The big picture of the Universe seems both gratifyingly well understood in gross fashion, yet puzzlingly unresolved in its devilish details. Present consensus suggests an evolutionary Universe that expands forevermore, but its origin, destiny, and basic composition remain concealed in lingering uncertainties. As we now enter a golden age of testable cosmology, many astrophysicists are inclined to say that we should expect a definite answer within a few years. This is perhaps overly optimistic, for the final solution requires the agreement of three often disparate groups of human beings:
First, there are the theoreticians, whose imaginative minds invent the model Universes while striving to stay within the bounds of good, solid, accurate science; they try to determine what the Universe is supposed to be like. Second, there are the experimentalists, constantly testing the theories while extending their observations to more distant realms within the real Universe; they try to determine what the Universe actually is like. And third, there are the skeptics, who regard the models of the first group as mere speculation, and the results of the second group as overinterpretation of the data without due regard for observational error.
In the end, all three attitudes are helpful, and even necessary, for only by their cooperation and counteraction can we ever hope to approach the truth. Fortunately, the cosmic-evolutionary story—a telling of natural history from shortly after the big bang until now—is largely independent of which specific cosmological model is correct or what may be the ultimate fate (of the models and of the Universe!). All models include universal expansion, as well they must given the indisputable fact of galaxy recession—and it is expansion, more than anything else, that drives the potential for the rise of order, form, and structure in the Universe.
By the end of the PARTICLE EPOCH, the Universe had evolved dramatically. The spectacularly bright fireball identified as the origin of all things had subsided. Energy, which underlies all change in the Universe, had itself changed with its dispersal over time. The physical conditions of temperature and density had undergone extraordinary change. Atoms, mainly hydrogen and helium, had been synthesized. And matter had wrested firm control from radiation, heralding a whole new era.
Major events in the Universe would thereafter occur less frequently. Change continued, though at a more relaxed pace. Key transactions between matter and radiation may well have occurred posthaste immediately after the bang, and especially in the first few minutes of the Universe. But these interactions eventually lessened, becoming few and far between by the end of the PARTICLE EPOCH. The average density fell enormously throughout this initial epoch, plummeting below ~10-20 g/cm3 before the epoch had ended—less than a million years after the Universe began. The average temperature of the cosmos had also dropped to a relatively cool ~3000 K, a pale, sluggish remnant of the intense heat prevalent at creation.
With time, the Universe had grown thinner, colder, and darker. It was destined to evolve much more slowly in later epochs, but it evolved nonetheless. The average physical conditions were on their way to becoming a billion times still less dense and a thousand times still less hot—tenuous and frigid conditions now present more than 10 billion years after the bang—the fossilized grandeur of a bygone era.
The history of the early Universe presented here represents the prevailing view among cosmologists. Most share this general outline, though consensus is lacking regarding the fine details. Scientists agree on events as far back as the first nanosecond of existence, but the earlier we explore beyond that, the more unsure our statements become. Accordingly, the temperature and density in the first instants of the Universe are quite obscure, mainly because their values depend upon poorly understood interactions among the heaviest elementary particles at some of the highest conceivable energies. This uncertainty shouldn’t surprise us, for the promordial moments of the Universe are long gone with cosmic expansion, forever lost to the march of time. We can fathom the most ancient realms of Nature only indirectly, aided by crutches of abstract formulae and logical symbols.
What is surprising is that science can address any of this at all, modeling times and events that are very much over and done, perhaps never to occur again. And what we find, in virtually all models that are based on real data, is an early Universe reasoned to have been exceedingly hot and dense, growing cooler and thinner with time, and basically changing in ways to set the stage to promote the successive appearance of galaxies, stars, planets, and life.
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