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A Polarized Universe

by Christopher Wanjek

Our universe is so cool you gotta wear polarized shades.

In the beginning, there was light. And now, scientists have discovered, the Creator might have needed sunglasses to shield the glare.

This image shows the intensity and polarization of the cosmic microwave background as seen by the DASI radio telescope. The black lines indicate the direction and strength of polarization.
This image shows the intensity and polarization of the cosmic microwave background as seen by the DASI radio telescope. The black lines indicate the direction and strength of polarization.

At a September 2002 science conference, researchers announced the first detection of polarized light from the cosmic microwave background (CMB), a blanket of ancient light that is essentially the afterglow of the Big Bang. Encoded within this polarized light is the lesson of how the universe grew from its quantum to cosmological girth in a fraction of a second, a period dubbed inflation, as well as the shape and mass-energy distribution of the universe.

More importantly for now, the detection of polarization reassures cosmologists that their fantastic theories are right on track and that this funky ride they have taken us on -- from inflation and dark energy to gravitational waves and hidden dimensions -- will continue along with full speed.

"The prediction is bang on," said discovery team leader John Carlstrom of the University of Chicago. "Polarization has been detected and it's in line with theoretical predictions. We're stuck with this preposterous universe."

Conversely, Carlstrom said that had we not found polarization in the CMB, cosmologists would have been back to square one.

Most light in the universe is unpolarized. Individual light waves traveling en masse tumble together, with waves propagating along a multitude of two-dimensional planes on a journey ending in our eyes. Light becomes polarized when it is reflected. Light reflected off a ski slope or the hood of your car, for example, is polarized largely along a horizontal plane, which magnifies once-random waves and produces a glare. Polarized sunglasses work by blocking glaring horizontal waves and allowing the passage of vertical waves only.

The CMB polarization was produced as light scattered off a primordial cloud of protons and electrons nearly 14 billion years ago, about 400,000 years after the Big Bang. This marks the moment of recombination, when the universe finally cooled enough to allow electrons to join protons. The CMB is the light that broke through the fog.

Scientists can study the early universe by scrutinizing the CMB in two ways. The first way is by looking for temperature fluctuations in the largely uniform and cool microwave light, which varies by only 0.00001?C across an average 2.73 Kelvin.

The temperature differences today point back to density differences early on, the seeds that grew into galaxies and voids of seemingly empty space. NASA's COBE mission discovered fluctuations in the 1990s; balloon-borne experiments such as BOOMERanG refined measurements a few years ago; and by January 2003, NASA's MAP mission, currently in orbit, is expected to reveal the clearest snapshot yet of the infant universe.

The second way scientists can learn about the early universe is by studying CMB polarization, which double-checks measurements attained though temperature fluctuations and provides a few more parameters to boot.

University of Chicago scientists detected the signs of polarization with DASI, the Degree Angular Scale Interferometer. This groundbased radio telescope sits on the South Pole about 3 kilometers above sea level. The team took in more than 200 20-hour nights of observations of the same patch of sky. The results don't say much about polarization, other than it's there. But that's enough for now. The mere detection of polarization is consistent with inflation theory. Yet much more is to come.

Theory predicts that the CMB polarization pattern is determined by both temperature fluctuations and gravitons, the quantum "particles" of gravity. So whereas COBE's fluctuations point to density, polarization points to how that matter reverberated with the thunder of gravitational waves from the period of inflation. Scientists hope to characterize these gravitational waves through intense measurements of CMB polarization. A major bump in sensitivity will be needed, however.

Scientists see the DASI result as a platform for a three-step process to observe not only inflation but the Big Bang itself. First comes MAP and Planck, a European Space Agency mission slotted for a 2007 launch. They will characterize CMB polarization so that scientists, finally knowing what to look for, can build an "inflationary probe" up to 100 times more sensitive to measure the imprint of inflationary gravitational waves.

The inflationary probe, in turn, would also search for the signature of gravitational waves from the Big Bang. This sets the scene for a big-bang observatory, which would not be a radio telescope but rather a gravitational wave detector more sensitive than a proposed gravitational wave mission called LISA.

Neat stuff. So to the daring team of scientists wintering over with DASI at the South Pole, I say thanks for keeping the concept of a preposterous universe alive.

Last Updated: 21 February 2011

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Last Updated: 21 Feb 2011