Before the Dark Ages

The universe created in the big bang was predominantly made of hydrogen - about 75% by mass; the remainder of the matter was in the form of helium with some small traces of other light elements. The first hydrogen took the form of bare protons and electrons, and these particles scattered the cosmic light like droplets in a fog. After about 380,000 years the expanding universe cooled down, and stable, neutral hydrogen atoms formed. Light was then able to travel unfettered, and that light is seen today as the cosmic microwave background radiation.

The cosmic era after neutral atoms formed is called the "dark ages" because although the universe was filled with the remnant light, it gradually cooled off as the universe expanded, but meanwhile no new, hot radiation was being produced. Only after massive clouds of hydrogen atoms came together under the influence of gravity to form new stars did nuclear reactions heat up matter enough to generate significant new sources of light.
At least this is the conventional picture, and it is supported in considerable detail by new satellite and ground-based observations. Although comprehensive in general, however, this scenario leaves many gaps, one of which is the particular story of the universe during those dark ages, and precisely how that greatest generation of first stars formed.

CfA astronomers Jonathan Pritchard and Avi Loeb, writing in the Physical Review, have suggested a new way to use the radiation from neutral hydrogen atoms to probe this epoch. Hydrogen atoms emit at a radio wavelength of 21 centimeters, and radio telescope studies of hydrogen in our local galactic neighborhood have been a powerful tool of astronomy since the emission was discovered in the 1930's. Today there are several teams of astronomers planning to study the 21 centimeter light from period of the dark ages. Most of these new experiments, however, plan to search for (and model) variations in that emission, both because such differences are interesting but also because it is easier to measure differences in a faint glow than it is to determine the absolute strength of that emission. In their new theoretical paper, the CfA astronomers emphasize the important constraints that direct measurements would provide -- monitoring the local temperatures for example -- and also show how it could be possible to achieve precise results in the presence of the bright hydrogen emission from the Milky Way and our local universe. The new paper is the first serious look at this problem, and offers encouragement to experimentalists.