? Does this make any sense? Well, detailed mathematical studies do predict that, without some agent to compete against gravity, massive objects are expected to instantaneously shrink to singular points of infinitesimal volume—singularities, much as posited in the Particle Epoch regarding the origin of the Universe—which is why some researchers consider black holes as “laboratories” in which to explore aspects of the big bang itself. Strange as these statements may seem, observational evidence mounts daily in good agreement with theory. Black holes apparently really do exist.

Though the messy mathematics needed to understand black holes intimately are beyond the scope of this book, we can still explore a few qualitative aspects of these extremely dense and bizarre regions of space-time. The details are sketchy, precisely because the behavior of matter at extreme densities is not well understood. Magnetism and rotation are also tricky to model for highly compressed objects; the laws of physics here are clearly incomplete. Whoever manages to decipher those details will surely become famous.

Consider first the concept of escape velocity—the speed needed for one object to escape from the gravitational pull of another. For any relatively small piece of matter—a molecule, baseball, rocket, whatever—that velocity is proportional to the square root of an object’s mass divided by its radius. For example, on Earth, with a radius of about sixty-four hundred kilometers (or four thousand miles), the escape velocity equals about eleven kilometers per second (or seven miles per second). To launch anything away from the surface of our planet, it must have a velocity greater than this, explaining why typical speeding bullets, fired at about two kilometers per second, return to Earth’s surface. Also, the Space Shuttle, for example, orbits Earth at a speed of about eight kilometers per second, but the interplanetary probes, such as Voyager that went to Jupiter or Viking to Mars, required a boost to eleven kilometers per second to physically escape Earth’s gravitational pull.

Consider now a hypothetical experiment for which the apparatus is a gigantic, three-dimensional vise. Imagine the vise to be large enough to hold the entire Earth and, as awful as it sounds, for Earth to be squeezed on all sides. As our planet shrinks under the assault, its density rises because the total amount of mass remains constant inside an ever-shrinking volume. Accordingly, the escape velocity increases.

Suppose that our planet is compressed to one-quarter its present size, thus doubling the escape velocity. Anything attempting to escape from this hypothetically compressed Earth would then need a velocity of at least twenty-two kilometers per second. Imagine compressing Earth still more. Squeeze it, for example, by an additional factor of a thousand, making its radius hardly more than a kilometer. Now its escape velocity increases dramatically, to many hundred kilometers per second.

. . . a hypothetical experiment for which the apparatus is a gigantic, three-dimensional vise.

And so it goes: as an object of any mass contracts, the gravitational force grows stronger at its surface, mostly because of increased density. In fact, if this frightful vise were to compact our home planet hard enough to crush it to merely a centimeter across (about half an inch), then the escape velocity would reach three-hundred thousand kilometers per second (or 186,000 miles per second). And this is no ordinary velocity; it’s the velocity of light, the fastest velocity allowed by the laws of physics as we now know them.

So if, by some fantastic means, the entire planet Earth could be shrunk to the size of a pea, then its escape velocity would have to exceed the velocity of light. And since that’s impossible, the compelling conclusion is that nothing—absolutely nothing—could get away from the surface of such a compressed “Earth.” There is simply no way to launch away a rocket, a beam of light, or anything whatsoever. Furthermore, no exchange of information would be permitted with such an astronomical object. It would have become invisible and uncommunicative, making the origin of the term “black hole” clear. For all practical purposes, such an ultra-compact object has disappeared from the Universe!

The above example is of course hypothetical. It’s likely (and fortunate) that no such cosmic vise exists that is capable of squeezing the entire Earth to centimeter dimensions. But in massive stars and galaxies, such a vise does in fact exist—the force of gravity. The relentless pull of gravity is truly strong enough to compress dead stars and galactic cores to extraordinarily small dimensions. The gravitational force in massively compact objects is not at all hypothetical; it’s real.

Gravity cannot crush Earth in this way because our planet simply lacks enough mass. The collective gravitational pull of every part of Earth on all other parts of Earth is not powerful enough. However, as we shall see in the next Stellar Epoch, when the nuclear fires have ceased at the end of a star’s life, gravity can literally crush a star on all sides, thereby packing a vast amount of matter into a very small sphere.

When stars have more than several solar masses, the critical size at which the escape velocity equals that of light is not, as for Earth, of centimeter dimensions. For typically massive stellar core remnants, this critical size is comparable to kilometers. For example, a ten-solar-mass star would have a critical size of about thirty kilometers. This critical size below which astronomical objects are predicted to disappear is given a special name. Astronomers call it the “event horizon,” a region within which no event can ever be seen, heard, or known by anyone outside. Accordingly, the event horizons of Earth and of a ten-solar-mass star equal one centimeter and thirty kilometers, respectively.

We might then claim that magicians really could make coins and rabbits disappear provided they squeezed their hands hard enough. Even people could disappear if compressed to a size smaller than 10^–23 centimeter! In English units, that’s a trillionth of a trillionth of an inch. Gravity won’t naturally do it to us, though, again because we are just not massive enough. The collective gravitational pull of all the atoms in our bodies falls far short of the force needed to compact us to this minuscule size. Nor does any technological device presently known come close to doing so.

The important point here is the following: Should no force, or counteracting agent of some sort, be capable of withstanding the self-gravity of a celestial object having several solar masses or more, then such a hulk will naturally collapse of its own accord to an ever-diminishing size. Theory demands that the infall of such a massive object will not even stop at its event horizon. An event horizon is not a physical boundary of any type, just a communications barrier. The massive object shrinks right on past it to smaller sizes, presumably on its way toward becoming an infinitely small point—singularity again. We say “presumably” because physicists are unsure if any undiscovered forces can halt the catastrophic collapse somewhere between the event horizon and the point of singularity. This, again, is the realm of the as-yet-unconceived subject of quantum gravity, the holy grail of the previous Particle Epoch.

Black holes are products largely of Einstein’s relativity theory, although a logical extrapolation of Newton’s law of gravity does permit their existence. Whereas the Newtonian theory of gravity describes many other odd phenomena in the Universe, only the Einsteinian theory of space-time can properly account for the truly bizarre properties of black holes where matter becomes extraordinarily dense. Of particular interest, and to make a connection with the spacetime concepts of the prologue, the mass contained within a black hole is expected to warp greatly both space and time in its vicinity. Close to the hole, the gravitational force becomes overwhelming and the curvature of spacetime extreme. At the event horizon itself, the curvature is so severe that spacetime folds over onto itself, causing objects within it to become trapped and disappear.

Several props can help us visualize the curvature of spacetime near a black hole. Each way is, however, only an analogy. The problem here, as earlier in the case of the whole Universe, is our inability to deal conceptually with four dimensions. Here’s one such fanciful analogy designed to elucidate the formation of black holes and the spacetime warp caused by them:

Imagine a large group of people living on an enormous rubber sheet—a gigantic trampoline of sorts. Deciding to hold a reunion, all except one person converge on a given location at a given time. Their reunion is to be an event in spacetime. The one person remaining behind can still keep in touch by means of “message balls” rolled out to him along the rubber sheet. These balls are the analogue of radiation traveling at the velocity of light, while the rubber sheet mimics the fabric of spacetime itself.

As the people converge, the rubber sheet sags under their growing weight. Their accumulating mass in a small place creates an increasingly large spacetime curvature. The message balls can still reach the lone person residing far away in nearly flat spacetime, but they arrive less frequently as the sheet becomes progressively more warped.

Finally, when enough people have gathered at the appointed spot, their mass becomes too great for the rubber to support. The sheet breaks and compresses them into a bubble, sending them into oblivion and severing communications with the lone survivor outside. Regardless of the speed of the last message ball, it cannot quite outrun the downward-stretching sheet.

Analogously, a black hole is theorized to warp spacetime completely around on itself, thereby isolating it from the rest of the Universe.

. . . a black hole is theorized to warp spacetime completely around on itself, thereby isolating it from the rest of the Universe.

Two important caveats pertain to black holes. The first is that they’re not cosmic vacuum cleaners; they don’t cruise around interstellar space, sucking up everything in sight. The movements of objects near black holes mimic those of any object near a region of concentrated mass. The only difference is that, in the case of a black hole, objects skirt or orbit about a dark, invisible region, where nothing at all can be seen. Neither emitted nor reflected radiation of any sort emanates from the position of the black hole itself.

Black holes, then, don’t go out of their way to drag in matter, but if some matter does happen to infall via the normal pull of gravity, it will be unable to get out. Black holes are like turnstiles, permitting matter to flow in only one direction—inward. Swallowing matter, they constantly increase their mass as well as their event horizons, for the region of invisibility also depends on the amount of mass trapped inside. Those black holes that really do exist in space are probably enlarging their mass and size, some more than others, all of them apparently gulping, eating, growing.

