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en:Wilkinson Microwave Anisotropy Probe

Il Wilkinson Microwave Anisotropy Probe (WMAP), conosciuto anche come sonda spaziale per l'anisotropia delle microonde ((EN) : Microwave Anisotropy Probe (MAP)), e Explorer 80, è un satellite che misura ciò che rimane delle radiazioni dovute al Big Bang, ovvero la radiazione cosmica di fondo. Diretto dal professore della Johns Hopkins University Charles L. Bennett, si tratta di un progetto che prevede la collaborazione tra il Goddard Space Flight Center della NASA e l'Università di Princeton.^[1] Il satellite WMAP è stato lanciato il 30 giugno 2001, alle ore 19:46 (GDT) dallo stato della Florida. Il WMAP è l'erede dei satelliti COBE e MIDEX previsti dal programma Explorer. Tale satellite è stato così chiamato in onore di David Todd Wilkinson (1935-2002).^[1]

Le rilevazioni del WMAP sono più precise di quelle dei suoi predecessori; secondo il modello Lambda-CDM, l'età dell'universo è stata calcolata in 13.73 ± 0.12 miliardi di anni, con una costante di Hubble di 70.1 ± 1.3 km·s^-1·Mpc^-1, una composizione del 4,6% di materia barionica ordinaria; 23 % di materia oscura di natura sconosciuta, la quale non assorbe o emette luce; 72% di energia oscura la quale accelera l'espansione; infine meno del 1% di neutrini. Tutti questi dati sono coerenti con l'ipotesi che l'universo abbia una geometria piatta, e anche con il rapporto tra densità d'energia e densità critica di Ω = 1.02 ± 0.02. Questi dati supportano il modello Lambda-CDM e gli scenari cosmologici dell'inflazione, dando anche prova della radiazione cosmica di fondo di neutrini.^[2]

Ma questi dati contengono anche caratteristiche inspiegate: una anomalia nella massima misura ngolare del momento quadrupolico, ed una grande macchia fredda nella radiazione cosmica di fondo. Secondo la rivista scientifica Science, il WMAP è stato il Breakthrough of the Year for 2003 (scoperta dell'anno 2003).^[3] I risultati di questa missione sono stati al primo e al secondo posto della lista "Super Hot Papers in Science Since 2003".^[4] Alla fine del 2008 il satellite WMAP era ancora in funzione, mentre è prevista la sua dismissione per il mese di settembre 2009.

Obiettivi

 

La linea del tempo dell'universo, dall'inflazione al WMAP

Lo scopo primario del progetto WMAP è la misurazione delle differenze di temperatura nella radiazione cosmica di fondo. Le anisotropie della radiazione vengono quindi utilizzate per calcolare la geometria dell'universo, il suo contenuto e l'evoluzione, e per testare i modelli del Big Bang e dell'inflazione cosmologica.^[5] Per questo, questo satellite sta creando una mappa completa della radiazione di fondo, con una risoluzione di 13 arcominuti tramite una osservazione multi frequenza. Tale mappa, per assicurare una accuratezza angolare pari alla sua risoluzione, richiede alcuni errori sistematici, pixel di rumore non correlati tra loro ed una calibrazione accurata.^[5] La mappa è formata da 3,145,728 pixel e usa lo schema HEALPix per trasformare in pixel la sfera.^[6] Il telescopio misura inoltre la polarizzazione E-mode della radiazione di fondo^[5], e la polarizzazione in primo piano. ^[2] La sua vita è di 27 mesi: 3 mesi per ricercare la posizione L2, ed i restanti 24 mesi di osservazione.^[5]

Sviluppo

 

Paragone tra le sensibilità del WMAP e del COBE. I dati sono simulati

La missione MAP venne proposta alla NASA nel 1995, selezionata per uno studio approfondito nel 1996 e approvata per lo sviluppo definitivo nel 1997.^[7][8]

Il WMAP è stato preceduto da altri due satelliti per l'analisi della radiazione di fondo:

• la sonda sovietica RELIKT-1, la quale ha riportato i limiti superiori dell'analisi delle anisotropie della radiazione di fondo;
• la sonda statunitense COBE, la quale ha riportato fluttuazioni su larga scala della radiazione di fondo.

