When space theories collide

EXPERIMENTAL observations have amply confirmed the Big Bang theory, but some of its shortcomings have led scientists to develop the so-called inflationary theory, to pursue their bid to trace the early history of the universe.

Proposed originally by Alan Guth of MIT in 1980, the inflationary theory predicts the density of the universe is far higher than can be accounted for by the visible mass in it, which implies the existence of invisible, or dark, matter. The two theories have several predictions in common but they differ profoundly in their description of what happened in the first fraction of a second. The behaviour of the universe in this period is crucial as it has important consequences for its subsequent evolution.

According to the inflationary theory, the universe had a brief period of extraordinarily rapid expansion, or "inflation", during which it increased in size by many orders of magnitude in a fraction of a second. In asserting this, the inflationary theory differs radically on such issues as the variation in temperature, energy density and size of the universe from predictions made in the Big Bang theory.

The Big Bang has successfully explained many aspects of the observed universe, including the cosmic microwave background radiation, the red-shifting of the light from distant galaxies and the primordial abundance of the lightest elements. But, all these features depend on the way the model details the first second and more after the Big Bang. The inflationary and the Big Bang theories are agreed on events during this period.

The main weakness of the Big Bang theory is that it is based on a set of more or less arbitrarily introduced assumptions about the initial conditions of the universe. For instance, one of the observed universe's most salient features, namely, its large-scale uniformity, is not explained by the Big Bang theory but has to be included in the model as an assumption.

The Big Bang theory also requires that exotic particles, called magnetic monopoles, predicted by modern particle physics, must be produced in large numbers in the early stages of the formation of the universe. These monopoles, however, have not been experimentally detected.

The inflationary universe model was developed to overcome these limitations in the Big bang theory. The state of the universe one second after its formation had to be assumed in the Big Bang model. But in the inflationary theory, it follows as a consequence of the inflationary process, starting from any set of initial conditions. Furthermore, the density of magnetic monopoles is small enough in the inflationary theory to account for their not being detected yet.