Wilkinson Microwave Anisotropy Probe

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Wilkinson Microwave Anisotropy Probe
General information
Alternative names	MAP; Explorer 80
Organization	NASA
Launch date	30 June 2001, 19:46 GMT
Launched from	Cape Canaveral Air Force Station
Launch vehicle	Delta II rocket
Mass	840 kg
Type of orbit	Lissajous orbit
Location	Lagrange 2
Instruments
K-band 23 GHz	52.8 arcminute beam
Ka-band 33 GHz	39.6 arcminute beam
Q-band 41 GHz	30.6 arcminute beam
V-band 61 GHz	21 arcminute beam
W-band 94 GHz	13.2 arcminute beam
Website http://map.gsfc.nasa.gov
References: [1][2][3]

The Wilkinson Microwave Anisotropy Probe (WMAP; also known as the Microwave Anisotropy Probe or MAP and Explorer 80) is a satellite mission to survey the sky and measure the temperature of the radiant heat left over from the Big Bang. The mission is led by Professor Charles L. Bennett of Johns Hopkins University, and is a joint project between NASA Goddard Space Flight Center and Princeton University.^[4] The satellite was launched by a Delta II rocket on June 30, 2001, at 19:46:46 GDT from the Kennedy Space Center in Florida. It is the successor to COBE and one of the series of medium-class (MIDEX) satellites in the NASA Explorer program. It is named after Dr. David Wilkinson, a member of the science team and pioneer in the study of cosmic background radiation, who died in September 2002.^[4]

WMAP has provided much higher accuracy measurements of many cosmological parameters than had been available from previous instruments. According to the Lambda-CDM model of the universe, WMAP data shows that the age of the observable universe is 13.73 ± 0.12 billion years old, with a Hubble constant of 70.1 ± 1.3 km s^-1 Mpc^-1. It also shows that the universe is composed of 4.6% of ordinary baryonic matter, 23% of an unknown type of dark matter, which does not emit or absorb light, 72% of a mysterious dark energy, which acts to accelerate expansion and <1% neutrinos. The data is consistent with a flat geometry, with the ratio of energy density to the critical density Ω = 1.02 ± 0.02. Its results support the Lambda-CDM model, as well as the cosmological scenarios of cosmic inflation. It has also provided independent evidence for cosmic neutrino background radiation.^[5].

There are several unexplained features within the WMAP data, including an anomaly at the largest angular measurements of the quadrupole moment, dubbed the "Axis of Evil", and a large cold spot. WMAP was the Breakthrough of the Year for 2003 according to Science magazine.^[6] Mission results papers were #1 and #2 on the list of "Super Hot Papers in Science Since 2003".^[7] As of 2008 the probe is continuing to take measurements, and is due to complete observations in September 2009.

Contents 1 Objectives 2 Development 3 Description 4 Launch, trajectory and orbit 5 Foreground subtraction 6 Measurements and discoveries 6.1 One-year data release 6.2 Three-year data release 6.3 Five-year data release 7 Future measurements 8 References 8.1 Technical pages 9 External links

[edit] Objectives

The goal of WMAP is to measure the minute temperature differences in the Cosmic Microwave Background (CMB) radiation. Measurements of these anisotropies can be used to measure the geometry, content and evolution of the universe, and can be used to test the Big Bang model of the formation of the universe and the theory of cosmic inflation.^[1]

In order to do this, the mission aimed to create a full-sky map of the CMB with a resolution of 13 arcminutes using observations at multiple frequencies. The map was required to have the least amount of systematic errors possible in it, with no correlated noise on the pixels and accurate calibration, so that it was accurate on all angular scales greater than its resolution.^[1] The created map contains 3,145,728 pixels, and uses the HEALPix scheme for the pixelization of a sphere.^[8]

The philosophy behind the design of the telescope was to minimize the systematic errors, and control those that could not be minimized. The philosophy was adhered to even if it meant sacrificing some sensitivity or simplicity, or increasing the cost of the instrument.^[2]

The instrument can also measure the E-mode polarization of the CMB,^[1] as well as the polarization of the foregrounds, although it was not designed to be a true polarimeter.^[5] The telescope had a design lifetime of 27 months, which consisted of 3 months getting to the L2 position, followed by 2 years of observing.^[1]

[edit] Development

Prior to WMAP, there had been two space-based missions to observe the CMB. RELIKT-1 provided upper limits on the CMB anisotropies before COBE measured the very large scale fluctuations. There were also a series of ground- and balloon-based experiments which looked at the small-scale fluctuations in small patches of sky; these included Boomerang, the Cosmic Background Imager and the Very Small Array.

