13.7 billion years ago, an event occurred. Space itself swelled terrifically into being. Suddenly, there appeared a burgeoning cosmos where before there had been nothing. The explosion left behind a molten sea of charged particles that would eventually give life to everything in the universe. The churning plasma expanded along with space for 300,000 years until its constituent protons and electrons had finally cooled enough to combine. Consequently, the entire universe was soon clouded by neutral hydrogen gas. Such gas absorbs radiation exceptionally well, and so space was plunged into darkness for another billion or so years.
At this point, it was gravity that dispelled the haze. Clumps of gas soon condensed under its influence, slowly becoming dense furnaces for nuclear fusion. The high energy UV radiation emitted by these young stars leaked out across the universe, heating the surrounding gas to temperatures they had not faced since the big bang. Electrons that had been trapped within the nuclei of light atoms were suddenly released, reionizing hydrogen and rendering space transparent once more. As the universe expanded, overdense regions of structure contracted under gravity. Over the course of a few billion years, a familiar cosmos took shape. Stars came together into galaxies, galaxies came together into clusters, wacky objects like quasars came and went, supernovae went off, our Solar System formed, and the universe painstakingly plodded into its current configuration.
You may have noticed that some details are a little murky. As of yet, astronomers have no idea how the first stars formed out of the ambient fog. One pair of scientists is now hoping to change that. Alan Rogers of MIT and Judd Bowman of Arizona State University have developed a method using the 21cm spectral line of hydrogen. This line appears in the spectra of hydrogen atoms that have undergone an excitation. According to Rogers and Bowman, the specific way in which the radio spectrum of hydrogen evolves with time tells us that reionization itself lasted about 5 million years. Thus, the first stars and galaxies were most likely born during this time. Understanding the stellar mechanics that occurred during reionization is one of the most active quests in cosmology today. The team hopes to learn more with future observation and fine-tuning of their instrument, the EDGES antenna at the MIT Haystack Observatory.