05.13
"We go about our daily lives understanding almost nothing of the world. We give little thought to the machinery that generates the sunlight that makes life possible, to the gravity that glues us to an Earth that would otherwise send us spinning off into space, or to the atoms of which we are made and on whose stability we fundamentally depend. Except for children (who don't know enough not to ask the important questions), few of us spend much time wondering why nature is the way it is;
where the cosmos came from, or whether it was always here; if time will one day flow backward and effects precede causes; or whether there are ultimate limits to what humans can know."
Carl Sagan
From an introduction to "A Brief History of Time"
by Stephen Hawking
Introduction
Big Bang Theory, currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 10 billion to 20 billion years ago, and the universe has since been expanding and cooling.
The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein.
In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum. This proved that the galaxies were moving away from each other. He found that galaxies farther away were moving away faster, showing that the universe is expanding uniformly. However, the universe's initial state was still unknown.
In the 1940s Russian American physicist George Gamow worked out a theory that fit with Friedmann's solutions in which the universe expanded from a hot, dense state. In 1950 British astronomer Fred Hoyle, in support of his own opposing steady-state theory, referred to Gamow's theory as a mere "big bang," but the name stuck. Indeed, a contest in the 1990s by Sky & Telescope magazine to find a better (perhaps more dignified) name did not produce one.
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).
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, something like the process that happens when liquid water freezes into ice. The strong nuclear 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.
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, something like the process that happens when liquid water freezes into ice. The strong nuclear 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.
