Origins: Fourteen Billion Years of Cosmic Evolution (2 page)

Overture

The Greatest Story Ever Told

The world has persisted many a long year, having once been set going in the appropriate motions. From these everything
else follows
.

—Lucretius

S
ome 14 billion years ago, at the beginning of time, all the space and all the matter and all the energy of the known universe fit within a pinhead. The universe was then so hot that the basic forces of nature, which collectively describe the universe, were merged into a single, unified force. When the universe was a roaring 10
30
degrees and just 10
-43
seconds old—the time before which all of our theories of matter and space lose their meaning—black holes spontaneously formed, disappeared, and formed again out of the energy contained within the unified force field. Under these extreme conditions, in what is admittedly speculative physics, the structure of space and time became severely curved as it gurgled into a spongy, foamlike structure. During this epoch, phenomena described by Einstein’s general theory of relativity (the modern theory of gravity) and quantum mechanics (the description of matter on its smallest scales) were indistinguishable.

As the universe expanded and cooled, gravity split from the other forces. Soon thereafter, the strong nuclear force and the electro-weak force split from each other, an event accompanied by an enormous release of stored energy that induced a rapid, fifty-power-of-ten increase in the size of the universe. The rapid expansion, known as the “epoch of inflation,” stretched and smoothed matter and energy so that any variation in density from one part of the universe to the next became less than one part in a hundred thousand.

Continuing onward with what is now laboratory-confirmed physics, the universe was hot enough for photons to spontaneously convert their energy into matter-antimatter particle pairs, which immediately thereafter annihilated each other, returning their energy back to photons. For reasons unknown, this symmetry between matter and antimatter had been “broken” at the previous force splitting, which led to a slight excess of matter over antimatter. The asymmetry was small but crucial for the future evolution of the universe: for every 1 billion antimatter particles, 1 billion+1 matter particles were born.

As the universe continued to cool, the electro-weak force split into the electromagnetic force and the weak nuclear force, completing the four distinct and familiar forces of nature. While the energy of the photon bath continued to drop, pairs of matter-antimatter particles could no longer be created spontaneously from the available photons. All remaining pairs of matter-antimatter particles swiftly annihilated, leaving behind a universe with one particle of ordinary matter for every billion photons—and no antimatter. Had this matter-over-antimatter asymmetry not emerged, the expanding universe would forever be composed of light and nothing else, not even astrophysicists. Over a roughly three-minute period, the matter became protons and neutrons, many of which combined to become the simplest atomic nuclei. Meanwhile, free-roving electrons thoroughly scattered the photons to and fro, creating an opaque soup of matter and energy.

When the universe cooled below a few thousand degrees Kelvin —somewhat hotter than a blast furnace—the loose electrons moved slowly enough to get snatched from the soup by the roving nuclei to make complete atoms of hydrogen, helium, and lithium, the three lightest elements. The universe had now become (for the first time) transparent to visible light, and these free-flying photons are observable today as the cosmic microwave background. During its first billion years, the universe continued to expand and cool as matter gravitated into the massive concentrations we call galaxies. Within just the volume of the cosmos that we can see, a hundred billion of these galaxies formed, each containing hundreds of billions of stars that undergo thermonuclear fusion in their cores. Those stars with more than about ten times the mass of the Sun achieve sufficient pressure and temperature in their cores to manufacture dozens of elements heavier than hydrogen, including the elements that compose planets and the life upon them. These elements would be embarrassingly useless were they to remain locked inside the star. But high-mass stars explode in death, scattering their chemically enriched guts throughout the galaxy.

After 7 or 8 billion years of such enrichment, an undistinguished star (the Sun) was born in an undistinguished region (the Orion arm) of an undistinguished galaxy (the Milky Way) in an undistinguished part of the universe (the outskirts of the Virgo supercluster). The gas cloud from which the Sun formed contained a sufficient supply of heavy elements to spawn a few planets, thousands of asteroids, and billions of comets. During the formation of this star system, matter condensed and accreted out of the parent cloud of gas while circling the Sun. For several hundred million years, the persistent impacts of high-velocity comets and other leftover debris rendered molten the surfaces of the rocky planets, preventing the formation of complex molecules. As less and less accretable matter remained in the solar system, the planets’ surfaces began to cool. The planet we call Earth formed in an orbit where its atmosphere can sustain oceans, largely in liquid form. Had Earth formed much closer to the Sun, the oceans would have vaporized. Had Earth formed much farther, the oceans would have frozen. In either case, life as we know it would not have evolved.