The MAP mission was proposed to NASA in 1995, and was selected in April 1996^[3] for a definition study. It was approved for development in 1997.^[9]

WMAP has 45 times the sensitivity, and 33 times the angular resolution of its predecessor, the COBE satellite.^[2]

[edit] Description

The primary reflectors on the telescope are a pair of nearly identical Gregorian 1.4 by 1.6 m dishes, which look in opposite directions. These focus the signal onto a pair of nearly identical 0.9 by 1.0 m secondary reflectors. The mirrors are shaped to optimize their performance, and are constructed from a carbon fibre shell on a Korex core, and are coated by thin layers of aluminium and silicon oxide. The signal is reflected from the secondary reflectors onto the corrugated feedhorns, which sit in a focal plane array box beneath the primary mirrors.^[1]

The receivers are differential radiometers, meaning that the difference between a pair of telescope beams is measured. They are polarization sensitive. The signal is amplified by HEMT low-noise amplifiers. There are 20 feeds in total, 10 looking in each direction. Each radiometer uses one feed from each direction, meaning that the signal measured is the difference in the sky signal from opposite directions. The directions are separated in azimuth by around 180 degrees, and in total angle by around 141 degrees.^[1]

To facilitate rejection of foreground signals from our own Galaxy, WMAP uses five separate frequency bands from 23 to 94 GHz.^[1]

Properties of WMAP at different frequencies^[1]

Property	K-band	Ka-band	Q-band	V-band	W-band
Central wavelength (mm)	13	9.1	7.3	4.9	3.2
Central frequency (GHz)	23	33	41	61	94
Bandwidth (GHz)	5.5	7.0	8.3	14.0	20.5
Beam size (arcminutes)	52.8	39.6	30.6	21	13.2
Number of radiometers	2	2	4	4	8
System temperature (K)	29	39	59	92	145
Sensitivity (mK s1 / 2)	0.8	0.8	1.0	1.2	1.6

The base of the instrument is a 5 m diameter solar array. In addition to hosting solar panels, the solar array also constantly keeps the instrument in shadow, and it is constantly angled at just over 22 degrees with respect to the sun during observations of the CMB. Upon this sits a bottom deck supporting the warm instrument's electronics and a top deck. The cold components of the telescope, namely the focal plane array and the mirrors, are separated from the warm components by a 33 cm long cylindrical, thermally isolating shell that sits on the top deck.^[1]

The instrument is cooled to around 90 K using passive thermal radiators, which are connected directly to the low noise amplifiers. The telescope uses a total of 419 W of power. The only heaters on the telescope are survival heaters for emergencies, and a heater located adjacent to the transmitters that is turned on when the transmitters are off to keep the thermal load constant. The temperature of the spacecraft is monitored using a series of platinum resistance thermometers.^[1]

Calibration is done using the dipole of the CMB and measurements of Jupiter, with the beam patterns measured using Jupiter. Data is relayed daily from the telescope via a 2 GHz transponder providing a 667 kbs downlink to a 70 m Deep Space Network telescope. There are two transponders, one of which is redundant. They are turned on for the minimum possible time—around 40 minutes per day—to keep radio frequency interference to a minimum. The telescope is kept in position in all three axes using three reaction wheels, gyroscopes, a pair of star trackers and sun sensors, and is steered using 8 hydrazine thrusters.^[1]

[edit] Launch, trajectory and orbit

The instrument arrived at the Kennedy Space Center on 20 April 2001; the following two months were spent testing the instrument and integrating it with the launch vehicle, a Boeing Delta II 7425 rocket. The telescope was launched from launch pad 17B of the Cape Canaveral Air Force Station on June 30, 2001, at 19:46:46 GMT, and was the 286th Delta launch. The observatory was exposed to space for the first time 5 minutes later, when the first-stage engine cut off and was ejected.^[2][3]

The observatory switched to internal power five minutes before launch, and relied on its internal batteries until the solar array was deployed at 21:03 GMT. The instrument was turned on at 21:43 GMT, and it was monitored while it cooled down. It started observing two days later, at 19:18 GMT on 2 July 2001, with in-flight testing running from the instruments launch until 17 August 2001, when the instrument began to constantly observe.^[2]

After launch, the probe went through three Earth-Moon phasing loops, during which 7 burns fine-tuned its trajectory. The loops were also used to measure the instrument's sidelobes. The probe then underwent a fly-by of the Moon at 16:37 on 30 July 2001, which put it on a course for the L2 Sun-Earth Lagrangian point. Two corrections to the trajectory were made^[2] during the 3 months it took for the probe to reach L2.^[1] It arrived at L2 on 1 October 2001.^[9] It is the first mission that uses L2 as a permanent observing station.^[3]

