String Theory: What Banged?
The original expansion of the universe is a major theoretical question for cosmologists, and many are applying the concepts of string theory in attempts to answer it. The big bang theory doesn’t offer any explanation for what started the universe. One controversial conjecture is a cyclic universe model called the ekpyrotic universe theory, which suggests that our own universe is the result of branes colliding with each other.
Well before the introduction of M-theory or brane world scenarios, there was a string theory conjecture of why the universe had the number of dimensions we see: A compact space of nine symmetrical space dimensions began expanding in three of those dimensions. Under this analysis, a universe with three space dimensions (like ours) is the most likely space-time geometry.
In this idea, initially posed in the 1980s by Robert Brandenberger and Cumrun Vafa, the universe began as a tightly wound string with all dimensions symmetrically confined to the Planck length. The strings, in effect, bound the dimensions up to that size.
Brandenberger and Vafa argued that in three or fewer dimensions, it would be likely for the strings to collide with anti-strings. (An anti-string is essentially a string that winds in a direction opposite the string.) The collision annihilates the string which, in turn, unleashes the dimensions it was confining. They thus begin expanding, as in the inflationary and big bang theories.
Instead of thinking about strings and anti-strings, picture a room that has a bunch of cables attached to random points on the walls. Imagine that the room wants to expand with the walls and floor and ceiling trying to move away from each other — but they can’t because of the cables.
Now imagine that the cables can move, and every time they intersect, they can recombine. Picture two taut cables stretching from the floor to the ceiling that intersect to form a tall, skinny X. They can recombine to become two loose cables — one attached to the floor and one attached to the ceiling.
If these had been the only two cables stretching from floor to ceiling, then after this interaction, the floor and ceiling are free to move apart from each other.
In the Brandenberger and Vafa scenario, this dimension (up-down), as well as two others, are free to grow large. The final step is that in four or more space dimensions, the moving strings will typically never meet. (Think about how points moving in two space dimensions will probably never meet, and the rationale gets extended to higher dimensions.) So this mechanism only works to free three space dimensions from their cables.
In other words, the very geometry of string theory implies that this scenario would lead to us seeing fewer than four space dimensions — dimensions of four or more are less likely to go through the string/anti-string collisions required to liberate them from the tightly bound configuration. The higher dimensions continue to be bound up by the strings at the Planck length and are therefore unseen.
With the inclusion of branes, this picture gets more elaborate and harder to interpret. Research into this approach in recent years hasn’t been reassuring. Many problems arise when scientists try to embed this idea more rigorously into the mathematics of string theory.
Still, this is one of the few explanations of why there are four dimensions that make any sense, so string theorists haven’t completely abandoned it as a possible reason for the big bang.