Within the chemically rich liquid oceans, by a mechanism unknown, simple anaerobic bacteria emerged that unwittingly transformed Earth’s carbon dioxide–rich atmosphere into one with sufficient oxygen to allow aerobic organisms to form, evolve, and dominate the oceans and land. These same oxygen atoms, normally found in pairs (O
2
), also combined in threes to form ozone (O
3
) in the upper atmosphere, which shields Earth’s surface from most of the Sun’s molecule-hostile ultraviolet photons.

The remarkable diversity of life on Earth, and (we may presume) elsewhere in the universe, arises from the cosmic abundance of carbon and the countless number of molecules (simple and complex) made from it; more varieties of carbon-based molecules exist than of all other molecules combined. But life is fragile. Earth’s encounters with large objects, left over from the formation of the solar system, which were once common events, still wreak intermittent havoc upon our ecosystem. A mere 65 million years ago (less than 2 percent of Earth’s past), a 10-trillion-ton asteroid struck what is now the Yucatán Peninsula and obliterated over 70 percent of Earth’s land-based flora and fauna-including all the dinosaurs, the dominant land animals of that epoch. This ecological tragedy opened an opportunity for small, surviving mammals to fill freshly vacant niches. A big-brained branch of these mammals, one we call primates, evolved a genus and species—
Homo sapiens
—to a level of intelligence that enabled them to invent methods and tools of science; to invent astrophysics; and to deduce the origin and evolution of the universe.

Yes, the universe had a beginning. Yes, the universe continues to evolve. And yes, every one of our body’s atoms is traceable to the big bang and to the thermonuclear furnaces within high-mass stars. We are not simply in the universe, we are part of it. We are born from it. One might even say that the universe has empowered us, here in our small corner of the cosmos, to figure itself out. And we have only just begun.

Part I

The Origin of
the Universe

CHAPTER 1

In the Beginning

I
n the beginning, there was physics. “Physics” describes how matter, energy, space, and time behave and interact with one another. The interplay of these characters in our cosmic drama underlies all biological and chemical phenomena. Hence everything fundamental and familiar to us earthlings begins with, and rests upon, the laws of physics. When we apply these laws to astronomical settings, we deal with physics writ large, which we call astrophysics.

In almost any area of scientific inquiry, but especially in physics, the frontier of discovery lives at the extremes of our ability to measure events and situations. In an extreme of matter, such as the neighborhood of a black hole, gravity strongly warps the surrounding space-time continuum. At an extreme of energy, thermonuclear fusion sustains itself within the 15-million-degree cores of stars. And at every extreme imaginable we find the outrageously hot and dense conditions that prevailed during the first few moments of the universe. To understand what happens in each of these scenarios requires laws of physics discovered after 1900, during what physicists now call the modern era, to distinguish it from the classical era that includes all previous physics.

One major feature of classical physics is that events and laws and predictions actually make sense when you stop and think about them. They were all discovered and tested in ordinary laboratories in ordinary buildings. The laws of gravity and motion, of electricity and magnetism, and of the nature and behavior of heat energy are still taught in high school physics classes. These revelations about the natural world fueled the industrial revolution, itself transforming culture and society in ways unimagined by generations that came before, and remain central to what happens, and why, in the world of everyday experience.

By contrast, nothing makes sense in modern physics because everything happens in regimes that lie far beyond those to which our human senses respond. This is a good thing. We may happily report that our daily lives remain wholly devoid of extreme physics. On a normal morning, you get out of bed, wander around the house, eat something, then dash out the front door. At day’s end your loved ones fully expect you to look no different than you did when you left, and to return home in one piece. But imagine yourself arriving at the office, walking into an overheated conference room for an important 10
A.M.
meeting, and suddenly losing all your electrons—or worse yet, having every atom of your body fly apart. That would be bad. Suppose instead that you’re sitting in your office trying to get some work done by the light of your 75-watt desk lamp, when somebody flicks on 500 watts of overhead lights, causing your body to bounce randomly from wall to wall until you’re jack-in-the-boxed out the window. Or what if you go to a sumo wrestling match after work, only to see the two nearly spherical gentlemen collide, disappear, and then spontaneously become two beams of light that leave the room in opposite directions? Or suppose that on your way home, you take a road less traveled, and a darkened building sucks you in feet first, stretching your body head to toe while squeezing you shoulder to shoulder as you get extruded through a hole, never to be seen or heard from again.

