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 there is a different plausible explanation.
An alternative being discussed by cosmologists is that our current universe was born from a previous universe that underwent a “Big Crunch” – gravity pulled everything into a huge singularity. In this scenario, the two universes would have been nearly identical in physical properties. The Big Bang would not have been a Bang, but a Big Bounce. This Big Bounce explanation has a few obvious problems associated with it. All evidence suggests that our current universe is open and continuously expanding. Conservation of energy seems to be violated, because where would the energy come from to convert a closed universe into an open one? Also, why would the gravitational collapse from the crunch suddenly stop? What caused the bounce?
More likely something else created the Big Bang.
According to M-theory (the most advanced theoretic model of our physical world, which postulates 11 dimensions), there must be a parallel 3-1 space/time universe directly on the opposite side of the 11th dimension from our own. These universes oscillate and can get extremely close to one another. The question has been asked, “What would happen if they collided with each other?” The answer is something called a “Big Crash.” This is different than the Big Bang scenario in that the formation of matter would not occur at a single point, but would expand exponentially from a region of space/time directly affected by the collision of the two 3-1 dimensional Universes. Furthermore, this might happen multiple times along different sections of the space/time continuum. This would certainly allow the current observable universe to be 78 billion light years across and yet only 13.7 billion years old without violating known physical laws. Furthermore, such a Big Crash would create a wave like distortion of space/time that would appear to accelerate the current expansion. These predictions match observed facts.