Using light from the most distant object known, astronomers have found
traces of the first generation of atoms in the universe, 14 billion light
years from Earth. The observations are the first of the cosmic “Dark Age”
between the Big Bang and the first visible stars and galaxies and allow
astronomers to set a date for the complete reionization of the universe.
The observations, by Robert Becker of the University of California,
Davis, and the Lawrence Livermore National Laboratory, Xiaohui Fan of
the Institute for Advanced Study in Princeton, Michael Strauss of
Princeton University, and Rick White of the Space Telescope Science
Institute, a team of scientists with the Sloan Digital Sky Survey, mark
the point when radiation from the first stars and quasars tore apart and
reionized — the neutral hydrogen atoms that filled space after the Big
Bang.
“The history of these atoms has been part of the unknown territory of
cosmology,” said UC Davis cosmologist Becker.
All space, time, and energy began with the Big Bang. As the universe
cooled, energy turned into matter. Quarks and electrons, then protons
and neutrons appeared in the first minute. But at temperatures of 1
billion degrees, it was too hot for complete atoms to form. Scientists
have found that it took another 300,000 years for the temperature to
cool off enough for whole atoms of hydrogen to appear.
The half billion years between the formation of these first atoms and
the creation of the most distant light-emitting objects detected by
astronomers remains shrouded in mystery. Astronomers have dubbed this
period the cosmic Dark Ages. We do know, however, that all was not quiet
in these early times. Cosmic background radiation from the period of
atom formation shows that matter was smoothly distributed through the
universe, said Becker. By the end of the Dark Ages, matter was clumped
into massive structures such as the billion-solar mass black holes
powering the quasars detected in the new observations.
In addition, while the Big Bang created only hydrogen, helium and a
trace amount of lithium, the quasar light reveals traces of more
complex atoms including carbon, nitrogen, oxygen and silicon, all of
which had to be cooked up in stars out of the primordial material.
At the end of the Dark Ages, hydrogen atoms were ionized again, or
stripped into atomic nuclei and electrons. This reionization was
probably caused by ultraviolet light from the first quasars and stars.
Until now, the timing of this reionization event has been uncertain.
In 1965, Jim Gunn and Bruce Peterson, then at the California Institute
of Technology predicted that the neutral atoms would be detected by
their light-absorbing signature, creating a trough in the spectrum.
Hydrogen atoms absorb all the light at a particular, characteristic
wavelength, Becker said. If at least one part in a 100,000 of the
hydrogen in intergalactic space were made up of whole atoms, all the
light at this wavelength would be blocked, he said.
“Seeing the effects of neutral hydrogen in the spectrum of this quasar
is telling us that we’re finally probing the universe to a time when the
first stars and quasars lit up,” said Gunn, now at Princeton University
and the project scientist of the SDSS. “Scientists have been looking for
this for a very long time; it is tremendously gratifying that we are
finally seeing it.”
Because light from objects that are distant in space and time is shifted
toward the red end of the spectrum, the Gunn-Peterson trough would also
be shifted, Becker said. By looking at where in the spectrum the trough
occurred, astronomers could tell how old those atoms were, he said.
To achieve this, astronomers have to look at objects billions of light
years away and therefore far back in time. But in 35 years, no one has
been able to detect the Gunn-Peterson effect, Becker said. The conclusion
was that almost all of the hydrogen in intergalactic space is ionized.
“Only now have we found a quasar that is far enough away,” Becker said.
Using the powerful Keck telescope on Mauna Kea in Hawaii, Becker and his
colleagues detected the imprint of neutral atoms on light from quasar
J1030+0524. J1030 is the most distant object yet found, at 14.5 billion
light years from Earth. It was discovered in April this year by the Sloan
Digital Sky Survey scientists, led by Fan.
The researchers spread out the recorded light into its constituent
wavelengths. They found that, over a region of the spectrum corresponding
to millions of years of cosmic history, no light from the quasar gets
through — it is all absorbed in intergalactic space.
“The culprits responsible are from the very first generation of atoms.
These observations provide our first glimpse at truly primordial material,
and constrain the time at which the reionization of the universe took
place,” Becker said.
The results will be submitted to the Astronomical Journal. The Keck
observatory, which houses the world’s largest telescope, is jointly managed
by the California Institute of Technology, the University of California,
and NASA. The Sloan Digital Sky Survey aims to map in detail one quarter
of the sky and 200 million celestial objects. Funding for the Sloan
Digital Sky Survey has been provided by the Alfred P. Sloan Foundation,
the participating institutions, the National Aeronautics and Space
Administration, the National Science Foundation, the U.S. Department of
Energy, the Japanese Monbukagakusho, and the Max Planck Society. The
Sloan Digital Sky Survey is a joint project of The University of Chicago,
Fermilab, the Institute for Advanced Study, the Japan Participation Group,
The Johns Hopkins University, the Max-Planck-Institute for Astronomy
(MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State
University, Princeton University, the United States Naval Observatory,
and the University of Washington.
More information:
* Sloan Digital Sky Survey
* W.M. Keck Observatory
http://www2.keck.hawaii.edu:3636/index.html
Media contacts:
Robert Becker, UC Davis Physics
(925) 423-0664 at the laboratory. Over the weekend of Aug. 4-5, Becker can be
reached via e-mail, bob@igpp.ucllnl.org , or at home at (925) 462-3189.
Rick White, STSCI
(410) 338-4899, rlw@stsci.edu
Michael Strauss, Princeton
(609) 258-3808 (he will check voice mail through the weekend.)
Lisa Lapin, UC Davis News Service
(530) 752-9842, lalapin@ucdavis.edu
Georgia Whidden, IAS Press Office
(609) 734-8239, gwhidden@ias.edu
Stephen Schultz, Princeton News Service
(609) 258-3601, sschultz@princeton.edu
Anne Stark, Lawrence Livermore National Laboratory
(925) 422-9799, astark@llnl.gov
