Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) – the radiant heat remaining from the Big Bang. Headed by Professor Charles L. Bennett of Johns Hopkins University, the mission was developed in a joint partnership between the NASA Goddard Space Flight Center and Princeton University. The current expansion rate of the universe is (see Hubble constant) . The content of the universe currently consists of ordinary baryonic matter; cold dark matter (CDM) that neither emits nor absorbs light; and of dark energy in the form of a cosmological constant that accelerates the expansion of the universe. Less than 1% of the current content of the universe is in neutrinos, but WMAP's measurements have found, for the first time in 2008, that the data prefer the existence of a cosmic neutrino background with an effective number of neutrino species of . The contents point to a Euclidean flat geometry, with curvature (<math>\Omega_{k}</math>) of . The WMAP measurements also support the cosmic inflation paradigm in several ways, including the flatness measurement.

The mission has won various awards: according to Science magazine, the WMAP was the Breakthrough of the Year for 2003. This mission's results papers were first and second in the "Super Hot Papers in Science Since 2003" list. Of the all-time most referenced papers in physics and astronomy in the INSPIRE-HEP database, only three have been published since 2000, and all three are WMAP publications. Bennett, Lyman A. Page Jr., and David N. Spergel, the latter both of Princeton University, shared the 2010 Shaw Prize in astronomy for their work on WMAP. Bennett and the WMAP science team were awarded the 2012 Gruber Prize in cosmology. The 2018 Breakthrough Prize in Fundamental Physics was awarded to Bennett, Gary Hinshaw, Norman Jarosik, Page, Spergel, and the WMAP science team. The 2019 Cocconi Prize of the European Physical Society was awarded to the WMAP collaboration (jointly with the Plank Collaboration).

In October 2010, the WMAP spacecraft was derelict in a heliocentric graveyard orbit after completing nine years of operations. All WMAP data are released to the public and have been subject to careful scrutiny. The final official data release was the nine-year release in 2012.

Some aspects of the data are statistically unusual for the Standard Model of Cosmology. For example, the largest angular-scale measurement, the quadrupole moment, is somewhat smaller than the Model would predict, but this discrepancy is not highly significant. A large cold spot and other features of the data are more statistically significant, and research continues into these.

Objectives

thumb|left|The universe's timeline, from the [[Big Bang to the WMAP]]

thumb|right|A comparison of the sensitivity of WMAP with COBE and Penzias and Wilson's [[Holmdel Horn Antenna|telescope (simulated data)]]

The WMAP objective was to measure the temperature differences in the Cosmic Microwave Background (CMB) radiation. The anisotropies then were used to measure the universe's geometry, content, and evolution; and to test the Big Bang model, and the cosmic inflation theory. The map contains 3,145,728 pixels, and uses the HEALPix scheme to pixelize the sphere. The telescope also measured the CMB's E-mode polarization,

The WMAP was preceded by two missions to observe the CMB; (i) the Soviet RELIKT-1 that reported the upper-limit measurements of CMB anisotropies, and (ii) the U.S. COBE satellite that first reported large-scale CMB fluctuations. The WMAP was 45 times more sensitive, with 33 times the angular resolution of its COBE satellite predecessor. The successor European Planck mission (operational 2009–2013) had a higher resolution and higher sensitivity than WMAP and observed in 9 frequency bands rather than WMAP's 5, allowing improved astrophysical foreground models.

Spacecraft

thumb|left|WMAP spacecraft diagram

thumb|right|Illustration of WMAP's receivers

The telescope's primary reflecting mirrors are a pair of Gregorian dishes (facing opposite directions), that focus the signal onto a pair of secondary reflecting mirrors. They are shaped for optimal performance: a carbon fibre shell upon a Korex core, thinly-coated with aluminium and silicon oxide. The secondary reflectors transmit the signals to the corrugated feedhorns that sit on a focal plane array box beneath the primary reflectors.

Foreground radiation subtraction

The WMAP observed in five frequencies, permitting the measurement and subtraction of foreground contamination (from the Milky Way and extra-galactic sources) of the CMB. The main emission mechanisms are synchrotron radiation and free-free emission (dominating the lower frequencies), and astrophysical dust emissions (dominating the higher frequencies). The spectral properties of these emissions contribute different amounts to the five frequencies, thus permitting their identification and subtraction.

Based upon the Lambda-CDM model, the WMAP team produced cosmological parameters from the WMAP's first-year results. Three sets are given below; the first and second sets are WMAP data; the difference is the addition of spectral indices, predictions of some inflationary models. The third data set combines the WMAP constraints with those from other CMB experiments (ACBAR and CBI), and constraints from the 2dF Galaxy Redshift Survey and Lyman alpha forest measurements. There are degenerations among the parameters, the most significant is between <math>n_s</math> and <math>\tau</math>; the errors given are at 68% confidence.

