At the moment of the Big Bang, the universe was bathed with light that quickly faded. But with the ending of the cosmic dark ages as the first stars began to shine, the universe moved out of the dark ages and into the age of illumination.

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. *Illuminating The "Dark Ages" Of The Universe*. May 3, 2004

related context
> nucleosynthesis in the universe, the process of creating elements.
> oldest light: milestone in cosmology. 'new cosmic portrait of the afterglow of the big bang, called the cosmic microwave background.' february 17, 2003
> life come from explosions of stars. 'when stars die in explosions that generate billions upon billions of watts of energy, elements necessary for life are strewn throughout the galaxy.' september 25, 2001
> clues about early universe. 'these observations will be important in understanding how galaxies form and evolve.' june 6, 2000
> first "map" of dark matter. 'while dark matter makes up at least 90% of the mass of the universe, both its composition and its distribution are unknown.' march 7, 2000

imago
> cosmic microwave background fluctuations and distortions,
but what is behind shadows?