Vi sono stati anche altri 3 esperimenti, basati però sull'utilizzo di palloni sonda, che hanno analizzato piccole porzioni di cielo ma in modo più dettagliato:

• il pallone BOOMERanG;
• il Cosmic Background Imager;
• il Very Small Array.

Il WMAP, rispetto al suo predecessore COBE, ha una sensibilità 45 volte superiore, ed una risoluzione angolare più precisa di 33 volte.^[9]

La sonda

 

Illustrazione della sonda WMAP

Gli specchi primari del WAMP sono una coppia di gregoriani, di dimensioni 1,4 metri e 1,6 metri, rivolti in direzioni opposte tra loro, i quali focalizzano il segnale ottico su degli specchi secondari grandi 0,9 m x 1,0 m. Questi specchi sono stati modellati per ottenere delle prestazioni ottimali: una corazza in fibra di carbonio che protegge un nocciolo in Korex, ricoperto ulteriormente da uno strato sottile di alluminio e ossido di silicio. Gli specchi secondari riflettono il segnale a dei sensori ondulati, posti sul piano focale tra i due specchi primari.^[5]

 

Illustrazione dei ricevitori del WMAP

I ricevitori sono costituiti da dei radiometri differenziali sensitivi alla polarizzazione elettromagnetica. Il segnale viene amplificato quindi da un amplificatore a basso rumore di tipo HEMT. Sono presenti 20 alimentatori, 10 per ogni direzione, dai quali i radiometri raccolgono i segnali; la misura finale corrisponde nella differenza tra i segnali provenienti da direzioni opposte. La separazione azimuth direzionale è di 180 gradi; l'angolo totale è di 141 gradi.^[5] Per evitare di captare anche segnali di disturbo provenienti dalla Via Lattea, il WMAP lavora su 5 frequenze radio discrete, da 23 GHz a 94 GHz.^[5]

Proprietà del WMAP a differenti frequenze^[5]

Proprietà	Banda K	Banda Ka	Banda Q	Banda V	Banda W
Lunghezza d'onda centrale (mm)	13	9.1	7.3	4.9	3.2
Frequenza centrale (GHz)	23	33	41	61	94
Larghezza di banda (GHz)	5.5	7.0	8.3	14.0	20.5
Misura del raggio (arcominuti)	52.8	39.6	30.6	21	13.2
Numero di radiometri	2	2	4	4	8
Temperatura del sistema (K)	29	39	59	92	145
Sensibilità (mK s${\displaystyle ^{1/2}}$ )	0.8	0.8	1.0	1.2	1.6

Passive thermal radiators cool the WMAP to ca. 90 degrees K; they are connected to the low-noise amplifiers. The telescope consumes 419 W of power. The available telescope heaters are emergency-survival heaters, and there is a transmitter heater, used to warm them when off. The WMAP spacecraft's temperature is monitored with platinum resistance thermometers.^[5]

The WMAP's calibration is effected with the CMB dipole and measurements of Jupiter; the beam patterns are measured against Jupiter. The telescope's data are relayed daily via a 2GHz transponder providing a 667kbit/s downlink to a 70m Deep Space Network telescope. The spacecraft has two transponders, one a redundant back-up; they are minimally active — ca. 40 minutes daily — to minimize radio frequency interference. The telescope's position is maintained, in its three axes, with three reaction wheels, gyroscopes, two star trackers and sun sensors, and is steered with eight hydrazine thrusters.^[5]

La base del WMAP è costituita da un pannello solare di 5 metri di diametro, il quale tiene la sonda costantemente all'ombra durante il rilevamento della radiazione di fondo.^[10] Al di sopra del pannello si trova l'apparato di raffreddamento della sonda. Tra questo apparato di raffreddamento e gli specchi, è posizionato un cilindro per l'isolamento termico, dellla lunghezza di 33 cm.^[5]

Launch, trajectory, and orbit

 

The WMAP's trajectory and orbit.

