As time elapsed, change continued. A few hundred years after the bang, the density had decreased to a value of ~10-10 g/cm3, while the average temperature had fallen to ~106 K—values hardly different from those in the atmospheres of stars today. A principal feature of this latter part of the PARTICLE EPOCH was the steady waning of the original fireball; the annihilations of hadrons and leptons had ended. Even as the fireball faltered, though, a dramatic change began.
The first hundred centuries of the Universe—including part of the so-called atom period—saw radiation reign supreme. Radiation was in absolute and firm control, as all space was literally inundated with it. The cosmos remained a structureless and highly uniform blob; astrophysicists say that matter and radiation were intimately coupled to each other, in a technical sense equilibrated. As the Universe expanded, however, the radiation density decreased faster than the matter density. (That’s because matter is diluted in proportion to the volume increase, but radiation, being additionally affected by the Doppler effect, decreases more than with mere volume growth.) This imbalance ultimately caused the early sphere of blinding light to thin gradually, thus diminishing the early dominance of radiation. It was as though a luminous fog of energetic photons (like the gas inside a glowing neon sign) had begun to dim and then lift. Matter and radiation thereafter decoupled, their thermal equilibrium unraveling and their particle symmetry breaking, as an evolutionary change of great importance began.
Sometime between the first few millennia and a million years after the bang—the exact moment cannot be pinned down much better since the process was inherently gradual—the charged elementary particles of matter began clustering into atoms. Their own electromagnetic forces pulled them together, sporadically at first and then more frequently. The weakening radiation could no longer break them apart as quickly as they combined. In effect, the authority of radiation had subsided as the previously charged matter (“plasma”) gradually became neutralized, a physical state over which radiation has little leverage. Matter had, in a sense, managed to overthrow the cosmic fireball while emerging as the principal constituent of the Universe. To denote this major turn of events, the latter portion of the PARTICLE EPOCH and all the remaining epochs that have occurred since are collectively termed the Matter Era. (See Figure 1.21.)
Once the radiative fog dispersed and the Universe became nearly transparent, most photons traveled unhindered through space. Radiation had, in effect, become uncoupled from matter. And as the Universe expanded, the radiation simply cooled, eventually becoming the cosmic background radiation now perceived all around us, as described earlier in this first, PARTICLE EPOCH. That last, general interaction of photons with matter occurred when the Universe was ~300,000 years old. Thus, by observing the cosmic background radiation now, astronomers can probe conditions in the early Universe more than 99% of the way back in time to the big bang.
The emergence of organized matter from chaotic radiation was a preeminent change in the history of the Universe. The Radiation Era had naturally and inevitably given way to the Matter Era. Some scientists regard it as the greatest change that ever occurred.
(Even if the mysterious “dark energy” noted earlier in this PARTICLE EPOCH does exist, indeed even if it now dominates the total cosmic density, its presence wasn’t likely of much consequence in the early Universe. Dark energy, assuming it’s real and most likely a function of space itself, probably grows in force and influence as the Universe expands. Only more recently, some 10 billion years after the big bang, would its large-scale effect have begun to manifest itself.)
The onset of the Matter Era saw the widespread creation of atoms; they were literally everywhere. The influence of radiation had grown so weak that it could no longer prevent the attachment of hadron and lepton elementary particles that had survived annihilation. Hydrogen atoms were the first type of element to form, requiring only that a single negatively charged electron be electromagnetically linked to a single positively charged proton. Copious amounts of hydrogen were thereby made in the early Universe, and it is for this reason that we regard hydrogen as the common elemental ancestor of all material things.
Heavier Elements Hydrogen (and its isotope, deuterium) was not the only kind of atom fashioned during the PARTICLE EPOCH . Before all the electrons and protons were swept up into hydrogen, atoms of the second simplest element, helium, began to form.
Heavy elements originate when two or more lightweight particles combine together. They do so by means of a dual process: First, a heavy nucleus of an atom is created whenever lighter ones collide violently enough to stick and fuse. Second, the newly formed positively charged nucleus then attracts a requisite number of negatively charged electrons, thereby yielding a neutral, albeit heavier, atom.
