Researchers using NASA’s Hubble Space Telescope reported
today they are seeing the conclusion of the cosmic epoch
called the “Dark Ages,” a time about a billion years after
the Big Bang, when newly-formed stars and galaxies were just
starting to become visible.
“With the Hubble Telescope, we can now see back to the epoch
when stars in young galaxies began to shine in significant
numbers concluding the cosmic ‘dark ages’ about 13 billion
years ago,” said Haojing Yan, a Ph.D. graduate student at
Arizona State University (ASU). The results are being
presented at the meeting of the American Astronomical Society
in Seattle.
Current theory holds that after the Big Bang that created the
universe, there was a time of expansion and cooling that lead
to what is known as the “Dark Ages” in cosmic terms. The
universe cooled sufficiently for protons and electrons to
combine to form neutral hydrogen atoms and block the
transmission of light. This epoch started about 300,000 years
after the Big Bang, and it may have ended about a billion
years later. Stars and galaxies started to form at some point
during this era. The omni-present neutral hydrogen in the
universe absorbed the ultraviolet light produced by stars and
cannot be seen by current telescopes.
The ASU team reports Hubble’s Advanced Camera for Surveys
(ACS) is revealing numerous faint objects that may be young
star-forming galaxies seen when the universe was seven times
smaller than it is today and less than a billion years old.
This was an important transition in the evolution of the
universe. Because ionized hydrogen does not absorb
ultraviolet light as easily as neutral hydrogen, the Dark
Ages came to an end. The Dark Ages ended when enough hot
stars had formed, so their ultraviolet light pervaded the
universe and re-ionized the neutral hydrogen. The shining
stars opened a window for astronomers to look very far back
in time.
“The objects we found are in the epoch when the universe
started to produce stars in significant numbers…the hard-
to-find young galaxies,” said Rogier Windhorst, professor of
astronomy at ASU. “These galaxies are at the boundary of the
directly observable universe,” he said.
The ASU team found the objects while examining a small
portion of the sky in the spring zodiacal constellation
Virgo. This particular area of the sky contains no known
bright galaxies, helping reduce light contamination in the
observations. The entire ACS field of view shows about 30
such faint red objects. The distances to the suspected young
galaxies are believed to be quite large, based on how red the
observed objects are compared with nearby galaxies.
Based on this sample, the ASU researchers estimate that at
least 400 million such objects filled in the entire universe
at this cosmic epoch to the limit of this Hubble image. The
researchers say they are able to see only the tip of the
iceberg with current telescopes such as Hubble. NASA’s
planned seven-meter James Webb Space Telescope is expected to
see the entire population of these proto-galactic objects
after it is launched in 2010.
Electronic images and additional information are available at
