A small amount of time between the big bang itself and the Planck time (about 10-43 seconds; time it takes light to travel the Planck length) remains unexplained by either the standard or inflationary big bang theories. General relativity when applied to the time between the big bang and the Planck time reveals that, working backwards, the universe shrinks to zero size and temperature, density, and spacetime curvature become infinite. These apparent physical impossibilities alert physicists to the breakdown of general relativity and the need for a fully quantized theory of gravity to understand objects this small and dense. Currently, this "message" means we must invoke string theory, the only quantum theory we have that incorporates gravity.
The use of string theory raises several complex issues. First, perturbative methods yield only very rough approximations since the extremes encountered at the moment of the big bang require precise calculation. Nonperturbative methods, still in their infancy, can present calculational difficulties and can only be used under certain circumstances. Still, physicists have determines several of the effects of string theory on the big bang model. In fact, string theory seems to modify the big bang theory in three important ways.
First, string theory implies that the universe has a minimum size. This effect, still being studied and refined, has profound implications in circumventing the troubles associated with zero size. Second, string theory has small/large radius duality, which further contributes to the size effect. Third, string theory incorporates more than the usual four spacetime dimensions.
Physicists Cumrun Vafa and Robert Brandenberger were the first to analyze in detail the implications of string theory on the big bang. They found that, if the big bang is again run in reverse, the temperature will continually increase until the universe is the Planck length, but will then hit a maximum and decrease. The reason behind this relates to the second effect of string theory on the big bang - the small/large radius duality. If we assume for simplicity that all the dimensions are circular, their radii will collapse toward and past the Planck length. However, string theory states that this event is physically indistinguishable from the radii collapsing to the Planck length and then reexpanding. The temperature decreases as the universe expands, so it is logical that the temperature would hit a maximum and then begin to decrease again. After careful calculations, Brandenberger and Vafa found that this scenario does in fact emerge.
This led to the following "stringified" big bang theory. All the spatial dimensions begin curled to the smallest possible spatial extent, roughly the Planck length. None of them have been distinguished in any way, so it is impossible to tell which will expand later or which will remain curled up. The universe then begins to expand and the first stage of symmetry reduction will occur at about the Planck time, when three of the spatial dimensions begin to expand while the others remain curled up.
Why do only three dimensions expand? Physicists have long speculated on this question, since there seemed to be no reason for this particular symmetry reduction. However, Brandenberger and Vafa also studied this question and have a possible explanation. They theorized that wrapped strings tend to constrict the size of their encircled dimensions. If a wrapped string and its antistring encounter each other, they should annihilate and produce an unwrapped string, allowing the dimension to expand. The two physicists suggest that this can happen in only three spatial dimensions.
To understand their reasoning, imagine two points moving along a line. There is a very high probability that they will intersect. However, if their arena is extended to two dimensions or more, the probability that they will intersect is significantly decreased and they will generally not meet. For strings, which have spatial extent, the number of dimensions in which they they become unlikely to collide is four (spatial). Click here for a more advanced explanation.
Physicists Gabriele Veneziano and Maurizio Gasperini of the University of Torino have developed their own, slightly different view of the origin of the universe. According to their theory, the universe began cold and infinite in spatial extent. An instability of some sort suddenly drove this pre-universe to vast inflationary expansion, resulting in a drastic upward trend in temperature and density. After some unknown time has passed, a small patch within this pre-universe emerges as the original hot, dense point of big bang cosmology. The expansion of this patch is what we recognize as the familiar big bang.
Physicists have been frantically researching the effects of M-theory, as opposed to the ordinary string theories, on the earliest moments of the universe. More information on this idea can be found in Symmetry Reduction.
Another recently proposed, hotly debated idea involves another variation of the many-worlds theory. Developed by André Linde, the theory postulates that the expansion of a patch within the pre-universe may have occurred more than once, giving rise to multiple universes within one larger framework (the multiverse) that may differ in fundamental physical properties so long as their light never reaches another universe. Some claim that this destroys the idea of ever having a theory with full power to predict the probabilities of various events, but others disagree. They argue that a theory that explains everything in our universe can be extended to the whole of the multiverse, determining why and how certain physical properties are distributed through the constituent universes.
Physicist Lee Smolin has contributed yet another variation on this idea. He theorizes that every black hole seeds another universe hidden from our view. Moreover, he suggests that the properties of this new universe are similar to, but not exactly the same as, those of the parnt universe. He goes on to speculate on a cosmological version of evolution. Knowing that the processes associated with star formation and destruction are closely associated with the physical constants of a universe, he has theorized that the universes most capable of generating more black holes and thus more universes will be continually selected by the fact that they will leave more "progeny." Those universes optimized for production of black holes will then grow numerous and overwhelm the other, less successful universes. Scientists have begun the process of calculating the validity of this proposal, but there is no consensus as yet.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.