Big Bang Confirmed: Seeing twists and turns of primordial light

The latest observations of the cosmic microwave background, the faint glow left over from the Big Bang, are giving cosmologists quite a turn.

PRIMORDIAL MAP. The intensity and polarization of the cosmic-microwave-background radiation recorded by the Degree Angular Scale Interferometer. Among the tiny variations in temperature, yellow is hottest and red is coldest. Each black line’s length represents strength of polarization at a location; its orientation reflects direction. DASI Collaboration

Revealing for the first time that microwave-background photons from adjacent patches of the sky vibrate in slightly different directions, the discovery confirms that by studying that background “we really are observing the universe as it was about 300,000 years after the Big Bang,” says theorist Wayne Hu of the University of Chicago. The finding verifies that just about everything astronomers thought they understood about the early universe and the emergence of galaxies is likely to be true, he adds.

Had astronomers not detected the polarization, “we would have had to go back to the drawing board” regarding our theories about the universe, says John E. Carlstrom of the University of Chicago. He led the new study of the cosmic microwave background and announced the results last week at the Cosmo-02 meeting in Chicago.

Two years ago, Carlstrom and his colleagues, including John Kovac of the University of Chicago, redesigned a ground-based detector they had built at the South Pole to study the cosmic microwave background. The researchers had already used the instrument, known as the Degree Angular Scale Interferometer, to record tiny temperature variations in the microwave background (SN: 4/28/01, p. 261: Available to subscribers at Sounds of the universe confirm Big Bang.). Next the team searched for polarization.

The hot and cold spots represent the slightly uneven distribution of photons and matter in the early universe, which scientists view as the seeds of galaxy formation. If that interpretation is correct, the cosmic microwave background would have to be polarized in the specific pattern that Carlstrom and his colleagues found.

The detection “is an important test of the physical conditions of the universe 300,000 years after the Big Bang,” says cosmologist David N. Spergel of Princeton University. Until that time, the universe was so hot that electrons and atomic nuclei were separate. Moreover, photons were constantly bouncing between closely spaced electrons and could not travel freely into space.

Each time a photon scattered off an electron, it became polarized. But for most of the first 300,000 years, the scattering of individual photons was so frequent that no net polarization developed.

That changed as the universe cooled and electrons began to combine with nuclei to make atoms. With fewer free electrons, photon scattering became less frequent. With more photons striking electrons from one direction than another, the polarization of the microwave background now could arise.

More detailed observations of polarization may shed light on conditions even earlier in the universe, such as the nature of inflation–the brief but stupendous growth spurt that seems to have generated the primordial lumps from which galaxies arose. Further polarization studies may also examine dark energy, the mysterious substance that scientists propose is revving up the expansion of the universe.


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