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?

just came across your site wanted to say its very interesting and i love your illustrations

great stuff

Big bang chronology. ...absence of ionized gas makes universe transparent to light for first time. At 3000 K, kT=0.26 eV*, above this temperature atom formation is hindered. ...

*(Local todays kT= 0.023 eV).

The blackbody radiation is seen as a remnant of the transparency point at which the expanding universe dropped below about 3000K so that radiation could escape. At temperatures higher than about 3000 K where the average kinetic energy of particles is about 0.26 electron volts, the formation of stable atoms is hindered. Above that temperature, matter exists in a plasma state of ionized atoms, which strongly absorbs electromagnetic radiation of all wavelengths, i.e., the plasma is opaque. When the plasma cools below that temperature, it is cool enough for hydrogen and helium nuclei to collect electrons and become stable atoms. Stable atoms absorb only those frequencies characteristic of those atoms or those high enough to ionize them. This means that a cooling gas cloud has a point at which it becomes transparent to almost all wavelengths, at least for photons with quantum energy less than the ionization energy of the atoms. This transparency point is a crucial concept in the modeling of the expanding universe and in the modeling of star formation. Key information about it is provided by the 3 K* background radiation.

(3 K = -270 ºC)

(...)determines the maximal class of conformally flat cosmological models which eventually absorb all the electromagnetic radiation they contain. This is the requirement of the Wheeler-Feynman absorber theory of radiation. The condition for complete absorption is more or less written down immediately, and the various cosmological models tested by inspection. Some of the existing errors concerning the refractive index of the intergalactic medium and the past absorber are examined in some detail. Of the Friedmann cosmologies, only the oscillating models are opaque. A number of ever-expanding nonrelativistic cosmologies such as the Dirac and the latest Hoyle-Narlikar models, are also opaque. An assessment of the present value of the Wheeler-Feynman theory is given in the light of recent work on its quantization.
Is the universe transparent or opaque?P C W Davies Univ. Cambridge, UK

:-)

"The Universe began not with a bang but with a low moan, building into a roar that gave way to a deafening hiss. And those sounds gave birth to the first stars. Cosmologists do not usually think in terms of sound, but this aural picture is a good way to think about the Universe's beginnings.

Contrary to its name, the big bang began in absolute silence. But the sound soon built up into a roar whose broad-peaked notes corresponded, in musical terms, to a 'majestic' major third chord, evolving slowly into a 'sadder' minor third, Whittle explained.

For those worried that you cannot have sounds in space, that is true today, but it was not so in the Universe's infancy. For perhaps its first million years, the Universe was small and dense enough that sound waves could indeed travel through it - so efficiently, in fact, that they moved at about half the speed of light." From primordial sounds: big bang acoustics. june 1, 2004 http://www.astro.virginia.edu/~dmw8f/sounds/aas/press_release.txt

Big Bang Acoustics: Movie and Sound Files
http://www.astro.virginia.edu/~dmw8f/sounds/cdromfiles/index.php