Astronomers See First Light in Universe, Lifting Cosmic

Fog

R. R. Britt, Senior Science Writer

Astronomers announced Tuesday they have seen through

the fog of the early universe to spy some of the first light 

emitted during a "cosmic renaissance" that occurred when

the first galaxies were born.

The announcement came just days after a different

research group said they had spotted the first evidence of

the cosmic dark ages, the period long thought to have

preceded this newly spotted cosmic brightening.

Together, the studies provide glimpses into the earliest

mechanisms of the universe, after the Big Bang.

Astronomers familiar with the studies called them

important for helping create a timeline of the universe's

evolution.

Evidence for the two epochs have long been sought by astronomers and cosmologists, who believe the universe began in a Big Bang some 12 to 15 billion years ago, after which the universe expanded rapidly but remained dark for millions and millions of years. Lumps and bumps were thought to form in an otherwise smooth distribution of matter during these dark ages, and the first galaxies were born after gravity caused these clumps of matter to grow larger.

The galaxies marked the end of the dark ages and the beginning of the cosmic renaissance.

"This was one of the key stages in the history of the universe," said S. George Djorgovski, a California Institute of Technology astronomy professor who led the team that discovered the cosmic renaissance evidence.

Djorgovski and his colleagues, like the other team that found evidence for the dark ages, used data collected by the Sloan Digital Sky Survey, a collaboration of several

universities and institutions, including NASA and the National Science Foundation.

In the beginning

Researchers say that shortly after the Big Bang, the universe was loaded mostly with hydrogen and helium that was ionized -- too hot to remain in a stable state. For about 300,000 years, the universe expanded and cooled, and the gases began to recombine and stabilize to neutral states.

The universe then entered the dark ages, estimated to have lasted about half a billion years. All the while, clumps of matter developed. Then the first stars, galaxies and quasars formed. Quasars are incredibly bright objects thought to harbor black holes with masses billions of times that of our Sun.

Like morning sunlight burning through fog, the radiation from these new objects made the opaque gas of the universe become transparent by splitting atoms of hydrogen into free electrons and protons, say researchers, who call this period one of re-ionization.

"The results are now telling us when this process was completed," said Sir Martin Rees, a cosmologist at Cambridge University who is familiar with the study. Rees told SPACE.com that the results are important for pinning down a time chart of the universe.

"It is as if the universe was filled by a dark, opaque fog up to that time," explains Sandra Castro, a postdoctoral student at Caltech and member of the team. "Then the fires -- the first galaxies -- lit up and burned through the fog. They made both the light and the clarity."

The researchers studied the spectra, or spectrum of light emissions, of a quasar called SDSS 1044-0125, discovered last year by the Sloan team. The spectra of the quasar were obtained at the W. M. Keck Observatory atop Mauna Kea, Hawaii.

In the Keck observations, researchers found not only clear areas of sky but also large, leftover dark regions caused by the opaque gas from the dark ages. Scientists know that certain wavelengths of ultraviolet light are either absorbed or pass through hydrogen depending on its atomic state. In this case, the light emitted by the quasar was absorbed by intervening "neutral" hydrogen -- the non-ionized hydrogen that was the supposed fog of the dark ages. In other patches, some of the hydrogen had apparently been re-ionized, because the ultraviolet light passed through.

They say the conversion from an opaque to transparent universe was not instantaneous. It may have taken tens or even hundreds of millions of years for the earliest quasars and galaxies to burn through the cosmic fog.

The data show the trailing end of this time of change, said Daniel Stern, a scientist at NASA's Jet Propulsion Laboratory who worked on the new study, which been submitted for publication in the Astrophysical Journal Letters.

"There were opaque regions in the universe back then, interspersed with bubbles of light and transparent gas," Stern said. "This is exactly what modern theoretical models predict."

Stern said the beginning of the cosmic renaissance could not be seen because it "seems to be just outside the range of our data." The quasar studied by Stern and his Caltech colleagues was slightly closer than the one studied by the team that found evidence for the dark ages, and it is estimated to have formed a hundred millions years later.

"We had not seen [the re-ionization] before," said Abraham Loeb, a Harvard University astronomer who was not involved in the work but is familiar with it. "This is the first indication that we're getting close to it or perhaps we have seen it."

In a 114-page paper recently published in Physics Reports, Loeb and colleague Rennan Barkana of Princeton University detailed the re-ionization process and predicted that observations were on the threshold of confirming it.

But more observational and theoretical study is now needed to confirm the new results, Loeb said in a telephone interview. "Before we can jump to conclusions we need more examples of these quasars," he said.

He expects other research groups will begin to study other quasars that have already been identified by the Sloan survey, and efforts will be boosted to find other undetected quasars.

Click here for more news and information about cosmology.

轉貼自︰

http://www.space.com/scienceastronomy/astronomy/cosmic_light_010808.html

本文於 修改第 2 次

Shadow of Cosmic Dark Ages Detected

SPACE.com Staff, 08/06/2001

The foggy dawn of the universe, a time even prior to the

shining of stars, has been detected by astronomers

conducting an extensive sky survey, it was reported this

weekend.

The discovery of the "cosmic dark ages" reaches back to

an epoch more than 12 billion years ago, after the Big

Bang, when the universe's expanding hydrogen gas

cooled and circulated through space in a huge, thick haze.

That haze is thought to have blocked light emitted by the universe's first stellar objects, as evidenced by a newly detected shadow seen in light from an extremely distant quasar, according to The New York Times. Quasars are some of the earliest star-like objects in the universe, burning with the energy of billions of Suns and difficult to detect as they are so far away.

"The expectation is that the universe started off, the first half-million years, being very hot," because of the initial explosion 13 billion years ago that birthed the universe, Sir Martin Rees, an astrophysicist at Cambridge University, told the newspaper.

As the universe cooled, it remained in darkness for hundreds of millions of years until stars started forming and ionized the gas, Rees said.

"And then the fog suddenly lifts, as it were," he said.

The quasar shadow effect, a result of follow-up research on a project called the Sloan Sky Survey, was like seeing clouds drift in front of the Moon, the newspaper said.

The findings will be submitted this week to The Astronomical Journal, said Princeton University astrophysicist Michael Strauss, who was among the project's leaders.

"We're in some sense probing the end of the dark ages, which means the universe has lit up," he said.

Other scientists involved in the finding were Xiaohui Fan of the Institute for Advanced Study in Princeton and Richard L. White of the Space Telescope Science Institute in Baltimore.

The Sloan Survey is an $80 million, five-year census of the cosmos in which scientists all over the world are participating. In the past few years, it has detected the four most distant objects ever found.

Those objects were combed out by astronomers intentionally, using the Sloan's telescope in Apache Peak, New Mexico. Then astronomers looked at them more closely with the Keck Telescope on Mauna Kea, in Hawaii. The light pattern emitted by the most distant object of the four showed the tell-tale swath, the presumed shadow of the cosmic dark ages.

Click here for more stories about cosmology and the birth of the universe.

轉貼自︰

http://www.space.com./scienceastronomy/astronomy/cosmic_dawn_010806.html