Coming of Age in the Milky Way (51 page)

To determine where the cell obtained the blueprint that told
it how to form, move into the nucleus and behold the lanky contours of the DNA macromolecules secreted within its genes. Each holds a wealth of genetic information accumulated over the course of some four billion years of evolution. Stored in a nucleotide alphabet of four “letters”—made of sugar and phosphate molecules and replete with punctuation marks, reiterations to guard against error, and superfluities accumulated in blind alleys of evolutionary history—its message spells out just how to make a human being, from skin and bones to brain cells.

The relationship between the sizes of basic natural structures and their binding energies (i.e., the forces needed to tear them apart) is thought to reflect their origins at differing stages of cosmic history. Quarks, for example, are said to be smaller than nucleons (i.e., protons and neutrons), and to have higher binding energies, because they were formed earlier in cosmic time, when the universe itself was small and relatively energetic.

Turn up the magnification some more and you can see that the DNA molecule is composed of many atoms, their outer electron shells intertwined and festooned in a miraculous variety of shapes, from hourglasses to ascending coils like lanky springs to ellipses fat as shields and threads thin as cheroots. Some of these electrons are new arrivals, recently snatched away from neighboring atoms; others
joined up with their atomic nuclei more than five billion years ago, in the nebula from which the earth was formed. Increase the magnification a hundred thousand times, and the nucleus of a single carbon atom swells to fill the field of view. Such nuclei were assembled inside a star that exploded long before the sun was born; the age of this one might be anywhere from five to fifteen billion years or more. Finally, looking closer still, one can perceive the trios of quarks that make up each proton and neutron in the nucleus. The quarks have been bound together since the universe was but a few seconds old.

In venturing to smaller scales, we have also been entering realms of higher binding energies. An atom can be stripped of its electron shell by applying only a few thousand electron volts of energy, but to split up the nucleons that constitute an atomic nucleus requires several million electron volts, and to liberate the quarks that make up each nucleon would require hundreds of times more energy still. Introduce the axis of history, and this relationship attests to the particles’ past: Smaller, more fundamental structures are bound by higher levels of energy because the structures themselves were forged in the heat of the big bang.

This implies that accelerators, like telescopes, function as time machines. A telescope looks into the past by virtue of the time it takes light to travel between the stars; an accelerator re-creates, however fleetingly, conditions that pertained in the early universe. The 200 KeV accelerator devised in the 1920s by Cockroft and Walton replicated some of the events that transpired at about one day after the beginning of the big bang. Accelerators built in the 1940s and 1950s hovered at around the one-second mark. The Fermilab Tevatron pushed back the boundary to less than a billionth of a second after the beginning. The superconducting super collider (had it been completed, rather than being canceled in mid-construction by Congress) would have provided a glimpse of the cosmic environment when the universe was less than one thousand billionth of a second old.

That’s pretty early: One ten thousand billionth of a second takes a smaller slice out of a second than a snap of the fingers takes out of all recorded human history. And yet, oddly enough, research into the evolution of the newborn universe indicates that a great deal happened even earlier, during that first tiny fraction of a second. The theorists, accordingly, endeavored to piece together a
coherent account of the first moments in cosmic history. Their ideas were of course sketchy and incomplete, and many of their conjectures will doubtless turn out to have been distorted or simply wrong, but they constituted a far more enlightening chronicle of the early universe than was available only a decade or so earlier, and hinted at the extraordinary beauty and explanatory power that could be expected from a more advanced theory once one could be worked out.

To review the story of cosmic history as depicted by the early-universe theories, imagine a staircase leading into the past—a stairway to heaven, if you will. We are standing at its base, in the present, at a time when the universe is some ten to twenty billion years old. (Most of the observational evidence suggests that the age of the universe is a little under fourteen billion years.) The first step upward will take us back to when the universe was only one billion years old, and each step higher will turn back the clock to a tenth of its previous reading—to only a hundred million years after the beginning, then ten million years, then one million, and so on.

Suppose that we ascend this staircase. One step, and the date is one billion years after the beginning of time (or ABT for short). The universe looks quite different. The nucleus of the young Milky Way galaxy burns brilliantly, casting the shadows of galactic thunderheads out across the murky disk; at its core shines a bright, blue-white quasar. The disk, still in the process of formation, is jumbled and thick with dust and gas; it bisects a spherical halo that will be dim in our day, but currently wreathes the galaxy in a glittering chandelier of hot, first-generation stars. Our neighboring galaxies in the Virgo Supercluster float relatively nearby; the expansion of the universe has not yet had time to carry them away to the distances, typically tens of millions of light-years, at which we will encounter them in our own era. The universe is highly radioactive: Torrents of cosmic rays rain through us every millisecond, and if anything lives at this time, it probably mutates rapidly. Indeed, the pace of most events is hectic, an urban bustle compared to the relative placidity of our more mature epoch.

With the second step, we are plunged into darkness. We have reached a time, one hundred million years ABT, before any but the most precocious stars have yet had time to form. Except for their scarce and smoky beacons, the universe is a dark soup of hydrogen and helium gas, whirlpooling here and there into protogalaxies.