A whirlpool is an apt analogy for the grip that black holes have on matter. Whirlpools of water, for example, tend to have attractive affects on nearby fish. Since the speed of the water is greater closer to the center of the whirlpool, fish entering an area where the water speed is faster than the fish can swim will be sucked inward. Those closest in will never make it out.

Another notable point is that strong gravity near black holes causes great tidal stress. An unfortunate person, falling feet first into a black hole, would find himself stretched enormously in height, all the while being squeezed laterally. He would, moreover, be literally torn apart, for the pull of gravity would be stronger at his feet than at his head. He wouldn’t stay in one piece for more than a fraction of a second after passing the event horizon. Similar distortion and breakup apply to any kind of matter near a black hole. Whatever falls in—gas, people, robots, whatever—is vertically stretched and horizontally compressed in the process of being accelerated to high speeds. The upshot is numerous and violent collisions among the torn-up debris, causing much heating of the matter that plunges into the hole.

This rapid destruction of infalling matter by tides and collisions is so efficient that, prior to submersion below the hole’s event horizon, even matter outside a black hole can be effectively converted to heat energy while falling inward. Although radiation ceases to be detectable once the hot matter dips below the event horizon, regions just outside black holes are expected to be energetically emitting, mostly in the X-ray part of the spectrum since the matter is so hot. To distant observers, contrary to popular belief, black holes can then appear as bright points of radiation and prodigious sources of energy.

With this in mind, and being only partly facetious, perhaps black-hole research may eventually result in practical applications after all. Through some marvel of technology, our descendants might someday learn how to compact garbage to an almost incredibly small size—after which it would disappear! Not only that, the crushed garbage would emit copious amounts of energy in return. Maybe black holes are just what the doctor ordered for technological civilizations long on pollution and short on energy. An ability to tap this energy safely may be a major milestone in the history of any long-lived civilization.

Of much interest is the obvious question, What lies within the event horizon of a black hole? What’s it like deep inside? The answers are simple: No one knows.

Some researchers maintain that the inner workings of black holes are irrelevant. In situ observations could conceivably be done by robots sent “down under” to test the nature of space and time beneath the event horizon, but that information could never reach the rest of us outside. Apparently, no theory offered to explain the hidden recesses of black holes could ever be put to the experimental test. Anyone’s guess seems as valid as anyone else’s. Perhaps the inner sanctums of black holes then represent the ultimate in the unknowable. For that very reason, though, other researchers argue that it’s of utmost importance to unravel the nature of black holes, lest we someday begin to worship them. Sounds ridiculous, but whole segments of humankind have often revered the unknowable, venerating that which cannot be tested experimentally. Come to think of it, many still do in twenty-first-century society.

What sense are we to make of black holes? The basis for these outlandish objects is the relativistic concept that mass curves spacetime—an admittedly weird phenomenon, yet one that has been partially tested locally in our Solar System. The larger the mass concentration, the greater the warp, and thus the stranger the observational consequences. Perhaps. Some theorists are convinced that relativity is incorrect, or at least incomplete, when applied to black holes. It does seem nonsensical to claim that very massive astronomical objects will collapse catastrophically to infinitely small points. Not even our wildest imaginations can visualize such phenomena; science-fiction stories fall short, mathematicians are baffled. Maybe the current laws of physics are inadequate in the vicinity of a singularity; precisely at the point of singularity, general relativity is probably absurd. On the other hand, perhaps matter trapped in black holes never does compress all the way down to that mathematically arcane singularity. Perhaps matter just approaches this most bizarre state in all of science, in which case relativity theory may still hold true.

This is where in many accounts, even by leading scientists, writers often launch into discussions of parallel universes, multiple universes, hyperspace, warp drive, worm holes, time travel, other dimensions, and a host of other “possibilities,” both remote and fanciful. But these and other like-minded speculations are not within the scope of this book. Here, we strive to stay on reasonably solid ground, appealing to empirical findings and acquired data while admitting our ignorance wherever it lay. And when it comes to the secluded sanctorum of black holes, the honest answer is that scientists just don’t know what to make of them—nor will we likely ever learn much until the frontier subject of quantum gravity is realized and mastered.

Despite their freakishness, black holes do seemingly populate Nature. In addition to the “smallish,” stellar black-hole candidates best assessed in the next Stellar Epoch, many astronomers contend that the much larger galaxies display convincing evidence for black holes. Particularly intriguing are the centers of galaxies, including the core of our own Milky Way, some thirty thousand light-years from Earth. Our Galaxy’s midsection should provide us with a stunning view, given that it’s teeming with so many billions of densely packed stars. But we don’t see its brilliance because its midst is completely obscured by dust, denying studies with optical telescopes; even the largest such devices can visually see only about a tenth of the way toward the galactic center. Fortunately, longer-wavelength, radio and infrared observations are possible, enabling us to probe more deeply into the heart of the Galaxy (much like radar cuts through thick fog on Earth). And what was found in the innermost few hundred light-years initially yielded spectacularly unexpected results; now, in retrospect some two decades later, the findings seem typical of the black holes probably lurking in the hearts of galaxies everywhere.

At the Milky Way’s core, infrared sensing shows thousands of stars swarming per cubic light year—a stellar density more than a million times greater than in our solar neighborhood. Giant nebulae tens of light-years across, rich in gas and embedded among even bigger clouds loaded with dust, reside in a ringlike structure more than a thousand light-years across, the whole formation housing some tens of thousands of solar masses and rotating at the fast clip of a hundred kilometers per second (or more than two hundred thousand miles per hour). And in the center of the ring is an intense radio source—the dynamical nucleus of our Galaxy.

On even smaller scales, high-resolution radio maps show an inner ring of gas less than ten light-years across, rotating even more rapidly (at more than a thousand kilometers per second) and resembling a colossal whirlpool at the very center of our Galaxy. This remarkable realm, quite unlike anything near Earth, has been closely monitored ever since it was first found some twenty years ago, including recent outbursts at X-ray wavelengths that imply the presence of a spinning, white-hot accretion disk of million-degree-Celsius gas right in the middle of it all. Magical and mysterious, yet not mystical, the enshrouded nature of this most perplexing piece of galactic real estate so far and foreign is slowly being deciphered.

Frustrated late one evening at the Harvard Observatory, some colleagues wandered to Cambridge Common, where we perched ourselves on a bench near the edge of the park. Straining to fathom the locations of crosswalks, benches, and trees, we gained some insight into the problem of trying to map the Milky Way while stuck inside it. Barring ourselves from walking, bicycling, or otherwise sauntering about, we soon discovered that charting the park’s layout is no easy task. Any resulting map would likely be subject to distortion, obscuration, and incompleteness. Statues and signposts—and especially the grand monument near the common’s center—seemed especially strange and intriguing from a distance, for they resembled none of the familiar shrubs and benches near the outer part of the park. And so it is with our Milky Way Galaxy. Relegated perhaps forever to the galactic boondocks, we strain to unravel the spread of stars, gas, and dust in that part of the Universe we call home.

Models capable of accounting for most of the galactic-center observations to date stipulate that a rapidly rotating halo of thin, hot, ionized matter surrounds a furiously spinning vortex of even hotter, denser matter. This swirling mess of stars, gas, and dust is apparently orbiting—and here’s the punch line—a tremendously compact object housing a few million solar masses, all packed into a region comparable to our Solar System. Such an enormously massive and compact blob is needed to give the maelstrom some structural integrity—to prevent the whirlpool of gas from dispersing into the outer regions of the Galaxy. Fast rotations doubtless produce strong centrifugal forces and, unless a huge mass were gravitationally pulling back, the gas would be flung away like mud from the edge of the spinning bicycle wheel. The implication that millions of stars are compressed to planetary-system dimensions follows from simple, well-understood physics, even if the result borders on surrealism.

Though the details are controversial, a consensus now seems at hand that a supermassive, ultracompact “something” resides at the hub of our Milky Way Galaxy. As best we can tell, that something can be only one thing—a black hole. Not to worry, the hole currently seems rather quiescent, if not dormant, and in any case is more than two billion times farther from Earth than is the Sun.

Our Milky Way isn’t alone in having a troublesome core. Recent observations imply the presence of supermassive objects in or near the middle of many other galaxies. The evidence here is much the same as for our own Galaxy, with gas and stars in the innermost regions of several normal galaxies, including perhaps nearby Andromeda, observed to be rapidly whirling—apparently, again, centered on black holes of millions of solar masses. And although the active galaxies cannot be seen as well owing to their greater distances, observations of them also suggest that highly compact regions lurk in their hearts, usually housing even more gyrating matter than at the cores of the normal galaxies. Astonishingly, for some of the most active galaxies, several billion—not million, but billion—solar masses are implied, all within a region less than a few light-years across. Perhaps these central whirlpools are remnants of the primordial eddies that gave rise to the galaxies, as noted below.

