Telescopes serve as time machines for sky surveyors

Princeton NJ -- Princeton scientists have glimpsed the universe as it was when there was almost no starlight -- analogous perhaps to seeing the cosmos at the biblical moment of "...let there be light."

The astronomers, studying the light of one of the earliest star-like objects, observed the last wisps of distinctive gas clouds that initially pervaded the universe.

The achievement is the result of work by the Sloan Digital Sky Survey, a sky-mapping project that has discovered the most distant objects in the universe. These objects are so far away that their light took more than 13 billion years to reach telescopes on Earth, nearly the entire history of the universe.

When scientists study the light from these objects they are literally seeing into the past, like finally receiving a letter that was posted eons ago.

"Because of the finite speed of light, the light we receive from distant objects was emitted in the past. The more distant the object, the further back in time we are looking. Our telescopes are like time machines, allowing us to see the distant past," said Associate Professor of Astrophysics Michael Strauss, a Sloan Survey collaborator.

In 2001, a Princeton-led team of Sloan survey scientists identified a quasar that existed when the universe was just a billion years old, a time earlier than had ever been observed. The current age of the universe is thought to be 12 to 14 billion years.

The researchers knew immediately that this object might allow them to see remnants of the universe as it existed before matter coalesced into stars and quasars. In that early period, called the cosmic dark ages, the universe was little more than an expanding cloud of hydrogen and helium atoms. When these atoms slowed and cooled, gravity drew them together in clumps, which accumulated greater and greater density until the intense heat and pressure of their own mass made them burst forth as stars.

The remaining free-floating atoms were bathed in starlight, which heated them and stripped off their electrons. Today, scientists observe only these stripped, or ionized, atoms floating in space. The intact, electrically neutral atoms from the original cloud are gone.

From dark to light

In 1965, Princeton astrophysicist James Gunn, who was then at the California Institute of Technology, and colleague Bruce Peterson predicted that neutral hydrogen atoms would block certain colors from any bright light shining through them. They showed that if any clouds of neutral hydrogen remained from the dark ages, they would be distinguishable by this light-changing effect. Scientists have been looking for this unique signature in the light of very distant quasars for nearly 40 years, but did not find it. As a result, they had no direct observations to help them pin a date on the end of the dark ages and the first light of stars.

That changed when Sloan survey researchers looked carefully at their most distant quasar using the powerful Keck telescope on a mountain, Mauna Kea, in Hawaii. Last year, they found clear evidence of the long-sought Gunn-Peterson effect in the quasar's light -- essentially the last streaks of neutral hydrogen wafting in front of the beacon.

"What we are seeing here is, in a very real sense, the transition from the dark ages to the universe as we know it today," said Strauss, "because the universe becomes ionized as the dark ages end and the light from the first generation of stars begins to shine."

The discovery has heightened the hunt for further glimpses of the neutral hydrogen in the light of very distant objects, said Strauss. It also has spawned new rounds of theoretical work aimed at better describing those earliest phases of the universe, he said.

For the scientists working on the Sloan survey, the discovery is an example of the payoffs that are expected to come from carrying out such a painstaking project. The Sloan survey is creating a detailed map of one-quarter of the sky using a telescope and instruments developed by Gunn and collaborators at other institutions around the world. The project now has 11 participating institutions.

The Sloan survey team has viewed about 50 million distant objects and analyzed the light content, or spectra, of 250,000 of them. Some of those points of light are quasars, which look like stars but really result from intense radiation that spews forth when matter falls into a very big black hole.

In its first years, the survey created excitement by breaking and rebreaking the record for the most distant quasar seen in the universe.

"Up until now it's as though we've just been breaking the land-speed record. Now we actually get to do something with it," said Strauss, paraphrasing a comment by John Peoples, the Sloan survey director.

Other Princeton collaborators on the dark ages study are Jill Knapp, professor of astrophysics; Vijay Narayanan, a postdoctoral fellow; Robert Lupton, a technical staff member; and Xiaohui Fan, who received his Ph.D. in astrophysics and is now at the Institute for Advanced Study.
 
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