The WMAP satellite arrived at the Kennedy Space Center on 20 April 2001, was tested for two months, mounted atop a Delta II 7425 rocket, and fired to outer space on 30 June 2001.^[9][7] It began operating on its internal power five minutes before its launching, and so continued operating until the solar panel array deployed. The WMAP was activated and monitored while it cooled. On 2 July, it began working, first with in-flight testing (from launching 'til 17 August), then began constant, formal work.^[9] Afterwards, it effected three Earth-Moon phase loops, measuring its sidelobes, then flew by the Moon on 30 July, enroute to the the L2 Sun-Earth Lagrangian point, arriving there on 1 October 2001, becoming, thereby, the first CMB observation mission permanently posted there.^[7]

 

WMAP's orbit and sky scan strategy

The satellite's orbit at Lagrange 2, (1.5 million kilometers from Earth) minimizes the amount of contaminating solar, terrestrial, and lunar emissions registered, and to thermally stabilize it. To view the entire sky, without looking to the sun, the WMAP orbits around L2 in a Lissajous orbit ca. 1.0 degree to 10 degrees,^[5] with a 6-month period.^[7] The telescope rotates once every 2 minutes, 9 seconds" (0.464 rpm) and processes at the rate of 1 revolution per hour.^[5] WMAP measures the entire sky every six months, and completed its first, full-sky observation in April 2002.^[8]

Foreground radiation subtraction

The WMAP observes 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.^[5]

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 also is reduced by using only the the full-sky map portions with the least foreground contamination, whilst masking the remaining map portions.^[5]

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

23 GHz	33 GHz	41 GHz	61 GHz	94 GHz

Measurements and discoveries

One-year data release

 

The first-year map of the CMB.

On 11 February 2003, based upon one year's worth of WMAP data, NASA published the latest calculated age, composition, and image of the universe to date, that "contains such stunning detail, that it may be one of the most important scientific results of recent years"; the data surpass previous CMB measurements.^[1]

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. Note that there are degenerations among the parameters, the most significant is between ${\displaystyle n_{s}}$  and ${\displaystyle \tau }$ ; the errors given are at 68% confidence.^[11]

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

Parameter	Symbol	Best fit (WMAP only)	Best fit (WMAP, extra parameter)	Best fit (all data)
Hubble's constant ( km⁄Mpc·s )	${\displaystyle H_{0}}$	0.72 ± 0.05	0.70 ± 0.05	${\displaystyle 0.71_{-0.03}^{+0.04}}$
Baryonic content	${\displaystyle \Omega _{b}h^{2}}$	0.024 ± 0.001	0.023 ± 0.002	0.0224 ± 0.0009
Matter content	${\displaystyle \Omega _{m}h^{2}}$	0.14 ± 0.02	0.14 ± 0.02	${\displaystyle 0.135_{-0.009}^{+0.008}}$
Optical depth to reionization	${\displaystyle \tau }$	${\displaystyle 0.166_{-0.071}^{+0.076}}$	0.20 ± 0.07	0.17 ± 0.06
Amplitude	${\displaystyle A}$	0.9 ± 0.1	0.92 ± 0.12	${\displaystyle 0.83_{-0.08}^{+0.09}}$
Scalar spectral index	${\displaystyle n_{s}}$	0.99 ± 0.04	${\displaystyle 0.93_{-0.07}^{+0.07}}$	0.93 ± 0.03
Running of spectral index	${\displaystyle dn_{s}/dk}$	—	-0.047 ± 0.04	${\displaystyle -0.031_{-0.017}^{+0.016}}$
Fluctuation amplitude at 8h−1 Mpc	${\displaystyle \sigma _{8}}$	0.9 ± 0.1	—	0.84 ± 0.04
Age of the universe (Ga)	${\displaystyle t_{0}}$	13.4 ± 0.3	—	13.7 ± 0.2
Total density of the universe	${\displaystyle \Omega _{tot}}$	—	—	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, ${\displaystyle 379_{-7}^{+8}}$  kyr); and the redshift of matter/radiation equality, ${\displaystyle 3233_{-210}^{+194}}$ . They determined the thickness of the surface of last scattering to be 195 ± 2 in redshift, or ${\displaystyle 118_{-2}^{+3}}$  kyr. They determined the current density of baryons, ${\displaystyle (2.5\pm 0.1)\times 10^{-7}cm^{-1}}$ , and the ratio of baryons to photons, ${\displaystyle (6.1_{-0.2}^{+0.3})\times 10^{-10}}$ . The WMAP's detection of an early reionization excluded warm dark matter.^[11]