Astronomers now sense that the center of virtually every galaxy is inhabited by a supermassive black hole. Normal galaxies such as our own probably have relatively small holes of “mere” millions of solar masses, most of them, as in our Galaxy, now relatively inactive for lack of fuel. Those galaxies considered more active have larger black holes, often on the order of billions of solar masses, as betrayed by their more intense radiation. It is this enhanced emission of energy that makes them “active,” largely because we see many of the distant, active galaxies in their youth, when fuel was more plentiful.

As antic as this scenario seemed when first proposed some twenty years ago, astrophysicists now generally agree that the great energetics of the active galaxies are naturally explained by matter perishing within the clutches of supermassive black holes. Thus, we discern one of the greatest paradoxes in science, as forewarned earlier in this section: black holes trigger some of the brightest objects in the Universe—all of it caused by matter being gobbled, distorted, accelerated, and heated before disappearing below their event horizons. How the enigmatic jets perpendicular to a black hole’s accretion disk manage to launch away matter despite the powerful gravity of the hole, however, remains one big puzzle.

Not inconceivably, the most energetic objects in the Universe—the innocuous-looking yet powerful quasars—might be ruled by hyper-massive black holes that regularly consume whole stars. Roughly ten stars devoured per year would do it for typical quasars; a thousand stars per year, or therefore a few per day, would be needed to explain the brightest of them. If true, then black holes in quasars might be even more massive, more compact, and more abnormal than the billion-solar-mass objects implied for the active galaxies. This idea, however hard to swallow intellectually—since it’s so foreign compared to the more mundane events near us in space and time—can seemingly explain most quasar observations. It also has the added advantage of resembling the process thought to power smaller-scale yet still violent regions, such as normal galactic centers and stellar X-ray sources within galaxies, implying that unifying principles may be at work on many scales in Nature.

Evidence of a supermassive black hole.

At left is a combination of an optical photo and a radio image of a giant elliptical galaxy (called NGC 4261). Its visible part is the blob at center, whereas the invisible radio-emitting lobes at top and bottom extend hundreds of thousands of light-years beyond. At right is a close-up photo of the galaxy’s core, revealing a whirling disk of hot gas surrounding a bright hub that likely harbors a black hole containing several million times the mass of our Sun. Source: National Radio Astronomy Observatory/Space Telescope Science Institute.

Clearly, a complete understanding of the powerhouse galaxies lies partly buried deep in their cores, awaiting future explorers to discover, unravel, and share their secrets. There, their central engines are both the instigators of change and the recipients of change; again akin to biological events broadly considered, black holes drive change and adapt to it. Yet the timescales for noticeable change differ so markedly—in biology on the order of thousands to millions of years for species change, in astronomy easily millions to billions of years for architectural change. Astrophysicists are still learning to decipher the clues hidden within invisible radiation emitted by alien environments near hugely massive and totally invisible black holes. We are only beginning to appreciate the full magnitude of these strange new realms deep in the hearts of galaxies.

Some final words of caution regarding black holes, large and small: Forces may yet be discovered capable of withstanding the relentless pull of gravity, even that near exceedingly massive and compact astronomical objects. Magnetism and rotation have not yet been fully incorporated into black-hole theory, and no one knows what to expect regarding the behavior of gravity on deeply submicroscopic, quantum scales. Massive clusters of dark stars and ultradense pools of elementary particles have been proposed as alternatives to black holes, as have queer and inventive groupings of more exotic kinds of dark matter. That these are all terribly hard problems to solve is an understatement, so much so that some of the best minds confess ignorance as to how to go about even attacking them. Serious research regarding realistic models of black holes is only beginning at many observatories around the world.

Skepticism is healthy in science. Unless astrophysicists can find direct, or compellingly indirect, evidence for the existence of black holes, neither of which is currently at hand, then the whole concept of black holes may well turn out to be no more than a whim of human fantasy—another case of mathematics gone awry without the check and balance of tested physical law. The nature of matter, energy, space, and time deep down inside event horizons may be no more significant than a challenging and amusing academic problem devoid of reality.

On the other hand, the Universe did emerge from what seems to have been a naked singularity some fourteen billion years ago. Of all the amassments now known or suspected to be part of our cosmic inventory, black-hole singularities might just be the keys needed to unlock an understanding of the creation state from which the Universe arose. By theoretically studying the nature of black holes, and especially by observationally probing their physical behavior, we shall perhaps someday be in a better position to address the most fundamental problem of all—the origin of the Universe itself.

Regardless of how galaxies populate space or how they emit their radiation, an even more basic question comes to mind: Where did the galaxies come from? How did the grandest of material structures arise from an early Universe comprising a uniform mixture of hot matter and intense radiation? Do galaxies form by engorging already-made stars, or do stars gestate in already-made galaxies? Which came first, stars or galaxies? Not least, how do galaxies evolve, once formed?

Fortunately, we can address these and other questions pertaining to the Matter Era with more assurance than the rather uncertain events of the Radiation Era previously described in the Particle Epoch. Even here, though, substantial puzzles remain about the details of the galaxy-formation process. Astrophysicists are now tackling the issue of galaxy origins and have identified its main problems, but they have not yet solved them.

Lack of good observational knowledge of the galaxies themselves creates the basic enigma. Galaxies can be classified according to their gross morphology and their energy budgets, as done above. But we have thus far no explanation for the observed properties of all the galaxies in terms of, for example, the simple gas laws that describe our rather detailed knowledge of stars, a topic of focus next in the Stellar Epoch. Not surprisingly, it’s hard to fathom how galaxies emerge and change when we don’t quite know what they are.

When put under bright lights and interrogated, astronomers admit they know only a few grand and mutual facts about galaxies. Together, these common denominators are helping us begin to understand the events that produced these most majestic of all objects in the Universe.

First, the galaxies are out there. That’s a telling datum, for we do know that the galaxies exist. And our civilization should be mightily proud of that fact; no other life form on Earth knows, or ever has known, of the presence of the galaxies. Yet their mere existence doesn’t help us much to decipher their origins. Given the galaxies’ expanse and magnificence—in vast numbers, in any direction, as far as our best telescopes can see—we are left perplexed and wondering: Just how did these awesome structures come into being?

Second, there are now no young galaxies. Said another way, no galaxies seem to be originating at the present time. Some may be still growing and developing as they accrete more matter, but none seems to have emerged within the past ten billion years or so. Since all normal galaxies contain some old stars, and since most active galaxies are far away in space (and thus in time), the bulk of the observed galaxies must have come forth long ago. Whatever the seminal mechanism, it was surely widespread in the early parts of the Matter Era. But if the galaxies originated so prolifically in the younger Universe, then why aren’t they still doing so now?

Yet another common factor derives from the finding that most galaxies house comparable amounts of matter. The capacity of virtually every individual galaxy thus far measured ranges between a billion and a trillion solar masses. Normal galaxies appear to have about that many stars and, as best can be determined, active galaxies also include roughly this much matter in some form. No known galaxy is much smaller and none much larger. They all seem to average a hundred billion stars, or their equivalent, much like our own Milky Way Galaxy, give or take a factor of ten for giant ellipticals or dwarf irregulars. Why should Nature’s grandest intact assemblies have such a narrow range of sizes? What precludes the construction of galaxies containing, for instance, a quadrillion stars?

To summarize, reiterate, and clarify: There is no evidence that galaxies are originating at the present time, nor have any done so within the past many billions of years. Galaxies do seem to be evolving currently, as noted toward the end of this Galactic Epoch, including additional growth as new matter occasionally falls into already established galaxies. But if new galaxies were emerging only now, astronomers should have spotted some objects having sizes and morphologies somewhere between well-defined galaxies and sheer empty space. We know of no such nearby, amorphous, “half-baked” objects. Furthermore, the regions beyond the galaxy clusters—the intergalactic voids—don’t seem to contain much matter, if any at all. Whenever and however the galaxies did form, they apparently did so very efficiently, sweeping up almost all the (normal) matter available and leaving little behind for further assembly.

What’s more, key theoretical ideas presented in the next Stellar Epoch strongly suggest that stars ought to be forming now within galaxies. The bulk of most galaxies most likely formed first, yielding environmental conditions ripe for the later formation of the stars we now see richly populating galaxies. These ideas have been handsomely verified in the past couple of decades by splendid observations of widespread locations throughout our Milky Way, where stars are known to be originating slowly but surely from the galactic hodgepodge of loose gas and dust. Recent stellar census implies that roughly ten new stars now form in our Galaxy each year.

