The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA satellite whose mission is to survey the sky to measure the temperature of the radiant heat left over from the Big Bang. The satellite was launched by a Delta II rocket on June 30, 2001, at 3:46 p.m. EDT from Cape Canaveral Air Force Station, Florida, USA.
The goal of WMAP is to map out minute differences in the Cosmic Microwave Background (CMB) radiation in order to help test theories of the nature of the universe. It is the successor to COBE and one of the series of medium-class explorer (MIDEX) satellites.
WMAP is named after Dr. David Wilkinson, a member of the science team and pioneer in the study of cosmic background radiation. The science goals of the WMAP broadly dictate that the relative CMB temperature be measured accurately over the full sky with high angular resolution and sensitivity. The overriding priority in the design was the need to control systematic errors in the final maps. The specific goal of WMAP is to map the relative CMB temperature over the full sky with an angular resolution of at least 0.3°, a sensitivity of 20 µK per 0.3° square pixel, with systematic artifacts limited to 5 µK per pixel.
To achieve these goals, WMAP uses differential microwave radiometers that measure temperature differences between two points on the sky. WMAP observes the sky from an orbit about the L2 Sun-Earth Lagrangian point, 1.5 million km from Earth.
This is on the line from the Sun to the Earth, but at a larger distance from the Sun than the Earth, where the sum of the (larger) Sun's gravity and the (smaller) Earth's gravity is equal to the centripetal force needed for an object to have the same orbital period in its orbit around the Sun as the Earth, with the result that the object will stay in that relative position. Gravity from the Sun is 2% (118 µm/s²) less than at the Earth (5.9 mm/s²), while the increase of required centripetal force is half of this (59 µm/s²). The sum of both effects is balanced by the gravity of the Earth, which is here also 177 µm/s².
This vantage point offers an exceptionally stable environment for observing, since the observatory can always point away from the Sun, Earth and Moon while maintaining an unobstructed view to deep space. WMAP scans the sky in such a way as to cover ~30% of the sky each day and as the L2 point follows the Earth around the Sun WMAP observes the full sky every six months. To facilitate rejection of foreground signals from our own Galaxy, WMAP uses five separate frequency bands from 22 to 90 GHz.
On February 11, 2003, the public relations group from NASA made a press release regarding the age and composition of the universe. This release included the most intricate "baby picture" of the universe taken so far. According to NASA, this picture "contains such stunning detail that it may be one of the most important scientific results of recent years". Note that the image was not the highest resolution image of the cosmic microwave background radiation at that time, but it was the all-sky image of the radiation which had by far the least noise.
The three-year WMAP data were released at noon on March 17, 2006. The data included temperature and polarization measurements of the CMB, which provide stronger confirmation of the standard Lambda-CDM model.
Findings so far from WMAP
WMAP provided higher accuracy measurements of many cosmological parameters than had been available from previous instruments. According to current models of the universe, WMAP data show:
- The universe is 13.7 billion ± 200 million years old .
- The universe is composed of:
- The cosmological scenarios of cosmic inflation are in better agreement with the three-year data, although there is still an unexplained anomaly on the largest angular measurement of the quadrupole moment.
- The Hubble constant is 70 (km/s)/Mpc, +2.4/-3.2
- The data are consistent with a flat geometry.
- CMB polarization results provide experimental confirmation of cosmic inflation favoring the simplest versions of the theory.
Instruments which also measured fluctuations in the Cosmic Microwave Background
Before WMAP, there were several incremental improvements in our maps of the Cosmic Microwave Background:
- COBE - measured the very large scale fluctuations
- Cosmic Anisotropy Telescope - measured the very small scale fluctuations in small regions of the sky
- Boomerang - measured fluctuations with improved precision
- Maxima - measured fluctuations with improved precision
- Cosmic Background Imager - measured the very small scale fluctuations with improved precision in small regions of the sky
- Very Small Array - measured fluctuations with improved precision in small regions of the sky
Future instruments are expected to make measurements with higher precision and/or resolution than WMAP. These include:
- Clover - improved precision and B-mode polarization measurements
- Planck Surveyor - improved precision
- WMAP Website at NASA GSFC
- NASA's February 11, 2003 press release
- local cosmological parameters - WMAP (1st Year) team
- Seife, Charles, BREAKTHROUGH OF THE YEAR: Illuminating the Dark Universe, Science 2003 302: 2038–2039. The article includes a bibliography and interesting web links.
- Sizing up the universe
- About WMAP and the Cosmic Microwave Background - Article at Space.com
- The 'Shape' of the Universe according to WMAP.
- Cosmology blog comments on the three-year data.
- NASA March 16, 2006 WMAP inflation related press release