Wilkinson Microwave Anisotropy Probe
|Charles L. Bennett (2007), Scholarpedia, 2(10):4731.||doi:10.4249/scholarpedia.4731||revision #137655 [link to/cite this article]|
The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA space mission that has put fundamental theories of the nature of the universe to a precise test. Since August 2001, WMAP has continually surveyed the full sky, mapping out tiny differences in the temperature of the cosmic microwave background (CMB) radiation, which is the radiant heat from the Big Bang. A fossil remnant of the hot big bang, the CMB permeates the universe and is seen today with an average temperature of only 2.725 Kelvin. Tiny variations about this average temperature were first discovered by NASA’s Cosmic Background Explorer (COBE) mission. WMAP followed up on the COBE results by characterizing the detailed statistical nature of the CMB temperature variations (called “anisotropy”), revealing a wealth of detail about the global properties of the universe. WMAP mapped the CMB with higher resolution (and greater sensitivity) than COBE.
The WMAP mission was proposed to NASA in 1995, selected in April 1996, and confirmed for development in 1997. The satellite, with its single instrument, was built, tested, and launched in only four years.
WMAP was launched on a Delta II rocket on June 30, 2001, at 3:46 p.m. EDT from the Cape Canaveral Air Force Station in Florida. Originally called the Microwave Anisotropy Probe (MAP), the satellite was renamed in 2003 to honor the memory and accomplishments of David T. Wilkinson, a member of the science team and a pioneer in CMB studies.
The WMAP science requirements dictated that the relative CMB temperature be measured accurately over the full sky. The overriding design requirement was to control systematic errors that would otherwise contaminate the measurements. To achieve this, WMAP uses differential microwave radiometers that measure temperature differences between pairs of spots on the sky.
To facilitate the separation of the CMB from foreground signals from our own Galaxy, WMAP uses polarization-sensitive radiometers at five separate frequency bands centered at 23, 33, 41, 61, and 94 GHz (wavelengths of 13, 9.1, 7.3, 4.9, and 3.2 mm). There are 4, 4, 8, 8, and 16 channels per frequency, respectively, with beam sizes of 0.88, 0.66, 0.51, 0.35, and 0.22 degrees. The radiometers are rapidly modulated with a 2.5 kHz phase switch. Amplitude calibration relies on the in-flight modulation of the cosmic dipole, and beam calibration relies on in-flight observations of Jupiter.
Dual back-to-back Gregorian (1.4 m x 1.6 m) primary reflectors focus the microwave radiation from two spots on the sky roughly 140° apart and feed these signals to 10 separate differential receivers that are in an assembly directly underneath the primary optics (Fig. 1). Large radiators, between the primary optics, passively cool the sensitive amplifiers in the receiver assembly below 90 Kelvin. The bottom half of the spacecraft provides the necessary avionics functions, such as command and data collection electronics, attitude (pointing) control and determination, power services, and a propulsion system. The entire observatory is kept in continuous shadow by a large deployed sun-shield, which includes the solar panels that power WMAP.
WMAP observes the sky from an orbit about the second Sun-Earth Lagrangian point (L2), 1.5 million km from Earth (Fig. 2). L2 is about four times further than the moon. This vantage point offers an exceptionally stable environment since the observatory can always point away from the Sun, Earth and Moon while maintaining an unobstructed view to deep space.
Scanning the full sky is an important part of WMAP’s mission. WMAP scans the sky in such a way as to cover ~30% of the sky each day; as the L2 point follows the Earth around the Sun, WMAP observes the full sky every six months.
The attitude is controlled using 2 star trackers, 2 gyros, coarse & fine Sun sensors, and 3 reaction wheels. The satellite spins at 0.464 rpm (~2 min per spin) and precesses at 0.017 rpm (1 hr per precession) about a 22.5° cone on WMAP-Sun line. Blow-down hydrazine propulsion with 8 thrusters were used to achieve the L2 orbit, and are also used for station-keeping.
The spacecraft structure is made of carbon composite and aluminum materials. The total observatory mass at launch was 840 kg. Communications employ two omnidirectional antennas and a fixed medium gain antenna, which is used at 667 kbps for daily downlinks to the 70 m Deep Space Network. A 3.1 sq. meter GaAs/Ge solar array oriented 22.5° off the full Sun line, and a 23 A-hr NiMH battery, provides the required 419 W. There are no eclipses in the observing phase of the mission. The design lifetime of 27 months has been exceeded.
The first results from the WMAP mission were reported on February 11, 2003. This release of the first year of flight data included the most detailed full sky "baby picture" of the Universe taken so far (Fig. 3). The WMAP data were compared and combined with other diverse cosmic measurements (based on galaxy clustering, supernovae, etc.), and a new unified and precise understanding of the universe emerged.
The universe is 13.7 billion years old, with a margin of error of about 1% (assuming the standard model with a cosmological constant and cold dark matter). The cosmic microwave signal in the WMAP map is from 379,000 years after the Big Bang. The expansion rate of the universe (the Hubble constant) value is H0= 71 (km/sec)/Mpc, with a margin of error less than 5%. The contents of the Universe include 4% baryons, 22% cold dark matter, and 74% dark energy. Fast moving neutrinos do not play a major role in the evolution of structure in the universe. In fact, they would have prevented the early clumping of gas in the universe, delaying the emergence of the first stars, in conflict with the new WMAP data.
The WMAP data confirm the existence of dark energy and aid in placing new constraints on its nature. One possibility for the dark energy, Albert Einstein’s "cosmological constant," is fully consistent with the WMAP data. In this scenario the Universe will expand forever. However, if the dark energy is due to some other physical effect, the conclusion about the fate of the universe could change.
The polarized data provide new evidence for inflation, the rapid expansion of the universe a fraction of a second after its birth. Many particular inflation models are ruled out; others are supported with this new evidence. When WMAP data are used in combination with large scale structure measurements, additional inflation models can be ruled out. The inflationary theory predicts that density is very close to the critical density, producing a spatially flat (Euclidean) universe. WMAP and other cosmological observations have determined that the universe is flat, within the limits of the measurements. Thus, the results support the Big Bang and inflation theories.
The WMAP data and related images are available on-line at the The Legacy Archive for Microwave Background Data Analysis (LAMBDA), http://lambda.gsfc.nasa.gov.
Marcus Chown, Afterglow of Creation: From the Fireball to the Discovery of Cosmic Ripples, University Science Books, 1996.
Alan H. Guth, The Inflationary Universe: The Quest for a New Theory of Cosmic Origins, Perseus Books Group, 1998.
Michael D. Lemonick, Echo of the Big Bang, Princeton University Press, 2003.
David N. Spergel, et al., “First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters,” Astrophysical Journal .Suppl, 148, 175, 2003. doi:10.1086/377226.
Charles L. Bennett et al., “First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Preliminary Maps and Basic Results,” Astrophysical Journal Suppl., vol 148, page 1, 2003. doi:10.1086/377253.
Charles L. Bennett et al., “The Microwave Anisotropy Probe (MAP) Mission,” Astrophysical Journal, vol 583, pages 1-23, 2003.
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