To address the issue of galaxy formation, imagine a giant cloud of hydrogen and helium atoms embedded in a weakening sea of radiation, some hundreds of millions of years after the big bang. This giant cloud should not be regarded as filling the entire Universe; rather, think of only a small sector of the cosmos, yet one still millions of light-years across. Although universal expansion continued apace, such a huge clump of mass would not have indefinitely expanded; local gravity had slowed the cloud to a maximum size, after which it began to fall back on itself. The cosmic temperature and density had dropped greatly since the onset of the Matter Era. Radiation was no longer sufficiently intense to shatter atomic matter, as fully formed hydrogen and helium atoms were then numerous enough to exert a collective influence of their own. Electromagnetic and nuclear forces bound elementary particles within atoms, while gravity in turn bound the atoms within the giant cloud. All the known forces that now direct the evolution of matter were already operating well enough to grant the cloud some structural integrity of its own. Vast parcels of matter were becoming distinguishable from one another, each isolated in a fragmenting cosmos, a state of affairs strongly contrasting with the uniform mixing and chaotic violence of the earlier Radiation Era.

Despite its growing stability locally and its steady recession globally, the initially homogeneous cloud would have surely experienced occasional fluctuations—small irregularities in the gas density that came and went at random. No cloud, whether a terrestrial fluffy cloud in Earth’s atmosphere, a tenuous interstellar cloud in our Milky Way, or the primordial cloud visualized here in the young and formative Universe, can remain completely homogeneous indefinitely. Eventually, as one atom somewhere in the cloud accidentally moved closer to another, that part of the cloud became just a little denser than the rest. The atoms might have then separated, dispersing this density fluctuation, or they might have acted together to attract a third atom to enhance it. In this way, small pockets of gas arose anywhere in the cloud simply by virtue of random atomic motions. Each pocket of enhanced matter was a temporary condensation in an otherwise rarefied medium. The whole process is not unlike the billowing clouds of a terrestrial thunderstorm, collecting, growing, and eventually forming condensation nuclei that give rise to rain.

Vast parcels of matter were becoming distinguishable from one another . . .

Provided some density fluctuations further developed by gravitationally attracting many more atoms, they could have conceivably grown into clumps of matter that became the seeds of galaxies. Theoretical calculations support the idea that such chancy gas fluctuations could have been the forerunners—protogalaxies—of today’s galaxies. But—and this is an important but—these same calculations suggest that, given the slow rate of chance encounters, the protogalaxies would only just be forming at the present time. Yet, as noted, astronomers have no evidence whatever that galaxies are now orginating; we have found few, if any regions caught in the act midway between full-fledged galaxies and intergalactic nothingness.

Extremely long times—typically several tens of billions of years—are needed for enough randomly moving atoms to coalesce into a large pocket of gas that can be rightfully called a galaxy. This lengthy duration is not surprising given the absolutely gargantuan quantity of atoms in a typical galaxy—namely, nearly a million billion billion billion billion billion billion billion atoms. How do we know that? Well, each galaxy houses about a hundred billion stars, each star averages a million billion billion billion grams, and each gram has a million billion billion atoms, all of which adds up to a very big number. That’s why astronomers prefer scientific notation, in which case the number is some 10⁶⁸ atoms—clearly an awful lot of atoms to collect regardless of the notation used. Consequently, it takes a great while for Nature to do it at random.

But—and this is an even bigger but—no scientist ever said that galaxies were built by random events, by chance and chance alone. Some philosophers of science or historians of science or others who, like postmodernists, tend to criticize the methodology of science yet have never practiced science themselves, have occasionally made such claims to champion the cause of pure chance. By contrast, few natural scientists have ever argued that chance and only chance plays a role in any physical phenomenon. Rather, Nature almost surely operates by combining chance with necessity, randomness with determinism—a basic, unifying issue to which we shall return several times in this book, especially when describing the origin and evolution of life in later epochs.

A time of several tens of billions of years is of course well longer than the current age of the Universe—meaning that no galaxies should now exist. So, despite the likelihood that random density fluctuations in an otherwise homogeneous gas could have eventually produced galaxies, it’s unlikely that the galaxies we now see emerged strictly in this way. Chance cannot be the sole factor governing the origin of these truly immense cosmic systems. Still, the idea of naturally arising spots of different gas densities remains a powerful concept, for it’s a reasonably well understood process not requiring any unknown forces or unique conditions. If we could find an agent or mechanism, some means or another, that would accelerate the growth of the gargantuan number of atoms needed to form a galaxy, then we might begin to understand their origins.

To clarify the oft-misunderstood issue of chance versus necessity: Chance surely does play some role in galaxy formation, especially as the initial trigger that starts the fragmentation of primordial clouds. Other, more deterministic agents in the early Matter Era, such as turbulence or shocks, likely enhanced the growth of the inhomogeneities so that myriad galaxies could have formed in a timescale shorter than the age of the Universe. Or, perhaps the seeds of the galaxies were sown at the quantum level much earlier, in the chaotic events of the Radiation Era as proposed below. Whatever it was and however it worked, the enhancement process must have been surprisingly effective since observations clearly imply that the bulk of virtually all galaxies formed long ago, apparently within the first few billion years after the big bang.

The problem of galaxy formation is currently a tough one for astrophysicists. Its solution has exasperated many brilliant minds and is still not yet in hand. The origin of galaxies appeals to theorists with fertile imaginations (to visualize conditions so long ago) and computing skills (to keep track of all those atoms), and especially to those willing to make unorthodox assumptions. This is one of the trickiest areas of astronomy to appreciate, for few hard facts are known about galaxies, and fewer still about the physical events that formed them long ago.

One hard fact that is clearly known, however, is the first one noted above: galaxies do exist. They populate the Universe in great abundance. Somehow they came into being, and somehow they got to be where and when we find them now in space and time. Let’s consider in greater detail some of the specific galaxy formation scenarios recently proposed by theoreticians.

Astrophysicists today seek to identify ways that random gas fluctuations might have been enhanced earlier in the Universe. If some factor could be found that might have speeded the growth of the density irregularities, the galaxy-formation problem might be solvable. One such possibility assumes that the Universe was quite turbulent long ago—a not altogether unreasonable idea since turbulence involves the inevitable “confusion” or disordered motion of matter (the gas) within a rapidly moving medium (space itself).

Once the Matter Era dawned, all the atoms within the vast primordial clouds were set into motion not only from the expulsion of the bang but also from the heat of the fireball. The gas then had some “directed” kinetic energy—outward, from the ordered expansion of the Universe. It also had some “undirected” thermal energy—random, from the disordered aftermath of the blazing inferno. Intact pockets of gas undoubtedly surged this way and that, whirling here and shearing there amid collisions with one other, in addition to the disarrayed agitation of each of the individual atoms. In particular, turbulence probably aided the growth of spinning eddies at those places where density fluctuations had already become established in the early Universe.

Turbulent eddies of this sort can be visualized by watching water swirl down the drain of a bathtub. In a sense, the swirling eddies them-selves are turbulence. Even better examples can be created by moving your hand gently through water, or a teaspoon through coffee; swirling eddies naturally emerge in the wake of this turbulence. Water flowing past rocks in a stream also gives an appreciation for the whirlpools that naturally arise in its aftermath.

Probably the best examples of the effects of turbulence are the fluffy clouds of Earth’s atmosphere. Especially vivid in photographs of the tops of clouds, taken with Earth-orbiting satellites, kilometer-sized eddies can be seen as density enhancements of the atmospheric gas. Such whirling eddies are known to become more pronounced whenever air currents are particularly turbulent. Should they grow, in this case by accumulating additional amounts of moisture, the eddies may well form stormy depressions and occasionally even hurricanes hundreds of kilometers across.

Here is a case, then, where studies of a terrestrial phenomenon—Earth’s weather—may help us understand one of today’s most vexing extraterrestrial problems. Planetary hurricanes roughly mimic the overall morphology, the pancake shape, the differential rotation, and the disposition of energy within spiral galaxies. Though these two systems are entirely unrelated and of vastly different sizes, their many resemblances might teach us something about galaxy formation via the study of hurricane formation. In particular, since most meteorologists agree that some sort of turbulent “priming” is needed to trigger hurricanes, the early stages of such storms could conceivably be used by astronomers to extract clues about the turbulence-enhanced density fluctuations that gave rise to protogalaxies long ago.

It’s worth pursuing this idea a little further. Despite the inevitable cooling caused by the expansion of the Universe, each localized eddy within a large gas cloud must begin to heat. It can’t avoid it. Eddies are sites not only of turbulence but also of rising heat within a steadily cooling cloud. The heat results from friction caused by frequent collisions among the increasingly dense collections of atoms within each eddy. The process is a simple one, not unlike the heat derived by rubbing our hands together on a cold wintry day.

