Explanations of the Big Bang' tend to be primarily speculative. All we know for sure is that the universe seems to be expanding at an accelerated rate from what appears to be a single point in space/time (roughly 13.7 billion years ago). The discovery of the cosmic microwave background radiation in 1964 and the proof of its relative uniformity through the consistent interpretation of both the COBE and the WMAP data appear to support this expansion scenario for the universe. Yet there are many things we still do not understand.

Why is the observable universe 78 billion light years across, if it is only 13.7 billion years old?

The seemingly simplistic and consensus answer is that the universe first expanded at faster than the speed of light, then slowed down significantly, and then started to accelerate again. We do not know whether the forces that caused the initial Inflation' period are the same forces that are currently causing the expansion to accelerate. In the context of expansion from a singularity (a black hole or white hole, which are two sides of the same coin), we do not have any good explanation as to why the initial expansion slowed.

How could the initial expansion be faster than light?

General Relativity breaks down within or near a black hole (in this case a white hole because of the expansion). Space/time itself expanded exponentially because of a negative-pressure vacuum energy density. As space/time was stretched, it pushed everything within it apart. The dimensions of our familiar 3-1 space/time were being created by this stretching, and therefore were not subject to the same cosmic speed limit as ordinary matter or energy.

What caused the Big Bang?

The original idea suggested that something came from nothing, a sort of "cosmic egg" or primeval atom. This is impossible given that matter and energy can neither be created nor destroyed - they can only be transformed. If the singularity hypothesis is correct, then our universe must have been born from a black hole in a different universe. The Big Bang would then be the white hole that would form on the other-side of a theoretic worm-hole connecting the two universes. This would suggest that the previous universe was likely very different from ours. Despite the attractive nature of this evolving theory, analysis of the equations involved suggests that a stable wormhole is not possible. It would close before any matter or energy could travel through. Perhaps

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