Користувач:Viktoriia Borshchenko/Чернетка/Вимір 2

Launch, trajectory, and orbit[edit source]

Animation of WMAP's trajectory

Oblique view

Viewed from Earth

  Earth ·   WMAP

The WMAP spacecraft arrived at the Kennedy Space Center on 20 April 2001. After being tested for two months, it was launched via Delta II 7425 launch vehicle on 30 June 2001. It began operating on its internal power five minutes before its launching, and continued so operating until the solar panel array deployed. The WMAP was activated and monitored while it cooled. On 2 July 2001, it began working, first with in-flight testing (from launching until 17 August 2001), then began constant, formal work. Afterwards, it effected three Earth-Moon phase loops, measuring its sidelobes, then flew by the Moon on 30 July 2001, en route to the Sun-Earth L₂ Lagrange point, arriving there on 1 October 2001, becoming the first CMB observation mission posted there.

Locating the spacecraft at Lagrange 2, (1,500,000 km (930,000 mi) from Earth) thermally stabilizes it and minimizes the contaminating solar, terrestrial, and lunar emissions registered. To view the entire sky, without looking to the Sun, the WMAP traces a path around L₂ in a Lissajous orbit ca. 1.0° to 10°, with a 6-month period. The telescope rotates once every 2 minutes 9 seconds (0.464 rpm) and precesses at the rate of 1 revolution per hour. WMAP measured the entire sky every six months, and completed its first, full-sky observation in April 2002.

Experiment[edit source]

Pseudo-Correlation Radiometer[edit source]

The WMAP instrument consists of pseudo-correlation differential radiometers fed by two back-to-back 1.5 m (4 ft 11 in) primary Gregorian reflectors. This instrument uses five frequency bands from 22 GHz to 90 GHz to facilitate rejection of foreground signals from our own Galaxy. The WMAP instrument has a 3.5° x 3.5° field of view (FoV).

Foreground radiation subtraction[edit source]

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.

Foreground contamination is removed in several ways. First, subtract extant emission maps from the WMAP's measurements; second, use the components' known spectral values to identify them; third, simultaneously fit the position and spectra data of the foreground emission, using extra data sets. Foreground contamination was reduced by using only the full-sky map portions with the least foreground contamination, while masking the remaining map portions.

The five-year models of foreground emission, at different frequencies. Red = Synchrotron; Green = free-free; Blue = thermal dust.

Measurements and discoveries[edit source]

One-year data release[edit source]

On 11 February 2003, NASA published the first-year's worth of WMAP data. The latest calculated age and composition of the early universe were presented. In addition, an image of the early universe, that "contains such stunning detail, that it may be one of the most important scientific results of recent years" was presented. The newly released data surpass previous CMB measurements.

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  and ; the errors given are at 68% confidence.

Best-fit cosmological parameters from WMAP one-year results

Parameter	Symbol	Best fit (WMAP only)	Best fit (WMAP, extra parameter)	Best fit (all data)
Age of the universe (Ga)		13.4±0.3	–	13.7±0.2
Hubble's constant ( km⁄Mpc·s )		72±5	70±5	71+4 −3
Baryonic content		0.024±0.001	0.023±0.002	0.0224±0.0009
Matter content		0.14±0.02	0.14±0.02	0.135+0.008 −0.009
Optical depth to reionization		0.166+0.076 −0.071	0.20±0.07	0.17±0.06
Amplitude	A	0.9±0.1	0.92±0.12	0.83+0.09 −0.08
Scalar spectral index		0.99±0.04	0.93±0.07	0.93±0.03
Running of spectral index		—	−0.047±0.04	−0.031+0.016 −0.017
Fluctuation amplitude at 8h−1 Mpc		0.9±0.1	—	0.84±0.04
Total density of the universe		–	–	1.02±0.02

Using the best-fit data and theoretical models, the WMAP team determined the times of important universal events, including the redshift of reionization, 17±4; the redshift of decoupling, 1089±1 (and the universe's age at decoupling, 379+8

−7 kyr); and the redshift of matter/radiation equality, 3233+194

−210. They determined the thickness of the surface of last scattering to be 195±2 in redshift, or 118+3

−2 kyr. They determined the current density of baryons, (2.5±0.1)×10⁻⁷ cm⁻¹, and the ratio of baryons to photons, 6.1+0.3

−0.2×10⁻¹⁰. The WMAP's detection of an early reionization excluded warm dark matter.