Eventually, individual eddies must rid themselves of some of this newly acquired energy, much as the Sun or any other heated object needs to unload energy, lest it blow up. The eddies in the protogalactic cloud did it by radiating away some of their heat. In this way, a large cloud containing lots of eddies can cool even faster than would the nor-mal, homogenous clouds of the expanding Universe. As it cools, the entire cloud contracts a little, thereby increasing the density and hence the heat within each eddy. Both the individual eddies and the whole cloud simultaneously radiate some of this newly gained energy into space, thereby allowing further contraction of the parent cloud and its smaller eddies. On and on, this cycle of contracting, heating, radiating, cooling, and contracting proceeds—all fundamentally driven by gravity. The cycle may operate at different speeds for each eddy, particularly since some eddies will be more successful than others at sweeping up additional gas from the parent cloud.

It’s easy to conceptualize a cluster of galaxies forming in this way, with each eddy becoming a member galaxy within that cluster. Alternatively, perhaps only one or a few galaxies formed within each of the vast primordial clouds of the early Matter Era, after which gravity gradually swept the galaxies into the very much larger galaxy clusters now seen scattered throughout the Universe. Either way, fragmentation models of this sort resemble a “top-down” approach to galaxy birth whereby huge clouds give rise to litters of young galaxies—a process known in the trade as “monolithic collapse.”

As nice as this galaxy-formation scenario seems, it, too, runs into some serious problems once mathematics are applied to it. Calculations show that timing is once again an issue but not, as above, because the eddies take too long to form. Here, it’s more a case of competing timescales between physical events affecting the eddies: the time needed for capture and contraction of the gas in a turbulent eddy is longer than the typical time for the random dissipation of that eddy. In other words, eddies tend to break up long before they have a real chance to bind tightly. Turbulent eddies do enhance the random gas fluctuations, but they don’t last long enough to form galaxies.

Any kind of eddy, then—in the bathtub or in the early Universe—comes and goes in iffy sorts of ways, all governed by the laws of statistical physics. Eddies appear, disappear, and reappear at different parts of either a terrestrial atmospheric cloud of moist air or an extraterrestrial galactic cloud of primordial gas. Occasionally, a terrestrial eddy does indeed grow to form a flourishing hurricane, or a primordial eddy, presumably a genuine galaxy. But the expected rarity of their rapid growth implies that turbulent eddies cannot be the sole solution to the problem of the ancient formation of the galaxies or of galaxylike objects.

Mainstream astrophysicists prefer to avoid radical theories of galaxy formation—such as a weird one postulating the ballooning of compact, primordial blobs (called by some “white holes”) for which there’s no evidence whatever. They head back to first principles and embrace once again the basic notion of random gas fluctuations developing into something bigger—a “bottom-up” approach that groups smaller chunks of matter to build galaxies. Still, some additional means must be found to speed the growth of such fluctuations in the gas-radiation mix of the early cosmic fireball. Current research therefore centers on other ways that might have enhanced, or accelerated, the growth of simple gas fluctuations.

The general scenario now favored by the astronomical community—an idea known as hierarchical clustering—postulates an early Universe that was not homogeneous. Instead, it’s imagined to have been peppered, even in the Particle Epoch, with minute density clumps. In other words, the eddies got a head start even in the Radiation Era and thereafter acted as seeds for the growth of galaxies early in the Matter Era. These already-formed pockets of gas would then have developed during the Galactic Epoch to fabricate at least the essential features of galaxies seen today. Although this idea initially sounded like a cop-out to many astronomers, observational evidence for these truly primeval inhomogeneities was marvelously confirmed in the first few years of the twenty-first century, allowing theorists to breathe a sigh of relief that they might be on the right track.

Our only direct probe into the early Universe is the cosmic background radiation noted near the end of the prologue and again briefly in the previous Particle Epoch. Launched at the end of the Radiation Era, some half-million years after the start of all things, the radio photons now engulfing us grant some inkling of the wild physical conditions prevailing at the time. Briefly explained, radiation is influenced by the gravity of growing clumps of matter, so that as the density of the clumps varied from place to place in the early Universe, the observed radiation—then launched and now observed—ought to show slight temperature variations from place to place on the sky. Such “ripples” in the temperature of the background radiation have indeed been spotted, though only weakly, at the level of parts per million. That is, given that the average temperature of the fossil radiation is about 3 degrees absolute, or –270 degrees Celsius, the minute thermal variations that have been detected by radio receivers aboard Earth-orbiting satellites, most notably the Wilkinson Microwave Anisotropy Probe (WMAP), are only on the order of millionths of one degree. Yet they are in accord with those expected for a wide range of theoretical models of galaxy formation, including the superclusters, voids, filaments, and bubbles observed all across the firmament.

Here, in a nutshell, is the basic idea of hierarchical clustering, considered more of an ongoing process than a single event: Extremely small-scale fluctuations in the matter density present before the time of inflation—an inevitable consequence of quantum physics operating in the very early Universe well less than a second old—would have been stretched and amplified by inflation to a size and scale typifying whole galaxies and even larger. The subsequent growth of those gravitational instabilities, already established when the Radiation Era gave way to the Matter Era, probably led to the gradual formation of self-gravitating collections of matter. Should this idea be correct, then the vast assemblages of matter we see today as galaxies, galaxy clusters, and even the gargantuan galaxy superclusters are the progeny of subatomic quantum effects prevalent when the Universe was a mere 10^-35 second old.

Evidence of galactic origins.

This is a map of temperature variations in the cosmic background radiation measured across the entire sky, much more sensitive than the one shown in the prologue. Here, the thermal changes are minute, amounting to mere millionths of a degree Celsius, yet they display clear departures from an otherwise uniform sea of radiation dating back to about a half-million years after the big bang. These variations, shown here as shades of grey and implying clumps of enhanced density, were probably the “seeds” from which galaxies began forming in the earlier Universe. Source: Wilkinson Microwave Anisotropy Probe.

The accepted mechanism of galaxy formation, still only roughly understood, is then a familiar one to experts of star formation, as will be examined in detail in the Stellar Epoch. Nature quite naturally selects mass-density fluctuations that gravitationally induce cycles of contracting, heating, radiating, cooling, and eventual flattening into disk-shaped objects. But, just when we feel good about getting closer to grasping reality, another complication sets in. For galaxies, unlike for stars, these events didn’t likely involve only normal matter; dark matter has been implicated to some (unknown) extent, and that clearly confuses things.

Given the prevailing conditions in the early Universe, specifically at the interface of the Matter and Radiation Eras, only regions of higher-than-average density containing more than a million times the mass of the Sun would have begun to contract. However, if galaxies grew long ago exclusively from the fluctuations within normal matter (in the absence of dark matter), those density fluctuations should manifest themselves now as a clear observable imprint of large temperature variations in today’s cosmic background radiation; that imprint is not observed.

Instead, if dark matter was involved, it might have acted as that long-sought agent, or gravitational scaffolding, to help normal matter clump earlier in the Universe. The reason is that dark matter—whatever its true nature—interacts only weakly with normal matter and with radiation. So, its natural tendency to gravitationally infall (for dark matter still exerts gravity) was neither hindered by radiation, nor expected to leave a large signature on the cosmic background radiation. Accordingly, dark matter, being ten times more abundant than normal matter, probably clumped first and then acted as an accelerant to draw normal matter into the regions of highest density. This scenario explains why so much dark matter seems to reside in the vicinity of the visible galaxies. That’s where the dark matter initially concentrated, thus attracting the normal matter that became the galaxies now so luminously seen. The brightly lit galaxies resemble the visible tips of mostly hidden icebergs, or the illuminated bulbs on an otherwise dark Christmas tree.

Of course, all this modeling is a little shaky given that astronomers don’t yet know the nature of that dark matter. To be honest, some of the uncertainty is welcome, allowing theorists much freedom in choosing dark matter’s properties while seeking to match galaxy-formation models with observed structures in the sky—and that, in turn, might imply valuable information about the dark matter. To be just as honest, the theorists may be—to make a bad pun—whistling in the dark, as their models depend on vast quantities of abnormal matter that is only inferred and has never been detected.

In any case, the seeds of galaxy formation were likely sown in the very early Universe when small density fluctuations in the primordial matter began to grow. The initial masses of these pregalactic blobs were quite small by galactic standards—perhaps only a few million, yet more likely a few billion, solar masses, comparable to those of the smallest, irregular galaxies. Those irregulars now seen scattered all around the edges of galaxy clusters may well be the building blocks of galaxies—the so-called baby galaxies. As we shall see in the final section of this Galactic Epoch, a growing consensus champions the idea that today’s big galaxies formed by the repeated merging and accumulation of smaller objects. This is indeed a “bottom-up” scheme, but not one that begins with objects as small as stars and planets, rather, with million-to-billion-solar-mass blobs that emerged near the start of the Matter Era.