The history of the universe, depicted in terms of a stairway leading exponentially backward in time, displays the evolution of natural structures from quarks to atomic nuclei to atoms and galaxies of stars.

In two more steps, the darkness is replaced by blinding white light. The time is one million years ABT, and the technical term for what has happened is photon decoupling. The ubiquitous cosmic gas has recently thinned sufficiently to permit light particles—photons—to travel for significant distances without colliding with particles of matter and being reabsorbed. (There are plenty of photons on hand, because the universe is rich in electrically charged particles, which generate electromagnetic energy, the quantum of which is the photon.) It is this great gush of light, much redshifted and thinned out by the subsequent expansion of the universe, that human beings billions of years hence will detect with radiotelescopes and will call the cosmic microwave background radiation.

This, the epoch of “let there be light,” has a significant effect on the structure of matter. Electrons, relieved from constant harassment by the photons, are now free to settle into orbit around nuclei, forming hydrogen and helium atoms. With atoms on hand, chemistry can proceed, to lead, eons hence, to the formation of alcohol and formaldehyde in interstellar clouds and the building of biotic molecules in the oceans of the early earth.

The ambient temperature of the universe rises rapidly as we continue up the stairway. It was less than 3 degrees above absolute zero on the bottom step, reached room temperature by the third step, and by the sixth step has risen to 10,000 degrees Kelvin—hotter than the surface of the sun. By the eleventh step, at which point the universe is a little under one month old, the temperature everywhere surpasses that of the
center
of the sun, and at the fifteenth step (five minutes ABT) it is fully a billion degrees Kelvin.

Energetic as this may be, the universe at the age of five minutes has already become cool enough for nucleons to stick together to make permanent atomic nuclei. We watch as protons and neutrons adhere to make nuclei of deuterium (a form of hydrogen) and deuterium nuclei pair off to form the nuclei of helium (two protons and two neutrons). In this fashion, one quarter of all the matter in the universe is rapidly combined into helium nuclei—along with traces of deuterium, helium-3 (two protons, one neutron), and lithium. The whole process is over in three minutes twenty seconds.

Above this point—prior to about one minute forty seconds
ABT—there are no stable atomic nuclei. The ambient energy level exceeds the nuclear binding energy. Consequently, any nuclei that form are quickly torn apart again.

Between the seventeenth and eighteenth steps, at about one second ABT, we encounter the epoch of neutrino decoupling. Though the universe at this time is denser than rock (and as hot as the explosion of a hydrogen bomb) it has already begun to look vacuous to the neutrinos. Since neutrinos react only to the weak force, which is extremely short in range, they now find that they can escape its clutches and fly along indefinitely without experiencing any significant further interaction. Thus emancipated, they are free hereafter to roam the universe in their aloof way, flying through most matter as if it weren’t there. (Ten million trillion neutrinos will speed harmlessly through your brain and body in the time it takes to read this sentence. By the time you have read
this
sentence, they will be farther away than the moon.) The flood of neutrinos released at one second ABT therefore persists ever after, forming a cosmic
neutrino
background radiation comparable to the microwave background radiation produced by the decoupling of the photons. If these “cosmic” neutrinos (as they are called, to differentiate them from neutrinos released later on by supernovae) could be observed by a neutrino telescope of some sort, they would provide a direct view of the universe when it was only one second old.

As we climb on, the universe continues to become hotter and denser, and the level of structure that can exist becomes ever more rudimentary. There are of course no molecules or atoms or atomic nuclei at this early time, and by about the twenty-second step, some 10
^-6
(0.000001) second ABT, there are no protons or neutrons, either. The universe is an ocean of free quarks and other elementary particles.

If we take the trouble to count we will find that for every billion antiquarks there are a billion and one quarks. This asymmetry is important: The few excess quarks destined to survive the general quark-antiquark annihilation will form all the atoms of matter in the latter-day universe. The origin of the inequity is unknown; presumably it involved the breaking of a matter-antimatter symmetry at some earlier stage.

We are approaching a time when the basic structures of natural law, and not only those of the particles and fields whose behavior they dictate, were altered as the universe evolved. The first such
transition comes at the twenty-seventh step, 10
⁻¹¹
second ABT, when the functions of the weak and electromagnetic forces are found to be handled by a single force, the electroweak. There is now enough ambient energy available to support the creation and maintenance of large numbers of W and Z bosons. These particles—the same kind the conjuring up of which in the CERN accelerator verified the electroweak theory—mediate electromagnetic and weak force interactions interchangeably, making the two forces indistinguishable. Prior to the twenty-seventh step the universe is ruled by only three forces—gravity, and the strong nuclear and electroweak interactions.

The next two dozen or so steps of our ascent are clouded in mystery. Some say that they traverse a “desert,” a bleak stretch of time in which little of importance occurred. But it remains to be seen, given further accelerator experiments and the development of more sophisticated theories, whether the desert will prove to have bloomed.