Illuminating The "Dark Ages" Of The Universe (Forwarded)

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For Release: May 3, 2004

Release No.: 04-15

Illuminating The "Dark Ages" Of The Universe

Cambridge, MA -- When the European dark ages ended with the coming
of the Renaissance in the 14th century, society was illuminated by new
"stars" of science, art and literature like Michelangelo, Leonardo da Vinci,
Giotto, and Dante. Oddly enough, the universe may have experienced the
same enlightenment. 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 -- like western
civilization -- 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.

Inflation and Dark Matter

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."

Studying CMB shadows also allows astronomers to observe much smaller
structures than was possible previously using instruments like the
Wilkinson Microwave Anisotropy Probe (WMAP) satellite. The shadow
technique can detect hydrogen clumps as small as 30,000 light-years
across in the present-day universe, or the equivalent of only 300 light-
years across in the primordial universe. (The scale has grown larger as the
universe expanded.) Such resolution is a factor of 1000 times better than
the resolution of WMAP.

"This method offers a window into the physics of the very early universe,
namely the epoch of inflation during which fluctuations in the distribution
of matter are believed to have been produced. Moreover, we could
determine whether neutrinos or some unknown type of particle contribute
substantially to the amount of 'dark matter' in the universe. These
questions -- what happened during the epoch of inflation and what is dark
matter -- are key problems in modern cosmology whose answers will yield
fundamental insights into the nature of the universe," said Loeb.

An Observational Challenge

Hydrogen atoms absorb CMB photons at a specific wavelength of 21
centimeters (8 inches). The expansion of the universe stretches the
wavelength in a phenomenon called redshifting (because a longer
wavelength is redder). Therefore, to observe 21-cm absorption from the
early universe, astronomers must look at longer wavelengths of 6 to 21
meters (20 to 70 feet), in the radio portion of the electromagnetic spectrum.

Observing CMB shadows at radio wavelengths will be difficult due to
interference by foreground sky sources. To gather accurate data,
astronomers will have to use the next generation of radio telescopes, such
as the Low Frequency Array (LOFAR) and the Square Kilometer Array
(SKA). Although the observations will be a challenge, the potential payoff
is great.

"There's a gold mine of information out there waiting to be extracted.
While its full detection may be experimentally challenging, it's rewarding
to know that it exists and that we can attempt to measure it in the near
future," said Loeb.

This research will be published in an upcoming issue of Physical Review
Letters, and currently is available online at http://arxiv.org/abs/astro-
ph/0312134.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin, evolution
and ultimate fate of the universe.

Note to editors: A high-resolution image to accompany this release is
online at:
http://cfa-www.harvard.edu/press/pr0415image.html