PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 769 March 17, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

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% of all energy---see
update 624, http://aip.org/pnu/2003/split/624-1.html).

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. The microwaves are partly polarized
(oriented) from the time of their origin (emerging from the so
called sphere of last scattering---see
http://www.aip.org/pnu/2002/split/591-1.html ) and partly polarized
by scattering (later on their journey to Earth) from the pervasive
plasma (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 nK), provide a new estimate for the
inhomogeneities in the CMB's temperature. The simplest model, called
Harrison-Zeldovic, 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 at:

http://map.gsfc.nasa.gov/m_mm/pub_papers/threeyear.html