If those scenes played themselves out in our daily lives, we would find modern physics far less bizarre; our knowledge of the foundations of relativity and quantum mechanics would flow naturally from our life experiences; and our loved ones would probably never let us go to work. But back in the early minutes of the universe that kind of stuff happened all the time. To envision it, and to understand it, we have no choice but to establish a new form of common sense, an altered intuition about how matter behaves, and how physical laws describe its behavior, at extremes of temperature, density, and pressure.

We must enter the world of
E = mc
2
.

Albert Einstein first published a version of this famous equation in 1905, the year in which his seminal research paper entitled “Zur Elektrodynamik bewegter Körper” appeared in
Annalen der Physik
, the preeminent German journal of physics. The paper’s title in English reads “On the Electrodynamics of Moving Bodies,” but the work is far better known as Einstein’s special theory of relativity, which introduced concepts that forever changed our notions of space and time. Just twenty-six years old in 1905, working as a patent examiner in Bern, Switzerland, Einstein offered further details, including his best-known equation in another, remarkably short (two-and-a-half-page) paper published later the same year in the same journal: “Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?” or “Does the Inertia of a Body Depend on Its Energy Content?” To save you the effort of locating the original article, of designing an experiment, and of thus testing Einstein’s theory, the answer to the paper’s title is yes. As Einstein wrote,

If a body gives off the energy E in the form of radiation, its mass diminishes by E/c
2
. . . .The mass of a body is a measure of its energy-content; if the energy changes by E, the mass changes in the same sense.

Uncertain as to the truth of his statement, he then suggested,

It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test.
*

There you have it: the algebraic recipe for all occasions when you want to convert matter into energy, or energy into matter.
E = mc
2
—energy equals mass times the square of the speed of light—gives us a supremely powerful computational tool that extends our capacity to know and understand the universe from as it is now, all the way back to infinitesimal fractions of a second after the birth of the cosmos. With this equation, you can tell how much radiant energy a star can produce, or how much you could gain by converting the coins in your pocket into useful forms of energy.

The most familiar form of energy—shining all around us, though often unrecognized and unnamed in our mind’s eye—is the photon, a massless, irreducible particle of visible light, or of any other form of electromagnetic radiation. We all live within a continuous bath of photons: from the Sun, the Moon, and the stars; from your stove, your chandelier, and your nightlight; from hundreds of radio and television stations; and from countless cell-phone and radar transmissions. Why, then, don’t we actually see the daily transmuting of energy into matter, or of matter into energy? The energy of common photons sits far below the mass of the least massive subatomic particles, when converted into energy by
E = mc
2
. Because these photons wield too little energy to become anything else, they lead simple, relatively uneventful lives.

Do you long for some action with
E = mc
2
? Start hanging around gamma-ray photons that have some real energy—at least 200,000 times more than visible photons. You’ll quickly get sick and die of cancer; but before that happens, you’ll see pairs of electrons, one made of matter, the other of antimatter (just one of many dynamic particle-antiparticle duos in the universe) pop into existence where photons once roamed. As you watch, you’ll also see matter-antimatter pairs of electrons collide, annihilating each other and creating gamma-ray photons once again. Increase the photons’ energy by another factor of 2,000, and you now have gamma rays with enough energy to turn susceptible people into the Hulk. Pairs of these photons wield enough energy, fully described by the power of
E = mc
2
, to create particles such as neutrons, protons, and their antimatter partners, each nearly 2,000 times the mass of an electron. High-energy photons don’t hang out just anywhere, but they do exist in many a cosmic crucible. For gamma rays, almost any environment hotter than a few billion degrees will do just fine.

The cosmological significance of particles and energy packets that transform themselves into one another is staggering. Currently, the temperature of our expanding universe, found by measuring the bath of microwave photons that pervades all of space, is a mere 2.73 degrees Kelvin. (On the Kelvin scale, all temperatures are positive: particles have the least possible energy at 0 degrees; room temperature is about 295 degrees; and water boils at 373 degrees.) Like the photons of visible light, microwave photons are too cool to have any realistic ambitions of turning themselves into particles via
E = mc
2
. In other words, no known particle has a mass so low that it can be made from the meager energy of a microwave photon. The same holds true for the photons that form radio waves, infrared, and visible light, as well as ultraviolet and X rays. More simply expressed, particle transmutations all require gamma rays. Yesterday, however, the universe was a little bit smaller and a little bit hotter than today. The day before, it was smaller and hotter still. Roll the clocks backward some more—say, 13.7 billion years—and you land squarely in the post–big bang primordial soup, a time when the temperature of the cosmos was high enough to be astrophysically interesting as gamma rays filled the universe.

