Scientists See Through the Dark Ages of the Universe

Cambridge, MA – Astronomers who want to study the early universe face a
fundamental problem. How do you observe what existed during the “dark ages,”
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 era of recombination. 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 (CfA) 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.