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The Big Bang
Most scientists agree that the universe began some 12 to 20 billion years ago in what has come to be known as the Big Bang (a term coined by the English astrophysicist Fred Hoyle in 1950. Hoyle, who championed a rival cosmological theory, meant the "Big Bang" to be a term of derision, but the name was so catchy that it stuck.). Though the Big Bang suggests a colossal explosion, it wasn't really an "explosion" in the sense that we understand it. Space itself exploded.
At the instant of the Big Bang, the universe was infinitely dense and unimaginably hot. Cosmologists believe that all forms of matter and energy, as well as space and time itself, were formed at this instant. Since "before" is a temporal concept, one cannot ask what came before the Big Bang and therefore "caused" it, at least not within the context of any known physics. (At least one cosmological theory, however, predicts that our universe's Big Bang is part of a chain reaction in which the demise of one universe spawns the birth of many, parallel, universes. According to this scenario, our universe may simply be part of a huge, infinitely growing fractal.)
Science tells us nothing about the way space, time and matter behaved in our universe's earliest instant, from the time of the Big Bang to 10^-43 seconds later. Space was certainly expanding--violently--and from this expansion of space was formed a highly energetic soup of particles and antiparticles.
The energy was so great during the this so-called Grand Unification Epoch--a fine-sounding name for the period from 10^-43 to 10^-35 seconds after the Big Bang--that all matter and energy was essentially interchangeable and in equilibrium. What's more, electromagnetism and the two nuclear forces were as one (gravity, the fourth and weakest force, had separated from the other three at the beginning of the Grand Unification Epoch).
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A Burst of Inflation
As the universe expanded, it cooled down. At 10^-35 seconds, the
temperature was a mere 10^27 degrees K (water boils at 373.16 K or 3.7316^2!). At this critical temperature, the universe underwent a phase transition,
the process that happens when liquid water freezes into ice. The strong
force--which acts at very short distances and holds protons and neutrons
together--split off from the other forces. Physicists call this process
"symmetry breaking," and it released an enormous amount of energy.
Then, in an extraordinary instant that theorists have dubbed "inflation," the universe expanded exponentially. During this time, the universe grew by a factor of 10^50 in 10^-33 seconds. Talk about runaway inflation!
This scenario, much as it strains credulity, neatly explains several different observations made during the last 20 years--the large-scale smoothness and apparent flatness of the universe among them--that had weakened the original Big Bang theory of cosmology based on a much more leisurely period of expansion.
Things slowed up a bit after the inflationary epoch. A number of observations, well supported by theory, suggest that our universe continued to expand, albeit more slowly, and that it is expanding still.
As space expanded, it continued to cool down. Matter--at first photons, quarks, neutrinos, and electrons, and then protons and neutrons--condensed out, all less than one second after the Big Bang. It was not until one billion years later, when the universe was one-fifth the size it is today, that the matter would form the first stars and galaxies.
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