The Horizon Problem

In one form or another, the Big Bang has become a staple of modern physics, but its traditional formulation raises a few troubling questions. The most important one, called the horizon problem, deals with the striking homogeneity of the universe. The cosmic background radiation from various directions in space is equal in temperature to better than one part in 100 000. How could regions of space separated by such vast distances be so close in temperature? The first reaction of nearly everyone who sees the problem is to reply that the two regions are separated now, but once were closely connected. Since they emerged from a common starting point, it is logical that they share physical characteristics.

There is, however, a problem with this rationale: standard big bang theory renders it impossible. Temperature homogenization relies on prolonged direct contact between two regions. Imagine viewing the big bang in reverse - you would see our current large, expanded universe rapidly contract until only a point - the original singularity - remained (this thought experiment will be conducted in ordinary, not imaginary, time). No exchange of information, and therefore no temperature homogenization, can occur between two regions of space separated by a distance greater than that traveled by light since the big bang. As our imaginary reverse big bang proceeds, getting closer and closer to the singularity, there is a "battle" between spatial distance between them and how far from the singularity we are. For example, they will be at a distance of 300 000 km - which light could travel in one second - at less than a second away from the singularity. Therefore, light cannot possibly have connected the regions. This remains true no matter how far back we go.

Alan Guth's Model

In 1979, Alan Guth of the Massachusetts Institute of Technology proposed a revision of the big bang model that corrected for this problem. His proposal was called inflationary theory. The core of the difficulty is this: the pull of gravity actually slows the expansion rate in the standard big bang model, so we must more than halve the time since the big bang in order to halve the separation. Guth found another solution to Einstein's equations (providing for an inflaton field that "stretches" space) that allowed for a short period of extremely rapid, exponential expansion followed by the standard slower expansion. Inflationary theory means that, in order to halve the distance between the regions, we can decrease the time by less than half, thus allowing the regions plenty of time to communicate and thus homogenize their temperatures.

Under inflationary theory, the time between 10^-36 and 10^-34 after the big bang (ATB) saw expansion by a factor of at least 10³⁰, as compared to a factor of about 100 in the standard big bang model. Inflationary theory thus is generally accepted by most cosmologists, but may be further refined by the ideas of imaginary time, the wave-function of the universe, and the big bang under string theory.