Astronomers who want to study the cosmic dark ages face a fundamental problem. How do you observe what existed before the first stars formed to light it up? Theorists Abraham Loeb and Matias Zaldarriaga (Harvard-Smithsonian Center for Astrophysics) have found a solution. They calculated that astronomers can detect the first atoms in the early universe by looking for the shadows they cast.

To see the shadows, an observer must study the cosmic microwave background (CMB) -- radiation left over from the birth of the universe. The Big Bang filled the universe with light and matter. As space expanded, it cooled, and the light from the Big Bang dimmed as it was stretched to longer and longer wavelengths, leaving the universe in darkness.

When the universe was about 370,000 years old, it cooled enough for electrons and protons to unite, recombining into neutral hydrogen atoms and allowing the relic CMB radiation from the Big Bang to travel almost unimpeded across the cosmos for the past 13 billion years.

Over time, some of the CMB photons encountered clumps of hydrogen gas and were absorbed. By looking for regions with fewer photons -- regions that are shadowed by hydrogen -- astronomers can determine the distribution of matter in the very early universe.

"There is an enormous amount of information imprinted on the microwave sky that could teach us about the initial conditions of the universe with exquisite precision," said Loeb.

To absorb CMB photons, the hydrogen temperature (specifically its excitation temperature) must be lower than the temperature of the CMB radiation -- conditions that existed only when the universe was between 20 and 100 million years old. Coincidentally, this is also well before the formation of any stars or galaxies, opening a unique window into the so-called "dark ages."

Image Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics