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The Big Bang. The “Big Bang” is a term we use to describe the very early seconds of the universe when, as far as we can tell, time and energy and matter and dimensions came into being. In those very early seconds (or picoseconds and nanoseconds) after the initial “Big Bang” the universe went through various epochs (e.g., the Planck epoch, the Grand Unification Epoch, the Inflationary Epoch, etc.) defined by the emergence of various fundamental forces of the physical universe (such as gravity, nuclear strong and weak forces, and so forth). Much of our understanding of the Big Bang is speculative, but based upon mathematics and observations of the universe as it exists and existed in the distant past (observations of the distant past are available to us by looking at distant objects). It is more difficult to speculate about what caused the Big Bang or what was going on before the Big Bang.

The current best estimates of when the Big Bang took place for this observable universe suggest this event most recently happened 13.75 billion years ago, plus or minus 110 million years. Different cosmologies suggest the big bang is a one-time singularity, or a repeating phenomenon. It is an interesting fact that the rough diameter of the known universe is much larger than 28 billion light years (it's approximately 90 billion light years in diameter).

Links about the Big Bang:
  1. NASA’s page on the Cosmology of the Big Bang Theory.
  2. NASA’s page describing the WMAP mission.
  3. NASA’s page on the age of the universe.
  4. The Big Bang countdown from the timeline made by John Baez.
  5. Diagram of the history of the universe from Fermilab.
   
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