The universe is growing colder and darker. These charts show the relative constituents of the universe today (top), and for when the universe was 380,000 years old, 13.7 billion years ago (bottom). Neutrinos used to be a larger fraction of the energy of the universe than they are now. Below: This image shows WMAP cosmic microwave fluctuations over the full sky with five years of data. Colors represent the tiny temperature fluctuations of the remnant glow from the infant universe: red regions are warmer and blue are cooler. Images: WMAP Science Team
A NASA satellite built in partnership with Princeton scientists has uncovered evidence that a sea of neutrinos -- almost weightless elementary particles that zip around at nearly the speed of light -- permeates the universe.
The discovery, announced March 7 by NASA, is part of a treasure trove of findings gleaned from five years of data collected by the Wilkinson Microwave Anisotropy Probe (WMAP). This includes clear evidence the first stars took more than a half billion years to create a cosmic fog and a far better understanding of the burst of expansion in the universe's first trillionth of a second.
The results are contained in seven papers submitted this week to the Astrophysical Journal.
"We are living in an extraordinary time," said Gary Hinshaw of NASA's Goddard Space Flight Center in Greenbelt, Md. "Ours is the first generation in human history to make such detailed and far-reaching measurements of our universe."
The so-called "cosmic neutrino background," detected by the probe, is a fossil from the very early universe, researchers said.
"We see these neutrinos on Earth, and it is wonderfully reassuring that we also see their effects at the edge of the observable universe," said Lyman Page, the Henry De Wolf Smyth Professor of Physics at Princeton and one of the lead researchers on the project. "Perhaps the biggest news is that we have a scientific model of the universe that explains virtually all observations of the cosmos."
Cosmic neutrinos originated in the universe's early moments, when enormous numbers of matter and antimatter particles annihilated one another. Millions of cosmic neutrinos pass through people every second.
WMAP measures the temperature of the cosmic light, also a remnant of the early universe. This oldest light in the universe, now seen as microwaves, has cooled substantially over the 13.7 billion years it has traveled through the universe.
By making accurate measurements of microwave temperature fluctuations across the entire sky, WMAP is able to precisely determine the composition and history of the universe. WMAP's measurements of this afterglow are helping to answer many questions about the universe's age, composition and development.
Cosmic neutrinos once existed in such huge numbers that they actually affected the universe's early expansion, which in turn influenced the microwaves that WMAP observes.
"The energy of these neutrinos is a million times lower than can be seen by existing neutrino detectors on Earth," said Eiichiro Komatsu of the University of Texas-Austin, a team member who was a postdoctoral fellow at Princeton from 2001 to 2003. He was also a visiting graduate student from 1998 to 2001 where he did his thesis work with David Spergel, the Charles Young Professor of Astronomy and chair of the Department of Astrophysical Sciences.
"A block of lead the size of our entire solar system wouldn't even come close to stopping a cosmic neutrino," Komatsu added.
The satellite measurements reflecting the traces of the mysterious particles also match precision measurements of neutrino properties made by particle colliders that simulate conditions present at the dawn of the universe. For the first time, evidence for the sea of cosmic neutrinos now comes from the ancient cosmic light as well.
The new WMAP data also provide crucial insights into the end of the "dark ages," when the first generation of stars began to shine. The glow from these stars ionized the surrounding gas, creating a thin fog of electrons that scatter microwaves, in much the same way that fog scatters the beams from a car's headlights.
"We now have evidence that the creation of this fog must have been a drawn-out process, starting when the universe was about 400 million years old and lasting for about half a billion years," said WMAP team member Joanna Dunkley, a visiting postdoctoral research associate in Princeton's physics and astrophysical sciences departments who also is a research fellow at the University of Oxford. "These measurements are currently possible only with WMAP."
The results also are providing the best data yet for examining the astonishing burst of growth in the first trillionth of a second of the universe, when ripples in space itself may have been created.
Measurements of sound wave relics from the Sloan Digital Sky Survey galaxy maps further confirm and extend WMAP's precision measurements, scientists said. The survey is a joint project of Princeton and a number of universities, institutions and federal organizations.
The WMAP results also show that the universe is growing colder and darker. Atoms now make up 4.6 percent of the current universe, dark matter makes up 23 percent and dark energy makes up 72 percent, according to its measurements. The microwaves that WMAP sees come from when the universe was only 380,000 years old, when atoms made up 12 percent of the universe, dark matter made up 63 percent, photons made up 15 percent, neutrinos made up 10 percent and the dark energy was negligible.
WMAP's 2003 determination of the need for a large percentage of dark energy, which is causing the universe's expansion to accelerate, erased the lingering doubts in the scientific community about its very existence. The 2003 measurements also led to WMAP's precision determination of the 13.7 billion year age of the universe.
"The new WMAP data rule out many mainstream ideas that seek to describe the growth burst in the early universe," said WMAP's principal investigator, Charles Bennett of Johns Hopkins University. "It is astonishing that bold predictions of events in the first moments of the universe now can be confronted with solid measurements."
NASA launched the sun-orbiting space probe in June 2001. The satellite is named for David Wilkinson, the Cyrus Fogg Brackett Professor of Physics at Princeton, who died in 2002. It is currently about 1 million miles from Earth. "It's exciting that more than six years after launch, WMAP is still operating well and producing significant cosmological results," said Norman Jarosik, a senior research physicist in Princeton's physics department.
Princeton students and scientists continue to play an important role in the WMAP satellite.
In addition to Page, Dunkley, Jarosik and Spergel, several other Princeton scientists participated in the project. Recent Princeton Ph.D. Michael Nolta led the analysis of the power spectrum.
"It is really exciting to see our former students and postdocs who worked on early WMAP analysis play a leadership role in this analysis," Spergel said. Three of the major analysis papers are led by current and former Princeton graduate students and postdoctoral fellows.
