ASTRONOMERS have at last seen the shadows of the first atoms that formed
after the big bang. The finding should allow them to pin down when the 鈥渃osmic
dark ages鈥� ended and the first stars and galaxies began to light up space.
The cosmic dark ages stretched roughly from 300,000 to 900,000 years after
the big bang. They began when the fledgling Universe cooled to 3000 kelvin, cold
enough to allow electrons and protons to stick together to make light-absorbing
hydrogen atoms. They ended when galaxies of stars formed from the hydrogen shone
brightly enough to ionise the remaining hydrogen gas, turning it back into a
thin, translucent plasma of electrons and protons.
No one has peered into the dark ages to see the first stars lighting up. But
now astronomers have found the shadows of the primordial hydrogen in light from
the most distant quasar known, which lies around 14 billion light years from
Earth. This quasar appears to have burnt towards the end of the dark ages, says
Robert Becker, an astrophysicist at the University of California, Davis.
Advertisement
The quasar鈥檚 light that reaches Earth lacks wavelengths in a telltale range,
Becker鈥檚 team says in a report submitted to the Astronomical Journal.
The observation implies that 14 billion years ago, space contained hydrogen gas
that soaked up this part of the quasar鈥檚 radiation.
Astronomers have hunted for a quasar with this hallmark gap in its spectrum
since it was predicted 35 years ago. But finding one is difficult because these
distant objects are extremely faint. 鈥淚t鈥檚 really been a question of getting
telescopes and databases big enough to see far enough back,鈥� Becker says.
Indeed, to bag their prize quasar, Becker and his colleagues had to use three
different telescopes. They first spotted the quasar using the Sloan Digital Sky
Survey鈥檚 2.5-metre telescope at the Apache Point Observatory in New Mexico. They
then studied its spectrum with a second 3.5-metre telescope at Apache Point and
with the 10-metre Keck II telescope at Mauna Kea in Hawaii.
The researchers would like to confirm their result by spotting other similar
quasars. But ground-based observatories may struggle to see any farther into the
dark ages. Because of the expansion of the Universe, light from more distant
objects is stretched to near-infrared wavelengths and beyond. Optical
telescopes don鈥檛 detect such radiation, and ground-based infrared telescopes
can鈥檛 see very faint objects through the warm glow of the Earth鈥檚
atmosphere.
So the task of finding the very first stars and galaxies will probably fall
to the Next Generation Space Telescope (NGST), the mammoth space-based infrared
telescope that NASA plans to launch in 2009. Mark Dickinson of the Space
Telescope Science Institute in Baltimore, Maryland, says the observations of the
distant quasar should help fine-tune the NGST鈥檚 design. 鈥淚t will be very
important for planning and designing the NGST,鈥� Dickinson says. 鈥淚t gives us a
better indication of what we should be looking for.鈥