Support for this hierarchical scenario is moderate and derives from two fronts. Theoretical backing is provided mainly by computer simulations stipulating how normal (baryonic) and abnormal (dark) matter might have interacted with radiation during the Universe’s first few billion years. These models demonstrate that merging was a viable phenomenon in the Galactic Epoch and could have conceivably led to the formation (and evolution) of the many varied galaxies observed today. Although the models have wide latitude among their input parameters, while at the same time suffer from computer codes obviously not as robust as the real cosmos, no “showstoppers” have yet intruded—nothing in the theoretical analyses that leads us to believe we are not on the right track, finally.

Observational support derives from the finding that some of the most remote galaxies (namely, those seen in their youth) appear distinctly smaller and less regular than those found nearby. Deep, long-exposed images acquired by the world’s most powerful telescopes—such as the Hubble Space Telescope in orbit, the Keck Observatory in Hawaii, and the Very Large Telescope in Chile—show evidence for distant and distorted spheroids containing a million to a billion solar masses (but no distinct stars) in regions typically a few thousand light-years across—roughly the size and scale expected for pregalactic building blocks. We seem to be seeing these blobs as they were some twelve billion years ago, perhaps poised to merge into larger, galaxy-sized objects. Alas, not all astronomers buy this interpretation, as the data are sketchy, the images fuzzy, and the modeling simplified. It remains unclear if anyone has yet seen a genuine “baby” galaxy or any luminous object caught in the act of galactic birth—another of science’s unachieved grails.

Which came first: black holes or galaxies? In other words, did super-massive black holes form initially and then accumulate around them matter that eventually became genuine galaxies, or did the galaxies form much as we see them now, after which they gave birth to holes at their cores as early matter migrated toward their centers? This is the first of several chicken-or-egg conundrums encountered in cosmic evolution, most of them unresolved or at least not solved satisfactorily to date. That’s probably because nothing in Nature is black or white, few solutions are clean and clear; rather, reality, and especially our models of it, possess shades of gray throughout.

Favoring the “inside-out” idea, whereby black holes form first, is the notion that in any gravitationally bound system the densest things tend to infall early on, followed by the host galaxy taking shape around the central hole. Some data bolster this idea demographically: given that quasars are more abundant than galaxies as we probe farther back in time—and where there are quasars, there are likely supermassive black holes—it would seem that black holes led the way.

By contrast, the radiative effects of the really big black holes might have actually hindered the formation of host galaxies, meaning that the galaxies probably formed first. If so, then the process was more “outside-in,” whereby the galaxies came first, at least in rough form, after which the stars, gas, and dust later trickled toward the cores to create the huge black-hole engines. Computer simulations do imply that powerful jets associated with young, massive holes would have blown away surrounding material, possibly preventing the formation of galaxies at all. Furthermore, many supermassive black holes are still actively accreting matter, implying that the process of creating them is actually quite slow, perhaps taking many billions of years to settle at the cores of already formed galaxies.

The answer, citing those shades of gray again, likely mixes aspects of both models—that is, massive black holes and enveloping galaxies may have formed together. Astronomers have discovered recently that the mass of the central black hole is proportional to the bulge of their host galaxy, so the construction of both might well have been tightly wedded and coeval in Nature.

We can pose this unsolved riddle in another related way: Did the galaxies precede the stars or was it the other way around? The answer is important for the cosmic-evolutionary scenario since, as told here, the Galactic Epoch precedes the Stellar Epoch. Is this justified? Most modern arguments do favor early origins for galaxies, followed by later formation of stars and then planets within those galaxies. But the latest data are beginning to soften that view or at least to muddy the waters a bit.

Recent findings suggest that some star formation must have occurred early in the Galactic Epoch, since traces of heavy elements, such as carbon, silicon, magnesium, and iron, are observed to have been present ten billion years ago. We know this because quasars, being typically a hundred times brighter than normal galaxies, act like thin-beam cosmic flashlights, illuminating that part of intergalactic space between the quasars and Earth. And in the quasars’ spectra—when their light is split into its component colors—is clear evidence for minute amounts of heavy elements (about a hundred times less than those in the Sun), implying that at least some stars lived and died back then, for stars are the only places known where heavy elements are made. (Astronomers take the “heavies”—sometimes also called metals, to the dismay of chemists—to mean any element more massive than helium.)

The idea that some massive stars preceded the galaxies is bolstered by evidence (from quasar spectra) that early in the Matter Era the Universe was reionized, separating atoms everywhere into ions and electrons, much as had been the case in the earlier plasma-rich Radiation Era. This would have been a relatively brief period, probably less than a billion years following the “cosmic dark ages” when no luminous objects anywhere—no quasars, stars, or any other kind of light-emitting bodies whatever—had yet graced the cosmos. All was completely and totally dark, from the first half-million years—when the Universe became neutralized and cosmic expansion redshifted the background radiation out of the visible and into the infrared part of the spectrum—to roughly a half-billion years after the bang, when gravity finally but only locally overcame expansion enough to begin clumping matter into spherical structures. As the first glowing objects—almost surely the building blocks of fledgling galaxies—began emerging from those dark ages, a renaissance of light began to flow through the Universe. The details are murky, but a consensus has emerged:

Surely quasars and possibly massive stars formed in the young Universe, starting no more than a billion years into the arrow of time. Objects smaller than a million solar masses would not likely have clumped, given the rapidly expansive conditions at the time; the thermodynamics in that warm environment would tend to dissipate smaller clumps. The quasars largely lit up (and ionized) matter near the start of the Galactic Epoch, possibly aided by ultraviolet radiation from the earliest stars. In addition, those “first stars” quickly created some heavy elements through the same kind of nuclear-fusion events that occur in stars today, as later explained in the Stellar Epoch—so quickly that all these first massive stars are now long gone, having dramatically expired as supernovae (or having been eaten by black holes—it’s possible!) within those first few billion years. Although we do see plenty of quasars in the earlier Universe, not a shred of observational evidence exists for those first stars—which means either that they did all disappear somehow (if theory is right) or that they never existed (if theory is wrong). Nor have astronomers ever found any stars with zero heavy-element content within them, as would be the case for any celestial objects among the first genuine stars. Perhaps the quasars themselves did all the reionizing, without the need for any early stars.

The upshot is that mostly big, million-to-billion-solar-mass blobs likely took shape early in the Galactic Epoch. These were the building blocks of galaxies—almost surely quasars and their black-hole engines, and possibly massive star groupings that resembled today’s globular star clusters, which still linger in the haloes of many nearby galaxies. The quasars were clearly there then, probably thousands of times more populous than now; our telescopes spy on them in the distant past, a few billion years after the big bang, when the number of quasars peaked. (None of them resides near us in space or time; the closest quasar is more than two billion light-years distant, the last of a dying breed.) As best we can explore those truly ancient times, the primordial blobs are mostly gone, presumably having merged together quickly to build the galaxies. Those blobs, either lit with stars or not, must have repeatedly merged to make virtually all the galaxies within the first few billion years—which is probably why, even in our own Milky Way, most globular star clusters in the halo average twelve billion years in age and none is younger than nine billion years. To what extent stars were already up and running in those formative blobs, or whether the stars originated mostly after the fledgling galaxies had formed, is frankly unknown.

Astronomers are closely examining the latest data from both stars and galaxies, struggling to get the timing and sequencing correct. The task is nontrivial, for we are looking way back in time, trying to decipher events long over and done. To date, these data imply major assembly of the galaxies from smaller blobs mostly within two to four billion years after the bang; later formation was not as robust, if only because universal expansion was continuing to carry those building blocks and the young galaxies away from one another, reducing the number of interactions. By contrast, stars’ formation rates peaked some five to ten billion years after the bang; this we know by tracking back in time their ultraviolet radiation—the hallmark of newborn stars. In the main, then, the origin of most galaxies definitely preceded that of most stars. Although star production has been declining during for the past several billion years, stars still do now originate—which means, again in the main (for these are averages over all the details), that the Galactic Epoch preceded the Stellar Epoch.

When did galaxy formation stop, or has it? Astronomers are divided on this issue, too, which may be more semantics than astrophysics. Some contend that at a fairly well-determined time in the past—given by the age of the globular clusters in our Galaxy, for example—most galaxy formation was over. If true, then all galaxies are old, in fact nearly equally old and on the of order twelve billion years. Other astronomers demur, citing evidence that many galaxies seem to experience repeated collisions and mixing with dwarf satellite galaxies over extended periods of time—perhaps even up to the present day. If so, then galaxies might be said to be still forming today.

