A faint signal from the primordial Universe has provided an important confirmation of theories that describe the period immediately after the Big Bang.

Researchers from the University of Chicago, speaking at the COSMO-02 workshop on particle physics and the early Universe, held last week in Chicago, presented the first evidence of a tiny polarization in the cosmic microwave background radiation. Such a polarization had been predicted by standard inflationary theories, which hold that the Universe underwent a rapid expansion immediately after the Big Bang. "This confirms, almost beyond doubt, that we really have understood the very young Universe," says Simon White, a director of the Max Planck Institute for Astrophysics in Garching, Germany.

The cosmic microwave background dates from some 300,000 years after the Big Bang, at which time the Universe was cool enough for protons and electrons to combine to form atoms. This allowed photons to travel freely through the Universe, as atoms interact less frequently with photons than free protons or electrons do. These photons form the microwave background.

One key prediction about the microwave background — that its temperature varies in different parts of the sky — has already been confirmed. The temperature differences are thought to be the remnant of small density fluctuations that seeded the formation of galaxies (see Nature 411, 880–881; 2001).

Theory predicts that the density fluctuations will also have caused some polarization of the microwave photons — a prediction now confirmed by the Chicago team. They studied 200 days' worth of data taken from two points in the sky by the National Science Foundation's Degree Angular Scale Interferometer radio telescope at the Amundsen-Scott South Pole Station. The effect is slight and has eluded scientists for decades, but the telescope was sensitive enough to identify a polarization that matches the predictions.

Researchers now want to increase the sensitivity of the polarization measurements. Several teams are now focused on this goal, including the consortium that runs Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERANG) — a balloon-borne telescope that navigates Antarctica.