The spacecraft's orbit at Lagrange 2, 1.5 million kilometers from Earth, was chosen in order to minimize the amount of contaminating emission from the Sun, Earth and Moon, and to help thermally stabilize the spacecraft. In order to view all of the sky without looking towards the sun, WMAP orbits around L2 in a Lissajous orbit of between 1 and 10 degrees,^[1] with a period of 6 months.^[3] Between the start of 2002 and mid-2006, a total of 12 station keeping burns were made to keep it in this orbit.^[2] The telescope also spins once every 2 minutes and 9 seconds (0.464 rpm) and precesses at a rate of 1 revolution per hour.^[1] WMAP measures all of the sky every 6 months,^[3] and completed its first full sky observation in April 2002.^[9]

[edit] Foreground subtraction

WMAP observes in five frequency bands so that the foreground contamination of the CMB from our own galaxy (and also from extragalactic sources) can be measured and subtracted. The main emission mechanisms are synchrotron radiation and free-free emission dominant at the lower frequencies, with emission from astrophysical dust dominant at the higher frequencies. Due to the spectral properties of these emission mechanisms, they contribute in different amounts to the five frequencies, allowing for their identification and removal.^[1]

There are a variety of methods to remove the foreground contamination. One method is to subtract existing maps of the emission from the WMAP measurements; another is to use known values for the spectra of the different components to identify them, and a third approach is to fit the data simultaneously for both the position and spectra of the foreground emission, optionally also using extra data sets. Foreground contamination can also be reduced by only using the parts of the full-sky map that have the least foreground contamination, whilst masking the rest.^[1]

The five-year models of foreground emission at different frequencies. Synchrotron is red, free-free is green and thermal dust is blue.

23 GHz	33 GHz	41 GHz	61 GHz	94 GHz

[edit] Measurements and discoveries

[edit] One-year data release

On 11 February 2003, based on the one-year WMAP data, NASA issued a press release regarding the age and composition of the universe. This release included the "best baby picture" of the universe taken up to that point. According to NASA, this picture "contains such stunning detail that it may be one of the most important scientific results of recent years". The new data far exceeded previous CMB measurements in both accuracy and precision.^[4]

The WMAP team produced a number of sets of constraints on cosmological parameters from the WMAP first year results, based on the Lambda-CDM model, using various combinations of data sets. Three of these sets are given below. The first set given is from the WMAP data alone, as is the second: the difference is the addition of another parameter: the running of the spectral indices, which is a prediction of some inflationary models. The third combines the constraints from WMAP with those from some other CMB experiments (ACBAR and CBI), as well as with constraints from the 2dF Galaxy Redshift Survey and Lyman alpha forest measurements. Note that there are degeneracies between the different parameters, the most significant of which is between n_s and τ. The errors given are at 68% confidence.^[10]

Best-fit cosmological parameters from WMAP one-year results^[10]

Parameter	Best fit (WMAP only)	Best fit (WMAP, extra parameter)	Best fit (all data)
Reduced Hubble constant h	0.72 ± 0.05	0.70 ± 0.05
Baryonic content Ωbh2	0.024 ± 0.001	0.023 ± 0.002	0.0224 ± 0.0009
Matter content Ωmh2	0.14 ± 0.02	0.14 ± 0.02
Optical depth to reionization τ		0.20 ± 0.07	0.17 ± 0.06
Amplitude A	0.9 ± 0.1	0.92 ± 0.12
ns	0.99 ± 0.04		0.93 ± 0.03
dns / dk	—	-0.047 ± 0.04
σ8	0.9 ± 0.1	—	0.84 ± 0.04
Age of the universe (Gigayears)	13.4 ± 0.3	—	13.7 ± 0.2
Total density of the universe, Ωtot	—	—	1.02 ± 0.02

Using the best fit to all data, and theoretical models, the WMAP team put constraints on the times that various important events happened within our universe. These include the redshift of reionization, 17 ± 4, the redshift of decoupling, 1089 ± 1 (as well as age of universe at decoupling, kyr) and the redshift of matter/radiation equality, . They determined the thickness of the surface of last scattering to be 195 ± 2 in redshift, or kyr. They were also able to determine the current density of baryons, , and the ratio of the number of baryons to the number of photons, . WMAP's detection of an early reionization ruled out warm dark matter.^[10]

The team also examined the emission from our galaxy at the WMAP frequencies, and produced a catalogue of 208 point sources. They also observed the Sunyaev-Zel'dovich effect at 2.5σ with the strongest source being the Coma cluster.^[8]

[edit] Three-year data release

The three-year WMAP data were released on March 17, 2006. The data included temperature and polarization measurements of the CMB, which provided further confirmation of the standard flat Lambda-CDM model and new evidence in support of inflation.

