Age of universe refined
Posted: February 11, 2003

NASA today unveiled the sharpest picture ever taken of the infant universe, a landmark set of data showing when the very first stars "turned on" a surprisingly brief 200 million years after the big bang birth of the cosmos.

Full-sky map of the oldest light in the Universe from WMAP. Colors indicate "warmer" (red) and "cooler" (blue) spots. The oval shape is a projection to display the whole sky; similar to the way the globe of the earth can be represented as an oval. Credit: NASA/WMAP Science Team
The data from NASA's Wilkinson Microwave Anisotropy Probe, or WMAP, has also given researchers a new, independent measure of the age of the universe: 13.7 billion years.

While that number is consistent with earlier studies - and in agreement with recent research indicating the expansion of the universe is accelerating, not slowing down - the WMAP-derived age is believed to be accurate to within 1 percent.

More important, the spacecraft has provided independent verification of the fundamental theory of how the universe came into existence, what it's made of today and how it will end in the distant future. And it has done so with astonishing precision.

"WMAP has returned a gold mine of new results," Charles Bennett, the WMAP principal investigator at NASA's Goddard Space Flight Center, said at a news conference today.

"Number one, we have produced a new detailed, full-sky picture of our infant universe, the afterglow of the big bang. Number two, we have discovered the era when the very first stars in the universe turned on, ignited. Number three, WMAP lays the cornerstone of a coherent cosmic theory with a new set of accurate and precise numbers. An example of that is a new determination of the age of the universe."

That treasure trove of data conclusively shows the big bang theory remains the best explanation for the universe we see around us. And it will allow researchers to confirm or rule out various versions of inflation theory, which describes an ultra-brief instant long before the universe was one second old when the cosmic expansion suddenly, exponentially, accelerated.

This brief epoch of inflation set many of the parameters that govern the universe we life in today and the WMAP data will give researchers a powerful new tool for studying the physics of the early universe and the processes necessary to create stars in the aftermath of the big bang.

"The temperature variations we see in our picture correspond to tiny fluctuations in the density of material in the early universe, the seeds of materials starting to clump to form the stars and galaxies," Bennett said in an interview. "How quickly the material can clump depends partially on its content."

Data from WMAP shows atoms of ordinary matter make up 4 percent of the cosmos. Twenty three percent comes from an unknown type of "dark matter" and the remaining 73 percent is made up of "dark energy," the repulsive force now thought to be accelerating the expansion of the universe.

Again, the results are in remarkable agreement with earlier predictions but in this case, derived from a new set of data.

"Every astronomer will remember when they first heard the results from WMAP. I know I will," said John Bachall of the Institute of Advanced Study at Princeton. "The announcement today represents a right of passage for cosmology, from speculation to precision science and I am thrilled by the precision of the results.

"But I have to confess, I'm astounded," he said.

"Before the WMAP results, astronomers and physicists over the past couple of decades ... put together a very plausible picture of our universe. It had a tiny amount of the right stuff, ordinary matter. It had a modest amount of the dark matter, whatever that is, and it had an overwhelming amount of dark energy, which is a very strange beast.

"Now I have to confess I was very skeptical of this picture. But the WMAP results have convinced me. The error bars are tiny, they're really tiny, and the experiment has been designed so there are multiple, redundant, repetitive ways of checking the experiment and they all give the same answer. So we have to believe WMAP. The way the universe is, is the way WMAP sees the universe. We have to learn to understand this unattractive universe because we have no other choice."

Said David Spergel of Princeton University: "This is a beginning of a new stage in our study of the early Universe. We can use this portrait not only to predict the properties of the nearby universe, but can also use it to understand the first moments of the big bang."

The Microwave Anisotropy Probe began its mission atop a Boeing Delta 2 rocket at 3:46 p.m. on June 30, 2001. The spacecraft ultimately used the moon's gravity to send it on to its final destination, a point about 1.2 million miles from Earth.

Operating in the shade of a 16-foot-wide shield, MAP needed two years to complete its initial set of scientific observations, taking two full-sky pictures. Bennett's team then needed six months to analyze the resulting data.