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! Parameter !! Symbol !! Best fit (WMAP only) !! Best fit (WMAP, extra parameter) !! Best fit (all data)

| Age of the universe (Ga) || <math>t_0</math> || || – ||

| Hubble's constant ( ) || <math>H_0</math> || || ||

| Baryonic content || <math>\Omega_b h^2</math> || || ||

| Matter content || <math>\Omega_m h^2</math> || || ||

| Optical depth to reionization || <math>\tau</math> || || ||

| Amplitude || A || || ||

| Scalar spectral index || <math>n_s</math> || || ||

| Running of spectral index || <math>dn_s / dk</math> ||—|| ||

| Fluctuation amplitude at 8h−1 Mpc|| <math>\sigma_8</math> || ||—||

| Total density of the universe ||<math>\Omega_{tot}</math> || – || – ||

Using the best-fit data and theoretical models, the WMAP team determined the times of important universal events, including the redshift of reionization, ; the redshift of decoupling, (and the universe's age at decoupling, ); and the redshift of matter/radiation equality, . They determined the thickness of the surface of last scattering to be in redshift, or . They determined the current density of baryons, , and the ratio of baryons to photons, . The WMAP's detection of an early reionization excluded warm dark matter.

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! Parameter !! Symbol !! Best fit (WMAP only)

| Age of the universe (Ga) || <math>t_0</math> ||

| Hubble's constant ( ) || <math>H_0</math> ||

| Baryonic content || <math>\Omega_b h^2</math> ||

| Matter content || <math>\Omega_m h^2</math> ||

| Optical depth to reionization || <math>\tau</math> ||

| Scalar spectral index ||<math>n_s</math> ||

| Fluctuation amplitude at 8h−1 Mpc ||<math>\sigma_8</math> ||

| Tensor-to-scalar ratio || r || <0.65

[a] Optical depth to reionization improved due to polarization measurements.

[b] <0.30 when combined with SDSS data. No indication of non-gaussianity.

The improvement in the results came from both having an extra two years of measurements (the data set runs between midnight on 10 August 2001 to midnight of 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 33-GHz observations for estimating cosmological parameters; previously only the 41-GHz and 61-GHz channels had been used.

Improved masks were used to remove foregrounds.

{| class="wikitable" style="margin:1em auto; text-align:center;"

|+ Best-fit cosmological parameters from WMAP five-year results Most were shown not to be statistically significant, and likely due to a posteriori selection (where one sees a weird deviation, but fails to consider properly how hard one has been looking; a deviation with 1:1000 likelihood will typically be found if one tries one thousand times). For the deviations that do remain, there are no alternative cosmological ideas (for instance, there seem to be correlations with the ecliptic pole). It seems most likely these are due to other effects, with the report mentioning uncertainties in the precise beam shape and other possible small remaining instrumental and analysis issues.

The other confirmation of major significance is of the total amount of matter/energy in the universe in the form of dark energy – 72.8% (within 1.6%) as non 'particle' background, and dark matter – 22.7% (within 1.4%) of non baryonic (sub-atomic) 'particle' energy. This leaves matter, or baryonic particles (atoms) at only 4.56% (within 0.16%).

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|+ Best-fit cosmological parameters from WMAP seven-year results

|- style="background:#b0c4de; text-align:center;"

! Parameter !! Symbol !! Best fit (WMAP only) !! Best fit (WMAP + BAO + H0)

| Age of the universe (Ga) || <math>t_0</math> || ||

| Hubble's constant ( ) || <math>H_0</math> || ||

| Baryon density || <math>\Omega_b</math> || ||

| Physical baryon density || <math>\Omega_b h^2</math> || ||

| Dark matter density || <math>\Omega_c</math> || ||

| Physical dark matter density || <math>\Omega_c h^2</math> || ||

| Dark energy density || <math>\Omega_\Lambda</math> || ||

| Fluctuation amplitude at 8h−1 Mpc || <math>\sigma_8</math> || ||

| Scalar spectral index || <math>n_s</math> || ||

| Reionization optical depth || <math>\tau</math> || ||

| *Total density of the universe || <math>\Omega_{tot}</math> || ||

| *Tensor-to-scalar ratio, k0 = 0.002 Mpc−1 || r || < 0.36 (95% CL) || < 0.24 (95% CL)

| *Running of spectral index, k0 = 0.002 Mpc−1 || <math>dn_s / d\ln k</math>|| ||

| Note: * = Parameters for extended models (parameters place limits on deviations from the Lambda-CDM model)

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|+ Best-fit cosmological parameters from WMAP nine-year results The map suggests the universe is slightly older than previously thought. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about 370,000 years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth (10−30) of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter. Based on the 2013 data, the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. On 5 February 2015, new data was released by the Planck mission, according to which the age of the universe is 13.799 ± 0.021 billion years and the Hubble constant is 67.74 ± 0.46 (km/s)/Mpc.

References

Primary sources

External links

Sizing up the universe
Big Bang glow hints at funnel-shaped Universe, New Scientist, 15 April 2004
NASA 16 March 2006 WMAP inflation related press release

Wilkinson Microwave Anisotropy Probe

Objectives

Spacecraft

Foreground radiation subtraction

See also

References

Primary sources

Further reading

External links