A New Triumph for Inflation

The inflationary big bang model has passed a crucial test as scientists working on the Wilkinson Microwave Anisotropy Probe released a long-awaited second set of data at a press conference held March 17.

WMAP was launched in 2001 to map the anisotropies in the cosmic microwave background (CMB) with far greater precision than the Cosmic Background Explorer, the predecessor that first discovered the anisotropies in 1990s.

The earlier release of WMAP data 3 years ago nailed down several grand features of the universe that had previously been known only very roughly, including: the time of recombination (380,000 years after the big bang, when the first atoms were formed); the age of the universe (13.7 billion years, plus or minus 200 million years); and the makeup of the universe (with dark energy accounting for 73 percent of all energy -- see PNU 624).

Since that 2003 announcement, WMAP researchers have painstakingly worked to reduce the uncertainties in their results. The big new thing in yesterday's announcement, based on three years of data, was the release of a map of the sky containing information about the microwaves' polarization (see image at Physics News Graphics).

The microwaves are partly polarized, or oriented, from the time of their origin (emerging from the so called sphere of last scattering -- see PNU 591) and partly polarized by scattering, on their journey to Earth, from the pervasive plasma of mostly ionized hydrogen created when ultraviolet radiation from the first generation of stars struck surrounding interstellar gas.

WMAP now estimates that this reionization, effectively denoting the era of the first stars, occurred 400 million years after the big bang, instead of 200 million years as had been previously thought. The main step forward is that smaller error bars, courtesy of the polarization map and the much better temperature map across the sky -- with an uncertainty of only 200 billionth of a degree Kelvin -- provide a new estimate for the inhomogeneities in the CMB's temperature.

The simplest model, called Harrison-Zeldovich, posits that the spectrum of inhomogeneities should be flat; that is, the inhomogeneities should have the same variation at all scales. Inflation, on the other hand, predicts a slight deviation from this flatness.

The new WMAP data for the first time measures the spectrum with enough precision to show a preference for inflation rather than the Harrison-Zeldovic spectrum -- a test that was long-awaited as inflation's smoking gun.

Papers available on the NASA Web page;
Image available at Physics News Graphics
High resolution images and more information available on the NASA page