The team also examined Milky Way emissions at the WMAP frequencies, producing a 208-point source catalogue. Also, they observed the Sunyaev-Zel'dovich effect at ${\displaystyle 2.5\sigma }$  the strongest source is the Coma cluster.^[6]

Three-year data release

 

A map of the polarization from the 3rd year results

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 (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.^[12]

Best-fit cosmological parameters from WMAP three-year results^[12]

Parameter	Symbol	Best fit (WMAP only)
Hubble's constant ( km⁄Mpc·s )	${\displaystyle H_{0}}$	${\displaystyle 0.732_{-0.032}^{+0.031}}$
Baryonic content	${\displaystyle \Omega _{b}h^{2}}$	0.0229 ± 0.00073
Matter content	${\displaystyle \Omega _{m}h^{2}}$	${\displaystyle 0.1277_{-0.0079}^{+0.0080}}$
Optical depth to reionization [13]	${\displaystyle \tau }$	0.089 ± 0.030
Scalar spectral index	${\displaystyle n_{s}}$	0.958 ± 0.016
Fluctuation amplitude at 8h−1 Mpc	${\displaystyle \sigma _{8}}$	${\displaystyle 0.761_{-0.048}^{+0.049}}$
Age of the universe (Ga)	${\displaystyle t_{0}}$	${\displaystyle 13.73_{-0.15}^{+0.16}}$
Tensor-to-scalar ratio [14]	${\displaystyle r}$	<0.65

[a] Template:Note Optical depth to reionization improved due to polarization measurements.^[15]
[b] Template:Note < 0.30 when combined with SDSS data. No indication of non-gaussianity.^[12]

Five-year data release

 

5 year WMAP image of background cosmic radiation (2008)

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 a billion years for the first stars to reionize the universe, and new constraints on cosmic inflation.^[16]

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 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.^[2]

 

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

Improvements to the spectra were in the 3rd acoustic peak, and the polarization spectra.^[2]

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.^[16]

The WMAP five-year data was combined with measurements from Type Ia supernova (SNe) and Baryon acoustic oscillations (BAO).^[2]

 

Matter content in the current universe

Best-fit cosmological parameters from WMAP five-year results^[2]

Parameter	Symbol	Best fit (WMAP only)	Best fit (WMAP + SNe + BAO)
Hubble's constant ( km⁄Mpc·s )	${\displaystyle H_{0}}$	${\displaystyle 0.719_{-0.027}^{+0.026}}$	0.701 ± 0.013
Baryonic content	${\displaystyle \Omega _{b}h^{2}}$	0.02273 ± 0.00062	0.02265 ± 0.00059
Cold dark matter content	${\displaystyle \Omega _{c}h^{2}}$	0.1099 ± 0.0062	0.1143 ± 0.0034
Dark energy content	${\displaystyle \Omega _{\Lambda }}$	0.742 ± 0.030	0.721 ± 0.015
Optical depth to reionization	${\displaystyle \tau }$	0.087 ± 0.017	0.084 ± 0.016
Scalar spectral index	${\displaystyle n_{s}}$	${\displaystyle 0.963_{-0.015}^{+0.014}}$	${\displaystyle 0.960_{-0.013}^{+0.014}}$
Running of spectral index	${\displaystyle dn_{s}/dk}$	−0.037 ± 0.028	${\displaystyle -0.032_{-0.020}^{+0.021}}$
Fluctuation amplitude at 8h−1 Mpc	${\displaystyle \sigma _{8}}$	0.796 ± 0.036	0.817 ± 0.026
Age of the universe (Ga)	${\displaystyle t_{0}}$	13.69 ± 0.13	13.73 ± 0.12
Total density of the universe	${\displaystyle \Omega _{tot}}$	${\displaystyle 1.099_{-0.085}^{+0.100}}$	1.0052 ± 0.0064
Tensor-to-scalar ration	${\displaystyle r}$	<0.20	—