The team also examined Milky Way emissions at the WMAP frequencies, producing a 208-point source catalogue.

Three-year data release[edit source]

The three-year WMAP data were released on 17 March 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 (Arcminute Cosmology Bolometer Array Receiver (ACBAR), Cosmic Background Imager (CBI) and BOOMERANG), Sloan Digital Sky Survey (SDSS), the 2dF Galaxy Redshift Survey, the Supernova Legacy Survey and constraints on the Hubble constant from the Hubble Space Telescope.

Best-fit cosmological parameters from WMAP three-year results

Parameter	Symbol	Best fit (WMAP only)
Age of the universe (Ga)		13.73+0.16 −0.15
Hubble's constant ( km⁄Mpc·s )		73.2+3.1 −3.2
Baryonic content		0.0229±0.00073
Matter content		0.1277+0.0080 −0.0079
Optical depth to reionization		0.089±0.030
Scalar spectral index		0.958±0.016
Fluctuation amplitude at 8h−1 Mpc		0.761+0.049 −0.048
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.

Five-year data release[edit source]

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

The five-year total-intensity and polarization spectra from WMAP

Matter/energy content in the current universe (top) and at the time of photon decoupling in the recombination epoch 380,000 years after the Big Bang (bottom)

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. Improvements to the spectra were in the 3rd acoustic peak, and the polarization spectra.

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. It also constrained the content of the present-day universe; 4.6% atoms, 23% dark matter and 72% dark energy.

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

The elliptical shape of the WMAP skymap is the result of a Mollweide projection.

Best-fit cosmological parameters from WMAP five-year results

Parameter	Symbol	Best fit (WMAP only)	Best fit (WMAP + SNe + BAO)
Age of the universe (Ga)		13.69±0.13	13.72±0.12
Hubble's constant ( km⁄Mpc·s )		71.9+2.6 −2.7	70.5±1.3
Baryonic content		0.02273±0.00062	0.02267+0.00058 −0.00059
Cold dark matter content		0.1099±0.0062	0.1131±0.0034
Dark energy content		0.742±0.030	0.726±0.015
Optical depth to reionization		0.087±0.017	0.084±0.016
Scalar spectral index		0.963+0.014 −0.015	0.960±0.013
Running of spectral index		−0.037±0.028	−0.028±0.020
Fluctuation amplitude at 8h−1 Mpc		0.796±0.036	0.812±0.026
Total density of the universe		1.099+0.100 −0.085	1.0050+0.0060 −0.0061
Tensor-to-scalar ratio	r	<0.43	<0.22

The data puts limits on the value of the tensor-to-scalar ratio, r <0.22 (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 10.9±1.4, the redshift of decoupling, 1090.88±0.72 (as well as age of universe at decoupling, 376.971+3.162

−3.167 kyr) and the redshift of matter/radiation equality, 3253+89

−87.

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

Main result[edit source]

праворуч|міні|Interviews with Charles Bennett and Lyman Page about WMAP The main result of the mission is contained in the various oval maps of the CMB temperature differences. These oval images present the temperature distribution derived by the WMAP team from the observations by the telescope during the mission. Measured is the temperature obtained from a Planck's law interpretation of the microwave background. The oval map covers the whole sky. The results are a snapshot of the universe around 375,000 years after the Big Bang, which happened about 13.8 billion years ago. The microwave background is very homogeneous in temperature (the relative variations from the mean, which presently is still 2.7 kelvins, are only of the order of 5×10⁻⁵). The temperature variations corresponding to the local directions are presented through different colors (the "red" directions are hotter, the "blue" directions cooler than the average).^{[citation needed]}