At today's news conference, NASA announced the probe had been renamed in honor of Princeton cosmologist David Wilkinson, who died last September.

Before WMAP was launched, astronomers believed the universe - space and time - exploded into existence about 14 billion years ago. Most astronomers believed it took the expanding fireball 300,000 to 500,000 years or so to thin out enough for light to travel freely. At some point after that, matter began to clump together and stars were born.

The radiation that emerged 300,000 to 500,000 years ago, which can be thought of as a measure of the current temperature of the universe, was first detected in 1965 and mapped by NASA's Cosmic Background Explorer satellite, or COBE, in the early 1990s.

COBE's instruments detected the first hints of structure in the 2.73-degree background radiation, variations of one part in 100,000. The MAP spacecraft is 15 times more sensitive.

All-sky images of the infant Universe, 380,000 years after the Big Bang, over 13 billion years ago. In 1992, NASA's COBE mission first detected patterns in the oldest light in the universe (shown as color variations). WMAP brings the COBE picture into sharp focus.The features are consistent and 35 times more detailed than COBE's. Credit: NASA/WMAP Science Team
"Those patterns are put in place very early in the history of the universe due to tiny differences in gravity from one place to the next," Bennett said before WMAP's launch. "Those same differences are what allowed the galaxies to form. So we try to put all the pieces of the puzzle together."

In an interview Feb. 9, Bennett said WMAP lived up to its advance billing. Not only did it determine when the first stars began shining, it also nailed down when the universe turned transparent: 380,000 years after the big bang.

"At the time, 380,000 years, the material in the universe is starting to clump and as it clumps, the radiation pressure tries to push it apart," Bennett said. "And that sets up an oscillation. We know the physics of that very well."

By studying those oscillations in the faint afterglow of the big bang, the WMAP team was able to pin down exactly when photons began traveling unimpeded through the universe, a key milestone in the history of the cosmos and a sort of starting point for the clumping that ultimately led to star formation.

To figure out exactly when stars began shining, the WMAP team looked for incredibly subtle variations in the polarization of the microwave background radiation. That polarization, in turn, was caused by light from new-born stars re-ionizing material in the surrounding environment.

It is this re-ionization phenomenon, the stripping of electrons from surrounding material and the effects of that on the amplitude of the observed polarization, that WMAP observed. The earlier the stars turned on, the larger the amplitude of the polarization.

"We were really surprised by the result, that's only about 200 million years after the big bang," Bennett said. "When I went through school, generally we thought the first stars formed around a billion years after the big bang. In more recent times, people have been getting indications it was earlier than a billion."

Based on deep images by the Hubble Space Telescope, astronomers in recent years have revised their estimates. Galaxies appear in the Hubble images that are already formed within a billion years or so of the big bang, implying stars must have formed hundreds of millions of years earlier.

But seeing evidence of the first stars at just 200 million years after the birth of the universe was a stunning surprise.

"We're seeing the result of the stars turning on," Bennett said. "What we actually measure is the amplitude of that polarization signal. The later you turn the stars on, the weaker that signal gets."

Data from WMAP not only opens a window on the early universe, the physics responsible for the processes being observed leads directly to a fresh, independent estimate of the age of the universe - 13.7 billion years - to an accuracy of 1 percent.

"Basically, we look for what combination of numbers simultaneously explains everything," Bennett said in the interview. "We do this all in one step. We have our measurement of the reionization when the stars first turned on, we put all that in and say tell us what set of numbers explains our observed universe."

Because the WMAP data is so ultraprecise, "it doesn't give a lot of wiggle room," Bennett said.

More important, the WMAP conclusion was reached using a completely different approach than those responsible for earlier estimates based on observations of remote type 1a supernovae and even variable stars in galaxies closer to the Milky Way.

"The technique for how it's being measured is entirely different," Bennett said. "The fact that we get the same answer is pretty remarkable."

In an email, Bennett was asked to discuss what the WMAP data says about the ultimate fate of the universe. As a joke, the writer added "in five words or less."

Bennett replied: "The universe will expand forever."

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