The data puts a 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, ${\displaystyle 1091.00_{-0.73}^{+0.72}}$  (as well as age of universe at decoupling, ${\displaystyle 375,938_{-3115}^{+3148}}$  years) and the redshift of matter/radiation equality, ${\displaystyle 3280_{-89}^{+88}}$ .^[2]

The extragalactic source catalogue was expanded to include 390 sources, and variability was detected in the emission from Mars and Saturn.^[2]

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

23 GHz	33 GHz	41 GHz	61 GHz	94 GHz

Future measurements

File:Planck satellite.jpg

Artist's impression of the Planck satellite

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.^[7]

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 in early 2009. 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.

References

1. New image of infant universe reveals era of first stars, age of cosmos, and more, su gsfc.nasa.gov, NASA / WMAP team, 11 February 2003. URL consultato il 27 aprile 2008.
2. Hinshaw et al. (2008)
3. ^ Seife (2003)
4. ^ "Super Hot" Papers in Science, su in-cites.com, in-cites, ottobre 2005. URL consultato il 15-02-2009.
5. Bennett et al. (2003a)
6. Bennett et al. (2003b)
7. news sul WMAP: fatti, su map.gsfc.nasa.gov, NASA, 22 April 2008. URL consultato il 27 aprile 2008.
8. WMAP News: Events, su map.gsfc.nasa.gov, NASA, 17 aprile 2008. URL consultato il 19 febbraio 2009.
9. Limon et al. (2008)
10. ^ Questo avviene mantenendo la sonda ad un'angolatura costante di 22 gradi rispetto al Sole
11. Spergel et al. (2003)
12. Spergel et al. (2007)
13. ^ a
14. ^ b
15. ^ Hinshaw et al. (2007)
16. WMAP Press Release — WMAP reveals neutrinos, end of dark ages, first second of universe, su map.gsfc.nasa.gov, NASA / WMAP team, 7 March 2008. URL consultato il 27 aprile 2008.

Technical pages

• C. Bennett, et al., The Microwave Anisotropy Probe (MAP) Mission, in Astrophysical Journal, vol. 583, 2003a, pp. 1–23, DOI:10.1086/345346.
• C. Bennett, et al., First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Foreground Emission, in Astrophysical Journal Supplement, vol. 148, 2003b, pp. 97–117, DOI:10.1086/377252.
• G. Hinshaw, et al., Three-Year Wilkinson Microwave Anisotropy Probe (WMAP1) Observations: Temperature Analysis, in Astrophysical Journal Supplement, vol. 170, 2007, pp. 288–334, DOI:10.1086/513698.
• G. Hinshaw, et al., Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results (PDF), in Astrophysical Journal Supplement (submitted), 2008, arΧiv:0803.0732.
• M. Limon, et al., Wilkinson Microwave Anisotropy Probe (WMAP): Five–Year Explanatory Supplement (PDF), su lambda.gsfc.nasa.gov, 20 March 2008. Formato sconosciuto: PDF (aiuto)
• Charles Seife, Breakthrough of the Year: Illuminating the Dark Universe, in Science, vol. 302, 2003, pp. 2038–2039, DOI:10.1126/science.302.5653.2038.
• D. N. Spergel, et al., First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters, in Astrophysical Journal Supplement, vol. 148, 2003, pp. 175–194, DOI:10.1086/377226.
• D. N. Sergel, et al., Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology, in Astrophysical Journal Supplement, vol. 170, 2007, pp. 377–408, DOI:10.1086/513700.
• E. Komatsu et al.: WMAP Cosmological Interpretation 2008

External links

Altri progetti

• Wikimedia Commons

•   Wikimedia Commons contiene immagini o altri file su Vale maio/Sandbox2
• 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
• Charles Seife, With Its Ingredients MAPped, Universe's Recipe Beckons, in Science, vol. 300, n. 5620, 2003, pp. 730–731, DOI:10.1126/science.300.5620.730.

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