What constitutes an origin in contrast to evolution? Most experts have reached a tentative consensus that billion-solar-mass protogalaxies became established in some form or another relatively early on, probably within the first couple of billion years of the Matter Era. Virtually all galaxies originated contemporaneously long ago as simply structured yet distinct objects. Their emergence was clearly the dominant feature of the Galactic Epoch. In addition, ongoing mergers, interactions, and rearrangements within and among the galaxies ever since are regarded as evolution—developmental changes that further bulked up the galaxies with each successive merger. To be sure, astronomers have ample evidence that galaxies have evolved in response to external factors, indeed that evolution continues among the galaxies today.

Much of the fascination felt by workers studying the subject of galaxy formation derives from our inability to disprove many contending theories. An array of ideas remains possible, there being only meager experimental data to discriminate among the details. However, this state of affairs will not likely last long when new data begin pouring in at rapid pace as telescopes of the twenty-first century become powerful “time machines” designed to probe the far away and long ago. New and ambitious projects—to name just one, the Sloan Digital Sky Survey now underway—are expected to map accurately several million galaxies in the northern sky in the next few years. Until galactic data become demonstrably better, however, researchers familiar with the sophisticated mathematics of notoriously tough subjects such as fluid mechanics, turbulent physics, and magnetohydrodynamics will continue to justify their interests by tinkering with the problem of the galaxies’ origins. For despite heroic efforts of the past few decades to unlock the secrets of galaxy formation, the specifics of a tested, plausible process have thus far eluded discovery.

However galaxies might have originated, either their formative stages or subsequent evolutionary events led to the myriad galaxies now seen in the nighttime sky. We observe loose and tight spiral galaxies with mixtures of old and new stars, large and small ellipticals containing only old stars, dwarf irregular, and explosively active galaxies, let alone the baffling quasars whose central engines may not house any stars at all.

With such a zoo of galaxylike objects littering the Universe, we naturally wonder if any overall pattern or evolutionary scheme interrelates all the various types of galaxies. The answer is, none discerned presently. As best we know, no identifiable physical mechanisms underlie all the galaxies and no clear developmental bonds relate one type of galaxy to another. Whoever does discover strong evolutionary links among the galaxies, akin to those connecting stars as discussed next in the Stellar Epoch, not to mention the elaborate relations among life-forms described later in the Biological Epoch, will get their names in textbooks forever.

Astronomers decades ago proposed an evolutionary progression among normal galaxies, starting with the nearly spherical ellipticals that gradually became squashed ellipticals, eventually changing into closed spirals, followed by open spirals, and finally culminating in irregular galaxies. The central idea here is that galaxies originate with a more or less spherical shape and, as they grow older, their rotation tends to flatten them, first producing some ellipticity and then some spiral arms, prior to their breaking up as aged irregular galaxies. However, therein lies a problem: this type of evolutionary notion requires all elliptical galaxies to be young and all irregular galaxies old—which isn’t the case at all. Observationally, elliptical galaxies are not young. They are populated with only old stars, nearly depleted of interstellar gas and dust, and display no evidence of active star formation.

On the other hand, given that the elliptical galaxies are so clearly old, then perhaps the evolutionary sequence runs in the opposite sense. Maybe irregulars are young and, having formed first, gradually evolve into ellipticals. It’s easy to imagine loose spiral galaxies wrapping up into tighter spirals and eventually becoming elliptical galaxies. But troubles abound here, too. Apart from the obvious puzzle of how beautiful spirals might have emerged from the contorted irregulars, it’s hard to reconcile this idea with the abundance of old stars observed in the irregular and loose-spiral galaxies. Simply put: If irregular and loose spiral galaxies are the starting point in a scheme of galactic evolution, then all of them should be young. But they’re not. Virtually all irregulars and spirals contain a mix of old and new stars. The existence of old stars is inconsistent with the nature of a youthful galaxy. The fact that astronomers know of no “dead galaxies” doesn’t help our understanding either.

Alas, normal galaxies do not likely evolve directly from one type to another. Spirals don’t seem to be ellipticals with arms, nor do ellipticals appear to be spirals without arms. No unambiguous parent-child relationships connect these huge cosmic systems—other than the idea that all galaxies are cousins that trace their birth to the same grandparent, namely, the turbulence of the gases in the aftermath of the big bang. Indeed, all galaxies’ dispositions probably result partly from the intrinsic physical conditions extant in the gas clouds from which they originated more than ten billion years ago and partly from environmental interactions with other galaxies ever since.

Frankly, this contrast between intrinsic and surrounding influences is not much different from the way that biological species evolve, combining aspects of their internal genes with those of their external environment. In the above paragraph, we could replace the word galaxy with the word organism and still be reasonably correct. Apparently, the nature-versus-nurture struggle extends beyond the living world. All through this book, we shall be confronted with the issue of whether systems change inherently or in response to external events. The answer for astronomical galaxies, as for biological life, is probably, both. And much like human life, wherein genes are estimated to influence well less than half of human behavior, environmental effects probably dominate changes among the galaxies too.

Astronomers do have ample evidence that galaxies change in response to external, environmental factors, long after the first pregalactic fragments originated. As already noted, given the size, scale, and groupings of galaxies, collisions and interactions among them are commonplace events. This is especially true for the dark-matter halos surrounding many spiral galaxies, including our own, and probably those around all galaxies. Computer simulations performed during the past decade show that these dark halos are strongly involved in, and influenced by, such galactic interactions.

As galaxies orbit or encounter one another, halo material from one galaxy can become stripped by tidal forces exerted by the other. The freed matter often ends up in a common envelope surrounding both galaxies; occasionally it’s lost entirely to (that is, flung out of) the system. In this way, even small galaxies can severely distort larger ones, depending upon the angle and proximity of interaction and the energy transferred between them. In some cases, over the course of hundreds of millions of years—a span of cosmic time that powerful computers can model in minutes—the simulations illustrate how close encounters between galaxies can cause spiral arms to appear where none existed before. The pinwheeling arms are literally drawn out of one or both galaxies, as they pass by in each others’ wakes like giant ships at sea.

Such environmental factors may be the sole source of galaxies’ spiral arms, implying that “arms” are evolutionary appendages, not products of birth. If so, then even our home Milky Way plausibly got its arms by interacting with another galaxy at some time in the past. Our Galaxy’s stellar census surely does contain evidence that it has feasted on its neighbors, now seen as remnant, elongated clumps of elderly stars captured into the Milky Way’s halo and disk billions of years ago. Perhaps the culprits were systems as small as the Magellanic Clouds now orbiting in the halo of the Milky Way, or the Sagittarius dwarf galaxy now being torn apart and subsumed by our Galaxy on its far side, opposite the Sun. Previous, long-ago encounters with a larger, comparable galactic system, such as the nearby spiral galaxy, Andromeda, is another possibility. Andromeda does currently have a component of motion toward us, meaning that our two giant galactic systems are destined for a close encounter that could cause both to become tidally disrupted and eventually more elliptical. Even more dire (or spectacular, depending on one’s viewpoint), these two grand spirals might merge together during their next encounter—the result often glibly called Milkyomeda—though that won’t happen for another several billion years.

Mergers and acquisitions may well be common among galaxies in clusters, triggering changes in shape well after their initial formation. To appreciate such evolutionary events, however, we need to contemplate extremely long durations of time. And that’s where computer simulations again come in handy. The simulations clearly show that interacting galaxies occasionally tend to gravitate toward one another, eventually merging. What’s more, those simulations imply that giant elliptical galaxies probably grew via generations of mergers with spiral galaxies, potentially explaining why the big ellipticals reside near the core of galaxy clusters and the somewhat smaller spirals toward their perimeter. Colloquially termed “galactic cannibalism,” or “galaxy gobbling,” these are the cases in which the galaxies experience very close encounters, often, in fact, direct collisions. Still, the interactions are sluggish, their explosiveness muted. The last big impacts seemingly occurred eight to ten billion years ago, after which most galaxies, dispersed somewhat by cosmic expansion, have enjoyed a relatively peaceful existence.

Despite these fanciful terms, astronomers have acquired remarkable observational support for such cannibalism, as actual imagery shows smaller galaxies at or near the central regions of large galaxies, apparently in the process of being “digested” as the larger galaxy gobbles them up and consumes them. Such cannibalism may also explain why super-massive galaxies—those having roughly ten times more mass than typical galaxies—are often found near the centers of rich galaxy clusters. The relatively nearby Virgo cluster of galaxies, some sixty million light-years distant, offers a prime example. There, a titanic, trillion-solar-mass galaxy known as Messier 87 resides in the middle of this cosmic archipelago, ostensibly ruling the cluster’s dynamics. Having dined on its companions, this supermassive galaxy now lies in wait, patiently awaiting more “food” to fall into the gravitational grip of its three-billion-solar-mass black hole. The other, smaller galaxies swarming around in the outskirts of this and other galaxy clusters like it are almost surely destined to be someday integrated into the swelling central “beasts” at the heart of their evolving systems.