The 3-year WMAP data alone shows that the universe must have dark matter. Results were computed both only using WMAP data, and also with a mix of parameter constraints from other instruments, including other CMB experiments (ACBAR, CBI and BOOMERANG), SDSS, the 2dF Galaxy Redshift Survey, the Supernova Legacy Survey and constraints on the Hubble constant from the Hubble Space Telescope.^[11]

Best-fit cosmological parameters from WMAP three-year results^[11]

Parameter	Best fit (WMAP only)
Reduced Hubble constant h
Baryonic content Ωbh2	0.0229 ± 0.00073
Matter content Ωmh2
Optical depth to reionization τ	0.089 ± 0.030
ns	0.958 ± 0.016
σ8
Age of the universe (Gigayears)

Tensor-to-scalar ratio < 0.65 (95% certainty) from WMAP alone; < 0.30 (95% certainty) when combined with SDSS data. No indication of non-gaussianity.^[11] 115 new point sources. Optical depth to reionization improved due to polarization measurements.^[12]

[edit] Five-year data release

The five-year WMAP data were released on February 28, 2008. The data included new evidence for the cosmic neutrino background exists, evidence that it took over half a billion years for the first stars to reionize the universe, and new constraints on cosmic inflation. ^[13]

The improvement in the results came from both having an extra 2 years of measurements (the data set runs between midnight on 10 August 2001 to midnight of the 9 August 2006), as well as using improved data processing techniques and a better characterization of the instrument, most notably of the beam shapes. They also make use of the 33GHz observations for estimating cosmological parameters; previously only the 41 and 61GHz channels had been used. Finally, improved masks were used to remove foregrounds.^[5]

Improvements to the spectra were in the 3rd acoustic peak, and the polarization spectra.^[5]

The measurements put constraints on the content of the universe at the time that the CMB was emitted; at the time 10% of the universe was made up of neutrinos, 12% of atoms, 15% of photons and 63% dark matter. The contribution of dark energy at the time was negligible.^[13]

The WMAP five-year data was combined with measurements from Type Ia supernova (SNe) and Baryon acoustic oscillations (BAO).^[5]

Best-fit cosmological parameters from WMAP five-year results^[5]

Parameter	Best fit (WMAP only)	Best fit (WMAP + SNe + BAO)
Reduced Hubble constant h		0.701 ± 0.013
Baryonic content Ωbh2	0.02273 ± 0.00062	0.02265 ± 0.00059
Cold dark matter content Ωch2	0.1099 ± 0.0062	0.1143 ± 0.0034
Dark energy content, ΩΛ	0.742 ± 0.030	0.721 ± 0.015
Optical depth to reionization τ	0.087 ± 0.017	0.084 ± 0.016
ns
dns / dk	−0.037 ± 0.028
σ8	0.796 ± 0.036	0.817 ± 0.026
Age of the universe (Gigayears)	13.69 ± 0.13	13.73 ± 0.12
Total density of the universe, Ωtot		1.0052 ± 0.0064

The data also puts limits on the value of the tensor-to-scalar ratio, r < 0.20 (95% certainty), which determines the level at which gravitational waves affect the polarization of the CMB, and also puts limits on the amount of primordial non-gaussianity. Improved constraints were put on the redshift of reionization, which is 10.8 ± 1.4, the redshift of decoupling, (as well as age of universe at decoupling, years) and the redshift of matter/radiation equality, . ^[5]

The extragalactic source catalogue was expanded to include 390 sources, and variability was detected in the emission from Mars and Saturn.^[5]

The five-year maps at different frequencies from WMAP with foregrounds (the red band)

23 GHz	33 GHz	41 GHz	61 GHz	94 GHz

[edit] Future measurements

The original timeline for WMAP gave it two years of observations; these were completed by September 2003. Mission extensions were granted in both 2002 and 2004, giving the spacecraft a total of 8 observing years (the originally proposed duration), which end in September 2009.^[3]

WMAP's results will be built upon by several other instruments that are currently under construction. These will either be focusing on higher sensitivity total intensity measurements or measuring the polarization more accurately in the search of B-mode polarization indicative of primordial gravitational waves.

The next space-based instrument will be the Planck satellite, which is currently being built and will launch towards the end of 2008. This instrument aims to measure the CMB more accurately than WMAP at all angular scales, both in total intensity and polarization. Various ground- and balloon-based instruments are being constructed to look for B-mode polarization, including Clover and EBEX.

[edit] References

[edit] Technical pages

[edit] External links

• Sizing up the universe
• About WMAP and the Cosmic Microwave Background - Article at Space.com
• Big Bang glow hints at funnel-shaped Universe, NewScientist, 2004-04-15
• NASA March 16, 2006 WMAP inflation related press release
• Seife, Charles (2003). "With Its Ingredients MAPped, Universe's Recipe Beckons". Science 300 (5620): 730-731. 

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