For a few hundred thousand years after the creation of the universe in the big bang, the universe was so hot that neutral atoms could not exist. Once the universe cooled to a few thousand degrees kelvin, however, neutral atoms formed and began to collect into primitive structures, perhaps proto-galaxies, as gravity gathered the matter together.
The first stars that formed heated the gas again, and ionized atoms. The period when the universe changed from containing mostly neutral atoms into mostly ionized ones is called the epoch of reionization, and is thought to have occurred a few hundred million years or so after the big bang. It is a particularly important period because it presumably holds the clues as to how the first stars formed, what kinds of stars they were, and the galaxy-like structures they comprised.
These stars were of crucial importance to us -- they began the process leading to the production of the chemical elements needed for life.
A team of six CfA astronomers, Matthew McQuinn, Adam Lidz, Oliver Zahn, Suvendra Dutta, Lars Hernquist, and Matias Zaldarriaga, has completed a computational study of the epoch of reionization that, for the first time, is able to produce a more realistic simulation of what likely went on in the early universe. They used a model with over a billion particles to study the properties of the first ionized regions that developed in this epoch from the stars. They find that the most important factor in determining the morphology of ionized regions is the fraction of ionized gas; other suggested factors like the degree of clumping of the matter were not found to be as important. The nature of the stars, if they are very massive for example, is another key determinant. Their results have immediate observational consequences, as several new cosmological experiments underway to study the epoch of reionization will be able to measure the detailed effects that these astronomers predict from their new models.