In the case of helium production, a temperature of at least 107 K is needed to thrust two hydrogen nuclei (protons) into one another. Each proton boasts a positive charge and at lower temperatures they would simply repel like identical poles of magnets. This minimum temperature ensures that the hydrogen nuclei collide with ample vigor to pierce the natural electromagnetic barrier that prevents them from interacting under ordinary circumstances. For a split second, the colliding particles enter the extremely small operating range of the powerful nuclear force. Once within ~10-12 cm of one another, the two hydrogen nuclei no longer repel. Instead, the attractive nuclear force seizes control, slamming them together ferociously and uniting them instantaneously into a heavier nucleus. Exactly the same process occurs now in the hearts of stars everywhere, as we shall see in the third, STELLAR EPOCH. And it’s the same process that humans have made operational, though on a much smaller and uncontrolled scale, in the form of modern thermonuclear weapons, especially the hydrogen bomb.
The superheat of the early Universe meant that the physical conditions were ripe for the creation of helium nuclei from protons of the primeval fireball. Thereafter, in the later stages of the PARTICLE EPOCH, pairs of electrons were electromagnetically attracted to each helium nucleus, thus fabricating neutral helium atoms. Given the rapid rate at which most models suggest the Universe expanded and cooled, only so much of the hydrogen could have been transformed into helium, leaving about a dozen hydrogen atoms for every one helium atom. That’s a helium abundance of about 8% by number, or 25% by mass.
Accordingly, all cosmic objects should contain at least 8% helium abundance. For two reasons, this is a minimum amount of helium that should "contaminate" virtually all objects in the Universe. First, ongoing nuclear events within stars have surely created additional amounts (a few percent more) of helium well after the bang; helium is, in fact, forming at this moment inside all stars. And second, since helium is chemically inert, there’s no easy way to change it into something else once it forms; helium atoms cannot even "hide" within other substances, like molecules, since helium doesn’t easily combine with any other elements.
Small rocky planets like Earth are exceptions. They usually have little helium because their gravity isn't strong enough to prevent lightweight helium atoms from escaping. Life is also an exception, for it originated from the matter of which our planet is made, indeed from matter that does readily combine. At any rate, the fact that the oldest stars, especially the stars of globular clusters, contain nearly 10% of their atoms in the form of helium lends support to the idea that most helium was in fact created in the violent moments of the early Universe.
By contrast, elements heavier than helium could not have been appreciably produced in the early Universe. (Nuclei of the third element—lithium—squeezed through a thermodynamic bottleneck, but only in exceedingly small amounts of a billion times less than helium.) The materials composing this screen (or page) of text, the oxygen and nitrogen in the air we breathe, as well as the copper and silver in the coins in our pockets were not made in the aftermath of the initial bang. Fusion of heavy elements, including all the way up to iron, for example, requires temperatures much higher than 107 K. Such syntheses also require lots of helium atoms. The trouble here is that, even though helium production was in high gear during those first few years of the Universe's existence, both the density and temperature were quickly falling. Theoretical calculations suggest that, by the time there were sufficient helium atoms to interact with one another to manufacture heavier elements, the cosmic temperature had dipped below the threshold value needed for the mutual penetration of doubly charged helium nuclei. That threshold value is ~108 K, for it takes even greater violence for multiply charged (and thus more strongly repelling) nuclei to collide, stick, and fuse. The Universe at the time was still hot, but not quite hot enough anymore to make the heavies.
Contrary to the progressive cooling and thinning of the early Universe, the compact matter within stars, none of which had yet formed by that time, is perfectly suited for the generation of hotter temperatures, greater densities, more brutal collisions, and thus heavier elements. The hearts of stars, as examined later in the STELLAR EPOCH, are indeed where the heavies were eventually created, albeit long after the PARTICLE EPOCH had ended. It’s where they are still being made today.