To understand the behavior of space, time, matter, and energy from the big bang to present day is one of the greatest triumphs of human thought. If you seek a complete explanation for the events of the earliest moments, when the universe was smaller and hotter than ever thereafter, you must find a way to enable the four known forces of nature—gravity, electromagnetism, the strong and the weak nuclear forces—to talk to one another, to unify and become a single meta-force. You must also find a way to reconcile two currently incompatible branches of physics: quantum mechanics (the science of the small) and general relativity (the science of the large).

Spurred by the
successful marriage of quantum mechanics and electromagnetism during the mid-twentieth century, physicists moved swiftly to blend quantum mechanics and general relativity into a single and coherent theory of quantum gravity. Although so far they have all failed, we already know where the high hurdles lie: during the “Planck era.” That’s the cosmic phase up to 10
-43
second (one ten-million-trillion-trillion-trillionth of a second) after the beginning. Because information can never travel more rapidly than the speed of light, 3 x 10
8
meters per second, a hypothetical observer situated anywhere in the universe during the Planck era could see no farther than 3 x 10
-35
meter (three hundred billion trillion-trillionths of a meter). The German physicist Max Planck, after whom these unimaginably small times and distances are named, introduced the idea of quantized energy in 1900 and generally receives credit as the father of quantum mechanics.

Not to worry, though, so far as daily life goes. The clash between quantum mechanics and gravity poses no practical problem for the contemporary universe. Astrophysicists apply the tenets and tools of general relativity and quantum mechanics to extremely different classes of problems. But in the beginning, during the Planck era, the large was small, so there must have been a kind of shotgun wedding between the two. Alas, the vows exchanged during that ceremony continue to elude us, so no (known) laws of physics describe with any confidence how the universe behaved during the brief honeymoon, before the expanding universe forced the very large and very small to part ways.

At the end of the Planck era, gravity wriggled itself loose from the other, still-unified forces of nature, achieving an independent identity nicely described by our current theories. As the universe aged past 10
-35
second, it continued to expand and to cool, and what remained of the once-unified forces divided into the electro-weak force and the strong nuclear force. Later still, the electro-weak force split into the electromagnetic and the weak nuclear forces, laying bare four distinct and familiar forces—with the weak force controlling radioactive decay, the strong force binding together the particles in each atomic nucleus, the electromagnetic force holding atoms together in molecules, and gravity binding matter in bulk. By the time the universe aged a trillionth of a second, its transmogrified forces, along with other critical episodes, had already imbued the cosmos with its fundamental properties, each worthy of its own book.

While time dragged on for the universe’s first trillionth of a second, the interplay of matter and energy continued incessantly. Shortly before, during, and after the strong and electro-weak forces had split, the universe contained a seething ocean of quarks, leptons, and their antimatter siblings, along with bosons, the particles that enable these particles to interact with one another. None of these particle families, so far as we now know, can be divided into anything smaller or more basic. Fundamental though they are, each family of particles comes in several species. Photons, including those that form visible light, belong to the boson family. The leptons most familiar to the nonphysicist are electrons and (perhaps) neutrinos; and the most familiar quarks are . . . well, there are no familiar quarks, because in ordinary life we always find quarks bound together into particles such as protons and neutrons. Each species of quark has been assigned an abstract name that serves no real philological, philosophical, or pedagogical purpose except to distinguish it from the others: “up” and “down,” “strange” and “charmed,” and “top” and “bottom.”

Bosons, by the way, derive their name from the Indian physicist Satyendranath Bose. The word “lepton” comes from the Greek
leptos
, meaning “light” or “small.” “Quark,” however, has a literary and far more imaginative origin. The American physicist Murray Gell-Mann, who in 1964 proposed the existence of quarks, and who then thought that the quark family had only three members, drew the name from a characteristically elusive line in James Joyce’s
Finnegans Wake
: “Three quarks for Muster Mark!” One advantage quarks can claim: All their names are simple—something that chemists, biologists, and geologists seem unable to achieve in naming their own stuff.