Nothing in this area of research is clear cut. The above ideas represent frontier thinking, which is itself evolving with each generation of astronomers. Puzzles abound at every turn: Some isolated elliptical galaxies reside in the “field” well outside clusters, which would seem hard to explain as the result of mergers. (Perhaps they have already gobbled up everything around them.) Spiral galaxies often populate the outskirts of galaxy clusters where encounters would seem to be rare and thus not conducive to the growth of spiral arms. (Perhaps they are in wide orbits about the cluster core, obeying Kepler’s laws and spending most of their time far from the center.) And the irregular galaxies don’t seem to fit into any evolutionary scheme—unless, ironically, they are the larger galaxies’ building blocks staring us right in the face. (Perhaps those irregulars that still exist are the survivors, having so far managed to avoid extinction.)

Simply stated, owing to their distance and therefore their dimness, galaxies are hard to observe and the observations even harder to interpret. Many galactic secrets still lurk within them, awaiting new probes and new insights by future generations of astronomers eager to solve one of the great unresolved riddles in all of science—the origin and evolution of normal galaxies, abundantly and ubiquitously scattered through the Universe.

Evolutionary links between normal galaxies and active galaxies are more robust, though they, too, are hotly debated. A time sequence starting with quasars and proceeding to active galaxies and finally to normal galaxies, implying a continuous range of cosmic energy, has been bolstered in recent years. Adjacent objects along this sequence are almost indistinguishable from one another, meaning that all galaxies, regardless of type, might have similar “engines” at various stages of activity—such as supermassive black holes, which virtually all galaxies do seem to have at their cores. For example, weak quasars have some commonality with the most explosive of the active galaxies, whilst the feeblest active galaxies often resemble the most energetic members of the normal galaxies. Such a chain of cosmic verve suggests that galaxylike objects originated as quasars some twelve billion years or so ago, after which their emissive powers gradually declined, becoming active galaxies and eventually normal galaxies. This continuity among all galaxies has been strengthened recently as astronomers have become convinced that the black-hole energy-generation mechanism can account for the luminosities of quasars, active galaxies, and the central regions of most normal galaxies.

This unifying idea maintains that the quasars are actually ancestors of all (or most of) the galaxies. Consistent with the observed fact that quasars were more common in the past than they are today, galaxies do seem to have been more active long ago than they are now. Far too remote for us to resolve any individual stars within them, the quasars are detectable at great distances only because of their tremendously energetic central engines. Precisely because of their great distances, we perceive them as they once were in their blazing youth. As their core activity decayed with time, quasars assumed forms closer to those of more familiar and nearby galaxies. They essentially “wound down” while running out of fuel to feed their central black holes, eventually becoming the relatively quiescent normal galaxies now observed closer to us in space and time.

Should this view be proved correct, then maybe even our Milky Way Galaxy was once a brilliant quasar. Most ironic, if true. For decades, astronomers have struggled to decipher the Herculean quasars, especially their prodigious energy emission, only to find, perhaps, that we live inside an old, burned-out one—a time-tamed version of a quasar that once lit up the far away and the long ago.

A time sequence starting with quasars, then active galaxies, finally normal galaxies . . .

For this quasar-evolutionary idea to hold, we ought to be able to see the vague outlines, however far away, of the more normal galaxies surrounding the quasars. Until quite recently, astronomers were hard-pressed to discern any galactic structure whatever in quasar images. However, the Hubble Space Telescope has done yeoman service since the mid-1990s by indeed finding “host” galaxies around some of the distant quasars. The evidence is in the form of very dimly glowing “fuzz” now seen to be faintly enveloping a few dozen of the brighter quasars studied to date. The quasars really do seem to be residents within the centers of normal galaxies, rich in ordinary matter beyond their bright cores; the fuzz is apparently the accumulated soft emission of innumerable unresolved stars or stars-to-be. Some of the deepest, long-exposure quasar images even show suggestive evidence for spiral arms.

Although attractive, this quasar → active galaxy → normal galaxy evolutionary sequence has its drawbacks. Not all astronomers have yet embraced the idea, arguing that evolutionary links may not exist at all. They suggest that the powerful quasars are merely extreme manifestations of the explosive phenomena seen in virtually all galaxies. After all, even the center of our own Milky Way is known to be expelling matter and radiation. The same can be said for active galaxies and quasars, though on vastly larger scales. Perhaps all these objects are part of the same family without there being any evolutionary sequence linking its members, just as evolutionary changes cannot be said to bridge different races within the human species. Each galaxy type or human race is distinctly different. One race of humans doesn’t evolve into another, and similarly one type of galaxy might not necessarily evolve into any other. Instead, all the galaxies might be quite ordinary galaxies that formed long ago, though some were endowed with especially explosive central regions. Those able to exercise their explosiveness more than others for some still unknown reason are called quasars, while those hardly able to fire up their cores much at all are called normal galaxies.

Why the quasars emit radiation so prodigiously, even violently, is also unknown, though there is the notion that more fuel was available at earlier times. And for how long the quasars endure in their bright phase, adequately supplied with fuel, is also unknown; certainly they cannot do so indefinitely, lest their central black holes consume their whole being. The answers presumably lay buried within the relatively uncharted centers of galaxies, including the startling idea, now subject to heated debate, that quasars originally formed and regularly flare as supermassive black holes themselves merge, especially during the Galactic Epoch within a few billion years after the big bang.

Future research focused on the cores of galaxies will probably provide the best insights for deciphering the secrets of the bright and shining quasars, whose troubling properties of huge energy yet small size once threatened to topple the laws of physics. Even if their details are sorely lacking, their main issues now seem reasonably solved and the laws of physics intact. Rather than jeopardizing our knowledge of the cosmos, these violent objects have become an integral part of the thread of understanding that binds our own Galaxy to the earliest epochs of the Universe in which we live.

Our knowledge of the galaxies, especially their origin and evolution, is inadequate. How each of them materialized, endowed with peculiar shapes and prodigious energies, remains largely unsolved. Their enigma is deepened by the fact that astronomers cannot find any galaxy unambiguously in the act of formation. Parts of all of them seem almost as old as the Universe itself, their youthful exuberance still beyond the clear reach of our best telescopes. Furthermore, even when galaxies do evolve, their changes are so agonizingly slow, compared to the duration of our technological civilization, as to make them appear immutable. If our understanding of galaxies seems sketchy, that’s because it is; in some ways, galaxy research is only now coming into its own.

Currently, the origin and evolution of galaxies pose more problems than the formation of stars, which we can observe directly; than the evolution of stars, which we can decipher clearly; than the origin of life, which we can test in our laboratories; than the evolution of life, which we can study in action; even than the origins of intelligence, culture, and technology, all of which we can probe tangibly with fossils and artifacts unearthed from layers of historical rubble. Practically everything else discussed in this book is on firmer ground than the origin and evolution of galaxies. Exempt those eternal perplexities about the origin of the Universe itself, the subject of galaxy formation is the foremost missing link in the scenario of cosmic evolution.

Galaxies, though, are so very important. Apart from the creation of atoms, the formation of galaxies (perhaps along with some massive, extinct stars) was the first great accomplishment of the Matter Era. Until we learn a great deal more about how gravity leverages even slight initial gas irregularities into conspicuous density contrasts, our understanding of galaxy origins, and hence of cosmic evolution, will remain incomplete and unsatisfactory. Yet the promise is great, the potential payoff even greater. With the physicists unable to build accelerators on Earth sufficiently energetic to reproduce the earliest instants of time, it is the astronomers who, by studying the macrorealm of galaxies and their large-scale structure, are beginning to provide tests, albeit indirect ones, of the grand unification of particles and forces in the microrealm.

Astronomers now stand on the threshold of a golden age of galaxy research, much of which is a century or so behind stellar research if only because the galaxies are so dim and distant and therefore tricky to study. The equipment scheduled to debut during the early years of the new millennium will have greater sensitivity to collect more radiation as well as higher resolution to clarify the spread of that radiation, thereby almost surely advancing our knowledge of the origin and evolution of galaxies. Over the entire range of phenomena—from the earliest onset of density fluctuations in the primordial Universe, through the emergence of activity in the centers of galaxies, and on to the slow conversion of galactic gas into stars and planets—observations with novel instruments on the ground and in orbit are poised to provide a wealth of new and exciting data that hold clues to nothing less than some of the most profound and ancient cosmic secrets.